Why aren’t US mosquitoes spreading Oropouche? New study uncovers weak transmission rates—even with historic and modern viral strains.
Research Letter: Lack of Competence of US Mosquito Species for Circulating Oropouche Virus. Image Credit: nechaevkon / Shutterstock
In a recent study published in the journal Emerging Infectious Diseases, researchers assessed the competence of mosquitoes as vectors for the Oropouche virus (OROV) in the United States.
OROV is a segmented, negative-sense ribonucleic acid (RNA) virus under the Orthobunyavirus genus in the Peribunaviridae family. It was first detected in 1955 in Trinidad and Tobago. More than 10,000 cases of OROV infection were reported in 2024. Additionally, the virus has been associated with vertical transmission, congenital abnormalities, and fetal death in recent outbreaks. The US recorded 108 imported cases associated with travel, with reports in Florida, New Jersey, California, Colorado, Kentucky, and New York.
While OROV generally causes a mild, self-limiting febrile illness, severe outcomes, including fatalities, were reported in the recent outbreak. Culicoides paraensis midges are the primary vectors of OROV, though mosquitoes, particularly Culex quinquefasciatus, have been implicated in limited transmission. Experimental studies on mosquito-borne OROV transmission, however, remain sparse.
The Study and Findings
In the present study, researchers investigated whether mosquito species in the US have the potential to maintain OROV using both historic (TRVL9760) and recent (240023) viral isolates. OROV 240023 was isolated from a reverse-transcription polymerase chain reaction (RT-PCR)-positive serum sample from a febrile case in Cuba. First- and second-strand complementary DNA (cDNA) was synthesized using random primers, followed by purification, library preparation, and sequencing.
High-quality reads were aligned to reference sequences to generate consensus sequences. OROV 240023 was compared to the 1955 strain (TRVL9760). OROV 240023 showed 6.3%, 5.2%, and 10.9% divergence at the nucleotide level and 0%, 1.8%, and 2% at the amino acid level in the small, medium, and large segments, respectively. The 69 unique amino acids were dispersed throughout the large and medium peptides.
Next, the researchers evaluated the competence of mosquitoes fed on blood meals containing OROV 240023 or TRVL9760 at 6.5 log10 plaque-forming units (PFUs)/mL. Four US species were assessed: Cx. pipiens, Cx. quinquefasciatus, An. quadrimaculatus, and Ae. albopictus. Mosquitoes were collected from Florida, New York, and Georgia at different times. Mosquitoes were anesthetized 14 days post-infection, and competence was assessed using 26 to 50 mosquitoes.
Legs were removed, and the proboscis was placed in a tube containing sucrose. Bodies and secretions were homogenized and centrifuged. Samples were tested via plaque assay to determine infectivity, dissemination, and transmission potential. A positive body indicated infectivity; positive legs suggested dissemination; positive saliva indicated transmission. The team found that all mosquito species lacked competence for both viral strains.
OROV 240023 infection rates were 0% in Ae. albopictus, An. quadrimaculatus, and Cx. quinquefasciatus, and 2% in Cx. pipiens. OROV TRVL9760 infection rates were 4% in An. quadrimaculatus, 2% in Cx. quinquefasciatus and Ae. albopictus, and 0% in Cx. pipiens. Two of the three mosquitoes positive for TRVL9760 had disseminated infections, though no transmission was detected.
Conclusions
The findings provide evidence of OROV infectivity in Anopheles mosquitoes, and modest transmission may be possible with this species. A more comprehensive evaluation of competence could be attained with larger samples. Notably, the study contrasts with prior research: McGregor et al. (2021) reported higher infection rates in Cx. quinquefasciatus, while de Mendonça et al. (2021) found no infectivity in the same species using a historic strain. Infection with OROV 240023 was only detected in Cx. pipiens, which also showed dissemination and transmission. Despite strain-specific variability, the likelihood of US mosquito species maintaining OROV remains low.
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