Parasites That Attack Animals and Man

BENJAMIN SCHWARTZ.

SOME of the parasites that occur in animals also attack people.

Some have to develop in an animal or in two different animals before they can invade human beings. Man then is the definitive, or final, host. He usually acquires the parasites by eating the intermediate animal hosts or (if they are small insects) by swallowing them accidentally. For some parasites, human beings are the intermediate hosts and animals are the final hosts.

Even the few parasites that live only in man are related closely to others that live in animals. Farm animals, dogs and cats, and rats and mice contribute to man's burden of parasites. That burden tends to become lighter as our civilization advances, however, largely because the sanitary barriers that have come into existence impede the spread of parasites from parts of the world where standards of sanitation and hygiene are imperfect.

Some of the major parasites that man shares with animals are discussed here. The examples illustrate that parasites can be acquired by eating infected food of animal origin and by being in close contact with animals that harbor the kinds of pests that can be transmitted to human beings.

THE BEEF TAPEWORM derives its name from the fact that in its immature, or cystic, stage it lives in the flesh of cattle. The adult (or strobilate) tapeworm, which arises from the cystic stage, lives in the intestine of man, where it can attain a length of 40 feet Or more. An infested person usually harbors only one tapeworm.

The adult tapeworm is whitish in color. It is a chain of hundreds of proglottids, or segments, which are anchored to the intestinal wall by four cup-shaped suckers on its head. A unique feature is that the head lacks the microscopic hooks that most tapeworms have. It is therefore known also as the unarmed tapeworm of man.

The immature stage, known as a bladderworm, or cysticercus, occurs mainly in the muscles, including the heart, and in other internal organs of cattle. It is grayish white. It is about one-fifth to three-fifths of an inch long and about half as wide. It has thin walls and contains a fluid into which the head of the future worm is pushed, just as the tip of a finger might be pushed into a glove.

Before the middle of the 19th century it was not known that a bladder-worm was the intermediate stage of a strobilate tapeworm. Bladderworms were regarded as distinct parasites without sex organs and were therefore given scientific names of their own. The name given to the bladderworm of the beef tapeworm was Cysticercus bovis, which no longer has any scientific validity. Parasitologists retain that name in their writings for convenience in differentiating the bladderworm from the adult tapeworm. The valid scientific name of the unarmed, or beef, tapeworm of man in all stages of its development is Taenia saginata.

The life history of the beef tapeworm involves an alternation of hosts. Man, the definitive host, discharges the ripe, or gravid, segments with the excreta, or the segments may pass out independently and be discovered in the bed linen of infected individuals. Once outside the body, the gravid segment begins to disintegrate and release the eggs it contains.

A single gravid segment may contain several thousand eggs. Several segments may become gravid and be expelled almost every day for several years by one tapeworm carrier. If the segments reach pastures, barnyards, feed lots, and other places where cattle graze or feed, the animals may swallow some of the eggs and become infected with bladderworms.

Inside the body of an animal, the microscopic egg hatches, and the tiny larva bores into the intestinal wall. From there it reaches the blood stream and is carried to all parts of the body. It localizes mainly in the flesh and heart. The parasite develops rather slowly and becomes a bladderworm in about 2 to 4 months.

A person who happens to eat raw or rare beef that harbors a single live bladderworm can become infected with a tapeworm. The tapeworm head inside the bladder is already fully developed. In the stomach it becomes evaginated, or everted, and the bladder wall is digested. When it gets into the intestine, the head of the future tapeworm attaches itself to the intestinal wall and grows by producing more and more segments. The newly formed segments are the ones right below the head. New ones, as they form, push the older ones down, so that the first segment to be formed comes to occupy the lowest position in the chain that constitutes the strobilate tapeworm.

About 10 to 12 weeks after the bladderworm has been swallowed, the oldest segment becomes gravid and detached from the rest of the chain. The others follow this pattern in rapid succession. The process continues as long as the tapeworm remains in the intestine. The affected individual meanwhile passes many thousands of eggs with the detached segments nearly every day.

The tapeworm carrier who happens to live on a farm, ranch, cattle-feeding establishment, or wherever he has contact with cattle is the source of infection for them especially if sanitary facilities are primitive. The carrier might deposit waste matter on a pasture or in a place where wind and rain can disperse it on a pasture or feed lot or wash it down to ponds and sloughs from which cattle drink.

"Beef measles" is the name applied to the condition in beef infected with these bladderworms. The economic loss from the condemnation of cattle on account of "measles" at times may be considerable, even though the percentage of infested cattle is small. About 16,500 to 27,000 beef carcasses were found annually in 1948-1954 to be infected with bladderworms out of a total annual slaughter of 12 million to 15 million head and 5.5 million to 7.5 million calves.

Infected carcasses in which the bladderworms are readily found are condemned as unfit for human food and consigned to the tank. If one dead or degenerated cyst, usually located in the heart, is discovered, the carcass is passed for food after the cyst is removed. Cattle carcasses that have a somewhat greater infestation also may be passed for food after the few visible cysts are cut away. Such carcasses, however, must be refrigerated at temperatures and for periods known to be fatal to all the parasites, or they must be thoroughly cooked at a temperature of 140 F. Such cooking also kills the parasites.

The control of "beef measles" will control the beef tapeworm in man. Detection by physicians of human carriers of tapeworm, especially those who live in rural areas, and prompt treatment to remove the parasites, however, will end the hazard of infection of cattle with bladderworms. In simpler terms: Improvement in rural areas in ways of disposing of human wastes will prevent cattle from becoming infected; that, in turn, will ultimately eliminate the carrying of tapeworms by people.

THE PORK TAPEWORM, Taenia solium, also lives in the human intestine. It is a long, whitish, strobilate tapeworm. It resembles the beef tapeworm in most respects, but it is usually shorter and is about 2.5 to 5 feet long. Another difference is that the pork tapeworm has a double row of tiny hooks, besides the four suckers, on its head. The pork tapeworm is also known as the armed tapeworm of man.

The cystic stage of the pork tapeworm, known for convenience only as Cysticercus cellulosae, is like that of the beef tapeworm. The bladderworms occur in swine, but they also can live in man. They occur in various parts of the body. They tend to localize in the muscular part of the diaphragm, the loin muscles, heart, the jaw muscles, muscles between the ribs, muscles of the hind legs and shoulders, brain, eyes, liver, lungs, and other organs.

The pork tapeworm alternates between man and the hog in its developmental cycle. Swine become infected by swallowing, with feed or water, the tapeworm eggs freed from the gravid segments passed in the excreta of a human carrier. Man, in turn, acquires the strobilate tapeworm by eating raw or incompletely cooked pork infected with the bladderworms.

Persons harboring the pork tapeworm in the intestine could get tapeworm eggs on their hands. The hands might transfer the eggs to the mouth and thus pave the way for an infection of the muscles and the heart, brain, and eyes. The pork bladderworm in the brain of man is known to produce symptoms like those of epilepsy.

Very few persons in this country harbor the pork tapeworm.

Hogs so infected also are rare in the United States, but they might harbor thousands of bladderworms in their organs and muscles. Only 11 infected swine carcasses were found by Federal inspectors in more than 57 million carcasses inspected in 1953. Only four such infected carcasses were found in 1954 in more than 50 million inspected. Federal meat inspectors condemn as unfit for human food all hog carcasses in which are bladderworms.

We can prevent bladderworms in swine and the pork tapeworm in man by not eating raw or undercooked pork and by improving rural sanitation.

THE HYDATID TAPEWORM, Echinococcus granulosus, lives as an adult parasite in the intestines of dogs and related wild carnivores and as a larva in man, cattle, sheep, goats, swine, horses, and wild animals. The larva localizes in the liver, lungs, spleen, kidney, brain, and other organs.

The adult tapeworm in the intestine of a dog or other carnivore is about one-eighth to one-fourth inch long and about one-eightieth inch wide. It attaches itself to the wall of the intestine by four cup-shaped suckers on its head, which also has small hooks. Behind the head and neck are two segments. The second segment contains the eggs, which start new infections.

Only rarely has the adult hydatid tapeworm been found in dogs in this country--perhaps because most dogs that have been autopsied were from pounds in cities, where they had little chance of acquiring the tapeworm. Country dogs could more easily become infected by eating offal from unsupervised slaughterhouses.

The larval stage of the hydatid tapeworm is the largest of its kind. The adult tapeworm is the smallest. The larva is several inches to almost a foot in diameter. Its size alone would make it unusually dangerous to man, because it localizes in the liver and lungs. Persons so affected require surgery.

Human beings acquire hydatids by fouling their hands in some way with the excreta of infected dogs or by picking up food or by swallowing food or water contaminated with the tapeworm eggs. Petting an infected dog is one way of contaminating the hands; dogs often roll in excreta, which may contain the eggs. The eggs hatch into larvae when they reach the human intestine. These larvae can penetrate the intestinal wall. They enter small blood vessels and so reach various organs and tissues, where they develop rather slowly and attain a diameter of about one-half inch or so after several months. Many tapeworm heads occur in a single hydatid cyst. Each head can develop into an adult tapeworm when the cyst or part of it is swallowed by a dog or other final host.