Is Bluetongue Virus a Risk Factor for Reproductive Failure in Tropical Hair Sheep in Brazil?
Background: Bluetongue is a vector-borne viral disease transmitted by midges from the genus Culicoides. The disease can infect most of the ruminant and camelid species, but the severe disease is most often seen in european wool and mutton sheep breeds. In this sense, there is a gap in the knowledge on BTV infection in hair sheep breeds from tropical zones. Thus, this study aimed at establishing whether exposure to BTV is a risk factor for reproductive failure in Santa Inês ewes, a hair sheep breed, reared under tropical conditions in Brazil.
Materials, Methods & Results: A retrospective cross-sectional study was carried out in sheep farms in São Paulo state, Brazil, after the rainy season. Serum samples from 110 Santa Inês ewes with a history of reproductive disorders, in the last 6 months, which were included: abortion, premature birth, stillbirth, retention of placenta, infertility, estrus repetition, fetal malformation, weak lamb birth and neonatal death were collected. The presence of antibodies against BTV was assessed by agar gel immunodiffusion method (AGID). Serology to the infectious agents Brucela ovis, Lepstopira spp., Toxoplasma gondii, Neospora caninum and Campylobacter sp. were also assessed. Bivariate associations between the outcome and individual explanatory variables were assessed using the Fisher's exact test. Abortion was the most common reproductive disorder (53%; 74/139) observed, followed by estrus repetition (12%; 17/139) and infertility (11%; 15/139). Other disorders related to the conceptus totaled nearly one fourth of the reported disorders. A total of 20% (22/110) of the ewes were seropositive to BTV. A higher frequency of BTV seropositive than BTV seronegative ewes with a history of abortion was found. Also, abortion with seroreactivity to BTV was tested for prevalence ratio that showed 1.38 [95% CI 1.10-1.74; P = 0.030]. With regards to the abortion involvement of other infectious diseases associated with the seropositive ewes to BTV, more than a half of ewes (53%; 10/19) were solely seropositive for BTV.
Discussion: In the current study, it was detected 20% (22/110) of seropositive ewes to BTV. These findings demonstrated that even though the BTV has been considered endemic in tropical countries such as Brazil, there are regions or microclimates in which the virus cannot be present or in varied prevalence. The history of abortion was identified as the potential factor associated with BTV seropositivity in Santa Ines ewes. Equally, the differential diagnosis for other infectious agents related to abortion demonstrated the unique presence of antibodies against BTV in more than half of all cases. Other studies with native sheep flocks in Iran and Nepal also demonstrated a strong positive correlation between abortion history and seropositivity for BTV. Thus, it is possible that in other continents of the world, under tropical conditions, the virus does not behave the same asymptomatic infection such as have been reported for native sheep breeds in Africa. In conclusion, it was demonstrated that one-fifth animals were positive for antibodies against BTV clearly implying the viral spreading in the local hair sheep flocks. These findings highlight the importance of surveillance related to BTV in endemic areas. Therefore, it is recommended to strengthen the surveillance system for BTV within Brazil and to educate farmers about the management and control of this disease.
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