IRWIN H. ROBERTS AND N. G. COBBETT.
SCABIES is a contagious skin disease caused by minute parasitic organisms known as mites. It affects cattle of all ages and breeds. Sometimes it is referred to as scab, mange, or barn itch. Similar infections attack other classes of livestock, wild animals, and birds, as well as people.
Scabies, the medical term for which is acariasis, is common throughout the world. It generally causes a severe inflammation of the skin and itching.
Mites are related to ticks, spiders, and scorpions, and are not true insects. Unlike insects, adult mites have 4 pairs of legs instead of 3. They are wingless and usually are so small that they can barely be seen with the naked eye.
Of the thousands of known kinds of mites, four are commonly parasitic to cattle. Each of the four produces a different type of skin reaction. Two, known as psoroptic and chorioptic mites, live on the surface of the skin and cause a condition generally spoken of as scabies. Two others, the sarcoptic and the demodectic mites, burrow under the surface of the skin and produce mange.
PSOROPTIC SCABIES, also known as common scab, was prevalent in range cattle in the Western States until about 1938. It is caused by Psoroptes equi var. bovis, a whitish mite, which can be seen as a minute speck if it is placed against a dark background. It spends its entire life cycle on the animal. Female mites deposit about 20 eggs on the surface of the skin. The eggs hatch in about 4 days, and minute larvae, each with three pairs of legs, emerge. These molt, or shed their skins, become nymphs, molt again, become adults, reach maturity, and mate, and the females of the new generation begin laying eggs. The entire cycle takes no more than 12 days.
Psoroptic mites attack the hairy parts of the body. They generally begin an infestation over the withers, but sometimes also over the back or around the tailhead. The mites prick the skin to obtain food. Tissue fluids ooze from the wounds. After many mites have fed, the fluids dry, become mixed with tissue debris, and form scabs.
The lesions made by the mites spread as the parasites increase in number and involve large areas of the back and sides. The condition may advance over practically the entire body if it is not checked. As the disease worsens, hair falls out, and the body is covered with thick, rough crusts. The skin becomes hard and thickened and it takes on a corrugated look.
CHORIOPTIC SCABIES occurs chiefly in farm herds. It is widely distributed from the Atlantic coast to the States east of the Rocky Mountains. A persistent disease, it spreads slowly over the infected animal, but may travel from one animal to most of the animals in a small herd in a year.
The mite responsible for it is Chorioptes bovis var. bovis. Like the psoroptic mites, it lives on the surface of the skin. Its life history is similar to that of the psoroptic mites, and it obtains nourishment in the same way.
Chorioptic scabies mites do not usually produce such extensive and prominent lesions as psoroptic mites, and the injury they cause is less severe. This form of scabies usually begins on the inside surface of the hind legs in the fetlock region, high on the rear surface of the udder or scrotum, or on the inside of the flanks and thighs. The infection is known commonly as leg or foot mange.
Chorioptic mites produce an infection by piercing the skin. The serum, or tissue fluid, that exudes from this wound forms a minute blister. The mites multiply, the blisters unite and break, and their dried contents slowly build up into little scabs or crusts, beneath which the skin is raw and bleeds easily.
The mites live in colonies under the scabs. If a scab is removed with the fingers and placed on a dark surface in some warm place, the flesh-colored mites will be seen, just about visible to the naked eye, crawling rapidly away in every direction. A reading glass will help one find the parasites.
The mites from one small lesion spread after a few months to various places about the hindquarters, establishing new colonies. The disease eventually may involve large portions of the underparts, side, and back.
Sometimes the disease progresses so slowly that scabs the size of a half dollar, in various places about the hindquarters, may go unnoticed for a year or two.
In highly susceptible animals, the condition may eventually spread to the underparts of the body, forward along the midline, around the outer surface of the legs, and over the sides and back.
SARCOPTIC MANGE used to be considered of no economic importance in cattle in the United States. By 1950, however, 30 percent of the scabies- infested cattle in one Northeastern State were infested with the sarcoptic variety of the disease, either alone or in combination with chorioptic scabies.
As in chorioptic scabies, sarcoptic mange often is found in purebred herds and may be spread about the country with the sale of breeding stock.
The mite that causes it is Sarcoptes scabiei var. bovis. It spends its entire life cycle on the body of the animal. The mature female makes long channels Within the horny surface of the skin. In the burrows the female deposits her eggs, which are almost as large as she is. The mature female usually stays in the burrow her entire life. The mite begins laying eggs within a few hours after starting her burrow. She deposits the eggs every second or third day for as long as 2 months, stringing the eggs out behind her as she lengthens her burrow. She usually lays about 30 eggs in her lifetime.
The eggs hatch in about 5 days, after which the larvae leave the channels. They move about on the skin, where they molt, become nymphs, and molt again to become adult males or immature females.
Larvae and nymphs can be found in the skin follicles. The adult males and young females make short burrows, in which they remain briefly. Mating takes place on the skin. After mating, the fertilized female burrows into the skin to lay her eggs and start a new generation. The full cycle takes about 14 days.
A female theoretically could have more than a million descendants in six generations, or 90 days.
When huge numbers of mites are present on an animal, they are rather easily found under scabs, which can be removed from the skin with the fingers or a dull knife edge. Their burrowing habits in the early stages make it rather hard to find the mites. However, skin scrapings made with a knife edge, deep enough to draw the blood, will usually disclose the active parasites. They are barely visible, but a reading glass will show them as moving specks if they are placed against a dark background.
Sarcoptic mange mites may produce lesions anywhere on the body of cattle. They appear to adapt themselves best in locations where the skin is thin and tender and the haircoat is thin. They are commonly found high on the rear surface of the udder or scrotum, and on the rear and inner surfaces of the thighs, where the lesions they produce may exist side by side with those caused by chorioptic mites.
Sarcoptic mites may also start infestations at the root of the tail or the lower parts of the neck and brisket. Before many months a large portion of the body surfaces may be involved. Itching is more intense than in other forms of scabies or mange.
Shortly after the onset of the disease, hairless spots appear, dandruff is abundant, and the skin may become thickened, hard, and covered with crusts or scabs. The skin may crack and ooze blood and pus. It may bleed where the scabs become detached. The disease may spread rapidly from one cow to practically every animal in a dairy barn in a winter.
DEMODECTIC, or follicular, mange of cattle is widespread in the United States, but few cattlemen are aware of its existence.
It is caused by the mite Demodex folliculorum bovis, a microscopic, cigar-shaped, sluggish organism that lives within the skin.
The lesions in the skin take the form of nodules, usually in the region of the neck, shoulders, and brisket, and sometimes on other parts of the body. The size of the nodules may vary from that of a match head to that of a hazelnut. The nodules appear to result from the formation of pus, which accompanies the mite. Frequently the nodules are pitted. Sometimes they break and discharge their cheesy-white contents over the surrounding hair.
The mite occasionally can be found in this material, but often one has to lance the nodule and extract the contents. It can be seen only with a microscope. Dairymen sometimes become aware of the swellings or nodules on the neck or brisket, which usually can be felt more easily than they can be seen. Little is known about the life history of this strange, wormlike mite.
THE SEASONAL OCCURRENCE of psoroptic and chorioptic scabies and sarcoptic mange follows a similar pattern. The mites multiply the most rapidly, produce the most severe skin lesions, and cause the greatest annoyance during the fall, winter, and spring. Owners have observed that the lesions appear to clear up and disappear spontaneously when an infested herd is turned out to pasture. Some mites, however, survive the summer, and infestations almost invariably become serious again when the weather gets cool.
Often there is much less change in the status of infestations on animals that remain housed or in close contact with each other during the summer. The presence of oils in the skin, the increased activity of skin glands, and the improved nutritional state of the animals when they are on pasture may be responsible for the diminished activity of the mites in summer.
Demodectic mange shows no response to the change of seasons. The nodules almost invariably discharge their contents or are reabsorbed on the body, regardless of time of year.
THE SPREAD OF SCABIES and mange mites from one animal to another nearly always takes place through direct contact. Infection in a herd usually starts when an animal with scabies lesions too small to be noticed is introduced into a clean herd. Mites spread most rapidly in a herd when the dairy barn, feed lot, or barnyard are crowded. A bull that has mites may infect many animals, even on the range.
Mites parasitic on cattle can live apart from their hosts for varying periods. The length of life varies according to species, humidity, and the temperature. When the weather is damp and cool, the scabies mites may survive up to 3 weeks, but direct sunlight and dryness may destroy them in 48 hours or less.
The eggs may persist on barn walls fence posts, railway cars, and cattle trucks, but it is not very likely that cattle will acquire infection in that way. It is possible, though, that infection may be spread by such objects as currycombs, brushes, and halters.