Screw worms (fig. 4) are the maggots of a fly (_Chrysomyia macellaria_), so called from their fancied resemblance to a screw. The adult fly (fig. 5) is about one-third of an inch long, with a bluish-green body, red eyes, and with three dark longitudinal stripes on the back (thorax). Attracted by odors of decay, it deposits its eggs, 300 to 400 at a time, in cuts, sores, castration wounds, etc. The crushing of a tick on the skin commonly results in screw-worm infection at that point. The eggs hatch in a few hours and the larvae or maggots, or so-called screw worms, begin to burrow into the flesh and continue burrowing and feeding from three to six days, after which they leave the wound and crawl into the earth, there transforming into the quiescent pupal stage. This stage is completed in three to fourteen days. The mature flies then emerge from the pupal envelope and are soon ready for egg laying. From two to three weeks are therefore required for the entire life cycle, although under certain conditions it is possible for the fly to undergo its full development in as short a time as seven days, and on the other hand as long as a month is often required.

Besides cattle, the screw-worm fly attacks sheep, horses, dogs, and man. In the case of hogs it is generally the ears which are affected. The fly also breeds in dead animals, and all carca.s.ses should therefore be buried deeply or burned. The complete destruction of all dead animals by burning has been found by the Bureau of Entomology to be by far the best method of controlling screw worms.

_Treatment for screw worms._--For proper treatment an animal suffering from screw worms should be caught and thrown. Chloroform is then poured into the wound, taking care that it penetrates thoroughly into all the burrows of the screw worm, if necessary using a slender stick or a small bunch of twisted hay as a probe. The animal should be held for several minutes in order to insure the continued action of chloroform. Instead of chloroform, gasoline may be used, and carbon tetrachlorid is said by some authorities to give good results. Finally, the dead or dying maggots may be removed with forceps, the wound washed with a weak carbolic or cresylic acid solution, and painted with pine tar to reduce the chances of further attack by flies. Finally the wound should be dressed with a carbolic or cresylic ointment to promote healing and thus prevent further infection, or the wound may be painted with pine tar. Dipping in the a.r.s.enical dips used for destroying cattle ticks is a convenient method of treatment if many animals are involved.

[Ill.u.s.tration: FIG. 4.--Screw worm (larva of _Chrysomyia macellaria_). Enlarged. (From Bureau of Entomology.)]

[Ill.u.s.tration: FIG. 5.--Screw-worm fly (_Chrysomyia macellaria_).

Enlarged. (From Bureau of Entomology.)]

GRUBS, WARBLES, BOTS.

Ox warbles are whitish or, when full grown, dark-colored grubs or maggots that develop from the eggs deposited on the hairs of cattle by certain flies known as warble flies. In the United States there are two species of ox-warble flies, technically known as _Hypoderma lineatum_ and _Hypoderma bovis._ These flies somewhat resemble bees in their general appearance, but like all flies have only two wings.

The first named, _H. lineatum_, is commonly called the heel fly and is more generally distributed over the United States than the other species. The tail has a distinctive reddish-orange color and the legs are rough and hairy. This fly commonly deposits its eggs about the coronet, whence the name of heel fly, and on the fetlocks, knees, and hocks. When cattle are resting, eggs are deposited along the line of contact of the body with the soil. Cattle are frequently indifferent to the activity of this fly in depositing its eggs. Commonly 8 to 10 eggs, sometimes as many as 14, are attached to a single hair.

In the United States the other warble fly, _H. bovis_, has been found only in the North (New England, New York, Pennsylvania, Maryland, Michigan, Iowa, Missouri, and Washington) and has not yet been found in the Southern States. The tail is orange-yellow, lighter in color than that of the other species, and the legs have but few hairs. This fly commonly deposits its eggs on the outside of the hind quarters and above the fetlocks when the animals are moving, or lower down if they are quiet. Cattle are usually much disturbed by the activity of this fly and not infrequently appear terror stricken. The eggs are attached singly, one egg to a hair near its base.

The eggs of the warble flies hatch as a rule in about a week, the time varying with local conditions. The young warble is about 1 mm. (1/25 inch) long. It crawls to the base of the hair and burrows into the hair follicle.

The entrance of the larvae frequently causes sudden appearance of swellings.

The larvae of _H. bovis_ in entering the skin rarely cause a flow of serum or pus formation, but those of _H. lineatum_ commonly cause a considerable flow of serum with pus formation resulting in a matting of the hair on affected areas of the skin. The warbles after penetrating the skin migrate through the body and ultimately reach the backs of the cattle, _H.

lineatum_ usually appearing there in northern lat.i.tudes about the middle of December and _H. bovis_ about a month later. During their migrations, before they reach the back, the young warbles spend a considerable period in the walls of the esophagus, or gullet, and may be found in this location as early as August 15. During the fall and winter a large proportion of the gullets of cattle that are slaughtered are found to be infested with warbles, and are known as grubby gullets or weasands in the packing houses.

When the warbles first appear in the back they are about 3/5 inch long.

They cause swellings about the size of pigeons' eggs, each swelling having a small hole in the center, which has been punctured in the skin by the warble to enable it to breathe. Through this hole the warble leaves the back of the cow when it has completed its parasitic stage of development, at which time it measures nearly an inch in length.

The full-grown larvae begin to leave the backs of the cattle early in the year, and in northern lat.i.tudes the last leave before the middle of July.

After leaving the backs of the cattle they burrow into the ground, lie quiescent for about a month, and become transformed into mature flies. In northern lat.i.tudes the mature flies of the species _H. lineatum_ may be observed during April and May, and those of the other species during June and July.

The damage caused by warbles includes injury to stampeding cattle frightened by the fly, decreased milk flow and diminished growth in infested animals, and injury to hides, the last item being especially serious.

_Treatment._--The best method of control known at present is to extract the warbles from the backs of cattle and kill them. If they are almost ready to leave the cattle, they may be squeezed from the backs with the fingers.

Forceps are useful in removing the warbles, but it is important to be careful in extracting warbles not to crush them, as the body juices of these parasites are sometimes poisonous to cattle if absorbed into their circulation. In the South herds may be examined in November or early in December and once a month during the next two or three months. In the North the first examination may be made six weeks to two months later, with two or, better, three following examinations at intervals of a month. If this procedure is carefully carried out there will be a noticeable diminution of warbles the following year, and if persisted in the warble can be almost if not completely eradicated. Where an entire community follows up the practice of removing and destroying warbles, the results are highly beneficial.

As a result of recent studies by various investigators it appears that the tiny grubs, newly hatched from the eggs, may gain entrance to the body by penetrating directly through the skin. Many observers, however, have held that the eggs or newly hatched larvae are taken into the mouth by the cattle licking themselves. It is possible, as in the case of several other parasites, that both modes of infection may occur and that the larvae may gain entrance to the body either by penetrating the skin or by being swallowed. From the evidence at present available it seems likely that the usual mode of entrance is through the skin. Irrespective of the mode of infection, the larvae evidently wander extensively through the tissues of the body, developmental stages being found in considerable numbers in the wall of the esophagus during the fall of the year. They have also been found in the spinal ca.n.a.l and in various other locations. Finally, about January they appear beneath the skin of the back, forming the well-known swellings. The posterior end of the grub is near the small opening in the hide, through which the grub breathes and discharges its excrement, and through which, when its development is complete, it finally escapes. The anterior end of the grub is at the bottom of the tumor, where the mucus collects upon which it feeds. By spring or early summer the grub is full grown and forces its way out of the skin, falling to the ground, into which it burrows for a short distance and transforms into the pupal stage. In about a month the mature fly emerges.

It has lately been discovered that a second species of warble fly (_Hypoderma bovis_) common in Europe is of not uncommon occurrence in Canada and the northern part of the United States, whereas it was formerly supposed that the ox warbles of this country were all of the one kind. The general appearance, life history, and effects of the European species are much the same as those of the American form.

Grubs weaken cattle, cause them to fall off in flesh and milk, and decrease the value of the hide. The beef in the immediate vicinity of a grub becomes slimy and of a greenish color, and is known to the butchers as "licked beef."

The total loss to this country on account of the warble fly is estimated at $35,000,000 to $50,000,000 a year, at the least, and may amount to considerably more.

_Treatment for warbles._--During the winter and spring examine the cattle for the presence of warbles. By pa.s.sing the hand over the backs of the animals the swellings marking the location of the grubs may be readily found. Pressure properly applied to the swellings will cause the grubs to "pop out" if they have reached a late stage of development. They may be more easily removed by means of slender forceps inserted into the opening of the warbles, and a still more certain method of removing them, particularly if the lumps are still very small, is to cut into the swellings with a sharp knife or bistoury, after which they may be pressed out. Care should be taken to crush all grubs removed, so as to prevent the possibility of their further development and transformation into flies. In order that none may escape it is advisable to examine the cattle every two weeks during the late winter and spring, at each examination removing the grubs which have developed sufficiently to cause perceptible swellings.

Another method of treatment is to force grease or oil into the openings of the warbles, which kills the grubs. This method is less certain than that of removing the grubs, and has the further objection that the dead grubs remain beneath the skin.

Cattle may be treated during the summer with fly repellents (p. 502) to keep off the warble flies. The efficacy of repellents against these flies is probably, however, not very great.

In localities where the character of the cattle industry is such as to render practicable the systematic examination of cattle and the removal of the grubs--that is, where the herds are comparatively small and subject to the close supervision of the owners--it is possible, by the exercise of a little care and with very little effort on the part of the cattle owners, provided they work together, each doing his share by seeing to the removal of grubs from his own cattle, so that as few as possible survive to transform into flies, to reduce the number of grubs within one or two seasons almost, if not entirely, to the point of extinction.

Investigations not yet completed indicate that grub eradication may sometimes be accomplished by the use of a.r.s.enical dips, which are extensively used at the present time for destroying cattle ticks. (See p.

497.) It is possible that the destructive action of a.r.s.enical dips upon warbles is more or less dependent upon the fact that a.r.s.enic is stored up in small quant.i.ties in and upon the skin of cattle that are repeatedly dipped in a.r.s.enical dips. The a.r.s.enical dip appears to act, not upon the well-developed grub beneath the skin, but upon the eggs or the newly hatched larvae, probably the latter. Accordingly the dipping of cattle to destroy grubs should be carried out during the fly season and repeated treatments should be given every two or three weeks, as in dipping cattle to eradicate ticks.

[Ill.u.s.tration: FIG. 6.--The warble fly (_Hypoderma lineatum_): _a_, adult female; _b_, eggs attached to a hair, 25; _c_, larva as seen in egg; _d_, larva from esophagus of an ox; _e_, later stage of larva from beneath the skin of the back; _f_, larva at the stage when it leaves the back of cattle and falls to the ground--all enlarged (after Riley).]

LICE.[16]

Cattle in the United States are commonly infested with three species of lice, two of them sucking lice (_Haematopinus eurysternus_, the short-nosed cattle louse, and _Linognathus vituli_, the long-nosed cattle louse), commonly known as blue lice, and one biting louse (_Trichodectes scalaris_), commonly known as the red louse.

The blue lice (figs. 7 and 8) suck the blood of cattle and are more injurious than the red lice (fig. 9). Unless very abundant the latter cause little injury. If numerous they irritate and worry their host probably more by their sharp claws than by their bites, as their food seems to consist entirely of particles of hair and dead skin.

Cattle lice reproduce by means of eggs or nits (fig. 10) which they fasten to the hair. The blue lice infest chiefly the neck and shoulders; red lice, when present, may be found almost anywhere on the body, but are usually most numerous on neck, shoulders, and at the root of the tail.

On account of the itching caused by the lice, infested cattle rub against posts, trees, etc., and lick themselves, the hair sometimes coming out and the skin becoming thickened so that mange may be suspected.

_Treatment for lice._--Cattle may be treated for lice by means of hand applications, spraying, or dipping. Dusting powders sold under various trade names are of value in helping to hold lice in check when the weather is too cold for dipping or spraying. The application of greases and insecticidal liquids by hand is fairly effective and practicable in cases in which there are only a few animals to be treated. The following remedies have proved effective when applied by hand, the treatment being repeated if necessary in about 16 days: (1) Cottonseed oil and kerosene, equal parts; (2) kerosene, 1/2 pint, mixed with lard, 1 pound; (3) crude petroleum; (4) any of the dips recommended for use in dipping, and diluted in the same proportions as for dipping. Oils or greases should not be used in very warm or very cold weather. The remedies mentioned may be applied with a brush or a cloth. They should be distributed in a thin, even coating over the surface of the body, taking care that there is no excess quant.i.ty at any point.

In spraying, any of the dips recommended for lice, properly diluted, are applied by means of a spray pump over the entire body. Thorough wetting of the skin and hair is important, and a second treatment should be given 15 or 16 days later.

When a considerable number of animals are to be treated the most satisfactory method of destroying lice is by the use of a dipping vat. Two dippings should be given 15 or 16 days apart. Dipping in the fall is good insurance against risk of loss from lice during the winter. All animals in the herd should be treated regardless of the number showing infestation.

Either coal-tar-creosote or nicotin dips may be used. These are sold under various trade names. The directions for dilution given by the manufacturer should be carefully followed. As coal-tar-creosote dips do not mix well with all kinds of water, they should be tested with the water to be used for making the solution by mixing some of the dip in the proper proportions with the water in a clean and clear-gla.s.s bottle or jar. If an oily layer or ma.s.s of globules collects either at the top or the bottom of the mixture after standing an hour, the dip is not suitable for use with that kind of water. Imperfectly mixed coal-tar-creosote dips are liable to poison animals even when not used in stronger solutions than that recommended by the manufacturers, and are also likely to be inefficacious.

The lime-sulphur dip, which is highly efficacious as a mange remedy, is of little value for destroying lice, especially blue lice. The a.r.s.enical dip used in tick eradication is a good louse remedy, but its use is not advisable on account of its poisonous nature, except under the supervision of capable persons who know how to use it and what precautions to take.

[Ill.u.s.tration: FIG. 7.--Short-nosed blue louse (_Haematopinus eurysternus_) of cattle. Enlarged. (From Bureau of Entomology.)]

[Ill.u.s.tration: FIG. 8.--Long-nosed bluelouse (_Linognathus vituli_) of cattle. Enlarged. (From Bureau of Entomology.)]

[Ill.u.s.tration: FIG. 9.--Red louse (_Trichodectes scalaris_) of cattle. Enlarged. (From Bureau of Entomology.)]

[Ill.u.s.tration: FIG. 10.--Egg of short-nosed blue louse (_Haematopinus eurysternus_) attached to a hair. Enlarged. (From Bureau of Entomology.)]

MANGE, ITCH, SCAB.[17]

Cattle are subject to four kinds of mange, of which common mange or psoroptic mange is the most important.

PSOROPTIC MANGE.

Psoroptic mange of cattle is caused by small mites (fig. 11) which multiply rapidly and are spread from diseased to healthy cattle by bodily contact, or by pens, stables, railroad cars, etc., recently occupied by mangy cattle. The mites attack the skin and cause it to become thickened and covered with crusts and scabs, with a consequent loss of hair. Intense itching accompanies the disease, and affected cattle are more or less constantly rubbing and licking themselves. Psoroptic mange commences at the root of the tail, or on the neck, or withers, and gradually extends over the back up to the head, over the sides, and may finally affect nearly the entire body except the legs. In serious cases the skin may become ulcerated; the animals are greatly weakened and emaciated, and finally die.

By taking sc.r.a.pings from the edges of scabby patches and placing them on a piece of black paper in a warm place the mites may be seen as tiny white objects crawling over the paper, more distinctly if a magnifying gla.s.s is used. Mange may be confused with lousiness, ringworm, or with any condition in which there is itching or loss of hair, but if mites are found there is no question of the diagnosis. The disease is worse during cold, wet weather. Mangy cattle when on good pasture during the summer often seem to recover, but in the fall the disease again appears in a severe form.

_Treatment._--The most generally used and most satisfactory method of treating cattle mange consists in dipping the animals in a vat filled with a liquid of such nature that it will kill the parasites without injuring the cattle. Vats for dipping cattle are built of wood, stone, or concrete, and vary in length from 30 to 100 feet or more. They vary in width from 3 to 7 feet at the top and 1-1/2 to 3 feet at the bottom, and the depth may be from 7 to 10 feet. A narrow chute through which the cattle are driven leads to one end of the vat, where a steep slide pitches the cattle into the dipping fluid, through which they swim, and climb out of the vat at the other end, which is built sloping and provided with cross cleats to give the animals a foothold. A draining pen with floor sloping back toward the vat is generally provided. The dip should be used warm, 100 to 105 F., and the cattle should be held in the vat for two minutes to insure thorough action of the dip. The head of each animal should be ducked at least once.

Care should be taken that the vat contains a sufficient depth of fluid to swim the animals to be dipped. The dipping fluid may be heated from a steam boiler by pipes or hose, or water heated in large iron cauldrons or tanks may be used for charging the vat, and hot water with a proper quant.i.ty of dip added from time to time as the dipping fluid becomes cool.