Transmission dynamics of Eimeria acervulina in broilers

Control of the intestinal disease coccidiosis, caused by infections with Eimeria species, is a major challenge, especially for the broiler industry. Effective control strategies require a comprehensive understanding of processes that lead to infection and disease in a population. One of the key factors that determine infection dynamics in a flock is the rate of transmission between hosts. Therefore, transmission experiments were carried out to increase the understanding of the underlying mechanisms of Eimeria acervulina infections in broilers, to facilitate improvement of control strategies. An important outcome of the experiments was that the excreted oocyst dose, which may be related to severity of clinical signs, increased during successive generations of infection in the flock, but that the transmission rate was independent of the oocyst dose. This suggests that transmission is not determined by the number of oocysts excreted with faeces of infected birds but, most likely, by the probability of birds to come into contact with infectious faeces. Factors influencing the degree and dispersal of infectious faecal material in the environment, such as movements and (litter pecking) behaviour of chickens, environmental conditions and faeces characteristics, may have a large impact on infection dynamics and efficacy of control measures. Furthermore, it was demonstrated that a previous infection with a wild-type E. acervulina strain significantly reduced oocyst output and transmission after re-infection. After infection with a live vaccine strain, oocyst output following an infection with a wild-type strain was also significantly reduced. However, a significant reduction of transmission of the wild-type strain was not found in groups of broilers that had been infected with the vaccine strain. Nevertheless, it was demonstrated that the live vaccine was efficiently transmitted to initially unvaccinated birds. Furthermore, the level of reduction of oocyst output was equally high for directly vaccinated and the “contact-vaccinated” chickens, that became infected due to ingestion of oocysts excreted by vaccinated birds. These results indicate that transmission of the vaccine can induce protection against high oocyst output for the entire flock, even when not all birds receive the vaccine during the initial mass application. The results of these experiments indicate that influencing the rate of transmission of wild-type and vaccine strains can be important for reducing the adverse effects of flock infections with Eimeria. Furthermore, this thesis has increased insight into some of the underlying factors that determine transmission dynamics of E. acervulina in a broiler flock. Further investigation of these factors may reveal novel targets or facilitate improvement of current strategies for coccidiosis control.