Lungworm infection and ovine visna–maedi: Real risk factor or a confounding variable?

Serum and fecal samples (n = 2081) were taken from 74 commercial meat ovine flocks in Galicia (NW Spain) in order to establish the influence of lungworm infection with protostrongylids and/or Dictyocaulus filaria on visna–maedi (VM) seropositivity. VM was tested by an indirect ELISA test and lungworm larvae were detected by the modified Baermann–Wetzel technique. In addition to lungworm infection (protostrongylids and D. filaria), flock, management and individual factors were included in univariate and multivariate statistical tests. Overall VM seroprevalence was 18.9% and the prevalence by protostrongylids and D. filaria was 11.6% and 10.7%, respectively. Protostrongylid infection was considered as a risk factor for VM with univariate statistical analysis (1.49 significant OR), but neglected in a logistic multivariate regression analysis when the age of the animals was introduced as variable. As VM, protostrongylid infection is a chronic disease with a cumulative pattern over time, so that these two infections are closely related to the age of the animal. A Mantel–Haenszel procedure was applied and lungworm infection was detected as a confounding variable with no real effect on visna–maedi infection.