Cache Valley Virus (CVV) has been in the news around here lately. This mosquito-borne virus can be found in various parts of North America, particularly in the southeastern US. It’s been present in Ontario for a while, but has hit the news recently because of a recent outbreak in sheep flocks in the province. CVV infection in sheep can cause abortions, stillbirths and birth deformities when ewes are infected during the first trimester of pregnancy. Some people are even drawing parallels with Zika virus, which is a completely unrelated (but also mosquito-borne) virus that has been implicated as a cause of birth defects in people. Different viruses, but some similar issues, I guess. (For the record, there is currently no evidence that animals are involved in the spread of Zika virus.)
The Ontario outbreak was recognized due to a sudden increase in abortions and births of deformed lambs starting in December 2015, but that’s not when the virus was making the rounds. Presumably, there was a spike in virus activity in the late fall when the sheep were in early pregnancy, and the consequences of those exposures were seen a few months later.
This virus typically cycles between mosquitoes and a variety of ungulates (i.e. hooved animals like sheep, deer, horses and cattle). It’s not one of the better-studied viruses so what species is/are really the primary main host isn’t well understood, and neither are all the implications for the various species the virus can infect.
What about people?
Since CVV is transmitted by mosquitoes, exposure of people is to be expected in areas where the virus is circulating. Antibodies against the virus can be found in a small percentage of healthy people, presumably just indicating that the virus was passed to the person via a mosquito bite and the immune system fought it off (making antibodies in the process).
Disease can occur in people, although only a handful of cases have been reported, such as a case of fatal encephalitis (brain inflammation) in a Wisconsin man in 2003 that was attributed to CVV. The big question always is whether this really is a rare disease in people or whether it’s just rarely diagnosed. Idiopathic (undiagnosed) encephalitis occurs regularly, and testing usually focuses on the “usual suspects” (e.g. herpesvirus, West Nile virus). Fishing expeditions for a long list of potential causes such as CVV aren’t usually performed, because they are low yield, expensive and often unrewarding. That makes it hard to say whether infection is rare or just not tested for.
There has also been a little investigation of the potential for birth defects in people. An older study suggested it could be associated with brain defects in babies, but limited subsequent study hasn’t confirmed that.
It’s likely that this is truly a very rare disease in people, but that can’t be stated with absolute confidence.
What about dogs?
Even less is known about CVV in dogs. Whether that’s because it doesn’t infect dogs or because dogs are even less likely to be tested for CVV isn’t known. Testing of dogs with encephalitis usually isn’t extensive. Rabies is the big one, but testing beyond that is pretty sporadic in part because it’s expensive and there’s little impetus for the owner of a dead dog to pay hundreds of dollars for low yield testing.
If CVV was a major problem in dogs, we’d hopefully see it through recognition of parallel issues in local dogs when virus activity was high in an area. So, if we hear reports of more dogs with undiagnosed neurological disease, abortions or stillbirths in areas where CVV cases is being diagnosed in sheep, we’d hopefully be able to get some testing done. However, we’re at the mercy of people in the field, including animal owners and veterinarians, identifying and reporting these issues. Unfortunately that isn’t usually a terribly robust surveillance system, but it’s the best we can get for now.
Bottom line (for now, at least…)
Cache Valley Virus is probably of very limited concern to anyone who doesn’t raise sheep, and the concerns are focused on what the virus does to sheep, not what it can do to owners or any other animals they may own.