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March 2005

Liver Fluke

As a clinical manifestation, liver fluke is not considered a significant problem in horses. However, it should be appreciated that horses grazing fluke infested pastures are as likely to be affected as are sheep and cattle, even though the effects are possibly not as damaging. Interestingly, an Irish survey once showed as many as 77% of horses affected, although UK figures, at 0.1%, disagreed. Probably, however, neither figure reflects the true situation. On the basis of my experiences, the former is too high and the latter too low.

The horse is not a natural host for fluke, which may influence transmission, clinical effect and disease expression. And diagnosis is a problem because of the difficulty of finding fluke eggs in horse faeces, partly because egg laying can be inhibited, meaning a negative faecal test does not ensure the horse is fluke free.

Life cycle

Liver fluke are found world-wide and affect horses in many countries. Adult fluke lay eggs in the liver and bile ducts which, when passed from the body, produce an immature fluke, called a miracidium, which penetrates a snail to continue its life-cycle. The fluke egg must fall into water for the miracidium to develop, a process that takes two to four weeks at summer temperatures. Within the snail, several stage changes take place before the infectious (cercaria)stage is reached and these are released and swim about until becoming encysted on grass that will be grazed by the final host.

Certain geographic factors are important. Primarily, there has to be a suitable habitat to sustain the snail host and this means wet areas that hold water, often by the side of streams or soggy ground that remains wet throughout the seasons. Naturally enough, the incidence is affected by prevailing weather and is likely to be greater in periods of persistent rain.

Clinical signs

The disease is not easily diagnosed in horses and the presence of fluke eggs in faeces is not a significant indicator of disease status. Neither do negative findings rule out infestation and sometimes only trial treatment with a flukicide drug will suggest an answer.

Although horses seldom appear to suffer from the acute condition as seen in sheep and cattle, there is little doubt that many, open to the same source of infection, suffer from the disease in a way that influences health and limits athletic performance. The most common sign is chronic anaemia of non-specific origin. There may also be chronically soft faeces and a tendency to dry coat and weight loss. Mild jaundice is a feature and it is not unusual to find abnormal heart sounds (murmurs) in horses with infestations.

The history of illness in horses with fluke is essentially of a chronic nature and, where anaemia is a factor, there is reduced exercise tolerance, often recurring mild infections due to resistance problems, inability to hold condition and dry coat.

Fluke in the Liver

Damage to the liver is caused by migration of immature fluke through the organ’s substance. Adult fluke inhabit the bile ducts where they cause local tissue irritation; they also consume blood.

In sedentary horses, the disease might have little significance, although a large intake of cercariae could seriously compromise health. The influences of fluke are far greater for working or athletic horse. Anaemia, recognised by pallid membranes, is easily detected through blood analysis, and is mostly expressed through exercise intolerance and accentuated heart sounds. Oedema can be a consequence and passive filling of the lower limbs is not unusual in chronic cases.

Diagnosis

A specific diagnosis of fluke infestation is difficult in horses, as negative faecal samples are not necessarily significant and positive results don’t always align with the cause of the symptoms seen. Frequently, it is only response to a fluke-killing drug that confirms the diagnosis, in hindsight. None of this should diminish the importance of the condition or the significance of its role as a disease generator, especially in athletic horses.

A contact in New Zealand informed me of performance problems in racehorses grazing land also inhabited by deer. After treatment, they recovered their form.

Treatment

In the New Zealand incident, triclabendazole (Fasinex) was used without side effects. Oxyclozanide (Zanil, or Nilzan) has been used effectively also, though heavily infested animals may show mild reactions to treatment. It needs to be stressed that neither drug is licensed for use in the horse though there would appear to be little hazzard in otherwise healthy animals.

Injectable fluke drugs are not advised as there is a risk of local or systemic reactions.