The response of Boer goats to natural and deliberate gastrointestinal nematode infection

Gastrointestinal nematode infection is one of the major diseases affecting sheep and goats but most of the studies have focused on sheep. As a highly productive meat breed of goats, it is important to study the response of Boer breed to gastrointestinal nematode infection. The aim of this study was to investigate the interaction between Boer goats and their gastrointestinal nematodes by evaluating the phenotypic (faecal egg counts (FEC), IgA, packed cell volume (PCV), peripheral eosinophil counts and bodyweight) and parasitological (nematode number, length and index) variables following natural and deliberate co-infections that were dominated by Teladorsagia circumcincta and Haemonchus contortus, respectively. This study also aimed to estimate the repeatability, which provides an upper limit on heritability, and heritability of phenotypic variables among Boer goats, in particular the FEC as this is an important marker of resistance to gastrointestinal nematode infection. The study on natural infection was conducted in a farm in England with a semi-intensive grazing system whereas a deliberate infection study was conducted in a farm in Malaysia with an intensive management system. The FEC was confirmed to follow a zero-inflated Poisson distribution after comparing the observed and predicted zeroes in a Poisson regression. Mixed model repeated measures analysis that was conducted in a Bayesian framework was then used to analyse the phenotypic data. The findings from the studies of the phenotypic variables showed that the repeatability was moderate for FEC and PCV but relatively high for IgA activity and peripheral eosinophil counts and decreased significantly as the interval between sampling increased. The repeated measures models showed that FEC variation among Boer goats was affected by eosinophilia through time with the presence of an interaction with IgA responses, but that FEC was not heritable among Boer goats. The bodyweight of Boer goats was shown to be highly heritable despite them being infected with gastrointestinal nematodes. Additionally, the unaffected PCV levels and the increase in bodyweight over the course of infection suggest that Boer goats may be relatively resilient to gastrointestinal nematode infection. Multiple linear regression analyses of the mean phenotypic variables and the parasitological variables measured at necropsy suggested that H. contortus and Trichostrongylus colubriformis affected each other during co-infection. Shorter T. colubriformis was associated with greater H. contortus index and number whereas longer T. colubriformis was associated with a reduction in H. contortus number. Additionally, increased in IgA activity was associated with increased H. contortus length but a reduction in T. colubriformis index during co-infection. Together the findings show that eosinophils and IgA do play a role in the response of goats to gastrointestinal nematode infection, but the effects are less pronounced than in sheep. Moreover, the finding of a very low heritability of FEC is in contrast to findings in sheep. Most of the findings also suggest that the Boer breed is relatively resilient to infection but further work is needed to confirm whether these finding might be due to low infection doses. In conclusion, this study has expanded knowledge of the host-parasite relationship in goats as well as demonstrating interspecific nematode interactions between H. contortus and T. colubriformis, which commonly co-infect goats under natural conditions. However, future studies that overcome the limitations in the present study are needed to confirm the resistance status of Boer goats against these nematode species