The current outbreak of dengue fever in central Tokyo underscores recent World Health Organisation warnings that climate change is increasingly extending the range of potentially fatal tropical diseases like malaria and dengue.
Dr Rabindra Abeyasinghe, a World Health Organisation entomologist based in the Philippines, told the Nikkei Asian Review that global warming is continuing to have a significant impact on the distribution of mosquitos around the world and the spread of insect-borne diseases.
“Increases in temperature result in a more conducive environment for mosquitos,” he said. “It has now already been shown that increases in temperature result in a geographical expansion of disease-affected areas, including the reporting of diseases such as malaria from higher elevations.” Higher temperatures, he added, also help mosquitos live for longer, as well as speeding the development of the disease causing agents they carry.
Dengue and other insect-borne diseases such as chikungunya and Zika fever are spreading to new geographical areas including in the Pacific, he explained. As well as the Tokyo outbreak, dengue has been reported from southern France and an outbreak of malaria was reported in Greece a couple of years ago. Dengue has also increasingly been seen in the southern states of the US, such as Florida, where 28 locally-transmitted cases were reported in a 2009-2010 outbreak, the first there in more than 40 years.
Scientists have been struggling for decades to find controls for these virulent diseases. The French pharmaceutical company Sanofi-Pasteur has recently announced it was refining the world’s first dengue vaccine, which should be commercially available next year. But the new drug has already been rejected by Singapore’s government; dubbed as “not good enough” by one of the city-state’s ministers.
Trials in Asia found the vaccine only offers limited protection (56 per cent), and no protection at all against one of the four dengue viruses carried by mosquitos. However, this week Sanofi announced the results of the vaccine’s trials in Latin America, which were a little more successful (60 per cent).
Most patients recover from dengue, but contracting the disease more than once can prove fatal. Often known as “breakbone fever”, dengue usually causes a high temperature, pains in the joints and behind the eyes and a rash. The more severe form of the disease is characterised by bleeding, seizures and death.
Dengue viruses are transmitted by various mosquitos, including the aedes egypti mosquito, which is found in most parts of the world including the US and Australia, but not yet in Japan, and the invasive and voracious aedes albopictus or Asian Tiger mosquito.
In other developments, scientists have been working on a vaccine for malaria, the disease spread by yet another species of mosquito, the anopheles. When the malaria-carrying female mosquito bites a human, the malaria parasites are injected into the human’s blood stream, and speed to the human liver to replicate, before attacking red blood cells.
The British healthcare company GlaxoSmithKline in July announced it was seeking regulatory approval for a malaria vaccine known as RTS,S, which has taken 30 years to develop. Designed to stop the malaria parasite from maturing and multiplying in the human liver, the vaccine requires three doses, administered monthly. While seen as a promising step forward, the vaccine has not been universally greeted as a long-awaited pharmaceutical saviour.
Dr Arjen Dondorp, the deputy director of the Mahidol Oxford Tropical Medicine Research Unit in Bangkok, said the vaccine was not all he had hoped it would be. “The protective effects – everyone is a bit disappointed,” he said. “It gives around 50 per cent protection, after three rounds. It’s quite expensive.” Dr Dondorp added that the vaccine provided about two years of protection against falciparum malaria; one year of very good protection; after which it slides dramatically.
One way to control the spread of malaria is to cure those infected with the parasite as quickly as possible, so they are unable to pass it on to other people, via mosquitos. But the malaria parasite continues to mutate, and drugs-resistant strains continue to emerge.
Artemisinin-based combination therapies, recommended by WHO as the first-line treatment for uncomplicated falciparum malaria, have been remarkably effective for many years, leading to a huge fall in malaria cases in Asia. Yet the days of success might be numbered.
Parasite resistance to artemisinin, the drug derived from the sweet wormwood plant, has so far been found in Cambodia, Laos, Myanmar, Thailand and Vietnam. It takes a lot longer to clear parasites with artemisinin-resistance from a patient’s blood, leaving the partner drugs to do much of the work. Now, though, the malaria parasites are developing resistance to the partner drugs as well.
“That is what we now see happening in western Cambodia and on the Thai-Myanmar border,” Dr Dondorp explained, adding that he considered it a much bigger threat than artemisinin resistance alone. So wide-scale malaria elimination programs have been set up to try and eliminate falciparum malaria in the region before it becomes entirely untreatable.
There are a few new drugs in the pipeline, Dr Dordorp says, but at least two of them are still in the development phase and it will take between three and five years before they are commercially available. “If (the spread of malaria) goes up, it will be an exponential curve,” he says. “We think we are still in this flat part, but once you cross the tipping point, we’re very much afraid that we will see it resurge.”