February 2017 was notable for not being very wintery across northern areas of the U.S. Headlines across the country (for example, this one from the Washington Post) alternately commented on the record warm temperatures, the guilty pleasure of people enjoying spring-like weather in what normally can be a dreary month, the early arrival of spring blooms, and whether this weather is a result of ongoing climate change due to rising carbon dioxide levels. (See this story in The Atlantic for one perspective on the big picture.)
In some parts of the country, reports rolled in about how even mosquitoes were making an early appearance, leading to expressions of indignation and concern on the part of many people. Always something of a nuisance as a visitor to backyards and recreational areas in the U.S. because of the itchy bites they cause, mosquitoes are of deeper concern to scientists and physicians as a vector for carrying diseases, especially in warm, tropical areas with plenty of standing water for mosquito breeding and a compact human population through which the diseases can spread. Malaria, Dengue fever, Chikungunya, and now West Nile and Zika all worry epidemiologists who track the spread of disease by mosquitoes. Most of these are viral diseases, but malaria is spread by a parasite of mosquitoes that spreads from person to person by the blood transferred by biting mosquitoes. Malaria affects millions of people every year, most of them in Africa and tropical areas (see this comprehensive website from the Centers for Disease Control and Prevention for more information about malaria).
This month, several mosquito-related science news reports caught my eye. Ongoing efforts to better understand and control or eradicate mosquito-borne illness continue to reveal new information about the intricacies of the underlying science.
Item #1: Creating a vaccine for malaria has been an enormous international goal, but so far efforts have not yet yielded a widespread success story. However, a recent report published in the journal Nature highlights one approach among the broader ongoing experiments. Researchers tested whether they could give healthy people the live Plasmodium falciparum malarial parasite, let it move to their livers (where it doesn’t cause active disease), and trigger their immune response. The scientists also gave the participants anti-malarial drugs to make sure the parasite was then killed in their body and wouldn’t be able to spread. This rather convoluted approach gave good results for protection against later infection in a small-scale trial. We’ll have to wait and see if it pans out in the long run in future vaccine trials.
Item #2: Many vaccine approaches have aimed to boost people’s immune responses to a particular disease agent, such as the malaria parasite or a virus carried by mosquitoes (such as Zika). What if we could help people mount a better immune response just to the action of being bitten by mosquitoes themselves, and possibly use the vaccine agent to help reduce the liklihood of other people being bitten by the same mosquito? That’s the strategy of a new vaccine trial testing whether compounds in mosquito saliva itself could be used to vaccinate people against a wide range of mosquito-borne diseases. If the vaccine ingredients also were toxic to mosquitoes who bit a vaccinated person, it could also reduce disease transmission by limiting mosquito numbers and reproduction. This idea seems pretty wild, but it’s an interesting approach that has the backing of the National Institute of Allergy and Infectious Diseases (NIAID) division of the National Institutes of Health. Read here for more information.
Item #3: Underscoring the fact that we don’t understand all the variables of mosquito-borned infection, a recent study identified that the P. falciparum malarial parasite that infects mosquitoes produces a chemical called HMBPP ((E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate) that may regulate release of carbon dioxide and other gases from the blood of infected people. Apparently, at least one species of mosquito that spreads the disease may then preferentially seek out the “tainted” blood of infected people for its meals, which then would in turn more easily cause the spread of malaria. For more information, see this report from ScienceNews or the original scientific report in the journal Science).
Can’t we just kill all the mosquitoes?
Humans have gone to great effort to swat and slap individual mosquitoes, wear DEET-containing mosquito repellents, spray pesticides, and distribute bed nets to prevent mosquito bites. But it’s very difficult to actually limit mosquito numbers in nature, because they’re very successful breeders, especially in habitats they share with large numbers of people.
One approach to eradicating mosquito-borne illness has focused on approaches to lower the reproductive rate of the mosquitoes themselves. A company called Oxitec has been developing targeted genetic modifications to several mosquito species to limit their ability to reproduce in the wild. Their aim is to gain approval for release of these engineered mosquitoes into disease-prone communities and cause the natural mosquito population to decrease after breeding with the newcomers. Here’s a link to a PBS NewsHour report from last August about one area in Florida that was considering the use of these mosquitoes to stave off the spread of the Zika virus, which is carried by mosquitoes. The mosquitoes have successfully decreased mosquito populations in other parts of the world in trial releases and are being considered for new applications, as well.
As we continue to wage war against these pesky vectors of disease, we must also be mindful of the broader roles mosquitoes play in the ecosystems which they inhabit. We must also take steps ourselves to manage our risk and exposure to mosquitoes and evaluate the various ways people can help minimize mosquito reproduction (such as limiting any areas of standing water on their properties). Efforts to eradicate global diseases such as malaria also require a large input of public and private funds and a dedicated scientific and medical workforce to test and implement new strategies. (For example, in addition to work through the U.S. National Institutes of Health and the Centers for Disease Control, the Bill and Melinda Gates Foundation has dedicated billions of dollars to global malaria research efforts.) Time will tell how effective we will be at solving the puzzles of this ongoing area of public health concern.
For more information on mosquito safety, visit this Centers for Disease Control website. And in the meantime, I hope spring finds its way to you on time next month!