Global Trends by Martin Khor

Monday 14 March 2016

Zika, dengue, malaria and the deadly mosquito

The mosquito may be tiny but it can certainly deliver a deadly bite, being the carrier of serious ailments like zika, dengue and malaria.  Can it be effectively controlled?


You are sleeping quite soundly when a sharp buzzing sound around your head wakes you up, forcing you to try to swap it away.

Or else you are walking in the garden or a park, and you only realise you were bitten when several swelling rashes appear on your arms and legs.

More worrisome is when the infant in your family develops red spots on her otherwise smooth skin on the face.

The mosquito is a nuisance which Malaysians and others in tropical countries have to live with.

If the only irritation is broken sleep and slightly painful bumps that go away in a few days, that pesky insect may considered a nuisance but tolerable.

However, its association with so many serious diseases has made the world take the mosquito very seriously.

Many types of mosquitos can pass on serious or deadly diseases.  The latest headline catching ailment is the zika virus, which is causing near public panic in some countries.

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.

There are a host of problems in tackling each of these diseases, thus enabling the mosquito to continue its reign.  The interest in the zika virus is still new, after reports since 2015 on its effects in some Latin American countries, so there is now haste and urgency in finding a vaccine.

Dengue fever cases have rising at an alarming rate but there is until now no cure yet nor an approved vaccine to prevent it.  Malaria has had the most deadly effect in terms of sheer numbers, and there are drugs to treat it, but also increasing resistance to the medicines by the parasite causing it mosquito, thus causing alarm.      

Since last year, the zika virus is reported to have been locally transmitted in 31 Latin American and Caribbean countries, and the WHO warns that it is likely to be transmitted in other countries where there are the aedes aegypti mosquito.   Imported cases have also been reported in all the world’s regions. 

The main concern is the zika virus’ apparent association with neurological disorders, especially microcephaly.   Eight countries have also reported increased incidence of Guillain-Barré syndrome (GBS) linked to the zika virus.

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

The WHO Director General told the Committee of evidence of the link between the zika virus and microcephaly, especially fetal malformations.  “We can now conclude that Zika virus is neurotropic, affecting tissues in the brain and brain stem of the developing fetus.”
There is also growing evidence of a link with GBS.  This expands the group at risk well beyond women of child-bearing age. GBS has been detected in children and adolescents but is more common in older adults and slightly more common in men.

The Committee recommended more research on the links between the zika virus and neurological disorders; vector control measures to reduce the mosquito population; and public education on the risks especially to women of childbearing age and pregnant women, and the risk of  sexual transmission. 

Pregnant women who have been exposed to Zika virus should be counselled and followed for birth outcomes based on the best available information and national practice and policies, In areas of known Zika virus transmission, health services should be prepared for potential increases in neurological syndromes and/or congenital malformations.

The WHO Committee thinks there should be no general restrictions on travel or trade with these countries, but pregnant women should be advised not travel to areas of ongoing Zika virus outbreaks.

Travellers to areas with Zika virus outbreaks should be provided with up to date advice on measures to reduce exposure to mosquito bites. There should be vector control at airports and disinfection of aircraft should be considered.

The Committee also recommended priority to be given to develop new diagnostics for Zika virus infection and to intensify research and development and evaluation of new drugs and vaccines.

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

Malaria is caused by Plasmodium parasites (the most deadly being P. falciparum) which are spread by the female Anopheles mosquitoes. 

According to the WHO estimates, there were 214 million cases of malaria in 2015 and 438, 000 deaths.

The best available treatment, particularly for P. falciparum malaria, is artemisinin-based combination therapy (ACT).

The problem is that the P. falciparum parasite is becoming resistant to the artemisinin-based drug.  It had earlier become resistant to two previous medicines – chloroquine and sulfadoxine-pyrimethamine – in the 1970s and 1980s.

This undermined the gains in malaria control, and the increasing resistance to artemisinin may see an alarming repeat.

In recent years, parasite resistance to artemisinins has been detected in Cambodia, Laos, Myanmar, Thailand and Viet Nam.

WHO says there are concerns that P. falciparum malaria in Cambodia and Thailand is becoming increasingly difficult to treat, and 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, known as RTS, S/AS01, is most advanced and pilot projects could pave the way for deployment of the vaccine in 3 to 5 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%. Dengue prevention and control solely depends on effective vector control measures.

Dengue virus is transmitted by female mosquitoes mainly of the species Aedes aegypti.   One estimate cited by WHO indicates 390 million dengue infections per year, of which 96 million manifest clinically.  An estimated 500 000 people with severe dengue require hospitalization each year, and about 2.5% of those affected die.

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

Unfortunately, there is no specific treatment for dengue fever.  For severe dengue, medical care (maintenance of the patient's body fluid volume is critical) by physicians and nurses can save lives.

There is yet no approved vaccine to protect against dengue but three vaccines are under clinical trials, and 3 other vaccine candidates are at earlier stages of clinical development. 

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; applying appropriate insecticides to water storage outdoor containers.

Also recommended are using of personal household protection such as window screens, long-sleeved clothes, insecticide treated materials, coils and vaporizers; improving community participation and mobilization for sustained vector control; applying insecticides during outbreaks and  monitoring and surveillance of vectors to determine effectiveness of control interventions.

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

According to the WHO, in recent years, mosquito resistance to pyrethroids (the main class of insecticides used for malaria control) has emerged in many countries. In some areas, resistance to all 4 classes of insecticides used for public health has been detected.  Rotational use of different classes of insecticides is recommended by WHO to manage insecticide resistance.

However, malaria-endemic areas of sub-Saharan Africa and India are causing significant concern due to high levels of malaria transmission and widespread reports of insecticide resistance. The use of 2 different insecticides in a mosquito net offers an opportunity to mitigate the risk of the development and spread of insecticide resistance.

Strategies to manage insecticide resistance in Aedes aegypti mosquitoes include prioritizing tools that do not use insecticides, such as: eliminating mosquito breeding sites – cleaning, emptying and covering domestic water storage containers; personal protection against insect bites – insect repellents, wearing clothes that cover as much of the body as possible; using wire mesh screens or treated netting materials on doors and windows and combining the use of insecticides with other non-chemical insecticide control methods.

(WHO has also published the Global Plan for Insecticide Resistance Management in Malaria Vectors in May 2012; and interim guidance on "Monitoring and managing insecticide resistance in Aedes mosquito populations").

According to Institute for Medical Research Malaysia’s Infectious Diseases Research Centre (IDRC) medical entomology unit head Dr Lee Han Lim, there is evidence that Aedes mosquitoes were becoming less susceptible to chemical insecticides because of extensive use (New Straits Times 7 March 2016).  

Dr Lee suggested rotating, every six months, the use of insecticides that had different mechanisms of killing mosquitoes and the use of non-chemical control agents.

Universiti Malaysia Sabah (UMS) professor Dr Chua Tock Hing said there was ample evidence to show that Aedes mosquitoes were resistant to pyrethroids insecticides (such as permethrin) and organophosphates (such as temephos and malathion).

The other resistance problem – the resistance of the pathogen to the medicine – also has to be addressed. The focus is on the malaria-causing parasite’s resistance to artemisinin.

Artemisinin resistance has occurred as a consequence of several factors: poor treatment practices, inadequate patient adherence to prescribed antimalarial regimens, and the widespread availability of oral artemisinin-based monotherapies and substandard forms of the drug.

In January 2011, WHO released the Global plan for artemisinin resistance containment, and  in April 2013, WHO launched an emergency response to artemisinin resistance (ERAR) in the GMS and set up a regional hub in Phnom Penh..

Urgent action now will deliver significant savings in the long run, improving the sustainability and public health impact of malaria interventions around the world.

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, big 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.