Coccidiosis is an acute infection which affects the mucus linings of the intestines, and is particularly important in poultry, pigs, sheep, goats, cattle and also rabbits (in which the liver can also become affected). It can affect dogs and cats, although in these species the disease is often sub-clinical and not diagnosed as often. The causative organism is usually a protozoa of the genera Eimeria or Isospora although each distinct form of coccidia is not only host-specific and so is not passed from one species to another, but it is also site-specific in that it only affects one specific part of the gut.

Coccidiosis is seen everywhere and is clinically most apparent in young animals in confined housing, although older animals can also be affected, particularly when they have not been previously exposed or where other environmental or disease stress weaken their immunity.

When trying to combat the effects of coccidiosis, it helps to understand the lifecycle of the parasite. The lifecycle starts with an unsporulated oocyst, which is similar to an egg, containing an undeveloped stage of coccidiosis. The oocyst has a thick wall which protects it from heat, cold and even common disinfectants and it will not develop until it is provided with the right conditions of warmth, oxygen and moisture; this usually occurs within 1-2 days and the oocyst becomes sporulated and thus infective. The ability to lie dormant awaiting the right conditions enables coccidiial oocysts to survive for up to several years and makes it difficult to destroy them, although they can be killed both by freezing and by excessive heat. Coccidia populations take time to build to critical levels, so typically the illness shows itself at between 3-8 weeks of age.

When the sporulated oocyst is eaten by a chicken, or other animal, enzymes and acids in the gut break down the outer wall of the oocyst and release the sporocyst – which is the infective form of coccidiosis. The sporocyst becomes a sporozoites, which quickly invades the cells of the gut wall and replicates. The sporozoites, in turn, release a further stage in the cycle, called merizoites – which then go on to invade and destroy even more cells of the gut wall. In due course, the infection runs full circle and produces more oocysts – which the chicken then passes out of the body with the faeces and which then lie in wait to infect another animal. The complete life cycle can take as little as 4-7 days.

Throughout its lifecycle, a single oocyst can end up destroying thousands of cells in the gut wall and if a large quantity of oocysts are consumed, so many millions of cells will be destroyed that it produces clinical symptoms, which are incredibly painful to the host, causing them to stop drinking and eating, lose weight, become deficient in nutrients and suffer diarrhoea and blood loss, causing anaemia. It can also change the balance of bacteria in the gut, allowing bacteria to cross through the gut wall and cause septicaemia. Typically, the animal looks extremely miserable. However, if the animal is lucky enough to only consume a few oocysts then the damage to the gut wall will be minimal, without producing clinical symptoms and it is possible to build up an immunity to the infection.

Given the universal presence of coccidia in the environment, best practice husbandry aims to minimise the quantity of oocysts to which the young animal is exposed, as far as possible, using specifically-targeted disinfectants, such as Smite Kokzi Des, prior to chick placement or introducing young stock, so that in the first few weeks of life, the animals are exposed only to low levels of infection which will allow them to build up an immunity. In general, older animals have already developed an immunity and will be resistant, though immunity to one species of Eimeria will not create immunity to any other variant of coccidia, so it is possible to become overwhelmed by a different, previously unencountered species later in life; one example of this is in ex-battery hens, where the birds were not exposed to their own faeces in early life and have therefore not developed immunity.

Animals and birds which have been affected by coccidiosis will rarely ever regain full productivity and in severe infections the gut remains scarred and impaired. Affected broilers do not catch up in weight gain. This means that the economic and financial losses associated with coccidiosis are considerable and well worth going to some lengths to avoid.