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Coccidiosis in lambs – what it is and how to spot it

lambs in a field. one unhealthy under a cloud, the other healthy in the sunshine

- What is coccidiosis?

- What are the clinical signs?

- Are my lambs at risk of coccidiosis?

- How do I diagnose coccidiosis?

- Treatment

Coccidiosis is one of the most important causes of scour (diarrhoea) in young lambs, which can result in reduced growth rates, persistent ill-thrift, and can sometimes even be fatal. It is becoming increasingly common in the UK, especially in more intensive systems.

Coccidiosis in lambs can be seen as clinical disease, presenting as lambs with diarrhoea, but also can have subclinical effects. These lambs will show no obvious symptoms but when monitored, will record reduced feed intakes and poorer growth. A study of housed lambs showed subclinical disease reduced feed conversion rates by around 10% which meant lambs took an extra week to fatten, despite showing no obvious signs of disease.1

Lambs which show clinical signs will take much longer to recover and can take many more weeks to reach slaughter weight long-term. A 2005 study estimated that an extra 3 weeks to slaughter would cost £4.40 per lamb in feed – not counting the extra cost of any handling and treatments.2 This is likely a significant underestimate in today’s prices.

What is coccidiosis?

Coccidiosis is caused by a parasite called Eimeria. This parasite is common in farm environments and most lamb flocks will encounter infection at some stage. Many different species can infect sheep, and most can live in the gut without causing damage or any clinical signs. The two species of importance to sheep in the UK are Eimeria crandallis and Eimeria ovinoidalis, as these can both cause disease.

Eimeria life cycle:

Oocysts (eggs) are shed in the faeces of sheep. They can survive in the environment for up to 18 months and mature in the environment for several days when temperature and moisture conditions are suitable. Once mature, if they are ingested by young lambs without immunity, they enter the cells lining the gut and reproduce, causing huge damage as they do so. They produce very large numbers of oocysts which are then shed in the faeces to contaminate the environment and start the cycle again.

What are the clinical signs?

Coccidiosis causes clinical signs due to the extensive damage to the intestinal lining. This prevents the absorption of water and other nutrients, leading to diarrhoea and poor growth. In many cases this damage will never fully heal, meaning suboptimal growth rates for the rest of the lamb’s life. The damage to the gut, along with reduced immunity (due to a lack of nutrients) can increase the severity of other gut diseases (such as Nematodirus, a gut worm which can affect young lambs at a similar time of year).

Clinical signs are most commonly seen in young lambs 4-8 weeks old, usually with most/all of the group affected:

• Profuse diarrhoea or scour, often with mucus or flecks of blood

• Staining of the tail and surrounding area due to the diarrhoea

• Abdominal pain causing a ‘tucked up’ appearance and straining when passing faeces

• Severe weight loss in the later stages

• Dehydration (sunken eyes, dull, weak lambs) which if untreated can lead to death

Clinical disease due to coccidiosis can occur both indoors and in grass-based systems. As they age, lambs develop immunity to Eimeria, and in most cases older lambs do not develop coccidiosis unless they were not exposed to infection at a young age.

Are my lambs at risk of coccidiosis?

Eimeria oocysts are present in the environment in most flocks, however this does not always mean every flock will have signs of disease. Two factors affect whether lambs in a flock are at risk of clinical coccidiosis.

1. Firstly, the level of contamination of the environment with oocysts. Low levels of contamination can be beneficial as it will stimulate the lamb’s immune system to develop immunity without causing clinical disease. However, if there is a heavy environmental challenge this can lead to clinical disease. High levels of oocysts in the environment can be caused by:

- Lambs housed or grazed at a high stocking density (intensive systems)

- Dirty conditions in fields or buildings

- Feed and water troughs that are contaminated with faeces (either due to their design, siting, or poor hygiene management)

- Mixing young, susceptible lambs with older lambs – the older lambs will have developed immunity to coccidiosis so won’t show clinical signs but can still be shedding oocysts into the environment, putting younger lambs at risk

- Grazing younger lambs on fields where older, earlier-born lambs have already grazed (and so contaminated the ground with oocysts)

2. Secondly, animals with poor immunity are most susceptible to clinical coccidiosis. This is a higher risk in certain cases:

- Young lambs (as they have never been exposed to Eimeria before and are still developing their immunity after birth)

- Lambs which have had inadequate colostrum intake and/or poor-quality colostrum at birth. This will also be influenced by the condition of the ewe

- Lambs which are under stress – for example due to poor nutrition, cold/wet weather, or other diseases (such as Nematodirus)

- Concurrent flock diseases such as Border disease which can lower flock immunity

How do I diagnose coccidiosis?

Early diagnosis is crucial as clinical coccidiosis can be a significant welfare concern and can be fatal. It also can lead to poor weight gain long-term which has huge economic impacts for sheep farms. Effective treatment can minimise these effects if diagnosis is made promptly.  

Testing, along with farm history and clinical signs, are all essential for a diagnosis of coccidiosis. The main diagnostic test for young lambs with diarrhoea is to check for coccidia oocysts in a faecal sample. The sample can be prepared and then assessed by microscope either in-house at a veterinary practice or at an external laboratory. On-farm testing systems such as FECPAK can identify coccidia eggs but are not yet validated to give oocyst counts. For farms with a known history of coccidiosis, a high oocyst count, along with clinical signs that are indicative of coccidiosis can be enough for your vet to recommend treatment. For farms that have not previously had coccidiosis confirmed on their farm, or for lambs with ambiguous clinical signs, it is essential to also send the faecal sample away for speciation. This allows confirmation that the oocysts found in the muck sample are from the two disease-causing species, E. crandallis and/or E. ovinoidalis (the other species can also produce eggs but do not cause disease, so are irrelevant). This is important as other diseases can also cause diarrhoea in young lambs and need to be differentiated to allow appropriate and effective treatment.

If lambs have died already in the outbreak, then a postmortem is also a valuable tool. Examination of the intestine will show diffuse inflammation and a thickened gut wall; it also allows sampling of gut content for oocyst testing. It is important to remember that Nematodirus, a gut worm, is also common in lambs at similar times of the year and can infect lambs concurrently with coccidiosis. Postmortem is often the most accurate way of diagnosing concurrent Nematodirus infection as this worm often causes disease well before adult worms start shedding eggs in faeces.


Prompt treatment is essential to minimise long-term gut damage in affected lambs. Treatment options for coccidiosis include oral drenches, such as Dycoxan, and in-feed medicines. The aim of treatment is to reduce clinical signs and reduce shedding of oocysts onto the pasture (to reduce spread to other vulnerable lambs). Optimal treatment timing is essential, and there are pros and cons of each type of treatment, but these will be covered further in the treatment blog .


1. Alzieu JP, Mage C, Maes L, de Mûelenaere C. Economic benefits of prophylaxis with diclazuril against subclinical coccidiosis in lambs reared indoors. Vet Rec. 1999 Apr 17;144(16):442-4. doi: 10.1136/vr.144.16.442. PMID: 10343376.

2. Nieuwhof GJ, Bishop SC. Costs of the major endemic diseases of sheep in Great Britain and the potential benefits of reduction in disease impact. Animal Science. 2005;81(1):23-29.

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