Vector-Borne Diseases

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VECTOR-BORNE DISEASES

From the perspective of infectious diseases, vectors are the transmitters of disease-causing organisms that carry the pathogens from one host to another. By common usage, vectors are considered to be invertebrate animals, usually arthropods. Technically, however, vertebrates can also act as vectors, including foxes, raccoons, and skunks, which can all transmit the rabies virus to humans via a bite. Arthropods account for over 85 percent of all known animal species, and they are the most important disease vectors. Arthropods may affect human health either directly by bites, stings, or infestation of tissues, or indirectly through disease transmission. Several genera of arthropods play a role in human disease, but mosquitoes and ticks are the most notable disease vectors. The most significant mode of vector-borne disease transmission is by biological transmission by blood-feeding arthropods. The pathogen multiplies within the arthropod vector, and the pathogen is transmitted when the arthropod takes a blood meal. Mechanical transmission of disease agents may also occur when arthropods physically carry pathogens from one place or host to another, usually on body parts.

The transmission of vector-borne diseases to humans depends on three different factors: the pathologic agent; the arthropod vector; and the human host (see Figure 1).

The majority of vector-borne diseases survive in nature by utilizing animals as their vertebrate hosts, and are therefore zoonoses. For a small number of zoonoses, such as malaria and dengue, humans are the major host, with no significant animal reservoirs. Intermediary animal hosts often serve as a reservoir for the pathogens until susceptible human populations are exposed. The vector receives the pathogen from an infected host and transmits it either to an intermediary host or directly to the human host. The different stages of the pathogen's life cycle occur during this process and are intimately dependent upon the availability of suitable vectors and hosts. Key components that determine the occurrence of vector-borne diseases include: (1) the abundance of vectors and intermediate and reservoir hosts; (2) the prevalence of disease-causing pathogens suitably adapted to the vectors and the human or animal host; (3) the local environmental conditions, especially temperature and humidity; and (4) the resilience behavior and immune status of the human population.

Vector-borne diseases are prevalent in the tropics and subtropics and are relatively rare in temperate zones, although climate change could create conditions suitable for outbreaks of diseases such as Lyme disease, Rocky Mountain spotted fever, malaria, dengue fever, and viral encephalitis in temperate regions. There are different patterns of vector-borne disease occurrence. Parasitic and bacterial diseases, such as malaria and Lyme disease, tend to produce a high disease incidence but do not cause major epidemics. An exception to this rule is plague, a bacterial disease that does cause outbreaks. In contrast, many vector viral diseases, such as Yellow fever, dengue, and Japanese encephalitis, commonly cause major epidemics.

There has been a worldwide resurgence of vector-borne diseases since the 1970s including malaria, dengue, Yellow fever, louse-borne typhus, plague, leishmaniasis, sleeping sickness, West Nile encephalitis, Lyme disease, Japanese encephalitis, Rift Valley fever, and Crimean-Congo hemorrhagic fever. Reasons for the emergence or resurgence of vector-borne diseases include the development of insecticide and drug resistance; decreased resources for surveillance, prevention and control of vectorborne diseases; deterioration of the public health infrastructure required to deal with these diseases; unprecedented population growth; uncontrolled urbanization; changes in agricultural practices; deforestation; and increased travel. Changes have been documented in the distribution of important arthropod disease vectors. The yellow fever mosquito, Aedes aegypti has reestablished in parts of the Americas where it had been presumed to have been eradicated; the Asian tiger mosquito, Aedes albopictus, was introduced into the Americas in the 1980s and has spread to Central and South America; and the blacklegged tick, Ixodes scapularis, an important transmitter of Lyme disease and other pathogens, has gradually expanded its range in parts of eastern and central North America.

Control measures for vector-borne diseases are important because most are zoonoses that are maintained in nature in cycles involving wild animals and are not amenable to eradication. Therefore, control methods generally focus on targeting the arthropod vector. These include undertaking personal protective measures by establishing physical barriers such as house screens and bed nets; wearing appropriate clothing (boots, apparel that overlap the upper garments, head nets, etc.); and using insect repellents. Environmental modification to eliminate specific breeding areas, or chemical biological control measures to kill arthropod larvae or adults may also be undertaken. Areas such as ports and airports should be rigidly monitored, with control measures utilized to prevent

Figure 1

important arthropod disease vectors from entering the country. Some efforts to control vectorborne diseases focus on the pathogen. For example, there are vaccines available for diseases such as Yellow fever, tick-borne encephalitis, Japanese encephalitis, tularemia, and plague. The vertebrate host and/or reservoir may also be the target for control measures. For example, vaccination of fox against rabies in Europe and Canada is an effective means to reduce the threat of rabies. In addition, reduction of host reservoirs, such as rodents and birds, from areas of human habitation may lessen the risk for contracting certain vectorborne diseases such as plague and St. Louis encephalitis.

It is clear that people will always have to live with vector-borne diseases, but maintenance of a strong public health infrastructure and undertaking research activities directed at improved means of control—possibly utilizing biological and genetic-based strategies, combined with the development of new or improved vaccines for diseases such as malaria, dengue and Lyme disease—should lessen the threat to human health.

Harvey Artsob

(see also: Arboviral Encephalitides; Communicable Disease Control; Malaria; Plague; Zoonoses )