Ectoparasites of cattle, sheep and horses

Lecturer: Dr. Donal O'Toole

Ectoparasites and their importance

Ectoparasites are agents that live on or close to the surface of its host - generally, skin.  They are important for two reasons:

1. They are a direct cause of disease and annoyance.  Some cause illness through blood loss (lice), severe dermatitis (mange mites; biting midge dermatitis ("sweet itch")), and economic damage to hides and fleece (keds and cattle grubs).  Others are unsightly and/or complicate existing conditions (maggots in wounds - "fly strike").

2. Arguably more important is their role as vectors (transmitters of diseases).  A listing of important diseases transmitted by insect vectors to food animals and horses in the Rocky Mountain-High Plains area is below.   Ectoparasites are of moderate importance in our area.  In warmer or more temperate parts the country, such as the Gulf Coast where there are long ectoparasite seasons, their importance grows.  As a general rule, ticks are the most versatile vectors, since they parasitize all vertebrate groups except fish, transmit some of the most significant animal pathogens, and survive for up to 3 years, permitting them to assume dual roles as disease vectors and reservoir hosts.  Some important diseases that we deal with, such as vesicular stomatitis, is presumed to be insect-vetored but the exact agent(s) has not been identified.

Ectoparasites of cattle

Lice (pediculosis; eggs = "nits")

Individual species tend to occur in specific parts of the body (face vs. trunk vs. distal limbs).  Transmission is direct (animal-animal contact).  Cattle and sheep have multiple species and lice are highly species-specific.  They most commonly occur in winter on young stock.  They are not transmissible to people and in animals they are generally not important as disease vectors (in people however they transmit typhus).  Lice infestations cause pruritis (= itching) and dermal irritation with scratching, rubbing, and biting of infested areas. This causes an unthrifty appearance and a rough coat.   In severe infestations, there may be loss of hair and local scarification. Extreme infestation with sucking lice causes anemia - this is rare. In sheep and goats, rubbing and scratching results in broken fibers, which gives the fleece a "pulled" appearance.  Control: Dips, sprays and pour on insecticides are effective  Usually two treatments two weeks apart effectively controls the problem.

MANGE

Mange is due to mites that  cause a severe pruritic dermatitis usually in winter.   Mites live on the skin's surface (Psoroptes sp. and Chorioptes sp.) or in tunnels within skin (Sarcoptes sp.  They are USDA-reportable diseases.  They can be treated by spray- or vat-dipping, or with ivermectin.  The mange mite, Sarcoptes scabiei, causes a highly contagious disease that is transmissible to people.  Psoroptes ovis causes mange in range and feedlot beef cattle in central and western states, with the largest numbers of outbreaks reported in TX, NM, NM, OK, KS, CO and NE.  It does not spread to people.  A third form of mange affects the legs ("leg mange"), especially the skin of the pastern; this is the most common form of mange in cattle in the US.

Cattle grubs (adults = heel flies; grubs in skin = warbles)

Two cattle grub species spend part of their life cycle in cattle.  Both are declining in prevalence in the US and have been eradicated in the UK. In late spring or early summer, heel flies attach eggs to the hair of cattle, particularly on the legs and lower body regions. Eggs hatch after several days and first-stage larvae penetrate the skin. During fall and winter, larvae migrate to and remain in two sites, depending on species: one goes to the wall of the gullet (esophagus), and the other to fat surrounding the spinal cord in the vertebral canal.  Damage is done when grubs eventually leave cattle, which they do by boring holes in the hide of the dorsal midline as they emerge in the spring (March-April in our area).  They are important because the adult flies cause gadding in cattle, and grubs damage hides and occasionally die in situ, causing inflammation.  Systemic insecticides, in various formulations, are available for treatment.

Screwworm

This is due to the maggot of the primary screwworm fly.  There are two types of screwworms fly, one species in the western hemisphere and several in the eastern; the New World screwworm fly exults in the name Cochliomyia hominivorax.  Flies lay eggs in fresh wounds, including the umbilicus of newborn calves, dehorning and castration wounds, which become greatly enlarged due to multiple infestation. They can also infest the oral cavity (gums).   They are most common in livestock and wildlife, but can infest people - one unlucky rancher reported expelling 385 larvae from his nostrils during a 9-day period; I'd like to think that after the first one came out my nose, I'd stop counting and see a physician.  Screwworms feed on live tissue, unlike the maggots of house flies and blow flies, which survive on dead or devitalized tissue. Larvae penetrate deeply into the wounds warm-blooded animals; often they will not be seen on the surface. Unless treated, infection can result in death. The disease was eradicated in the United States in 1966 and is now eradicated in Mexico, but pockets persist in Central and Southern America.  Imported cattle, horses, dogs and even one hapless paratrooper who got a head wound after a jump in Panama occasionally enter the country quarantine stations with screwworms.  As recently as In June 1997, a Rottweiler imported from Panama into Utah was found with screwworm myiasis.  Control has been achieved by releasing irradiated (sterile) flies that mate with native flies so that no eggs result (these are released after accidental importations).  This works because females mate once in their life, whereas males mate promiscuously.  There have been several inadvertent releases of non-sterile screwworm flies in Mexico and Panama, most recently in 2003.  It is estimated that if this gets back into the US it would cost the livestock industry $750 million (2002 USDA APHIS estimate).  In the unlikely event you see a peculiarly aggressive form of myiasis and wonder whether it might be screwworm, submit the maggots to a laboratory for identification which can be done on physical characteristics, or by a genetic test (PCR).  Some of the avermectin compounds are effective at killing the parasite.  There is a interesting review of screwworms in a recent issue of JAVMA.

Ectoparasites of sheep

Sheep scab (Sarcoptes scabei)

Sarcoptes scabiei ovis is an historically important disease that is now rare in sheep.  It is a USDA-reportable condition. It affects skin, starting on the head and face.  The parasite burrows through the superficial part of the skin, form tunnels and causing marked inflammation. It used to be common in Wyoming flocks at the turns of the 1900s when the state was the leading wool producer.  Reports by the Wyoming state veterinarian had many accounts of outbreaks and attempts to control them.

Lice

Sheep and goat have both blood-sucking and chewing lice that cause dermatitis and damage the fleece.  They tend to be more prevalent in winter-spring, and in colder climates.  Treatment is with insecticides.

Keds (Melophagus ovinus)

The sheep ked is one of the most widely distributed external parasites of sheep. It is common in Wyoming flocks, particularly in winter - shearing in the summer markedly reduces the problem during warm month.  Keds are wingless flies that spend their life in the fleece.  They are not transmissible to people.  Adults are brown, covered with short hairs, and have legs armed with stout claws.  To feed, keds pierce the skin with mouthparts and suck blood. They feed on the neck, breast, shoulder, flanks, and rump. Bites cause pruritis over much of the sheep's body, causing them to bite, scratch, and rub, damaging the wool. The fleece becomes thin, ragged, and dirty. Keds' excrement causes permanent discoloration of the fleece. Infested sheep, particularly lambs and pregnant ewes, may loose vitality and become unthrifty. Heavy infestations can cause anemia.  Keds do not transmit any major diseases, although they do transmit an interesting non--pathogenic trypanosome (trypanosomes cause sleeping sickness in Africa).  Control:  Shearing removes pupae and adults. Shearing before lambing and treatment of ewes with insecticides to control remaining keds greatly reduces the possibility that lambs will be heavily infested.  Best results are obtained when an insecticide with residual activity of ≥3 - 4 weeks is used. This ensures that keds emerging from pupae are killed.  Dipping, spraying, jetting, powder and spot-on insecticides work well

Maggots (fly strike; myiasis)

The larvae (= maggots) of house flies, blow flies and flesh flies can mature in the subcutaneous tissues of the skin or organs of multiple domestic species, particularly sheep and rabbits, producing myiasis.  The most common type of myiasis is when maggots that normally develop in feces or carcasses are found in devitalized tissue or wounds.  Eggs are laid and hatch within 24 hours when conditions are right (moist + warm). Moisture and nutrients from serum and/or feces are necessary for larvae to survive.  Mild strikes cause loss of condition.  Severe strike can be fatal. The behavior of sheep is a good index of myiasis: they are depressed and stand with their heads down, do not feed, and attempt to bite infested areas. A common site is the breech (perineum), where flies are attracted to urine- or feces-scalded skin. Maggots irritate, injure, and kill successive layers of skin and produce exudate (pus). Maggots tunnel through damaged skin and form cavities underneath. Unless the process is halted by therapy, infested sheep die from shock, intoxication and secondary bacterial infections. Advanced lesions contain thousands of maggots and the total lesion may be much more extensive at necropsy than is evident from looking at the surface of the skin.  Control:  Blowfly infestation of the breech are effectively controlled for ~6 - 8 weeks by “tagging” or “crutching” (wool shorn between legs and around tail). Complete shearing helps to control outbreaks involving other parts of the body, since early strike will be noticed early and the absence of wool means a less hospitable (drier) environment for maggots.  Wool removed from around the head and the prepuce prevents strike in these areas. Fecal contamination can be reduced by docking tails.  Scouring should be controlled, since the damp and fecal bacteria compromise the skin of the perineum.. Odors and associated moisture attract flies, particularly in hot, humid weather.  Maggots can be killed by saturating fly strike tissues with suitable insecticidal and larvicidal preparations, such as organophosphates.  The main source of maggots for flock mates is the struck sheep.  If secondary bacterial infections are present, broad-spectrum antibiotics need to be given.  Sheep with fresh skin wounds should be held in fly-free areas, if available. 

Ticks

Ticks are large mites (arachnids).  All ~850 species are exclusively blood-sucking in all feeding stages.  Ticks are important because they vector a large number of infectious diseases, and can cause some disease themselves.  Among the diseases they transmit or induce are anaplasmosis (see other parasitology notes), tick-borne fever, tick-paralysis, and foothills abortion.  When we had the large die off of elk (~400) in the Red Rim area west of Rawlins in Jan-Feb 2004, an important rule-out was tick-paralysis.  There are no effective assays for the disease and twe excluded by collecting ticks from downer elk and putting them on sheep.  No paralysis occurred so tick paralysis was not the basis for the problem; it turned out to be lichen poisoning.  Tick paralysis is poorly characterized but assumed to be due to a neurotoxin formed by the tick and injected into hosts.  Epizootic bovine abortion (EBA) is a tick-borne disease that occurs in California, Nevada and Oregon; we have not recognized it in Wyoming. The bacterial agent causing abortion has not been identified but it is spread by a specific tick species that is consistently found in areas where abortion occurs.  Tularemia (rabbit fever) is a rare but - due to its human health risks - important cause of abortion in sheep in WY, MT and ID.  We've seen several large scale outbreaks in sheep.  We've occasionally seen rabbit fever in horses and cattle.  The Rocky Mountain wood tick is responsible for transmission in our area.

Ectoparasites of horses

Flies

Flies transmit various diseases, as well as cause annoyance, hypersensitivity reactions, and myiasis.  Horse and deer flies (tabanid flies) are painful biters and blood feeders, as well as a primary vector for equine infectious anemia (EIA).  Culicoides (biting midges) are blood suckers and the cause of sweet itch hypersensitivity.  Mosquitoes are also blood feeders, and transmit several important insect-borne viruses (= arboviruses).  House, stable and deer flies are nuisance species.

Summer sores (cutaneous habronemiasis)

As the name suggests, this is a seasonal skin disease due to larvae of three nematode species that are laid in the "wrong" location (near the eye, prepuce and in wounds).  The worms (Habronema spp.) have an indirect life cycle involving flies and horses.  The adult worms live in the horses stomach where they do minimal harm. Flies become infected from feces.  The parasites develop in house and stable flies. The normal life cycle is completed when infective larvae are deposited by flies around the horse's lips, and are then swallowed.   Cutaneous habronemiasis is a form of aberrant parasitism in which larvae gain access to deeper layers of the skin via wounds.  The disease is seasonal, first appearing in spring and, in most cases, regressing in winter. The occurrence of the disease is sporadic, only a few horses in a given area are affected, supporting the hypothesis that it is a hypersensitivity.  Once the disease develops it recurs each succeeding summer unless preventative measures are taken. The disease can be controlled by fly control and wound care.  Ivermectin can be used to kill newly deposited larvae.

Sweet itch (cutaneous hypersensitivity to Culicoides spp.)

Biting midges, particularly species of the genus Culicoides, are incriminated in the transmission of multiple viral, protozoal, and filarial agents.  The USDA's Arthropod-Borne Animal Disease Research Laboratory on campus and beside the Wyoming State Veterinary Laboratory is focused on diseases caused by biting midges.   Only female Culicoides feed, usually in early evening and early morning hours. Culicoides are active during hot, humid, still environmental conditions - they are not good flyers.  Sweet itch is a common skin disease of horses throughout the world and is known by various names.   It is an allergic reaction to the bites of Culicoides, and usually only one horse in a group will be affected.  It is rare or nonexistent in horses under one year of age. The age of onset is 2 - 4 years   Initially the disease is seasonal and appears in spring, worsens in summer, and regresses in fall.   The disease is recurrent and with each succeeding year clinical signs tend to become more severe and prolonged. The disease eventually may be present year round, especially in temperate climates.   Two major patterns of disease distribution occur.  One is a dorsal pattern primarily affecting the mane and tail and the other involves the ventral body surface. Additional patterns of distribution have been documented.  Different species of Culicoides are responsible for specific patterns. Control is by stabling sensitized horses from dusk till  midmorning and by periodic 'painting' surfaces with insecticides that have good residual activity.

Mosquitoes are important vectors of WNV, EEE and WEE, which are discussed elsewhere in this course.

 

Insecticidal compounds

Dr. Donal O'Toole

Last updated: 01/29/2010