Transmission and excretion of Eimeria acervulina in vaccinated broiler flocks
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Coccidiosis is an economically important disease in chickens, caused by infection with Eimeria species. Due to increasing resistance against anticoccidials and European legislation to forbid prophylactic in-feed medication by 2012, alternatives for the current control strategy with anticoccidials in feed should be considered. Vaccination might be a good alternative, but is not often applied in broilers due to high costs and disappointing results. Suboptimal results of vaccination might be caused by insufficient uptake of vaccine by part of the flock, due to mass application. However, this was not studied to this date. Studying transmission of vaccine or field strains can increase knowledge on infection dynamics in vaccinated flocks, which might contribute to optimalisation of vaccine programs. Therefore, an experiment was carried out where transmission and excretion of vaccine and field strains were examined and effects on body weight were assessed. The experiment consisted of a vaccination period from days 2-27 and a challenge period from days 28-49 of age. On day 2 of age, 88 SPF broilers were divided in four groups of 22 animals: group 1 remained unvaccinated (NV, control group). In groups 2-4 half of the flock (11 animals) was vaccinated (Vaccinated, V birds) with 600 sporulated E. acervulina oocysts of the Paracox vaccine (Intervet). The other 11 birds of each group were sham vaccinated with water (Contact-Vaccinated, CV birds). In the challenge period, birds from groups 1-4 were divided into eight groups of 10 birds. Groups 2A, 3A, 4A consisted of V birds and groups 2B, 3B and 4B of CV birds. Groups 1A and 1B consisted of NV birds. In all groups, except 4A and 4B (unchallenged vaccinated controls), five birds per group were challenged (inoculated, I birds) with 5000 sporulated oocysts of a field strain of E. acervulina and five were not (contact-inoculated, CI birds). Individual droppings were examined daily from days 5-49 to determine OPG (number of oocysts per gram of faeces). The total infection delay time, i.e. time between expected and actual start of excretion added for all CV or CI birds per group, was determined as a measure for transmission of the Eimeria strain. Body weights were determined on day 0 and weekly from day 2-49. In the vaccination period, all 11 CV birds became infected and total infection delay times were, 16, 12 and 18 days for groups 2, 3 and 4 respectively. The peak of output was significantly higher for CV birds compared to V birds. In the challenge period, all CI birds became infected and total infection delay time was zero days for groups 1A, 1B (NV) and 3A (V), one day for groups 2B and 3B (CV) and nine days for group 2A (V). Total oocyst output after challenge infection by NV birds was significantly higher compared to CV or V birds but was not significanty different between CV and V birds or between CI and I birds. A significant difference in body weight between V and CV birds was only found at day 7 after vaccination, when V birds showed a slight growth check. During the challenge period, body weights could not be compared due to large differences in cage conditions. From these results it can be concluded that direct or contact-vaccination apparently could not reduce the (rate of) transmission of the field strain of E. acervulina, but did reduce oocyst output after challenge compared to non-vaccinated birds. When birds were not directly vaccinated but were into contact with vaccinated birds, i.e. contact-vaccinated, a similar degree of protection against high oocyst output after challenge infection occurred. The results of this study suggest that vaccinating only part of the flock might be applicable in vaccination programmes. The absence of differences in rate of transmission between non-vaccinated and (contact)-vaccinated birds suggest that the probability of coming into contact with infectious material might play a more important role for spread of vaccine strains than the number of oocysts in the environment. Consequently, in future vaccination efficacy studies, effects of bird density and available space for the birds should be evaluated as well.