Once bitten...

MOSQUITOES, apart from spoiling a pleasant evening or causing a sleepless night, also transmit diseases like malaria, dengue fever, Japanese encephalitis, filariasis and other viral infections. Interestingly, the incidence of these diseases is on the rise at a time when there have been advances in the field of science and medicine. I shall try to highlight some aspects of this issue in a question and answer format.

What are the diseases transmitted by mosquitoes?

Malaria, dengue fever, Japanese encephalitis and filariasis are the most common in India. Sandflies are similar to mosquitoes because they are the vector of diseases such as kala azar (visceral leishmaniasis). Sandfly fever, West Nile fever and a few other viruses are much less important.

Why are mosquito borne diseases important?

Malaria, caused by the parasite Plasmodium, ranks fifth among all infectious diseases as a cause of mortality worldwide. It is an overwhelming public health problem and causes one or two million deaths every year. Eradication programmes, most notably by a DDT spraying programme in the 1950s and 1960s, were attempted but did not succeed in any long lasting way. The incidence of malaria today is far greater than it was 40 years ago. The reasons include resistance of the mosquito vector to DDT, and of the malarial parasite to the drug chloroquine, the most common anti- malarial in use for the last half century. Severe malaria is fatal if untreated or when improperly treated with chloroquine.

Dengue fever, which is caused by a virus, is the other major problem. After starting in East Asia in the 1950s, the epidemic reached India in the 1990s in a major way. Dengue shock syndrome and dengue haemorrhagic fever reflect increased transmission and can be fatal. Unlike malaria, there is no effective therapy against dengue.

Filariasis is caused by the worms Wuchereria and Brugia resulting in fever and swelling of the limbs. It requires intense exposure to mosquitoes, unlike malaria and dengue which can be contracted by a single mosquito bite. While not life threatening, it causes disability and chronic illness. Small flies (sandflies) transmit kala azar which has reached epidemic proportions in Bihar and adjoining Uttar Pradesh. An emerging problem is resistance to antimony compounds, the traditional treatment.

Japanese encephalitis is caused by a mosquito-transmitted virus. It is common in rice farming areas, especially where pigs abound (these serve as reservoir hosts). The disease can cause coma and death.

Where do mosquitoes breed?

The three common species of disease-transmitting mosquitoes (Aedes, Anopheles and Culex) breed in water. Only the female mosquito feeds on human blood. Poor drainage following rains results in pools of stagnant water, which serve as an ideal breeding ground and the reason for the dramatic increase in mosquito borne diseases during the rainy season.

Are vaccines or preventive medicines available against mosquito- borne diseases?

Unfortunately, the only disease for which a vaccine exists is Japanese encephalitis. The vaccine is used in Korea but is expensive and not widely available in India. It is generally advised only for those who live in rural areas, especially in rice farming areas and where pigs abound.

Taking anti-malarials periodically (usually once a week) is effective in preventing malaria. This strategy is widely used by travellers from Western countries to malaria-endemic areas. Long term preventive therapy is not generally recommended for those native to and residing in India on account of concerns about side-effects and the emergence of resistance.

What are the symptoms of these diseases and how are they treated?

The two most important diseases (malaria and dengue) cause fever, often without other symptoms. It is important to seek medical attention early on as both diseases can be rapidly fatal if diagnosis and treatment are delayed. Malarial parasites are becoming increasingly resistant to chloroquine and it is not advisable to use this drug in a person with malaria caused by the virulent falciparum strain. Luckily, several effective anti- malarials (quinine, mefloquine and artemesinin derivative) are available against malaria. Many physicians, however, continue to prescribe chloroquine without either confirming the diagnosis of malaria or making sure the dangerous falciparum strain is not present.

Dengue causes a non-specific fever but can develop into either a severe bleeding state or shock. Only supportive therapy exists for dengue and Japanese encephalitis. Filariasis usually causes fever and swelling of an extremity, and sometimes asthma-like symptoms. It responds to the drug di-ethyl carbamazine.

How can you prevent mosquito bites?

As these diseases are serious and have no effective treatment or a specific vaccine, it is important to prevent bites. Most malaria-transmitting mosquitoes bite between dusk and dawn. Use a mosquito net at night. This is the most important single measure and has been shown to reduce the incidence of malaria in studies conducted in Africa. Spraying the net with a pyrethrin spray is even more effective as sometimes moquitoes bite body parts in contact with the net.

Screen windows with mosquito netting. Screens should not be removed during the day and entrances should either be kept closed or fitted with a screen door. Try to wear clothes that cover the wrists and ankles when outdoors after dark to reduce exposed skin surface.

Using a mosquito repellent containing di-ethyl toluamide (DEET) on exposed skin is also useful. The optimum strength of DEET for adults is 25 or 35 per cent and for children is 10 or 20 per cent. Creams, lotions and sprays are effective.

Spraying indoor areas with products containing permethrin further improves protection against mosquitoes. Commercial vapourisers or coils do the same, but for maximum effectiveness, doors and windows should be kept closed. This in turn increases room heat and humidity while some persons also develop respiratory symptoms and allergy to these products. These measures should greatly reduce the incidence of mosquito borne diseases.

DR. RAM GOPALAKRISHNAN

The writer is Consultant Physician, Infectious Disease and Tropical Medicine, Apollo Hospital and Apollo Speciality Hospital, Chennai.

Send this article to Friends by E-Mail

Section  : Features
Previous : Some real 'heavy' post this
Next     : The Shashi Tharoor column: Birth of the 21st
           century