Bluetongue disease BT in Latin America remains relatively underexplored, with limited information on its spread. Given its vector-borne nature, BT prevalence is expected to align with favourable environmental conditions for its primary vector. In Ecuador, BTV presence has been limited, highlighting a gap in comprehensive surveillance. We hereby aim to provide scientific grounds for future targeted surveillance and control strategies.
Bluetongue (BT) is a noncontagious disease affecting domestic and wild ruminants caused by the Bluetongue virus (BTV), which belongs to the genus Orbivirus in the Reoviridae family. The virus is capable of infecting cattle, sheep, deer, goats and camelids1,2 and is transmitted by insects from the genus Culicoides As the disease causes substantial economic losses and is a major concern for international trade3, it is notifiable to the World Organization for Animal Health (WOAH). Many countries have adopted regulatory control measures addressing Bluetongue; however, the effectiveness of these measures depends on the capacities and resources of their veterinary services, including laboratory facilities4.
The disease has multiple manifestations that depend on the host and viral factors, and its clinical presentation ranges from salivation to depression, dyspnea, and asymptomatic to mild fever, as well as abortion and death1. According to the phylogenetic analysis of the more variable region of the BTV genome (Seg-2 region), at least 26 distinct serotypes have been identified around the World, each of which is able to cause disease5. In South America, BTV serotypes 1 to 4, 6 to 10, 14, 17 and 24 have been found previously in Argentina6, Brazil, Colombia, Guyana, and Peru7. In Ecuador the identified serotypes in cattle are 9, 13 and 188.
Among the wild species, collared peccaries (Tayassu tajacu) have been found to be infected in Brazil9 and Peru, marsh deer (Blastocerus dichotomus), pampas deer (Ozotoceros bezoarticus) and Tapir (Tapirus terrestris) in Brazil10; grey brocket (Mazama gouazoubira) in Bolivia, guanaco (Lama guanicoe) and vicuna (Vicugna vicugna) in Argentina11. Currently, there is no available information on BT in wildlife in continental Ecuador and the virus is absent in the Galapagos islands12.
According to the WOAH, midges (genus Culicoides) are the only significant competent vector of BTV. They are also vectors of vesiculovirus (vesicular stomatitis)13, Schmallenberg virus, African horse sickness virus, Aino virus and Akabane virus14. As a vector-borne disease, the natural distribution and prevalence of BT are governed mainly by ecological factors modulating vector populations (e.g., rainfall, temperature, humidity, and soil characteristics).
Transmission mainly occurs through the bite of infected midges (females)15. BTV requires a minimum temperature between 10°C and 15°C to replicate inside the Culicoides vector, as RNA polymerase activity is positively influenced by temperature2. Higher temperatures may increase the biting rate, favouring vector host transmission15. The temperature that maximises the chance of a midge surviving and consuming a blood meal are 23°C, and 13°C is the temperature that results in the greatest expected number of lifetime midge bites16. The inventory of Culicoides fauna in Ecuador comprises 70 species, including C. insignis and C. paraensis17.
Rainfall is also a determinant of the survival and activity of midgets, and the abundance of the vector is often related to rainfall (and suitable temperatures). After rainfall, their feeding frequency18–20. Rainfall also governs the availability of larval habitat, survival, and dispersal of adults. The pupae of most species will float if submerged; however, the pupae of some species, such as C. imicola, can be drowned if inundated20.
The long-distance spread can also be attributed to animal movements in the case of insufficient veterinary control. Little evidence for contact transmission has been found only in goats, sheep and deer, and transplacental infection has been reported in cattle, sheep, and elk14.
Knowledge of BTV dispersion in Latin America is limited mostly to serological surveillance and few molecular characterization reports, and risk analysis or systematic surveillance that could guide prevention and control strategies is lacking. In Ecuador, BTV is not targeted by a specific surveillance approach but is included as a differential diagnosis of vesicular diseases in the foot and mouth disease (FMD) surveillance programme. Serological findings have been reported in the national surveillance system, scientific reports8,12,21, as well as in international reporting systems since 2015 (https://wahis.woah.org/#/home/).
In this paper, we present the first analysis of ecological factors such as temperature and rainfall dependency of past detections of antibodies against BTV in Ecuador. We hereby provide scientific grounds for future targeted surveillance and control strategies addressing BT.