How Diseases and Parasites Are Spread

H. W. SCHOENING, BENJAMIN SCHWARTZ, AND ARTHUR W. LINDQUIST

WHEN disease-causing bacteria and viruses establish themselves in an animal, they attack certain tissues, multiply, and set up a disease process.

The infected animal eliminates with its excreta the causative organisms, so that its surroundings become contaminated and other susceptible animals are exposed to the contagion.

The ability of the infection to survive outside the animal body until it can gain entrance to a new host largely determines its ability to spread disease.

CERTAIN BACTERIA can live for indefinite periods outside the animal body and still retain a high degree of virulence.

An example is the anthrax organism. In the final stages of anthrax, the animal's blood stream and tissues contain large numbers of the organisms in a vegetative stage. The vegetative forms develop into spores when they become exposed to the air through the opening of the carcass or the escape of blood from the natural openings of the body. The organisms then are hard to kill. They can withstand high and low temperatures and retain their infective capacity for years. Once the soil becomes infected with anthrax spores, therefore, it may remain dangerous for susceptible animals for long periods.

Another disease-producing organism that is quite resistant to adverse influences is the germ that causes swine erysipelas. It does not go into a spore stage, but it can live in certain soils for lone, periods, and the suggestion has even been made that active multiplication may take place in suitable types of soil. Here again the infected animal contaminates its surroundings through its discharges.

THE VIRUS DISEASES are alike in several ways, but their mode of transmission may vary considerably, depending on the nature of the disease and the extent to which the virus has multiplied within the body of the affected animal. Infection takes place when the virus in a proper state is introduced into a susceptible animal.

Rabies, for example, is caused by a filterable virus that is found in the saliva of an affected animal and under natural conditions it is transmitted through the bite of a rabid animal, usually a dog. The disease causes degenerative changes in the brain, which cause affected animals to become aggressive and attack or bite any available object. After the virus is deposited in the tissues, usually through the injury made by the bite, it is believed to move from the site of deposit by way of the nerves. Eventually it reaches the the central nervous system the brain and spinal cord.

Equine encephalomyelitis, which sometimes is known as sleeping sickness or blind staggers of horses, is another virus disease that attacks mainly the brain and spinal cord, although it can propagate also in other tissues, including the skin. Early in the infection the virus of equine encephalomyelitis lives for a short time in the blood stream. It then disappears from the blood and is found only in the brain and spinal cord. Nervous symptoms of the disease are noted early. It can be transmitted by the bites of a number of different species of mosquitoes and other insects. The disease is therefore considered to be spread chiefly through biting insects.

The virus of foot-and-mouth disease is an example of the viruses that affect skin membranes. Vesicles, which form in the mouth and on the soft parts of the feet, contain the infective agent. When they rupture, the virus is disseminated in the litter and on the ground, where the susceptible animals easily pick up the infection. In the early stages of the disease, the virus is found in the blood, milk, saliva, urine, and perhaps the feces. Because of the large quantities of the virus thus eliminated by affected animals and its highly infectious nature, a wide dissemination of the disease in a short time is possible.

A CARRIER ANIMAL is one that carries an infective agent within its body without showing evidence of illness. It may eliminate the infective agent from time to time and act as a source of infection to other animals; or the carrier animal itself through some adverse condition may develop a frank attack of the disease; or the infection may be carried from a carrier to a normal animal through the bite of an insect.

Certain animals that have recovered from infection with the virus of foot-and-mouth disease may retain active virus within their bodies for months or years and discharge the virus from time to time in sufficient amounts to cause the disease in susceptible animals.

In infectious anemia of horses so-called swamp fever a carrier stage is seen in which an apparently normal animal carries the infective agent in its blood stream for many years, perhaps during the rest of its life. It seems that this disease is transmitted mainly by biting insects.

Wild animals and birds also may be reservoirs and spreaders of diseases as, for example, foot-and-mouth disease, rinderpest, and trypanosomiasis.

FARM ANIMALS acquire parasites by grazing on contaminated pastures; by ingesting feed or hay contaminated with the manure of parasitized animals; by swallowing contaminated soil, water, insects, and other small forms of animal life that harbor infective stages of parasites; by eating the flesh or other tissues of animals that harbor infective stages of parasites; or by being bitten by infected insects and ticks.

Among the practices that contribute to the dissemination of parasites are: Bringing parasitized stock to a farm or ranch from the outside; using permanent pastures year in and year out; spreading manure on pastures; placing feed on the ground of barns, stalls, and other shelters; and allowing parasitized dogs to be near livestock.

To INSURE their perpetuation, parasites eliminate their reproductive elements in the form of eggs or larvae, which as a rule are produced in enormous numbers to compensate for the likelihood that many of them will perish before they can get into another host. Furthermore, the reproductive elements often are encased in shells and other tough membranes, which enable them to survive for a long time outside the body even when conditions are unfavorable.

Many protozoan and worm parasites of animals are transmitted through the excreta. Parasites that inhabit the digestive tract of cattle, sheep, goats, swine, horses, poultry, and wild animals are transmitted in that way.

Some protozoan parasites transmitted through the excreta of the host have a resistant form known as a cyst a resting stage, microscopic in size, usually rounded, and enclosed in a membrane. The membrane protects the parasite from unfavorable influences after it has been eliminated from the body. Sooner or later the cyst becomes infective to other animals of the kind in which it originated. It gets into them by being ingested with grass, dry feed, or water that has become contaminated with the droppings of infected animals. Coccidia of livestock and poultry, parasitic amebas of animals and man, and the ciliates, which parasitize swine, are so transmitted.

Other protozoans are transmitted by the active stage, known as a trophozoite, which passes out of the infected host animal with the droppings.

Trophozoites have to be swallowed soon after they have been eliminated from animals, because their ability to live outside the host is sharply limited.

Trichomonads in the intestines of swine and other animals and in poultry are transmitted in that way.

WORM PARASITES of the alimentary canal, or of the lungs and other organs that have a direct or indirect connection with the alimentary canal, produce eggs, which are eliminated with the droppings.

The swine kidney worm is one of the few parasites that eliminates its eggs into the ureter, from which they are voided with the urine. Once the eggs reach the outside, regardless of how they are eliminated from the host's body, they develop and become infective as eggs, or they hatch into larvae, or the eggs have to be swallowed by intermediate hosts. Cold weather retards the development of parasite eggs, but during favorable weather the eggs develop rapidly. Those that hatch yield larvae, which undergo their transformations to the infective stage in about one week or more, depending on the temperature.

The larvae that first emerge from eggs feed almost constantly on bacteria and bits of organic matter, grow rapidly, and molt in a day or two. Before they become infective they undergo at least a partial second molt, after which they are ready to invade the kind of hosts from which they came.

During the favorable weather that prevails in most parts of the United States in late spring, summer, and early fall, the development of the egg to the infective larva takes about a week or less. The infective larvae climb upward on grass and other vegetation, especially after rains and dews when the vegetation has a film of moisture on it. The upward migration places the larvae in a situation where they are likely to be swallowed by grazing animals. In barns the larvae climb up on litter, hay, moist walls, and posts and may be eaten or licked off by animals housed therein.

Cattle, sheep, goats, horses, and ;wine acquire many of their internal parasites by grazing on pastures that have become contaminated with the eggs and larvae of the parasites we mentioned. Stomach worms of cattle, sheep, and goats and most of the intestinal hairworms of ruminants that produce enteritis and scouring are acquired in this manner, as are lung-worms of cattle and the common lung-worm of sheep.

Other parasites, however, including hookworms of farm animals and pets, intestinal threadworms of all domestic animals, kidney worms of swine, and other worm parasites can gain entrance into the host by boring into its skin. That can readily happen when an animal lies down in a pasture, barnyard, or corral that is contaminated with skin-penetrating larvae.

SOME PARASITE EGGS that develop outside the host but do not hatch there are infective as eggs when a suitable animal swallows them. Among them are the large intestinal roundworm or ascarid of cattle, horses, and swine; whipworms of cattle, sheep, goats, and swine; and intestinal roundworms or ascarids and whipworms of pet animals. The infective eggs are swallowed with contaminated feed or water in kennels, runs, yards, corrals, pastures, barns, stables, and elsewhere. The larvae hatch in the alimentary canal of the host, reach their preferred location in the digestive tract or elsewhere in the body by active migration, and develop there to maturity.