Vector borne diseases are a heavy burden on human populations, a major cause of work loss, and a serious impediment to economic development and productivity. They require an intermediate living agent for their transmission. Their epidemiology is influenced by attributes of their vectors, which in turn are closely linked to environmental conditions.
Over the past decades, the increased demands on the landscape for food and shelter and an increased number of by-products of man’s living environment have led to unparalleled changes. Some of these changes have led to an increase in the distribution of several vector borne diseases, including malaria
What are vectors?
Vectors are living organisms that serve as vehicles to transmit a pathogen (a disease-causing agent like a virus or parasite) from a host to a human or to an animal or both. They are predominantly pests, such as insects and ticks. Birds and mammals such as rats and mice can also be considered vectors, as they both can carry and transmit disease.
What are the methods of vector control?
Vector control involves using preventive methods to eradicate or control vector populations, in order to limit the transmission and spread of diseases. Preventative measures include:
• Habitat control: Removing or reducing the number of places where the vector can breed helps to limit populations from growing excessively. For example, by removing stagnant water, removing old tires and empty cans which serve as mosquito breeding habitats and through good management of used water.
• Reducing contact with vectors: Reducing the risk of exposure to insects or animals that are vectors of diseases can limit the risk of infection. For example, using bed nets, adding window screens to homes, or wearing protective clothing can help reduce the likelihood of coming into contact with vectors.
An important component of exposure reduction is also the promotion of health education and raising awareness of risks. Bed nets treated with insecticide can reduce the risk of insect bites and infection.
• Chemical control: Insecticides, larvicides, rodenticides and repellents are used to control pests and can be used to control vectors. For example, larvicides can be used in mosquito breeding zones; insecticides can be applied to house walls (indoor residual spraying); bed nets treated with insecticide and use of personal skin repellents can reduce the risk of insect bites and thus infection. The use of pesticides for vector control is supported by the World Health Organization (WHO) and has proven to be highly effective.
• Biological control: The use of predators (natural enemies of the vectors), bacterial toxins or botanical compounds can help control vector populations. For example, using fish that eat mosquito larvae or the introduction of sterilized male tsetse flies in order to reduce the breeding rate of these flies are methods to control vectors and reduce the risk of infection.
All these measures are important elements for an integrated approach to control the spread of vectorborne diseases. The choice of the most appropriate method(s) to use depends on the disease pattern and behavior of the vector.
Side effects of pesticides used in public health
In spite of the millions of houses sprayed with DDT for malaria control, no accidental deaths of spray men or householders due to DDT poisoning have been reported. However, some domestic animals were killed, especially cats, with the result that rat populations increased in some sprayed areas. Bedbugs, cockroaches, and other household pests soon developed resistance to DDT, and became more abundant because the DDT had killed many of their predators. This led some householders to oppose spraying.
In the past, it was claimed that environmental contamination by DDT used in malaria control was relatively minor because it was sprayed inside houses, and the quantities were much smaller than those used in agriculture. However, washing of equipment, containers, and overalls, and the unauthorized use of DDT for other purposes (e.g., fish poisoning) spread the substance outside the houses.
DDT used for malaria control accounted for an estimated 8% of global DDT contamination. Where pesticides are stockpiled for use in epidemics (e.g., dengue), environmental contamination due to leaking containers (some insecticides are highly corrosive), fire, theft, war, or natural disasters is always a danger.
Extensive studies in the effects on non-target organisms of aerial spraying of endosulfan against tsetse flies and temephos used in rivers against blackfly larvae have revealed no permanent damage to treated ecosystems. In the case of tsetse fly control, it has been argued that any changes to the ecosystems caused by spraying are insignificant compared to the changes that will follow human settlement of tsetse-cleared land. However, there is no need for complacency in these matters, and further studies are required.
Limitations of disease vector control
• Environmental management requires cooperation and willingness of all members.
• Bush clearing in tsetse fly control is not regarded as environmental friendly and goes against environmental conservation.
• High initial costs, especially when applying environmental modification methods
• Procurement of chemicals and procedures are expensive.
• Vectors may develop resistance to chemicals
• Chemicals can accumulate in food chain • Need regular applications
• Can cause irritation, toxicity and other undesirable effects.
• Expensive (e.g. application of male sterile technique to control tsetse).
• Availability of biological agents (e.g. larvivorous fish to control mosquito larva)
Delimitations of disease vector control
• Application of integrated methods of vector control.
• Tax relief /exemption for chemicals
• Community mobilization
• Monitoring insecticides resistance
• Restricted use of pesticides (e.g. restriction of DDT for in indoor use)
• Health education on the importance use of pesticides (e.g. use of treated nets)
• To determine the limits of boundaries for vectors control the community should be involved right from the initial stage of the project.