Fighting the mosquito scourge

AT THE turn of the century a visitor from Africa came to our shores, apparently to complicate our lives.

It all began up in the trees of the Malayan jungle where some of the monkeys (mostly macaques) had to contend with a disease that gave them high fever, a disease supported by a group of mosquitoes known as the Aedes niveus.

When the monkeys ventured down to ground level, they were fair game for the Asian tiger mosquito (Aedes albopictus), which got two for the price of one - monkey blood and monkey disease: dengue.

Culex tritaeniorhynchus a vector of Japanese encephalitis (JE) in Malaysia, breed in padi fields.

On the ground, however, the orang asli were also a food source for the albopictus mosquito. And the disease spread to them.

When the orang asli ventured into urban areas, our African visitor, the Aedes aegypti, got the dengue virus and thus began what has been a scourge for Malaysians ever since.

This little story was actually proven, though in much greater detail, by a survey conducted jointly by the Institute for Medical Research (IMR) in Kuala Lumpur and the Hooper Foundation of the University of California, in the United States.

The survey, which ran from 1962 to 1980, proved that dengue was originally a monkey disease that Aedes passed on to us, says IMR medical entomology division chief Lee Han Lim.

It also showed that human encroachment on the jungles may have been a factor in forcing the monkeys to ground level.

It has been speculated that the aegypti may have hitched a ride on ships and ended up in Malaya at the turn of the 20th century.

Fact is,surveys by IMR in the early 1900s show that the aegypti mosquito "used to exist just along coastal towns and villages. More surveys in the 50s confirmed its ubiquitous presence in Peninsula Malaysia. By the 80s, it was everywhere."

City slicker

Lee says that the African Aedes aegypti, also called the yellow fever mosquito as it can transmit yellow fever (and the chikungunya viral disease), may have a competitive edge over the Asian tiger (albopictus)which explains why it proliferates and spreads dengue so successfully.

This edge has something to do with the way it breeds. Lee says that though both the aegypti and albopictus have the "same habits and the same hardiness," the latter breeds mainly in outdoor natural water receptacles such as coconut shells, tree holes or on leaves with still water.

The aegypti, however, is "a highly domestic mosquito, following man and breeding in artificial indoor containers wherever it goes ... and one can never find it in drains, ponds or rivers," says Lee, adding that the mosquito prefers concrete tanks. Sometimes, however, the reverse happens, especially in suburban areas.

Essentially, human behaviour is connected to the mosquito's breeding.

Lee goes on to say that the larvae of both types of mosquitoes prefer clear, though not necessarily clean, water to breed in and neither species can be found in drains or ponds.

Drains bear non-Japanese Encephalitis-carrying Culex quinquefasciatus (the common house mosquito), as well as other mere "nuisance mosquitoes" (those that do not transmit disease).

And both the albopictus and aegypti mosquitoes do not need a lot of water to breed either, just enough for the larvae to swim freely and develop pupae. "For example, a plastic sheet (discarded and crumpled) can provide an adequate breeding site," says Lee.

Surveys conducted by IMR a decade ago showed that the yellow fever mosquito had acquired some liking for outdoor water containers and that it may supplant the Asian tiger, though it is too soon to tell as "the interspecies competition is complex," says Lee

Industrialisation and modern living, though tough on other carriers of diseases like malaria, has led to the aegypti being called an "urban mosquito".

Ministry of Health epidemiologist Dr Satwant Singh says that with industrialisation, mosquito breeding has been flourishing since the 80s due to the increase in the number of breeding sites. Dr Satwant points to improper disposal of water and garbage, especially in squatter areas, as well as the abundance of construction sites, as leading factors in the rise of the aegypti.

In children's playgrounds, rain water that collects in old tyres for example, form ideal breeding ground. So too are discarded plastic bags and bottles or broken jugs containing stagnant water.

Dr Satwant adds that overcrowded conditions in some living areas provide for "convenient transmission" of the dengue virus as the carrier does not have far to go between people. Both the albopictus and aegypti mosquitoes transmit dengue from human to human.

Battling infection

In dengue haemorrhagic fever (DHF), loss of body fluids can lead to shock and death. Most people do not progress to that stage. In fact, "most doctors do not admit a patient into the hospital if they only have dengue fever," says Dr Satwant.

He stresses, however, that if someone suspects he has even the mildest form of dengue fever, that person should consult a doctor to determine if it is indeed dengue, what stage it is in, and if there is a need to be warded. A person who has DHF would definitely be warded.

He says that even if a patient does not need to be warded, the doctor will monitor his blood platelet level (an indicator of the progression of the disease).

If a person is found at a late stage of dengue and is going into shock, doctors would attempt to rehydrate the patient, which is the normal treatment for dengue fever (mild or otherwise), in addition to trying to keep the fever down.

"But at such a late stage, it is difficult to bring a person back from the state of shock," says Dr Satwant, adding that the most dangerous part of the infection cycle is the one to two days after the fever subsides.

"People think that when the fever goes down they must be getting better, but this is when shock sets in," warns Dr Satwant. All the more reason to be monitored by a doctor.

Close-up of a mosquito sucking blood from a human being.

A very good reason to check in with your doctor at the first sign of dengue fever is that there is no effective vaccine.

As Lee says, there is one virus, but four serotypes. This means that if a population develops immunity to one serotype, it can still can be infected by the other three.

Tham says that though we can develop life-long immunity to one particular serotype, we can acquire only short-lived immunity to the other serotypes.

There are other theories however:

"There is the double infection theory in which a person contracts classical dengue and recovers; the body is then primed against infection. When that person gets a second infection, there is a massive immune system response, and so the symptoms become haemorrhagic," says Lee.

There is also the theory that the albopictus transmits the virus that causes classical dengue, while the aegypti is responsible for dengue haemorrhagic fever.

The bottom line, however, is that vaccination is impractical and ineffective at this time.

War strategies

There are various approaches to combating dengue and its Aedes carriers.

Besides preventing bites with adequate clothing and repellents, and destroying mosquitoes with fogging, people are advised to avoid high risk areas during "peak biting times".

The aegypti is active during the day, particularly mid-morning and late afternoon. The albopictus, on the other hand, is most active about 6pm, though it has been "observed to possess some night activity."

Ministry of Health senior entomologist Tham Ah Seng says another way to do mosquitoes in, is to destroy the larvae with a natural larvicide, called Bacillus thuringiensis H-14, a bacteria.

Sometimes, a combination of the bacteria and a suitable chemical insecticide is used to kill both the larvae and the adult mosquito.

One of the most important approaches is that of denying the mosquito a place to breed, and this is where public co-operation is imperative.

Tham says that people in the community need to play their part in the war against mosquitoes by keeping their living areas clean and free of breeding sites. Plastic containers, and garbage in general, should be disposed of properly as these can collect rainwater.

Emptying containers of water, however, is not enough as the aegypti eggs can survive in a dry state for a very long time, says Lee.

"Even after six months, about 60% of the eggs are capable of hatching, so you cannot just drain water out, you need to scrub the insides of containers to get rid of the eggs," he adds.

Public apathy

Tham laments that the public is generally lackadaisical about efforts to eradicate dengue by preventing aedes breeding. Which is a pity really. Lee says that it was found not long ago that mama mosquito can pass on the disease to her children (transovarian transmission), so getting rid of dengue won't be an easy affair.

Tham says that the cleanliness habit must be inculcated in schoolchildren. Are adults truly too recalcitrant to change their ways, then?

But the bigger question is: Can humanity learn from its mistakes, so that in future we no longer have to wage species genocide just to save our necks?

This page is hosted by

Get your own free homepage.