The bluetongue virus, which causes a serious disease that costs thecattle and sheep industries in the United States an estimated $125million annually, manages to survive the winter by reproducing in theinsect that transmits it, report veterinary scientists at theUniversity of California, Davis.
The findings solve a century-old mystery and are particularlysignificant as global climate change brings more moderate wintertemperatures around the world. The new study appears Sept. 12 in thejournal PLOS ONE.
"By conducting this epidemiological study on a commercial dairy farmin Northern California, we were able to demonstrate that the virusoverwinters in female midges that had fed on an infected animalduring the previous season," said lead author Christie Mayo, aveterinarian and postdoctoral researcher in the UC Davis School ofVeterinary Medicine.
"This discovery has important ramifications for predicting theoccurrence of bluetongue in livestock and, we hope, for eventuallydeveloping controls for the disease," said co-author JamesMacLachlan, a UC Davis veterinary professor and viral disease expert.
About bluetongue
Bluetongue disease, first identified during the 1800s in southernAfrica, is transmitted by the Culicoides biting midge, a tiny gnatsometimes referred to as a "no-seeum."
The disease mostly sickens sheep but also infects cattle and goats,as well as deer and other wild ruminants. In the U.S., the virus'greatest economic impact is in the cattle industry, because it isbigger than the domestic sheep industry and most adversely impactedby international trade barriers related to bluetongue. The diseasedoes not pose a threat to human health.
The name bluetongue derives from the swollen lips and tongue ofaffected sheep, which may turn blue in the late stages of the disease.
The virus that causes bluetongue was first isolated and identified inthe Western Hemisphere in the early 1950s at the UC Davis School ofVeterinary Medicine.
A seasonal mystery
In California, bluetongue is most prevalent when midges are abundantin late summer and fall, but there has been speculation over how thevirus survives through the winter. When temperatures turn cold andthe biting-midge populations plummet, transmission appears to ceasefor more than six months, but the virus reappears when temperatureswarm the following season.
Findings from California dairy
The researchers monitored cows and midges on a Northern Californiadairy farm for more than a year. They documented, for the first time,the presence of genetic material for the bluetongue virus in femalemidges that were collected during two consecutive winter seasons.
The bluetongue virus was widespread in both the dairy cows and themidges from August to November. Surprisingly, however, theresearchers discovered that the virus was also present in femalemidges captured in February of both 2013 and 2014. There was no signof infection in the dairy cattle being studied.
The researchers concluded that those long-lived female midges hadbeen infected with the bluetongue virus during the previouswarm-weather season. They were carrying the virus through the wintermonths and would later in the season once again transmit it to cowson the dairy.
The research team notes that the bluetongue virus may also haveadditional, yet-to-be discovered, modes of overwintering in temperateregions.
Source: University of California - Davis