Zika in the Minnesota: Ecological Limitations for Mosquito-borne Viruses

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!

Asian Tiger Mosquito (Aedes albopictus)
Image credit: Susan Ellis, Bugwood.org

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:aedes aegypti distribution

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:

overlap dengue and aedes

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

Hutchinson collecting insects at Cherryhinton Chalk Pits
Image credit: Yale University Archives

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.


When the Stingless Bee F. schrottkyi Made Headlines, We Made Generalizations

Sure, honey bees are important. They’re worth about $1.6 billion annually to U.S. agriculture. But take the blueberry plant, for example: southeastern blueberry bees, bumble bees, carpenter bees, orchard bees, Bradley’s andrena bee, and many others lend a “tarsus” in pollinating flowers. Together, all these “other” pollinators are more important than honey bees economically (by about $5 billion) and ecologically (enhancing productivity and stability).

While honey bees are the center of public attention—partly because they’re dying from Varroa destructor mites, the Nosema apis fungus, and insecticides—native bees modestly continue to pollinate in the face of similar environmental challenges.

operaria_inteiro_friesella_schrottkyiHowever, the stingless bee Friesella schrottkyi (Hymenoptera: Apinae) made headlines this week because of new research by Rodrigues et al. identifying copper sulfate-containing leaf fertilizers as mind-altering and lethal when consumed by F. schrottkyi. (Image credit: Sylvia Maria Matsuda, Laboratório de Abelhas)

Science Today

Ento Today

Wait a second. The research article only investigated the effects of certain leaf fertilizers on one species of stingless bee. Our friend F. schrottkyi’s only been famous for a week, and already we’re speculating and generalizing.

A key point to this research is in understanding the differences in pollination behavior and agrochemical toxicity among bee species. And stingless bees are not a single species, but rather a very diverse group in and of themselves. The effects of biopesticides, fungicides, and other agrochemicals on all kinds of bees (as well as other pollinators, and the surrounding ecosystem as a whole) is incredibly important.

So let’s take a quick look at what the Rodrigues et al. paper actually says:

  • The authors are implicitly concerned about the effects of all agrochemicals (especially non-insecticides) on a wide diversity of native pollinators.
  • Their research focused on Friesella schrottkyi and two commonly used fertilizers.
  • A series of bioassays (experiments run in the laboratory, not in the field) assessed the toxicity of a micronutrient mix and a fertilizer containing 24% copper sulfate.
  • The copper sulfate-containing fertilizer was surprising deadly when consumed, while contact exposure affected flight and foraging behavior.
  • Consuming the micronutrient mix did not affect F. schrottkyi survival, but did reduce respiration rate.
  • Copper, manganese, molybdenum, zinc, sulfur, and boron may all be involved in the toxic effects of leaf fertilizers on bees.

It’s important to take a step back and think about what this research really means. We should not be making broad conclusions, but we do need to ask questions about how other leaf fertilizers and other agrochemicals are affecting a wider spectrum of animals. While in a general sense it’s true that at high enough concentrations, all leaf fertilizers are probably toxic to all stingless bees, this conclusion is beyond the scope of the Rodrigues et al. article.

I also feel it shouldn’t come as a surprise to anyone that the heavy metals in fertilizers, administered orally or in high concentrations, have a negative affect on, well, any insect, plant, or animal. Heavy metals play an important role as cofactors for proteins and enzymes (think hemoglobin, in blood). But at higher levels, heavy metals start to interfere with enzymes, cellular structures, and even DNA.

The point is to take these broad ideas—like the stuff we think can be toxic, the pollinators we think are useful—and learn as much as we can about it, which is exactly what Rodrigues et al. did: figuring how and why a commonly used fertilizer affects one of their native bees. The best way we can preserve our ecosystems and protect our agriculture is to have detailed knowledge of how it works—and then we can ask even more questions and create new hypotheses about how stuff works.

Tweet: Native pollinators are important, too! #savethebees is not just for honeybees anymore. #saveALLthebees via @rudimik


DNA Thief: How a Braconid Wasp Used a Stolen Virus to Hack Its Prey

In a long con lasting nearly 100 million years, a crafty braconid wasp stole a virus’s DNA to control the immune system of its prey.

Braconid wasp (Aleiodes indiscretus) parasitizing a gypsy moth larva (Lymantria dispar)
Image credit: Scott Bauer, USDA Agricultural Research Service

Braconid wasps are unique parasites that lay their eggs inside unsuspecting butterfly or moth larvae, in which newly hatched wasps feed on relatively nonessential organs. Normally the immune system of the host defends against these attacks, but the wasps have a secret weapon in their arsenal: a domesticated virus from the Cretaceous period used to ensure the prey stays incapacitated but alive…until the young wasps are old enough to search out alternative food sources, that is.

The viruses coopted by braconid wasps are a type of polydnavirus (PDV), an infectious agent barely recognizable as a virus at all. Typical “everyday” viruses—like influenza or the common cold—contain all of the genetic information needed to replicate, but the genes involved in PDV reproduction are stored within the wasp’s DNA, which is an odd habit for a virus. As a result, the wasps more or less control the manufacture of the viruses and produce them at will.

Nearly 100 million years ago when the DNA coding for PDV replication was integrated into the wasp’s genome, virulence genes encoding wasp venom were transferred into the virus. Like Cold War-era microfilm, these DNA sequences contain sensitive information capable of great destruction. With these genes packaged into the shell of the stolen virus, braconid wasps literally invented nanotechnology for use in predator–prey warfare.

Think not what your virus can do for you, but what your virus can do for your baby wasps. After the mother wasp transmits the modified virus while laying its eggs, the prey’s blood cells read the DNA message inside the virus. Traditionally, this is how viruses hijack cellular machinery in order to reproduce, but after the braconid wasp’s slight-of-hand millions of years ago, the host’s cells are tricked into producing wasp venom instead of new viruses—effectively silencing the host’s own immune system so the young wasps can develop in secret.

By tricking larvae into poisoning themselves, braconid wasps save a lot of time and energy they would have otherwise used to produce their own venoms. They also gained more control over venom delivery—if the PDV integrates into a host larva’s genome, infected cells can be reprogrammed to deliver small doses of venom days or even weeks after the initial attack.

Using viruses for genetic hacking is unique among braconid wasps and several species of ichneumons, but many other wasps transmit viruses as well. Ascoviruses and baculoviruses, for example, are two types of contagion that infect butterfly and moth larvae. The parasitic wasp is only a disease vector in this case, just as mosquitoes transmit West Nile Virus to humans without infecting themselves. Ascovirus and baculovirus infections lower the immune defenses of the prey, making it easier for immature wasps to feed and grow without interference.

The relationship between wasps and viruses is symbiotic. The wasp’s young are able to develop without fear of an immune system attack and the viruses have better access to hosts. But for braconid wasps and their PDVs, the virus completely relies on the wasp for reproduction. PDVs can be selfish, though. If a caterpillar survives being parasitized by young wasps, small pieces of the virus may join with the host’s DNA to be inherited by future offspring. This is not an ideal situation for the virus, but it does remove some the virus’s reliance on the wasp for reproduction and transmission. However, these fragments of virus DNA would need to hijack DNA from another virus in order to become infectious again…but that’s just science fiction.

Or is it science fiction? In the laboratory, scientists have successfully made super-viruses from pieces of Langet and mosquito-borne dengue viruses. These and other “Frankenviruses” are typically inactive and used to create vaccines against dangerous diseases, but as for our friendly neighborhood PDVs…perhaps over the next 100 million years another insect will reengineer them in newer, even more unimaginable ways.

Further Reading:

Herniou, Huguet, Thézé, Bézier, Periquet, & Drezen. (2013). When parasitic wasps hijacked viruses: genomic and functional evolution of polydnaviuses. Phil Trans R Soc B, 368, 20130051. doi: 10.1098/rstb.2013.0051

Gunderson–Rindal & Pedroni. (2005). Characterization and transcriptional analysis of protein tyrosine phosphatase genes and an ankyrin repeat gene of the parasitoid Glyptapanteles indiensis polydnavirus in the parasitized host. J Gen Virol, 87(2), 311–322. doi: 10.1099/vir.0.81326-0

Pletnev, Bray, Hanley, Speicher, & Elkins. (2001). Tick-borne Langat/mosquito-borne dengue flavivirus chimera, a candidate live attenuated vaccine for protection against disease caused by members of the tick-borne encephalitis virus complex: evaluation in rhesus monkeys and in mosquitoes. J Virol, 75(17), 8259–8267. doi: 10.1128/JVI.75.17.8259-8267.2001

How African Arctiid Moths Evolved to “Jam” Bat Sonar

If you’re a moth, you don’t want to get in the way of a hungry bat. Well, as long as they’re not the nectar-sipping kind. Bats are swift, agile predators. They soar quietly through the air, nearly invisible in the darkness, hunting not by sight but by sound. Moths need to be creative to avoid these skilled predators… otherwise they’ll be moth à la mode.

How does a moth evolve to outsmart these flying bundles of rabies? The modern structure of a trait evolves as long as there is some functional and beneficial use of a previously existing feature on which natural selection can work.

Noctuid moths take evasive flight maneuvers when they hear the ultrasonic pulses emitted by bats. Ears—or, rather, specialized tympanic membranes—are not unheard of for an insect. Over thousands of years, the auditory range of noctuid moths was refined to be able to detect the incredibly high-pitched ultrasonic frequencies used by bats. Believe it or not, moths able to listen in on “batspeak” are 60% less likely to be a midnight snack.

Okay, so now that moths can eavesdrop in on ultrasonic frequencies, what happens next? Taking defenses one step further, African arctiid moths evolved to respond to bat sonar with an ultrasonic pulse of their own. This signal acts as an aposematic warning of their unpalatability, just as the bright color of monarch butterflies warns birds they’re toxic.

Wait!! Back up a second. Moths can “talk” now, too? This is unbelievable!

Not really. Natural selection would have favored moths that could emit the whatever frequencies would startle bats and jam their echolocation reception, since this helps the moth to survive. A vocal apparatus previously used for attracting mates or communicating with other moths could have been repurposed to create an ultrasonic clicking sound, used by arctiid moths in response to bat echolocation. This way, the arctiids trick bats into thinking the echo is reflected from prey located somewhere else, so when bats dive in for the kill, all they find is empty air.

Arctiid moth (Hypercompe scribonia)
Credit: Vern Wilkins, Indiana University

Use it or lose it: High Arctic moths living in the absence of bats have diminished reaction to ultrasonic frequencies. Without natural selection pressures, their tympanic membranes have effectively atrophied. The structures and neural pathways are not as developed because they simply aren’t needed. Rather, these moths have evolved to be better at staying warm in cold environments and have a higher reproductive output.

To wrap things up, I want to stress the lack of direction in evolution. Arctiid moths able to jam bat sonar are no more evolved than noctuid moths, nor are they superior to the High Arctic moths with reduced defensives. All three kinds of moths are equally well adapted to their environmental niches, and the structures they have evolved all come from the functional usage of preexisting ancestral characteristics. The variability within a population coupled with selection pressures has shaped the modern form of an organism based on the functions they needed to accomplish within their unique ecological niche.