Distribution of Nasal Myiases Affecting Roe Deer in Spain and Associated Risk Factors

This study aims to assess the current situation of the main nasal bot flies (<i>Cephenemyia stimulator</i> and <i>Oestrus ovis</i>) affecting roe deer in Spain and to identify the factors influencing their spread. Between 2018 and 2025, 1600 roe deer heads were examined. The overall prevalence of <i>C. stimulator</i> was 39.9% (CI 95% 37.5-42.3), with a mean larval intensity of 44.1 (SD 71.99). In contrast, a low mean larval burden of <i>O. ovis</i> (2.6; SD 1.90) was found in 1.6% of the animals (CI 95% 1.0-2.3). Our findings revealed the first case of mixed myiasis caused by both oestrids. Roe deer abundance was the main risk factor for the prevalence of <i>C. stimulator</i>, whereas larval burden was significantly highest in young animals. In addition, the percentage of infested animals and larval intensity was generally higher in northern areas. The high prevalence of <i>C. stimulator</i> in Spain and its notable increase in recent years indicate that control measures must be taken. Our results reveal that preventing roe deer overpopulation, especially in northern regions of the country, would be key to decreasing the likelihood of nasal bot fly infestation.