Creepy Crawlies, Part 1: Mighty Mosquitoes

They might be small, but these flying fiends can spread some deadly diseases

The original intent of this article was to relay how malicious and deadly some mosquitoes are, but according to Charles Calisher, PhD, professor emeritus of microbiology, immunology, and pathology at Colorado State University, mosquitoes are neither spiteful nor a menace to society. “Mosquitoes are not deadly,” he explains. “It is the viruses and microbes they transmit that are deadly.”

So although mosquitoes do not willfully harm horses (aside from getting a snack), it is still important for owners to minimize contact between mosquitoes’ mouthparts and their horses.

“Mosquitoes transmit some of the deadliest and most debilitating diseases in the world, including malaria, dengue fever, and lymphatic filariasis,” says Nicholas Ledesma, DVM, PhD, now a veterinary medical officer in the serology section at the USDA National Veterinary Services Laboratories, in Ames, Iowa; he researched mosquito ecology during his studies at Cornell University, in Ithaca, New York. “In addition to the human mortality and decreased quality of life they cause, mosquito-borne diseases (arboviruses) such as Eastern equine encephalitis afflict both domestic and wild animal populations.”

In this article we will describe how mosquitoes transmit viral infections, identify the most important viral infections horses can acquire from these annoying arthropods, and describe methods for protecting horses from infection through vaccination and control measures.

How Mosquitoes Spread Disease

Many consider mosquitoes to be one of the world’s most annoying creatures. They have six long, spindly legs, scaly wings and bodies, antennae, and the pièce de résistance: long, needlelike mouthparts able to pierce both human and animal skin.

“The protein and other nutrients in blood are necessary for female mosquitoes to produce eggs,” explains Ledesma. “After taking a blood meal and mating, female mosquitoes lay their eggs on or near water. Different mosquito species have their own preferred places to lay their eggs, such as temporary floodwater, snowmelt pools, catch basins, marshes, swamps, lagoons, ponds, stagnant waters, and water found in both natural and artificial containers.

“The eggs hatch in approximately two to seven days, depending on water availability and temperature, and the larvae (immature mosquitoes) that emerge eat and grow over a period of five to seven days before becoming pupae,” he continues. “The pupae, which are also still in the water, are nonfeeding but mobile. They emerge from the water as adult, flying mosquitoes in two to three days.”

Most mosquito-borne diseases are spread by female mosquitoes that become infected with viruses, protozoan parasites, or even worms after ingesting blood from an infectious animal. Once a susceptible mosquito has ingested the infectious agent in a blood meal, the pathogen (disease-causing organism) replicates and begins its next stage of development.

According to Ledesma, “Viruses, protozoan parasites, and filarial worms differ in the mosquito organ location, duration, and process by which they develop, but all must eventually be passed on to another vertebrate host when the female takes another blood meal. Most, but not all, organisms do this by invading the mosquito’s salivary glands.”

There are actually 2,500 to 3,000 different mosquito species worldwide, and many prey on particular animal hosts. Many Culex mosquitoes, for example, prefer to feed on birds. Birds serve as reservoirs for common arboviruses, which means infected birds can have high enough levels of a virus in the bloodstream to infect mosquitoes that feed on them. Many infected birds appear healthy and serve as ideal “hosts” for the virus. Horses and humans become infected when an adult female mosquito infected with the virus feeds on these (and sometimes other) mammals instead of another bird. (These viruses are considered zoonotic, meaning humans can become infected in addition to animals.) Because humans and horses do not develop high enough virus concentrations in their blood for a mosquito to become infected by taking a blood meal from them, horses and humans are not reservoirs and are referred to as “dead-end hosts.”

Arbovirus Rundown

West Nile virus (WNV), Eastern equine encephalitis (EEE) virus, Western equine encephalitis (WEE) virus, and Venezuelan equine encephalitis (VEE) can each cause encephalomyelitis—inflammation of the brain and/or spinal cord.

“In horses, these equine encephalitides are the most important central nervous system virus diseases spread by mosquitoes in North America,” says Calisher. “These three viruses are similar in how they persist, move from place to place, and cause disease. They are all spread by mosquitoes from birds, which are the natural hosts of the virus.”

WNV occurs almost everywhere in North America, whereas EEE is predominantly seen in states east of the Mississippi River. WEE is usually only seen west of the Mississippi River. Veterinarians see VEE predominantly in South and Central America, but they must report any suspected cases of VEE in the United States to their state veterinarians so officials can implement a quarantine (the last major outbreak in the United States occurred in 1971).

Signs of disease caused by these viruses are similar and include:

  • Fever;
  • Ataxia (incoordination), stumbling, and hind limb weakness;
  • Depression;
  • Anorexia;
  • Recumbency with the inability to rise;
  • Muscle tremors;
  • Teeth grinding and dysphagia (inability to swallow);
  • Head pressing;
  • Signs of colic;
  • A flaccid paralysis of the lower lip;
  • Aimless wandering;
  • Excessive sweating;
  • Behavioral changes; and
  • Convulsions, coma, and death.

Viral Encephalitides: Under Control, but Still Problematic

According to the USDA Animal and Plant Health Inspection Service’s (APHIS) disease surveillance website, in 2011 there were 83 horse deaths due to WNV and 58 due to EEE (there were no reported cases of WEE). The number of horses infected each year varies and cannot be predicted.

There is no specific treatment or “cure” for the viral encephalitides, although veterinarians can provide supportive care. Approximately 90% of horses infected with EEE ultimately die or are euthanized after their condition deteriorates. The mortality rates for WEE and WNV are 50% and 35%, respectively.

“Even if the patient does not die, however, a significant proportion of survivors will have some form of neurologic deficits for the rest of their lives, which may be brief,” warns Calisher.

The U.S. Centers for Disease Control (CDC) reports that in 2011, officials confirmed 690 cases of human WNV in addition to six cases of EEE.

Approximately 35% of adults and 70% of infants infected with EEE die. If the patient recovers, he or she often has lingering neurologic deficits, including mental retardation, behavioral changes, seizure disorders, and paralysis.

Preventing Infection

Because mosquitoes are essentially miniature flying hypodermic needles, it’s important to give these “nettles” a wide berth. There are two main ways for you and your horse to ward off a potentially disabling or deadly infection: avoidance and vaccination.


Because all immature mosquitoes develop in water, the No. 1 mosquito control method is to identify and target areas and structures where water accumulates on your property and either remove or change the water frequently. Fill in depressions and ruts in the ground, discard cans, birdbaths, old tires, and any other water-harboring object, and ensure roof gutters are not clogged. Cover all rain barrels with a tight-fitting fine mesh screen.

Personal protection is another way to evade infection. Reduce your horse’s mosquito exposure by using light sheets and fly masks and applying a chemical-based product that repels mosquitoes. DEET is the most effective mosquito repellent, and there are now commercially available products safe for use on both horse and rider (be sure to read and follow the label directions, however, before applying any product). Avoid riding or turning horses out at dusk and dawn, and use screens, fans, and fluorescent lights inside the barn to help keep mosquitoes at bay.

Although there are a variety of “chemical-free” mosquito control options (e.g., inviting natural mosquito predators such as bats, swallows, and dragonflies to hunt on your property), researchers attest that currently no scientific evidence supports these practices. Mosquito zappers and traps can actually kill many insect species that eat mosquitoes, so scientists don’t advocate these either. Finally, there are no studies showing that feeding garlic or apple cider vinegar to horses will help repel mosquitoes (and garlic can potentially cause anemia—a decreased number of red blood cells).

If DEET or other natural options seem ho-hum, then consider these “next generation in mosquito control” options currently in various phases of development:

1. The Chemical Invisibility Cloak As highlighted in the June 2, 2011, edition of Nature, University of California, Riverside, scientists have discovered chemicals that can inhibit or disrupt mosquitoes’ abilities to hunt their next blood meal. According to the researchers, one of the most powerful chemicals to attract mosquitoes to animals is carbon dioxide in exhaled air. Certain chemicals can imitate carbon dioxide and can be used as a lure to draw mosquitoes away from a particular area. The researchers also identified a handful of compounds that either stop the mosquitoes’ carbon dioxide sensors from working at all or that overwhelm and inactive them.

“The three classes of CO2 response modifying odors offer powerful instruments for developing new generations of insect repellents and lures, which even in small quantities can interfere with the ability of mosquitoes to seek humans,” wrote the researchers. They believe that in a few years they will be able to produce a cloak of odors that makes exhaled carbon dioxide undetectable.

2. Photonic Fence Currently, inventors at Intellectual Ventures are designing a mosquito-honing laser that can track a female mosquito by the beating of her wings and blast her from the sky before she can get close to an animal.


EEE, WEE, and WNV are almost 100% preventable through appropriate vaccination. The American Association of Equine Practitioners (AAEP) considers immunizations for these three viruses to be core vaccines, defined by the American Veterinary Medical Association as those “that protect from diseases that are endemic to a region, those with potential public health significance, required by law, virulent/highly infectious, and/or those posing a risk of severe disease. Core vaccines have clearly demonstrated efficacy and safety and, thus, exhibit a high enough level of patient benefit and low enough level of risk to justify their use in the majority of patients.” For horse owners, this means unless there is a particular reason (e.g., previous reaction to a vaccine), all horses should be vaccinated against all three viruses according to the AAEP’s guidelines. Despite the vaccines’ widespread availability, each year many horse owners elect not to vaccinate their horses against said viruses. “Failure to vaccinate puts horses at risk for these potentially deadly diseases and results in unnecessary horse deaths,” emphasizes Calisher.

Take-Home Message

Mosquitoes aren’t just annoying while camping; they are like little machines capable of spreading disease to millions of people and horses worldwide. “The equine encephalitides … are important, and they are only a scant few of the many viruses transmitted by mosquitoes that can cause diseases (in both humans and horses),” concludes Calisher.