DISEASES AND PARASITES AFFECTING--dogs and cats

Internal Parasites of Dogs and Cats

F. D. ENZIE AND EMMETT W. PRICE.

EVERY seventh American in 1956 owned a dog. At least 20 million cats lived in American homes that year, but anybody who knows cats knows that it is not entirely correct to say they were "owned."

Along with the ownership of dogs and association with cats goes the responsibility for their proper feeding, housing, grooming, and protection from preventable disease. We are here concerned with the last responsibility the duty and responsibility of the owner to become familiar with the common parasites that affect dogs and cats, particularly with respect to the sources of infection, modes of transmission, and methods of control.

The internal parasites of dogs and cats include a few one-celled organisms, the protozoa, and a much larger and more important group known as helminths or worms. All of these parasites live within the body of the host as compared with the arthropods, or external parasites, which usually infest the surface of the body.

Many of the internal parasites of dogs and cats occur infrequently and are of little concern to most owners. A few are widespread and produce serious losses, particularly among puppies and kittens.

This discussion includes a description of only the more important species. We give particular consideration to the sources of infection, manner of transmission, and methods of control.

The external parasites, including fleas, lice, ticks, and mange mites, are discussed in the chapter that follows. The nematodes the roundworms--comprise the largest group of internal parasites affecting dogs and cats, and they are by far the most injurious.

Some species, notably hookworms and ascarids, have a simple, direct life history and are distributed widely wherever dogs and cats are raised. They usually are acquired at an early age and may be responsible for the loss of entire litters before the animals are weaned.

Other roundworms, such as heart-worms, the kidney worms, and certain lungworms, require intermediate hosts to complete their life cycles. These species generally occur in older animals and are restricted mostly to certain geographical areas. Although spectacular death losses are seldom associated with these nematodes, the usefulness of infested animals, particularly hunting dogs and animals serving with the Armed Forces, is largely destroyed.

THREE KINDS OF HOOKWORMS may be found in dogs and cats in the United States. Ancylostoma cannium is widely distributed throughout the country. A. braziliense is limited primarily to the Southern States. Uncinaria stenocephala occurs in the more northerly areas.

Hookworms are white or grayish roundworms about one-half to three-fourths inch long and about as thick as an ordinary straight pin. The front end is bent slightly upward to give a hooklike appearance, and the mouth is provided with teeth or cutting plates. Hookworms generally are firmly attached to the lining of the small intestine, but in heavy infestations they may occur also in the cecum, colon, and rectum.

The life histories of the hookworms are comparatively simple and are similar for all species. The adult females produce many eggs, which are eliminated with the fecal material of the host. When temperature and moisture are favorable, wormlike larvae hatch from the eggs within a few days.

In a week or so the young larvae molt twice and are transformed into the infective stage. Susceptible animals become parasitized when the infective larvae are swallowed in contaminated feed or water or when they are brought into contact with the mouth on balls, rubber bones, or other objects. Infective hookworm larvae may also penetrate the skin of susceptible hosts. In pregnant females they may pass by way of the circulatory system to the developing young.

Larvae that penetrate the skin are carried with the blood to the lungs, where they escape to the air passages. They are eventually coughed up and swallowed and pass to the intestine, where they develop to maturity.

Infective larvae that are swallowed with contaminated feed and water pass directly to the intestine without going through the lungs.

Hookworm eggs first appear in the feces 3 to 6 weeks after the larvae reach the intestine, the worms generally maturing more rapidly in young animals. In prenatal infections, however, eggs may be found in the feces as early as 13 days after birth. In the absence of reinfection, hookworms may persist in the intestine of the host for as long as 2 years, although most of them are eliminated within 6 months.

Because these parasites are voracious bloodsuckers, the principal symptoms of hookworm disease are the ones associated with chronic hemorrhage. There is a pronounced anemia, manifested by extremely pale mucous membranes, marked depression, and a reluctance to move about.

The animal may have a persistent diarrhea in the early stages of the disease. The feces may be streaked with blood. As the condition advances, the feces contain progressively larger amounts of blood and mucus. Shortly before death the bowel movements may consist almost entirely of pure blood. Since the bitch generally keeps the puppies and bedding free of fecal material during the first few weeks, prenatal infections may end fatally before hookworm disease is suspected.

Forty hookworms may withdraw as much as an ounce of blood every 24hours. Infections of several hundred worms are not uncommon, even in nursing puppies. But that is only part of the blood loss: The worms have a tendency to migrate to new areas, and the abandoned sites continue to bleed for some time. That is what makes hookworm infections so serious, particularly in puppies and young dogs.

The common hookworm, A. caninum, is the species most frequently encountered in cats, but cats seldom harbor sufficient numbers to produce clinical hookworm disease. This parasite is the same species that occurs in dogs, but it is a strain especially adapted to cats, and the two strains are not readily transmitted from one host to the other.

The infective larvae of A. braziliense occasionally penetrate the skin of man and produce a condition known as creeping eruption. Tortuous burrows are formed in the superficial layers of the skin. The itching is intense. The larvae move at the rate of an inch or so a day and may persist for several days or weeks. The larvae may be destroyed in the skin by spraying the lesions with ethyl chloride or carbon dioxide snow, although the larvae die before they reach the intestine, even without treatment.

The larvae of the common hookworm may produce a transient inflammation of the skin of people, but they do not produce the typical lesions associated with A. braziliense larvae.

LARGE INTESTINAL ROUNDWORMS, or ascarids, are second only to hookworms in their injuriousness to puppies and kittens.

The dog is parasitized by two species, Toxocara Canis, which occurs principally in puppies and young dogs, and Toxascaris leonina, a species generally found in older animals. The latter occasionally is found also in cats, although cats, particularly kittens, usually are infested with T. cati, a species closely related to the common ascarid of young puppies.

Large roundworms are white or yellowish and 2 to 8 inches long. They occasionally are passed in large numbers in the feces or vomitus of young animals, and while still alive they have a tendency to coil in a spiral.

The life histories of dog and cat ascarids are similar to that of the large roundworm of man. Numerous eggs are passed in the feces of the host and become infective within a few days under favorable conditions. The infective eggs are swallowed in contaminated feed or water and hatch in the small intestine. After penetrating the wall of the digestive tract, the young worms pass to the lungs with the blood and escape from the blood vessels to the air passages. They are subsequently coughed up, swallowed, and develop to maturity in the small intestine within a few weeks.

The larvae of T. leonina do not pass through the lungs but penetrate the intestinal lining, where they grow and develop. They return to the lumen of the intestine after about 10 days and mature in a few weeks. Ascarid larvae, like those of the hookworms, may pass from the circulatory system of the pregnant female to the developing embryos. This is particularly common in mature animals that are resistant to (and seldom harbor) the adult worms. The need for preventing exposure of the pregnant animal to sources of infection therefore is apparent.

The most common symptoms of ascarid infection in puppies and kittens are marked enlargement of the abdomen, unthriftiness, and digestive disturbances. The animals appear listless. Large numbers of worms may be eliminated in the feces or in vomitus when the infections are heavy. Coughing may be noted when larvae are passing through the lungs. Pneumonia commonly follows massive invasion of the lungs.

The larvae, of dog and cat ascarids, like those of the hookworm, may invade the body tissues of people and produce a condition known as visceral larva migrans. The disease generally occurs in children less than 3 years old and may be mild or severe, depending on the numbers of larvae involved. After the eggs hatch in the small intestine, the larvae penetrate the gut wall, enter the circulation, and invade the liver, lungs, heart, kidney, brain, spinal cord, and other internal organs.

Because no treatment is known for this condition, every effort should be made to prevent exposure of children to the infective eggs of these parasites.

THE WHIPWORM of dogs, Trichuris vulpis, is a white or gray-colored worm that is usually found in the cecum, or blind gut. It is 2 or 3 inches long when mature. It resembles a small whip. Closely related species have been reported in cats in other parts of the world, but they are not known to occur in this country.

The life cycle of the whipworm is direct that is, infection occurs when susceptible animals swallow embryonated eggs in contaminated feed or water. The eggs hatch in the small intestine, and the young worms reach maturity in the cecum about 3 months later.

The damage produced by the whip-worm is not well understood, although a variety of symptoms have been associated with heavy infestations. Symptoms commonly attributed to whip-worm infections are abdominal pain, unthriftiness, and chronic diarrhea, or alternate periods of diarrhea and constipation. In many cases, however, the worms appear to do little or no harm.