Another Emerging Mosquito-borne Infection Has Arrived in the Americas!

In 2013 it was Chikungunya virus moving from across the oceans into the Caribbean before spreading extensively through warmer climates of the Western Hemisphere. In 2014 it was Zika virus entering Brazil from across the Pacific and from there on to locations through South and Central America and the Caribbean, then into the southern United States. Now, as of November 2016, another infectious pathogen has made its way for the first time into the Western Hemisphere. This one is a parasitic worm, long established in Europe, but appearing this month in Mexico and posing a threat of further expansion into a wide range of the Americas. In this case, the range may extend even into temperate areas with cold climates. The new disease-causing invader is a filarial worm, Dirofilaria repens, probably brought to Mexico in a dog reservoir host; and like the viral pathogens that preceded it, the transmitting vectors are mosquitoes that are common throughout the Americas. In Europe, our recent reports have shown that that the worms infect humans even as far north as Ukraine (link here to our detailed published report from Ukraine) and Russia (link here to our brief published report from the Russian Federation).

Unlike Zika and Chikungunya, which primarily infect only humans in the regions where they have been introduced recently, D. repens worms prefer to live in domestic dogs (Canis familiaris) and other canine hosts. However, there is frequent spillover into human hosts, partly because of the close association between dogs and humans. Human cases, like those of dogs, are usually subcutaneous, with large adult worms living in nodules under the skin (see image above), from which they produce microscopic microfilariae larvae that can be picked up by blood-feeding mosquitoes, in which they develop further before becoming infectious to a new human or dog host (see diagram below from U.S. Centers for Disease Control and Prevention, or link here for more information from CDC).

So far, no human cases of D. repens have been reported in the Americas, though the disease has been suspected to be present. However, a serological survey of dogs in Guanajuato, Mexico using precise molecular techniques (PCR) yielded a 100% match for D. repens, unequivocally distinguishing it from other Dirofilaria species that parasitize dogs, wild mammals, and occasionally humans in North America. This novel finding was published by Sandra Ramos-Lopez and collaborators in the November 2016 issue of the journal, Vector-Borne and Zoonotic Diseases (link to PubMed abstract here).

The two viral pathogens that invaded the Americas in the past few years are transmitted almost exclusively by certain mosquitoes of the genus Aedes, especially the human-associated Aedes aegypti, most famous for its role in spreading the devastating Dengue virus and the deadly Yellow Fever virus. These mosquitoes are especially abundant in warm climates, but do not fare as well in cooler regions, thus limiting the spread of the diseases somewhat. However, D. repens is not so particular, and can develop to the infectious stage in many mosquitoes that live in a wide range of temperatures and breed in a wide range of habitats. These may include species of the genera Aedes, Anopheles, Culex, and Mansonia; and unlike many viruses, the filarial worms that these diverse species harbor regularly develop to the infectious stage either in tropical environments or where temperatures are much cooler. Dirofilaria repens has been gaining attention as an aggressively emerging parasite for several decades across its natural but expanding range in Europe. Our recent extensive report from Ukraine showed a correlation between climate zones and distribution of human dirofilariosis repens over a period of 16 years, as shown in the diagrams below (link to full publication here).

In North America, it is likely that, once established, the disease could spread even more rapidly and extensively than it has in Europe, due to the combination of our large mosquito populations and our large populations of feral dogs (C. familiaris) and susceptible wild canids such as foxes, wolves, and especially coyotes (Canis latrans), which are now common even in densely populated urban areas across the United States. Other Dirofilaria species that are already endemic in North America and are also transmitted by mosquitoes include Dirofilaria immitis, the heartworm of dogs, and Dirofilaria tenuis, a subcutaneous worm from raccoons; both are also zoonotic and can infect humans, but the spillover to humans may occur more readily with D. repens in some situations. I predict that we will see human and canine cases of dirofilariosis repens in diverse locations across the Americas soon, and that these will become more common in the coming years. Indeed, human cases are possibly already present but undiagnosed, especially in situations where people have limited or suboptimal access to healthcare services. 

When humans become infected, the disease is generally rather mild, due to the typical locations under the skin not involving any vital organs. However, as seen in the diagram at the top of the page, human lesions may include ocular sites that may damage the eye, head lesions that can be disfiguring, or troubling nodules in the breast and scrotum that may be complicating factors in the diagnosis of malignancies in those sites. So, here is yet another reason to adopt One Health strategies to monitor and control mosquito populations (link here to my recent review on this topic), as well as to maintain surveillance in populations of domesticated and wild animals in addition to human clinical cases. Movement of either humans, other animal hosts, or insect vectors, contributes to the invasion and emergence of diseases in new locations around the world.

Prof. Dr. David Bruce Conn is a biomedical scientist who has conducted research, taught university students, and advised governments and industries about parasitic and other infectious diseases, invasive species, and environmental health for 40 years. (Disclaimer: Dr. Conn is a senior advisor to the U.S. Department of State on issues of health and foreign policy, but the contents of this post are his own views and do not officially represent any unit of the U.S. federal government).

Copyright © 2016 by David Bruce Conn. Free use and distribution permitted with attribution.