Rodents are mammals of the order Rodentia, with rats and mice belonging to the suborder Myomorpha. Members of the Muridae family are the dominant species in every region of the world, due to their ability to adapt to and exploit new situations. Commensal rats and mice, that is those that live at the expense of humans, invade their dwellings, eat their food, upset their comfort, and frequently transmit diseases to them, belong to this family. Three species of commensal rodents are the most widely distributed: the Norwegian rat, Rattus norvegicus; the roof rat, Rattus rattus; and the common mouse, Mus musculus.
Rodents and Public Health
Rats and mice (commensal and wild) have significance for public health chiefly due to their role as carriers or reservoirs for infections and diseases that can be transmitted to humans (zoonoses). The diseases present in the Americas include: the plague (Yersinia pestis), salmonellosis (S. typhimurium; S. enteritidis); leptospirosis (L. icterohaemorragiae); murine typhus (Rickettsia typhi); rickettsialpox (R. akari); lymphocytic choriomeningitis (arenavirus); rat-bite fever (Spirilum minus, Streptobacillus monilifomis); Hantavirus hemorrhagic pulmonary syndrome; hemorrhagic fevers (Arenavirus); Venezuelan equine encephalitis (alphavirus); Powassan encephalitis (Flavivirus); rabies; Rocky Mountain spotted fever (R. rickettsii); and tularemia (Francisella tularensis), as well as parasitisms, such as trichinosis (Trichinella spiralis); eosinophilic meningitis (Angiostrongylius cantonensis); and taeniasis (Hymenolepis nana or H. diminuta).
The transmission of these infections to humans occurs by indirect contact. Some are transmitted through contact with infected rodent urine or feces, others through fleas and lice, and still others through mosquito bites.
Rodents and Mice in Natural Disasters
When natural disasters strike, rats and mice endure the same suffering as humans do. Many of them are crushed to death or drowned, die of starvation, or fall prey to infections. Their populations are frequently decimated. Moreover, there is often displacement among the survivors, who wander to new areas in search of protection, shelter, and food. Fearful and disorganized, it takes time for them to regroup and reorganize their social behavior, become familiar with their new environment, find safe havens, locate food and water, and memorize their movements. All this occurs before they reproduce again. Reproductive activity among wild rodents and commensal rats usually declines during the rainy season.
Colony building and reproduction only will begin when their new ecosystem has stabilized. Thus, the reestablishment of a rodent population after a disaster will take six to ten months under favorable conditions.
The infections carried by the new populations can be transmitted to people when the conditions favor contact with contaminated rodent urine or feces or with their ectoparasites, vectors of the infections. This contact also occurs between various species of commensal and wild rodents, permitting transmission and cases of emerging or heretofore unknown infections in new geographical areas.
In conclusion, during the emergency the spread infection by rodents will not be an immediate concern. However, the activity and growth of rodent populations should be monitored as part of the surveillance of these diseases.
MANAGEMENT AND CONTROL OF RODENT POPULATIONS
Decisions on rodent control after a natural disaster should be analyzed within an epidemiological and economic context, and steps to eliminate rodents should not be taken impulsively and haphazardly with the mass application of rodenticides. This measure takes time and money and exposes other animals (domestic animals) and people to an additional risk, which may be the chemical or biological components of the product. The indiscriminate elimination of rodents poses a greater risk, since the ectoparasites of these species, the vectors of the infections they carry, immediately seek out other hosts--among them humans-and transmit the infections to them. This is what happens with the plague and rickettsiosis.
Moreover, the disease vectors for people are not necessarily rodents. For example, the last leptospirosis outbreaks occurred in Peru and Ecuador during El Niño; in Nicaragua, after Hurricane Mitch, where the leptospire involved was L. canicola, whose principal reservoir is the dog; and in Belize, where the predominant leptospire was L. harjo, for which cattle are the principal reservoir. Consequently, the mass application of rodenticides would not be an appropriate measure.
What to Do:
PREVENTION OF DISEASES TRANSMITTED BY RODENTS
Rodent control activities should consider the epidemiological implications, ecology, and dynamic population of the rodents. When selecting the control methods, safety as well as public health and environment protection will be taken into account.
Department of Emergency Preparedness and Disaster Relief
Tel: (202) 974-3434
Regional Office of the World Health Organization