First molecular evidence of potential Culicoides vectors implicated in bluetongue virus transmission in Morocco.

BACKGROUND

Bluetongue is a non-contagious viral disease that affects both domestic and wild ruminants. It is transmitted primarily by small hematophagous Diptera belonging to the genus Culicoides (Diptera: Ceratopogonidae). The current study represents the first molecular investigation into the potential role of Culicoides imicola, Culicoides paolae, Culicoides newsteadi, Culicoides spp., and Culicoides circumscriptus as bluetongue virus (BTV) vectors in Morocco. Additionally, the study aimed to evaluate the vectorial activity of midges during the survey seasons.

METHODS

Parous females of these species were captured from several regions of Morocco (6 out of 12) from 2018 to 2021 using Onderstepoort Veterinary Institute (OVI) traps. A total of 2003 parous female specimens were grouped into 55 batches. The midge body of each batch was dissected into three regions (head, thorax, and abdomen), and these regions were analyzed separately using reverse transcription quantitative polymerase chain reaction (RT-qPCR).

RESULTS

BTV RNA was detected in 45 out of the 55 batches tested, indicating a positivity rate of 81.8%. The RT-qPCR-positive pools of the studied Culicoides species exhibited high levels of BTV positivity in each body part (head, thorax, and abdomen), confirming the successful replication of the virus within midge bodies. The BTV circulation was substantial across all three survey seasons (spring, summer, and autumn). High infection rates, calculated using the minimum infection rate (MIR) and maximum likelihood estimation (MLE), were observed during the collection seasons, particularly in autumn and spring, and for all investigated Culicoides species, most notably for C. imicola and C. newsteadi. These increased infection rates underscore the significant risk of Culicoides transmitting the BTV in Morocco.

CONCLUSIONS

The detection of BTV positivity in Culicoides spp. (lacking wing spots that allow their differentiation according to morphological identification keys) suggested that other Culicoides species are competent for BTV transmission in Morocco. The study results indicated, for the first time at the molecular level, that C. imicola and C. newsteadi are the primary potential vectors of BTV in Morocco and that C. paolae and C. circumscriptus are strongly implicated in the propagation of bluetongue at the national level.