Coccidia are protozoal parasites which can grow and multiply in the intestine of the chicken. They are host specific so the chicken has its own set of coccidia different from those which might infect turkeys, game birds, sheep or other animals. There are seven strains of coccidia which may infect chickens. These are Eimeria acervulina, Eimeria brunetti, Eimeria mitis, Eimeria necatrix, Eimeria praecox, Eimeria tenella and Eimeria maxima.

Gut damage
The different strains of coccidia affect different parts of the intestine. For example, Eimeria tenella causes caecal coccidiosis whereas the other strains cause more damage in the small intestine. Depending on the level of infection, coccidia may cause very severe damage to the intestine leading to death of the bird. In lighter infections, sometimes called “subclinical infection”, the effect may be depression in growth and unthriftiness, or loss of egg production in adult birds.

The life cycle
The infective stage of the coccidial parasite is called the “oocyst” or egg. It is passed out in the faeces of infected birds. The oocysts, when eaten by a chicken, releases sporocysts and then sporozoites which invade the cells lining the intestine and undergo several stages of multiplication ultimately resulting in the release of oocysts into the droppings of the bird again. The enormous multiplication cycle which the coccidial parasite undergoes in the intestine means that if a bird eats only one oocyst, this can result in the shedding of hundreds of thousands of oocysts in the droppings.

The life cycle takes around 7 days from the bird first ingesting the oocyst to passing out the oocysts in the droppings. This varies somewhat between different species of coccidia. When the oocyst is first passed in droppings, it needs to develop further to reach the infective stage. This development happens most quickly under warm, moist conditions when it takes only 1 to 2 days to reach the infective stage.

Resistant bugs
Coccidial oocysts are protected by a thick wall and are very resistant to disinfectants and to a fairly wide range of environmental temperatures. Oocysts can be killed by very high temperatures (above 56o C) by freezing or by drying. They can be killed using ammonia based disinfectants.

What are the symptoms of coccidiosis?
Birds with coccidiosis may appear dull and depressed with ruffled feathers and are often reluctant to feed. Droppings may be loose and/or contain blood, particularly in the case of infections with Eimeria tenella, Eimeria brunetti and Eimeria necatrix. In severe infections, mortality can be high and recovered birds may be unthrifty due to chronic gut damage.

Coccidiosis can affect chickens from around two weeks of age onwards. If treatment is required in the rearing period, drugs such as Amprolium, sulphonamides or Toltrazuril can be used. When coccidiosis occurs on the laying site, treatment is more difficult as the medications used are not licenced for use in laying birds and prolonged egg withholding periods may be required.

How do we protect our chickens from coccidiosis?
Pullets need to be exposed to coccidia to develop their own natural immunity - if birds are not immune to coccidia, they may suffer high mortality if suddenly exposed to the parasite at any point later in their life. Allowing the bird to develop immunity means that a balance must be achieved by giving the birds exposure to a small number of coccidial parasites – enough to trigger immunity but not to massive numbers which would cause severe disease and mortality.

Anticoccidial drugs can be administered in the feed during the rearing period. These drugs need to be effective in controlling coccidia so that they don’t cause acute disease in the bird but not so effective that the bird receives no exposure to the parasite at all. Sometimes the level of the anticoccidial drug in the feed is reduced gradually during the rearing period as birds get older. On other programmes, the treatment is stopped two to three weeks before birds are moved to the laying accommodation. This approach can work well and is usually quite adequate for birds intended as cage layers as these birds are at very little risk of exposure to coccidial parasites in the laying period.

Free range
For birds intended for the free range situation, however, this type of approach during the rearing period does not give any guarantee that the bird will have met all the strains of coccidia which it might meet once out on the range. This is becoming more of a problem with increased biosecurity on the rearing site – reducing the level of coccidial exposure during rear.

Vaccination
In order to address this problem, coccidiosis vaccines were developed. In the UK, there is only one licenced coccidiosis vaccine available - Paracox. This contains mild strains of all the coccidia which birds might be at risk from in the UK. The vaccine is administered to the birds during the first two weeks of life, usually via the drinking water. These mild strains of coccidia colonise the birds’ intestine and allow the bird to develop a natural immunity to all seven strains of coccidia but because they are very mild strains they do not cause disease in the bird. When properly administered, this type of vaccination ensures that birds should have a lifelong immunity to all the strains they might be likely to meet. Use of this vaccine also removes the risk of subclinical coccidiosis which can result in poor growth in pullets and difficulty in achieving eveness and optimum weights at point of lay. The use of the vaccine means that no anticoccidial drugs are required in the feed. In fact, care must be taken to make sure that anticoccidial products are not inadvertently included in the feed given to vaccinated birds as these will destroy the vaccine strains of coccidia making the vaccine ineffective. Although vaccination seems a more expensive option than feed medication during the rearing period, in our opinion, this provides the best protection for the free range layer.