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Warmer wetter weather and the growth of cities is increasing the spread of mosquito borne diseases

The Aedes aegypti mosquito which transmits the Zika virus (Image by Sanofi Pasteur)

The Aedes aegypti mosquito which transmits the Zika virus (Image by Sanofi Pasteur)

The explosion in the number of Latin American cases of microcephaly – a congenital condition associated with maldevelopment of the brain – has become an international emergency due its “strongly suspected” link with the rapidly spreading Zika virus, according to the World Health Organisation (WHO). The WHO has called for urgency in finding a vaccine and better diagnostics, and has urged the world to be alert.

Researchers of infectious diseases are still trying to understand how Zika in pregnant women causes congenital malformation. “Everyone agrees on the urgent need to coordinate international efforts to investigate and better understand this relationship,” said Margaret Chan, the head of the WHO.

The Aedes aegypti mosquito, the species that also transmits the dengue and chinkungunya viruses, is responsible for infecting a growing number of humans with Zika.

For now, the only apparent defense against Zika is to avoid getting bitten by mosquitoes. But as global temperatures rise and Aedes aegypti moves to new habitats and breeding grounds in urban areas where water is stored in open containers, this is becoming increasingly difficult. Scientists from all over the world are concerned.

“Global warming affects the abundance and distribution of disease vectors [small disease-carrying insects], increasing the incidence of infectious diseases and expanding the geographical areas which are at risk,” says Fiona Armstrong, executive director of the Australia-based Climate and Health Alliance.

Increased heat and humidity associated with climate change create ideal breeding conditions for mosquitoes, Armstrong adds. “As regions that used to be drier and colder start to register higher temperatures and more rain, mosquitoes expand their breeding areas, which increases the number of populations at risk.”

Rapid spread

The WHO estimates that up to four million cases of Zika will be recorded in the Americas over the next 12 months. About 80% of those infected do not exhibit symptoms but when they manifest, patients notice fever, a red rash and conjunctivitis.

At least 22 Latin American countries have reported cases of Zika, and Brazil has been hit hardest. In November 2015, the Ministry of Health confirmed a correlation between the outbreak of the virus and the increased incidence of microcephaly in the northeast region of the country. Since then, 3,448 suspected cases of microcephaly are being investigated across the country.

It is not known for sure how Zika arrived in Brazil, but it is thought to have been introduced by either an Asian tourist during the 2014 World Cup or a canoeist from Polynesia who participated in a 2014 event in Rio de Janeiro.

Researchers Davidson Hamer and Lin Chen of Boston Medical Center in the US fear the Zika virus will spread quickly there too. Their study affirms the risk is high due to the presence of the Aedes aegypti and Aedes albopictus mosquitoes in several states. Concerns are growing as summer – the mosquito breeding season – nears.

According to CCTV, local authorities are monitoring the chances that the virus reaches China, whose southern regions are most at risk. No cases have been reported there so far. The Chinese foreign ministry has already expressed its willingness to help Latin American authorities in any way it can, including searching for vaccines to combat the disease.

The Zika virus was discovered in Uganda, Africa in 1947, but the first outbreak in humans was recorded in 2007. The virus then spread to Asia and French Polynesia in 2013, where it infected around 19,000 people and where the first associations with microcephaly were made.

In addition to congenital malformations, it is also believed that Zika causes Guillain-Barre syndrome, a neurological disease that can result in temporary or permanent paralysis.

More pressure

Authorities still have difficulty predicting how far the Zika outbreak could spread. Christovam Barcellos, a researcher at the Fundação Oswaldo Cruz Health and Climate Observatory, says the history of dengue fever can provide some clues.

“We have observed that the areas where dengue is transmitted are expanding throughout the world. In Brazil, dengue is moving further south and to cities at higher elevations, which used to be more protected,” says Barcellos. “This could be an effect of climatic conditions.”

Studies have already indicated southern China and Southeast Asia as areas where dengue could be transmitted, along with northern Australia, since the mosquito vector inhabits these regions.

“It’s remarkable how Aedes is adapted to the city. It has a life cycle of one to two weeks, during this time it hatches, flies away and bites people. And cities have an ideal environment for it to reproduce,” Barcellos adds.

Climate change will continue to put pressure on public health systems and resources allocated to fighting new diseases, especially in underdeveloped regions of the globe, researchers say.

“Political leaders and health authorities are underestimating the breadth and complexity associated with the risks to human health that come along with a warmer planet,” Armstrong warns.

This article was previously published by our sister website Diálogo Chino.

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