By Marlene Cimons
For nearly a century, scientists thought that malaria could only spread in places where it is really hot. That’s because malaria is spread by a tiny parasite that infects mosquitoes, which then infect humans — and this parasite loves warm weather. In warmer climates, the parasite grows quickly inside the mosquito’s body. But in cooler climates, the parasite develops so slowly that the mosquito will die before the it is fully grown.
At least that was the prevailing understanding. New research suggests parasites don’t mind the cold as much as scientists thought.
It appears that parasites can thrive in slightly cooler temperatures that were once believed to be inhospitable for them. This means that slightly warmer weather in more temperate regions could prompt parasites to reach their full potential—a situation that will put thousands or more people in danger of contracting malaria. Not only will rising temperatures spur mosquitoes to move to new areas that were formally too cold to inhabit, hotter weather will also nurture the growth of the disease-carrying parasites that live inside those mosquitoes.
“Our results show that not only is it possible for the mosquito to become infectious with malaria parasites at cool temperatures, but that it happens considerably faster than has been previously thought,” said Jessica Waite, a senior scientist at Penn State and a co-author of the study with Matthew Thomas, professor and Huck scholar in ecological entomology at Penn State. Their findings could help predict the spread of malaria in cooler climates. The paper, which also included researchers from the University of Exeter, appears in the journal Biology Letters.
Malaria is a life-threatening illness caused by Plasmodium parasites that are carried by the female Anopheles mosquito, and passed on to humans through her bite. More than 400,000 people died of the disease in 2017, with sub-Saharan Africa bearing the largest brunt of malaria cases — more than 90 percent — according to the World Health Organization. It is also a problem in parts of South America and South Asia, according to the Centers for Disease Control and Prevention (CDC).
Malaria was eradicated from the United States in 1951, largely due to effective control measures, such as spraying and air conditioning. Today, only around five people a year die of malaria in the United States. Most of those infected with malaria contracted the disease while traveling, according to the CDC. “It is unlikely that developed countries with active detection and prevention programs, such as ours, would ever become areas of active malaria transmission again,” Waite said.
The disease typically begins with fever, chills and headache, but can lead to death if untreated. “I’m not a medical doctor, but anecdotally I’ve heard it makes people feel pretty miserable — like a bad flu with cyclic fevers,” Waite said. It is particularly dangerous for small children and “can lead to coma and death for severe infections,” she said.
The researchers wanted to know how long it took for mosquitoes to become infectious, believing that older studies were flawed, including some that relied on a Russian species of mosquito.“We wanted to get some good data to improve models,” Waite said. This is especially important, she said, in places wear mosquitoes might thrive if it were only a little warmer.
For example, “imagine a mountain, and at the peak of the mountain, it’s too cold for mosquitoes to live or for parasites to complete their transmission cycle,” she said. “However, at the base of the mountain there is a lot of malaria and mosquitoes.” Many people live between the base and the peak and could become vulnerable to malaria if it becomes just a little bit warmer in those areas, she said.
In conducting their study, scientists infected mosquitoes in the lab with human malaria parasites. They did this by feeding them blood containing the parasite. The blood was held in place by a layer of waxy, stretchy film. “The mosquitoes pierce through this material like they would for human skin and take up a blood meal with the parasites,” she said, explaining the experiments.
After the mosquitoes finished dining, the scientists placed them into different incubators, some warm, some cold. They exposed some mosquitoes to fluctuating temperatures — as in nature, where it’s cool at night and warm at midday — and others to constant temperatures. They then dissected them and examined their guts and salivary glands, looking for malaria parasites.
Open an infected mosquito and you will find bags filled with replicating parasites. When these bags burst, the parasites will spill out and find their way to the mosquito’s salivary glands. Infected mosquitoes then pass on these parasites to humans when they draw blood. It takes time for the parasites to grow and then make their way to the salivary glands — but not as much time as was previously thought.
“So, for example, at [64 degrees Fahrenheit] the [earlier] model would predict mosquitoes become infectious at around 56 days, but we found at constant temperatures infectious mosquitoes were found as early as 33 days,” Waite said. “With fluctuating temperatures — similar to real world conditions — this was as fast as 27 days. This makes a big difference because mosquitoes don’t live forever.”
Thus, if climate change continues to cause cooler regions to get warmer — even just a little bit — mosquitoes will get a jump start toward becoming fully infectious before they die. Waite is counting on that not happening. “I’m hopeful our work will lead to better estimates of malaria spread, and better prevention strategies,” she said.
Marlene Cimons writes for Nexus Media, a syndicated newswire covering climate, energy, policy, art and culture.