There is conflicting data as to whether males are more susceptible to gastrointestinal nematodes than females. It has been suggested that testosterone may have an immunosuppressive effect and that females may have a better immune response due to the role of prolactin. Another theory is that the more aggressive grazing behavior of males may result in increased levels of infection. However, most lambs are castrated at an early age and the differences between castrated males and females are not clear.
Over a three year period (2001-2003), Scottish researchers used three generations of Scottish Blackface lambs (n=758; equal numbers of castrated male and female lambs) to evaluate the role of lamb gender on susceptibility to natural nematode infection. After weaning at four months of age. the lambs were treated with an anthelmintic. Fecal samples were collected from the lambs at 4, 5, and 6 months of age from August through September.
The majority of the lambs had relatively low egg counts. Egg counts varied by time and gender. Egg counts in the male lambs were consistently higher than the females lambs, but differences were only significant in October, at 6 months of age. Since the male lambs were castrated, male hormones cannot explain the observed differences. An alternative hypothesis is that males are growing more quickly and they allocate greater resources to growth.
Source: Journal of Veterinary Advances. 2014