Researcher sleuths vectors for vesicular stomatitis

Wyoming

A microbiologist casting about to find a disease vector used a fish to catch a biting midge.

Barbara Drolet utilized her doctoral research that tracked the path of a virus through a fish as a basis to determine if the biting midge--of "no-see-um" fame--could be a vector for vesicular stomatitis (VS).

Drolet is with the U.S. Department of Agriculture's (USDA) Agricultural Research Service, Arthropod-Borne Animal Diseases Research Laboratory (ABADRL) in the College of Agriculture at the University of Wyoming. She earned her bachelor's and master's degrees at UW, and she received her doctorate degree in genetics at Oregon State University. Drolet is also an adjunct professor with the Department of Veterinary Sciences.

VS is a viral disease that infects cattle, horses and swine and whose outward signs in cattle are similar to Foot and Mouth Disease. The western United States suffered VS outbreaks in horses and cattle in 2004 and 2005.

The disease was noted at two New Mexico locations by April 30 last year. Progressive USDA Animal and Plant Health Inspection Services (APHIS) maps with red dots show the disease blossoming in Arizona and Texas. Utah was dotted by June 19 and Colorado by July 17. Wyoming's first case was found in a horse August 10.

VS suffered a slow death in the Rocky Mountain region. According to APHIS, the last Wyoming location was released from quarantine Dec. 28. The end of the 2005 outbreak wasn't officially confirmed until April 11 of this year.

Unusual to see this disease three consecutive years, no one knows what awaits producers this year, said Drolet, who has been sleuthing the disease for five years.

"Part of the problem is we don't understand all the variables that have to come together just right for an outbreak to occur," she said.

Outbreaks occurred in 1982-1983, 1985-1986, 1995, 1997-1998, and 2004-2005--sporadically and never across three successive years. No one knows why, but Drolet noted it is commonly accepted, although not condoned, producers and horse owners underreport the disease. This could affect the "recognized" outbreak pattern.

Transportation of animals could be one reason why the disease appears to skip across large geographic areas. The initial 2005 VS case in Wyoming was caught in Torrington. The animal had been in Colorado and Nebraska before restrictions had been put in place in those areas.

Despite VS being of high importance in the United States and the international community, no one knows all the insect players, said Drolet.

Is it sand flies? Black flies? Mosquitoes? Midges? Grasshoppers? All of the above?

After looking at more than 1,500 insect sections, each five-microns thick (that's less than one 10 thousandth of an inch), she determined all the criteria for being a biologically competent vector for VS are met with Culicoides sonorensis midges.

Midges are pool feeders, which mean they get their blood meal from livestock by macerating the flesh with specialized cutting mouth parts and sucking up blood that pools into the wound.

Competent insect vectors must be able to become infected with the virus from the blood meal, amplify the virus and pass it on from the salivary glands, all without suffering ill effects from the virus.

No basic biology had been published on the virus for this possible insect vector. She relied on experience gained during her Ph.D. research, where she painstakingly tracked a virus infection progression through a fish.

There was a problem. "I had never taken an entomology class in my life," she said. "I didn't know anything about insects other than they were generally annoying. That first year required a lot of learning about what I was looking at and making sense of what I was seeing. It actually was good, internal control for me. I didn't know what to expect as far as infection pathways in the insect were concerned. Then the more I learned about insect physiology, the more the pathways I was seeing started to make sense."

She fed insects an infected meal and tracked the virus. It infected the mid-gut and then spread through the insect to the salivary glands and the ovaries. The virus presence in the salivary glands, the eggs and in the excrement suggests it may be transmitted in the midge's bite, to the insect's offspring and to other midges from excretions.

"If the virus is transmitted in all three ways, it's likely the insect is a significant player in VS transmission and outbreaks," said Drolet.

All her work has been conducted in the lab. The next step would be animal testing, but the facility's large animal facility is currently decommissioned. The ABADRL is considering leasing space in a facility in Athens, Ga.--the closest available place with the needed space and containment level, noted Drolet.

Ranchers can help control VS spread by controlling midge habitat, she said. Midges lay eggs in standing water areas high in organic waste, such as spillover areas around tanks and troughs. Pockets of water caused by hoof prints and other ground depressions where cattle or horses are grazing, are also possible habitat areas.

Filling such collection sites with dirt or sand decreases biting midge habitat.

Date: 5/24/06

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