Long-tailed macaques leave the safety of Taman Negara National Park to rummage through trash in a nearby resort.  Macaques were believed to be the only host of Plasmodium knowlesi until the first documented human case in 1965. (Photo credit: Laura Kraft)

By Laura Kraft

This post is the eighth in the “Travel Bug” series by Laura Kraft, a recent graduate from the University of Georgia, who is chronicling her travels in Asia from an entomological perspective. See earlier posts from the series.

Laura Kraft

The Fifth Malaria

I can imagine the scene. An American surveyor traipsing through the lush primary rainforest of Malaysia, perhaps stopping to watch some macaques groom each other in the canopy above. The sound of insects is so deafening he fails to hear the high-pitched whine of a mosquito coming in for a landing, delivering in its saliva a tiny parasite to this unsuspecting human. This was the first known natural case of the malaria pathogen Plasmodium knowlesi, sometimes referred to as the fifth malaria, infecting a human. The parasite had been found and studied before but was believed to infect only primates, especially macaques.

After this first human case, there were no others in the literature for a long period. Then, in 2004, a large number of malaria cases arose in Sabah, the eastern province on Malaysian Borneo. Dr. Balbir Singh was the first to suggest that this malaria may be due to the same P. knowlesi that had first infected the American in 1965. A quick test proved him right, and P. knowlesi has since been attributed to many human cases in Malaysia. In some regions of Malaysia, especially on Malaysian Borneo, it is the most common species of malaria affecting humans.

The vector of P. knowlesi in Malaysia, Anopheles balabacensis, is typically found in the forest, where it readily bites primates. Recently, an increase in cases in Sabah and Sarawak, the Malaysian provinces on the island of Borneo, made Dr. Indra Vythilingam of the Universiti Malaya believe that the vector might have started biting humans in more urban areas. Her team tested that hypothesis and found that An. balabacensis attacks around homes, though they did not find a significant number attempting to bloodfeed within the home. This changes how humans should avoid the mosquito, which has been found to bite from 6 p.m. until dark. Where insecticide-treated bed nets were one of the only control methods being used before, Malaysians in these regions may benefit more from using repellents when they are out in their gardens in the evening.

Trapping Dengue in its Tracks

In the lab of Dr. Indra Vythilingam, principal investigator in the Department of Parasitology at the Universiti Malaya, mosquitoes caught in the field are regularly tested in an effort to detect dengue fever before it shows up in infected humans. (Photo credit: Laura Kraft)

 “[Dengue virus] has become out of control here. Too many cases,” Dr. Vythilingam confesses. The biggest challenge to controlling the disease, in her opinion, is that it is only found when feverish humans enter the hospital. At this point, it has already spread too far. Once patients have symptoms, they typically have already been infectious for about seven days. Any mosquitoes that have bloodfed on these humans during the pre-symptomatic seven-day period can pick up the disease and later transmit it. In a normal outbreak, the government steps in once a cluster of cases in a region hints that an outbreak might be starting. At that time, the government airs commercials on television about reducing standing water and, more importantly, about visiting a hospital if symptoms arise. In this region, many people get severely ill or die simply because they think they have a serious cold or the flu and do not go to the hospital to get treatment, which is costly. In addition, the government starts to use insecticidal fogs in the areas with cases in order to decrease the number of vectors carrying and transmitting the disease.

For Dr. Vythilingam, these measures come too late and at too great a cost. Her lab is working to uncover dengue outbreaks before they expand by regularly capturing and testing adult mosquitoes in an attempt to make use of that seven-day period in which humans are infected but asymptomatic.

To do this, she and her students are testing a new trapping method. While some trapping methods already exist for the adult vector in Malaysia, they are expensive and sometimes are even damaged or stolen when left in public areas to trap mosquitoes.

The trap she shows me in her lab is simple and much more affordable, less than 25 cents USD. It involves infused water in the bottom of a cup to attract pregnant female mosquitoes. Then, a top part has sticky paper lining the inside of the cup where the mosquitoes are likely to land and rest. If they happen to avoid the sticky paper and oviposit in the water, they get stopped from exiting by a mesh covering the water source. Easy to get in, hard to get out. The adult mosquitoes caught in the trap can then be tested regularly onsite for the dengue virus with a simple kit. If dengue is found, the government can be alerted and start insecticidal fogging before the problem spreads further.

While funding is becoming harder to come by, these small, focused studies looking at the basic biology and sampling methods of these disease vectors can be a simple and affordable way to decrease outbreaks of malaria and dengue virus.

Laura Kraft is a recent graduate from the University of Georgia who is taking a year off to travel the world before returning home to start a Ph.D. program in the fall of 2017.