PREDATORS and Rodents are Factors in the Spread of Disease
That wild animals may be carriers of human diseases, notably bubonic plague, spotted fever, and rabies, has long been recognized by medical authorities. Investigations during the past few years have added other diseases to the list, and now it is becoming more generally appreciated that wild animals play an important role in the health as well as the economic life of man, The Bureau of Biological Survey has done much to learn of the relationship of wild animals to man and to aid in dealing with outbreaks of various diseases by controlling the wild-animal hosts. These diseases have included tularemia, Rocky Mountain spotted fever, endemic typhus fever, rabies, and bubonic plague.
Tularemia has been found to be transmitted, usually by insects, from infected rodents—principally wild rabbits—to man. One of the most recent of the outbreaks, which have been rather common throughout the West, occurred in Meagher County, Mont., late in April and early in May of 1934. Jack rabbits died in great numbers, and dead ground squirrels also were noted. ~Approximately 200 head of a band of sheep grazing in the area died before the cause was discovered to be tularemia. Investigation by specialists of the Public Health Service, the State board of health, and the Bureau of Biological Survey demonstrated that wood ticks, present in great numbers, were responsible for the transmission of the tularemia from the diseased rodents. The sheep were sheared, dipped, and moved to another range, and the Biological Survey inaugurated a campaign to eliminate the rabbits and ground squirrels, labor and funds being supplied for the purpose by the State emergency relief administration.
Rocky Mountain spotted fever, long one of the dreaded diseases of the West, has been transmitted to humans by wood ticks, with rodents and other wild animals acting as intermediary hosts. Alarm has been felt by health authorities in the Eastern States because of the recent occurrence there of this disease, heretofore considered as a western malady only. Cases have been reported in Pennsylvania and Maryland. A few deaths occurred near the District of Columbia.
Endemic typhus fever, while less important as a cause of death than epidemic typhus, has for many years been a serious disabling disease in the South, and it increased af an alarming rate from 1931 to 1933. The State health departments of Alabama, Georgia, and Texas reported a total of 250 cases in 1931,772 in 1932, and 1,747 in 1933. As the result of intensive investigations, at the bedside, in the field, and in the laboratory, medical officers of the United States Public Health Service found that this disease has an animal reservoir, chiefly in the common rat, and that under suitable conditions the disease is transmitted from rat to man by certain of the rat fleas. It had been previously shown by specialists of the Bureau of Entomology and Plant Quarantine and of the Baylor University College of Medicine that the tropical rat mite also is capable of transmitting this disease.
The number of typhus-fever cases had increased 300 percent in Alabama during 1932 and 1933, and again in 1934 up to the conclusion of the rat-control campaign. Had the same ratio been maintained there would have been 630 cases reported from the close of the campaign, in March, to July 1. Instead there were only 47 cases, or an indicated decrease of 93 percent. In Georgia and Texas a corresponding decrease also had taken place. The economic saving of produce and property in the 136 counties covered has been estimated to be approximately $8,750,000.
Rabies is frequently contracted by coyotes and other predators probably largely from dogs, and may thus be spread among other wild animals and to man, Outbreaks are difficult to control, and it is only through constant vigilance and the work of the expert force of trappers maintained by the Biological Survey that it has been possible to check them. One of the most serious outbreaks in several years occurred in southern Lea County, N. Mex., in February 1933, and within a month it had assumed alarming proportions. In one case, 18 of 22 sheep bitten by coyotes showed symptoms of rabies and were killed by the owner. Several bulls held in a feed lot were attacked by a rabid coyote, but recovered after being given serum treatment. A milk cow at Mesquite, N. Mex., developed the disease, and an entire family that had been using its raw milk was given Pasteur treatment. One trapper bitten by a rabid coyote also received treatment, and another attacked by a coyote killed the animal before it could bite him. Bureau workers, in cooperation with local authorities, instituted a vigorous coyote-trapping and poisoning campaign, and within a few months the epizootic was stamped out. In Nevada about the same time the loss of 23 cattle from rabies in Paradise Valley led to prompt coyote-control measures that stamped out the disease and prevented further serious losses.
In August 1934 officials of the health and game departments of Maine urged that aid be given in controlling an outbreak of rabies near Farmington. The Biological Survey’s expert learned that the trouble was localized in a largely wooded farming section, not over 8 miles in diameter, where 10 foxes with evidence of rabies had been killed since March. One boy, 3 cows, and 4 dogs were known to have been bitten by the foxes, and 2 of the cows had died. A rapid spread of the disease among the numerous large and small wild animals was threatened, but acting on the Bureau’s recommendation the State game department immediately employed 10 trappers to remove the possible carriers from the locality. By October 1 these men had taken 162 foxes, 107 raccoons, 510 skunks, 117 porcupines, 9 minks, 67 woodchucks, and numerous squirrels, muskrats, weasels, and vagrant cats. This action brought the situation under control.
Bubonic plague has long been prevalent among ground squirrels in California, but Federal and State health and agricultural officials have cooperated in controlling these rodents about resorts, campgrounds, and other places frequented by people, and the human cases have been exceptionally few. It has been definitely demonstrated in California that systematic, intensive rodent-control campaigns must be carried on each year if the health and welfare of the State are to be protected, and recent control work made possible by E. C. W. and P. W. A. allotments has thus been of great benefit.
Disease control, in addition to its importance to public health and man’s economic interests, is part of wildlife management. Tularemia epizootics, for instance, have virtually wiped out cottontail rabbits over large areas, and muskrats, gray foxes, quail, and grouse have been affected with this disease, which has caused widespread alarm among hunters and trappers and reduced the sale of hunting licenses.