The current outbreak of dengue fever in central Tokyo underscores recent World Health Organization warnings that climate change is extending the range of potentially fatal tropical diseases such as malaria and dengue.
Dr. Rabindra Abeysinghe, a WHO entomologist based in the Philippines, told the Nikkei Asian Review that global warming is having a growing impact on the distribution of mosquitoes around the world and the spread of insect-borne diseases.
"Increases in temperature result in a more conducive environment for mosquitoes," 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 mosquitoes live longer, as well as accelerate development of the disease-causing agents they carry. Dengue and other insect-borne diseases such as chikungunya and Zika fever are spreading to new areas, including in the Pacific, he explained.
As well as the Tokyo outbreak, which began in August in the city's central Yoyogi Park and numbered 60 infections by early September, dengue cases have been reported in southern France and elsewhere in Europe, while 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 U.S., such as Florida, where 28 locally transmitted cases were reported in a 2009-2010 outbreak, the first there in more than 40 years.
Not good enough
Scientists have been struggling for decades to find solutions to these virulent diseases. The French pharmaceutical company Sanofi Pasteur 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 and was dubbed as "not good enough" by one of its ministers.
Trials in Asia have found the vaccine only offers limited protection (56% of those vaccinated), and no protection at all against one of the four dengue viruses carried by mosquitoes. However, in early September, Sanofi announced the results of the vaccine's trials in Latin America, which were a little more successful, with success levels of 60%.
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, pain in the joints and behind the eyes and a rash. The more severe form of the disease is characterized by internal bleeding, seizures and death.
Dengue viruses are transmitted by various mosquitoes, including the Aedes aegypti mosquito, which is found in most parts of the world, including in the U.S. 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, which carries them to the human liver to replicate before attacking red blood cells.
The British health care 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 savior.
Dr. Arjen Dondorp, 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% protection, after three rounds. It's quite expensive." He added the vaccine provides about two years of protection against falciparum malaria but only one year of very good protection, after which it slides dramatically.
Waiting for better
One way to control the spread of malaria is to cure those carrying the parasite as quickly as possible, so they are unable to pass it on to other people via mosquitoes. But the malaria virus continues to mutate, and drugs-resistant strains continue to emerge.
Artemisinin-based combination therapies, recommended by the WHO as a 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 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," Dondorp said, adding that he considers it a much bigger threat than artemisinin resistance alone. Such concerns have hastened moves to set up wide-scale malaria elimination programs to try to eliminate falciparum malaria in the region before it becomes entirely untreatable.
There are some new drugs in the pipeline, according to Dondorp, although at least two of them are still in the development phases 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 said. "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."
Sian Powell is a Hong Kong-based Australian reporter.