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Vectors are carriers of disease agents

ALBUQUERQUE, N.M. — Q: With all the current concern about Zika virus infection, we keep hearing that it’s a “vector-borne disease.” I think of a vector as being a physical force with direction and magnitude, and don’t understand how that can apply to a disease.

A: In medicine and epidemiology, “vector” means “a carrier of a disease-causing agent.” Vectors are usually insects and other arthropods, and Zika is one of several nasty viruses carried by two types of mosquito (dengue, chikungunya, and yellow fever are three others) named Aedes aegypti and Aedes albopictus. These two Aedes species have now been found in several southern New Mexico counties.

According to New Mexico Department of Health epidemiologist Sandra Melman, with whom I spoke recently, no locally transmitted Zika or cases of those other unpleasant diseases had been seen yet in New Mexico, though Puerto Rico and Florida have suffered cases caught locally as I write this changing story. Melman is a zoonotic disease epidemiologist, which means that she studies diseases transmitted between humans and other animals.

Melman and I talked about a number of the many other vector-borne infections; she emphasized the importance of prevention, as only some of the diseases carried from other species to humans are treatable. Probably the historically most important vector-borne disease is plague — though uncommon everywhere in the U.S. now, plague has been more common in New Mexico than elsewhere over the past half century. Carried by fleas (the vector), the agent (a bacterium called Yersinia pestis) causes severe infection of lymph nodes and/or lungs and/or blood, and is often fatal. For literary discussions of plague, consult Albert Camus’s The Plague, Barbara Tuchman’s A Distant Mirror, or Alessandro Manzoni’s The Betrothed. I digress: get the books, but don’t get the disease. Plague can be deadly, though when diagnosed in a timely manner, it usually can be successfully treated with antibiotics.

In contrast, there is no treatment other than support for hantavirus infection, a serious lung disease more often seen in adults than children and first discovered in New Mexico in 1990. Here the vector is a cute little critter named a deer mouse, which excretes the hantavirus in saliva, urine, and feces. You probably don’t have deer mice salivating on you, but you may be exposed to deer mouse urine and feces if cleaning up after them, especially in outdoor buildings. When my wife discovered a dead deer mouse we had trapped in my tool shed, we called the Department of Health and got the same advice that Melman gave just now: avoid stirring up clouds of virus-containing dust, wear a respirator, and disinfect with a bleach-water mixture or other chemical before attempting to clean up the droppings.

New Mexico may have more plague and hantavirus infection than most other states, but it may be surprising to know that we have very little Rocky Mountain Spotted Fever or other tick-borne disease, despite the fact that we live at the south end of the Rockies. In the U.S., most RMSF occurs in the Appalachians and northern Midwest, not in the Rocky Mountains. Occasional cases of other tick-carried disease like tick-borne relapsing fever, Colorado tick fever, and ehrlichiosis are seen in New Mexico. We are very fortunate that ticks seen in New Mexico are not vectors for Lyme disease, so we have never had a case in this state not acquired through travel, according to Melman. Ticks aren’t very pleasant creatures, often found on dogs and cats, as are the fleas that can carry plague. Melman recommends that our animals wear insect protection and not be allowed to hunt to reduce their risk of transmitting disease.

In the past, dogs have been the main source of the deadly disease rabies, though thanks to immunization, rabies hasn’t been detected in dogs more than a few times in the last decade in this state. I can’t say the same about skunks and bats, though; a bite from either of these animals — and occasionally from foxes — has led to rabies injections in 29 New Mexicans in 2014 and 19 in 2015. This so-called “post-exposure prophylaxis” is vital because once rabies has taken hold in a human, almost no one has survived.

Back to Zika and the Aedes mosquitos: Melman tells me that we are at most risk of Aedes stings at dawn and dusk, but that is not the case with the much more common Culex mosquitos that carry West Nile virus — they’re happy to feed on us humans and the birds that are the “reservoir hosts” of West Nile at any old time, moving the virus from bird to human quite efficiently. Although it most often causes no symptoms, infection with West Nile can be serious, causing neurological disease in one percent of its victims. So use those insect repellants — more information is available from Ms. Melman and her colleagues at https://nmhealth.org/about/erd/ideb/zdp/. ZDP=Zoonotic disease program.

Lance Chilton is a long-time Albuquerque pediatrician. Send your questions to him at lancekathy@gmail.com.

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