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Christiaan ter Veen is a poultry veterinarian and expert in the field of coccidiosis. He keeps you up to date on the latest developments. Any questions?

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Coccidiosis is a disease caused by the Eimeria parasite, which is generally present and survives for a long time in the house surroundings in a form that is extremely resistant to environmental influences (known as an oocyst). Such oocysts can remain infectious even after weeks (in faeces) or months (in soil).

Animal disease information Coccidiosis

  1. General
  2. Clinical signs
  3. Diagnostics
  4. Prevalence
  5. Approach to infected farm



Coccidiosis is an umbrella term for several clinical signs, each caused by a different Eimeria species. In chickens, the main species are Eimeria acervulina, E. maxima, E. tenella, E. brunetti and E. necatrix. There are a further two species of Eimeria (E. mitis and E. praecox), which do not generally present clearly recognisable clinical signs, but can still cause retarded growth. Recently, three new Eimeria species were discovered in chickens known as E. lata n. sp., E. nagambie n. sp. and E. zaria n. sp.

The bacterium

Eimeria spp. are single-celled parasites, also called protozoa. The species has a direct cycle, meaning no intermediate host is needed and oocysts excreted into the environment by an infected chicken can directly reinfect chickens following a short developmental step (sporulation). Several asexual and sexual reproduction steps take place in the chicken after which the parasite is thus re-excreted by the chicken as an oocyst. All in all, the cycle from ingestion to sporulated oocyst takes about 4 to 8 days, depending on the Eimeria species.

Susceptible animal species

Eimeria parasites are quite animal-specific and each species can often infect only one or a few species. The Eimeria species found in chickens do not infect other animal species.

Public health

The Eimeria species found in poultry cannot infect humans.


Oocysts are highly resistant to environmental influences and can remain infectious for weeks to months in a barn environment or free-ranging situation. Oocysts are especially sensitive to ammonia and are killed off when temperatures above 56 degrees Celsius are applied for several minutes.


Oocysts are not very sensitive to common disinfectants. Disinfection against Eimeria should therefore be specifically included in the disinfection programme. Ammonium hydroxide, Cresol, caustic soda and formaldehyde are suitable agents when used in sufficient concentration in an effective cleaning process. Usually, however, disinfection is insufficient to get completely rid of the parasite, but if done properly, the amount of infectious oocysts is reduced to the extent that infections occur later in life and are milder.

Clinical signs

Clinical presentation

The severity of the disease depends partly on the infection pressure, and there are often no clinical signs in chickens infected with a low number of oocysts. However, coccidiosis is a risk factor for the development of non-specific intestinal disorders in such cases. When there is a infection pressure, clinical disease develops; symptoms vary from species to species. As the infection pressure develops in the barn during the growth-out period, clinical disease often occurs in some of the animals, while other animals in the barn are already immune to the disease at that time. Multiple coccidiosis species may be present simultaneously.

  • E. acervulina causes a relatively mild intestinal disorder. The animals suffer (limited) growth retardation, and there is often no illness within the flock. Diarrhoea and ruffled feathers will occasionally be seen over a number of days.
  • E. maxima usually causes a mild infection though with rather strong growth retardation due to impaired digestion and reduced nutrient absorption. In more severe cases, E. maxima can cause small intestinal haemorrhaging, resulting in severe weight loss in the chickens, anaemia, ruffled feathers and sometimes death. E. maxima is also more significant than other Eimeria species with regard to an increased risk of developing necrotic enteritis.
  • E. tenella causes bloody inflammation of the appendix. The animals become listless and anaemic and may die. Fresh blood can often be found in the faeces. There can also be considerable losses in case of a 'slight' E. tenella infection.
  • E. brunetti usually causes mild illness accompanied by diarrhoea. In severe cases, the animals can die within a few hours.
  • Animals infected with E. necatrix will lose weight quickly and have watery, bloody and/or slimy faeces. They become anaemic and can die from the infection. In some cases, losses can be as high as 25 percent.
  • E. praecox, E. mitis, E. lata n. sp., E. nagambie n. sp. and E. zaria n. sp. do not generally cause clinical signs, though they do cause growth retardation.


Coccidiosis is typically a disease where the infection pressure increases during the round. Some of the animals will become infected early in the round, often without actually becoming sick. However, due to the excretion of oocysts by these animals the infection pressure in the environment builts up as time goes by. Animals infected later on in the round will therefore suffer a more severe infection potentially resulting in growth retardation, clinical disease or mortality. In some Eimeria species, the disease is mainly subclinical, while in others mortality can be as high as 25 percent.

Coccidiosis often occurs at a young age. Rapidly reproducing species such as E. acervulina, E. maxima and E. tenella are often seen before the fifth week of life. Slowly reproducing species such as E. brunetti and E. necatrix are mostly seen later in life, but these species can also cause disease at a younger age if there is a high infection pressure at a young age. There is no real age resistance, as adult animals can also be susceptible to coccidiosis, but in practice, outbreaks are mostly seen in young animals. This is because many chickens are exposed to the parasites at an early age and build up immunity after going through an infection.

Excretion of the bacterium

The bacterium is excreted in the faeces.

Differential diagnostics

The disease is often mainly characterised by abnormal faeces, which is rather atypical since there are many other diseases that present with more or less the same symptoms. Pathological examination or additional laboratory tests, such as an oocyst count or a PCR test, can usually distinguish coccidiosis from other diseases.



Most of the main species of coccidiosis can be diagnosed on the basis of the clinical signs and the necropsy results. This is due to these species showing fairly specific necropsy results. Pathological examination at GD or by the farm veterinarian will indicate the overall severity of the disease and will identify the most likely Eimeria species. Scientific research has developed a lesion score system to denote the severity of the coccidiosis, with abnormalities given on a scale of 0 (absent) to 4 (extremely severe).

  • E. acervulina causes infections in the upper small intestine (duodenum). White coloured lesions become visible, which can vary from a few lesions per square centimetre (score 1) to overlapping lesions over the entire surface of the intestine (score 4).
  • E. maxima infects the middle section of the small intestine (jejunum) and the abnormalities vary from orange content with occasional pinpoint haemorrhaging (score 1) to copious blood in the lumen (score 4).
  • E. tenella occurs only in the appendix, and the damage varies from occasional pinpoint haemorrhaging (score 1) to copious blood or clots in the appendix of dead, anaemic animals (score 4). In intermediate infections caecal cores are visible.
  • E. brunetti is characterised by occasional haemorrhaging in the lower section of the small intestine (score 1) to a bloody and extremely damaged intestinal wall, which can sometimes also be covered with an extra layer of inflammatory cells, intestinal cells and parasites, or in which caecal cores are visible (score 4).
  • E. necatrix occurs in the middle section of the small intestine; the abnormalities vary from occasional red and white pinpoints (score 1) to extensive bleeding with slimy intestinal contents and a gaseous intestine (score 4). E. necatrix can also cause caecal cores.
  • E. praecox, E. mitis, E. lata n. sp., E. nagambie n. sp. and E. zaria n. sp. infections do not generally cause specific abnormalities, though abnormal intestinal contents may be seen. However, the oocysts of these coccidiosis species can be found in an intestinal wall scraping.

Faecal testing

To gain insight into the risk of infection in a flock, faecal samples can be tested using the oocyst count (OPG) or quantitative (q)PCR. A pooled sample of at least 50 individual fresh faecal samples per barn must be tested for this purpose, preferably containing 40 samples of large intestinal faecal matter and 10 appendix samples. In the case of the qPCR, it is important that samples are stored cool, as development of the oocysts is affected by temperature, which can strongly influence the test results. Both testing techniques identify the number of oocysts per gram of faeces. An advantage of the oocyst count method is that this test is cheaper than the qPCR. However, distinguishing between the various Eimeria species is more difficult in the oocyst counting method, because the oocysts of the various species are not always visually distinct. The qPCR is capable of distinguishing and identifying all main species that occur in chickens.

When interpreting the oocyst excretions, variations in the volume of the various Eimeria species excreted must be taken into account. Indeed, an E. acervulina infection excretes more oocysts than E. maxima or E. tenella infections, even though it need not lead to severe disease. Moreover, older animals can excrete low numbers of oocysts despite being immune to disease. For a correct diagnosis of coccidiosis it is therefore important to assess the general health of the animal or flock in addition to the oocyst excretion count.

By regularly determining the oocyst excretion count (every 2-3 days during the risk period) and combining this with other data (including the growth curve and necropsy results), we are able to gain effective insight into the coccidiosis infection suffered by a flock. This data can then be applied to evaluate the anticoccidial programme. Samples can also be pooled on a weekly basis to save costs: the samples should be collected every 2-3 days, and stored cool. However, this will give a more general picture, as the sensitivity of the test is reduced in a pooled sample.


The Netherlands

Coccidiosis is common in both commercial and backyard poultry. Every flock probably suffers an infection with one or more coccidiosis species. For the purpose of animal disease monitoring, GD keeps track of the incidence of coccidiosis in animals submitted for necropsy; this is reported in the monitoring report. As normal monitoring only examines diseased flocks a 2012 project studied the occurrence of coccidiosis in average flocks of Dutch broilers. Five animals were examined weekly by necropsy, taken from a large number of flocks, for this purpose. E. acervulina was found in 94% of the flocks, E. maxima in 49% of the flocks and E. tenella in 40% of the flocks. E. mitis and E. praecox could not be detected, because the study looked at abnormalities in the intestine. Subsequent monitoring with qPCR showed that E. mitis occasionally occurs in broiler flocks. E. praecox was not found. E. brunetti and E. necatrix were not found in broilers in the 2012 study. These species are nowadays occasionally found in flocks of slower-growing or organic broilers.

No prevalence studies have been conducted in rearing breeding poultry or rearing layers, but it is assumed that most flocks suffer infections with a variety of coccidiosis species. However, exactly which species occur can vary from farm to farm.

The three new Eimeria species known as E. lata n. sp., E. nagambie n. sp. and E. zaria n. sp. have not yet been encountered in the Netherlands.

Other countries

The coccidiosis species affecting poultry all occur worldwide, although the prevalence of certain species varies.

Approach to infected farms

In the event of coccidiosis, the clinical signs depend on the number of oocysts ingested and the immunity already developed by chicks. The plan of approach to coccidiosis is therefore primarily aimed at reducing the risk of infection and facilitating the development of immunity. There are two ways of doing so: by preventively adding anticoccidial agents to the feed, or by vaccination.


Vaccination is widely used in rearing animals, but can also be used in broilers. Vaccines provide protection against the Eimeria species present in the vaccine. Almost all vaccines contain the E. acervulina, E. maxima and E. tenella species, which are common in poultry. E. mitis and E. praecox are not included in all vaccines. Vaccines developed for replacement hens additionally contain E. brunetti and/or E. necatrix, but these coccidiosis species may also be important for (organic) broilers. Vaccines are not yet available for E. lata n. sp., E. nagambie n. sp. and E. zaria n. sp.

The coccidiosis vaccines registered in Europe are all live vaccines containing mild Eimeria strains. In other words, these strains in themselves are not highly pathogenic (causing disease). When using these vaccines, two issues are important for effective development of protection: administration of the vaccine and suffering repeated infections.

The vaccine is administered in the first week of life. There are several ways to administer coccidiosis vaccinations. The main methods of administration are via spray on feed or in the hatchery, gel spray, drinking water and in-ovo; individual administration via eye drops is also possible. The most suitable administration method depends on the vaccine chosen and the farm situation.

After vaccination, animals suffer a uniform infection and oocysts are excreted into the environment. The animals will subsequently reinfect themselves. This is important because repeated infection is necessary to build up sufficient immunity. To sporulate, oocysts need sufficient heat, moisture and oxygen. Normal barn conditions are fine for this, though reinfection may be delayed in very dry barns. Any moving of animals should be delayed as long as possible until the animals have built up immunity. If moved during the first ten days of life, chicks may not experience repeated infections and therefore may not develop immunity. Vaccination cannot be combined with the use of anticoccidial agents due to the vaccine strains being sensitive to these agents.

During the period when the vaccine is circulating in the flock, the animals are also susceptible to field infections. The animals may therefore suffer simultaneous infections with field and vaccine strains. It is therefore important to reduce the risk of infection in the barn before starting vaccination. Consequently, when transitioning from anticoccidial agents to vaccination, strong anticoccidial agents are often used to reduce oocyst excretion, and thus environmental contamination, as much as possible. We recommend keeping a close eye on coccidiosis infections during this round, as some of the agents used can lead to late and severe coccidiosis infections under certain circumstances, which is obviously the opposite of the intended effect. In addition, we recommend reducing the risk of infection between rounds as much as possible by additional cleaning and disinfection.

Anticoccidial agents

Over the years, several agents have been developed that can be used preventively against coccidiosis. Some agents particularly suppress the severity of the infection, while at the same time allowing development of immunity, so that the animals are protected later on in life. Other agents interrupt the cycle of the parasite, so that the infection is delayed to a later point in time, as it were. If the cycle is interrupted at an early stage, no immunity can be developed, and (severe) infections may occur after all, later in the round. Such agents must therefore be administered until late in the round, leaving no time for the risk of infection to increase. There are also agents that kill the parasite at a later stage, which will result in immunity developing, but without the excretion of oocysts.

The parasite may lose sensitivity and possibly even become resistant to anticoccidial agents over the course of time. The number of rounds that an agent can be used depends very much on the active ingredient. Ionophore anticoccidial agents can be used for a number of consecutive rounds, whereas the intensive use of quinolones will quickly result in resistance being developed. When switching anticoccidial agents, it is important to remember that the new agent should be from a different (sub)group and should have a different active ingredient, induction point or operating mechanism. It is also possible to alternate anticoccidial agents and vaccination.

Should a clinical outbreak of coccidiosis occur, treatment can be administered using agents based on sulphonamides, toltrazuril or amprolium. In the case of broilers, the long withdrawal period must be taken into account, which means that such agents often cannot be administered during the most important risk period (fourth week of life).

Other measures

Good biosecurity is important to keep coccidiosis out. While it is virtually impossible to completely avoid contact with Eimeria spp. In most commercial settings, the flock will benefit from fewer coccidiosis species being present and the lowest possible risk of infection.

Cleaning and disinfection also help reduce the risk of infection in the barn. Check the topic on Disinfection above.

Coccidiosis infection has quite an effect on intestinal health. Dysbacteriosis results from impaired digestion and absorption or leakage from the gut. In some cases, necrotic enteritis may develop. Preventive or therapeutic treatments may be required to combat dysbacteriosis or necrotic enteritis in coccidiosis. Several treatments are available 

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