How Diseases and Parasites Are Spread

40 Yearbook of Agriculture 1956 Tumors are another important and pines from ig2i to ig2j. He became staf common cause of noninfectious dis- assistant and consultant in parasitology in eases of livestock and poultry. the Animal Disease and Parasite Research These and some of the other causes Branch of the Agricidtural Research Service of noninfectious diseases are discussed in ig§4. in later sections. E. F. KNIPLING, a graduate of Texas Agricultural and Mechanical College and H. W. ScHOENiNG retired in ig^§ after Iowa State College, became chief of the ^Syears of service in the Department of Ag~ Enio?nology Research Branch in igjj. He riculture. At that time he was assistant to was formerly in charge of the Insects Affect- the chief of the Animal Disease and Parasite ing Alan and Animals Section of the Ento- Research Branch. He was chief of the mology Research Branch. He has been with Pathological Division of the former Bureau the Department of Agriculture since igji. of Animal Industry for 20 years. AUBREY M. LEE is head of the Nonin- BENJAMIN SCHWARTZ has been engaged fectious Diseases Section of the Animal in parasitological research since igi^. He Disease and Parasite Research Branch, was chief of the geological Division of the Agricultural Research Service. He received Department^ s former Bureau of Animal In- his degree in veterinary medicine at Kansas dustry from igj6 to ig^j and professor of State College in ig22 and has been with the parasitology in the University of the Philip- Department of Agriculture since igj^*

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

WHEN disease-causing bacteria and large numbers of the organisms in a viruses establish themselves in an ani- vegetative stage. The vegetative forms mal, they attack certain tissues, mul- develop into spores when they become tiply, and set up a disease process. exposed to the air through the opening The infected animal eliminates with of the carcass or the escape of blood its excreta the causative organisms, from the natural openings of the body. so that its surroundings become con- The organisms then are hard to kill. taminated and other susceptible ani- They can withstand high and low tem- mals are exposed to the contagion. peratures and retain their infective ca- The ability of the infection to survive pacity for years. Once the soil becomes outside the animal body until it can infected with anthrax spores, therefore, gain entrance to a new host largely it may remain dangerous for suscep- determines its ability to spread disease. tible animals for long periods. Another disease-producing organism CERTAIN BACTERIA can live for in- that is quite resistant to adverse influ- definite periods outside the animal ences is the germ that causes swine body and still retain a high degree of erysipelas. It does not go into a spore virulence. stage, but it can live in certain soils for An example is the anthrax organism. long periods, and the suggestion has In the final stages of anthrax, the ani- even been made that active multiplica- mal's blood stream and tissues contain tion may take place in suitable types of How Diseases and Parasites Are Spread 41 soil. Here again the infected animal ground, where the susceptible animals contaminates its surroundings through easily pick up the infection. In the its discharges. early stages of the disease, the virus is found in the blood, milk, saliva, urine, THE VIRUS DISEASES are alike in sev- and perhaps the feces. Because of the eral ways, but their mode of transmis- large quantities of the virus thus elim- sion may vary considerably, depending inated by affected animals and its on the nature of the disease and the ex- highly infectious nature, a wide dis- tent to which the virus has multiplied semination of the disease in a short within the body of the affected animal. time is possible. Infection takes place when the virus in a proper state is introduced into a sus- A CARRIER ANIMAL is One that carries ceptable animal. an infective agent within its body Rabies, for example, is caused by a without showing evidence of illness. filterable virus that is found in the It may eliminate the infective agent saliva of an affected animal and under from time to time and act as a source natural conditions it is transmitted of infection to other animals; or the through the bite of a rabid animal, carrier animal itself through some ad- usually a dog. The disease causes de- verse condition may develop a frank generative changes in the brain, which attack of the disease; or the infection cause affected animals to become ag- may be carried from a carrier to a gressive and attack or bite any avail- normal animal through the bite of an able object. After the virus is deposited insect. in the tissues, usually through the in- Certain animals that have recovered jury made by the bite, it is believed to from infection with the virus of foot- move from the site of deposit by way and-mouth disease may retain active of the nerves. Eventually it reaches the virus within their bodies for months or the central nervous system—the brain years and discharge the virus from and spinal cord. time to time in sufficient amounts to Equine encephalomyelitis, which cause the disease in susceptible ani- sometimes is known as sleeping sick- mals. ness or blind staggers of horses, is an- In infectious anemia of horses—so- other virus disease that attacks mainly called swamp fever—a carrier stage is the brain and spinal cord, although it seen in which an apparently normal can propagate also in other tissues, in- animal carries the infective agent in its cluding the skin. Early in the infection blood stream for many years, perhaps the virus of equine encephalomyelitis during the rest of its life. It seems lives for a short time in the blood that this disease is transmitted mainly stream. It then disappears from the by biting insects. blood and is found only in the brain Wild animals and birds also may be and spinal cord. Nervous symptoms of reservoirs and spreaders of diseases as, the disease are noted early. It can be for example, foot-and-mouth disease, transmitted by the bites of a number rinderpest, and trypanosomiasis. of different species of mosquitoes and other insects. The disease is therefore FARM ANIMALS acquire parasites by considered to be spread chiefly through grazing on contaminated pastures; by biting insects. ingesting feed or hay contaminated The virus of foot-and-mouth dis- with the manure of parasitized ani- ease is an example of the viruses that mals; by swallowdng contaminated affect skin membranes. Vesicles, which soil, water, insects, and other small form in the mouth and on the soft forms of animal life that harbor infec- parts of the feet, contain the infective tive stages of parasites; by eating the agent. When they rupture, the virus is flesh or other tissues of animals that disseminated in the litter and on the harbor infective stages of parasites; or 42 Yearbook of Agriculture 1956 by being bitten by infected insects soon after they have been eliminated and ticks. from animals, because their ability to Among the practices that contribute live outside the host is sharply limited. to the dissemination of parasites are: Trichomonads in the intestines of Bringing parasitized stock to a farm or swine and other animals and in poultry ranch from the outside; using per- arc transmitted in that way. manent pastures year in and year out; spreading manure on pastures; placing WORM PARASITES of the alimentary feed on the ground of barns, stalls, and canal, or of the lungs and other organs other shelters; and allowing parasi- that have a direct or indirect connec- tized dogs to be near livestock. tion with the alimentary canal, produce eggs, which are eliminated with To INSURE their perpetuation, para- the droppings. sites eliminate their reproductive ele- The swine kidney worm is one of the ments in the form of eggs or larvae, few parasites that eliminates its eggs which as a rule are produced in enor- into the ureter, from which they are mous numbers to compensate for the voided with the urine. Once the eggs likelihood that many of them will per- reach the outside, regardless of how ish before they can get into another they are eliminated from the host's host. Furthermore, the reproductive body, they develop and become in- elements often are encased in shells fective as eggs, or they hatch into and other tough membranes, which larvae, or the eggs have to be swal- enable them to survive for a long time lowed by intermediate hosts. Gold outside the body even when conditions weather retards the development of are unfavorable. parasite eggs, but during favorable Many protozoan and worm parasites weather the eggs develop rapidly. of animals are transmitted through Those that hatch yield larvae, which the excreta. Parasites that inhabit the undergo their transformations to the digestive tract of cattle, sheep, goats, infective stage in about one week or swine, horses, poultry, and wild ani- more, depending on the temperature. mals are transmitted in that way. The larvae that first emerge from Some protozoan parasites transmit- eggs feed almost constantly on bacteria ted through the excreta of the host and bits of organic matter, grow rap- have a resistant form known as a cyst— idly, and molt in a day or two. Before a resting stage, microscopic in size, they become infective they undergo usually rounded, and enclosed in a at least a partial second molt, after membrane. The membrane protects which they are ready to invade the the parasite from unfavorable influ- kind of hosts from which they came. ences after it has been eliminated from During the favorable weather that the body. Sooner or later the cyst be- prevails in most parts of the United comes infective to other animals of the States in late spring, summer, and kind in which it originated. It gets into early fall, the development of the ç^gg them by being ingested with grass, dry to the infective larva takes about a feed, or water that has become con- week or less. The infective larvae climb taminated with the droppings of in- upward on grass and other vegetation, fected animals. Coccidia of livestock especially after rains and dews when and poultry, parasitic amebas of ani- the vegetation has a film of moisture mals and man, and the ciliates, which on it. The upward migration places parasitize swine, are so transmitted. the larvae in a situation where they Other protozoans are transmitted are likely to be swallowed by grazing by the active stage, known as a tropho- animals. In barns the larvae climb up zoite, which passes out of the infected on litter, hay, moist walls, and posts host animal with the droppings. and may be eaten or licked off by Trophozoites have to be swallowed animals housed therein. How Diseases and Parasites Are Spread 43 Cattle, sheep, goats, horses, and serve as intermediate hosts are insects swine acquire many of their internal (particularly those that cat dung or parasites by grazing on pastures that breed in it), snails, slugs, earthworms, have become contaminated with the free-living mites, and other inverte- eggs and larvae of the parasites we brate creatures found in pastures, mentioned. Stomach worms of cattle, yards, corrals, barns, and other places sheep, and goats and most of the in- to which animals and poultry have testinal hairworms of ruminants that access. When the infective interme- produce enteritis and scouring are diate hosts are swallowed by suitable acquired in this manner, as are lung- definitive hosts, either accidentally or worms of cattle and the common lung- because they are choice morsels, the worm of sheep. parasites they contain are freed in the Other parasites, however, including digestive tract of the new host, migrate hookworms of farm animals and pets, to their favorite location, and develop intestinal threadworms of all domestic there to maturity. animals, kidney worms of swine, and A mode of development peculiar to other worm parasites can gain en- the liver flukes and certain other flukes trance into the host by boring into its involves the hatching of the ç:gg out- skin. That can readily happen when an side the host, the eating of the larvae animal lies down in a pasture, barn- by snails, and finally the emergence of yard, or corral that is contaminated the transformed larvae from the snails with skin-penetrating larvae. onto vegetation. Livestock become infected by swallowing the larvae with SOME PARASITE EC;GS that develop grass, water, and sometimes even dry outside the host but do not hatch there feed. The lancet fluke of ruminants arc infective as eggs when a suitable requires a second intermediate host— animal swallows them. Among them a common pasture ant—before it can are the large intestinal roundworm or infect cattle and sheep. ascarid of cattle, horses, and swine; whipworms of cattle, sheep, goats, and IN ADDITION to the passive transfer of swine; and intestinal roundworms or parasites by eating insects and other ascarids and whipworms of pet ani- low forms of animal life, others are mals. The infective eggs are swallowed injected into the blood of their host by with contaminated feed or water in biting insects or other arthropods, kennels, runs, yards, corrals, pastures, especially ticks. Tick fever and ana- barns, stables, and elsewhere. The plasmosis of cattle are two parasitic larvae hatch in the alimentary canal diseases caused by protozoans that of the host, reach their preferred loca- are transmitted in this manner. Tick tion in the digestive tract or elsewhere fever is transmitted by the bites of in the body by active migration, and ticks, which transfer the causative develop there to maturity. organisms they harbor to the cattle Other parasites require intermediate whose blood they suck. Anaplasmosis hosts to convey them from one animal also is transmitted in this manner and to another. Among them are certain may also be transferred from cattle to stomach worms of horses, mules, and cattle by horseflies during the inter- swine; gullet worms of swine and rupted feeding of the flies on the blood ruminants; swine lungworms; sheep of the hosts. lungworms; intestinal tapeworms of Outstanding examples of the trans- ruminants; and other parasites of live- mission of parasitic diseases by biting stock, pet animals, and poultry. The flies are trypanosome diseases of live- eggs pass out of the host and develop stock. In Africa tsetse flies transmit into larvae only when they are eaten trypanosomes, which they acquire by certain intermediate hosts. Among while they are obtaining their blood the small forms of animal life that can meal from infected animals. Later the 44 Yearbook of Agriculture 1956 flies can transmit the trypanosomcs to WE NEED TO KNOW how parasites are other suitable host animals during spread from animal to animal before another blood meal. In South America we can develop rational control meas- trypanosomes of livestock are trans- ures. Before the existing knowledge mitted by other bloodsucking flics came to light, little of practical value since tsetse flies do not occur there. could be done to curtail the spread of Many kinds of worm parasites also parasites and minimize their efl'ects. are transmitted by biting flies and The discoveries made since 1890 or so other insects that attack their host and on how parasites arc transmitted by feed on its blood. Stablcflies, for ex- ticks, mosquitoes, and other insects ample, transmit certain stomach worms make a fascinating chapter in the his- to horses. The larvae of the stomach tory of parasitology and preventive worms, which are located near the medicine. The discoveries have helped mouth parts of the insect, arc stimu- us to curb and even to eradicate some lated to activity during the biting proc- devastating parasites. ess by the heat of the horse's body and Another important discovery stem- enter into the skin that has been broken med from the work of Arthur Looss in by the fly's bite. Various long and Egypt early in the 20th century. Looss slender nematodcs, known as Alarias or determined that the human hookworm fiiarids, which occur outside the diges- followed a pattern of development out- tive tract in practically all classes of side the host; that was found to be the farm animals, are transmitted by development pattern also of some in- bloodsucking insects. jurious nematodcs in the digestive tract of farm animals. Looss determined, STILL ANOTHER METHOD of parasite moreover, that hookworm larvae can transmission is through the eating of enter human and canine hosts by bur- the flesh or internal organs of one ani- rowing through the skin. That, too, was mal by another. Man and animals, for later shown to be true of a number of example, acquire trichinae and the injurious parasites of livestock. disease trichinosis, which trichinae produce, by eating the flesh of infected LNSEGTS spread from place to place hogs or other animals. Man, dogs, and in many and devious ways. Man him- other animals acquire intestinal tape- self transports from farm to farm, from worms by eating the flesh of animals State to State, and even from foreign infected with bladderworms. Hogs ac- lands some destructive insect pests. quire lung flukes by eating crayfishes Flics and ticks can be carried on com- infected with the larval flukes. modity shipments in railroad cars, A more unusual method of trans- ships, trucks, airplanes, and automo- mission is through copulation. Dourine biles. Lice, mites, cattle grub larvae, of horses, a disease that occurs in the and some flies remain on an animal Western Hemisphere and is caused by during shipments and are thus spread. trypanosomes, is transmitted in this Changing conditions favor establish- manner. Another disease transmitted ment of new pests, such as mosquitoes through copulation is bovine tricho- in irrigated regions, when the habitat moniasis, caused by trichomonads is suitable in the new areas. which aflfect the reproductive system of The growth of livestock auctions has bovines and cause reproductive fail- intensified the spread of some insect ures in cattle in the United States and pests. Livestock may be brought many other countries. The infected bull miles to an auction and within a day transmits the causative organisms to or two be transported an equal or the cows and heifers he serves. An un- greater distance to some other section. infected bull acquires these organisms Shipping livestock to agricultural fairs by serving a cow that harbors them in occasionally hastens a reinfcstation of the vagina. parasites. The larvae of flies are trans- How Diseases and Parasites Are Spread 45 insect, originally from Europe, became established in the United States about 1890. Within a decade it had spread throughout most of the United States and became one of our most important pests of cattle. Cattle lice have been reported to at- tach themselves to hornflies and thus be transported to other animals. Many insects are strong fliers and can migrate far. The black blowfly, Phormia regina^ which produces fleece- worms in sheep, can fly at least 28 miles. The houseflies will fly at least 20 miles. Horseflies and deerflies will move several miles from their breeding places. Lice of cattle, sheep, and goats and other animals are spread by contact of Biting sheeY> louse—female. one animal with another. A clean herd can become lousy by the introduction ported in the straw and in manure of of one infested animal. It is therefore trucks and railway cars. Moving live- prudent to eradicate lice on acquired stock to new ranges or feed lots spreads animals before the animals are placed many of these pests. with clean livestock. Animals harboring screwworm in- Ticks, which are wingless, spread by festations are shipped from southwest- contact between animals. They rest on ern Texas to the uninfested Northern plants until a warm-blooded host passes States every spring. Infestations ha by, and they drop on it. They also are Northern States have become more transported by manmade shipments. common presumably because of more rapid methods of transporting live- H. W. ScHOENiNG retired in ig§5 after stock. Although the screwworm fly 48 years of service in the Department of migrates by natural flight, the shipping Agriculture. At the time of his retirement of infested animals hastens spread in he was assistant to the chief of the Animal Texas and other Southwestern States. Disease and Parasite Research Branch, The screwworm was unknown in the Prior to that assignment^ he served as Chief Southeast until 1933, when it was in- of the Pathological Division of the former troduced in infested cattle from Texas. Bureau of Animal Industry for 20 years. Pie The seasonal history of cattle grubs entered the Department shortly after his in some localities is being altered by graduation from the School of Veterinary frequent cattle shipments. Because Medicine of the University of Pennsylvania. grubs appear earlier in the backs of BENJAMIN SCHWARTZ in ig^^ became cattle and persist longer than formerly, staff assistant and consultant in parasi- the period during which control must tology in the Animal Disease and Parasite be conducted is extended. The heel- Research Branch of the Agricultural Re- fly, the adult of the grub, also is active search Service. He has been engaged in para- for a longer period. Since 1930 the sitological research since igi^. northern cattle grub, Hypoderma bovis^ ARTHUR W. LINDQUIST is leader of the has spread from the East Coast through Insects Affecting Man and Animals Section^ all of the Northern and Western States. Entomology Research Branch. He is a grad- Most of this dispersal was caused by uate of Bethany College, Lindsborg, Kans., shipment of infested cattle. The rapid and Kansas State College. Since igji he has spread of an insect species is exempli- conducted and directed research on the biology fied by the spread of the hornfly. This of pests that affect man and livestock.