Coccidiosis in Sheep
Introduction
Coccidiosis is caused by microscopic, protozoal (single-celled) parasites that infect and multiply within the gut wall. These parasites are highly host adapted, meaning sheep will only become infected with sheep-specific species of coccidia. Whilst many of these are harmless, some species can cause severe enteric disease. Such infections are of greatest significance in young lambs.
This disease is commonly associated with high intensity husbandry systems and stocking densities as well as stress factors such as poor colostrum supply, adverse weather conditions when grazing wet muddy paddocks and/or extended housing periods.
Figure 1: Oocysts survive in the environment for long periods, lambs act as multipliers before they develop immunity. Image credit: Fiona Lovatt, Flock Health Ltd.
Cause
Many coccidia infecting UK sheep are considered harmless, but infection with either Eimeria ovinoidalis and Eimeria crandallis can lead to severe disease. Where this occurs, it is associated with either heavy infection and/or a lowered immune status.
Infection occurs through the ingestion of infective “oocysts” in the environment which have been passed in the faeces of already infected animals such as older lambs. Adult ewes can also act as infected carrier animals, shedding low numbers of oocysts whilst showing no sign of infection. Unlike roundworm and fluke eggs, oocysts can develop and survive in housed and indoor environments as well as on pastures. They may also survive for long periods, including over winter, being very resistant to drying out, freezing, heat and several disinfectants.
Young lambs aged 4-8 weeks are typically considered at greatest risk of coccidiosis, particularly those born later in the lambing season. After ingesting infective-stage oocysts, several rounds of replication occur inside the gut wall of the infected animal. The new stages then erupt from the infected host cells in large numbers causing damage to the lining of the gut. In heavy infections this gut damage can be extensive, resulting in sudden onset, severe disease. After around 7-28 days lambs will begin to shed oocysts in their faeces in vast numbers even following a light, sub-clinical infection. Once in the environment, these new oocysts can develop to become infective within 1-3 days, meaning the level of environmental contamination and subsequent disease risk can go from low to high very quickly, with further ingestion of large numbers of oocysts resulting in extensive intestinal damage and disease.
Figure 2: Classic clinical signs include dullness, weight loss and dark-coloured diarrhoea
Figure 3: Diarrhoea associated with coccidiosis often contains blood and mucus
Welfare and Economics
Coccidiosis can have severe impacts on animal health, welfare and production. These parasites are found everywhere. Consequently, disease prevalence is high across the UK. Cases on farms may range from mild, sporadic to severe, widespread outbreaks depending on the conditions present and control measures in place. Affected animals do not grow well and take a long time to recover delaying finishing, and lead to poor carcase size and quality. Furthermore, diarrhoea caused by coccidiosis can also lead to heavily soiled fleece and an increased risk of flystrike.
The true prevalence and cost to the industry is unknown, but due to its widespread presence on farms, the severity of clinical disease, production limiting effects, costs of preventative treatments etc. the cumulative economic impacts are likely to be substantial.
Figure 4: Staining of the wool around the back end is suggestive of coccidiosis
Clinical Signs and diagnosis
Diarrhoea is the predominant clinical sign associated with coccidiosis due to the extensive damage caused to the lining of the small intestine, caecum and large intestine resulting. This may contain mucus and blood, and is often accompanied by straining, pain, weight loss and sometimes death of the lamb. Ongoing poor growth rates may be apparent in older lambs after clinical or subclinical disease as the gut has a reduced ability to absorb nutrients from food and takes time to recover.
Severe, bloody diarrhoea accompanied by straining in lambs aged four to eight weeks is highly suggestive of coccidiosis. Clinical disease may appear in affected lambs at progressively younger ages as the season progresses due to a continued build-up of infective oocsysts on pastures and/or in buildings previously occupied by infected older lambs.
Huge numbers of oocysts can be identified in faecal samples of patently infected animals using a standard salt flotation technique for faecal worm egg counts. However, it is important to note that in cases where animals are exposed to very high levels of oocyst contamination from the outset clinical disease may occur before oocysts are shed. Additionally, since there are a number of species of coccidia which do not cause disease, diagnosis of coccidiosis should not be based solely on presence of oocysts in faecal samples. Where there is uncertainty, samples should be submitted for species identification at a specialist laboratory.
Figure 5: Diarrhoea and straining can lead to rectal prolapse and a hunched stance due to pain
Where there is cause for doubt, alternative causes of diarrhoea and weight loss in older lambs, including roundworm burdens (particularly nematodirosis) and salmonellosis should also be investigated. These infections can also occur concurrently with coccidiosis resulting in further complications and disease severity. Similarly, fertilised, lush grass and high protein creep feed can also cause scour, whilst malnutrition including vitamin and mineral deficiencies may also predispose lambs to coccidiosis in addition to being a primary cause of poor condition.
Prevention and Control
There are two aspects to preventing clinical coccidiosis. The first is minimising exposure of at-risk animals to high levels of oocyst contamination through good biosecurity, hygiene and management. The second is through the administration of timed and targeted preventative treatments.
In general, reducing stocking density, batch rearing of lambs by age and avoidance of heavily contaminated pastures/premises previously occupied by older animals can help reduce build-up of oocysts and disease risk to a certain extent. Objects lambs will routinely interact with such as milk feeding equipment, water buckets and feed troughs should be cleaned and disinfected regularly, and water provided from a fresh, clean supply. Elevating buckets and troughs above ground level helps to reduce faecal contamination.
In a housed environment, level of oocyst contamination can be lowered further by ensuring adequate ventilation and drainage, and providing regular, plentiful bedding. Furthermore, prior to lambing, and between different groups of animals, all housing and pen materials should be thoroughly cleaned to remove all organic material, then disinfected with a product effective against coccidia oocysts. Very few disinfectants are capable of killing oocysts, so seek advice where necessary, check the product claims and always use the chosen product at the correct concentration and apply following the manufacturers recommendations
Figure 6: Poached areas are likely to become highly contaminated with oocysts: move the feeder regularly to reduce the risk
For animals at pasture, contamination levels can be reduced by regularly moving animals to clean grazing and avoiding use of fields that carried young lambs in the previous season where possible. Avoid build-up of contamination in specific locations, such as areas around creep feeders, by moving these to fresh ground regularly (every 2-3 days).
Figure 7: Regular application of clean bedding helps reduce faecal contamination of feed troughs
Even with such control measures, group treatment to prevent clinical disease may be required when farm history and conditions indicate a high coccidial challenge is anticipated. A number of anticoccidial products including feed medication are available for both prevention and treatment of coccidiosis. It is important to consider what constitutes a high-risk situation to ensure treatment is appropriate. Examples include:
Susceptible lambs moved onto fields or into a contaminated building previously occupied by older lambs have been.
- Lambs which have been kept housed in well-bedded, clean conditions for an extended period after birth then turned out onto contaminated pasture.
- Sudden withdrawal of medicated creep feed, or medicated ewe feed.
Various studies have shown that in the face of a heavy coccidial challenge lambs treated with either diclazuril or toltrazuril shed fewer oocysts, have less or no diarrhoea and grow faster than untreated lambs. In sheep, toltrazuril has been shown to be more effective at reducing the numbers of oocysts shed than diclazuril, although both are effective in reducing contamination of pasture and lowering the challenge facing the current and future batches of lambs.
Traditionally, decoquinate has been used in the ewe ration to suppress the number of oocysts shed by the ewes. However, this approach is no longer recommended for a number of reasons. Firstly, this constitutes medication of clinically unaffected animals. Secondly, this practice has no impact on overwintered oocysts on the pastures and, lastly, due to the benefits of lambs being exposed to low levels of coccidia in building their immunity.
In addition to limiting exposure and prophylactic treatments, there are a number of additional factors which can influence lamb resilience and the risk posed by coccidiosis on farm. Lambs that receive an adequate colostrum intake at birth have a decreased risk of coccidiosis. Twins and triplets are at greater risk disease, as are lambs which receive lower milk volumes, since these animals graze more extensively and at a younger age. Malnutrition through a lack of energy or protein intake, deficiency of vitamins or minerals also increases susceptibility to coccidiosis. Poor, underfed lambs have been shown to be more severely affected than well fed animals with the same level of oocyst challenge. Other stressors such as extreme weather, transportation, mixing, docking and castration will all lower lambs’ resilience.
Lambs typically develop a long-lived protective immunity following exposure to even low levels of infection that prevents further risk of clinical disease. It should be noted, however, that there is no cross protection of immunity to different species of coccidia. Maintaining a closed flock or following robust isolation and quarantine procedures can reduce the risk of purchasing animals shedding novel species.
Treatment
Treatment with coccidiocidal medicines are administered to allow infected animals to gain immunity while their gut recovers and reduce further shedding of oocysts into the environment. Treatment is most effective when used before severe gut damage has occurred and is indicated for the whole of the group in contact with the affected animals. Diarrhoea may often continue in the worst affected lambs following treatment since the damaged gut takes time to recover.
There are three licensed drugs available for the treatment and/or prevention of coccidiosis in sheep - diclazuril (e.g. Vecoxan), toltrazuril (e.g. Baycox, Toltranil and Cavezuril) and decoquinate (e.g. Decoxx). Whilst to date no issues have been found in UK sheep relating to drug resistance and coccidiosis, there have been reports from abroad, including a recent case of toltrazuril resistant Eimeria ovindalis and Eimeria crandalis in sheep in Norway.
Diclazuril has no residual activity so timing of treatment is critical and it is rarely appropriate to be used on the day of turn-out. When susceptible lambs are moved onto contaminated land, it is recommended that the product is given 10-14 days later. Depending on the suspected level of oocyst contamination and/or oocyst shedding, it may then be necessary to give a second dose three weeks after the first.
Toltrazuril has a longer duration of action, making a single dose sufficient in most instances. Consequently, timing of dosing is also less critical, although one week after turn-out onto contaminated land or a week before expected clinical disease is usually considered optimal.
Decoquinate is an in-feed medication which can only be added to commercial creep feeds by the manufacturer following receipt of an appropriate prescription from the farmer’s vet. It should be incorporated and provided at the recommended concentration of 100 mg decoquinate/kg feed (100 ppm) for the duration of the period of the risk period and for a minimum of 28 days.
Each 10kg lamb must ingest a minimum of 100g of creep a day to maintain adequate levels of decoquinate medication. Sick lambs with poor appetites may therefore be at increased risk of concurrent coccidiosis. Decoquinate is also only active in the small intestine. Infected lambs may therefore continue to shed oocysts due to infection in the large intestine despite showing no signs of disease.
Lambs with severe diarrhoea and dehydration may require additional supportive therapy such as oral electrolyte solutions and intensive nursing care. These animals should be protected from adverse weather conditions and predation, monitored for feed intake and given supplementation where necessary. Protection from blowfly strike may also be necessary in animals with heavily soiled back ends, and any other concurrent disease e.g. nematodirosis must be investigated and treated appropriately.