Almost any animal species can be infected by trematode parasites. Trematode eggs are often too heavy to float in a flotation solution. Similarly, Eimeria leuckarti, a coccidian parasite of horses and donkeys is very heavy and can only be detected in sediments.
Infection with Eimeria leuckarti, a coccidian parasite of the small intestine of horses and donkeys, occurs throughout the world. However, it is a rarely reported infection because the oocysts do not rise to the surface on the usual flotation media and infection is often overlooked. The infection has only been reported twice in Australia, once in Tasmania and once in Victoria, but is believed to be much more common. The infection is usually symptomless, however, severe diarrhoea and deaths have been reported. The Department of Primary Industries and Regional Development (DPIRD) Diagnostics and Laboratory Services (DDLS) have only seen this parasite once in more than 2000 horse samples examined for the presence of liver fluke infection.
There are several important diseases of trematodes in domestic animals and the most important ones are briefly described below:
Fascioliasis, the disease caused by infection with liver fluke, occurs in all herbivorous animals including pigs, kangaroos and humans. It is found in temperate high-rainfall regions all over the world, according to the distribution of freshwater snail intermediate hosts.
Recorded prevalence rates of liver fluke infection in cattle in endemic areas vary from about 10% in Europe to 95% in southern Latin America. Liver fluke infection in humans can reach a prevalence of 90% (the Lao People’s Democratic Republic).
In eastern Australia, liver fluke infection occurs from south-eastern Queensland to southern Victoria and Tasmania. In South Australia, small endemic areas occur in the high-rainfall areas (irrigated pasture adjacent to the Murray river). In Western Australia outbreaks have occurred, but the parasite has not become established.
Fascioliasis is caused by flukes of the genus Fasciola. F. hepatica occurs in most countries and is the only species present in Australia. F. gigantica is the more common liver fluke in Africa and Asia and Fascioloides magna infects wild and domestic ruminants in North America.
F. hepatica is a leaf-shaped, greyish-white parasite with one ventral sucker and can grow to 20–50 millimetres in length and 4–13mm in width. F. hepatica is hermaphroditic and only one fluke is necessary to establish a patent infection. Each adult fluke can produce 20,000 eggs per day and may live for 10 years or more. Adults produce 200 times more eggs in sheep than in cattle where the egg production per adult fluke is only about 100 eggs per day. Eggs deposited in faeces hatch and release a miracidium, which infects freshwater snails. Cercaria develop and are released from the snail 4–7 weeks later. Cercaria encyst on vegetation as metacercaria, which are eaten by the host. The metacercaria excyst, immature flukes travel through the wall of the small intestine and migrate through the liver parenchyma to the bile ducts. They develop to egg-laying adult flukes in the main bile ducts about 8–10 weeks after infection.
The main intermediate hosts for F. hepatica are snails of the genus Lymnaea. Lymnaea (Pseudosuccinea) columella is an important intermediate host in many warm countries and until recently was the only intermediate host for F. hepatica present in WA. L. viridis is an important intermediate host in New Guinea and in the Philippines, has been recorded in Brisbane (Queensland) and more recently also in WA. L. tomentosa is an important intermediate host in Europe and also the eastern jurisdictions of Australia.
Sheep may suffer severe liver damage and death is not uncommon. Affected animals are anaemic, hypo-proteinaemic, jaundiced and show signs of peritonitis. Fascioliasis in cattle is usually a chronic disease associated with low weight gains and decreased milk production. It can be the cause of significant economic losses through liver condemnation at slaughter.
Pathological changes due to migration of immature flukes are termed acute or subacute fascioliasis, and the response to adult flukes in the bile ducts, chronic fascioliasis. Both changes can be present in the same animals. Migratory flukes cause traumatic lesions to the liver parenchyma forming tortuous tracts that appear in cross sections as 2–3 mm diameter haemorrhagic foci. Microscopically, there is haemorrhagic necrosis infiltrated with eosinophils, histiocytes and giant cells. Repair of the lesion results in irregular hepatic fibrosis. In cattle, there may be massive fibrous thickening of the bile ducts in response to adult flukes. This is associated with cholangio-hepatitis and sometimes mineralisation. In most species, however, there is little thickening of the bile duct walls.
Diagnosis in individual animals relies on the demonstration of F. hepatica eggs in the faeces using the sedimentation technique. Eggs are thin-walled, golden yellow, have an operculum and measure 140 x 80 microns. They can be difficult to differentiate from paramphistome eggs. The sedimentation technique used in cattle only detects about 30% of all animals shedding eggs. Diagnosis of infection during the pre-patent period (8 weeks) is not possible.
Intestinal paramphistomiasis caused by paramphistomes, or stomach flukes, is a severe debilitating disease of young cattle between 6 and 18 months of age.
The clinical disease of severe ill-thrift and diarrhoea is caused by massive numbers of immature flukes that colonise the anterior small intestine causing severe damage to the intestinal mucosa. Adult flukes are found in the rumen and reticulum and generally cause no harm. The parasite has an indirect life cycle with planorbid snails as intermediate hosts.