Features
Nematodirus Disease
by Dr Frank Jackson and Dr Bob Coop
Scouring in young lambs can result from the acquisition of a variety of organisms but frequently coccidia and/or the intestinal roundworm Nematodirus are involved. Coccidia infections tend to occur in lambs of around 4-6 weeks of age and Nematodirus disease typically in lambs of 6-10 weeks-old although summer and autumn cases of nematodirosis have also been reported. The two species of Nematodirus that are commonly found in sheep in the UK are N.battus and, N.filicollis, but N.battus is the most often associated with outbreaks of disease as a result of its unique epidemiology.
Epidemiology
The important features of N.battus which differ from the other roundworms that cause disease in lambs are:-
- Development to the infective third stage larvae (L3) takes place wholly within the egg.
- The egg, which contains the infective L3 larvae, is highly resistant to drying out and to freezing and as a result can survive on pasture for up to two years.
- Most eggs require an extended period of cold exposure followed by rising temperatures (mean day/night temperature above about 10° C) to stimulate the infective L3 to hatch from the egg.
- Following a period of exposure to infection lambs acquire a strong immunity against N.battus which persists even in adult lactating ewes so that adult animals play a negligible role in the epidemiology of infection.
- Under most circumstances there is only one generation of N.battus parasites each year.
N.battus is a lamb to lamb infection, eggs deposited by one seasons lamb crop hatching the following spring to infect the next crop of lambs. If no preventive measures are taken N.battus infectivity tends to build up on the pasture over 2-3 years. Although N.battus is primarily a parasite in lambs young calves can sometimes become infected and thus contaminate pasture with N.battus eggs. As the epidemiology of N.battus is driven by climatic factors such as temperature and availability of moisture, changing weather patterns will invariably impact the occurrence of disease. Recently, there have been reports of atypical nematodirosis with disease occurring in older lambs in late summer, autumn and even in the early winter months.
Clinical signs and diagnosis
Disease associated with Nematodirus is most often seen in May or early June. The migration and development of immature worms in the gut causes acute enteritis and profuse watery diarrhoea often with loss of appetite and rapid loss of weight. The fleece may be dull and coarse and lambs become thirsty. If untreated, severe dehydration can occur with lambs showing a typical 'tucked up' appearance and some deaths may occur. Lambs which recover develop a good immunity but the check in growth is significant as often there is little compensatory growth over the summer period.
Diagnosis is normally based on the grazing history and previous experience of Nematodirus disease on the farm. Post- mortem examination will often confirm large numbers of N.battus in the small intestine. Faecal egg counts are of limited value as lambs frequently start scouring before the main egg shedding phase begins. However, egg counts can be useful in cases of atypical late season nematodirosis and persistent scour to differentiate between Nematodirus and coccidial infections.
Prevention and treatment
Nematodirus disease can usually be prevented by avoiding grazing young lambs on pasture which has been grazed by young lambs over the preceding seasons. Farmers can break the cycle of N.battus by grazing next season's lambing pasture with older cattle (not young calves) or by incorporating some arable crops to produce 'safe pasture'. Although small numbers of N.battus eggs can survive these management practices the numbers are unlikely to be sufficient to cause disease. N.battus can pose a considerable challenge to sheep farmers who are converting to organic production on permanent pasture with little option for grazing management. Anthelmintic intervention may be necessary in the initial period until some form of alternate grazing can be incorporated.
Most anthelmintics are effective against N.battus and to date there have been no reported cases of resistance in the UK (see Manufacturers data sheets for recommended dose rates and efficacy against different stages). Since none of the current anthelmintics have a persistent effect against Nematodirus farmers may have to treat lambs on two or three occasions over May/June depending on the challenge from pasture, in most average years two treatments in May should suffice. Where practical, sheep should be moved to 'safe' pasture after dosing. Modern anthelmintics will act rapidly on N.battus so that if scouring continues following treatment then veterinary advice should be urgently sought as the underlying problem may be coccidiosis.
Footnote: This article originally appeared in the Sheep Farmer magazine and is reproduced with their permission