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close this bookAssessing Needs in the Health Sector after Floods and Hurricanes (PAHO)
View the document(introductory text...)
View the document1. Introduction
View the document2. Critical decisions
View the document3. Magnitude of the impact
View the document4. Morbidity and mortality
View the document5. Environmental sanitation
View the document6. Vectors
View the document7. Food and nutritional status
View the document8. Evacuation camps
View the document9. The health center
View the document10. Surveillance systems
View the documentBibliography

6. Vectors

Floods are more likely to cause vector problems than any other type of disaster because of their effect on vector habitats.

Vector and breeding sites such as puddles and water collected in discarded containers and old tires may increase as a direct result of the floods or they may be altered indirectly when uncollected garbage increases. The flood may also destroy the normal habitat of animals such as dogs and cats that are likely to be parasitized by ticks, fleas, lice, and mites. In seeking safer ground, these animals may come closer to human population groups and transmit their ectoparasites. Population migration is yet another risk factor, as it may cause overcrowding that enhances the transfer of vectors. Finally, the disaster may interrupt ongoing vector control programs (PAHO Sci Pub No. 419).

The increase in the vector population will happen gradually. In fact, some mosquito population indices tend to rise as flood waters recede and leave pools of stagnant water. It also takes some time for other vectors and rodents to multiply to nuisance or hazardous levels.

Public health problems hence will not appear at the peak of the flood, which is precisely when health authorities are under the greatest pressure from the population for action. Table 6.1 has been adapted from PAHO Scientific Publication No. 420; it lists the illnesses produced by the vectors most likely to be encountered during or after a disaster.


When flooding occurs, health authorities are pressed by public opinion, the mass media, and ill-advised politicians to initiate emergency spraying campaigns. These are often badly designed, ill-timed, and use inappropriate insecticides. It is important to consider that (a) it generally takes about two months for the flood's effects to be evident in mosquito populations; (b) certain types of mosquitoes are essentially "domestic" while others are sylvatic; (c) the effect of the insecticide or larvacide depends on the mosquito's life cycle; (d) certain varieties of mosquitoes have become resistant to some insecticides; (e) poorly planned campaigns may actually cause insecticide resistance; and (f) if it is still raining, residual sprays will be washed away.

There are laws regarding the sale, transport, and use of certain insecticides. For example, it is almost impossible to buy DDT in the United States, and passenger airlines are permitted to transport only very small amounts of some insecticides, like malathion.

TABLE 6.1. Vector- and rodent-related diseases.






Yellow fever (urban)



Filth flies




Typhoid fever



Epidemic typhus

Louse-borne relapsing fever

Trench fever

Fleas (dogs, cats, rats)


Endemic typhus

Mites (dogs)


Rickettsial pox

Ticks (dogs)


Tick-borne relapsing fever

Rocky Mountain spotted fever


Chagas' disease

The assessment team should compare the size of the vector population to its preflood size, and project what it will be in two or three months if no corrective measures are taken. An apparent increase in breeding sites or vector density may be nothing more than seasonal variation.

To get an overview of the situation and devise the best control strategy, the team should plot the following on a map: mosquito population size and breeding sites; their geographic distribution, migration pattern, and accessibility; number and availability of trained vector control personnel; and location of insecticide and equipment stocks. This data should be correlated with epidemiologic findings (e.g., case distribution, location of the outbreaks) and with meteorological observations about expected rainfall patterns.

Sources of Information

There is probably an already-existing malaria or Aedes aegypti eradication program in the country that can provide a picture of the normal situation, the frequency and regularity of spraying cycles, and the mosquitoes' resistance to the insecticides. If there is no such program, the local epidemiologist will probably have some information on the density of the mosquito population by type. In either case, the team should visit disaster areas to check the location, quantity, type, and condition of stocks of existing equipment and insecticides.

A visit to the ovitrap (mosquito traps) sites (if any) and breeding places may be useful. Health centers will know if there is an active case finding for malaria. In endemic areas, blood is sometimes taken in all febrile cases of unknown or doubtful etiology. It is important to know whether each center has facilities for reading the blood samples or if they are sent to a central laboratory. If the latter is the case, the team should find out how long the results take and whether the system has been interrupted by the disaster.

The data collected in this first assessment will give a broad view of the general status. However, a detailed report should then be made on the amounts and types of insecticides needed for at least the next six months, as well as a list of required equipment and personnel.

The assessment team may have to test the sensitivity of the mosquitoes to the insecticide and the best time of the biological cycle in which to apply it. In disaster situations, adult control is preferable, although larvicides may also be indicated, especially if larval control was practiced before the flood.

Data on such factors as population, migrations, rainfall patterns, and destruction of houses should be checked by field observation to see whether displaced populations are more exposed to the mosquitoes. This can happen when migration is toward breeding areas, when houses have been destroyed and their inhabitants are less protected, or when intense rainfall is washing away residual insecticide.



Sources of information

· Vector control programs

· Aedes aegypti eradication program

* national
* regional

· Malaria eradication program

* national
* regional

· Epidemiologist
· Regional
· Malariologist
· Program warehouses
· Institutes of entomology
· Ovitraps/breeding sites
· Health centers

Entomologic data

· Type of mosquito
· Distribution
· Population density
· Seasonal variations in density
· Effect of floods on habitats
· New flood-created breeding sites or reservoirs

- Habits
- Resistance to insecticides

Control program data

· Has it been interrupted?
· Is there any entomological surveillance program?
· Personnel: type, number, location
· Insecticides

- Types in stock
- Amount
- Location

· Equipment

- Types: slides, microscopes, reagents
- Amount
- Working condition

· Surveillance of cases

- Passive
- Active case finding
- Spraying cycles
- General data

*geographical area affected
*geographic distribution of population
*exposure to the vector
*destruction of housing
*settlements and evacuation camps
*distance to breeding sites
*road destruction
*diversion of water and sewerage systems (creation of new breeding sites)
*amount of rain
*sq. kms. or number of houses to be treated
*estimated amount of insecticide to be used

transport regulations
means of transport
local agents

*equipment needed to carry out the estimated spraying

personnel to be trained


The survey of morbidity and mortality (see Chapter 4) will show whether vectors other than mosquitoes should be a public health concern. Furthermore, from observation of ecological changes and field investigations, the team will get some clues as to the likelihood of future problems.

Comparative data for before and after the disaster may be available. Whether or not this is so, the observer should note prevalent conditions and check rumors while carrying out the survey.

Sometimes merely noting such things as the presence of stray dogs, dead rats, and uncollected garbage in the streets; the state of dumps; the cleanliness of street markets; the abundance of flies; the hygiene condition of the evacuation camps; and complaints about mosquitoes, fleas, and lice will suffice for a rough Judgment of the situation.


During floods, reports and rumors are common about problems created by such animals as dogs, rats, mice, and, in rural areas, snakes. Sightings of these animals may increase as they compete with man for dry space and food after their habitats are disturbed by the flood. Moreover, the disaster may cause a breakdown in the garbage collection system, and the accumulation of rubbish will attract rodents and dogs. There have been rumors of packs of dogs attacking humans after a disaster, although these rumors are more frequent after earthquakes than floods. While few actual rabies cases have been confirmed, it is a problem in most Latin American countries, and closer contact between dogs and men may increase the number of dog bites. Dogs and rodents may also carry ectoparasites such as fleas and ticks which can be vectors of disease. In addition to destroying food, rodents may cause certain diseases such as leptospirosis and rat-bite fever.

During the Bolivian floods of 1982, many families reported seeing snakes in their homes but could not say whether they were poisonous. Among the more than 100 families interviewed bimonthly over one year (during which time more floods occurred), there was only one "heard-of" case of a snake bite. Similarly, during the 1983 floods in Ecuador, there was an increase in the sighting of snakes: the report of one herpetologist indicated that 18 snakes had been captured in only one house during the two-week period. Most of them were not poisonous and even the poisonous ones posed very little risk to man because of their habits or the anatomic position of their teeth (Touzet, 1983). Nonetheless, there were numerous rumors of poisonous snake bites (which exhaustive investigation failed to confirm in most cases).

In assessing the situation, the team must determine whether there has been an increase in the number of pests and, if so, whether a public health hazard exists, either directly (e.g., bites, food contamination) or indirectly (plague, scabies, etc.).

Sources of Information

Emergency departments and health centers should know of increases in the number of animal bites (and corresponding fatalities). The checking procedure is the same as for any other process described in Chapter 4.

If one exists, a national institute of hygiene or health and/or the epidemiology division at the health ministry or at the local level will have data on the number of rabies cases and whether an anti-rabies campaign exists. The veterinary division of the ministry of health or agriculture may have more detailed information.

There may also be a national serpentarium or a division or department of herpetology in the museum of national history that will have data on the poisonous snakes in the region. If there is reason for serious concern, the team should enlist the assistance of a herpetologist to carry out a special assessment.

Where plague is endemic, it may increase, but this will probably happen months after the floods have passed.


Rodents, Dogs, Snakes

Sources of information

· Emergency department
· Health centers
· National institute of health/hygiene
· Epidemiology division
· Veterinary division
· National serpentarium
· Division/department of herpetology; natural history museum


· Increase in number of animals seen
· Increase in number of bites treated
· Dead rata
· Stray dogs
· Poisonous snakes in the disaster area
· Availability of serum (rabies, snake venom)

- Production
- Distribution
- Control

· Existing control programs
· Surveillance system for diseases indirectly due to rodents and dogs