Subacute fasciolosis
Subacute fasciolosis is characterised by jaundice, some ill thrift and anaemia. The burrowing fluke causes extensive tissue damage, leading to haemorrhaging and liver damage. The outcome is severe anaemia, liver failure and death in 8–10 weeks.
Chronic fasciolosis
Chronic fasciolosis is the most common form of liver fluke infection in sheep, goats and cattle – and particularly in more resistant hosts, such as horses and pigs. It occurs when the parasites reach the bile ducts in the liver. The fluke ingests blood, which produces severe anaemia and chronic inflammation and enlargement of the bile ducts. The clinical signs develop slowly. The animals become increasingly anaemic, appetite is lowered, the mucous membranes of the mouth and eyes become pale and some animals develop oedema under the jaw (‘bottle jaw’). Affected animals are reluctant to travel. Black disease Black disease is an acute and fatal liver disease which can affect sheep and cattle. It is usually associated with the liver damage caused by the migrating young fluke; the damage provides a suitable environment for the germination of spores of Clostridium novyi type B bacteria in the liver.
Parasite–host relationship
In sheep, there is no evidence of any acquired resistance to Fasciola hepatica. Acute and chronic fasciolosis can occur at any age.
Cattle have a natural resistance and under normal conditions the clinical disease is only likely in young cattle.
Chronically infected cattle can spontaneously recover, and previously infected animals can partially resist reinfection. However, this resistance is only possible because of chronic fibrotic changes in the liver, so with even a small number of fluke present, there may be production losses.
Diagnosis Fasciolosis should be considered when there are deaths, anaemia or ill thrift in sheep or cattle grazing on fluke-prone country. In live animals, chronic fasciolosis is indicated by fluke eggs in faecal samples. The sampling technique is generally reliable in sheep but much less so in cattle. Diagnosis in dead animals relies on seeing mature or immature fluke in the liver. Necropsy will also identify other conditions that may be contributing to the problem. A serological test (ELISA) is also available for fasciolosis. It detects infection with both immature and adult fluke in a flock or herd, but it is not sensitive enough for diagnosis in individual animals.
Treatment The treatment recommended will depend on the nature of the disease. Some of the available anthelmintics are not effective against immature fluke and so are not recommended in acute fluke outbreaks. Also, they are less efficient for the strategic control of fasciolosis. The best prevention and control can be achieved with drugs such as triclabendazole, which are effective against early immature and adult fluke. Table 1 (below) summarises the efficacy of drugs registered for treatment of fasciolosis in sheep and cattle.
Strategic control
Due to the great biotic potential of Fasciola hepatica and their intermediate host snails, only a continuous and coordinated strategic application of all available measures can provide economic control of the disease. Control should be on a preventive rather than a curative basis. For effective control: • use strategic anthelmintic treatment, to reduce the number of fluke in the host and the number of fluke eggs in pasture; • reduce the number of intermediate host snails; • manage fluke-prone areas, to reduce exposure to infection. These three strategies are detailed as follows.
Subacute fasciolosis Subacute fasciolosis is characterised by jaundice, some ill thrift and anaemia. The burrowing fluke causes extensive tissue damage, leading to haemorrhaging and liver damage. The outcome is severe anaemia, liver failure and death in 8–10 weeks. Chronic fasciolosis Chronic fasciolosis is the most common form of liver fluke infection in sheep, goats and cattle – and particularly in more resistant hosts, such as horses and pigs. It occurs when the parasites reach the bile ducts in the liver. The fluke ingests blood, which produces severe anaemia and chronic inflammation and enlargement of the bile ducts. The clinical signs develop slowly. The animals become increasingly anaemic, appetite is lowered, the mucous membranes of the mouth and eyes become pale and some animals develop oedema under the jaw (‘bottle jaw’). Affected animals are reluctant to travel. Black disease Black disease is an acute and fatal liver disease which can affect sheep and cattle. It is usually associated with the liver damage caused by the migrating young fluke; the damage provides a suitable environment for the germination of spores of Clostridium novyi type B bacteria in the liver.Parasite–host relationshipIn sheep, there is no evidence of any acquired resistance to Fasciola hepatica. Acute and chronic fasciolosis can occur at any age.Cattle have a natural resistance and under normal conditions the clinical disease is only likely in young cattle.Chronically infected cattle can spontaneously recover, and previously infected animals can partially resist reinfection. However, this resistance is only possible because of chronic fibrotic changes in the liver, so with even a small number of fluke present, there may be production losses.Diagnosis Fasciolosis should be considered when there are deaths, anaemia or ill thrift in sheep or cattle grazing on fluke-prone country. In live animals, chronic fasciolosis is indicated by fluke eggs in faecal samples. The sampling technique is generally reliable in sheep but much less so in cattle. Diagnosis in dead animals relies on seeing mature or immature fluke in the liver. Necropsy will also identify other conditions that may be contributing to the problem. A serological test (ELISA) is also available for fasciolosis. It detects infection with both immature and adult fluke in a flock or herd, but it is not sensitive enough for diagnosis in individual animals.Treatment The treatment recommended will depend on the nature of the disease. Some of the available anthelmintics are not effective against immature fluke and so are not recommended in acute fluke outbreaks. Also, they are less efficient for the strategic control of fasciolosis. The best prevention and control can be achieved with drugs such as triclabendazole, which are effective against early immature and adult fluke. Table 1 (below) summarises the efficacy of drugs registered for treatment of fasciolosis in sheep and cattle.Strategic controlDue to the great biotic potential of Fasciola hepatica and their intermediate host snails, only a continuous and coordinated strategic application of all available measures can provide economic control of the disease. Control should be on a preventive rather than a curative basis. For effective control: • use strategic anthelmintic treatment, to reduce the number of fluke in the host and the number of fluke eggs in pasture; • reduce the number of intermediate host snails; • manage fluke-prone areas, to reduce exposure to infection. These three strategies are detailed as follows.
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