Parasitic, bacterial, or viral diseases

Vector-borne diseases are infectious diseases transmitted by vectors. These vectors are blood-feeding arthropods that actively transmit (mechanically or biologically) an infectious agent from one vertebrate to another. They are primarily blood-feeding insects and mites.

They transmit diseases:

• parasitic (such as malaria, Chagas disease);

• bacterial (such as Lyme disease, rickettsioses, and plague);

• or viral (such as dengue, chikungunya, and West Nile virus):

  • viruses transmitted by blood-feeding arthropods are called arboviruses. This term derives from the English term “arthropod-borne virus.”

Strictly human diseases or zoonoses

These diseases may be strictly human (such as malaria), but many are zoonoses (diseases transmissible from animals to humans and vice versa), such as West Nile virus infection or Lyme borreliosis.

Among blood-feeding arthropods, only a few can act as vectors. These are primarily blood-feeding insects and mites, specifically mosquitoes, sandflies (small flies), lice, bedbugs, and ticks. Infectious agents are transmitted by vectors specific to them. Thus, malaria is transmitted by certain Anopheles mosquitoes, dengue by mosquitoes of the genus Aedes, and Lyme borreliosis by ticks of the Ixodes complex.

The infection is transmitted by the vector after it has become infected itself during a blood meal on a host carrying the infectious agent. Following this infecting blood meal, the infectious agent replicates or transforms within the vector over a period of 5 to 15 days (known as the extrinsic cycle). At the end of this extrinsic cycle, the vector can transmit the disease. Transmission routes vary; most commonly, transmission occurs through a bite (malaria, chikungunya, sleeping sickness, Lyme disease), but transmission can also occur through mosquito feces (Chagas disease, rickettsioses) or regurgitation (plague).

Epidemiology Depends on Several Factors

The epidemiology of vector-borne diseases depends on:

• the vectors: their distribution, competence, and capacity. A vector’s competence is its ability to become infected on a vertebrate host, to support the development of an infectious agent, and to transmit that agent to another host. Capacity takes environmental conditions into account. It depends on competence, the vector-host contact rate—which itself depends on trophic preference (choice of vertebrate species for a blood meal)—and abundance (vector density), as well as the vector’s lifespan (the longer a vector lives, the greater its chance of becoming infected during a blood meal);

• infectious agents: their infectivity, host specificity, and resistance to anti-infective agents, for example;

• human activities, the environment, and climatic conditions: these factors influence the distribution and activity of vectors and affect interactions between vectors and humans as well as animal reservoirs.

Spreading Diseases

The spread of vector-borne diseases today stems primarily from the intensification and globalization of trade and the movement of people. Human interactions with the environment, as well as climate change, also contribute to the spread of these diseases.