The influence of sheep age group on the seasonal prevalence of oestrosis in the island of Majorca

Oestrosis is highly prevalent in Mediterranean countries. Understanding the life cycle of Oestrus ovis is crucial to design effective control measures of this myiasis, largely based on the use of macrocyclic lactones. We carried out a survey of ovine oestrosis in the island of Majorca (Spain) and found that 46.03% of animals were infested in a 13-month period. Interestingly, we found significant differences in oestrosis prevalences in winter and autumn when separating the animals by group of age (P < 0.001). Pearson correlation analysis showed that prevalence in lambs younger than 4 months was significantly affected by changes in air temperature (P < 0.05), but this association was not significant in adult sheep (P = 0.081). Chronic infestations or unsystematic treatments may explain confusing results in adult sheep. Observing the evolution of the disease in young lambs, we determined that the hypobiotic period took place from October to February and the beginning of fly activity occurred between May and June. Interannual variations in oestrosis prevalence indicate the need of monitoring the disease to establish the appropriate timing of treatments. We hypothesize that lambs are better indicators of the seasonality of oestrosis than their older counterparts. Furthermore, we propose that observing O. ovis infestations in young lambs can be used as an efficient early warning system of fly activity, to be applied in future control programs.