Deer Parasites

Deer Parasites


Although internal and external parasites are common in most deer populations, they only become a problem when deer are under stress for other reasons. At such times, parasite levels may increase and start to affect reproduction or survival or even kill the host animals—deer in this case. Parasites are not a major source of mortality, but they can be signals to other stresses in the population like over-abundance and nutritional stress. For these reasons it’s important to know the more important parasites common to whitetails.


Nasal bot fly larvae

Nasal bot fly larvae are one of the most commonly encountered parasites in deer, with infection rates varying widely. A survey of Texas deer in the 1960s found them in 76 out of 446 (17 percent) deer. Adult bot flies lay eggs on the wet skin around the nose or mouth of deer. The eggs hatch and small larvae migrate into the moist nasal passages and molt into a fully developed tan or yellowish grub nearly one inch long. The larvae drop out of the nose or mouth and complete their development into adult flies on the ground. The presence of nasal bots generally does not affect the survival of the deer, but can be a source of irritation if they are present in high numbers. These nasal bots are harmless to humans, but usually cause concern among hunters when they are discovered in the nasal passages or throats of deer.


Although rarely reported in deer, several varieties of mites cause related, but different illnesses. Some types of mites infest the ears of deer and produce a waxy crust in the ear canal, called “scabies.” Most deer do not appear sick, but a secondary bacterial infection of the inner ear can accompany heavy infestations, causing uncoordinated body movements. Heavy accumulations of crust in the ear canal may also affect the animal’s ability to hear predators or approaching vehicles.

Other mites cause mange by burrowing into, or feeding on, the skin or hair follicles, causing intense irritation and excessive scratching. The intense scratching causes large areas of hair to fall out and the underlying skin becomes thick and crusty (dermatitis). Mange and ear mites may reduce survival of individual deer, but neither plays a major role in population fluctuations. Although extensive mange is unsightly, there are no health risks reported to humans. Each host species usually carries a different type of mite that is host-specific and seldom infests a different species. Since the mites reside in the skin, the underlying meat is not affected.


Most people recognize ticks because they’re so common. Some ticks complete their life cycle with only one host, while others may require three years and three different host animals. For example, the winter tick develops from the larval stage to adulthood on the same host. Then the adult female mates, takes a large blood meal, drops off, lays her eggs, and dies. The larval ticks then lie dormant through the summer and find a host of their own in the fall. Other species use small mammals or reptiles as intermediate hosts when they are developing through three life stages (larvae, nymph, adult).

Ticks usually attach themselves in or behind the ears, along the neck, back, chest, and the anal area. Infestations are typically light and rarely cause significant problems. Severe tick infestations can reduce the survival of individual deer by causing weakness or even paralysis and further complications due to bacterial infection at tick attachment sites. There is no human health hazard in consuming deer with ticks; however, tick bites in some parts of the country can transmit diseases to humans such as Lyme disease, tick paralysis, and Rocky Mountain spotted fever.

Louse flies

Louse flies, sometimes called “keds,” are harmless to deer and humans. They are noticed by deer hunters and mistaken for ticks. But, louse flies differ from ticks because they have only six legs rather than eight. Adult forms of louse flies hatch on the ground and the winged hatchlings fly off in search of a host. When a suitable host is located, the louse fly lands on it and shortly thereafter its wings fall off. This parasite lives and breeds on the deer producing a single egg at a time, which matures within the female. The female then pushes the egg out and it falls to the ground and hatches.

Fleas and lice

Fleas are sometimes noticed on deer, but they do not cause outright mortality. Native lice are normally merely a source of minor irritation to deer. Like most parasites, the species of lice that occur on deer will not readily survive on humans. Two kinds of exotic lice seriously affected black-tailed and mule deer beginning in the mid-1990s in Washington and Oregon. Since the cause was originally unknown, it was named “hair loss syndrome” because deer chewed and scratched so much their hair fell out in patches. There was evidence of reduced fawn recruitment and higher mortality in the most-affected areas. In recent years, this syndrome seems to be affecting fewer deer. A more recent survey of parasites has now detected one of these exotic lice in many other areas such as South Dakota, California, Idaho, Nebraska, Wyoming, and Vancouver Island. For hunters, there are no known health risks from eating the meat, but as always, cook your venison completely.


Arterial worm

A disease sometimes called “sorehead” or “clear-eyed blindness” is caused by Elaeophora schneideri, a small, white, round, blood worm about two to five inches long that is usually found inside the carotid arteries of the neck. Adult worms residing there produce microscopic offspring called microfilariae that are carried in the blood to the small blood vessels in the forehead skin. When a horsefly bites a deer, they pick up some of the microfilariae, which then develop into larvae while in the fly. The larvae then find their way to the fly’s mouth and, with the next fly bite, are transferred to another deer.

Like many diseases and parasites, Elaeophora infections seem to only kill or debilitate animals that are not the natural host, such as whitetails, domestic sheep, bighorn sheep, elk, and some exotic deer species. When unnatural hosts are infected, the worms block main arteries in the neck and head, causing food impaction, malformed ears, muzzle, antlers, and sometimes blindness.

Stomach worms

Most deer are parasitized by at least one of several species of round worms inside the digestive tract, but alone these do not cause sickness or death. Sickness due to a heavy infestation is most frequent in young deer—less than 1 year—that show a loss of body condition, rough fur, and diarrhea. The adult worms produce eggs in the stomach that are deposited on the ground when the deer defecates. The eggs hatch into larvae that molt a few times before being accidentally eaten by another deer and the cycle begins again. This parasite is not a significant source of mortality except in conjunction with malnutrition on over-populated deer ranges.

Abdominal worm

The adult abdominal worm is a relatively long—five to 10 inches—white, thread-like worm that is sometimes noticed when hunters field dress their deer. This worm differs from stomach worms because these are found on the outside of the stomach and not attached to any organs. Adult worms produce tiny larvae that enter the blood stream and circulate through the deer’s body. Worms are spread to other animals when an insect bites infected deer and takes in these larvae along with a blood meal. When present, deer are most likely to average one to five adult worms. It presents no serious harm to the deer and poses no health hazard to humans handling or consuming the meat.


Lungworms are white or reddish brown round worms less than 2 1/2 inches long that infect the windpipe and lungs of deer. Heavy worm infestations can block airways and cause patchy pneumonia that shows up as dark red or grayish firm areas in the lungs. Adult worms produce eggs that hatch in the lungs; the larvae make their way up the windpipe and are swallowed and passed in the deer’s feces. With sufficient moisture, they molt on the ground into an infectious stage and are then consumed accidentally by other deer. The larvae then migrate to the lungs and develop into adults in the new deer host.


Deer are intermediate hosts, or “middle men” for several different types of larval and adult tapeworms. Deer accidentally eat tapeworm eggs, which hatch in the small intestine and either develop into an adult tapeworm or remain as a larvae and migrate through the intestine wall and into the body cavity. The tapeworm larvae appear as fluid-filled sacs in the liver or other internal organs. These cysts do not develop into tapeworms in deer, but instead reside in the deer until eaten by a carnivore, such as a coyote.

The cysts then develop into adult tapeworms in the coyotes’ small intestine and may reach 13 feet in length. Larval and adult tapeworms are not harmful to their host, nor a human health hazard for those eating properly cooked venison.

44 Tape worm larvae
Meningeal worm

Whitetails are the normal host for meningeal—or brain worms—and not seriously affected; but this parasite is fatal to other members of the deer family. The life cycle is complex involving snails as intermediate hosts, and because of that it is more prevalent in areas of moist soil conditions and high deer densities. Symptoms include weakness, lameness, disorientation, and loss of fear of humans.

Foot, leg and eye worm

The foot or leg worm is a very common, but apparently harmless, round worm. This parasite is 2-10 inches long and thread-like, residing mostly under the skin of the lower legs, but can be found elsewhere such as the brisket, neck, and shoulder. Hunters while skinning deer sometimes notice it coiled or extended in connective tissue. Like Elaeophora, foot worms produce tiny microfilariae larvae that are found under the skin (especially of the ears) and are spread by biting flies. Although common, they often go unnoticed because they are mostly found in the skin of the legs.

Eye worms are commonly found in many different species including white-tailed deer. These parasites are small round worms that live in the eye or associated tissue and feed on eye fluid. As irritating as they may sound, they seem to cause no apparent problems for deer.

Liver flukes

The 1-3 inch liver fluke is an internal parasite that is more common in wetter parts of the country. The liver fluke has a very complex life cycle, but basically the adult flukes reside in the liver encapsulated in a cyst. They mature and lay eggs that then travel into the bile ducts of the host animal and are shed in feces. The eggs then infect a particular species of snail where they develop further and are eventually deposited onto vegetation where a new deer host picks them up to complete the cycle. They do not affect the quality of the venison and may not even be noticed by hunters.