Seasonal variations in the larval burden distribution of Oestrus ovis in sheep in the southwest of Spain
Introduction
Oestrosis is a myasis caused by flies belonging to the genus Oestrus (Diptera: Oestridae). It is considered a severe parasitosis and is common in domesticated small ruminants. The parasite is found worldwide, specially in Mediterranean areas of Europe and Africa, and even in tropical areas, due to the easy adaptation of Oestrus ovis to different environments (Cepeda-Palacios, 2001).
The pathogenic effects are particularly serious in sheep and goats. The nasal bot flies give rise to serious economic losses in hosts, mainly in herds reared for meat and dairy produce. The disturbance caused by adult flies and the development of larvae in the nasal-sinus cavities can produce severe consequences for the health and production of sheep.
Numerous works regarding the epidemiology of O. ovis have been carried out in Mediterranean countries: in Italy (Caracappa et al., 2000, Scala et al., 2001), France (Bergeaud et al., 1994, Dorchies et al., 2000), Greece (Papadopoulos et al., 2001), Morocco (Dakkak et al., 2001), and Jordan (Abo-Shehada et al., 2000). There studies show a high prevalence of oestrosis in Mediterranean sheep, between 33.15 and 91%. In spite of those results and the importance of ovine breeding in Spain, there are no works to know the real prevalence and intensity of the infestation in this area. Therefore, this study was conducted to discover the seasonal variations in the life cycle of O. ovis in southwestern Spain and examine the association between the presence of larvae in the adult sheep and weather conditions (temperature and rainfall) of this region.
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Animals
A total of 477 adult sheep skulls were examined from November 2000 to September 2002. Data on the origin, sex and age were recorded from each animal. The heads of slaughtered sheep were separated from the body, incised sagitally using an electric saw and examined in order to find O. ovis larvae in the nasal-sinus cavities according to previously described methods (Yilma and Dorchies, 1991). The larvae found in the nasal and sinus cavities were recovered, counted and identified according to keys
Prevalence and intensity
The number of sheep infested with some O. ovis larval stages was 339 (71.1%). Only one flock was free of infected animals, reaching a percentage of 97.99%. The monthly incidence of infestation with O. ovis is shown in Table 1. The month of October 2001 had the lowest rate of infestation (22.22%), while July 2002 had the highest percentage prevalence (100% of infested animals).
The mean intensity of infestation was 18.54 larvae by infested head. The highest number of larvae recovered from a
Discussion and conclusion
The results of this epidemiological and chronobiological study demonstrate that O. ovis was a common parasite of the flocks checked (97.99%). The intensity of infestation was widespread within sheep population, with a high prevalence (71.1%). The mean infestation intensity was 18.54 larvae per host, much higher than results obtained by Ruiz-Martı́nez et al. (1992) in the sheep of southern Spain with a mean larval burden of 3.2 per animal.
Dorchies and Alzieu, 1997, in Piédmont pyrénéen,
Acknowledgements
The authors wish to express their gratitude to the slaughterhouses veterinarians and the rest of staff, from Coria and Cáceres for their indispensable help in the sampling process.
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