Sudden increase in virus a cause for worry in Ontario's sheep industry

Friday, March 4, 2016

by SUSAN MANN

Researchers with the University of Guelph and provincial agriculture ministry are trying to find out if a mosquito-borne virus causing deformities and stillbirths in lambs has recently changed or just became more prevalent in the fall.

Cache Valley virus is not new and was found in Ontario’s sheep flocks in other years, such as 2011, 2012 and 2013. Researchers have concluded the cause of the stillborn and deformed lambs submitted to the provincial Animal Health lab towards the end of 2015 and the beginning of this year is also Cache Valley virus.

What has researchers puzzled, however, is why there has been a sudden increase in virus cases.

Previous outbreaks “didn’t tend to be very severe,” says veterinarian Paula Menzies, a professor in the population medicine department at the Ontario Veterinary College, University of Guelph. “A producer might have two to three lambs that are affected and everybody else is healthy.”

Menzies says she is analyzing the results of an online voluntary survey from sheep marketing agency to determine how widespread the virus is this time. “We do know that based on submissions to the Animal Health Lab and information from our disease surveillance network, there are at least seven flocks (spread across Ontario) that have had problems,” she says.

Rob Scott, chair of the Ontario Sheep Marketing Agency, says Cache Valley virus “is a big concern for us. Since this virus is caused by a mosquito, it’s pretty hard to protect against it.”

Menzies says mosquitos that are the offspring of infected mosquitos or that have fed on infected white tail deer transmit the virus.

Non-pregnant ewes bitten by infected mosquitos mount “an immune response to the virus” and “it doesn’t really cause too many problems,” she says. Sheep exposed to the virus don’t show any signs of disease, she explains.

However, the virus causes problems for the lambs of pregnant ewes that are at less than 28 days and up to about 48 days of gestation. After 50 days of gestation, the virus doesn’t cause any damage to lambs.

The virus can kill the embryo in ewes that are at less than 28 days of gestation, Menzies says. If an ewe is infected between 28 to 48 days of gestation, the virus attacks the quickly developing fetus and causes deformities related to its musculoskeletal system, such as bent and fused legs, decreased muscles and a bent spine. The virus also affects the fetus’ nervous system development.

The infected lambs are either stillborn or die soon after birth because of the severity of the deformity, she explains. In ewes with two or more fetuses, the virus can attack one and not the others, meaning one lamb is born deformed and the others are normal.

The normal gestation length of an ewe is about 150 days or five months. Lambs born at the beginning of the year would have been infected around September 2015, she says.

The amount of virus that’s circulating in mosquitos builds up over the summer and is the highest in the fall. “That’s why we tend to see sheep that are bred in the early fall being the most susceptible,” Menzies says. “If they’re bred later in the fall, the mosquitos are gone. If they’re bred in the early summer, the mosquitos just aren’t carrying that much virus.”  

The virus has been circulating in Ontario for several years and is “common across North America,” she notes.

Why the virus appears to be causing more problems may indicate it has changed and is now more pathogenic, Menzies says. If there’s a new strain of the virus, “the animals don’t have the same level of immunity to it.”

Another possible explanation for the increased problems is the longer and warmer autumns provide more opportunity for the virus to build up in the mosquito population.

Compounding the difficulty of the situation for farmers is there are no available vaccines or treatments. Furthermore, Menzies says, “by the time the deformed lambs are born between Christmas, New Year’s and the first few weeks of January, the infection happened months before.”

For this year, farmers should work with their flock veterinarians to help reduce the risk of fetal losses caused by infection from the virus, Menzies says.

Researchers are trying to get provincial government funding to continue to do Bluetongue surveillance, which includes setting out insect traps to catch the midges causing that disease. The traps also catch mosquitos “and our plan is maybe we can test them for the virus,” Menzies says. That will help researchers answer the question of whether there is a new strain of the virus in Ontario or there’s “just more of it (the virus).” The answer is critical to “understanding what we can do to prevent more disease.”

Scott says the Cache Valley situation in Ontario really speaks to the need for more disease and vector surveillance.  BF