The use of genetically marked infection cohorts to study changes in establishment rates during the time course of a repeated Ascaridia galli infection in chickens.

This study investigated the changes in establishment rates during the time course of a 6 week trickle infection of chickens with Ascaridia galli at two different dose levels, using a molecular marker. To differentiate early and late infection, two different egg cohorts (haplotype a and haplotype b, genetically identified using PCR-linked restriction fragment length polymorphism on the cox1 gene of the mitochondrial DNA) were used. Cohort-specific egg batches were produced by harvesting eggs from the uteri of female worms of the specific cohort. Fifty-six 8 week old Lohmann Brown Lite chickens were divided into seven groups and the infectivity of the egg batches was compared between two groups of chickens (P=0.6). The remaining chickens were allocated to four infection regimes and one control group. Group ab100 was trickle infected for 3 weeks with 100 eggs of haplotype a (twice weekly) followed by the same dose of eggs of haplotype b for another 3 weeks. Group ba100 was treated similarly but in the opposite order (haplotype b preceding a). A similar infection regime was applied for groups ab25 and ba25 but with a lower inoculation dose (25 eggs). All of the birds in these five groups (four infected and one control) were euthanased 2 weeks after the last inoculation. It was found that in the low-dose groups both the early and late infections established equally well, whereas in the high-dose groups the early infection was recovered in a significantly (P<0.001) higher proportion of chickens than the late infection, irrespective of genetic cohorts. Moreover, relatively higher proportions of the larvae from both the early and late infections were found in the posterior section of the small intestine. This result indicates the presence of dose-dependent resistance against reinfection and this resistance seems to act by reducing the establishment of late infection and by relocating the larvae from early infection.