Coccidiosis is one of the more common and costly diseases in poultry. It is characterised by droopiness, paleness of the comb, diarrhoea and occasionally blood in the droppings. The death rate may be quite high, both in chicks and in adults.

Coccidiosis in chickens is caused by seven different species of coccidia (genus Eimeria), which are single celled parasites that live in the gut wall of their host. These coccidia are host specific: turkeys and other species are not infected by fowl coccidia and vice-versa. The different species of coccidia live in different parts of the gut and can be divided into those causing intestinal coccidiosis (the majority) or caecal coccidiosis (one species).

Coccidiosis is spread when one bird eats faecal material from an infected bird, which contains the infective stage of the coccidia (small egg-like bodies called oocysts). The oocysts in the droppings need moisture and warmth to mature before they can infect other birds, but in the right conditions, can do so very quickly (24 hr). Oocysts can remain alive in poultry sheds for more than a year and they are very resistant to most disinfectants.

Birds of almost any age may be affected, but problems are not common in chicks under three weeks of age because the parasites take time to build up in sufficient numbers to cause problems. Affected birds become depressed, lose condition and are very pale. The feathers are ruffled, the wings droop and the shanks become pale and dry. A slight whitish soiling may be present around the vent. There is usually diarrhoea and there may be blood in the droppings. Often, a large percentage of the chickens are sick. Birds may die suddenly before the above symptoms are obvious or the performance of birds may be affected without the disease causing obvious signs.

Post-mortem findings will vary depending on the type of coccidia responsible. In caecal coccidiosis, which is caused by the species Eimeria tenella, the blind gut (caeca) are swollen and filled with blood and cheesy plugs. In intestinal coccidiosis, the damage will vary depending on the species of coccidia. Findings may include: white streaks or spots in the upper part of the intestine, a ballooned and blood-filled intestine, or reddish spots, inflammation and dead tissue in the lower part of the small intestine.

Definite diagnosis can be confirmed only by laboratory examination. Material scraped from the lining of the gut is examined under a microscope and the coccidia are identified based on shape, size and location in the gut.

Coccidiosis can be confused with similar diseases such as blackhead, salmonellosis and necrotic enteritis.

Because coccidia require moisture to become infective, the litter must be kept dry. Ventilation must be good and the birds should not be overcrowded. Birds gradually become immune if they are exposed to a low level of infection, but clinical disease occurs if the coccidiosis challenge is too great. Immunity to one species of coccidia does not protect poultry against other coccidial species.

It is very risky to rely on hygiene alone to produce satisfactory control.

Effective live vaccines are now available in Australia. These ensure the birds are exposed early in life and develop immunity to the most virulent species of coccidia. For effective vaccination, it is essential to closely follow the manufacturer´s recommendations.

Medication programs may be used as an alternative to vaccination.  A wide selection of drugs (coccidiostats) is available for prevention and treatment. The choice of drug will depend on the type of flock, the type of coccidia and the aim of the medication program. Most coccidiostats have withholding periods and medication programs must take this into account.

Low dose rates of coccidiostat may be used to slow down a major build-up of coccidia, reducing the challenge to the bird, and thus preventing outbreaks while allowing immunity to develop. Minimising the build-up of oocysts using other management strategies will greatly assist the efficiency of a coccidiostat program since coccidiostats can be overwhelmed by heavy infections.

Outbreaks of coccidiosis may occur if the level of coccidiostat in the feed is too low, if the birds are not eating enough or if the coccidiostat is withdrawn too early (before immunity has developed). Lack of vitamins A and K will cause the outbreak to be more severe as will other diseases which reduce the general resistance of the bird.

Treatment should start immediately when coccidiosis has been diagnosed. Whichever drug is used, the manufacturer's instructions must be followed.

Coccidiosis will often respond to treatment using coccidiostats delivered in the drinking water. These are available from veterinarians, chemists or produce merchants.