Use repellents to reduce bluetongue risk in sheep
Persistent drought conditions in Wyoming have increased the risk of bluetongue disease in sheep, which is vectored by biting midges.
Scott Schell, assistant entomologist with University of Wyoming Extension, said the early spring and hot, dry conditions Wyoming has experienced favor the reappearance of bluetongue.
"Drought conditions create a lot of mucky edges around receding water holes, which is biting midge larval habitat," said Schell. "Drought also concentrates livestock around fewer water sources in late summer when the biting midge population peaks."
According to Schell, the only way to reduce the risk of bluetongue to sheep in Wyoming is the use of repellents. He said that long-lasting, insecticidal repellents, properly applied to sheep in the summer, provide economical protection frombluetongue.
The repellents work by reducing the number of bites sheep receive from infective midges in the weeks just before the first frost, when the chance of infection is highest, he said.
"Timely application of these repellents when the flocks return from summer pastures can protect sheep for a few weeks until the first frost kills off the biting midges," said Schell. "Rams on late summer pasture should also be protected with repellents."
Cooperative research between UW Extension, the former USDA-Agricultural Research Service Arthropod-Borne Animal Diseases Research Laboratory and Montana State University showed that insecticidal repellents, applied as low-volume belly sprays or via treated ear tags, significantly reduced biting midge feeding.
"The larval habitat for the biting midges cannot be treated successfully with insecticides," said Schell. "The insecticidal ear tags provided longer protection but were more expensive to purchase and were slower to work due to the time needed for the insecticide to spread over a sheep's body."
Schell said sprays are faster acting, but the protection provided from insect bites was of shorter duration.
The last large outbreak of bluetongue occurred in Wyoming in 2007.
"Symptoms can include high fever, excessive salivation, nasal discharge, swelling of the face and tongue, and, in some cases, a bluish coloration of the tongue," said Schell.
Many species of livestock and wildlife can get the disease from the bite of an infective biting midge. In Wyoming, the vector species is Culicoides sonorensis, an insect less than one-eighth of an inch long that prefers to blood feed on hoofed animals. However, the severity of the disease varies widely between species.
"Cattle can get bluetongue but exhibit little in the way of negative symptoms but may have a high level of the virus that persists in their blood," said Schell. "This may spread the virus into uninfected Culicoides midges that feed on an infected cow and then feed on other animals."
Sheep, on the other hand, can get very sick with bluetongue with 30-percent mortality of infected individuals possible, according to Schell.
For more information, contact Schell at 307-766-2508 or email@example.com.