Biogeography of the spread of bluetongue disease in sheep in Brazil

Abstract

Bluetongue is caused by the Bluetongue virus (BTV), a member of the genus Orbivirus within the family of the Sedoreoviridae. The virus can spread among sheep herds, increasing economic losses mainly due to the viral transmission being carried out by a biological vector directly affected by environmental conditions. In addition to Brazil having different biomes, it is also one of the countries with the most significant environmental commitment in recent years, which may interfere with the biogeography of the disease. Thus, the present study analyzed the spatio-temporal distribution of Bluetongue (BT) in sheep and the detection of regions with a higher risk for the disease in different regions of Brazil between 2013 and 2021. During this study period, five states with official notification for BT were verified: Rio Grande do Sul (228 cases; Incidence Risk/100.000 [IR] = 5.7), Rio de Janeiro (116 cases; IR = 251.7), Amazonas (17 cases; IR = 41.12), Paraná (12 cases; IR = 2.0), and Minas Gerais (7 cases; IR = 3.7). When evaluating the annual distribution, Rio de Janeiro stood out in 2013, with an IR = 217.3, whereas Amazonas had the highest mortality rate (37.13%) in 2017. However, fatality rates of 100% were found in Rio Grande do Sul (for 53 cases) in 2017 and in Paraná (for 1 case) in 2019. The months with the highest case notifications were February and April, highlighting Rio Grande do Sul and Rio de Janeiro as the main states that contributed to these notifications. We observed the formation of two clusters of high risk of the disease: i) cluster A (Relative Risk [RRs] = 63.04 between 2013 and 2015), formed by six states comprising all states from the South and part of the Southeast and Midwest, and, ii) cluster B (RRs = 180.52, in 2017), formed only by the state of Amazonas. Based on the results, BT is not evenly distributed among Brazilian states and concentrated in the southern region, especially Rio Grande do Sul. Important control and surveillance measures must be instituted to optimize national animal health programs in Brazil.