Ectoparasites, most of which are arthropods, are those parasites that live on the body of the host. Some spend their entire life on the host, others spend only parts of their life on the host, while still others only occasionally visit the host. The ectoparasites most veterinarians deal with direcdy are those that live all or much of their life on the host. Those that are periodic visitors to the host are not usually on the animals when presented to the practitioner. Although called upon to treat the effects of these periodic parasites, most veterinarians are not direcdy involved in their control. Consequendy, this chapter focuses on those parasites spending all or much of their life on the host with only brief mention of the periodic parasites.

Classification of the Arthropods

Kingdom: Animalia

Phylum: Arthropoda

Class: Arachnida (scorpions, spiders, mites, and ticks)

Order: Acarina (mites and ticks)

Class: Insecta (insects)

Order: Mallophaga (chewing or biting lice)

Anoplura (sucking lice)

Siphonaptera (fleas)

Diptera (two-winged flies)


• Intermediate hosts for various parasites.

• Vectors for bacteria, viruses, and other pathogens.

• Direct causal agents of disease.

• Produce venoms that may be toxic.


• Arachnids: body divided into two parts (cephalothorax [fusion of head and thorax] and abdomen) or completely fused; adults with four pairs of legs; antennae absent; wingless.

• Insects: body divided into three parts (head, thorax, abdomen); adults with three pairs of legs; antennae present; wings present or absent.





Myiasis-Producing Flies

Keds (Insecta)

This is a group of dorso-ventrally flattened insects that may or may not have wings. Although there are numerous species of keds in North America, only one is generally encountered in veterinary medicine.

Melophagus ovinus (Sheep Ked)

• Entire life cycle spent on sheep or goats; transmission is by direct contact although fed females may live up to a week off the host.

• No eggs present on host; larvae retained in abdomen until ready to pupate; time until adult emergence depends on ambient temperature.

• Most numerous in cold months (fall, winter) with fewer present during warm months; more prevalent in northern United States and Canada.

• Feed on blood which may cause anemia; bites are also pruritic leading to biting, scratching, and rubbing which damages wool; ked feces stains wool, decreasing value.

• Coumaphos, malathion, and other insecticides are effective against this parasite.

Biting Gnats and Mosquitoes (Insecta)

These dipteran insects are periodic parasites; that is, the only role vertebrate hosts have in the insects’ life cycle is as a food source for the adult females. However, these insects have a primary role as biological vectors of various disease-causing agents. Because the insects are only periodic parasites, they are usually not found on the animals.

Life Cycle

• Complex metamorphosis with egg, up to five larval stages, pupa, and adults.

• Separate sexes with adult females laying eggs in aquatic or semi-aquatic habitats.

• Larvae hatch from eggs; final larval stage pupates; adults emerge.

• Adult females need bloodmeal for egg development; males do not feed on blood.

Simulium spp. (Black Flies; Buffalo Gnats)

• Tiny flies (1-6 mm long) that tend to swarm; require well-aerated water for eggs; limits geographic distribution to areas of swiftly running water.

• Serrated, scissor-like mouthparts; lacerates tissue to form a pool of blood; bites very painful; ears, neck, and abdomen are favored feeding sites.

• Swarming and biting can cause annoyance, resulting in decreased production in livestock.

• Transmit Leucocytozoon spp. (hemoparasites of birds), Onchocerca gutterosa.

Culicoides spp. (No-See-Ums, Biting Midges)

• Tiny gnats (1-3 mm long); habitat requirements vary with species; strong fliers that tend to remain close to breeding grounds; active at dusk or dawn.

• Bites are very painful; favored feeding sites are either on dorsal or ventral aspect of host, depending on species involved.

• Bites cause annoyance.

• Allergic dermatitis in horses; begins as discrete papules on dorsum; areas of alopecia form as hair mats, crusts, then falls off; intensely pruritic leading to scratching and rolling behavior which may lead to injury or secondary infection.

• Transmit bluetongue virus, Onchocerca cervicalis, Haemoproteus, and Leucocytozoon spp. (hemoparasites of birds).

Anopheles spp., Aedes spp., Culex spp. (Mosquitoes)

• Small flies (3-6 mm long) that tend to swarm; lay eggs on water or in dry places that flood periodically; complete entire life cycle in as little as 1-2 weeks.

• Have piercing-sucking mouthparts; pierces blood vessel and feeds from the lumen; bites can be painful.

• Swarming and biting can cause annoyance leading to decreased production in livestock; rarely causes anemia.

• Transmit eastern and western equine encephalitis, Plasmodium species (malaria), Dirofilaria immitis.

Treatment and Control

Because these pests are not found on the host except when feeding, insecticidal treatment is ineffective unless repeated every few days; this becomes too expensive and impractical. Consequently, control is aimed at killing pre-adult stages.

Horse Flies, Deer Flies (Insecta)

Like mosquitoes and biting gnats, horse flies and deer flies are periodic parasites in which only the adult females feed on blood.

Life Cycle

• Complex metamorphosis with egg, larval stages, pupa, and adults.

• Separate sexes with adult females laying eggs in aquatic or semi-aquatic habitats.

• Larvae hatch from eggs, drop into the water or mud; first- and second-stage larvae do not feed; later stages feed on insect larvae, snails, young frogs, organic matter, etc.; in temperate regions, larvae may overwinter and pupate the following spring.

• Pupae are found in dry soil; adults are active only during warmer months in temperate regions.

• Adult females need blood for egg development; interrupted feeders — feeds several times in multiple sites on one or more hosts until replete; preferred feeding sites are ventral abdomen, legs, neck, withers; prefers feeding on larger animals.

Genera Involved

Tabanus (horse flies)

Chrysops (deer flies)


• Large flies (up to 3.5 cm with horse flies being bigger than deer flies).

• Bite very painful; scissor-like mouthparts lacerate tissue to form pool of blood; bite causes restlessness, annoyance, avoidance behavior, which interferes with grazing and resting resulting in decreased production.

• Mechanical vectors of anaplasmosis, anthrax, equine infectious anemia virus.

Tabanus spp. are intermediate hosts for Elaeophora schneideri (arterial worm of deer, elk, sheep).

Treatment and Control

Difficult to kill or repel. Flies rarely enter roofed areas so stabling during hours of peak fly activity helps. Keeping animals inside a fence 2.4 m in height with a 0.6 cm mesh helps reduce the attack rate. Keep grazing animals away from the edge of wooded areas also helps reduce the attack rate.

Stable Flies, Horn Flies, Face Flies (Insecta)

These flies are periodic parasites with different feeding habits. Both male and female stable and horn flies feed on blood whereas only female face flies feed on mucus, saliva, and tears.

Life Cycle

• Complex metamorphosis with eggs, larvae, pupae, and adults.

• Separate sexes with adult females laying eggs in decaying organic matter (stable fly) or fresh cow manure (horn fly, face fly).

• Larvae hatch from eggs; final larval stage pupates; adults emerge.

Stomoxys calcitrans (Stable Fly)

• Flies similar in size to house fly (6-7 mm long); piercing-sucking mouthparts.

• Distributed primarily in central and southeastern United States.

• Horse preferred host, but will feed on most domestic animals and humans; feed 1-2 times per day depending on ambient temperature; feed primarily on legs and flanks of cattle and horses, ears of dogs, ankles of humans.

• Bites are painful; annoyance can lead to decreased production in livestock.

• Mechanical vectors of anthrax and equine infectious anemia virus; intermediate host for Habronema microstoma (stomach nematode of horses).

Haematobia irritans (Horn Fly)

• Dark-colored, small flies (3-6 mm long); piercing-sucking mouthparts.

• Distributed throughout North America.

• Feed on cattle; rarely on horses, sheep, dogs.

• Adults spend most of life on host, leaving only to lay eggs; adults cluster on shoulders, back, and sides; if ambient temperature < 70°F, cluster around base of horns; if quite hot, cluster on ventral abdomen.

• Irritation associated with feeding activities results in lost beef and/or dairy production; of all blood-sucking flies in the United States, this fly is most responsible for reduced weight gains and milk production.

• Intermediate host for Stephanofilaria stilesi (filarid nematode of cattle).

• Can cause a focal, midline dermatitis in horses.

Musca autumnalis (Face Fly)

• Medium-sized flies (about 6 mm long); sponging mouthparts.

• Generally distributed in North America except for the southwestern United States; adults will hibernate in large groups inside buildings.

• Feed on all types of livestock, horses, and bison.

• Flies’ feeding activity is irritating to the host; can lead to decreased production.

• Mechanical vectors of infectious keratoconjunctivitis (pinkeye).

• Intermediate host for Thelazia spp. (eyeworms) of cattle.

Treatment and Control

Treatment with insecticides is possible; read the label and follow directions carefully. Insecticidal ear tags can be effective fly control aids. Pour-on avermectins are effective against horn flies