How Mosquito-borne Illness Could Become A Daily Reality in North America

Photo Credit: by Dr. Bradley van Paridon

In the wake of natural disasters such as the recent hurricanes in Central America and the US, the potential for severe disease outbreak is at a maximum. Infrastructure is destroyed, water and medicines are in short supply and sanitary conditions are alarming. In these situations, the immediate concern is usually waterborne infections such as cholera—as was the case after the earthquakes in both Haiti in 2010 and Nepal in 2015. In the southern US though, the bigger threat may be the long-term threat: invasive mosquitos.

Now that the clouds have parted and clean-up has begun, mosquitos may seem like a small concern to those in Houston and Florida. Yet mosquitos carry some of the world’s deadliest diseases and they thrive in places with large amounts of standing water. Mosquitos require standing water to breed and after a hurricane, ample flooded areas and water-filled debris strewn about creates a perfect mosquito breeding ground.

Thankfully, in the southern US, we don’t currently find a lot of devastating mosquito-borne diseases like malaria, dengue or yellow fever. One new mosquito transmitted virus, however, did make landfall in the US last year. The Zika virus swept across the southern hemisphere last year, causing disease and grabbing major headlines when it was linked to the abnormally high number of infants born with microcephaly. While Zika was reported in both Florida and Houston it hasn’t yet become a sustained epidemic in the US. This is in large part due to control efforts aimed at mosquitos. Now though, events could be conspiring to create the conditions necessary for a true outbreak.

Previously, Zika was not considered a sustained epidemic because the majority of cases in the US occurred in people who traveled elsewhere, became infected and then returned home. According to the Centers for Disease Control and Prevention (CDC), only a single case of Zika out of all 278 reported cases in 2017, so far, came from a local source. Fortunately, when people return carrying the virus from abroad it has nowhere to go. This is because the main mode of transmission is through two mosquito species, Aedes aegypti, and Aedes albopictus, both of which do not occur naturally in the continental US. Without these mosquitos, the virus has no way to spread out of the bodies of those who have brought it into the US. (There are reports of sexually transmitted Zika infections but these are rare: three of 278 reported in the US in 2017.) However, as any biologist will tell you, the potential for animals to spread and colonize new territory is increasing, mosquitos included.

Invasive species, as they are known, are animals that have spread to new habitats and subsequently thrived in the new area. These new homesteaders are often very destructive because they’ve entered habitats that are not used to their presence and cannot present any natural boundaries for their growth. Popular North American examples are the invasive zebra mussel and Asian carp that are decimating rivers and lakes. Of course, invasive species also bring with them any diseases they happen to be carrying. In the case of Zika virus in the US, if one or both of the Zika carrying mosquitos become regular members of the mosquito population then they can bring Zika virus along with them, plus any travelers carrying the virus can now spread it back to mosquitos. In this way the virus will always have a host to live in, and it too will become established on US soil.

So, what are the chances that these specific mosquitos will find their way into the US? This year alone, both mosquitos have been reported in at least eight different states and even as far north as Michigan and Canada. It is unlikely at the moment that they can survive that far north but their presence there demonstrates just how far these tiny pests can travel; they are normally found in tropical and sub-tropical areas. California and New Mexico, however, both have multiple reports this year: seven and three respectively. Since 2013, 13 counties in California have found the disease-carrying mosquitos.

Even more disturbing though is the fact that, in 2016, the only two cases of local Zika transmission in the US occurred in Texas and Florida. Brownsville, Texas, which lies 350 miles south of Houston, and Miami-Dade County were both designated by the CDC as “yellow areas” for Zika prevention and transmission in 2016. In short, this meant that pregnant women were advised not to travel to these areas and warnings were in place concerning donated blood and tissue, as these too carry the virus. Both areas had their yellow status rescinded this summer since experts think the problem mosquitos do not survive the winter in these locations—but this too could change. Climate change leading to a warming planet means areas that were previously too cold for Zika carrying mosquitos may soon become habitable. The CDC has published two maps showing the potential range expansion of both mosquitos. The areas identified in both maps are large swaths of the southern US, along with both Texas and Florida.

Take all of these factors into consideration—perfect breeding conditions post-hurricane, continued introductions of the mosquitos and rising temperatures (which according to some could lead to increased storm activity too)—and it seems like Zika becoming established in the southern US is a matter of when and not if. Admittedly, this seems like a very troubling scenario but we can’t forget that areas like Puerto Rico, the Dominican and Haiti will likely fare much worse. Not only did they receive more damage from the hurricanes, but they also have less money for recovery and mosquito control. It will be in these less developed countries to the South that we will see the first signs of what may come. Dealing with mosquito-borne illness is a fact of life for many people living in tropical areas and it may too become a future reality on the North American continent.

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Dr. Bradley van Paridon is a writer and science communicator who holds a PhD in Parasitology.