Infection is initiated when susceptible lambs ingest sporulated ooeysts, the environmentally resistant form of the parasite, which have been shed in the faeces of previously infected ovine livestock and which contaminate the environment such as feed, feeding troughs, bedding or the teats and udders of ewes.

Once ingested each oocyst which hatches in the small intestine releases 8 infective stages (sporozoites) which invade and penetrate epithelial cells lining the intestinal mucosa and then undergo several stages of asexual multiplication within the mucosa. The damage to the gut lining occurs when these endogenous parasite stages emerge from the epithelial cells. After several cycles in which the parasite repeatedly infects these intestinal cells and rapidly divides a phase of sexual reproduction (gamonts) occurs which results in the formation of oocysts which are shed into the lumen of the intestine and pass out in the faeces. These oocysts require several days outwith the host under suitable conditions of temperature and moisture to become fully developed and infective (sporulated). This life cycle takes approximately 2-3 weeks from ingestion of the ooeysts to shedding of new oocysts in the faeces. Thus ingestion of a few oocysts can result in the shedding of several million ooeysts in the faeces.

top

Clinical outbreaks of coccidiosis are usually seen in lambs of about 4-7 weeks of age but can also occur in older livestock. Usually the majority of lambs will be affected to some extent in an outbreak.

The main signs of infection are profuse diarrhoea, loss of appetite, check in growth, dullness and open-fleeced appearance, weakness and abdominal pain which often results in a tucked-up stance. If untreated, infection can lead to marked dehydration, severe weight loss and eventually death.

In severe outbreaks where extensive damage to the gut lining occurs and particularly those in which the most pathogenic species, Eimeria ovinoidalis, are involved the diarrhoea may contain streaks of blood. On some farms lambs may be simultaneously exposed to both coccidial infection and Nematodirus batrus worms and there is evidence that under these circumstances the clinical signs may be more severe.

At post mortem there is frequently diffuse inflammation and a thickened oedematous gut wall with destruction and loss of epithelial cells in the caecum and sometimes the ileum and colon are also affected. In the case of infection with E. ovinoidalis the mucosa of the small intestine may show small white pin-point spots or haemorrhages.

top

Lambs become infected by ingesting oocysts from other ovines. Host specificity of Eimeria rules out cross-infection from other ruminants. Sources of infection are three fold.

1. The primary source of infection is lamb to lamb
2. Environmental contamination which has overwintered (oocysts can persist and some may survive freezing).

Ewes can shed low numbers of oocysts, particularly around the periparturient period when their immune status is lowered, but are not considered a major source of infection, thus the ewe plays a role in the epidemiology of ovine coccidiosis by providing a reservoir of infection which will be multiplied through the lambs.

These sources of low numbers of oocysts may cause asymptomatic infection in the first batch of early lambs but these lambs will rapidly multiply the infection through a finite number of replications and contaminate the environment with very high numbers of infective ooeysts (several thousand million). This will result in severe challenge of later lambs and invariably lead to clinical outbreaks of coccidiosis. Thus coccidiosis is generally associated with intensification and high stocking densities on lowland farms. The disease is lessTrequently seen in hill and upland farms where stocking rates are lower and where there is a higher propensity for single lambs.

In grazing lambs outbreaks of clinical coccidiosis are most frequently sect] at around 4-8 weeks of age, in particular where stocking rates are high. Coccidiosis can also occur in lambs about 3 weeks after turnout. Lambs which survive coccidial infection usually develop a good immunity by the time they are about 2 months old and thus the recurrence of clinical disease is unlikely. However, they can carry an asymptomatic infection as ewes and shed low numbers of oocysts in their faeces although they will be resistant to the pathogenic effects of the parasites. Outbreaks of clinical coccidiosis can occasionally occur in adult animals particularly where they have been stressed through management changes, transport, intercurrent disease or environmental influences or where their cellular immunity is impaired.

One of the problems of coccidiosis is its unpredictability, varying in its occurrence from year to year and between different management systems. The predisposing factors are variable but there is evidence that stress and cold wet conditions can precipitate an outbreak and also that nutrition may play a role as clinical coccidiosis is less common in single lambs than in twins or triplets.

top

Diagnosis should be based on farm history, clinical signs, faecal analysis and, where animals have died, from a post-mortern examination of the gut for lesions and the presence of masses of ooeysts and gamonts in gut scrapings. Where lambs are dying it is essential to obtain an early confirmation. Faecal samples are analysed for the presence of coccidial ooeysts but speciation of ooeysts is also essential as only two of the eleven species are pathogenic. A simple salt floatation technique is used to concentrate oocysts from faeces followed by storage in 2 per cent potassium dichromate for several days to allow sporulation of the oocyst. Identification of the pathogenic species of coccidia incorporates oocyst dimensions, thickness of the oocyst wall and the presence or absence of a polar cap. E. crandallis and E. ovinoidalis are of similar size but the former has a polar cap and broad sporocysts whereas the latter species has no polar cap, a moderately thin oocyst wall and elongate sporocysts. Severe diarrhoea (with or without blood) in 4-6 week old lambs and a high oocyst count in which E. crandallis or E. ovinoidalis predominate is indicative of clinical coccidiosis. Generally the onset of clinical symptoms coincides with oocyst shedding but occasionally animals may be severely affected without passing oocyts in their faeces. With such acute infections the damage caused by the rapid proliferation of endogenous stages in the epithelial cells of the gut can be so severe that clinical symptoms appear prior to oocyst shedding. A corollary is that sometimes apparently healthy lambs can pass large numbers of oocysts of non-pathogenic species in their faeces. Thus oocyst counts on their own are of limited diagnostic value. Many other organisms such as bacteria, viruses, Cryptosporidium, roundworms (Nematodirus) or nutritional changes and management practices can cause diarrhoea in young lambs which is similar to that resulting from coccidial infection and veterinary advice will be required for a differential diagnosis.

top

The aim is to reduce the chance of lambs encountering a high oocyst challenge through good husbandry, to avoid overcrowding and stress and where appropriate by drug intervention. Lambs reared indoors on damp soiled bedding or overstocked on pasture are likely to be most at risk from coccidial infection. Prevention of coccidiosis is dependent on good hygiene and lamb management to reduce the environmental contamination with oocysts and also ensure that lambs receive adequate colostrum which will provide passive protection during the first few weeks of life and increase their resistance to coccidial infection. The aim is to ensure that lambs build up a good active immunity during the first 4-8 weeks after birth. Sheep of all ages are susceptible to coccidial infection but generally only young lambs will be susceptible to disease.

It is essential to provide clean dry bedding and good drainage for the lambing pens. Feeding troughs should be steam-cleaned or treated with disinfectant and raising them above the floor will assist in reducing contamination with faeces. Troughs in fields should where possible be sited on drier areas and moved regularly to prevent build-up of muddy wet areas with heavy faecal contamination.

If possible later born lambs should be housed or grazed on a different area from that used for the earlier lambs to reduce contact with contaminated areas. The same paddocks used sequentially for young lambs can result in a high level of infectivity. Movement of lambs to fresh pasture will also assist in reducing the build up of contamination.

The feeding of an adequate level of nutrition to ewes prior to parturition and provision of creep feed to lambs will assist in giving lambs a good start and increasing their resistance to coccidiosis. Rearing lambs of similar ages on a group basis will assist in limiting the spread of coccidial infection to younger lambs which can occur in a mixed rearing system. Avoid mixing younger lambs with older ones in the same fields and also using 'holding' paddocks for sequential grazing groups of lambs as a high challenge infection may build up for the later born lambs. From the above considerations it is apparent that the first line of defence against coccidial infection is good livestock management. A further preventive measure is to incorporate a coccidiostat (decoquinate, "Deccox") into the creep feed (this requires a veterinary prescription). The success of this treatment also depends on each lamb consuming sufficient creep feed at a young age to provide adequate protection against challenge and allowing immunity to develop. This blanket medication could possibly lead to the occurrence of coccidiosis in some cases once the drug is removed as adequate immunity may not have developed under this treatment regime. Decoquinate can also be administered in feed to ewes over at least a 28 day period as a preventive treatment to reduce the shedding of coccidial ooeysts around the periparturient period. The timing of the treatment will vary between management systems but should be administered when occyst shedding by ewes, is likely to be a hazard to lambs (ie before, during or after lambing). However, medication of ewe feed alone may not prevent coccidiosis occurring in the lambs and thus should be given in conjunction with lamb medication.

Recently an anticoccidial (diclazuril, "Vecoxan") has been licenced for the prevention and control of coccidiosis in sheep which is given as a single oral dose via a drench gun which causes interruption of the coccidial cycle and eliminates excretion of oocysts for up to 2 weeks. Treatment,of lambs should be on a flock basis rather than individuals as asymptomatic infection in some lambs will continue to contaminate the environment with oceysts. For prevention of disease diclazuril should be administered to lambs just before the peak oocyst challenge is predicted (usually at about 4-6 weeks of age). In circumstances where infection pressure is high a second treatment may be required about 3 weeks after the first dosing. Where possible treated lambs should be moved to clean bedding or uncontaminated grazing such as fields which did not run lambs in the previous season.

top

When outbreaks of coccidiosis occur there are a limited number of licenced anticoccidial drugs available for treatment of sheep (see Table 1 on page 7). For details of the dose rates reference should be made to the manufacturers data sheets.

Decoquinate can be used as an in feed medication to treat outbreaks of coccidiosis but the rapidity of effect will be less than that obtained with the injectable or oral drugs.

The sulphonamides may be administered therapeutically as injectable drugs by the subcutaneous, intramuscular or intravenous routes (see Table 1) but treatment in many cases will need to be administered over several consecutive days which involves additional handling of young stock. Diclazuril can be administered as a single oral drench for therapeutic use against coccidial infection.

Lambs with a severe coccidial infection will invariably become dehydrated and in addition will require supportive rehydration therapy to aid replacement of fluid and electrolytes using products such as Ion-Aid (Merial Animal Health Ltd) or Lectade (Pfizel Ltd). It is important to note that the damage to the gut caused by coccidial infection can persist for some time and therefore it is important to treat lambs for coccidiosis as soon as the disease is suspected.

It is worth emphasising again that on some farms coccidial infection may occur concurrently with the roundworm Nematodirus battus and this will exacerbate the clinical symptoms of disease. Any coccidia control programmes for outdoor lambs needs to take Nematodirus infection into consideration and where appropriate lambs may also need to be treated with anthelmintic.

top