Hematophagous Diptera collected from a horse and paired carbon dioxide-baited suction trap in southern California: relevance to West Nile virus epizootiology.

The research studies the prevalence and behavior of blood-sucking flies on a horse in Southern California, especially those known to transmit the West Nile virus. The findings may help us better understand and combat mosquito-borne illnesses in horses.

Methods and Results

This research presents an intensive study on hematophagous dipteras – a group of insects including mosquitoes, biting midges, and black flies – that landed on a horse and those found with the use of a CO2-baited suction trap.

• The species of insects were collected from the horse through vacuum application at three different areas in Southern California where a high rate of West Nile virus activity had been recorded in the previous year.
• Nine species of mosquitoes, biting midges (Culicoides sonorensis), and black flies (Simulium bivittatum) were collected directly from the horse.
• Majority of mosquitoes were found on the head, crest, withers, necks, chest, and shoulders of the horse, whereas biting midges and black flies were predominantly collected from the ventral midline of the horse.
• The horse biting rate was very low for Culex pipiens quinquefasciatus, intermediate for Culex tarsalis, and very high for Culex erythrothorax.
• Among the mosquito species collected, Culex erythrothorax was the most abundant, with biting frequency varying among mosquito species from zero to 58.3%. Culex pipiens quinquefasciatus had the least feeding rate (16.7% or one out of six mosquitoes).

Comparison with the Suction Trap

• The number of insects captured at the horse and paired CO2-baited suction trap was not different for Anopheles franciscanus, Culex tarsalis, and Simulium bivittatum.
• However, Culex pipiens quinquefasciatus was captured in greater numbers by the CO2-baited suction trap, while Anopheles hermsi, Culex erythrothorax, Culiseta inornata, and Culiseta particeps were collected in greater numbers from the horse itself.

Relevance to West Nile Virus Epizootiology

In the context of the West Nile virus:

• Both Culex tarsalis and Culex erythrothorax should be considered likely epizootic vectors, i.e., they are likely to transmit the disease to horses in rural southern California.
• This information is critical to managing and combating the spread of such mosquito-borne diseases, particularly in horses.