Notorious disease vectors

Mosquitoes are some of the most notorious disease vectors in the world. Because of their worldwide distribution (except for Antarctica), proximity to humans, and inclination to feed off of humans, mosquitoes have been able to spread viral (e.g. West Nile, Zika, Chikungunya) and parasitic (e.g. Malaria) diseases to people throughout the world. Annually, over one million human deaths are attributed to mosquito-borne diseases.

But how do they do this?! It turns out female mosquitoes inject some of their own saliva into the host – humans, to name one – to stop the host’s blood from coagulating before the mosquito can retrieve the blood (Image 1). If that mosquito has previously fed on a human or other animal infected with certain diseases, those diseases may have been able to replicate within the mosquito without harming it.

Yikes!
Thus, when an infected mosquito injects saliva into a host, that host can in-turn become infected.

Spectrum of Viruses.

So, are all viruses and parasites able to be passed from mosquitoes to humans? The short answer is no. Now for the longer answer: some viruses and parasites cannot survive the mosquito’s gut (like HIV). Because of that inability, they’re unable to establish within the mosquito’s cells and replicate. Environmental conditions, predominantly temperature, can also affect how a capable a virus or parasite is at infecting and replicating within a mosquito.

Warmer temperatures generally mean that a pathogen is able to replicate at a higher rate within a vector. Finally, the amount of the virus or parasite ingested by the mosquito also determines the ability for the mosquito to transmit the pathogen – the vector competence. The greater the dose, the greater the vector competence (assuming the pathogen is able to infect and replicate within the mosquito).  he main diseases that can be transmitted by mosquitoes within Canada are the California serogroup viruses, eastern equine encephalitis, and West Nile virus (WNv).

WNv is the most commonly transmitted mosquito borne disease in Canada. In 2018, a total of 432 human cases of WNv were reported in Canada – the highest total since 2007 (Image 2). Large-scale, nation-wide surveillance efforts are conducted to keep track of WNv incidence in horses, birds, and humans. These data give program managers the ability to direct mosquito control efforts.

The primary mosquito targets are Aedes vexans and Aedes sticticus. Their biology is quite specific to floodwater habitats. Both are somewhat easily identifiable in the field as adults and, when combined with habitat, also easily identifiable in the field as larvae. Generally, if the mosquitoes are so numerous that you can’t help but breath them in, and if they are biting during the heat of the day, then you could very well have at least one of these mosquito types.

Adult mosquitoes of both species have pointed abdomens. One has cool white bands on its legs and the other has black legs. That’s good enough 9 times out of 10 (that is, if you’re near a flooded area and the mosquitoes are aggressive).

A. vexans tend to come on first as rising water levels flood low-level grasslands associated floodplains. The trigger for their hatching is a rise in water temperature (in northern climes this can be as early as 4C but typically when the water reaches 6-8C).

A. sticticus tend to come on later as water levels reach the wooded riparian zones of the floodplain. The trigger for their hatching includes a decreased DO (and can happen in 1- 2 hours from wetting).

Both species lay their eggs on soils prone to seasonal flooding. As water levels rise the eggs are wetted (covered) and in appropriate conditions are triggered to hatch, beginning the progression to annoying biting adult. In cooler waters this process can take 2-3 weeks, from egg to adult. That provides a 10- 14 day window for effective treatments (we try to target 3rd instar larvae).

As the weather, and water, warms up this timeline can be shortened considerably. From the time that eggs are first wetted until the end of the effective treatment window can be as little as 60-70 hours (both species can, under certain circumstances, skip the 4th instar and moult directly from 3rd to pupa).

Each time the water levels progress higher, or as sustained water levels increases seepage areas not contiguous with the main river body, new batches of mosquitoes are hatched.

A proportion of A. vexans eggs do not require diapause before they are able to hatch. The only requirement is that they are dry for a 3-5 day period. This means that, in suitable conditions, subsequent peaks in water levels that don’t reach the previous high can still produce mosquitoes.
These mosquitoes are extraordinarily successful. When field techs are looking for them it’s not uncommon to find them at densities of more than 1000/250ml of water. That can translate to roughly 80 million mosquitoes per hectare of water. Believe us, that’s a lot!! In our experience, if you aren’t finding 100+ larvae per dip then you aren’t in the right place!