A soldier distributes bed nets in Cambodia. Photo: Flickr/DVIDSHUB

Scandinavia seems like an unlikely place to be looking for ways to help eradicate malaria, but new research culled from historical data in the region suggests there may be an element of malaria transmission that scientists haven’t been looking at: household size.

There has long been an established link between malaria and poverty, but researchers think that correlation may actually be a link between the number of people sleeping in an average household and malaria rates, rather than being specifically linked to hygiene or home construction techniques.

“My coauthors were comparing the history of malaria in Scandinavian countries and noticed that some countries had taken eradication efforts and some hadn’t. It turns out that it didn’t really matter. What they observed was a correlation between declining average household sizes and declines in malaria,” Ross McKitrick, a Canadian economist and coauthor of a report published in the Journal of the Royal Statistical Society Series A, said. “Part of what has been counted as an ‘income effect’ is really a household size effect.”

McKitrick says it all comes down to how malaria is spread. Anopheles mosquitos, which most frequently carry malaria, tend to feed on sleeping people during the night. In order for the parasite to spread, it has to encounter, with the help of the mosquito, uninfected people. In countries where one-roomed structures housing one or multiple families, malaria is still prevalent. In countries where average household size has decreased, the disease has been eradicated or has become much less prevalent.

Of course, telling people to build more houses or have smaller families isn’t exactly a feasible strategy, and there may be something else at play here. But McKitrick says that the team’s theory may be one reason why bed nets have been so successful at lowering malaria rates worldwide. 

Graph via Journal of the Royal Statistical Society Series A

“If you have a country with a high malaria rate, it’s not much help to tell them to ‘get rich,’” he said. “But you can artificially segment the sleeping quarters with bed nets, which allows you to mimic households of smaller sizes.”

The study took a look at demographic, sociological, and economic factors in 232 countries over the course of much of the 20th century. McKitrick found that having a lower number of people living under one roof was so important that, historically, it has outpaced any other measures thought to be important in rising or falling malaria rates. Those factors include population density, deforestation, urbanization, and even temperature and things like DDT spraying. Even after controlling for inequalities such as household income, the average number of people living in a household remained the most important factor. 

“Although right now, the disease is concentrated in tropical countries, that wasn’t always the case. It used to be a problem right up to the Arctic circle. It remains a tropical disease because that’s where poorer countries have tended to be,” he said. “The countries that have lowered household size have set up a situation where the parasite can’t remain established in human populations.”

To further back up his data, McKitrick found one region that averages a relatively high number of people per household and has managed to keep malaria rates down: Islamic countries in the Middle East. In those countries, McKitrick says, strong traditions that keep men and women separated at night have artificially created situations similar to those in countries that have smaller household sizes.

“In those countries with strict domestic Islamic practices, they build separate rooms for males and females to sleep in,” he said. “Even though they may have a large family living in the same house, they’ve effectively segregated the sleeping quarters.”

McKitrick’s study isn’t the first to notice a correlation between household sizes and malaria rates. But it is the first to do so for more than 80 years. In 1930, an author named Sidney Price James recognized that malaria seemed to be more prevalent in areas where families tended to sleep in one room. That study received little attention, but McKitrick’s seems to suggest there may have been something to it.

“We’ve only found a couple of fairly obscure papers based on field studies that have looked into this issue,” he said. “But as far as we know, we’re the first ones to put a global sample of the data together.”