Understanding resistance and differential vaccine responses to Eimeria in the chicken - novel biomarkers and genetic control.

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One of the main underpinning factors for a profitable large-scale poultry industry is the fact that the disease coccidiosis, caused by species of the protozoan parasite Eimeria, is controlled primarily through the use of drugs, or coccidiostats. Vaccines do exist, but these are currently primarily produced by passage of Eimeria through birds and therefore not a cheap nor practical solution to replace coccidostats. Reliance on a single main control measure is not ideal, particularly with political pressure in some parts of the world to ban the use of coccidiostats.Resistance to Eimeria infection has long been known in inbred lines of chickens, but attempts to map this have been largely unsuccessful. Chromosomes associated with resistance have been identified, but we have yet to identify causative genes, or better still causative mutations. Differential responses to vaccines have also been described, presumably due to similar mechanisms, although this has yet to be formally proven. Resistance has always been described as oocyst output - the fewer oocysts excreted, the more resistant the bird. However, resistance is associated with a stronger innate and adaptive immune response in these birds, and it is unclear, particularly in broilers, if this stronger immune response compromises other parameters, such as feed conversion efficiency (FCE). In other words, do birds that produce fewer oocysts, due to a stronger gut immune response, perform better than birds that produce more oocysts, or is the obverse the case?We plan to revisit mapping disease resistance, and differential responses to vaccines, using modern techniques, in particular the newly available 600K SNP chip. Similarly to funded work on Campylobacter resistance, we will use both inbred birds in a backcross design, and commercial birds in a genome-wide association study, or GWAS, experiment. We will then assess FCE in resistant versus susceptible birds.The adaptive immune response is that which clears the pathogen causing the infection, and delivers immunological memory against reinfection. For many years, the adaptive immune response has been split into two arms, each involving a different subset of CD4+ Thelper cells. Th1 responses control infection with intracellular pathogens, such as viruses. Th2 responses control infections with extracellular pathogens such as worms. Adaptive responses are now known to be more complicated, with more subsets of CD4+ T cells involved.It is well established that infection with Eimeria, an obligate intracellular pathogen, requires a strong inflammatory, Th1 response to control it. However, little has been done recently to investigate the role, if any, of other T cell subsets which have only recently become known in mammals and for which reagents have only recently become available in the chicken - i.e. Th17, Th9 and Treg. We will investigate these arms of the adaptive immune response in both the response to primary infection and to vaccination, with the expectation that this will lead to novel tools to defining disease biomarkers and phenotypes.