Vesicular Stomatitis Virus Detected in Biting Midges and Black Flies during the 2023 Outbreak in Southern California

Abstract

Vesicular stomatitis (VS) is a viral disease that affects horses, cattle, and swine that is transmitted by direct contact and hematophagous insects. In 2023, a multi-state outbreak of vesicular stomatitis New Jersey virus (VSNJV) occurred in California, Nevada, and Texas, infecting horses, cattle, and rhinoceros. To identify possible insect vectors, we conducted insect surveillance at various locations in San Diego County, CA, including at a wildlife park. CO₂ baited traps set from mid-May to mid-August 2023 collected 2357 Culicoides biting midges and 1215 Simulium black flies, which are insect genera implicated in VSNJV transmission. Insects were pooled by species, location, and date, then tested for viral RNA. Nine RNA-positive pools of Culicoides spp. and sixteen RNA-positive pools of Simulium spp were detected. Infectious virus was detected by cytopathic effect in 96% of the RNA-positive pools. This is the first report of VSNJV in wild-caught C. bergi, C. freeborni, C. occidentalis, S. argus, S. hippovorum, and S. tescorum. The vector competency of these species for VSNJV has yet to be determined but warrants examination. Active vector surveillance and testing during disease outbreaks increases our understanding of the ecology and epidemiology of VS and informs vector control efforts.

Keywords: California; Culicoides; Simulium; VSNJV; VSV; arthropod-borne virus; vector surveillance; vesicular stomatitis New Jersey virus.

Figure 1

Insect collection sites in San Diego County, California, USA. (A) Location of insect collection sites (black circles) and the wildlife park (yellow circle). The red highlighted box indicates the area of San Diego County (entire county boundary in black) where sampling was conducted. The red circles indicate a 10 km buffer around confirmed or suspected VS premises. (B) Trap locations (n = 6; A–F) at the wildlife park.

Figure 2

Insect collections, VSNJV-positive insect pools, and positive premises during the VSNJV 2023 outbreak in San Diego County. (A) Abundance of individual Culicoides (pink) and Simulium (blue) collected between May and August 2023. (B) Prevalence of Culicoides (pink) and Simulium (blue) VSNJV-positive pools and number of newly confirmed or suspected VSNJV-positive premises by date, as reported by APHIS (gray line).

Figure 3

VS-positive premises and insect collections during the 2023 outbreak in San Diego County. Collection locations in San Diego County of VSNJV-positive Culicoides (A) and Simulium (B) pools. Circle size indicates the total number of VSNJV-positive pools collected at each location. Color of circles designates the insect species, as indicated in the legends. Trap sites are numbered in yellow. The large red circles indicate a 10 km buffer around confirmed or suspected VS-infected premises.

Figure 4

Collection dates of VSNJV-positive Culicoides (A) and Simulium (B) pools during the 2023 outbreak in San Diego County. Color designates the insect species, as indicated in the legends. VSNJV prevalence by species and date is located in Table S3.

Figure 5

VSNJV-positive insect pools collected from 29 May to 21 August while rhinoceros were symptomatic. (A) Lesion onset and PCR-positive dates for probable and confirmed VS rhinoceros cases. Bars indicate the lesion time period for probable (light gray) and confirmed (dark gray) cases. Circles indicate when swabs from the rhinoceros were positive (yellow) or negative (black) for VSNJV RNA via RT-qPCR. Rhinoceros are categorized by habitat, indicated by the background color (teal, purple, and peach) and the closest trap to each habitat is specified by a letter in the top right corner (E, A). (B) Total number of VSNJV-positive pools by collection location (n = 6; A–F) and insect species at the wildlife park. Circle size indicates the total number of pools collected at each location that tested positive for VSNJV RNA. Color of circles indicates the insect species. Colored rhinoceros indicate the approximate location of the three rhinoceros habitats. (C) Species specific (Culicoides and Simulium) total number of VSNJV-positive pools by date collected at the wildlife park.

Figure 6

Environmental conditions leading up to and during the 2023 VS outbreak compared to previous 21-year ranges. Black lines indicate (A) minimum temperature, (B) maximum temperature, (C) precipitation, and (D) vegetation greenness (NDVI) for months in water year 2023 (spanning October 2022 to September 2023). Dark gray areas indicate the interquartile ranges for water years 2001–2022. Light gray areas indicate the minimum and maximum monthly values over the 2001–2022 period. Red rectangle from May to September indicates the 2023 VSNJV outbreak.