Zika, dengue, malaria and the deadly mosquito

You are sleeping quite soundly when a sharp buzzing sound around your head wakes you up, forcing you to try to swat it away. Or else you are walking in the park and only realise you were bitten when rashes appear on your arms and legs. More worrisome is when the infant in your family develops red spots on the otherwise smooth skin on the face.

The mosquito is a nuisance which Malaysians have to live with. But its association with so many serious diseases has made the world take the mosquito very seriously. The latest headline-catching ailment is the zika virus. In Malaysia, dengue fever and malaria are still more worrisome. And other mosquito-related diseases include yellow fever, filariasis, Chikungunya and West Nile fever. No wonder the mosquito has even been called the world’s most deadly species.

The interest in the zika virus is still new, after reports since 2015 that it causes birth defects, and there is urgency in finding a vaccine.

Dengue fever cases have risen at an alarming rate but there is, until now, neither a cure nor an approved vaccine. There are drugs to treat malaria, but increasing resistance by the parasite to them.

Since last year, the zika virus is reported to have been locally transmitted in 31 Latin American and Caribbean countries, and the World Health Organisation warns that it is likely to be transmitted in other countries (like Malaysia) where there areAedes aegypti mosquitoes.

The main concern is the zika virus’ apparent association with neurological disorders, especially microcephaly (a birth defect where the baby’s head is smaller than normal). Eight countries have also reported increased incidence of Guillain-Barré syndrome (affecting people of all ages) linked to zika.

On March 8, the WHO’s Zika Emergency Committee concluded that “there is increasing evidence that there is a causal relationship between microcephaly cases and other neurological disorders with the zika virus”.

The committee recommended more research on the links between the zika virus and neurological disorders, vector control measures, public education on the risks, and priority to develop new diagnostics, new drugs and vaccines.

Regarding malaria, global action has yielded good results. Between 2000 and 2015, among populations at risk, the malaria incidence fell by 37% and death rates by 60% globally.

Malaria is caused by Plasmodium parasites (the most deadly being P. falciparum) which are spread by Anopheles mosquitoes. According to WHO, in 2015 there were 214 million malaria cases and 438,000 deaths.

The best treatment is artemisinin-based combination therapy (ACT). But the P. falciparum parasite is becoming resistant to artemisinin and the partner drugs, just as earlier it had become resistant to chloroquine and sulfadoxine-pyrimethamine.

In recent years, this resistance has been detected in Cambodia, Laos, Myanmar, Thailand and Vietnam. WHO warns that multi-drug resistance could spread to other regions “with dire public health consequences”.

There are currently no licensed vaccines against malaria. One research vaccine against P. falciparum is most advanced and hopefully the vaccine can be deployed in three to five years.

With regard to dengue, this is probably the most feared mosquito-related disease in Malaysia due to the rapid increase in cases from 19,884 (36 deaths) in 2011 to over 120,000 (322 deaths) in 2015.

There is no specific treatment for dengue or severe dengue, but early detection and access to proper medical care lowers fatality rates below 1%, according to WHO. Thus, dengue prevention and control solely depends on effective vector control measures.

While dengue seldom causes death, severe dengue is potentially deadly, due to plasma leaking, fluid accumulation, respiratory distress, severe bleeding, or organ impairment.

There is yet no approved vaccine to protect against dengue but three vaccines are under clinical trials.

At present, the only method to control or prevent the transmission of dengue virus is to combat vector mosquitoes.

WHO-suggested actions include preventing mosquitoes from accessing egg-laying habitats, disposing of solid waste properly, removing artificial man-made habitats, covering, emptying and cleaning of domestic water storage containers, and applying appropriate insecticides to water storage outdoor containers.

Also recommended is use of personal household protection such as window screens, long-sleeved clothes, insecticide-treated materials, coils and vaporisers; mobilising the community for vector control; applying insecticides during outbreaks; and monitoring and surveillance of vectors to determine effectiveness of control interventions.

Although vector control is most important for all the mosquito-related diseases, one major problem is that the mosquitoes are becoming resistant to insecticides.

In recent years, mosquito resistance to pyrethroids (the main class of insecticides used for malaria control) has been found in many countries, and in some areas, resistance to all four classes of insecticides used for public health has been detected. To counter this, rotational use of different classes of insecticides is recommended.

Other strategies to manage insecticide resistance include prioritising tools that do not use insecticides, such as eliminating mosquito breeding sites, personal protection against insect bites and combining the use of insecticides with other non-chemical insecticide control methods.

In Malaysia, scientists at the Infectious Diseases Research Centre and various universities have reported evidence that Aedes mosquitoes were becoming less susceptible to chemical insecticides because of extensive use.

All in all, the tiny mosquito is causing medical problems, even havoc, around the world. A lot of resources have been used to counter the mosquito and its effects.

In some areas, such as reducing the incidence of malaria, there has been good effect. In other areas, such as dengue which experienced an explosive rise, there has been an adverse development. And in another area – the zika virus – there is a new emerging crisis, causing millions of people sleepless nights.

In health matters at least, size does not matter. The smaller the animal or biological entity, the deadlier it can be. Thus, the battle continues between humans, mosquitoes and the pathogens they carry.

Martin Khor is executive director of the South Centre. The views expressed here are entirely his own.