Infectious Diseases Common to Animals and Man

JAMES H. STEELE.

ZOONOSES are diseases that are transmitted among animals and from animals to man.

Of more than 200 communicable diseases of animals, one-half are considered infectious to man, and more than 80 are transmitted naturally between vertebrate animals and man.

They are grouped according to their morphological characteristics that is, viral, rickettsial, bacterial, fungal, protozoal, helminthic, and arthropod-insect.

Many other parasitic diseases of animals have occurred only rarely or on single occasions. Of the 120 parasites found both in animals and man, 76 are rare in man. Some of the latter may occur more commonly than is supposed, but others must be thought of as accidental infections.

Animals also are passive carriers of disease organisms, the most common of which are the bacilli of tetanus, gas gangrene, and botulism. Animals also transmit toxins, or chemicals, which may be injurious to persons who eat the meat.

Wild animals and birds are important reservoirs of zoonoses. They may be of even greater concern than domestic animals in some countries.

Among them are wild dogs, wildcats, and rodents.

Rabies is the most serious disease that can be attributed to the dog.. The dog, as far as man is concerned, is the major reservoir of rabies. Each year in the United States more than 600,000 persons are bitten by dogs; 5 to 10 percent of the victims have to have antirabies vaccine because the biting animal is proved rabid or is suspected of being rabid. Even if the dog is not captured or if for some other reason laboratory examination is not possible immediately, the victim must be given the vaccine.

The cost of rabies in our country exceeds 10 million dollars a year.

In the Western Hemisphere, rabies is a problem in the United States and in Alaska, parts of Canada, Mexico, Central America, and most of South America. The only places free of rabies are eastern Canada, the Northeastern States, some localities in the Northwestern States, some parts of the West Indies, Panama, southern Chile, and Argentina. The elimination of canine rabies in the Americas would save millions of dollars. The same probably can be said for the parts of Europe, Africa, and Asia where canine rabies is enzootic, that is, constantly present but of low incidence.

The control of canine rabies in cities is possible in most of the world if people recognize the problem and believe it can be done.

An effective program to control rabies is based on three principles:

Public health education, which is necessary to have public support of the technical phases; vaccination of the resident dog population with an effective canine rabies vaccine; and the elimination of stray and homeless dogs and, sometimes, wild animals.

The success of such a program depends on public support, the experience and training of the technical staff, and the effectiveness of the immunizing agent that is used. The cost of a preventive program is not comparable to the cost of living with the disease. Human treatment costs much more than canine immunization; and if the indirect or inestimable values such as mental anguish and loss of productive time are included there is no logic in the apathy that surrounds this disease in many cities, States, and countries.

MANY INVESTIGATORS consider the dog to be the principal mammalian reservoir of the infections that have been grouped under African tickborne fever (rickettsial). Among the diseases included in this group are Boutonneuse fever, South African tick-bite fever, Kenya typhus, and Brazilian and Colombian spotted fever. Dogs also may harbor the ticks that transmit Rocky Mountain spotted fever. Dogs have been known to develop infection, with signs of disease. Under such circumstances, uninfected ticks could become infected from the host dog and give rise to infected progeny, which would transmit the disease among various animals and to man.

The dog harbors a variety of bacterial diseases that occasionally cause human illness. The most important probably is leptospirosis. Leptospira Canicola is the causative organism commonly found in dogs, but L. icterohemorrhagiae, the rat type, has been found in infected dogs and farm animals. The infection is spread through the urine of diseased animals. Various methods of control have been attempted. The infection in canines responds to treatment with streptomycin and penicillin. Vaccines also have seemed to be of value.

The most common fungal disease transferable to man from the dog and the cat is ringworm caused by Microsporum Canis. We must control the disease in pets if we are to stop its spread to man. Because no tried and proved medication is available, each series of cases must be treated according to the experience of veterinarians in the area. It is recommended that people avoid intimate association with pets that may have the disease and that communities cooperate in destroying stray dogs. Those measures are important also in the control of other canine zoonoses.

Creeping eruption, an irritating form of dermatitis, plagues man in regions where the dog and cat hookworm, Ancylostoma braziliense, exists. It is common in the Southeastern States and parts of South America, Africa, and Asia. The larvae of A. braziliense have the ability, as do all hookworm larvae, to penetrate the skin of the host or accidental host, i. e., man in which it causes an inflammatory reaction manifested by intense itching.

In man the invasion of the organism is limited to the skin, in contrast to the more extensive invasion of the human type of hookworm, Ancylostoma duodenale. In regions where A. braziliense is common, many persons become infected at work and play at beaches and playgrounds, in gardens, and in the close quarters under buildings where plumbers, carpenters, electricians, and other persons sometimes have to work. Often the soil in such places is contaminated with infected dog and cat feces, and exposure is almost unavoidable.

The problem is serious. In one Southeastern State 8,000 cases were reported in one summer, but that number probably was a fraction of the cases that actually occurred.

Prevention in human beings requires that dogs and cats be. barred from all recreational areas. Pet owners should make sure that infected animals are treated with an effective vermifuge. Contamination of gardens, yards, and households thereby can be prevented.

THE INFESTATION by the tapeworm Echinococcus granulosus is serious in dogs.

Echinococcis is transmitted directly to man by the accidental ingestion of the eggs, which are found in dog feces and which in turn contaminate the immediate vicinity. The infection may sometimes be airborne the eggs are blown around in dust and thus contaminate food, food utensils, and man himself. The disease is one of the major zoonoses in southern South America, the Mediterranean area, the Middle East, Australia, New Zealand, and parts of Alaska and Canada. Some authorities have estimated that more than 50,000 persons in those areas are in some stage of infection at any time.

Echinococcus granulosus has an interesting life cycle between herbivorous and carnivorous animals. The adult worm inhabits the lower intestines of dogs and other Canidae wolves, foxes, coyotes, and wild dogs. The eggs are passed in the feces. Herbivore (cattle, sheep, goats, deer, and other grass eaters) become infected through the ingestion of contaminated vegetation. Swine also are frequently infected. The larval form of the infection is found in the herbivorous animals and swine, as in man. The infection in cattle, sheep, and other such animals produces hydatids, or cysts, in the lungs, liver, and other sites. The cysts are filled with fluid and infectious daughter cysts. When a dog or other definitive final host ingests cyst-bearing flesh or offal from diseased animals, the germinal tissue develops within 50 to 60 days into a sexually mature tapeworm, thus completing the organism's life cycle.

Echinococcosis is a costly public health problem in areas where it is enzootic. If only one-half of the estimated cases are treated, the cost of medical care runs into tens of millions of dollars. The economic losses due to this disease are also sizable when one considers the mortality among farm animals, loss of production resulting from human illness, and condemnation of meat animals or parts of their carcasses because of infection. An intensive control campaign would save millions of dollars and prevent untold illness and death and contribute to an improved animal husbandry and economy.

Control of hydatid disease is based on the elimination of the infection in dogs and the prevention of reinfection. Both in cities and in rural areas dogs should be kept under control. Generally speaking, in areas where the infection is known to occur, the numbers of dogs should be reduced as far as possible, and people should be discouraged from keeping pets within their home.

Dogs that must be kept within the home should be examined periodically if they are found to be infected, they should be treated.

ONE CANINE roundworm, Toxocara canis, causes severe and sometimes fatal liver damage in young children.

Other parasitic infections of dog that cause human disease include various other roundworms, tapeworms, and flukes. Strongyloidiasis has been reported frequently in dogs and man The dog tapeworm, Dipylidium caninum, is acquired by man when he accidentally swallows an infected flea Ctenocephalides canis, the intermediate host. Human infections with fluke are major health problems in man areas of the world. Dogs may have part in maintaining the life cycle of the fluke. The infection of dogs with Schistosoma japonicum has often been reported in the Far East.

Prevention of all these parasitic infections is based on proved hygienic principles, on effective control of dogs, and on veterinary medical care of animals closely associated with man.

This discussion would indicate that the dog is probably the most important animal reservoir of disease as far as man is concerned. Probably it is, because of its intimate association with man and its worldwide distribution. But man is exposed to the diseases of many other animals with which he has daily contact. After the dog, the cow is probably the most important of these animals. Unlike the dog, the products of the cow are the usual means of disease transmission, although contact may also be a route of infection.

BOVINE TUBERCULOSIS is a major zoonosis throughout the world. The disease is encountered frequently in cattle-raising areas of the world except in the United States, Canada, and Scandinavia, where the prevalence of bovine tuberculosis in cattle has been reduced to the point that it is no longer a serious animal problem or a public health problem.