Protection Against Transmissible Diseases and Parasites

B. T. SIMMS.

LONG before anybody knew about germs, farmers were trying to keep diseases away from their animals. They did not know the meaning of "bacteria" and "virus," but they did know that a sick animal might spread disease. They tried to stop the spread by doing two things just as farmers today do. One was to isolate healthy animals from animals that were sick or had been in contact with sick animals. The other was to increase the resistance of healthy animals so they would not get a disease if they were exposed.

When the number of farm animals was comparatively small and farmsteads were farther apart than now, a farmer could do much on his own farm to keep his stock healthy. But as farming and the population expanded and more and more livestock were shipped to and from markets, individual action was not enough.

Thinking people knew that group or government action was needed to stop international and regional movement of disease-spreading livestock and poultry. So quarantines to prevent the spread of some of the more serious diseases were set up some of them many years before anybody knew what caused the disease.

Kansas and Nebraska had quarantine laws to control hog cholera 20 years before research workers in the Department of Agriculture discovered that it is caused by a virus too small to be seen with a microscope. Massachusetts set up a quarantine against pleuropneumonia of cattle long before the germ causing it was discovered. A hundred years before scientists of the Department of Agriculture discovered the blood parasite that causes cattle tick fever and proved that ticks spread the disease, North Carolina tried to stop the spread of cattle tick fever by prohibiting the movement of cattle from the eastern part of the State (which was tick-infested) to the western part (which was tick-free).

None of those quarantine measures stopped the spread of the diseases they were aimed at. Because people lacked exact knowledge of how the diseases spread, it was impossible to include the most effective preventive measures in the quarantine regulations. Neither adequate staffs nor enough funds were available to enforce the quarantines strictly enough. The significant point is that quarantines were invoked so long ago.

Long ago, also, livestock owners tried to make animals more resistant to disease by various measures. They fed them sulfur, wood ashes, or charcoal. They gave drugs that were said to act as tonics, and put pine tar in drinking troughs. Some farmers burned feathers in their barns and stables as a protective measure. None of these measures or similar ones helped much.

One factor in curbing the spread of disease before the development of the steam engine was the lack of transportation. Most livestock moved on foot and the sick animals spread disease only as far as they would walk. Two examples:

Hog cholera first appeared in the United States about 1833 in the Ohio Valley. It did not become widespread for 20 or 30 years because hogs did not go far from home. Our first outbreak of foot-and-mouth disease occurred along our northern border in 1870 in winter. Because animals and people did not move about much in cold weather, the disease spread very little and died out before spring.

Steam and internal-combustion engines changed all that. Railroads, water transportation, trucks and automobiles on good highways, and airplanes made rapid and long-distance shipments of animals commonplace. They also presented an almost unlimited possibility of spreading disease.

If our methods of preventing disease were no better than they were a hundred years ago, we might now be spending ever so much more time than we do in the elemental fight against malnutrition and hunger. But during the years that transportation was developing, science also was moving forward against the scourge of transmissible diseases.

THE FIRST big accomplishment was proving that living organisms cause disease and that the organisms always originate from other organisms. The French veterinarian Delafond in 1863 demonstrated that the blood of sheep dead of anthrax contained microscopic organisms that would multiply if kept under suitable conditions. The German bacteriologist Koch in 1876 proved the organisms Delafond had seen and grown were the cause of anthrax in the animals and in people.

In 1869 Moritz, another French veterinarian, noticed "granulations" in chickens that had died of fowl cholera. Perroncito, an Italian veterinarian, saw and made sketches of the germs. Then the French veterinarian Toissant proved that those organisms caused the disease. He sent Pasteur, the famous French chemist, the head of a cock that had died of cholera. From it Pasteur grew the germs of cholera and produced a vaccine that would protect against it. These were epoch-making results.

Scientists now knew how to grow the germs of a disease, how to produce disease with them, and how to make from them a protective vaccine. But that was only the beginning of a program for protecting animals from transmissible diseases. Many practical questions needed answers: Where and how do germs grow in sick animals? How do they escape from such animals? How long will they live outside an animal? What will kill them? How do they enter susceptible animals? Do recovered animals continue to carry and spread germs? Each question had to be answered for each disease, because each was different from all others.

Research workers had to learn how to produce each disease studied so they could have sick animals with which to work. That in itself was quite a task.

Some examples: Scientists found out that anthrax organisms grow in practically all organs of the body. Most coccidia (the minute organisms that cause coccidiosis) multiply in the lining of the intestines only. The bacteria that cause brucellosis in cattle multiply principally in the pregnant uterus (womb) and in the udder. Anthrax germs may escape from infected animals in discharges from the nose, in urine, or in feces (dung). Coccidia are passed out mainly with the feces. Brucella organisms escape with the calf (at the time it is aborted or born), in the discharges from the uterus a few days or weeks after aborting or calving occurs, and in the milk. Anthrax bacteria can live for 50 years or more in soil even when exposed to bright sunlight and dry air. Some types of coccidia may live for many months in pastures or corrals. Brucella organisms are usually destroyed in a few days or a few weeks if they are exposed to direct sunlight and dry air.

RESEARCH WORKERS learned that animals that appear healthy may spread the organisms that cause such diseases as tuberculosis, brucellosis, glanders, fowl cholera, and coccidiosis. This discovery clarified the disappointing results that had followed attempts to eradicate disease from the infected herds and flocks by removing all animals that were showing symptoms: The carrier animals were not removed, and they continued to spread infection.

Scientists have done a great deal of research to find methods of detecting carrier animals that show no symptoms. They have given us many new methods and tests for diagnosing disease. Every cattle producer knows what tuberculin is. Every poultryman knows about the agglutination test for pullorum disease. Horse breeders do not know how to make a complement-fixation test, but they know it can be used to determine if a horse has glanders.

Active and successful research with methods of spread of disease paralleled the work with diagnostic procedures. The results of such research are discussed in another chapter.

Information and facts developed through research efforts during the last 100 years make it possible to prevent many diseases by preventing exposure. The basic principles concerned in accomplishing this consist of two steps. The first is learning where infection is. The second is prevention of any direct or indirect contact between disease-free animals and infected animals, or contaminated premises, feed, equipment, or vehicles.

The first learning where infection is may be accomplished in some instances by making careful and repeated examinations of the suspected animals. That, for example, is the basis of locating sheep scab infection. Trained men examine flocks and herds for symptoms and lesions changes brought about in tissues by the disease.

If they find suspicious lesions, they examine scrapings from them with a lens for scab mites. A positive diagnosis is made only when mites are found. Sometimes a combination of a physical examination and a diagnostic test is the standard diagnostic procedure. Diagnosis of foot-and-mouth disease is an example of this technique. Veterinarians examine the mouths and feet of cloven-footed animals in areas where the disease exists or is suspected. If they find suspicious cases, they may collect material and send it to a laboratory for a test. Sometimes, however, symptoms and lesions are so typical that the diagnosis is made without laboratory tests.

Sometimes one relies almost entirely on a test as the diagnostic method. Veterinarians rely on the tuberculin test rather than symptoms to diagnose tuberculosis.

The second step preventing contact between disease-free animals and any source of infection often is difficult. The easiest and most economical way to accomplish that is to separate all the diseased and exposed animals from healthy animals, give them a curative treatment, hold them in isolation until they are free of infection, and clean and disinfect all contaminated premises. That would eradicate disease by destroying the infectious agent at its source. It can be done with such diseases as sheep scab. Unfortunately, though, only a few diseases can be handled this way because only a few really effective treatments are known. Other methods of handling infected animals besides treatment must be resorted to in most diseases.

Sometimes the infected animals may be held in isolation and quarantine on the premises with little danger. Sometimes such procedures are unsafe. Sheep infected with lungworms may be kept on farms where lungworm-free sheep are held if they do not use the same pastures, corrals, or sheds, and if there is no drainage from them to the premises used by the lungworm-free sheep. But any attempt to hold animals infected with foot-and-mouth disease on premises with susceptible animals probably would be disastrous.

INFECTED ANIMALS are the source of all infection. They are always potentially dangerous.

Slaughter of infected animals is usually the best procedure for actually preventing exposure and spread of easily transmitted diseases like foot-and-mouth disease, which is so infectious that it is well-nigh impossible to prevent its spread if sick and healthy animals are kept on the same premises.