India’s elusive mosquito war

India’s elusive mosquito war
Descending slyly and softly at night, she spreads all her six long, filament-thin legs and stoops on uncovered flesh. She pierces the skin with the snout, which is a set of complicated tools comprising saw-toothed blades for cutting and a feeding tube powered by two tiny pumps.  Drilling through a thin layer of fat, then into blood-filled capillaries, the she mosquito begins to drink. She drools into the hole made by her to keep the flow of blood from clotting.

At first, it is a painless bite. Slowly, an itchy red mark appears which could possibly lead to a serious illness that may be malaria. The disease is caused by Plasmodium parasites. The parasites are transmitted exclusively through the bites of female mosquitoes (Anopheles) called ‘malaria vectors’— males do not bite. The intensity of transmission depends on factors related to the parasite, the vector, the human host and the environment. Plasmodium falciparum and Plasmodium vivax are the most common. Plasmodium falciparum is the most deadly. Mosquitoes have been around for 170 million years.

As the mosquito feeds on human blood, Plasmodium cells living in her salivary glands leak into the bloodstream. They cause shivering, vomiting, high fevers and severe fatigue. In the worst case—in a disease commonly called cerebral malaria—parasites attack the brain and destroy so many red blood cells that there are not enough left to run the body’s functions. Neurological damage, coma and death often follow.

Anopheles mosquitoes breed in water--shallow collections of fresh water, such as puddles and rice fields. Transmission also depends on climatic conditions that may affect the number and survival of mosquitoes, such as rainfall patterns, temperature and humidity. In many places, transmission is seasonal, with the peak during and just after the rainy season. Malaria epidemics can occur when climate and other conditions suddenly favour transmission in areas where people have little or no immunity to malaria. In some parts of India, due to deficient rainfall, there has not been any spike in mosquito-borne diseases.

The malaria parasite, which is highly adaptive parasite, has a very complex life cycle. It can live in the blood, in liver, in the stomach of a mosquito. Given its complexity, scientists have been struggling for decades to develop a malaria vaccine, but it is still a work in progress. Currently, there are no licensed malaria vaccines.

More than 20 vaccine projects are in clinical trials, according to the WHO. Of these, the most advanced vaccine is RTS,S/AS01, being evaluated in a Phase 3 clinical trial. The clinical testing of RTS,S is at least 5-10 years ahead of other candidate malaria vaccines. However its efficacy is limited to one species, Plasmodium falciparum, with no protection expected against Plasmodium vivax (the second most important species causing human malaria).

The RTS,S vaccine nearly halved the cases of malaria experienced by children aged between five and seven months and reduced the number of cases by a quarterin babies aged 6 to 12 weeks. The treatment’s protection lasted for 18 months, although it waned slightly over time.

Despite these efforts, the disease still kills around 660,000 people, most of them small children under the age of five (90 per cent of cases occurring in Sub-Saharan Africa), and leaves more than 219 million sick every year. About 3.4 billion people, half of the world’s population, are at risk. Children who survive the serious illness can suffer damage to their health and development in their lifetime afterwards. India has halved the number of malaria deaths per 100,000 during 1990-2013. But, malaria still remains a tremendous health burden for Indians. About 61 million cases were reported in 2013, and it killed more than 116,000 people.

The parasite has become resistant to some treatments. Methods to eradicate the disease have been limited to controlling mosquitoes, distributing pesticide-laced nets for homes and spraying indoors of
buildings with insecticides- known as vector control.

Pregnant women and children, who are more vulnerable, can take antimalarial medications during seasons when the cases of malaria are highest. These increased prevention and control measures have led to a reduction in malaria mortality rates by 42 per cent globally since 2000.

Scientists from Imperial College, London, have taken a different route to eradicate the disease by modifying mosquitoes to produce sperm that will only create males, according to a study published recently in the Nature Communications journal. They have tested a new genetic method that distorts the sex ratio of Anopheles gambiae mosquitoes, the main transmitters of the malaria parasite, so that the female mosquitoes that bite and pass the disease to humans are no longer produced. In the first laboratory tests, the method created a fully fertile mosquito strain that produced 95 per cent male offspring. The researchers believe the work paves the way for a pioneering approach to controlling malaria.

Hopes have also risen that a vaccine could be introduced in the world’s worst-hit countries in 2015. The expectations follow successful results of trials, announced last October. But, if India is to eradicate the disease, a vaccine alone would not be enough. It is important to find a permanent solution to remove garbage that piles up on streets and gets soaked in rain water; and also cover open drains in front of residences. The most crucial measure is to ensure that a proper drainage system is in place so that water does not stagnate.

The author is an independent journalist

K V Venkatasubramanian

K V Venkatasubramanian

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