The role of Culicoides vectors in the transmission of the bluetongue virus in Kazakhstan and adjacent regions

Abstract

The bluetongue virus (BTV) is a transmissible pathogen whose circulation in natural and economic systems is determined by the triad "susceptible hosts – competent Culicoides woodlice – seasonal and climatic conditions". The non-contact nature of transmission (mainly through Culicoides bites) means that the risk of outbreaks is determined not so much by the "density of animal–animal contacts" as by the intensity of host-vector contacts and the timing of host viremia with vector activity. Epizootologically cattle are often the key "hidden" link: infection in cattle is usually asymptomatic, but viremia can be relatively long lasting, which increases the likelihood of infection by woodlice and maintenance of transmission when co-grazing with sheep (clinically the most vulnerable). Complementary evidence of BTV circulation has been published for Kazakhstan: a 1997-1998 serological examination revealed widespread seropositivity in cattle/sheep/goats (~21-25%), which was interpreted as endemic in the absence of clinical recognition, and a 2021 review of the model risk assessment showed pronounced seasonality of potential transmission (spring-summer, peak in July) and a spatial gradient risk assessment, molecular research 2022-2024. The detection of BTV RNA in animals and in some Culicoides species in the southern regions was confirmed, while typing indicates the presence of at least separate genetic lines (BTV-9 "western topotype" was reported). Together, this justifies the transition from "serological confirmation of contact" to risk based continuous surveillance with simultaneous monitoring of hosts and vectors.