h. Rat Odor. The characteristic musty odor of rats sometimes lingers for
several days in closed spaces.
i. Live or Dead Rodents. The occasional observation of a single rat is not
enough to indicate the need for a control program, but it does indicate need for
investigation. It has been estimated that one rat seen abroad in daylight represents as
many as nine others unseen. It should, therefore, be determined whether the
occasional rat is an explorer or the representative of a stable population in the area.
Inspection of the suspected area with a spotlight at night may disclose feeding rats.
Finding dead rats in the open, in the absence of recent poisoning, may indicate disease
and always demands thorough investigation by Medical Department personnel. Such
rats should not be handled with the bare hands but should be picked up with gloves or
tongs, placed in bags immediately and held under refrigeration until an investigation is
SURVEYS OF INFESTED AREAS
Figure 2-4 will assist in converting signs of rat infestation into degree of
infestation. It is desired to conduct live trapping of rodent specimens in order to obtain
an accurate ectoparasite survey.
a. Determination of the Problem. A survey should be made of any area where
a rodent population is suspected in order to discover: (1) the particular locations
infested, and the relative number of rodents and the species involved; (2) the sources of
food and water and possible means of denying them these necessities; and (3) the
available harborage or shelter.
b. Sampling. While the evidences of infestation enumerated above are
important guides in making a survey, it is also desirable to set a number of traps in
suspected areas in order to collect specimens and thus to confirm the results of the
inspection. In city work, one man servicing 200 traps each day over an area of about 10
blocks, should be able to obtain a satisfactory sample within 1 week. He should inspect
all premises and set traps in those where evidence of active rat infestation exists. Traps
should be visited each day and removed in 3 days, if there is no catch. More than 10
rats caught per 10 traps set will indicate heavy infestation. Whenever possible, the
animals caught and their ectoparasites should be examined for disease in a qualified
laboratory. Inquiries should be made among the civilian population near military camps
and stations to assess local experience with the rodents concerned. Questioning may
bring reports of large concentrations of rodents adjacent to the control area, such as at
a city dump. Control on a military post is not likely to be successful if such opportunities
for reinfestation occur.