Vector borne diseases (VBD) are often the scourge of a nation’s economy thanks to the frequent epidemics they cause. Some of the VBDs are Malaria, Dengue, Chikungunya and Japanese Encephalitis. A strong interaction between the reservoir of infection, transmitting agent, causal organism and the environment leads to successful transmission of any VBD. Malaria, Dengue and JE are known for both mortality and morbidity, whereas Chikungunya results in morbidity leading to loss of working hours and also high cost of treating patients.

Dengue, or 'Break Bone Fever' once known as an urban disease has now become rural, and is known for its lethal results. Four viral serotypes are known to cause the infection. Infection with one serotype provides immunity for lifetime only against that serotype and partial immunity against other serotypes. Double infection with other serotypes may lead to Dengue Haemorrhagic Fever (DHF) and Dengue Shock Syndrome (DSS) which may prove fatal if not treated on time.

The first epidemic was recorded in 1812 in India, since when epidemics and sporadic cases have been recorded time and again.

Dengue usually presents as high grade fever, head-ache, severe body-ache, joint and muscle pain, skin rashes and vomiting. A single case of Dengue is alarming as it may take epidemic proportions. Lab diagnosis includes the detection of antibody levels in the blood samples.

Treatment is symptomatic. Since the treatment regime does not change much even after confirmation of the disease, preventive measures play a key role. Along with treatment, isolation of the patient under the bed net is important as it prevents further transmission.

Tiger mosquito
Aedes aegypti, commonly called as tiger mosquito qualifies as the principal vector of dengue and chikungunya as it is highly susceptible to these virus. It thrives close to humans and prefers human blood and is predominantly a day time feeder. Its bite is painless, hence not noticed easily. Even the slightest movement disturbs the feeding and sends it looking for a new host. Therefore, fever cases flock in clusters during an outbreak since a large number of people receive infective bites in a short time.

Mosquitoes becomes infective after feeding on a patient from the day before onset to the fifth day of illness. Extrinsic incubation in the vector is 8 to 10 days after which it becomes capable of disease transmission. Transmission cycle is Man-Mosquito-Man. There is no evidence of man to man transmission. Once infected, the mosquito remains so for its life time.

Trans-ovarian transmission (of virus from infected female mosquito to the progeny) of dengue has also been demonstrated in the lab.

The tiger mosquito (called so due to the presence of obvious dark and white stripes on legs) preferentially breeds in household cement tanks, discarded tyres, tins, coconut-shells, disposable cups.

After completing the four larval stages and a pupal stage in the aquatic stage the terrestrial adult stage emerges. It mates first and then goes for a blood meal. Adults have a very short flight range. After the blood meal, the adult mosquito rests in the cool, dark corners, beneath the hung clothes and also underside of the furniture and hence is not easily noticed.

Since this mosquito breeds, feeds and rests in close proximity to humans, it is easy to eliminate. Simple and routine practices like complete emptying, scrubbing and drying the water containers once a week, proper disposal of unwanted containers and  tyres would relieve the mosquito menace.

Fogging fails
Fogging operation carried out during early morning and late evenings gives only temporary relief as it knocks down the adult mosquitoes. The fresh batch emerging out of breeding site does not get affected by fogging.
Desert coolers, artificial fountains to be emptied and cleaned periodically would eliminate the possible breeding sites. Eggs of these mosquitoes have a peculiar feature of withstanding the desiccation for months together. Therefore scrubbing the water storage containers would prevent the mosquitoes from establishing an ideal breeding site. Hence presence of larva and pupa is taken as an indicator for an impending out-break.

Prevention of Aedes breeding provides the dual profit of containing both Chikungunya and fatal Dengue. Is it not time to act?

The writer is an assistant entomologist, dt surveillance unit, Chikmagalur.