6.3  Vector-borne disease transmission mechanisms

There are two ways that vector-borne diseases are transmitted:

  • a.Mechanical transmission takes place when a vector simply carries pathogenic microorganisms on their body and transfers them to food, which we then consume. Flies and cockroaches are in this category. Flies like to rest on faecal matter and then may move on to fresh food. They can carry infectious agents through their mouth and on their legs and other body parts. They deposit these agents on ready-to-eat foods and the recipient gets infected if they consume the contaminated food.
  • b.Biological transmission involves the multiplication and growth of a disease-causing agent inside the vector’s body.

Malaria is a good example of biological transmission. The female mosquitoes take the malaria infectious agent (Plasmodium) from an infected person with a blood meal. After sexual reproduction in the gut of the mosquito, the infectious agent migrates into the salivary gland of the insect, where it grows in size, matures and becomes ready to infect humans. When the mosquito next bites a human the saliva is injected into the skin and transfers the infection in doing so. An infectious agent may be passed from generation to generation of vector — this happens mostly in ticks and mites.

The methods of transmission for some known vectors are shown in Table 6.1.

Table 6.1  Important vectors and disease transmission mechanisms.

VectorDiseasesMechanism
HouseflyDiarrhoeal diseases, TB, polio, worms, food poisoning, infective hepatitisMechanical
MosquitoMalaria, yellow fever, filariasis, dengue feverBiological
LouseTyphus fever, relapsing fever, dermatitisBiological
MiteScabies, chiggerBiological
FleaPlague, murine typhus/endemic typhusBiological
SandflyLeishmaniasisBiological
BlackflyOnchocerciasisBiological
BedbugDermatitis, Chagas diseaseBiological
CyclopsGuinea worm, fish tapewormBiological
Tsetse flySleeping sickness (trypanosomiasis)Biological
Freshwater snailSchistosomiasisBiological
DogRabiesBiological

6.2  Public health importance of vectors

6.4  Classification of vectors and their life cycles