Demodex Bovis

Also known as

Cattle follicle mite, demodicosis, demodectic mange

Description

Infestation with Demodex mites is common in cattle but most of them do not develop signs of mange.

Three species of Demodex infect cattle: D. bovis, D. ghanensis, and D. tauri. D. bovis is the most common and infests hair follicles of cattle worldwide. Species of Demodex are host specific and not zoonotic (they do not spread to humans).

Demodex mites are unique among parasitic mites; they are very elongated (cigar-shaped) with four pairs of short, stumpy legs, possibly because of their adaptation to living in the hair follicles and sebaceous (oil) glands of their hosts.

All life cycle stages are found on the host and include egg, larvae, two nymph stages, and adults. These mites feed on sebum (oil produced by the sebaceous glands that help keep skin moist and healthy) and old skin cells.

Transmission of mites from one animal to another occurs through close contact of infested and susceptible hosts. The most common route is transfer of mites from infested dams to their young calves.

Cattle of any age are susceptible to demodectic mange, but this disease is more evident in young animals. Most cases are seen in dairy cattle in late winter or early spring.

Infestation with D. bovis is usually subclinical (no visible signs) and infestation may continue for many months.

Lesions consist of follicular papules (tiny, dome-shaped solid bumps) and nodules, especially over the withers, neck, brisket, forearm, back, armpits and flanks, and around the eyelids, but occasionally appear on the udder.

Invasion of the hair follicles by these mites results in chronic inflammation, formation of ulcers, abscesses, and fistulae (tiny drainage tunnels) due to follicular rupture or secondary staphylococcal infection. The lesions are not itchy, in contrast to many other skin diseases.

The tiny lumps (pustules which contain white cheesy pus) can usually be felt under the skin and feel like particles of bird-shot in the hide. In severe cases there may be some hair loss and thickening of the skin in that area, but usually the hair loss is not very noticeable.

In large abscesses the pus is more fluid. These larger lesions are visible, but very small lesions may only be detected by rolling a fold of skin through the fingers.

These mites can damage the hide. Even though the lesions are generally not visible (except in advanced cases), the rupture of follicular abscesses creates small holes in the hide and limit its use. Lesions in hides can be detected as dark spots when a fresh hide is viewed against a strong light source. However, these lesions may not be readily seen until the hair has been removed and the skin has been soaking for some time.

The mites are often found in cattle that have increased stress, such as from pregnancy or lactation. Natural and acquired immunity may reduce the number of mites on a cow, as well as decreasing the severity of signs.

Signs

• Palpable nodules and papules over neck, withers, shoulder, and flank - number and size varies, but most are 0.5 to 1.0 cm in diameter and covered with normal hair
• Crusting
• Folliculitis, drainage, or ulceration in advanced lesions
• Occasional secondary bacterial infection

Cause

Demodectic mange is caused by a microscopic mite, Demodex bovis, which lives in the hair follicles and oil glands in the skin of cattle. These mites are host-specific and spend their entire life cycle on the host animal.

Cattle infested with Demodex bovis commonly develop a papulo-nodular form of demodicosis (irritation and itching caused by a sensitivity to and/or overpopulation of mites) if the host’s immune system is unable to keep them under control. Natural and acquired immunity have a role in reducing mite numbers and clinical signs in infested cattle.

The life cycle of the mite is about 10 to 15 days. Adult female mites deposit eggs in hair follicles; mite populations may build to hundreds or thousands of mites per follicle, causing chronic inflammation. The follicles become dilated and enlarged, to form skin papules, nodules or cysts. As they enlarge, they can be felt beneath the skin even if they are difficult to see. If bacteria enter, they may create abscesses.

The papules and nodules usually last about a month and then are replaced with new ones. Greatest numbers occur during spring and summer, and their size can range from as small as a pinhead to as large as a chicken egg. Large ones are formed when cysts grow in size, forming nodules that block the opening of the follicle.

These large cysts can burst, which causes open, draining sores. The thick, pus-like substance that comes out of these sores has the consistency of toothpaste and contains large numbers of mites that can be spread to other animals.

Some female mites leave the follicular cysts to invade other hair follicles, also spreading the infestation. This is probably when the mites can transferred to other animals by direct contact, such as from cow to calf (when the calf’s body is in contact with the infected cow), or transferred between cattle with body contact during breeding.

Mites and eggs can survive away from the host for only short periods of time. Depending on temperature and humidity, mites may survive for up to 15 days outside the skin.

For an accurate diagnosis, deep skin scrapings or oozing material from lesions can be examined under a microscopic to look for mites. Other causes of folliculitis (inflammation of the follicles) such as dermatophytes (fungi that feed on skin), staphylococci, and Dermatophilus infection (bacterial infection known as rain scald) must be ruled out.

Asymptomatic cases (without clinical signs) also may look like insect bites. The lesions are also sometimes mistaken for urticarial (hives), deep ringworm, staphylococcal dermatitis, filarial dermatitis (caused by tiny parasitic worms) or sporotrichosis (a fungal infection).

Prevention

If animals in a group are infected, some people try to clean contaminated areas and equipment with insecticide spray to kill the mites--to prevent infecting healthy animals. Many formulations of insecticides are available commercially.

Another tactic is to isolate affected animals from susceptible ones. Preventing transmission is difficult, however, because demodectic mites are considered to be normal inhabitants of hair follicles and sebaceous glands of cattle.

Demodectic mite infestation of calves is thought to occur naturally through contact with the dam during the first few days of life. If dairy calves have very little exposure to the dam after birth, this may decrease transmission.

Since most cattle with demodectic mites are asymptomatic, those with clinical signs may have genetic or immunologic defects that allow the host–parasite relationship.

Treatment

Treatment is generally not necessary. Recovery is usually spontaneous. If an animal is severely affected and therapy is needed, the macrocyclic lactones listed for treatment of scabies or chorioptic mange should be considered. These include the Avermectins and Milbemycins: avermectins abamectin, doramectin, eprinomectin, ivermectin (most commonly used), milbemycins, moxidectin, and selamectin.

Organophosphates insecticides used topically include coumaphos, diazinon, dichlorvos, famphur, fenthion, malathion, trichlorfon, stirofos, phosmet, and propetamphos.

Repeated dipping or spraying is more to prevent spread than to cure existing lesions, but ivermectin has been reported to cure 98% of beef bulls when used at proper dosage.

Control of clinical demodectic mange is difficult, and even more so in dairy cattle, with limited insecticides approved for lactating cows. Individual cattle with obvious disease caused by these mites should be assessed for immunologic compromise or genetic predisposition.

Antibiotics such as long-acting oxytetracycline can be used to prevent/treat secondary bacterial infections.