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Vector-borne Diseases

Vector-borne Diseases

Vector-borne diseases are infections that are spread to humans via another animal (a vector). These animals are often mosquitos, ticks or other insects.

These diseases include some of the world’s biggest health threats — such as malaria and sleeping sickness. Other important vector-borne diseases include schistosomiasis, lymphatic filariasis, river blindness and Chagas disease.

According to the World Health Organization, hundreds of millions of people worldwide have a vector-borne infection.

Environmental conditions such as temperature and humidity may affect the parasites and vectors that cause such diseases.

This means that local weather conditions as well as climate change could affect the range (both altitude and latitude), intensity, and seasonality of many major vector-borne diseases.

According to the Intergovernmental Panel on Climate Change, there is some evidence of climate-change-related shifts in the distribution of tick vectors of disease, and of some (non-malarial) mosquito vectors in Europe and North America.

As with all diseases, prevention is preferable to having to cure an infection.

People can take steps to prevent vector-borne diseases by sleeping under bed-nets, using pesticides, eliminating vector habitats – such as still water that mosquitoes can breed in – and taking prophylactic drugs that protect against future infections.

Great care should be taken with chemical sprays as these products can also pose risks to human health and to other animals than the target vectors.

The World Health Organization supports the use of DDT – a controversial endocrine disruptor – in the fight against malaria. The Stockholm Convention, which bans DDT, provides a specific exemption for countries that choose to use the chemical to control malaria.

There are relatively few drugs and vaccines for vector-borne diseases because pharmaceutical companies and researchers pay them little attention, despite the fact that these ‘neglected diseases’ affect large numbers of people in developing nations.

This is essentially because poor people in poor countries represent an unprofitable market for drug companies, which would need to recover the costs of researching and developing any new drug or vaccine.

In recent years a number of partnerships have been set up to help plug this gap.

One is the Drugs for Neglected Diseases initiative. This collaborative, non-profit drug research and development (R&D) organization develops new treatments for malaria, visceral leishmaniasis, sleeping sickness and Chagas disease.

The Drugs for Neglected Diseases initiative and the World Health Organization’s Special Programme for Research and Training in TropicalDiseases (TDR) are both good sources of news and expertise.

Journalists should take care when reporting on possible links between climate change and vector-borne diseases, as the research in this field is still quite new and few solid conclusions exist yet (see Climate Change and Health).

SciDev.Net has a useful resource about these links called Climate change and insect-borne disease: Facts and figures.

For journalists who report on malaria, a useful sources include the Malaria Atlas Project and the African Media and Malaria Research Network (AMMREN). This helps build bridges between journalists and scientists and publishes Eyes on Malaria magazine.

Another useful source of stories about new research is the open-access journalist PLoS Neglected Diseases.

CASE STUDY – Malaria and deforestation
global assessment of closed forests, deforestation and malaria risk, published in 2006, provided evidence that deforestation increases malaria risk in Africa and the Americas and diminishes it in South-east Asia.

One study published in 2006 showed that when the forest was cleared to create new settlements in Brazil, this created more areas of standing water in which mosquitoes could lay their eggs, and this increased the risk of malaria.

The same study found, however, that once agriculture and urbanization became established in these areas, the habitat for mosquitoes to lay eggs declined, and so did the rate of malaria.

Also in 2006, another study showed that deforestation in the Peruvian part of the Amazon rainforest led to a 300-fold increase in the rate at which the commonest malarial mosquito bit people there.

The studies showed that alterations to the landscape can have a big effect on disease dynamics, and that health, conservation and agriculture can be closely linked.

SciDev.Net – neglected diseases
Intergovernmental Panel on Climate Change – 2007 Fourth Assessment Report section on vector-borne diseases
SciDev.Net – malaria
Nature journal’s special collection on Chagas disease

WHO on Vector-Borne Diseases