Изображения страниц
PDF
EPUB

studied and demonstrated from the eggs to the adult parasites. It was shown that mites are always the offspring of ancestors, the same as are the larger animals, and it has in later years come to be admitted that there is no such thing known as spontaneous generation of any living thing under any circumstances. The demonstration was repeatedly made that the disease always developed if mites were taken from diseased sheep and placed upon healthy ones, and that diseases of the skin resembling scab are not contagious unless the mite is present.

Questions are still frequently asked, by persons not conversant with the investigations of the subject, as to whether the scab is the cause of the mite or the mite is the cause of the scab, and also whether the disease can develop without the presence of the scab mite. The investigations which have been referred to answer these questions and also show that the treatment must consist in external applications for the destruction of the parasites and not internal remedies to "purify the blood."

Is scab hereditary?-An impression has arisen among some sheep raisers that scab is hereditary. This impression is, however, erroneous. Scab is no more hereditary than are sheep ticks or sheep lice, for the parasites which cause it live on the external surface of the body and do not reach the womb. It is possible, however, for a lamb to become infected from a scabby mother at the moment of birth or immediately after. Lambs are occasionally born with white spots on their skin, and this possibly has given rise to the idea that scab is hereditary.

LOSSES CAUSED BY SCAB.

Losses in home industry.-The losses from sheep scab have been and are still very severe in most sheep-raising countries. They are due to the shedding of the wool, the loss of condition, and the death of the sheep.

Although laws were made for the control of the disease as early as the beginning of the eleventh century, general ignorance in regard to its nature and proper treatment has prevented the successful administration of such laws even to the present day. The disease exists in most of the countries of Europe, and also in Asia and Africa, and until recently in Australia. Most civilized countries now control the disease to a certain extent and limit the losses by the enforcement of stringent sanitary regulations; but the extent of its prevalence is nevertheless surprising. It is a disease not difficult to cure and eradicate, and an accurate knowledge of its characteristics with attention to details are all that is needed to secure this result.

In the United States some sections have been overrun with sheep scab, and many persons engaged in the sheep industry have been forced to forsake it because of their losses from this disease. It is

probable that in its destruction of invested capital sheep scab is second only to hog cholera among our animal diseases. The large flocks of the Plains and Rocky Mountain region and the feeding stations farther east have suffered severely and are constantly sending diseased animals to the great stock yards of this country. As a consequence of this marketing of affected sheep, the stock yards are continually infected, and any sheep purchased in these markets are, unless properly dipped, likely to develop the disease after they are taken to the country for feeding or breeding. There is in this way a constant distribution of the contagion, and thousands of persons who know little of its nature or the proper methods of curing it find that they have introduced it upon their premises.

Losses in export trade.—In addition to the direct losses in wool, in flesh, and in the lives of our sheep, we have suffered immensely in our foreign trade because of the prevalence of this disease. Great Britain appears to have been the first country to prohibit live sheep coming from the United States, by an order issued in 1879. Upon representations that there was no foot-and-mouth disease in the United States this order was rescinded in 1892, but only to be again enforced in 1896 on account of the many scabby sheep sent abroad by our exporters. Our sheep are consequently slaughtered on the docks where landed, the market being restricted and the prices much less favorable than would otherwise be obtained. The markets of Continental Europe have been entirely closed to American sheep, as even the privilege of slaughtering at the landing places is denied. For a long time it was impossible to send our pure-bred sheep to Australia, where there is a demand for them for breeding purposes, because the Australian law required them to be transshipped and quarantined in British ports, and the British authorities declined to grant this privilege. Arrangements have since been made for the direct shipment of sheep to Australia, if accompanied by the certificate of a veterinarian appointed by the Australian authorities.

On the whole, it is seen that the existence of this disease in our flocks has prevented the development of our export trade in many directions, and has caused no end of trouble and loss to our exporters.

CAUSE OF SCAB.

1

Sheep scab is a strictly contagious disease. Common sort.-Common sheep scab is caused by that species of mites technically known as Psoroptes communis. Parasites of this species cause scab in horses, cattle, sheep, goats, and rabbits; but for each of these species of animals there appears to be a distinct variety

The technical term Psoroptes is derived from the Greek, and means that the mites hide under the crusts. The parasite is sometimes called Dermatocoptes, which means that the mites wound the skin. A third name, Dermatodectes, indicates that the mites bite the skin.

of this parasite.

Although it is more or less difficult to distinguish between these varieties, they differ somewhat in size, and it is found that the Psoroptes communis of the sheep does not cause scab of the horse, ox, or rabbit; nor, on the other hand, does the Psoroptes communis of the horse, ox, or rabbit cause scab of the sheep. Naturalists, therefore, distinguish the parasite of sheep scab by the name Psoroptes communis var. ovis.'

The parasite of this disease is one of the larger mites, and is quite easily seen with the naked eye. The adult female is about one-fortieth

[graphic]

FIG. 19.-A comparatively early case of common scab, showing a bare spot and a tagging of the wool.

inch long and one-sixtieth inch broad; the male is one-fiftieth inch long and one-eightieth inch broad. These mites are discovered more readily and more clearly on a dark than on a light background, and for that reason the crusts from the affected skin are often placed upon black paper and kept in the sunshine for a few minutes in order to reveal the parasites crawling about.

The psoropt inhabits the regions on the surface of the body which are most thickly covered with wool; that is, the back, the sides, the rump, and the shoulders. It is the most serious in its effects upon

1 Var. is the abbreviation of the Latin word varietas, meaning variety.

sheep of any of the parasitic mites, and it is the cause of the true body scab.

Other forms.-Sheep are also affected with three other forms of scab, likewise caused by parasitic mites. One of these is the sarcoptic scab (head scab, or black muzzle), which is limited almost entirely to the head, and is caused by the mite known as the Sarcoptes scabiei var. ovis.' The second is the symbiotic scab (foot scab), which affects the limbs, scrotum, and udder, and is caused by the Chorioptes communis var. ovis. Lastly may be mentioned an extremely rare affection, the so-called follicular, or demodectic, scab, affecting the eyelids, caused by a mite known as Demodex folliculorum var. ovis.3

The sarcoptic, symbiotic, and demodectic forms of scab are with sheep mild diseases compared with common scab, and appear to be rather rare.

DESCRIPTION OF SHEEP SCAB.

(1) COMMON SCAB, BODY SCAB, OR PSOROPTIC SCAB.

Although the symptoms of common scab are familiar to most farmers, they will here be briefly reviewed.

The mites of common, or body, scab-that is, the Psoroptes-prick the skin of the animal to obtain their food, and probably insert a poisonous saliva in the wound. Their bites are followed by intense itching, with irritation, formation of papules, inflammation, exudation of serum, and the formation of crusts, or scabs, under and near the edge of which the parasites live. As the parasites multiply they seek the more healthy parts, spreading from the edges of the scab already formed, thus extending the disease. The sheep are restless; they scratch and bite themselves, and rub against posts, fences, stones, or against other members of the flock. This irritation is particularly noticeable after the animals have been driven, for the itching is more intense when the sheep become heated. The changes in the skin naturally result in a falling of the wool; at first slender "tags" are noticed; the fleece assumes the condition known as "flowering;" it looks tufty or matted, and the sheep pulls out portions with its mouth, or leaves tags on the objects against which it rubs. Scabs fall and are replaced by thicker and more adherent crusts. The skin finally becomes more or less bare, parchment-like, greatly thickened, furrowed, and bleeding in the cracks. With shorn sheep especially a thick, dry, parchment-like crust covers the greatly tumified skin. Ewes may abort or bear weak lambs.

2

Parts of body affected.-When sheep are kept in large numbers the

1Sarcoptes, from the Greek, means that the mites wound the flesh.

? Chorioptes signifies that the mites hide in the skin. Another name, Symbiotes, signifies that a number of the mites live together; and a third name, Dermatophagus, means that the mites eat the skin.

3 Demodex signifies that the mites have a worm-like body.

[merged small][graphic][merged small][merged small][graphic][merged small][merged small]
« ПредыдущаяПродолжить »