Several microorganisms, or bacteria, found originally in diseased rats
or mice, have been exploited for destroying rats. A number of these
so-called rat viruses are on the American market. The Biological Survey,
the Bureau of Animal Industry, and the United States Public Health
Service have made careful investigations and practical tests of these
viruses, mostly with negative results. The cultures tested by the
Biological Survey have not proved satisfactory.
The chief defects to be overcome before the cultures can be recommended
for general use are:
1. The virulence is not great enough to kill a sufficiently high
percentage of rats that eat food containing the microorganisms.
2. The virulence decreases with the age of the cultures. They
deteriorate in warm weather and in bright sunlight.
3. The diseases resulting from the microorganisms are not contagious and
do not spread by contact of diseased with healthy animals.
4. The comparative cost of the cultures is too great for general use.
Since they have no advantages over the common poisons, except that they
are usually harmless to man and other animals, they should be equally
cheap; but their actual cost is much greater. Moreover, considering the
skill and care necessary in their preparation, it is doubtful if the
cost can be greatly reduced.
The Department of Agriculture, therefore, does not prepare, use, or
recommend the use of rat viruses.