Abstract:
Bluetongue (BT) is an important viral disease of ruminants that is transmitted by
hematophagous Culicoides midges. We examined the seasonal patterns of abundance and
infection of Culicoides sonorensis at four dairy farms in the northern Central Valley of California
to develop estimates of risk for bluetongue virus (BTV) transmission to cattle at each
farm. These four farms were selected because of their similar meteorological conditions
but varying levels of vector abundance and BTV infection of cattle. C. sonorensis midges
were collected weekly at each farm during the seasonal transmission period, using three
different trapping methods: traps baited with either carbon dioxide (CO2) alone or traps
with CO2 and UV light, and by direct aspiration of midges from sentinel cattle. Analysis of
BTV-infected midges using group and serotype-specific quantitative reverse-transcriptase
polymerase chain reaction (RT-qPCR) assays confirmed that BTV serotypes 10, 11, 13 and 17
are all present in the region, but that midge infection rates and the number of BTV serotypes
circulating differed markedly among the individual farms. Furthermore, more serotypes of
BTV were present in midges than in sentinel cattle at individual farms where BTV circulated,
and the virus was detected at each farm in midges prior to detection in cattle. BTV
infection rates were remarkably lower among female C. sonorensis midges collected by CO2
traps with UV light than among midges collected by either animal-baited aspirations or
in CO2 traps without light. A subsample of female midges examined from each collection
method showed no overall differences in the proportion of female midges that had previously
fed on a host. Findings from this study confirm the importance of using sensitive
surveillance methods for both midge collection and virus detection in epidemiological studies
of BTV infection, which is especially critical if the data are to be used for development
of mathematical models to predict the occurrence of BTV infection of livestock.
