The incidence of coccidia in broilers is increasing

Coccidia are single-celled intestinal parasites that currently represent one of the greatest challenges to the broiler industry. To keep the level of infection low, farmers commonly add coccidia-inhibiting chemicals (coccidiostats) to broiler feed. While this does not kill the parasites, it greatly reduces the incidence of overt sickness and death from infection. While clinical coccidiosis is therefore not a problem, veterinary authorities have never been able to gauge the extent of subclinical coccidiosis and the consequences this may have for animal welfare issues and production costs.

In her doctorate, Anita Haug looked at the incidence, epidemiology and significance of coccidiosis in the broiler industry in Norway. In order to complete such an extensive study, it was necessary for her to use diagnostic tools that could identify relevant coccidia strains quickly and reliably.

Existing test methods proved inadequate, and in several instances, intestinal changes characteristic of coccidia were not specifically identified by existing test methods. Haug therefore developed new test methods by simplifying traditional ones, and also developed a robust, effective and sensitive molecular-biological test.

Two large survey studies showed an increase in the incidence of coccidia-infected broiler flocks from 42% to 76% during a three year period and a strong swing in the type of dominant coccidia strain toward less pathogenic forms and away from more pathogenic ones. The total parasite load, country-wide, did not alter significantly during this period, but there were large regional differences in the numbers of infected flocks, the level of infection and the dominant species.

This survey study revealed that three coccidia species predominate in Norwegian broiler production. A relatively benign species was present in all flocks examined. The two other species were, however, extremely pathogenic, and were demonstrated in 77% and 25% of the flocks. Haug points out that twenty years' use of the same type of coccidiostat in broiler the broiler industry may have contributed to the increased incidence of coccidiosis on Norwegian farms. It will therefore be important to monitor the development of coccidia in Norwegian broiler production in the years to come.

The economic significance of milder coccidia infections may prove very difficult to evaluate. Haug studied the relationship between parasite load and production efficiency, and found that parasite load alone was not a good measure of the economic significance of infection. Reduced production occurred when there was over 50,000 parasites per gram of faeces and the pathogenic strains dominated. A corresponding level of infection of more benign coccidia strains did not have the same effect on production.

Haug's studies has given us effective tools for survey and routine diagnosis of coccidia in the poultry industry, enabling flocks at risk to be more easily identified.

Cand. med. vet. Anita Haug defended her thesis for the degree of Philosohiae Doctor on June 26, 2008 at the Norwegian School of Veterinary Science, with the title "Coccidiosis in broiler chickens - identification, epidemiological aspects and evaluation of gross intestinal lesions of infected birds".

The thesis work was carried out at the National Veterinary Institute in Oslo, and the National Veterinary Institute at Uppsala, Sweden, and was financed by the Research Council of Norway.

Mixed Eimeria species infection
Photo: Per Thebo

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The incidence of coccidia in broilers is increasing