Diseases borne by mosquitoes, flies, ticks, bugs and other organisms cause more than one million deaths each year. The deaths, though alarming, vastly underestimate the human misery and hardship caused by these vector-borne diseases, as many people who survive infection are left permanently debilitated, disfigured, maimed, or blind, says the World Health Organization (WHO) as it marks World Health Day on 7 April, with the tagline, ‘Small Bite: Big Threat’.
Medical definition of ‘vectors’ is that they are living organisms which transmit infectious diseases between humans or from animals to humans. Many of these vectors are bloodsucking insects that ingest disease-producing micro-organisms during a blood meal from an infected host (human or animal) and later inject them into a new host during their next blood meal. Mosquitoes are the best known disease vector. Others include certain species of ticks, flies, sandflies, fleas, bugs and freshwater snails.
Vector-borne diseases are one of the greatest contributors to human mortality and morbidity in tropical settings and beyond. Every year more than one billion people are infected and more than one million people die from vector-borne diseases. Malaria, which is spread by mosquitoes, is the biggest killer among vector-borne diseases, but other diseases, such as dengue and yellow fever, tend to erupt in large outbreaks that can paralyze health systems and cause considerable economic and social disruption. For instance, each year, around half a million patients with severe dengue require hospitalization.
Others like, onchocerciasis or ‘river blindness’, which prevails across sub-Saharan Africa, is caused through infection by the black-fly and often leads to blindness in the victims. Chagas disease, a tropical illness caused by parasitic insects, in its late stage can cause heart failure and early death in young adults. Japanese encephalitis can permanently damage the central nervous system and chikungunya, a disease spread by virus-carrying mosquitoes, causes severe joint pain that can last for weeks.
Schistosomiasis, the most widespread of all vector-borne diseases, caused by parasitic worms and leading to infection of urinary tract or intestine, contributes to poor nutritional status and poor school performance among children. Some forms of leishmaniasis, a parasitic infection spread by sandflies, can be rapidly fatal, while others cause severe facial disfigurement. Meanwhile, around 120 million people are currently infected with lymphatic filariasis, and about 40 million of them are disfigured and incapacitated by the disease.
One sixth of the illness and disability suffered worldwide is due to vector-borne diseases, with more than half the world’s population currently estimated to be at risk of these diseases. The poorest segments of society and least-developed countries are most affected. Lost productivity is one consequences of vector-borne diseases, but the stigma and social exclusion resulting from their manifestation on the body are additional sources of misery, especially for women.
These diseases also exacerbate poverty, as illness and disability prevent people from working and supporting themselves and their family, causing further hardship and impeding economic development. According to studies from eight countries, an average dengue episode represents 14.8 lost days for ambulatory patients at an average cost of US$ 514 and 18.9 days for non-fatal hospitalized patients at an average cost of US$ 1491. An econometric model for malaria suggests that countries with intensive malaria have income levels of only one third of those that do not have malaria.
Many of these diseases, which were historically confined to distinct geographical areas, have become more fluid due to a host of ills, including climate change, intensive farming, dams, irrigation, deforestation, population movements, rapid unplanned urbanization, and phenomenal increases in international travel and trade. These changes create opportunities for vectors and the diseases they spread to take up residence in new areas. For instance, the primary vector for dengue, the Aedes aegypti mosquito, is now found in more than 20 European countries. This same mosquito species recently carried chikungunya to the Caribbean islands; the first cases of this debilitating disease seen in the Americas.
Although significant progress is being made in combating some diseases such as malaria, lymphatic filariasis and Chagas disease, other diseases such as dengue continue to spread at an alarming pace. However, the good news is that in most cases simple preventive measures can lead to marked decrease in infection rates.
The massive use of insecticides in the 1940s and 1950s successfully brought many important vector-borne diseases under control. But this initial success led to complacency and many control programs were dismantled. Resources for prevention and protection dwindled and expertise in the field became scarce. Soon many of the diseases that were believed to be under control or eradicated returned with a vengeance and easily overran the limited infrastructure available for their control.
Another worrisome trend witnessed in the resurgence of vector-borne diseases in recent times is that many of them have developed resistance to affordable insecticides that were proven to be effective in the past. If existing insecticides lose their effectiveness this could erase all the gains made against malaria and other vector-borne diseases especially in parts of Africa.
If we do not take action now, the implications are extremely serious for the entire globe, says Dr. Margaret Chan, director-General of World Health Organization. She warns that for many vector-borne diseases, there are no vaccines, and drug resistance is an increasing threat. Vector-control plays a vital role and is often the only way to prevent disease outbreaks. Many existing interventions, such as insecticide treated bed-nets and indoor spraying, are simple and proven. These vector-control tools can be particularly effective when used in combination with interventions such as mass drug administration involving large-scale treatment of affected communities.
Taking action is entirely feasible. WHO promotes integrated vector management, which uses a range of interventions, from indoor residual spraying to the use of natural insect predators, in combination and in a value added way. Integrated management makes sense as many vector-borne diseases overlap geographically, some vectors cause several diseases, and some interventions provide protection against several vectors. In many cases, increased funds and political commitment are needed to scale-up access to existing vector-control tools, as well as medicines and diagnostic tools, says the Director-General.
At the same time, more investment in research is also urgently needed to find improved solutions for fighting vectors and the diseases they transmit.
The world needs to recreate the momentum for vector control and to boost the fundamental capacities that underpin it, including staff with technical expertise, stronger surveillance systems, and better laboratory infrastructure.
For vector-borne diseases, control programs have been found to either surge ahead or sink, so countries and their development partners must appreciate the urgent need to act before an alarming situation deteriorates leading to death and suffering for millions.
The world can no longer afford to be complacent. It is my sincere wish that this World Health Day will invigorate vector-control and give it the high profile it deserves. “No one in the 21st century should die from the bite of a mosquito, a sandfly, a blackfly or a tick,” adds Dr. Chan.