Coinfection affects the phenotypic but not genetic resistance of cattle to common parasites

Background
Genetic variation in host resistance to individual parasites is well documented in cattle; however, the influence of coinfection on these genetic responses to selection remains poorly characterized. In particular, it is unclear how concurrent exposure to multiple parasite species alters phenotypic expression, heritability estimates, or genetic correlations between resistance traits. To address these gaps, we evaluated the impact of coinfection on the genetic architecture of parasite resistance in yearling Nellore calves naturally challenged with ectoparasites (ticks) and endoparasites (gastrointestinal nematodes and Eimeria spp.). Using longitudinal parasite count data, we estimated genetic parameters and examined how coinfection modifies both individual parasite resistance and the genetic correlations among traits.

Results
Our results confirmed that coinfection is a common phenomenon (almost ¾ of samples contained multiple parasites) and that resistance to individual parasites is a heritable trait. Furthermore, coinfection with Eimeria spp. reduced the phenotypic resistance to nematodes, and vice versa. We observed diverse genetic associations for resistance to different parasites, including positive, negative, and nonsignificant correlations. Notably, coinfection had no significant effect on genetic resistance to individual parasites, nor did it alter genetic variances or associations between resistance to different parasites.

Conclusions
While coinfection may influence the outcomes of nongenetic parasite control programs, its impact on genetic control strategies appears minimal. In other words, genetic resistance of Nellore cattle to three key parasite species appears to be robust and unaffected by the presence of coinfection.