Microcephaly. Neurological disorders. Guillain-Barré. Described by the World Health Organization as a “Public Health Emergency of International Concern.”
…and up to 80% of adults are asymptomatic.
The Zika virus carries with it some pretty nasty health complications. It is mosquito-borne, although sexual transmission has been documented, which carries unknown epidemiological implications. In the United States, there are two mosquitoes capable of transmitting Zika virus: Aedes aegypti and Aedes albopictus.
Ae. albopictus, that cute little guy pictured below, can be found as far north as Minnesota. Which, according to the media, means: Expect Zika in Minnesota! It’s coming! Run!
But before you read any further, let me assure you—it’s unlikely Zika will become endemic to the northern US. There may be the possibility of transmission by mosquitoes, but there are many factors that will limit the spread of the virus to mosquito populations in temperate climates.
These maps from the CDC (below) show the ranges of Ae. aegypti and Ae. albopictus in the United States. Although both species are capable of transmitting Zika virus, Ae. aegypti is a better vector than Ae. albopictus (even better news for Minnesotans!). Yet there have been zero cases of Zika in the US (to date) that were locally acquired (from our own mosquitoes).
Let’s zoom out from the U.S. for a minute. Here is a map from the newsletter In Homeland Security, which shows the global distribution of Ae. aegypti:
Okay, so this is essentially where we can expect the mosquito to live. But where does Zika live? Can it be found wherever the mosquito is found?
Many researchers and health organizations have analogized Zika virus to dengue, which is a health risk wherever Ae. aegypti is found. Since dengue and Zika both share mosquito vectors, have similar pathology, and origins near the tropics, we use what we know about dengue to create epidemiological models for Zika.
But can we actually do that, using dengue by proxy?
Well, no, we really can’t. Recent studies have shown that Zika and dengue occupy different ecological niches. But wait—how can that be? They’re carried by the same mosquitoes that the scary maps tell me live in Minnesota!
This last map, from the Virginia Department of Health, shows the global range of Ae. aegypti with an overlay of dengue epidemics:
As the image suggests, dengue virus has a complex ecology than can’t simply be defined by the distribution of its arthropod vector. There are biological, environmental, climatic, and cultural restraints that keep dengue more “in check” in some areas rather than others—and we can expect some kind of ecological limitations for Zika virus as well.
Okay, so what restricts the range of a mosquito-borne virus besides, well, the mosquito? Limitations include the exact type of cell receptors required to promote infection in the mosquito midgut, the availability of alternate hosts (which may amplify the viral population), and large scale climatic variations that not only affect a vector population, but also affect viral activity and viral population dynamics.
And what does this mean for Zika in Minnesota? Well, not to spoil the surprise for anyone, but we shouldn’t expect Zika to become established as a mosquito–borne illness in temperate Minnesota. The virus will likely remain confined to the tropics. Our mosquito, Aedes albopictus, is just not as good a vector as Ae. aegypti. While most of the ecology of Zika is unknown, a huge limiting factor for a Minnesota mosquito-borne endemic stems from the temperate climate, unlikely persistence of the virus during the winter, and absence of known alternative hosts (for amplifying the viral population) or of seasonal reintroduction (e.g. migratory birds).
But, for now, it is better to play safe with mosquitoes until we know more about Zika. Avoid mosquito bites when traveling abroad and practice safe sex if you or a male partner have Zika or traveled to an area with Zika. If you’re returning to the US from a country with Zika, continue to avoid mosquito bites so local mosquitoes remain uninfected. Talk to you doctor if you are pregnant and you or a sexual partner has been to an area with Zika, even if both of you feel healthy. And don’t forget: mosquitoes can carry other diseases, too!
How To Lose a Virus in 10 Days: Defining a Niche for Insect-Borne Pathogens
I am a fan of G. Evelyn Hutchinson’s 1957 description of an ecological niche as an n-dimensional hypervolume. I could devote an entire blog to niche concepts (actually I think I will), but for now consider this basic description: an organism’s niche consists of the abiotic and biotic factors needed for it to survive—stuff like nitrogen, temperature, moisture. For viruses (whether we consider them “alive” or not), their mosquito vector describes only one dimension of their ecology.
The intra-cellular environment. A virus needs to make it way to a particular region of the cell to reproduce. The correct cellular machinery must be present to replicate DNA, build viral proteins, and export the virus from the cell. New viruses won’t be produced if the codes aren’t right—foreign proteins could be broken down prematurely, packaged incorrectly, or abandoned in situ. Humans, mosquitoes, birds, fish, turtles (and everything else) differ in cellular mechanics, and while viruses are extremely versatile and adaptable, they simply aren’t compatible with cells in every animal.
The extra-cellular environment. Blood, stomach acid, and connective tissue can be difficult for a virus to traverse. Between acidity, antibodies, and dense, impenetrable fibers, navigation throughout the body of a host can be challenging, if not deadly.
Fantastic Voyage: Mosquito Edition. Viruses must evade the immune system, enter the cells of the midgut, and replicate before migrating to the salivary glands and other body tissues. Within a vector population, there is a genetic diversity in cell receptors, encapsulation/melanotic immune responses, and permissibility to dissemination throughout tissues. Higher initial doses of viruses can overcome density-dependent responses by the mosquito, while viruses infecting younger mosquitoes are more likely to complete their development, ultimately being transmitted to other animals or humans.
Life outside of the mosquito. Viruses are affected by a number of variables, both directly and indirectly. Some viruses can survive on non-living objects for a period of time (fomites). Others may infect alternate hosts. Extrinsic higher temperatures and climatic oscillations, i.e. El Niño, can affect a virus’s ability to migrate to other tissues in a mosquito’s body, while also contributing to vector species range expansion and increased disease incidence. Overwintering is another challenge, especially in temperate climates. Viruses may remain in the tissues of hibernating adult mosquitoes during the winter, or become locally extinct, only to be reintroduced every spring by birds migrating from the tropics.
Alternate hosts. Other suitable host species can provide overwintering grounds for viruses and help maintain viral populations at very low levels in unsuspecting places. For example, the Western equine encephalitis virus, which is spread by the mosquito Culex tarsalis to horses and humans (birds are a reservoir), has also been found to infect frogs, snakes, and mosquito species feeding on cold-blooded animals. Other interesting aspects of virus–host ecology are the dilution and spillback effects of having alternate hosts present. A dilution effect occurs when alternate hosts “steal” the viral load, so other species are infected less—essentially, the same amount of virus is spread out over a larger number of hosts. Spillback effects, however, are the result of alternate hosts providing a hospitable refuge for the virus (at a cost to their own health, of course), which increases the viral population of an ecosystem.
There was an idea, way back when, that malaria transmission could be reduced via the dilution effect by introducing cattle into an ecosystem. In theory, the mosquitoes bite cattle instead of humans, but in reality the cattle didn’t have much of an effect on human biting rates (or just made things worse). In a different example, a spillback effect was observed for Buggy Creek Virus when invasive house sparrows (an alternate host) were present, resulting in increased prevalence of the virus in cliff swallows, the native host of Buggy Creek Virus.
Cultural variables. Lifestyle can limit exposure to mosquitoes; in Texas, air conditioning keeps people indoors, and there are only rare dengue outbreaks because of reduced mosquito–human interaction. Without this type of cultural control in Mexico, however, dengue occurs in epidemics. Urban environments, which can be less-than-ideal for mosquitoes, can reduce the presence of the viral population to a point where it becomes locally extinct, but sustained urban cycles of mosquito–human interactions can maintain a robust viral population.