Infective larvae enter the dog’s body when the mosquito bites the dog. They migrate through the animal’s tissues and then into the bloodstream and eventually find their way to the lungs and adjacent blood vessels where they mature to adults; mating and reproducing microfilariae within 6 – 7 months after the bite of the infected mosquito. The target host of interest to us is the dog or cat, but a wide variety of other animals can also be infected, including wild felids, mustelids, pinnipeds, beaver, horses, and humans.

Vector-borne diseases account for more than 17% of all infectious diseases of humans, affecting more than 1 billion people and causing more than 1 million deaths annually, globally (CDC, 2016)7.

Distribution of these diseases is determined by a complex dynamic of environmental and social factors. Globalization of trade and travel, unplanned urbanization and environmental challenges such as climate change are having a significant impact on disease transmission in recent years. Some diseases, such as Zika, Dengue, Chikungunya and West Nile virus, are emerging in countries where they were previously unknown.

Mosquitoes are the best known disease vectors for mankind rendering them the deadliest animal family in the world. Not only are these pesky insects devastating to human health, they also affect companion animals and livestock, carrying parasites and viruses that spread disease in the hosts they infect.

Mosquitoes are members of the family Culicidae in the order Diptera (true flies). Mosquitoes are insects that develop through four distinct life stages – egg, larva, pupa and adult. Adult mosquitoes are distinguished from other flies by the presence of a long proboscis and scales on the margins and veins of the wing. Female mosquitoes’ mouthparts form a long piercing-sucking proboscis. Their mouthparts are needle-like, and their saliva is both anesthetic, so you can’t feel the bite, and anti-coagulant, which keeps the blood flowing. Males differ from females by having feathery antennae and mouthparts not suitable for piercing skin. A mosquito’s principal food is plant nectar or similar sugar source. Only adult females feed on blood which is a required source of protein and iron for egg development.

Because mosquitoes are such a problem, one that is potentially getting worse with warmer temperatures, larger cities (and their inevitable water-sources, in which the insects lay their eggs), and ease of global movement, these insects are seriously studied by experts across a variety of scientific, healthcare and government groups.

Mosquitoes cause more human suffering than any other organism, over one million people worldwide die from mosquito-borne diseases every year.

Not only can mosquitoes carry diseases that afflict humans, they also transmit several diseases and parasites to which dogs and horses are very susceptible. These include dog heartworm, Eastern equine encephalitis (EEE), and West Nile virus (WNV). In addition, mosquito bites can cause severe skin irritation through an allergic reaction to the mosquito’s saliva. Mosquito-vectored diseases include protozoan diseases, like malaria, filarial diseases such as dog heartworm and human lymphatic filariasis, and viruses such as Chikungunya, Dengue, St. Louis Encephalitis, LaCrosse Encephalitis, Zika and Yellow Fever.

In contrast to human medicine, veterinarians have NOT routinely targeted the mosquito vector when fighting heartworm infection in pets. Why is this?

In veterinary medicine, an unusual thing happened. At almost the same time that veterinarians became aware of how serious infection with heartworms was for pets, some very effective drugs were introduced by the pharmaceutical industry. These drugs, called macrocyclic lactones, or heartworm preventives, are designed to be given to pets orally or by injection (in a time-released product) on a regular basis to kill the young worms after they have entered the pet’s body but before they have developed to the stages that cause harm.

For over 25 years these drugs seemed to be doing a good job, as long as pet owners used them absolutely according to their label instructions, year-round. Because these drugs did such a good job under ideal circumstances, the veterinary profession had little reason to worry about vector control, as human medicine did when it lacked any good drugs or vaccines.

Unfortunately, we now see an alarming increase in the number of heartworm infection cases in pet dogs and cats over the past several years. This increase is believed to be the result of owners not using the oral (or injectable time-release) preventives as compliantly as they should but also because strains of heartworms that are resistant to the preventives have now been detected in a large area of the mid-US. Even when pets receive their preventive medications perfectly compliantly, these strains are able to evade the drugs and develop to adults in the great vessels and lungs of the pet.

Most experts agree that it will be many years before we have new preventive drugs to fight the resistant strains of heartworm that may continue to emerge and spread during that time. Until recently, we have not had the kind of data showing that commercial topical ectoparasiticide products can effectively repel and kill mosquitoes and, thus, block the transmission of heartworm organisms from mosquitoes to pets and from pets to mosquitoes. But, in the past couple of years, studies have been conducted to assess the effectiveness of such products as repellents and insecticides for mosquitoes of many species. These studies show promise for a multi-modal approach that includes use of macrocyclic lactones in combination with repellent insecticidal topical products (McCall, 2015)11. We will discuss in depth the science behind the preventive synergy of macrocyclic lactone drugs in combination with repellent insecticidal topical in a later module.

The integrated mosquito management methods currently employed by organized control districts and endorsed by the Center for Disease Control (CDC) and the Environmental Protection Agency (EPA) are comprehensive and specifically tailored to safely counter each stage of the mosquito life cycle. Larval control through water management and source reduction, where compatible with other land management uses, is a prudent pest management alternative – as is use of the environmentally friendly EPA-registered larvicides currently available. When source elimination or larval control measures are clearly inadequate, or in the case of imminent disease, the EPA and CDC have emphasized in a published joint statement the need for considered application of adulticides by certified applicators trained in the specialized handling characteristics of these products (EPA, 2016)10.

Additionally, biologists and environmental scientists are evaluating new ways to leverage biologic controls such as sterile insect techniques and genetic engineering as a means to control mosquito populations. These efforts, while effective in controlling the mosquito population, are receiving a great deal of scrutiny due to the lack of safety and environmental studies to show that introducing biologically altered or genetically modified mosquitoes into nature is a safe alternative for mosquito control.

One particular group, Oxitec has applied a method developed by Luke Alphey, a British geneticist specializing in vector control. Partially supported by the Bill & Melinda Gates Foundation, Oxitec has applied the method in Brazil, Malaysia and Panama, and claims to have reduced the A. aegypti population in small test areas by at least 90 percent (Alphey, 2013)12. That’s a far better percentage than spraying, which usually hits about 50 percent and has a tendency to breed resistance, requiring more and more spraying to get the same low result. Recently, the U.S. Food and Drug Administration approved an experiment commissioned by Oxitec in an unincorporated area of South Florida. The locals in the area have protested and are scrutinizing the ethics and handling of such experiments. Their primary concern being the unproven effect of genetically modified mosquitoes that could further concerns about drug resistance and other unintended consequences. (Oxitec, 2016)13.

As the human and pet populations continue to expand and our world continues to shrink, mosquito control in the United States will assume a more critical public health function, well beyond its quality of life role. While new ways to control mosquito populations may take years to research and fully understand both efficacy and safety, today the veterinary profession has an opportunity to embrace a new way to control mosquito-borne diseases through effective multi-modal protocols. Because mosquitoes are the only known vector of Dirofilaria immitis, controlling mosquitoes through the use of an on-pet, mosquito repellent insecticide should help control heartworm transmission in much the same way that targeting vectors has been used successfully to control vector-transmitted diseases in people.

Congratulations, you have finished the content for this CE course. Click the button below to launch the quiz. If you would like more time feel free to go back and peruse the content until you feel comfortable enough to continue.