Keeping dengue at bay
By LOH FOON FONG
There are four types of dengue viruses and a person who is infected by one strain is not immune to the others.
Dr Ramlee Rahmat, director of Disease Control at the Health Ministry, says all four dengue serotypes can be found in the community at any one time, but one will be more dominant than the others during one season which can last three to four years.
All these viruses can cause dengue or dengue haemorrhagic fever. In haemorrhagic fever, a person’s body goes through dengue shock syndrome and the blood vessels begin to leak. The patient has high and persistent fever that cannot be treated with paracetamol.
Two or three years after the dominant strain has infected a community, the community’s immunity will build up against that strain and most who are infected will only suffer from dengue and not haemorrhagic fever, says Dr Ramlee.
“It is when another serotype becomes more dominant that we get most cases with haemorrhagic fever because the community’s immunity to that strain is generally low,” he says.
The virulence of the four dengue viruses is somewhat similar, he says. The rainy season and humid weather make it easy for mosquitoes to breed. However, if a certain area has a lot of mosquitoes but the virus is not present, we will not have any case.
If there are a lot of mosquitoes and there is also the virus, the spread of infection is fast and can contribute to a high number of cases, he adds.
He says that dengue viruses are transmitted during the feeding process. When the Aedes aegypti mosquito bites a person with dengue, it will be infected and will spread the virus to another person.
“If there are not many mosquitoes in the area, the transmission would be lower even if the virus is present,” he adds.
Once infected, the dengue patient carries the virus for about one month. That explains why patients take about a month to fully recover, he says.
Only 10% of dengue patients infected will be detected on the first day of fever, 30% on the third day, 50% on the fifth day and 90% after the seventh day, he says
There is no specific treatment available. Intravenous fluids and oxygen therapy are often used for these patients during their illness.
Untreated hemorrhagic dengue can result in death. The case-fatality rate of haemorrhagic fever in most countries is about 5%; most fatal cases are among children and young adults, says the US Centers for Disease Control and Prevention (CDC) website.
Besides fever, severe headache, backache, joint pains, nausea and vomiting, eye pain and rash are among the symptoms of dengue infection.
Does the number of mosquito bites make any difference to whether a person is infected or not?
“If a mosquito bites once and cannot get enough blood for the day, it might bite another time. If in the first bite, the mosquito injects a lot of virus, the person can get infected. It depends on the volume of the virus that gets into the body. If the second bite does not release much virus, you won’t get infected,” he says.
The black and white striped Aedes mosquitoes usually feed twice a day, once in the morning between 6am-8am and once in the evening 5pm-8pm.
The mosquitoes usually travel within a 200m distance and do not go higher than a six-storey building. Nevertheless, they can still travel higher if they accidentally get trapped in a lift and travel further when they happen to fly into a car, says Dr Ramlee.
There are two types of antibodies – one at the acute stage that lasts for six months and the other that provides long-term immunity when the person has recovered from the symptoms.
“Although the body is immune, the virus mutates, for there are subserotypes of the virus. That is why our immunity won’t last long,” he says.
It is thus important to sustain the efforts carried out to fight against dengue, he says. He adds that the Health Ministry has carried out fogging but this effort, although ongoing, is inadequate because it only reduces the amount of adult mosquitoes for a week. “It is only a stopgap measure.”
He urged the public to work together with the Health Ministry. He advised the public to use larva poison such as Abate to kill larva in pools of stagnant water in houses, work places and construction sites.
While construction workers can set up a committee to carry out such efforts, the public should organise gotong-royong efforts to clean up affected places, he says.
According to the CDC website, the first reported epidemics of dengue fever occurred in 1779-1780 in Asia, Africa, and North America; the near simultaneous occurrence of outbreaks on three continents indicates that these viruses and their mosquito vector have had a worldwide distribution in the tropics for more than 200 years.
During most of this time, dengue fever was considered a benign, nonfatal disease of visitors to the tropics. Generally, there were long intervals (10-40 years) between major epidemics, mainly because the viruses and their mosquito vector could only be transported between population centres by sailing vessels.
A global pandemic of dengue began in South-East Asia after World War II and has intensified during the last 15 years. Epidemics caused by multiple serotypes (hyperendemicity) are more frequent, the geographic distribution of dengue viruses and their mosquito vectors has expanded, and dengue haemorrhagic fever (DHF) has emerged in the Pacific region and the Americas.
In South-East Asia, epidemic DHF first appeared in the 1950s, but by 1975 it had become a leading cause of hospitalisation and death among children in many countries in that region, says the CDC website.