Oestrus ovis Infestation in Sheep Examined at Al-kharj Slaughterhouses, Saudi Arabia: Epidemiological Associations, Clinical Signs and Serum Mineral Concentrations

Background: Oestrus ovis is an important cause of myiasis in sheep and goats, mainly affecting the upper respiratory tract and causing sneezing, nasal discharge and respiratory discomfort. Infection may also reduce animal productivity through poor weight gain, decreased milk yield and, in severe cases, mortality. Methods: The study was conducted as an abattoir based observational study in Al-Kharj, Saudi Arabia, from December 2024 to August 2025, where a total of 480 sheep were examined for Oestrus ovis infestation using random sampling. Ante-mortem examination was performed to record clinical signs, including sneezing and different types of nasal discharge; however, sampling was not restricted to clinically suspect animals. Age and breed were documented for each animal. Post-mortem examination involved opening the nasal cavities and frontal sinuses, recovering larvae and preserving them in 70% ethanol for morphological identification and staging. For the biochemical assessment, blood samples were collected from 16 sheep with O. ovis infestation confirmed by post-mortem larval detection and 16 non-infested sheep (n = 32). Serum was separated by centrifugation and calcium, phosphorus and potassium concentrations were measured using a commercial veterinary diagnostic kit according to the manufacturer’s instructions. Result: A nine-month survey conducted in Al-Kharj, Saudi Arabia, demonstrated the occurrence of ovine nasal myiasis caused by Oestrus ovis among the examined sheep, with an overall infestation frequency of 11.46% (55/480; 95% CI: 8.91-14.62%, wilson method). Infestation was significantly more frequent in sheep older than 2 years (24.38%) than in those aged 2 years or younger (5.00%; P less than 0.001). Harri sheep showed a higher prevalence (13.79%) than naimi sheep (7.89%), with an unadjusted prevalence ratio of 1.75 (95% CI: 0.99-3.07; P = 0.047). Seasonal prevalence was highest in spring (26.88%), followed by winter (5.63%) and summer (1.88%; P less than 0.001). Mucous nasal discharge was the predominant clinical sign (45.45%), followed by sneezing, mucopurulent discharge and blood-tinged nasal discharge, while larvae were associated with inflammatory lesions and mucous accumulation in the nasal sinuses. Infested sheep had lower serum calcium and potassium concentrations than non-infested sheep (P less than 0.001), whereas serum phosphorus concentrations did not differ significantly between the groups (P = 0.488).