Community Composition of Black Flies During and After the 2020 Vesicular Stomatitis Virus Outbreak In Southern New Mexico, USA

Abstract

Abstract Background: Vesicular stomatitis virus (VSV), a vector-borne pathogen of livestock, emerges periodically in the western United States. In New Mexico (NM), USA, most cases occur close to the Rio Grande, implicating black flies (Simulium spp.) as a vector. In 2020, VS cases were reported in NM from April to May, although total black fly abundance remained high until September. We investigated the hypothesis that transience of local VSV transmission results from transient abundance of key, competent black fly species. Additionally, we investigated whether irrigation canals in southern NM support a different community of black flies than the main river. Lastly, to gain insight into the source of local black flies, in 2023 we collected black fly larvae prior to the release of the Rio Grande. Methods: We randomly sub-sampled adult black flies collected along the Rio Grande during and after the 2020 VSV outbreak. Further, we collected black fly adults along the Rio Grande in 2021 and 2022 and at southern NM farms and irrigation canals in 2022, and we collected black fly larvae from dams in 2023. All collections were counted, and individual specimens were subjected to molecular barcoding for species identification. Results: DNA barcoding of adult black flies detected four species in 2020: Simulium meridionale (N = 158), S. mediovittatum (N = 83), S. robynae (N = 26), and S. griseum/notatum (N = 1). Simulium robynae was only detected during the VSV outbreak period, S. meridionale showed higher relative abundance, but lower absolute abundance, during the outbreak than post-outbreak period, and S. mediovittatum was rare during the outbreak period but predominated later in the summer. In 2022, relative abundance of black fly species did not differ significantly between the Rio Grande sites and farm and irrigation canals. Intriguingly, 63 larval black flies comprised 56% S. vittatum, 43% S. argus, and 1% S. encisoi, species that were either extremely rare or not detected in previous adult collections. Conclusions: Our results suggest that S. robynae and S. meridionale could be shaping patterns of VSV transmission in southern NM. Thus, field studies of the source of these species as well as vector competence studies are warranted.