Tularemia is an infectious plague-like disease distributed throughout much of the world. The disease has been reported in man and more than 80 other species of mammals most of which are rodents and lagomorphs. Although birds have been infected experimentally they are not considered important hosts under natural conditions. In wildlife of the United States the disease occurs mainly in cottontail rabbits. One of the earliest records of the disease in wildlife of Ontario was a report from 1937 of tularemia in a snowshoe hare. Additional hosts of tularemia in Ontario include beaver, muskrat and man.
Tularemia is caused by the bacterium, Francisella tularensis. Upon gaining entrance to the body the bacterium multiplies rapidly, spreads through the body in the bloodstream and invades cells of the liver, spleen, lungs, kidneys and lymph nodes or glands. Due to the action of the bacteria and their toxins on these cells, tissue changes occur which give rise to small white areas (foci) consisting of dead cells. This condition is called focal necrosis.
Infection with F. tularensis is usually fatal to beaver, muskrat and cottontail. On postmortem examination the liver and spleen of these animals are found to be enlarged in addition to being dotted with foci of necrosis. In all three species lymph nodes may be enlarged and in some cases abscessed. The lungs are not affected as frequently but when involved they possess lesions similar to those seen in the liver and spleen.
Focal necrosis is not characteristic only of tularemia. It is observed with infection of wildlife by a variety of other serious bacterial diseases such as pseudotuberculosis and listeriosis. For this reason tularemia cannot be diagnosed in the field. A definite diagnosis of tularemia is dependent on successful isolation of F. tularensis using laboratory techniques. If a carcass possesses lesions characteristic of tularemia the carcass should be refrigerated and delivered or sent to the nearest diagnostic laboratory for bacteriological examination. If the carcass is frozen when received, it should be kept frozen for delivery to the lab.
In man, tularemia occurs most commonly in trappers, hunters, market men, butchers, farmers and laboratory workers. Although the infection is less virulent in humans, it is important to recognize the symptoms in man, as early treatment will, in most cases, successfully eliminate the infection.
The disease in man can follow any one of several courses resulting in the development of four more or less distinct clinical types of disease. General symptoms such as high temperatures, headaches, chills, sweats, nausea, vomiting and body pains occur in varying degrees with each of the following clinical types:
(1) Ulceroglandular type: This form of the disease is characterized by the formation of an inflamed papule (small swollen area) at the site where the infection entered the body, generally on the hands, arms, face or neck and is known as the primary lesion. The papule quickly fills with pus and then develops into an ulcer. Soon, this is followed by a swelling of the lymph nodes draining the lesion. The affected nodes or glands become quite tender, undergoing necrosis, suppuration and ulceration.
It is estimated that approximately 87% of all cases of tularemia are of this nature.
(2) Oculoglandular type: In this form of the disease, a conjunctivitis (inflammation of certain membranes of the eye) develops and is accompanied by an enlargement of the neighbouring lymph nodes. Approximately 3% of all tularemia cases fall into this category.
(3) Glandular type: In these cases, which comprise about 2% of all cases of tularemia, skin and eye lesions are absent but there is a general enlargement of all the lymph nodes of the body.
(4) Typhoid type: When the disease follows this course, which it does in approximately 8% of the cases, there is no obvious primary lesion nor is there any enlargement of the lymph nodes. There is, however, a high temperature and superficially, the symptoms resemble influenza. This form of the disease represents the septicemic or bacteremic stage of the disease and is the form usually seen in laboratory workers.
(5) Pneumonic type: This form of the disease, in which the infection reaches the lungs causing a pneumonia, is an extension of one of the previous four types.
In some instances tularemia occurs in a mild form and there is a tendency for the affected person to ignore the illness or treat the symptoms in a casual fashion. It is even possible that a person might be infected and not realize it. If a person has a history of handling animal carcasses or skins or having been bitten by insects, any illness similar to influenza or blood poisoning should be treated with suspicion and a physician should be consulted.
F. tularensis may be transmitted from animal to animal and from animal to man in several ways:
(1) Ingestion
(a) By drinking water or eating ice contaminated by either the carcass or the body discharges of animals infected with tularemia.
(b) By eating insufficiently cooked meat from infected animals.
(2) Inoculation
Biting arthropods such as blood-sucking flies, ticks, lice and fleas are an important means of transmitting tularemia among wild animals and also to man.
(3) Contact
Direct contact with infected animals is the main source of infection to man. The fur, internal organs, body fluids and discharges of an infected animal carry the bacteria which cause tularemia. When any of these are handled by man, the organism can be transmitted. Usually the organism enters the body by way of cuts and scratches on the skin although in some instances it may apparently penetrate the unbroken skin. F. tularensis can be carried into the eye by rubbing the eye with fingers soiled with infective material. It is unlikely that animal skins which have been dried over a period of three or four weeks will carry organisms capable of causing the disease.
Wild animals, especially beaver and muskrat, should be handled with care, particularly if they come from areas in which tularemia is known to occur. In locations where the disease is present or has occurred in the past, protective clothing should be worn and any ticks attaching to the body should be promptly removed. When skinning animals or handling fresh skins from an area in which tularemia is suspected (1) rubber gloves should be worn; (2) soap and water should be used liberally on hands and
arms and should be followed by a 1% cresol solution; (3) a 5% cresol solution should be used to clean up and disinfect all instruments and working space.
F. tularensis will be destroyed by thoroughly cooking wild birds and mammals to be used as food. In areas where F. tularensis is known to occur it could be of value to disinfect water which will be used for consumption.
Selected Rcferences:
Jellison, W.L., C.R. Owen, J.F. Bell and G.M. Kohis. 1961. Tularemia and animal populations: ecology and epizootiology. Wildl. Dis., No. 17, 22 pp. (micro-card).
Labzoffsky, N.A. and J.F.A. Sprent. 1952. Tularemia among beaver and muskrat in Ontario. Can. J. Med. Sci. 30:250-255.
Merchant, I.A. 1967. Veterinary bacteriology and virology. 7th ed. The Iowa State University Press, Ames.
