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CHAPTER 1 - MAJOR LIVESTOCK AND COMPANION ANIMAL PARASITES
All major livestock species are exposed to a variety of internal and external parasites which, if not treated, can have a major impact on levels of productivity, as well as the health and welfare of host animals. Left unchecked, heavy parasite loads can become life threatening, especially in young stock. Parasites can also jeopardise the health of livestock indirectly, by acting as vectors for potentially deadly diseases, including some zoonotic infections.
Companion animals are also prone to infestation by both internal and external parasites. Aside from the niche sector populated by competition animals, infestations do not have economic consequences, but most companion animal owners are keen to preserve the health and welfare of their charges, while the majority of household pet owners prefer to keep dwellings free from parasites such as fleas for hygiene reasons.
Parasite control strategies include management of the environment in which animals are kept, and the application of products designed to prevent infestation or to rid animals and premises of existing parasite populations. This chapter describes the most important parasites of major livestock and companion animal species, and outlines preferred methods of treatment and control.
1.1 Cattle parasites
Cattle are raised commercially or as part of subsistence agriculture systems across most of the world. Parasite challenges encountered by cattle vary according to both the prevailing climate and methods of production. Pasture-grazed animals are exposed to worm and liver fluke infestations that are usually at their heaviest in warm, moist climates, for example, while housed cattle are often affected by nuisance flies and lice. Flies and ticks also plague pasture-grazed herds, especially in tropical and subtropical regions.
1.1.1 Endoparasites
Gastrointestinal nematodes are the most important internal parasites affecting cattle in most countries where they are raised commercially. Lungworms, tapeworms and liver flukes can also affect the health and productivity of grazing stock, while protozoan parasites such as Neospora caninum and Eimeria spp. can also cause problems for producers and their animals.
Table 1.1: Major internal parasites of cattle
Source: Compiled from information in The Merck Veterinary Manual and Black’s Veterinary Dictionary.
Moniezia benedeni
Neospora caninum
. . . Gastrointestinal nematodes
Haemonchus spp, Ostertagia spp and Trichostrongylus spp are among the most prevalent gastrointestinal parasites of cattle. Larvae from all three species hatch shortly after eggs have been passed in the faeces of host animals, and become infective some two weeks later. Their ingestion perpetuates infection in animals grazing affected pasture. Infective larvae can survive on pasture for almost a year if climatic conditions are favourable. This means that eggs deposited towards the end of one grazing season can result in the presence of infective larvae on pasture grazed the following spring.
Ostertagiasis is a recognised condition resulting from infection with the brown stomach worm, O. ostertagi. Ingested larvae enter the cavity of the abomasum (fourth stomach), where moulting usually occurs within four days. Larvae grow over the following two to four weeks (the pre-patent period) before undergoing a final moult and emerging as young adult worms.
Two types of the condition are recognised. Type I ostertagiasis, which is diagnosed most often in calves up to one year old, involves predominantly adult worms and is most responsive to anthelmintic treatment. Type II ostertagiasis can be more difficult to treat, since it involves the emergence of previously dormant or inhibited fourth-stage larvae in large numbers. Some anthelmintics possess limited efficacy against inhibited larval stages of Ostertagia spp and other worms capable of infecting animals after lying dormant through the winter. The ability to deal effectively with these parasite populations is a favoured trait in anthelmintics as a result.
Although the impact of Haemonchus, Ostertagia and Trichostrongylus infestations is usually most pronounced in calves, older animals not previously exposed to significant challenge may also be affected. Persistent scouring, anaemia and anorexia are common signs of infection. Progressive weight loss, listlessness and poor coat condition are other signs. Particularly heavy worm burdens in susceptible animals can be fatal before clinical signs become apparent.
Cooperia spp inhabit the small intestine of cattle. Heavy infestations with C. punctata are usually most significant, causing scouring, anorexia and emaciation. C. oncophora infections are usually less serious, but can result in significant weight loss and reduced productivity. Cooperia spp have become increasingly significant in some regions, including Australia, where losses resulting from infection have risen sharply in recent years.
Unlike Cooperia spp, the hookworm, Bunostomum phlebotomum, is a bloodsucking parasite, and can cause anaemia and rapid weight loss in affected animals. Like Cooperia spp, hookworms usually occupy the upper reaches of the small intestine, where adult worms attach themselves to the mucosa. They are voracious feeders, and relatively low adult worm burdens can cause death in susceptible calves. Infection can follow the ingestion of larvae, but larval penetration of the lower limbs is more common. This can affect both the behaviour and condition of affected animals.
Like hookworms, the intestinal threadworm, Strongyloides papillosus, infests cattle as a result of either ingestion or larval skin penetration. Young calves can also be affected by the transmission of larvae in colostrum. Dairy calves are most at risk. Only female Strongyloides spp are actively parasitic in cattle, embedding themselves in the mucosa of the small intestine. Clinical signs of infection are rare, except in cases of heavy infestation. Weight loss and reduced productivity are common, however.
. . . Tapeworms
Tapeworm infestation (Moniezia expansa, M. benedeni) is relatively common in calves during their first summer at pasture, but while heavy worm burdens can affect weight gain and productivity, tapeworms are generally regarded as a less serious threat to the health of young cattle than many other gastrointestinal parasites. Infection is difficult to prevent, since the Moniezia lifecycle includes free-living mites as an intermediate host.
. . . Lungworms
The cattle lungworm, Dictyocaulus viviparus, inhabits the lower respiratory tract of host animals. Infection is often subclinical, but heavy worm burdens can cause persistent coughing and respiratory problems. The syndrome is commonly referred to as ‘husk’, or parasitic bronchitis. Secondary bacterial infections may develop in untreated animals, increasing the likelihood of fatalities in infected herds from conditions such as pneumonia.
D. viviparus eggs laid by adult females hatch directly in the bronchi of host animals, but infective larvae may also be present in faeces and on pasture. Some infective larvae may survive through the winter, infecting animals grazed on the same pasture in the following year. Persistently infected animals are a more important source of transmission, however, acting as carriers of the disease.
Lungworm infection can cause significant economic losses in affected cattle herds. Its impact is greatest in dairy cows grazed on pasture in warm, moist conditions such as those prevailing in Ireland and western areas of the UK. Affected animals lose weight and deteriorate in condition. Milk yields can be reduced substantially in adult dairy cows.
. . . Liver fluke
The common liver fluke, Fasciola hepatica, is endemic in large areas of Europe and North America, while the giant liver fluke, F. gigantica, affects livestock in parts of Asia and Africa. The economic impact of these parasites is at its greatest in sheep, and they are described in more detail in section 1.2.1. They also cause substantial losses to cattle producers, however, and the global impact of liver fluke infestation across the livestock sector as a whole is estimated at around $3 billion a year. In cattle, losses result from declines in the condition and the performance of affected animals, including poor feed conversion and reduced weight gain, lower rates of fertility in breeding cows, liver condemnation in slaughtered beef animals and reduced milk yield in dairy cows.
. . . Protozoa
Cattle are exposed to a range of protozoan parasites, some of which cause distinct problems in affected animals, while others play a key role in the development of multifactorial disease conditions.
Eimeria spp, which are the most significant parasites affecting poultry, also affect cattle, and while the economic impact on production is much less serious, coccidiosis can reduce rates of feed conversion efficiency and weight gain in growing calves. Clinical signs of infection include fever, diarrhoea and rapid weight loss in severe cases, though death is uncommon.
Toxoplasmosis and neosporosis are both caused by protozoan parasites. Toxoplasma gondii infection is more important in sheep and pigs, but Neospora caninum – previously confused with T. gondii but recognised as a distinct and economically important source of losses in the cattle sector since the 1980s – is a major cause of abortion in dairy cattle.
The complete lifecycle of N. caninum is still not understood fully, but it can be transmitted transplacentally in cattle, and surviving calves may be affected. Affected cows may abort at any point during gestation, but abortion is most common at 4-7 months. Infection can also result in stillborn, weak or paralysed calves. Abortion in affected herds is often sporadic, but occasional abortion ‘storms’ involving up to one-third of pregnant animals have been observed.
In some areas, N. caninum is thought to be responsible for around one-third of all abortions in cattle. Losses resulting directly from abortions are the most visible economic impact of infection, but research has also shown that milk production in first-lactation heifers can be reduced by up to 5% where infection is present.
. . . Treatment and control of endoparasites
Problems resulting from infection with parasitic worms and fluke are observed most frequently in calves during their first season at pasture. Older animals gradually build up a degree of natural immunity to internal parasites, while rotational grazing programmes, which involve running older animals on pasture vacated by susceptible calves, is a recommended method of controlling worm burdens.
Broad-spectrum anthelmintics and endectocides are effective against a range of internal parasites, and are administered regularly to grazing cattle. Macrocyclic lactones such as ivermectin, doramectin and moxidectin are the most widely used endectocides in cattle, while levamisole and active ingredients from the benzimidazole class dominate bovine anthelmintic use.
Animals are generally treated prior to turnout, and may require further dosing at regular periods throughout the grazing season. Long-acting formulations have reduced the regularity with which such products must be administered, and sustained-release bolus formulations offering protection for the entire grazing system have been developed for some active ingredients, including ivermectin, oxfendazole and morantel.
Young animals arriving at feedlots from pasture are treated routinely with anthelmintics or endectocides, while treatment of all animals bought into holdings is advised in order to avoid importing parasite burdens. This has become increasingly important following the development of parasite populations that are resistant to particular chemical classes. Guidance for producers in some regions now involves the routine administration of products from more than one class to incoming stock in order to prevent the spread of resistant worm populations.
Few broad-spectrum anthelmintics and none of the endectocides commercialised to date possess activity against liver fluke. As a result, specialist flukicides (including active ingredients such as clorsulon or triclabendazole) must be used to deal with fluke infestations. Combination products containing a flukicide and an endectocide or broad-spectrum anthelmintic have been developed, and are used widely by cattle producers in most developed markets. Merial’s Ivomec Plus brand, which contains ivermectin and clorsulon, is a prominent example.
An oral lungworm vaccine containing partially inactivated infective D. viviparus larvae has been available for some time, and is now part of Intervet’s cattle product portfolio following its acquisition of Hoechst Roussel Vet. Immunological protection against the protozoan parasite, N. caninum, is also available now, but chemical-based prophylaxis remains the preferred method of coccidiosis control in cattle. The ionophore anticoccidials used widely in poultry production are also administered on an in-feed basis to calves. Growth promotion is a side effect of prophylactic dosing with anticoccidials which, as a result, are used much more widely than might otherwise be the case in the beef cattle sector.
1.1.2 Ectoparasites
Cattle are exposed to a broad range of ectoparasites, including flies, mites, ticks and mosquitoes. Some cause little direct damage, but can affect productivity through their nuisance effect, which causes unsettled, irritable behaviour. Others have a direct impact on the health of host animals, while some, such as ticks and mosquitoes, act as vectors for more serious infections.
. . . Flies
Biting or nuisance flies cause intense annoyance to livestock where they are present in large numbers. In extreme cases they may prevent cattle from grazing, or may cause stampedes. Production losses result from loss of condition, reduced weight gain and, in dairy animals, reduced milk output. Bite wounds may also become infected, while further damage can be caused by affected animals rubbing or scratching, especially where they do so against barbed wire fencing. Flies are also responsible for the transmission of disease, while the larvae of some species – notably the screw-worm fly and warble flies – cause serious damage to host animals.
Table 1.2: Major external parasites of cattle
Source: Compiled from information in The Merck Veterinary Manual and Black’s Veterinary Dictionary.
H. irritans exigua
Morellia spp
Buffalo fly
Stomoxys calcitrans
Musca autumnalis
Hydrotaea irritans
Musca domestic
Mosquitoes
Stable fly
Face fly
Head fly
House fly
Lucilia spp
Hypoderma lineatum
Common cattle grub
Linognathus vituli
Solenopotes capillatus
Long-nosed cattle louse
Little blue cattle louse
Psoroptes ovis
Chorioptes bovis
Psoroptic mange
Chorioptic mange
Chrysomyia bezziana
Old World screw-worm
Amblyomma spp
Dermacentor spp
Haemaphysalis spp
Hyalomma spp
Ixodes spp
Black flies (Simulium spp), the horn fly (Haematobia irritans), the buffalo fly (H. irritans exigua), the stable fly (Stomoxys calcitrans) and mosquitoes are among the most significant blood-feeding members of the diptera order that affect cattle. Heavy infestations can have a major impact on the wellbeing and productivity of affected animals if measures are not taken to control parasite numbers. The health of host animals can also be compromised by the transmission of disease directly into the bloodstream by biting parasites.
Male black flies feed on nectar from flowers, but females must feed on the blood of host animals prior to egg laying. The ears, neck, head and abdomen of cattle are favoured feeding sites. Black fly swarms can inflict severe damage, and mortality is possible in untreated cattle exposed to heavy attack. Local reactions or infections at feeding sites are more common, however.
Like black flies, female mosquitoes feed on the blood of host animals prior to egg laying. Where mosquito populations are particularly high, affected animals may suffer from anaemia, while mosquitoes are also important vectors for the transmission of several serious diseases affecting cattle, including zoonoses such as malaria and yellow fever.
Both male and female adult stable flies (Stomoxys calcitrans) are blood feeders. Horses are the preferred host, but cattle are often affected, with blood loss and irritation causing reduced feed efficiency and subsequent reductions in the productivity of affected animals.
The horn fly, Haematobia irritans, is a significant parasite in many of the world’s major cattle-producing regions. Unlike some other biting flies, adult horn flies spend their entire life on host animals. They are also frequent feeders, and the damage, pain and irritation suffered by affected cattle causes substantial economic losses. In the worst affected areas of the US, weight gain in beef cattle can be reduced by up to 15%, while milk production in dairy herds may decline by up to 20%. The buffalo fly (H. irritans exigua) is similar in habit to the horn fly, but is distributed less widely. It is found most commonly in Australia and parts of south east Asia.
Tsetse flies (Glossina spp) are responsible for the transmission of trypanosomes, which cause fatal zoonoses such as African sleeping sickness. They are the subject of large-scale control efforts across extensive areas of the African continent and parts of southern and central America. Cattle production with animals other than resistant native breeds is impossible in badly affected areas.
Non-biting flies can also affect the productivity of cattle as a result of their nuisance impact, and can inflict indirect physical damage by acting as intermediate hosts for other parasites. The face fly (Musca autumnalis) and head fly (Hydrotaea irritans) are among the most common examples. Face flies are vectors for eyeworm parasites (Thelazia spp), and contribute to the spread of infectious bovine keratoconjunctivitis (pinkeye), while head flies are implicated in the spread of summer mastitis.
Screw-worm flies are highly damaging parasites found in central and south America and parts of Africa and Asia. Adult females lay their eggs on fresh skin wounds, where larvae hatch within a few hours and burrow into the flesh at wound sites to feed on fluids and live tissue. They emerge after several days, detaching themselves from host animals and burrowing into the ground, where they pupate.
Infestation can result in the rapid enlargement of originally minor wounds, and untreated animals will be targeted continuously, resulting in almost certain death unless effective treatment is administered. The New World screw-worm, Cochliomyia hominivorax, has been eradicated from the US and Mexico, but remains a problem in central and south America. The Old World screw-worm, Chrysomyia bezziana, inflicts similar damage in parts of Africa and southern/south-eastern Asia, where efforts to control and eradicate the parasite are also under way.
Like screw-worm flies, the damage caused by warble flies (Hypoderma spp) to host animals is inflicted largely by larval stages. Female flies attach their eggs to the legs or lower body of host cattle, and larvae burrow into the skin after hatching. H. bovis larvae migrate through the skin to the spinal canal, forming cysts or ‘warbles’ on the hide of affected animals. Larvae eventually emerge through these cysts and drop to the ground to pupate. The value of meat and hides of affected cattle can be reduced significantly where warbles have inflicted damage, while the death of larvae within host animals can cause severe, sometimes fatal reactions.
. . . Lice
Sucking lice of the order Siphunculata are blood-feeding parasites that usually infest the head and shoulder areas of host animals. Biting lice (order Mallophaga) feed on skin debris, and can affect all areas of host animals. Lice are largely host-specific, with the biting louse (Damalinia bovis), the long-nosed cattle louse (Linognathus vituli), short-nosed cattle louse (Haematopinus eurysternus) and the little blue cattle louse (Solenopotes capillatus) inflicting most damage to bovines.
. . . Mites
Mite infestation is responsible for the development of mange on cattle. The disease is highly contagious, and can be a serious problem in housed animals. Infestation causes intense itching and irritation, resulting in rubbing, scratching and damage to the hides of affected cattle. Productivity is also compromised.
Sarcoptic mange, caused by Sarcoptes scabiei var bovis, is transmissible to humans (as scabies). Clinical signs of infection in cattle include lesions to the head, neck and shoulders, with intense pruritis. Lesions spread rapidly to other areas of the body if the condition is not treated.
Psoroptic mange, caused by the Psoroptes ovis mite, is observed widely in feedlot cattle in south and central areas of the US. Pruritis associated with infection is usually seen first on the shoulders and rump of affected animals, but spreads rapidly if untreated. Secondary bacterial infections are common in severe cases. Psoroptic mange can claim the lives of untreated calves, while weight loss and reduced milk production are common in older animals.
Chorioptic mange, caused by Chorioptes bovis, affects the legs of host animals in early stages, but can spread if left untreated. It is the most commonly diagnosed type of mange in US cattle, though clinical signs are not always evident and infection, which is more prevalent in winter, often regresses without treatment in the summer months.
Infection with demodectic mange, which is caused by Demodex bovis, is transmitted from cows to suckling calves. The condition is not associated with pruritis, but can result in substantial damage to the hide of affected animals. Lesions are usually most prevalent on the withers, neck, back and flanks.
. . . Ticks
Ticks are among the most important external parasites of domesticated livestock, causing massive economic losses through the transmission of serious disease agents as well as a direct impact on the health and welfare of host animals. Babesiosis, anaplasmosis and a broad range of serious viral diseases are spread by ticks, which are the focus of widespread control and eradication efforts.
Boophilus microplus is among the most important ticks affecting cattle, and is a major problem for producers in many tropical and subtropical regions. It has been eradicated successfully in the US and parts of Argentina, but eradication efforts in several other countries have been less successful – usually because B. microplus can survive on a range of other hosts.
In economic terms, B. microplus remains the most damaging bovine ectoparasite in Australia, despite the fact that it is not prevalent in large parts of the country. It also inflicts major losses in many less developed countries, causing weight loss and reduced milk production in affected animals, and acting as a vector for diseases such as babesiosis and anaplasmosis. Indigenous breeds of cattle often develop a degree of resistance to ticks and tick-borne diseases, but the continued prevalence of these parasites effectively prevents the introduction of otherwise genetically superior breeds in large parts of the world.
. . . Treatment and control
Protecting cattle effectively against ectoparasites is difficult, partly because of the fact that many parasites feed only intermittently on bovine hosts, and partly because most can also make use of alternative hosts. Eradication of some parasites has been achieved in areas of the world, but is not a viable option elsewhere. Notable successes include the eradication of screw-worm in the US and Mexico, which was based on the use of biological control methods (the sterilisation and release of sterile male fly populations), while the cattle tick, Boophilus microplus, has also been eradicated in the US and isolated areas of south America.
The development of resistance to widely used insecticide classes has complicated chemical-based efforts to limit the impact of ectoparasites on livestock production, but this remains the only approach available to producers in many areas of the world. Chemical control programmes include the environmental treatment of premises and ground or aerial spraying in areas where infestations are particularly heavy. Repellents are also used widely, while insecticidal products are applied directly to livestock in a variety of forms, including ear-tags, pour-ons, sprays and dips.
Parasites such as the horn fly are easier to control than many other diptera, since adults live almost exclusively on host cattle. Sprays and insecticidal ear tags are among the most common treatment methods, though resistance to some insecticide classes (including the pyrethroids) has developed in some regions. Ear tags have also been used widely to control nuisance fly populations, but their impact in badly affected areas is often limited. The movement of stock away from heavily infested areas is often the only completely effective method of control, though for many producers this is not an option.
Pour-on, line-on and spot-on insecticide formulations have been developed to control and eradicate warble fly populations. Organophosphates were traditionally among the products of choice, but products from more recently developed chemical groups are also effective. New delivery routes for the administration of warble fly treatments have also been developed, including injectable, oral paste and topical pour-on formulations of avermectin and milbemycin endectocides that are effective against Hypoderma larvae.
The avermectins and milbemycins are also effective against both sucking and biting lice, and pour-on formulations of products such as ivermectin, doramectin and moxidectin are used widely for this indication. Lice control was previously achieved through dipping or spraying with insecticidal chemicals. Sprays and dips are also used to control mites responsible for mange in cattle, though the avermectin and milbemycin classes have once again offered producers effective alternatives and new routes of administration, including injectable formulations that are effective against both psoroptic and sarcoptic mange.
A variety of strategies has been pursued in an effort to control tick populations that inflict heavy losses on cattle. These include environmental management, biological controls and the use of acaricides. Cattle breeds such as the Zebu, which have developed resistance to ticks and tick-borne diseases, have also been introduced successfully in some countries where indigenous breeds are badly affected, while immunological approaches to tick control have also been attempted. A vaccine against the cattle tick, Boophilus microplus, was made available to producers in Australia during the early 1990s.
1.2 Sheep parasites
As grazing ruminants, sheep are exposed to many of the same parasites that affect cattle, though the impact of individual parasites in the two species varies widely and sheep are also affected by some parasites that do not affect cattle.
1.2.1 Endoparasites
Gastrointestinal nematodes, tapeworms, lungworms and liver flukes are the most significant parasites affecting the health and productivity of sheep. As in the cattle sector, climatic factors have a major impact on the type and extent of parasite challenge.
Table 1.3: Major internal parasites of sheep
Source: Compiled from information in The Merck Veterinary Manual and Black’s Veterinary Dictionary.
Ostertagia circumcincta
Trichostrongylus axei
Nematodirus spp
Bunostomum trigonocephalum
Brown stomach worm
Stomach hair worm
Intestinal worms
Hookworm
Fasciola gigantica
. . . Gastrointestinal nematodes
Like cattle, the most important gastrointestinal nematodes affecting sheep are Trichostrongylus spp, Ostertagia spp and Haemonchus spp. Ostertagia spp and T. axei usually represent the main challenges in temperate climates, while Haemonchus contortus is a major problem in many tropical and subtropical regions. Losses resulting from infestation are attributable mainly to reduced weight gain and poor wool growth, though gastrointestinal parasitism can be fatal in young lambs if it is not treated at an early stage. Losses from mortality are generally highest in extensively reared flocks, where regular monitoring of individual animals is difficult.
Like cattle, sheep are susceptible to ostertagiasis (see section 1.1.1). They are also prone to haemonchosis (caused by infestation with Haemonchus contortus), which may take hyperacute, acute or chronic forms. Chronic infections are most common, and may often be subclinical, though weight gain and wool growth are reduced in affected animals. Hyperacute infections, which result from heavy infestation, can cause rapid death with few prior signs.
Sheep are also affected by Bunostomum spp (hookworms) and Nematodirus spp (intestinal worms). Nematodirus infestation is a significant problem for producers in major sheep rearing countries such as New Zealand, Australia and the UK. Mortality of up to 20% has been reported among lambs in badly affected flocks. Onset of clinical symptoms is sudden, and includes profuse diarrhoea and extreme dehydration. Deaths can occur within two days of clinical symptoms becoming apparent.
. . . Tapeworms
Clinical disease symptoms previously attributed to the tapeworm, Moniezia expansa, are now believed to be the result of nematode infestation, and tapeworms are believed to have few clinical effects in sheep. Heavy burdens still affect productivity, however, and some tapeworm species do have a more significant impact in particular regions. The fringed tapeworm, Thysanosoma actinioides, is responsible for significant losses in parts of the western US, for example. These stem from the propensity of T. actinioides to inhibit the small intestine, bile ducts and pancreatic ducts. Livers from affected sheep are condemned when tapeworms are found in the bile ducts.
. . . Lungworms
Like cattle, grazing sheep are exposed widely to infestation by lungworms (Dictyocaulus spp). The impact of these parasites on sheep production is more modest, however, chiefly because the most common sheep lungworm, D. filaria, is less pathogenic than D. viviparus, which is responsible for substantial losses in the cattle sector. Nevertheless, lungworm infection can affect the productivity of sheep, and may have a significant impact on the productivity of animals in badly affected flocks. Primary pneumonia caused by lungworm infestation can also cause secondary viral and bacterial respiratory problems.
. . . Liver flukes
While tapeworms and lungworms generally cause less serious problems in sheep than in cattle, the reverse is true where liver fluke (Fasciola spp) infestation is concerned. The common liver fluke, Fasciola hepatica, is most prevalent, but F. gigantica is the main cause of losses in parts of Asia and Africa.
Chronic fascioliosis is a year-round problem, with adult fluke able to live in the bile ducts of sheep for several years (most are shed from infected cattle within six months). It can be fatal if untreated. The acute form of the disease is seasonal, and is caused by exposure to high fluke burdens on infected pasture. Liver fluke infestation is also recognised as a contributing factor to infectious necrotic hepatitis, an often fatal disease caused by the clostridial organism, C. novyi, which multiplies in areas of the liver where necrosis has been caused by fluke migration.
. . . Protozoa
Toxoplasmosis, caused by the protozoan parasite, Toxoplasma gondii, is a zoonotic infection that can have a significant impact on the reproductive performance of affected sheep flocks. Members of the cat family are the only known definitive hosts for T. gondii, and toxoplasmosis in sheep is usually acquired by the ingestion of faeces containing T. gondii oocysts. Toxoplasmosis is a recognised cause of early embryonic death, abortion and stillbirth in affected flocks.
Many of the products used to treat and control endoparasite infestations in cattle (see section 1.1.1) are also used in sheep, though dosages, formulations and recommended administration regimes vary. Levamisole, benzimidazole anthelmintics, the macrocyclic lactones and specialist flukicides are among the most widely used active ingredients.
Clinical disease resulting from internal parasitism is more common in sheep than in cattle, and treatment regimes should be managed accordingly. Ewes are particularly vulnerable to clinical infection in the weeks leading up to and following lambing, for example.
Managing the use of existing anthelmintic and endectocide classes is particularly important for sheep producers in countries such as Australia, where resistance to all of the major product groups is an increasing problem. Combination products containing active ingredients from two or more groups are being used on an increasingly widespread basis there.
1.2.2 Ectoparasites
Ectoparasites inflict major losses on the commercial sheep production industry worldwide. Economic losses are more serious than those sustained in cattle because of the value attached to the wool and hides of sheep. Damage caused by external parasites can also be seriously debilitating, however, and effective treatment and control measures are essential in order to protect the health and welfare of sheep. As in cattle, external parasites are also important vectors of other serious diseases in sheep.
. . . Flies
Blowfly infestation is a major problem for sheep producers in many countries. Lucilia cuprina is the most significant species in the southern hemisphere, while L. sericata is the main species encountered in northern hemisphere markets such as the UK.
Table 1.4: Major external parasites of sheep
| Parasite type | Species | Common name(s) |
| Flies | Lucilia cuprina L. sericata Phormia regina Protophormia terraenovae Melophagus ovinus Hydrotaea irritans Oestrus ovis | Blowfly Greenbottle fly Black blowfly Black blowfly Sheep ked Head fly Nasal fly, nasal bot |
| Lice | Damalinia (Bovicola) ovis Linognathus ovillus Linognathus pedalis | Sheep biting louse Blue louse Sheep foot louse |
| Mites | Psoroptes ovis Chorioptes bovis Sarcoptes scabiei var ovis | |
| Ticks | Ixodes ricinus I. rubicundus I. scapularis I. pacificus a | Sheep tick; pasture tick Paralysis tick Black legged tick na |
Second-stage larvae are the cause of most damage, following their development from eggs laid below the tip of the fleece. Larvae hatch and begin feeding around the breech areas, burrowing beneath the upper layers of skin. This process, known as ‘fly strike’, can be lethal if treatment is not administered at an early stage, since large numbers of maggots rapidly cause significant damage to affected areas. Annual losses suffered by Australian sheep producers as a result of fly strike by L. cuprina are estimated at up to Aus$200 million ($154 million).
The sheep ked, Melophagus ovinus, is a wingless member of the diptera order. Female keds produce only one larva, but M. ovinus is nevertheless one of the most economically important ectoparasites of sheep, spreading rapidly through flocks in the winter and early spring if left unchecked. Keds are bloodsucking parasites that favour the neck, shoulder, flanks and rump of host animals as feeding sites. Bitten animals suffer from pruritis, causing scratching and rubbing that damages the fleece and can open fresh wounds. Infestation also causes discolouration of the fleece, and can damage hides. Heavy burdens can have a significant impact on the productivity of affected animals.
The head fly, Hydrotaea irritans, is a non-biting fly, but its presence in large numbers can have a significant impact on sheep, and H. irritans is regarded as a significant parasite in many European countries. The health and wellbeing of affected animals is affected by constant irritation, while rubbing by affected animals to ease irritation can cause the formation of open wounds around the head and neck area. Subsequent feeding by flies on blood and other secretions exacerbates wounds and can result in the development of secondary infections.
. . . Mites
Economically, mite infestation has a bigger impact on sheep than other major livestock species due to their effect on the quality of wool and hides. The Psoroptes ovis mite is responsible for psoroptic mange in sheep, commonly referred to as sheep scab. This is a notifiable condition in some countries, and producers are required to treat sheep on a regular basis as part of control or eradication programmes. P. ovis pierces the skin of host animals, feeding off tissue fluids. Affected sheep scratch and rub irritated areas, causing significant damage to both skin and wool.
. . . Lice
Lice infestations cause reduced productivity in affected sheep, and also have an impact on wool growth and quality. Losses resulting from infestation in the Australian sheep flock are believed to be of a similar magnitude to those caused by blowfly strike (up to Aus$200 million a year; $154 million a year). Sheep are affected by the biting louse, Damalinia (Bovicola) ovis, and by three types of sucking louse (Linognathus spp).
. . . Ticks
Ticks are a cause of both direct and indirect health problems in sheep. Heavy infestations can cause anaemia, loss of appetite and weight loss, while secondary infections may develop at biting sites or around wounds caused by rubbing or scratching. Other parasites are also attracted to wounds left by tick bites. Toxins injected by female ticks while they suck blood from host animals can cause paralysis, which may be fatal if parasites are not removed.
Ticks also act as vectors for a broad range of pathogens that can affect sheep, including louping ill (ovine encephalomyelitis), tickborne fever, piroplasmosis, heartwater and Q fever. While sheep are prone to parasitism by several tick species, the prevalence of individual parasites varies widely according to geographical location and climate.
Ixodes spp are a major cause of losses to commercial sheep producers in most regions of the world. The sheep or pasture tick, I. ricinus, which is prevalent across much of Europe, is recognised as a vector for the virus that causes ovine encephalomyelitis (louping ill), an acute, sometimes fatal CNS infection. The paralysis tick, I. rubicundus, is a problem in some southern hemisphere sheep producing countries, while I. Scapularis and I. pacificus are among a range of ticks that affect sheep in parts of the US.
Amblyomma spp parasitise all domestic livestock species, including sheep, in central and southern Africa, parts of the southern US and central/south America. Haemaphysalis spp are distributed more widely, and cause problems across much of Europe, Asia, Africa and Australasia, while Rhipicephalus spp are a significant problem in southern Africa.
. . . Treatment and control
Organophosphate and pyrethroid insecticides have been used widely in the treatment and control of sheep ectoparasites, but resistance to these chemical groups is a problem, and safety concerns associated with the organophosphate class have seen their popularity wane with the availability of alternative products. The macrocyclic lactones are effective against a range of sheep ectoparasites, including parasitic stages of the warble fly, H. bovis, biting and sucking lice, mange mites and horn fly.
Measures to prevent blowfly strike include shearing of sheep around the breech, tail and other favoured egg-laying sites. Prophylactic treatment with insecticidal products is also practised widely in some regions. Methods of application vary, with dips, sprays and jet formulations among the most popular. High-pressure jet applicators enable deep penetration of insecticides with residual activity.
Shearing removes a substantial proportion of ked populations on sheep, and shearing prior to lambing is advised in some countries. The administration of insecticide treatments after shearing is also practised, with residual compounds generally favoured in order to kill adult parasites emerging from pupae in the weeks following treatment. Dips, sprays, jets and pour-on formulations are all widely available. The use of pour-on products for the treatment of lice infestations has also increased significantly since the widespread availability of macrocyclic lactones in this form.
1.3 Pig parasites
The intensive systems that dominate modern pig production have a significant impact on the type and extent of parasite challenge faced by animals reared in an enclosed environment. Extensive production systems have gained in popularity during recent years, while extensive production dominates in many developing markets. Animals reared out-of-doors face a much broader range of parasite challenges.
1.3.1 Endoparasites
. . . Gastrointestinal nematodes
The large roundworm, Ascaris suum, inhabits the small intestine of pigs, but may migrate into the stomach or bile ducts of affected animals. Ingested larvae hatch in the intestine before migrating to the liver and lungs and returning to the gastrointestinal tract. Infestation with adult worms, which may often be 30cm long, can have a significant impact on growth rates in young pigs. Heavy infestation may also cause intestinal obstruction, while large numbers of larvae can result in pulmonary oedema and may exacerbate respiratory infections such as swine influenza and pneumonia.
Trichinella spiralis infection may often go undiagnosed in pigs, since effects are usually subclinical. The consumption of poorly cooked pork can cause clinical illness (trichinellosis) in humans. Treatment of the infection in pigs is considered impractical, and prevention in humans relies largely on meat inspection at slaughter and adequate processing controls.
Stomach worms found widely in pigs include Hyostrongylus rubidus (the red stomach worm), Physocephalus sexalatus and Ascarops strongylina (both thick stomach worms). All three are more common in grazing pigs than in housed animals. Heavy infestation can cause anaemia, diarrhoea and weight loss. Severe gastritis is sometimes observed in sows close to parturition, as reduced immunity enables the development of previously inhibited larvae. Piglets are exposed to infestation at an early stage in this scenario.
Neonatal pigs can be exposed to infection with the intestinal threadworm, Strongyloides ransomi, as a result of larval transmission in colostrum. Challenge at such an early stage can have serious implications for the health of young piglets, with heavy infestations causing diarrhoea, anaemia and emaciation. Deaths may occur if the condition is not treated.
Table 1.5: Major internal parasites of pigs
| Parasite type | Species | Common name(s) |
| Gastrointestinal nematodes | Ascaris suum Physocephalus sexalatus Hyostrongylus rubidus Ascarops strongylina Strongyloides ransomi Trichuris suis Trichinella spiralis | Large roundworm Thick stomach worm Red stomach worm Thick stomach worm Intestinal threadworm Whipworm Roundworm |
| Kidney worms | Stephanurus spp | |
| Lungworms | Metastrongylus apri | |
| Protozoa | Isospora suis Eimeria spp Cryptosporidium parvum Toxoplasma gondii | |
. . . Protozoa
Infection with the protozoan parasites, Eimeria spp and Isospora spp, causes coccidiosis in pigs. Isospora suis causes enteritis and diarrhoea in piglets during the first two weeks of life. Weakness and dehydration result in poor growth rates, and badly affected animals may die. Eimeria spp, which are responsible for coccidiosis in poultry, usually cause subclinical infection in pigs. The productivity of infected animals may suffer, however.
Toxoplasma gondii infection in pigs is also usually subclinical in nature, but may be detected using enzyme-linked immunosorbent assay (ELISA) diagnostic tests. Incidence of the parasite in pigs has been reduced significantly, and no more than 2% of animals sent to market in the US are said to carry the infection now. Monitoring of the disease is still carried out widely, however, because of the zoonotic effect of toxoplasmosis.
. . . Treatment and control
Like sheep, the immunity of breeding sows against endoparasites declines in the weeks prior to and immediately after reproduction. As a result, young pigs are potentially exposed to infestation at an early stage, and basic anthelmintic programmes usually include treatment of sows prior to farrowing.
On large, intensive pig production units, thousands of animals may need to be treated simultaneously as part of worming regimes. In-feed formulations of widely used anthelmintics and endectocides have been developed in order to minimise the cost of such treatments. They also minimise levels of stress in treated animals. In-feed products containing levamisole, a number of the benzimidazole anthelmintics and macrocyclic lactones such as ivermectin are available. Injectable formulations of the macrocyclic lactones offer additional activity against some key external parasites such as lice and mites, but treatment of large herds is more labour intensive.
Anticoccidial drugs are not used on the same scale in pig production as in the poultry industry. Some producers administer anticoccidials prophylactically to sows from around two weeks prior to farrowing until piglets are weaned, however. Treatment of clinical coccidiosis usually involves the administration of sulphonamide drugs.
1.3.2 Ectoparasites
Lice and mites are the most significant external parasites affecting pigs. The biting louse, Haematopinus suis, is a common problem where animals are reared at high densities. Rubbing and scratching by affected pigs can cause damage to the skin, leaving animals open to secondary infection and attracting other parasites. Heavy infestations affect growth rates and can cause anaemia. H. suis is also implicated in the spread of swinepox virus between housed animals.
Sarcoptic mange, caused by Sarcoptes scabiei var suis, is the only form of mange encountered widely in pigs. Infective mites can spread rapidly between animals in intensive rearing units. Mange usually affects the ears and head of pigs first, but can spread widely if untreated. Intense itching causes affected animals to rub and scratch, resulting in damage to the skin.
. . . Treatment and control
The avermectins and milbemycins commercialised for use in pigs are effective against major external parasites affecting the species, as well as gastrointestinal worms and lungworms. Premix formulations are approved for use against both Haematopinus suis and Sarcoptes scabiei var suis, for example. Many ectoparasiticides from older chemical classes were originally developed in spray formulations for use on pigs, but injectable and pour-on products have also been developed.
1.4 Horse parasites
Most horses are allowed to graze freely and, as a result, are exposed to a broad range of pasture-associated endoparasites – notably gastrointestinal worms. Horses are also favoured hosts for some significant external parasites, including flies, lice and mites. Flies are also a common problem in stable premises.
1.4.1 Endoparasites
Grazing horses are exposed to potential infection by a range of nematode parasites. Strongyles spp are the most significant internal parasites affecting the species. Strongylus vulgaris, S. edentatus and S. equinus are the three major species of large strongyle. Infestation results from the ingestion of infective larvae, which migrate extensively before developing to maturity in the large intestine. S. vulgaris is the most significant, because larvae migrate extensively in the cranial mesenteric artery, and can cause parasitic thrombosis or arteritis. Colic and enteritis are among the other serious conditions that can result from the migration of S. vulgaris larvae.
Small stronglyes (Triodontophorus spp) inhabit the caecum and colon of equine hosts. Many species have been identified, but most are of much less clinical significance than large strongyles, mainly because larvae do not appear to migrate within host animals. T. tenuicollis can cause severe ulceration of the colon wall, however, and the mass emergence of larvae from the intestinal wall in temperate spring climates has been known to cause acute, sometimes fatal enteric syndrome in young animals.
Table 1.6: Major internal parasites of horses
| Parasite type | Species | Common name(s) |
| Gastrointestinal nematodes | Habronema muscae Parascaris equorum Strongylus spp Triodontophorus spp Oxyuris equi Strongyloides westeri Gasterophilus spp | Stomach worm Equine ascarid Large strongyles Small strongyles Pinworm Threadworm Bot flies |
| Lungworms | Dictyocaulus arnfeldi | |
| Tapeworms | Anoplocephala spp | |
Adult bot flies (Gasterophilus spp) do not parasitise the horse directly, but adult females lay eggs that attach to hairs on the host animal, often close to the mouth and nose. On hatching, larvae migrate to the mouth and tongue before passing into the stomach. Their natural lifecycle involves expulsion in faeces several months later. Larvae then pupate in soil before emerging as adult flies several weeks later. Bots can cause significant damage to the stomach lining, including ulceration. Oral stages can cause stomatitis. They may also be responsible for nasal discharges, and can affect the appetite of host animals.
Stomach worms (Habronema spp) are a common parasite of horses. The larvae of house and stable flies act as an intermediate host for stomach worm eggs and larvae, and horses are usually infested following ingestion of host larvae. Heavy infestations of adult worms can cause gastritis and very occasionally nodular lesions may rupture, causing fatal peritonitis. Few clinical signs of infestation are usually apparent.
Pinworms (Oxyuris equi) inhabit the terminal section of the large intestine. Perineal irritation may be observed in affected animals following egg laying by females close to the rectal area. Adult pinworm infestations are found most commonly in young horses under two years of age.
The equine ascarid, Parascaris equorum, affects mainly young animals, in which respiratory problems, lethargy and poor growth may be observed as a result of heavy intestinal infestations.
The threadworm, Strongyloides westeri, inhabits the small intestine of young foals. Activation of larvae in mares close to parturition can result in the transmission of infection to suckling foals. This may be a factor in neonatal diarrhoea, but the exact role of threadworm infection in such conditions has not been properly established.
Tapeworm infection can cause gastrointestinal problems in horses subject to heavy infestation, and colic has been associated with Anoplocephala spp infections. Research has shown that affected animals are more susceptible to colic than worm-free peers.
. . . Treatment and control
Regular treatment of horses with anthelmintics is recommended while animals are at pasture. Products from several classes are still used widely, including benzimidazole anthelmintics and pyrantel, while newer products such as ivermectin and moxidectin have also found a ready market in the field. These products are effective against all of the major strongyles and a range of other parasites, including both oral and gastric stages of Gasterophilus spp. Most equine wormers were traditionally presented in liquid form, but paste and gel-based products dominate the market now.
Resistance to some established anthelmintic classes is widespread in equine parasite populations – partly due to poor compliance by owners with recommended treatment regimes. Surveys have shown that under-dosing and failure to complete recommended courses of treatment are widespread in the sector, while some owners fail to follow rotation programmes designed to preserve efficacy levels of established product groups.
1.4.2 Ectoparasites
Horses are a common target for biting and nuisance flies, which affect animals both at pasture and in stables. Aside from their direct impact on host animals, some diptera species also act as vectors of serious equine diseases. Midges, lice and mites are among the other significant external parasites that affect horses, while bot flies present an internal parasite challenge to the species in the form of migrating larvae (see section 1.4.1 above).
Table 1.7: Major external parasites of horses
| Parasite type | Species | Common name(s) |
| Flies | Stomoxys calcitrans Tabanus spp | Stable fly Horse fly |
| Biting midges | Culicoides spp | |
| Lice | Damalinia (Bovicola) equi |