DISEASES AND PARASITES AFFECTING swine

Lung Parasites of Swine

JOHN S. ANDREWS.

SEVEN species of roundworms and one species of flatworm may exist in the lungs of pigs.

Three of the roundworms, Metastrongylus elongatus, M. salmi, and Choerostrongylus pudendotectus, are the common lungworms of swine and grow to maturity in the lungs, as does the much less common flatworm or lung fluke, Paragonimus westermanii.

The four remaining roundworm species, the large roundworm, the intestinal threadworm, the hookworm, and the kidney worm, live in the lungs only during their early development. They complete their growth in other organs.

The effect of pulmonary parasitism On the growth and health of pigs is hard to evaluate on the farm because it is often complicated by other disease-producing agents, like bacteria, viruses, and other species of parasites. In fact, lungworms have been reported to act as reservoir and intermediate hosts for swine influenza virus. The affected pigs may suffer also from malnutrition and other conditions brought about by poor management and lack of sanitation.

The common lungworms of swine are whitish, threadlike, and up to 2 inches long. They occur in the smaller air passages of the lungs (bronchioles) and in heavy infections in the larger. air passages (trachea and bronchi).

Except for M. salmi, which are chiefly in swine in the South, lungworms are among the most widespread of swine parasites and occur in all parts of the United States. Investigators in the Southeast found that 70 percent of the swine they examined had lungworms.

The lungworms of swine require a second or intermediate host in order to complete their development. Female lungworms produce large numbers of thick-shelled eggs, which the infected hog coughs up, swallows, and ejects with the feces.

Earthworms, particularly the red-striped worms of the genus Helodrilus that exist chiefly in manure piles and rich soil, swallow the eggs, which hatch in the earthworms' intestines. A single earthworm may harbor 2,000 lung-worm larvae. The tiny worms bore into the glandular part of the intestine (the part that contains the calciferous glands) and also get into one or all of the five to seven pairs of hearts of the earthworm. The larvae become infective to pigs 3 or 4 weeks later. They are freed from the earthworms by the process of digestion.

Pigs become infected with lung-worms by swallowing the earthworms that harbor the larvae. The larvae penetrate the pig's intestinal wall and are carried by the lymphatic and circulatory systems to the lungs. There they burrow through the walls of the capillaries into the alveoli, become localized in the bronchioles, complete their development, and mate. The females produce eggs about 33 days after the pig ingests the larvae.

The outstanding symptom of lung-worm infection in pigs is a cough. Even lightly infected animals may be subject to fits of coughing, which can weaken them and lower their vitality. The coughing seems to be caused by the presence of the lungworms in the bronchioles and by their moving about in the air passages. This movement irritates the mucous linings of these tubes. The worms block the air passages, interfere with breathing, and cause greater secretion of mucus. The pig therefore has to breathe faster, and that in turn makes the cough worse.

Hemorrhages appear on the surface of the lungs during the early stages of lungworm invasion. The constant irritation of the lungs by the worms and the plugging of the bronchioles subsequently can bring about the consolidation of the lung tissue around the sites occupied by the worms. The tips of the lungs become grayish or whitish and very hard in some cases of long-standing infection.

Pigs infected with lungworms tend to go off feed, become unthrifty, and fail to grow normally.

NO EFFECTIVE MEDICINAL treatment is available for removing lungworms from swine. A change in diet may be helpful by temporarily stimulating the appetite of the affected pigs.

Infected swine should be removed from the lots on which they acquired the parasites and put in dry, clean pens that have concrete floors. They may be placed on temporary pastures that have not been used for pigs for several years so as to insure against further infestation from swallowing infected earthworms. Sick pigs, while they are kept in isolation, should be supplied with nutritious feed, safe drinking water, and good bedding, which should be changed fairly often.

A pasture or lot that has many infected earthworms may remain dangerous for several years. In soil at Beltsville, Md., on which lung-worm eggs were deposited 4 years earlier, investigators found earthworms in which were infective larvae. The parasites and their intermediate hosts undoubtedly survived because a dense plant growth had kept the soil moist. Eggs remained viable for several months in soil in which the roots of vegetation growing on the surface penetrated to the level at which the eggs were located. But in soil bare of vegetation the period of survival was much shorter; in dry soil lungworms failed to survive more than a month in the summer.

Lungworm eggs can withstand low temperatures when adequate moisture is present. They have survived continuous freezing at 14 to 20 F. for more than a year.

Lungworm infection in swine can be prevented by keeping pigs in lots where they cannot come in contact with infected earthworms. Because the worms thrive in old hog lots in which manure and litter have accumulated, in old strawstacks, on permanent pastures, and in low fields that receive drainage from higher ground, a farmer who avoids grazing swine on such areas will do much to control lungworm infection. If he has to use such places for raising pigs, he can discourage rooting by putting rings in the noses of the animals and so reduce the number of earthworms ingested by the pigs.

THE LUNG FLUKE, Paragonimus westermanii, is a thick, oval worm, about one-fifth to three-fifths inch long and about one-fifth inch wide. It occurs in sacs, or cysts, in the lung tissue of many mammals, including man. It has been found in swine in Kentucky, southern Ohio, and southern Georgia. It has a limited distribution because the completion of its life cycle in the, United States depends on the presence of various species of crayfish and a small amphibious snail, Pomatiopsis lapidaria, which live along the edges of small, sluggish streams less than 30 feet across.

The lung fluke has been of limited economic importance in swine, but it could spread rapidly in areas favorable to its propagation.

The eggs produced by the adult fluke, which is hermaphroditic, are coughed up, swallowed, and discharged with the feces of the infected pig. The larval flukes (miracidia) develop in a warm, Moist environment to the stage infective to the snail host in about 3 weeks. The eggs hatch, and the miracidia penetrate the soft body of the snail. After about 12 weeks and two stages of reproduction within the snail, in which the number of individual parasites has been vastly increased, the second infective stage, or cercaria, is produced. These actively motile organisms emerge from the snails in the greatest numbers in the late afternoon or night when the crayfish also is most active. The cercariae penetrate the underside of the tail of the crayfish.

The fully developed stages (metacercariae), which are infective to pigs (the final host), develop in the crayfish 46 days after exposure to cercariae. Pigs can also become infected with the lung fluke by eating another final host in which immature forms of the parasite have not yet reached the lungs.

Pigs acquire lung flukes by eating infected crayfish, which they bring to the surface of the ground by rooting in wet and boggy pastures. The encysted flukes (metacercariae) are freed by digestion in the pig's stomach and small intestine. They then bore through the intestinal wall and migrate through the body cavity, penetrate the diaphragm, and enter the lung tissue. There they may form cysts, or pockets, which usually contain two worms, and develop to maturity in about 6 weeks.

No special symptoms have been noted in affected hogs. In other hosts, heavily infected with lung flukes, the pulmonary mucus is tinged yellowish-brown by the thousands of fluke eggs in it. Finding these eggs in the bronchial secretions or in the feces is the only sure way of diagnosing this infection in the living animal.

The presence of the flukes in the lungs produces an inflammatory reaction. When the worms are located near the surface of the lung, the cysts appear as bluish-gray protuberances. If the cysts are deep in the lung, the surface of the lung may show only a swelling. Sometimes there is a brownish cast because of the large number of eggs that lodge there.

There is no known treatment that will remove lung flukes from swine or any other animal.

Infection can be prevented by keeping hogs off wet and boggy areas along small creeks or rivers, where infected crayfish may live.

JOHN S. ANDREWS, a parasitologist in the Animal Disease and Parasite Research Branch of the Agricultural Research Service, has studied the internal parasites of the larger domestic animals since 1930.