When I got into vet school, I didn’t imagine that I’d have to know much about mosquito biology. (I probably didn’t really imagine that when I graduated either, to tell the truth). However, as I focused on infectious diseases, it became apparent that understanding certain aspects of inner-workings of mosquitoes (and ticks, and various other annoying critters) was critical.
We have a lot of mosquitoes in Ontario (a few billion of which live in the “protected wetland” (aka swamp) part of our property). We also have people from Ontario traveling to Zika-affected areas. Infected people can bring Zika virus home, where they could be bitten by Ontario mosquitoes. Why are we not concerned about Zika transmission here? The same could be said about dengue, chikungunya and a multitude of other mosquito-borne viruses that we don’t see transmitted here.
For sustained transmission of a mosquito-borne virus, you basically need two things:
- A “reservoir host”: a population of infected individuals that infect mosquitoes. For Zika, that’s people (and as far as we know). For West Nile virus, it’s a variety of bird species.
- A “competent vector”: In this case, that’s a mosquito species that can be infected when it feeds on a reservoir host, and then pass the virus on to the next person (or animal) it bites.
As people with travel-associated infections come home, there are/will be people in the province with the virus circulating in their bloodstreams, just waiting for a competent mosquito vector to which they can pass it along (or a sexual partner, but that’s another story). While rare, we do have the “host” component here.
The vector component is our saving grace here, at least at the moment. While all mosquitoes may seem the same to us – buzzing, biting, annoying – there are many different mosquito species. Zika is mainly spread by Aedes aegypti, a mosquito species that doesn’t live this far north. Another species, Aedes albopictus, is also a potential vector, and it has a broader range. The map below is from CDC, and while it looks like A. albopictus is either here or knocking on the door, it’s very rarely identified in Ontario, so the risk of it causing Zika problems (for now, at least) is very low.
The reason I said “for now, at least” is that vector ranges are changing over time, probably due to climate change. While it will presumably be a long time before A. aegypti makes it this far north, it probably won’t take A. albopictus as long. So, over time, the risk might increase, especially if Zika establishes itself in the US.
The other question that often comes up when dealing with vector-borne diseases is the fact that we say “no known competent vectors.” That means that we don’t know of any insects in a region are definitely able to transit the pathogen. Does that really mean that none can? While the risk is probably low, there’s always some concern that there may be competent vectors that we don’t know about (or don’t realize are competent) just waiting to be infected. In Ontario, some ongoing work is testing Ontario mosquito species to see if any can be infected with Zika virus. They probably can’t, but it would be nice to know for sure.