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CHAPTER 1 - COMPANION ANIMAL HEALTH PROBLEMS
Products for the treatment, control or prevention of companion animal health problems generate global sales of more than $5 billion. Parasite controls, vaccines and anti-infectives are among the most widely-used product groups, but treatments for an increasing range of health problems are being made available as the knowledge and understanding of other companion animal diseases improves. This chapter offers a brief overview of some major health problems affecting dogs, cats and horses, and outlines current methods of prevention or treatment. Products used to treat or control these conditions are discussed in detail in Chapter 2.
1.1 Infectious diseases
Companion animals are exposed to challenge from a broad range of infections, with causative agents including viruses, bacteria, mycoplasmas and protozoa. Prophylaxis against many viral infections is available in the form of vaccines. Multivalent products offering protection against the most common viral pathogens affecting companion animals have been available for many years, but vaccines against additional targets such as Lyme disease, feline leukaemia, feline infectious peritonitis and feline immunodeficiency virus and West Nile virus are more recent innovations. Vaccines are also available against mycoplasma and some protozoan infections.
Bacterial infections are common in all major companion animal species. They often occur as secondary or opportunistic infections following challenge with a viral disease. Common sites of infection include the respiratory, urogenital and gastrointestinal tracts, the oral cavity, skin and soft tissues, and traumatic or surgical wounds. Anti-infective drugs are administered to combat bacterial infections. Skin infections are common in both dogs and cats. Pyoderma, which often involves infection with more than one bacterium, is among the most widely diagnosed skin diseases. Allergic conditions can also cause skin infections, with flea allergy dermatitis (FAD) among the most significant examples.
1.1.1 Canine infectious diseases
Routine vaccination of dogs against a range of common pathogens is recommended, while vaccination against rabies is compulsory in a number of countries. Protection is conferred initially through the vaccination of puppies. Booster shots are administered to adult dogs, which may also be vaccinated against a range of other ‘non-core’ infections according to prevailing disease patterns and the lifestyle of individual animals.
The veterinary profession in most developed countries has begun to take a more discerning approach to the vaccination of both dogs and cats, following concerns that ‘over-vaccination’ of pets may be contributing to health problems. As a result, annual boosters are being replaced by regimes updating protection against some key diseases every two or three years.
Basic vaccination regimes usually include protection against coronavirus, adenovirus distemper, hepatitis, parainfluenza and parvovirus. Leptospirosis is also widely covered.
Coronavirus
The canine coronavirus is an RNA virus related to the feline coronaviruses, feline infectious peritonitis (FIP) virus and feline enteric coronavirus. Coronaviral gastroenteritis, which is highly infectious, is characterised by vomiting and diarrhoea. Asymptomatic coronaviral infection can cause damage to the intestinal epithelium in dogs, and can also increase the severity of opportunistic enteric infections.
Distemper
Canine distemper is caused by a paramyxovirus related closely to the measles and rinderpest viruses. The canine distemper virus has only one antigenic type, but differential syndromes can be associated with particular strains. Some strains suppress or impair the immune system, leading to complicating secondary infections.
Distemper is highly contagious, since the virus is spread via aerosol droplets from infected animals. Viral replication occurs initially in the lymphatic tissue of the respiratory tract. Infection is characterised by a diphasic fever, inflammation of the gastrointestinal and respiratory tracts, and leukopenia.
Hepatitis
A non-enveloped DNA virus, canine adenovirus type 1 (CAV-1), causes canine viral hepatitis. It is associated with liver, kidney, eye and respiratory disease. Most adult dogs infected with CAV-1 recover, but hepatitis is often fatal in young animals, for which vaccination is routinely recommended.
Viral hepatitis often occurs simultaneously with canine distemper. A related virus, canine adenovirus type 2, is responsible for tracheobronchitis, or ‘kennel cough’. CAV-1 and CAV-2 provide cross protection against each other.
Kennel cough
Canine infectious tracheobronchitis, or kennel cough, is an acute respiratory disease usually resulting in laryngitis, tracheitis and bronchitis. Any one or a combination of several viruses, bacteria and mycoplasmas can cause it. Canine parainfluenza virus, CAV-1, CAV-2, canine herpesvirus and Bordetella bronchiseptica are among the pathogens most commonly associated with the condition. B. bronchiseptica may act as a primary pathogen, but can often cause secondary infections where other pathogens are involved.
Kennel cough usually takes the form of a relatively mild but often persistent respiratory infection. It affects animals of all ages, causing loss of appetite, lethargy and loss of condition. Secondary infection can lead to fatal bronchopneumonia in puppies, or to chronic bronchitis in older dogs. The infection spreads rapidly among susceptible animals housed in close confinement (hence its common name). Stress and climatic extremes can increase the severity of the disease. Adenoviral components are included routinely in canine vaccination regimes.
Leptospirosis
Leptospirosis is an acute bacterial disease that can cause haemorrhagic enteritis, kidney failure and stomatitis. Common causative organisms in dogs include Leptospira canicola and L. copenhageni. Infection is characterised by weakness, vomiting, fever, jaundice, loss of appetite, depression and pinpoint haemorrhages around the lips and mucous membranes.
Rodents, which are common carriers of the disease, excrete leptospira organisms in their urine. Warm, damp conditions encourage survival of the organisms in the environment. Within the canine population, infection is usually spread in the urine of affected animals, which remain contagious for some time after they have recovered from the disease.
Vaccines are available against the Leptospira icterohaemorrhagiae serogroup, of which L. copenhageni is a member. The efficacy of vaccines in providing cross protection against other serovars is a matter of debate, but while they may not prevent infection they do reduce the severity of clinical disease. The re-emergence of leptospirosis in the US towards the end of the 1990s was attributed to a reduction in the number of dogs being routinely vaccinated against the disease.
Lyme disease
Lyme disease is a tick-borne, immune-mediated inflammatory disease caused by the spirochete, Borrelia burgdorferi. Major vectors include Ixodes dammini, I. pacificus, I. ricinus and I. scapularis. Lyme disease, which is also a zoonosis, can affect all dogs, cats, horses, cows and wild animals. Infection rates are highest in the summer and autumn months, when tick activity is at its peak.
Clinical signs of infection include arthritis and fever, but anorexia and fatigue are also common. Arthritis is usually episodic, but may become chronic. Neurological, cardiac, renal and reproductive signs have also been observed. Removal of ticks can prevent infection, since they do not transmit B. burgdorferi immediately. Lyme disease vaccines for use in dogs have been available since the early 1990s, and are administered widely in parts of the US where dogs are at particular risk of exposure.
Parvovirus
Canine parvovirus is a single-stranded DNA virus closely related to feline panleukopenia virus. Virus can remain viable in the environment for long periods, and vaccination against parvovirus in all dogs is recommended as part of routine protection regimes. Infection is often asymptomatic, and clinical disease may be triggered by stress. Puppies aged between six and 20 weeks are most at risk of infection, in the period when levels of maternal antibody decline and vaccination has not yet conferred protection.
Vomiting is often the first clinical sign of infection, with diarrhoea usually occurring within 24–48 hours. Anorexia, lethargy and rapid dehydration are also common, though clinical signs vary and may be exacerbated by the presence of opportunistic enteric infections. Extreme cases can result in death within hours of clinical infection, but most dogs recover within a few days.
Rabies
Rabies is caused by a lyssavirus of the rhabdovirus group. It affects all warm-blooded animals and is present in all continents with the exception of Australasia and Antarctica. Transmission of the virus usually occurs through the bite of infected animals. The disease is one of the most serious zoonotic infections monitored by international health agencies, and vaccination programmes involving both domesticated and wild animals are undertaken regularly in many countries.
Dogs are important vectors in Asia and South America, while wild animals are the most common causes of viral spread in Europe and North America. Baits containing rabies vaccine have been used widely in parts of mainland Europe, where foxes are the main vector.
1.1.2 Feline infectious diseases
Like dogs, cats are vaccinated routinely against a number of common infectious diseases, including calicivirus, Chlamydophila, panleukopenia and rhinotracheitis. Feline leukaemia preventatives have been added to vaccine protocols in many developed countries, and FeLV components are included in some multivalent vaccines. More recently, vaccines against feline infectious peritonitis and feline immunodeficiency virus have been made available.
Feline influenza
Feline viral rhinotracheitis and/or calicivirus are common causes of highly contagious, often multi-factorial respiratory infections referred to as feline influenza. Rhinotracheitis is caused by feline herpesvirus-1 (FHV-1). Feline caliciviruses and FHV-1 are host-specific, and account together for around 80% of all feline upper respiratory tract infections. Transmission of these viruses occurs via aerosol droplets. Calicivirus and rhinotracheitis coverage is included in most multivalent feline vaccines.
Rhinotracheitis is associated with fever, frequent sneezing, conjunctivitis and rhinitis. Mortality is relatively uncommon, but infection is often more severe in kittens and older cats, and is often complicated by secondary bacterial infections. There are many related strains of feline calicivirus, ranging from some that are non-pathogenic to others that cause pulmonary oedema and intestinal pneumonia. The epithelium of the oral cavity and deep lung tissues are the main sites of infection, and ulceration of the oral mucosa is common in affected animals. More virulent strains can destroy bronchial epithelial cells and cause pulmonary oedema.
Chlamydophila
Chlamydial conjunctivitis is caused by Chlamydophila psittaci. Infection usually affects the eye and upper respiratory tract, causing rhinitis, sinusitis or pharyngitis. Fever may occur as the disease progresses, and some animals may experience relapses after apparent recovery. Feline vaccine protocols usually include chlamydia components, while antibiotics are prescribed for treatment of existing infections.
Feline infectious peritonitis
Feline infectious peritonitis (FIP) is a progressive, contagious viral disease that is usually fatal in clinically infected animals. Infection is characterised by fibrinous polyseositis, accumulation of fluid in body cavities, disseminated pyogranuloma formation, hypergammaglobulinaemia and immune-mediated phenomena.
The feline infectious peritonitis virus (FIPV) is a mutant of the more common form of feline coronavirus, and is often generated during primary or secondary coronavirus infection. Virus is shed in the faeces, saliva and urine of affected animals, through direct contact, and possibly by maternal transmission. The vast majority of cats exposed to coronavirus become infected with FIP, but only about 10% develop the clinical FIP syndrome. Vaccines against FIPV have been available commercially since the early 1990s.
Feline leukaemia
Feline leukaemia virus (FeLV) remains a major cause of morbidity and mortality in cats, despite the widespread use of vaccines against the disease. Three key subgroups of the virus are recognised:
• type A viruses, which are found in all naturally infected cats, and which are generally less pathogenic than other subgroups
• type B viruses, which are isolated in around half of infected animals
• type C viruses, which are relatively uncommon, being isolated in only 1% of cases
Persistently infected animals are the main reservoir of the virus, the incidence of which is related directly to feline population densities. Virus is excreted in saliva, tears, urine and faeces.
FeLV causes several different disease syndromes, including leukaemia, anaemia, tumours, reproductive failure and neurological problems. The simultaneous presence of viral subtypes A and B is often associated with tumour development, while subtype C is associated with erythroid hypoplasia, leading to severe anaemia. Infected animals either develop a persistent infection or recover and acquire immunity. Persistently infected cats frequently become emaciated, suffering from anaemia, lethargy and anorexia. Most persistently infected cats eventually die as a consequence of the acquired immunodeficiency syndrome, feline immunodeficiency virus (see below).
Diagnostic tests for FeLV are available, but are not capable of differentiating between viral subtypes. Vaccines against the disease were introduced for the first time in the 1980s, and second-generation products available today offer significantly improved levels of protection against the development of persistent viraemia.
Panleukopenia
Feline panleukopenia, also known as infectious enteritis or feline distemper, is caused by a highly infectious parvovirus similar to the mink enteritis virus. Panleukopenia is a highly contagious, often fatal condition, with mortality rates especially high in kittens. Clinical signs of infection include fever, depression and anorexia, followed by the development of vomiting and diarrhoea. Virus is shed in the urine and faeces of affected animals for several weeks after apparent recovery, and is present in all secretions.
Antibodies to feline panleukopenia virus (FPV) are found in a high proportion of unvaccinated cats, and most infections are subclinical. Panleukopenia is often more serious in young animals, however, with mortality rates among young kittens sometimes reaching 90%. The disease is observed relatively infrequently in developed countries, however, thanks to the widespread use of effective vaccines.
Feline immunodeficiency virus
Feline immunodeficiency virus (FIV) is a lentivirus that was first isolated in 1986. It is endemic in free-roaming cats, with rates of infection exceeding 10% in some areas. FIV often develops in cats with persistent FeLV infection, but FIV can also be transmitted independently of feline leukaemia infection. Virus is shed mainly in the saliva of affected animals, and bites are the principal mode of transmission. As a result, the lifestyle of individual animals affects the degree of risk to exposure, with free-roaming, male and elderly animals most widely affected.
Most cats recover from an initial syndrome that includes fever, lymphadenopathy and neutropenia, and may show no clinical signs of disease for months or years thereafter. However, infection is permanent, and an unknown proportion of affected animals eventually succumb to immunodeficiency, resulting in chronic secondary or opportunistic infections of the respiratory, gastrointestinal and urinary tracts as well as the skin. Diagnostic tests for FIV have been developed, and a vaccine against the disease is now available.
1.1.3 Equine infectious diseases
Respiratory syndromes caused by infections such as equine influenza virus and the equine herpesviruses 1 and 4 (EHV-1 and EHV-4), are among the most common infectious diseases of horses. Vaccination against these and a number of other common infectious diseases is practised widely. Prophylactics against some other infections have been commercialised recently.
African horse sickness
African horse sickness (AHS) is an acute or sub-acute viral disease characterised by signs of respiratory and circulatory impairment. Transmission of the orbivirus responsible for AHS is via arthropods, with Culicoides spp representing the principal vectors. Nine immunologically distinct viral types have been identified.
AHS outbreaks usually follow periods of heavy rain that alternate with hot and dry conditions. The disease is endemic to the African continent, and outbreaks have been recorded in the Middle East and Spain. Mortality can reach up to 90%, depending on the virulence of the viral strain involved and the susceptibility of affected animals. Rates of mortality are highest where the acute respiratory form of the virus is involved. High fever is followed by dyspnea and spasmodic coughing in affected animals, with death usually occurring within a week of first clinical signs.
Insecticides are used to control arthropod vector populations in susceptible areas, while vaccines are available for the protection of horses against all nine serotypes of the virus. Affected animals are slaughtered immediately following confirmation of outbreaks, while unaffected animals are vaccinated. Most countries not affected by the disease impose quarantine requirements on horses imported from areas where it is prevalent.
Equine encephalomyelitis
Arboviruses are the most common cause of equine encephalomyelitis, but Sarcocystis neurona and Neospora spp may also cause encephalitis. Virus is carried by mosquitoes and other insect vectors, and can affect a range of vertebrate hosts, including humans. In horses, infection is characterised by signs of CNS dysfunction, and results in moderate to high mortality.
The most pathogenic viruses affecting horses are alphaviruses of the family Togaviridae, including Eastern, Western, Highlands J and Venezuelan viruses. The North American variant is the most pathogenic and antigenically homogenous of two distinct antigenic variants of Eastern equine encephalomyelitis (EEE). It is found in eastern Canada and the US, and in the Caribbean islands. The South American variant is less pathogenic. Mortality resulting from infection with the North American variant can reach 90% on occasions. Clinical signs include lymphopenia, leukopenia and fever. Neurological signs become evident at a later stage.
Insecticides and other insect control strategies are used to reduce the risk of infection in areas where the virus circulates. Vaccines are also available in both mono- and multivalent form. Vaccination protocols usually consist of an initial two-shot regime followed by annual or biannual boosters, depending on geographical risk factors.
Equine infectious anaemia
Equine infectious anaemia (EIA) is an acute or chronic viral disease affecting horses worldwide. The virus responsible for the disease is related to the human immunodeficiency virus (HIV), but is not known to be zoonotic. Transmission occurs via the transfer of blood cells from infected animals. EIA is usually detected sporadically, but epidemics may occasionally be witnessed when blood-sucking fly populations are particularly abundant.
Clinical signs of EIA include intermittent fever, depression, weakness, weight loss, oedema and progressive or transitory anaemia. Mortality is relatively infrequent, however, and many affected animals may not exhibit clinical signs of infection. Nevertheless, testing and control programmes are operated in many countries. Animals testing positive for EIA in the US must either be quarantined or slaughtered.
A vaccine against EIA has been developed by researchers in China, and is understood to have been trialled in the US. No vaccine is yet available on a commercial basis, however, and providing complete protective immunity remains a challenge.
Equine influenza
Equine influenza is an acute, highly contagious respiratory disease caused by Orthomyxoviridae type A influenza viruses. Two distinct viruses (Orthomyxovirus A/Equi-1 and Orthomyxovirus A/Equi-2) have been found in equine populations across much of the world. They are endemic in many countries, causing sporadic clinical cases and mild infection in susceptible horses. Viral transmission occurs through contact with infective respiratory secretions.
Onset of the disease is abrupt, with a fever that usually lasts less than three days in uncomplicated infections. Coughing is a common clinical sign, and may last for several weeks, especially where secondary bacterial infections are involved. Mildly infected animals usually recover in 2–3 weeks, but severely affected horses may take several months to recover fully. Mortality is rare, and is usually the result of complications caused by secondary infections.
Vaccines against both the A/Equi-1 and A/Equi-2 viruses are available, and are used widely, especially in areas where equine populations are significant, such as training centres. Levels of efficacy and duration of immunity that are offered by some vaccines are relatively limited, however, due in part to antigenic drift, which has resulted in the emergence of new viral strains. Several vaccine manufacturers have introduced improved, updated equine influenza vaccines recently, offering cover against major strains encountered in both Europe and North America.
Equine protozoal myeloencephalitis
Equine protozoal myeloencephalitis (EPM) is a neurological disease caused by the protozoan parasite, Sarcocystis neurona. The disease is prevalent in North America and parts of South America, but has not been recorded elsewhere except in horses exported from affected countries.
Infection is via the ingestion of sporocysts contained in contaminated feed or water. S. neurona can infect all parts of the CNS, and neurological signs of infection vary. Treatment with antifolate drugs such as sulfadiazine or sulfamethoxazole, in combination with pyrimethamine, is indicated in affected animals. Bayer launched a new EPM treatment, ponazuril (a derivative of the anticoccidial, toltrazuril), in the US during 2001.
Equine viral arteritis
Equine viral arteritis (EVA) is an acute, contagious disease caused by an RNA virus of the Arterivirus family. Only one serotype of the virus has been identified, but antigenic variation has been observed in isolates examined from different countries. Transmission is largely via respiratory or venereal routes, with aerosol transmission from infected animals implicated most widely. Outbreaks of EVA are relatively uncommon, but are often associated with the movement of horses.
Most cases of acute infection are sub-clinical, and clinical signs of the disease vary widely in both nature and severity. Leukopenia, depression, anorexia and limb oedema may all be associated with infection, with clinical signs usually more severe in young, old or weak animals. Abortion may occur in the acute phase of the disease, and may also affect sub-clinically infected mares. Abortion rates are often less than 10% in affected populations, but have been known to reach 50%.
Modified, live virus vaccines are available for protection against EVA, but are not recommended for use in pregnant animals. Symptomatic treatment in severe cases where clinical signs are evident usually includes the administration of antipyretic, anti-inflammatory and diuretic drugs.
Equine viral rhinopneumonitis
Infection with equine herpesvirus-1 (EHV-1) and EHV-4 can cause respiratory disease, abortion, neonatal death and paresis. The respiratory form of the infection, known as equine viral rhinopneumonitis (EVR), is characterised by fever, coughing and nasal and ocular discharge. The severity of the disease is dependent on the viral strain involved, the immune status of affected animals, pregnancy status and, sometimes, age.
Transmission is via direct or indirect contact with infective nasal discharges, aborted foetuses, placentas or placental fluids. Some infected animals appear to be latent carriers of both virus types. Vaccines are available against both EHV-1 and EHV-4, but reliable humoral immunity is not long lasting, and not all strains to which horses may be exposed are covered.
Strangles/distemper
Strangles, or equine distemper, is a bacterial infection caused by Streptococcus equi equi. Infection, which is characterised by inflammation of the upper respiratory tract and the development of abscesses on adjacent lymph nodes, is by inhalation or ingestion. Toxins released by the S. equi equi organism cause inflammation of the respiratory tract, followed by lymphadenitis. Mortality is rare, but morbidity can reach 100% in equine populations not previously exposed to infection.
Vaccines are available, but do not always prevent infection. The disease takes a milder form in vaccinated animals, however. Opinions regarding the use of antibiotic therapy in affected horses vary. High doses of penicillin are recommended where antibiotics are prescribed.
Tetanus
Horses are particularly susceptible to the neurotoxin of Clostridium tetani, which can cause tetanus in a broad range of species. Infection of wounds with clostridial spores is the most usual cause of the disease, with clinical signs of infection including hyperaesthesia, tetany and tonic convulsions. Vaccination with the toxoid confers immunity against infection, and tetanus toxoids are often administered as part of equine vaccination regimes.
West Nile virus
West Nile fever is a zoonotic disease caused by infection with the flavivirus, West Nile virus. Wild birds and horses are among the most commonly infected species, with virus transmission occurring through insect vectors (often mosquitoes). WNV is common in parts of Africa and Asia, but had never been detected in North America until 1999. Less than 1,000 cases were reported in the US equine population in 2001, but that figure rose to almost 15,000 in the following year.
Some affected animals show few or no signs of infection, but clinical symptoms usually include weakness, ataxia, depression, fever, tremors and reduced appetite. Paralysis, recumbency and eventual death may occur in severe cases. Approximately 30% of US horses in which the disease has been confirmed to date have either died or been euthanised.
A vaccine against WNV developed by Fort Dodge Animal Health was granted a conditional licence by US regulators in 2001. Full approval of the product was registered in February 2003.
1.2 Parasitic infections
Effective control of internal and external parasites is essential to the health and welfare of both small animal species and horses. Even modest parasite populations can affect the condition and wellbeing of host animals while, left unchecked, some parasites can eventually cause severe health problems and even death. Regular administration of parasite control products is recommended for all animals, though the type of medication and frequency of dosing may be dependent on geographical and climatic factors, as well as the lifestyle of the host animal.
1.2.1 Endoparasites
Gastrointestinal nematodes, tapeworms, lungworms, heartworm, protozoa and larval stages of arthropods are among the internal parasite challenges faced by companion animal species. Endoparasites may damage the host animal directly, forming cysts, feeding on tissue or blood, or by larval migration. Damage to tissues and internal organs causes loss of condition and increases host animal susceptibility to secondary infections. Young animals are most susceptible to infection, but regular treatment of both young and adult animals with worming products is recommended.
Broad-spectrum anthelminthics from a range of chemical classes are used to treat and control infections. Many broad-spectrum products are not effective against tapeworms, but other active ingredients with specific activity against cestodes are available, and are often included in combination treatments. Avermectin and milbemycin products are effective in the prevention of adult heartworm infestation.
| Parasite | Dogs | Cats | Horses |
| GI nematodes | Toxocara canis Toxascaris leonine Ancylostoma caninum Uncinaria stenocephala | Toxocara cati Toxascaris leonine Ancylostoma tubaeforme Uncinaria stenocephala Trichuris spp | Paracaris spp Strongylus spp Trichostrongylus axei Oxyuris spp Habronema spp |
| Tapeworm | Dipylidium caninum Echinococcus granulosus Taenia hydatigena | Dipylidium caninum Taenia taeniaeformis | Anoplocephala perfoliata Anoplocephala magna Anoplocephaloides mammillana |
| Lungworm | Oslerus osleri Angiostrongylus vasorum | Aelurostrongylus abstrusus | Dictyocaulus arnfeldi |
| Heartworm | Dirofilaria immitis | Dirofilaria immitis | |
| Protozoa | Leishmania spp Giardia spp | Toxoplasma gondii Giardia cati | Sarcocystis neurona |
| Bots | | | Gasterophilus spp |
Gastrointestinal nematodes
The large roundworms, Toxocara canis (dogs) and T. cati (cats), are common in small animals, especially in puppies and kittens. T. canis is most significant, causing occasionally fatal infections in untreated puppies. It can also be passed on to humans if infective eggs are ingested. In puppies, transplacental transfer is the most usual method of infection with T. canis.
Hookworm infestation (Ancylostoma spp and Uncinaria spp) may result from the ingestion of infective larvae from the environment, but A. caninum, which is the most significant hookworm parasite in most tropical and subtropical regions, is also passed to offspring through the colostrum and milk of infected bitches. U. stenocephala is the most common canine hookworm in cooler, more temperate regions. Hookworms feed on host blood, and congenital infection can cause anaemia and general debility in young animals.
The whipworm, Trichuris vulpis, usually inhabits the caecum of host animals. Eggs are passed in faeces and can remain viable for long periods in suitable conditions. Heavy worm burdens cause inflammatory reactions in the caecum, with weight loss and diarrhoea often observed.
Horses are affected by a broad range of gastrointestinal parasites, including large and small strongyles, stomach worms and tapeworms. The three major species of large strongyle are Strongylus vulgaris, S. edentatus and S. equinus. Infection is through the ingestion of infective larvae, which migrate extensively before developing to maturity in the large intestine. S. vulgaris larvae migrate extensively in the cranial mesenteric artery, where they can cause parasitic thrombosis and arteritis. More than 40 species of small strongyles have been identified in horses. Most are significantly smaller than Strongylus spp, but some may be almost as large, and one of these – Triodontophorus tenuicollis – can cause severe ulceration of the colon wall.
Other common gastrointestinal parasites affecting horses include Strongyloides westeri, which is a problem in young foals; the small stomach worm, Trichostrongylus axei, which can cause chronic catarrhal gastritis (a condition also associated with stomach worm (Habronema spp) infections); and the pinworm, Oxyuris equi.
Heartworm
Heartworm disease (dirofilariasis) is caused by Dirofilaria immitis, a filarial nematode whose immature stages are transmitted via insect hosts (usually mosquitoes). The parasite can affect a number of species, but its impact is most significant in dogs. Cats are also affected, however, and heartworm preventatives are administered to many small animals in countries where the parasite is prevalent. These include the US, Australia, Japan and parts of southern Europe.
Adult females of the Dirofilaria immitis parasite inhabit the right ventricle of the heart and adjacent large blood vessels, causing damage to the heart, blood vessels and lungs. Adult worms shed microfilariae, which are ingested by mosquitoes biting infected hosts. Clinical signs of infection include respiratory disease, exercise intolerance, weight loss and fever. Heavy worm burdens can cause heart failure.
Adulticides may be administered to eliminate adult worms in infected animals, but this can cause serious complications in animals with heavy parasite burdens. The avermectins and milbemycins are used widely to prevent the development of adult worms.
Horse bots
Horse bots are the larvae of botflies (Gasterophilus spp). Several minor species are recognised, but G. intestinalis (the common bot), G. haemorrhoidalis (the nose or lip bot) and G. nasalis (the nose bot) are most important. The larvae of all three species remain embedded in the tongue or oral mucosa before migrating to the stomach, where ulceration may be caused at attachment sites. They eventually pass out in the faeces of host animals and pupate in the soil for a period of several weeks before emerging as adult botflies.
Lungworm
Lungworms infect the lower respiratory tract of host animals, resulting in bronchitis or pneumonia. Dictyocaulus arnfeldi is the parasitic nematode responsible for lungworm infection in horses. It can cause severe coughing and lung lesions. Differential diagnosis with problems caused by other respiratory diseases can be difficult.
In dogs, the tracheal worm, Filaroides (Oslerus) osleri is the most commonly observed lungworm parasite. Infected bitches often pass on infection to puppies through saliva. Larvae pass into the bloodstream and are carried to the lungs and bronchi. A dry cough is the most common symptom of infection.
Aelurostrongylus abstrusus in the most common lungworm found in cats. Worms become deeply embedded in the lungs, and eggs are forced into alveolar ducts where they form small nodules. Once hatched they are coughed up, swallowed or passed out in the faeces.
Protozoa
Of the protozoan parasites that affect companion animals, Leishmania spp, which are responsible for the zoonotic disease, leishmaniasis, are among the most significant. Cats and a number of other domestic species are rarely affected, but dogs are a common host. Leishmania donovani and L. infantumi are implicated in the Mediterranean region, while L. chagasi, L. brazilensisi and L. mexicana are most important in central and south America. Infection in dogs is prevalent in both regions, while several areas of the US are also affected. Leishmaniasis is a visceral, often fatal disease characterised by cutaneous or mucocutaneous lesions, intermittent fever, anaemia, enlarged lymph nodes, weight loss, lameness and renal failure.
Toxoplasmosis caused by the zoonotic protozoa, Toxoplasma gondii, is often spread from cats to sheep, and is a common cause of abortion in the latter species. In cats, infection can be fatal, though subclinical infection is more common. Infected pregnant women can pass the parasite on to their children, where it can cause blindness and mental retardation.
Giardiasis is a chronic intestinal infection caused by Giardia spp. Infection is common in dogs and cats, but is also seen in other species, including horses. Giardia infections in dogs and cats can result in weight loss and chronic diarrhoea, especially in young animals. A vaccine against Giardia lamblia is now available for the protection of dogs against Giardiasis.
Tapeworm
Dipylidium caninum (dogs) and Taenia taeniaeformis (cats) are the most common tapeworms affecting housed pets, but hunting dogs and both dogs and cats allowed to roam freely in rural areas may be exposed to a range of other tapeworm species. Adult cestodes rarely cause serious health problems in dogs and cats. Clinical signs of infection with heavy infestations include loss of condition, irritability, poor appetite, colic and mild diarrhoea.
Fleas and lice act as intermediate hosts for D. caninum. Eggs are ingested and passed on to end hosts by the adult parasite during the larval stage. Taenia spp have more than one larval stage, and require two or three hosts, including humans and livestock as well as dogs and cats.
1.2.2 Ectoparasites
Dogs and cats are commonly affected by fleas, ticks, lice and mites, all of which can cause intense irritation, leading to dermal infections where skin is damaged as a result of constant scratching or chewing. Ticks can also spread infectious agents such as the spirochete, Borrelia burgdorferi, which causes Lyme disease. The most significant ectoparasites affecting horses are mites, lice, flies and biting midges.
Table 1.2: Common ectoparasites by host species
| Parasite | Dogs/cats | Horses |
| Fleas | Ctenocephalides spp | |
| Ticks | Ixodes dammini Ixodes holocyclus | |
| Mites | Demodex canis Sarcoptes scabiei var canis Notoedres cati Otodectes cynotis Cheyletiella spp | Psoroptes spp Sarcoptes scabiei var equi Chorioptes spp |
| Lice | Felicola subrostrata Trichodectes canis Linognathus setosus | Bovicola spp Haematopinus spp |
| Flies | | Haemotabia spp Hydrotaea spp Musca spp Stomoxys spp Culicoides spp |
Fleas
Ctenocephalides felis and C. canis are the most common fleas affecting small animal species. C. felis is the most significant, feeding and breeding on both cats and dogs. The flea life cycle averages 18–21 days, including egg laying, three larval stages and the emergence of adult fleas when conditions are favourable.
Fleas are blood sucking parasites which cause severe irritation to host animals. Scratching and biting by animals trying to relieve that irritation can result in skin damage and subsequent infection. A significant proportion of animals also experience an allergic reaction to flea bites, and flea allergy dermatitis (FAD) is among the most commonly diagnosed skin diseases in companion animals. Allergic reactions to flea saliva cause inflammation and itching. In hypersensitive animals, protracted clinical signs have been observed following a single exposure to fleas.
Ectoparasite controls are available for administration both directly to host animals and for use on bedding or other materials in the home of affected pets. Many are insecticidal, but some act as insect growth regulators, interrupting the flea life cycle.
Ticks
Ticks are responsible for the transmission of a wide variety of infectious agents. In dogs, these include Lyme disease, canine babesiosis and erlichiosis. In horses, piroplasmosis is caused by Babesia spp spread by ticks. Simultaneous infection with multiple tick-borne pathogens is common in dogs exposed to high tick populations. Ticks may also harm host animals directly, causing skin wounds that are susceptible to secondary infections, and in some cases by inducing toxicosis or even paralysis.
Of the three tick families, Ixodidae and Argasidae are most significant as companion animal ectoparasites. Major vectors of Borrelia burgdorferi, the spirochete responsible for Lyme disease, include Ixodes dammini, I. pacificus, I. ricinus and I. scapularis. The tropical horse tick, Dermacentor nitens, is another species that has emerged as an important parasite in the US following its spread from more southerly regions. D. nitens is responsible for the transmission of Babesia caballi in horses – an infection that is of substantial importance in the US horseracing industry.
Ticks are difficult to control because they exist on a variety of hosts and have a natural resistance to some chemicals. High concentrations of acaricidal chemicals and rapid action are required to eliminate infestations successfully. Control is achieved using long-acting acaricides in spot-on, spray and collar formulations. The novel insecticide, fipronil, possesses activity against some tick species as well as fleas, while the recently introduced avermectin compound, selamectin, possesses claims against both fleas and ticks as well as certain other ecto- and endoparasites.
Mites
Mites cause irritation, rubbing, scratching, restlessness and hair loss on host animals. They are responsible for mange in a broad range of species, including dogs, cats and horses.
Demodectic mange is caused by Demodex canis in dogs. Mites infest the hair follicles and sometimes the sweat glands of host animals, transmitting mange when they move from an infested to a susceptible host. Puppies can be exposed at an early age if an infected bitch nurses them. Symptoms of demodectic mange range from small patches of hairlessness around the eyes or over the body, to extensive bloody lesions covering the entire body. Demodectic range is often associated with immunosuppression in dogs.
Sarcoptic mange, caused by the Sarcoptes scabiei mite, is more severe than demodectic mange. Individual sub-species affect dogs, cats and horses, respectively. Sarcoptes mites gather initially on the head, throat and shoulders of host animals, and are readily transmitted by contact to other animals. Eggs are laid in tunnels formed by females burrowing into the skin. Infestation causes intense itching, prompting affected animals to scratch, chew or rub affected areas. This causes inflammation and can result in secondary infections. Affected skin becomes dry, thickened and wrinkled. In untreated animals, emaciation, debilitation and even death can result.
Otodectic mange is caused by the burrowing mites, Otodectes cynotis and Psoroptes cuniculi. Otodectes affects aural and facial areas of both cats and dogs. In horses, Psoroptes is found on either the body or the ears. Chorioptes spp are found on the lower legs and hocks of horses.
Lice
Lice are wingless, flattened insects with legs adapted for clinging to hairs and feathers on host species. Species affecting domestic animals are generally host-specific. The biting louse, Trichodectes canis, and the sucking louse, Linognathus setosus, are common ectoparasites of dogs. Felicola subrostrata is a biting louse found on cats, while the biting louse Bovicola and the sucking louse Haematopinus affect horses.
Lice infestation causes irritation, and affected animals often have an unthrifty (poor growth or development) appearance and rough coats. In horses, early sites of infestation include the mane and tail. Bovicola spp cause hair loss and irritation, with consequent rubbing, while infection with Haematopinus spp causes loss of condition and, in some cases, anaemia.
Flies
Flies are a common parasite of horses. Most species cause intense irritation when present in large numbers, but some biting flies can inflict direct damage and are also vectors for other parasites. The stable fly, Stomoxys, is a blood sucking species, which acts as an intermediate host of the nematode, Habronema, for example. Non-biting flies such as Hydrotaea and Musca are often the source of intense irritation in horses.
Horses are also prone to dermatitis resulting from hypersensitivity to the saliva of biting midges (Culicoides spp). Exposure to this parasite and some fly species is reduced during the summer months by the application of fly repellents, spraying with insecticides or stabling in the afternoon hours.
1.3 Musculoskeletal problems
Bone and joint disorders are the most common musculoskeletal problems diagnosed in companion animals. In treatment terms, lameness is the most frequent condition observed in horses, with problems especially common in racing and other competition animals. In the small animal sector, arthritic conditions are being diagnosed more often as the longevity of pets increases, with osteoarthritis particularly prevalent in the canine population.
1.3.1 Canine osteoarthritis
Osteoarthritis can affect animals of all ages, but is more common in older individuals. Primary osteoarthritis is caused by ‘wear and tear’ associated with age, while secondary osteoarthritis is often diagnosed following episodes involving infectious diseases, maligned fractures or hip dysplasia. Surveys in the US indicate that around one-fifth of all adult dogs are affected by osteoarthritis, with that figure rising to 40% in animals aged seven or over. The condition is the most common cause of chronic pain in dogs, and is being recognised increasingly as a problem in cats. It is still under-reported, with many owners attributing mild or even moderate symptoms to general ageing without seeking veterinary attention.
The condition initially affects the articular cartilage, which begins to crack and fibrillate. Cartilage softens and connective tissue within the cartilage matrix degrades, destroying the matrix. Inflammation of the synovial membrane contributes to cartilage degradation, while surrounding muscles atrophy and joints begin to thicken. Affected animals experience constant dull pain, are less mobile and can bear less weight on affected limbs. Pain is exacerbated by low temperatures, sudden changes of climate and heavy exercise.
Walking and swimming can help to keep affected muscles and joint capsules mobile, while rest and weight loss can also be beneficial. Anti-inflammatory drugs are prescribed widely in animals with osteoarthritis. While they cannot halt the progression of the underlying disease, they relieve pain associated with the condition; helping to keep affected animals mobile.
1.3.2 Equine lameness
Horses are prone to a broad range of musculoskeletal problems that often result in lameness. Competition animals are at particularly high risk, with exercise-related injuries such as fractures or tendon damage diagnosed much more frequently in performance horses. Degenerative joint diseases such as osteoarthritis, and conditions such as laminitis and navicular disease are among the other causes of equine lameness.
Surgical intervention is required for serious fractures or tendon injuries, but anti-inflammatory drugs are used widely in the treatment of acute or chronic inflammatory conditions. Phenylbutazone and flunixin meglumine are among the most established equine anti-inflammatory drugs, but several new generation non-steroidal anti-inflammatory drugs (NSAIDs) have been commercialised for equine use since the early 1990s.
1.4 Cardiovascular disease
Like other age-related diseases, cardiovascular problems are being diagnosed more commonly in pets as companion animal longevity increases. New and effective treatments for cardiovascular conditions have been also been made available to the veterinary profession over the past 20 years, while an increasing proportion of owners are willing to spend significantly on extending the active life of their pets. These factors have combined to raise the profile of companion animal cardiovascular disease.
1.4.1 Congestive heart failure
Heart failure is caused most often by reductions in the volume of blood pumped to the heart, or by abnormal heart rates. Three stages (asymptomatic, mild-to-moderate and advanced) and three types (left-sided, right-sided and generalised) are recognised. All follow a similar path, with reduced cardiac output triggering increased heart rate and cardiac contractility, and the constriction of arteries and veins.
Declining blood pressure prompts the release of renin by the kidneys, which converts angiotensinogen to angiotensin I. This in turn is transformed to angiotensin II by angiotensin-converting enzyme (ACE), a potent vasoconstrictor which also promotes sodium retention by the kidneys and stimulates the secretion of aldosterone by the adrenal cortex, resulting in further sodium and water retention. This compensatory chain of events is beneficial in the short term, but causes problems in chronically affected animals. It is responsible for a number of conditions associated with further reductions in cardiac output and the eventual progression to terminal heart failure.
Dietary management of animals with congestive heart failure (CHF) is usually recommended, but drug therapy is also indicated in many cases. Advanced cases may entail the use of several drug types to treat or control the various conditions associated with the syndrome, which can include pulmonary congestion and oedema, cardiac arrhythmias, reduced cardiac output and vasoconstriction.
Diuretics are widely prescribed in the treatment of pulmonary oedema, while vasodilating drugs – notably ACE inhibitors – are used to block the vasoconstriction that occurs as a result of chronic reductions in cardiac output. Veterinary-approved versions of several ACE inhibitors have been commercialised since the early 1990s.
1.4.2 Degenerative valve disease
Degenerative valve disease, or endocardiosis, is the most common form of heart disease encountered in older dogs, affecting more than half of all animals aged eight years and over. It is particularly common in some small breeds.
The condition is a degenerative disorder of the heart valves, which become thickened and distorted, leaking blood into other chambers when the heart contracts. The mitral valve is most commonly affected, but both the mitral and tricuspid valves are affected in a significant proportion of cases. Pulmonary oedema is a common result of increases in pressure within the left atrium where the mitral valve is affected, while atrial arrhythmias are also likely, resulting in further reductions in cardiac output. Excessive vasoconstriction and sodium and water retention are characteristic in chronic cases.
Symptoms of endocardiosis develop gradually, and affected animals can live with the condition for years if treatment is administered to slow the progression of early clinical signs. Four stages are recognised, with vasodilator therapy recommended at stage two, diuretic treatment at stage three and inotropic agents at stage four, when vasodilators and diuretics become less effective.
1.4.3 Cardiomyopathy
Primary muscle disorders such as dilated hypertrophic cardiomyopathy (HCM) are among the most common diseases of the myocardium in dogs, while HCM is also recognised as the most common acquired cardiac condition in cats. HCM is most common in middle aged male dogs, and is also more prevalent in large breeds. Incidence of the disease in cats has been reduced significantly since the discovery that taurine deficiency was responsible for a large proportion of cases. Prescription diets containing appropriate concentrations of taurine are now widely available.
HCM is characterised by loss of cardiac contractility, resulting in reduced cardiac output. Like other degenerative cardiac conditions, this triggers compensatory mechanisms including excessive vasoconstriction and retention of sodium and water, precipitating congestive heart failure. Diuretics are often prescribed to control congestion; positive inotropes to improve contractility; and vasodilators to reduce cardiac stress. Beta blockers may also be used to control tachydysrhythmias such as atrial fibrillation.
1.5 Cancer
Recent surveys indicate that cancer is now the leading disease-related cause of mortality in US dogs. Advances in the diagnosis and treatment of cancer in companion animals, increases in the incidence of tumour-related problems in ageing companion animal populations. Requests from a growing number of pet owners that their pets be treated for cancer rather than euthanised have all combined to raise the profile of neoplastic disorders in dogs and cats.
Oncology drugs prescribed for companion animals are all off-label human medicines to date, and many widely used human drugs are suitable for use in pets because of the similarities that exist between cancers in humans and those observed in dogs and cats. As a result, companion animal studies are performed widely on development-stage oncology drugs for use in human medicine. Cancer treatment is a major focus of human drug research, and the veterinary sector will benefit in future from the availability of new, more effective treatments. Veterinary-specific cancer drugs will also be brought to market.
1.5.1 Mast cell tumours
Skin cancers are among the most common tumour types observed in dogs, while mast cell tumours are the most widely-diagnosed skin cancer, accounting for around 20% of all canine skin cancers. Incidence is particularly high in certain breeds, including Boxers and Labrador Retrievers. Cutaneous mast cell tumours are common in cats, but are generally benign, with less than one in five recurring after initial surgical removal of the tumour mass. Mast cell tumours are diagnosed most often in middle-aged or elderly animals. They can occur at any point on the body surface as well as in internal organs, though the limbs, thorax and abdomen are most commonly affected.
Preferred treatment options depend on the clinical stage at which disease is diagnosed, but surgical removal of the tumour mass is usually indicated in early-stage cases. Surrounding tissue and regional lymph nodes may also be removed. Radiotherapy is administered where tumours are believed to have spread beyond surgical margins, and to prevent recurrence of the tumour at the surgical site.
Late-stage or inoperable tumours are usually treated with a combination of radio- and chemotherapy, though partial removal of inoperable tumours may also be indicated in some cases. Treatment with prednisone followed by injections of triamcinolone at the lesion site is one recommended regime. Chemotherapy with vinca alkaloids (vincristine or vinblastine), L asparaginase and cyclophosphamide has also been practised with a degree of success.
1.5.2 Lymphosarcoma
Lymphosarcomas or lymphomas are solid tumours that form in the organs of the lymphatic system. They are caused by the invasion of these organs by cancerous lymphocytes. Malignant canine lymphosarcoma is among the most commonly diagnosed cancers in dogs.
In cats, a substantial proportion of lymphosarcoma cases are thought to be caused by infection with the feline leukaemia virus (FeLV). It is possible that viral infections may also trigger the development of tumours in some canine cases, but genetic factors and other immune system abnormalities are also thought to be involved. Lymphosarcoma can affect animals of all ages, but is more common in middle-aged and older dogs.
Standard protocols for the treatment of canine lymphosarcoma include combination chemotherapy, which is used to affect remission. This can last for a year or more, depending on the characteristics of individual tumours and the stage at which the condition is diagnosed. Lymphomas formed by T lymphocytes are believed to be more aggressive than those developed from B-lymphocytes, and the prognosis for affected animals is less positive as a result.
A five-drug chemotherapy regime, including the administration of vincristine, L-asparaginase, prednisone, cyclophosphomide and doxorubicin has been practised widely in dogs, eliciting tumour regression in the majority of patients for at least six months. Maintenance therapy is administered thereafter, with asparaginase and prednisone usually withdrawn. Single drug therapy with doxorubicin or prednisone can elicit briefer remissions.
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