Health Watch

Mosquitoes arrive with the start of the rainy season. The rains that nourish plants help hatch mosquitoes, those little airborne blood suckers that cause irritation -- and sometimes disease -- when they bite. Mosquito eggs can survive in a dried state for years before being reactivated by water. Any outdoor place with standing water is a breeding ground for mosquitoes.

When a mosquito bites, it injects a bit of saliva to make the blood flow. It's that saliva that causes the itching and redness of a mosquito bite. Most of the time, irritation is all a mosquito bite will cause. People who have a sensitivity to mosquito bites also may have serious swelling. But mosquitoes also can transmit diseases like encephalitis, a potentially fatal disease that causes swelling of brain tissues.

Health Watch:(THE Hindu sep 30,2001)

Once bitten...

MOSQUITOES, apart from spoiling a pleasant evening or causing a sleepless night, also transmit diseases like malaria, dengue fever, Japanese encephalitis, filariasis, skin irritation, rashes and other viral infections. Intrestingly, 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 thecapy 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 epi-demic proportions in Bihar and adjoining Uttar Pradesh. An emerging problem is resistance to anti-mony 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 reser-voir 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 mosquitoborne diseases?
Unfortunately, the only disease for which a vaccine exists is Japanese-encephalitis. The vaccine isused in Korea but is expensive and not widely available in India. It is generally advised only for those who live in rural areas, especiany m nee rarmmg 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 conSrming 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 sometimesasthma-like symptoms. It responds to the drug diethyl carbamazine.

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

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