Features
Controlling Coccidiosis
by Dr Steve Wright
Coccidiosis is a disease caused when large numbers of protozoan parasites of the genus Eimeria invade the cells lining the intestinal tract. These single celled organisms are obligate parasites, and so can only reproduce within their host. They are extremely host specific – although other species of mammals and birds may suffer from coccidiosis, for example cattle and chickens, they have their own species of coccidia, and those which cause infection in sheep can only have originated from other sheep.
Coccidia are common in the farm environment, and the majority of flocks will be exposed to infection at some stage. If the numbers of parasites ingested are small, animals tend to experience a sub-clinical infection and show no symptoms. However, if larger numbers of parasites are consumed, severe clinical symptoms will be experienced. It follows that coccidiosis is more usually associated with intensive husbandry systems and high stocking densities, where the supply of susceptible hosts in close proximity favours spread of infection. Clinical coccidiosis is most commonly seen in lambs aged between 4 – 8 weeks, but can occur in older livestock.
Though there are 11 species of Eimeria which infect sheep in the UK, however only two of these are considered to be of economic significance, Eimeria crandallis and Eimeria ovinoidalis. Other species may be present in very large numbers, but do not seem to cause any clinical symptoms.
Infection occurs as a result of ingestion of oocysts, the environmentally resistant form of the parasite, shed in the faeces of infected individuals, which may contaminate pasture, bedding, feeding troughs and even the udders of ewes. Once ingested, each oocyst hatches in the small intestine, releasing 8 infective stages (sporozoites) which penetrate the cells lining the intestinal mucosa. Several generations of asexual replication occur, causing a rapid increase in the numbers of parasites present. Extensive damage to the gut lining occurs when these parasites emerge from the intestinal cells. A phase of sexual replication follows, resulting in the formation of oocysts, which are shed into the lumen of the intestine and passed out in the faeces. The oocysts are initially undeveloped, and require several days out with the host under suitable conditions of temperature and moisture before they become infective (sporulated). The life cycle takes 2-3 weeks to complete, from the ingestion of oocysts to detection of oocysts in the faeces. The ingestion of just a few oocysts may result in the shedding of several million oocysts in the faeces.
Animals suffering from coccidiosis exhibit a variety of symptoms, the most obvious of which is a profuse diarrhoea leading to soiling of the fleece, but may also include loss of appetite, reduced growth rate, open fleece, dull demeanour and abdominal pain. Untreated animals can experience dehydration, weight loss and eventually death. In very severe outbreaks where damage to the gut lining is extensive, streaks of blood may be present in the diarrhoea. Post mortem examination often reveals a thickened oedematous gut wall with evidence of diffuse inflammation and destruction of epithelial cells. Damage may be confined to the caecum, but may also involve the ileum and colon. With E. oviniodalis infections, the mucosa of the small intestine may show small white pin-point spots or haemorrhages.
The primary route of infection is lamb to lamb, since these susceptible animals can shed abundant oocysts. However, since oocysts can persist in the environment in cool, moist conditions, and may even survive freezing, some animals may become infected from environmental contamination which has over wintered. Ewes may also be a source of infection, since they can shed small numbers of oocysts, particularly around parturition in response to their lowered immune status. Thus the ewe may act as a reservoir of infection, which is subsequently amplified by the lamb crop.The consumption of small numbers of over wintered or ewe-derived oocysts by early lambs may result in sub-clinical infections. However, these lambs are rapidly multiplying the infection, producing large numbers of oocysts to contaminate the environment. Later lambs then face a severe challenge, resulting in clinical infections. The higher stocking densities on lowland farms tend to be associated with coccidiosis, while upland and hill farms with their lower stocking densities and increased percentages of single lambs tend to have fewer problems.
Clinical coccidiosis is usually seen in grazing lambs at 4-8 weeks old, but can also occur in lambs around 3 weeks after turnout. Animals which survive infection usually develop a good immunity to clinical disease, but like adult ewes may still shed low numbers of oocysts. Clinical coccidiosis can occasionally occur in adult animals, usually in response to stresses such as transport, management changes, intercurrent disease or impaired immunity.
Where coccidiosis is suspected, it is important to confirm the diagnosis by faecal analysis. Oocysts are concentrated by saturated salt floatation, and then stored for several days to allow development so that speciation may be attempted. This is important since only 2 of the 11 species of coccidia infecting sheep are pathogenic, and the presence of large numbers of oocysts of a non-pathogenic species may confuse diagnosis of an altogether different problem. Identification of species is based on oocyst dimensions, wall thickness, presence/absence of a polar cap and the appearance of the internal structures (sporocysts).
Onset of clinical symptoms generally coincides with onset of oocyst shedding, but in some very severe infections the damage caused to the gut by the endogenous stages can result in symptoms appearing before any oocysts have been shed. If animals have died, then diagnosis can be confirmed by examination of the gut for lesions, and of gut scrapings for the presence of oocysts or endogenous stages.
When trying to prevent or control coccidiosis, the aim should be to prevent lambs experiencing a high oocyst challenge. This is best achieved through good husbandry; ensuring lambs are not overcrowded or stressed. Lambing in clean, well-drained pens, with dry bedding and clean feeding troughs will assist in reducing contamination. If possible, later lambs should be housed or grazed in different areas to earlier lambs, to reduce contact with potentially infected areas. The regular moving of feed troughs will also help to prevent a build up of contamination in these heavily used areas. It is possible to have a coccidiostat incorporated into creep feed – this must be done under veterinary prescription. Coccidiostats may also be administered to ewes prior to lambing to attempt reduction of environmental contamination.
Coccidiosis is a flock problem, and once the diagnosis has been made treatment should be administered on a flock basis to all susceptible animals. A limited number of anticoccidial drugs are licensed for use in sheep. They should be used strictly in accordance with the manufacturers instructions. You should discuss possible options with your veterinary surgeon, as your local circumstances may influence choice of treatment regime. Where levels of oocyst contamination are high, it is important to monitor treated animals, since they may require a second treatment 3 weeks after the first dosing.
Footnote: This article originally appeared in the Sheep Farmer magazine and is reproduced with their permission