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close this bookEmergency Vector Control after Natural Disaster (PAHO)
close this folderPart II: Control measures for specific vectors
close this folderChapter 8: Flies, rodents and other vectors
View the documentSynanthropic Fly Problems
View the documentRodent Problems
View the documentOther Vector Problems

Rodent Problems

The environment of the rodent undergoes the same change as that of man after a natural disaster since their harborage and food sources are also damaged or destroyed. The rodent will consequently be in competition with man for whatever food and shelter remain. Commensal rodents and other animals are more visible to man following a disaster and may migrate into his environment. Unfortunately, what the rodent does not directly, consume, it may damage and contaminate.

The rodent species that are of concern are the Norway or brown rat (Rattus norvegicus Berk), the roof rat (Rattus rattus L.), also known as the ship or black rat, and the house mouse (Mus musculus L.). Rodents have been involved in transmission of a number of infectious diseases to man. The most important ones are:

(1) Plague, which is endemic to Brazil, Bolivia, Ecuador, Peru and Western United States, frequently involves rodents other than domestic rats
(2) Murine typhus, cases of which occur throughout the world in areas of warmer climates where commensal rats, especially R. norvegicus, are the chief reservoirs
(3) Leptospirosis, with a worldwide distribution, is maintained in reservoirs of commensal rodents, dogs, pigs and cattle
(4) Salmonellosis, which occurs when commensal rodents are infected with Salmonella and the infection is transmitted to man in contaminated foods and liquids, spread by infected fecal droppings or by urine; the house mouse probably plays a greater role than rats in the transmission of food-borne illnesses.

The economic and nutritional importance of loss of foodstuffs because of rodent contamination must also be considered. Damages and losses caused by rodents are substantial, and during natural disasters this extra burden can be serious.

In many cases, since rodent control is not the direct responsibility of the central government, it may be difficult to obtain predisaster information. Ro dent surveys and control work are usually undertaken by the port authority in seaports, by the local governments of cities, or by the Ministry of Agriculture. These agencies may be sources of background information, supplies and materials for rodent control, and expertise regarding surveys and the organization of control activities. In the private sector, pest control operators may be an excellent source of assistance and relevant information.

Rodent Surveys

Information about rodents can be collected in interviews with people who live in temporary housing and refugee camps after natural disasters. The location and relative density of rodents sighted should be determined at this time.

If an individual familiar with the signs and traces of rats and mice can be located, a survey of large areas can be performed fairly rapidly. The major signs are fecal droppings, rodent runways, rodent footprints or tail marks in dust and tracking powders, gnaw marks of rats and mice, burrows, and nests. Rodent odors, especially of house mice, and urine stains that can be seen under ultraviolet light are also indicative.

Sightings increase when the cover of the rodents is disturbed. After natural disasters it is possible to obtain information through daylight surveys in affected residential areas and near rescue centers. Additional information can be obtained during dusk and early evening surveys. These may be undertaken at random, or through the selection of potential trouble spots. Strong flashlights can be used to search in such places as under buildings, and in refuse disposal areas. Maps are essential for this type of work and if they are not available, sketch maps should be made by the workers. Record should be kept of potential habitats, such as temporary refuse dumping areas and harborage, and of the number of sightings.

A more detailed survey can be accomplished when pest control operators, biologists or personnel from a rodent control program are available. Forms should be reproduced for recording information in such surveys as: the location of the survey, the type of premise, the condition of the structure, the construction materials, the number of occupants, and the presence or absence of food, water, harborage, rodent signs and traces.

Some surveyors use traps to assess the density and determine species of rodents in an area. Live traps can be used when available; otherwise, snap traps serve the purpose. If food markets or hardware stores are still standing after a disaster, snap traps can usually be purchased locally. Care should be taken not to use large numbers of snap traps if a rodent associated disease exists in which an ectoparasite is a vector.


The World Health Organization document (WHO/VBC/79.726) should be consulted for the selection of rodenticides for control purposes. There are two general types of rodenticides in use. The first is the chronic type, a multiple dose, slow-acting compound. The second type is the acute, or single dose, quickacting compound. In general control operations, the rodenticide of choice is considered to be a slow-acting anticoagulant poison. Many acute rodenticides are toxic to man and other animals. Thus, in a disrupted environment, such as that which follows a natural disaster, extreme care should be taken in using acute rodenticides. They should be used only in extreme emergencies and by well-trained control operators. Red squill is an exception that can be used against R norvegicus, but it is not as effective against R. rattus. Anticoagulant rodenticides such as diphacinone, difenacoum, brodifacoum or chlorophacinone are available in a number of areas and have been used in emergency rodent control. The immediate needs of the control program should be determined with survey information or through estimates of experts. Locally available supplies of rodenticides should be located, and if inadequate, they should be supplemented immediately.

Rodenticides either come as preprepared bait or as concentrate. The concentrates might prove less expensive to order, but in emergencies either formulation is acceptable. In preparing food baits, it is necessary to know the rodents' food preferences. Contrary to popular belief, rats and mice prefer fresh, palatable food. Food dyes and other coloring materials that do not affect the flavor of the bait can be used as a warning to humans. Great care should be taken in mixing baits, especially those in which acute rodenticides are used. It is best to have a single individual responsible for mixing and/or packaging the bait.

Control operations should be based on the findings of the rodent surveys. Members of field teams need to be trained in placement of the bait and in public relations. Control personnel must be careful to develop positive working arrangements with the populace after a natural disaster. They should carry identification; and they should be trained well enough to understand what they are doing and why, and to communicate this information to the people.

Bait must not be haphazardly placed. Care must be taken to put the bait where the rodent will find it, but where children and other animals cannot. When the supply of rodenticides is inadequate, only the more hazardous areas should be treated. These areas include rescue centers, refugee camps, food warehouses, markets, ports and hospitals.

In areas of potential outbreaks of rodent-borne diseases where rodent control is necessary, special consideration should be paid to controlling ectoparasites. Before a trapping program is underway, rodent runs must be dusted with DDT, carbaryl, diazinon, pirimiphos-methyl or some other approved insecticide powder. Special care should be taken in handling and disposing of rodents in these areas.

The use of rodenticides is only a small part of a well-organized rodent control program. During a natural disaster, however, it is of greater importance than during routine operations. Sanitation is another important aspect, end' it must be remembered that by creating harborage, the accumulation of garbage and debris encourages the establishment of rodent populations. Refuse and debris should be thoroughly incinerated when sanitary landfills are unavailable, because on-site burning is of limited value.

No control program can be successful without the cooperation of the people it serves. Such programs should always incorporate sanitary education, and other campaigns to enlist the help of community groups and individuals.