Asthma is often thought of as a disease of developed nations clean living conditions saddle us with overactive immune systems, or so the theory goes but, in fact, more people die of the condition in low- and middle-income counties than in high-income countries.

While asthma is more common in developed countries, we might be underestimating its occurrence in other parts of the world, researchers say.

"There's also good evidence that we don't really understand the prevalence patterns" of asthma , said David Van Sickle, a medical anthropologist at the University of Wisconsin Madison. "We haven't really figured out how to measure it yet."

The problem, Van Sickle said, is that epidemiologists traditionally measure asthma's prevalence by asking, "Have you ever been told by physicians you have asthma?"

But his studies suggest that physicians vary in how they diagnose asthma, depending on where they live.

"The epidemiology of asthma is a reflection of the diagnostic habits of physicians," he said at the annual meeting of the American Association for the Advancement of Science in Washington, D.C., on Feb. 19. "It's possible that prevalence of asthma will change as awareness changes."

Who has asthma?

One way to assess if someone has asthma is to show them a video with scenes depicting the symptoms of asthma, such as wheezing, and ask them if they have ever experienced these symptoms.

Van Sickle and his colleagues showed such a video to 112 physicians in Wisconsin and 70 in India.

Doctors in Wisconsin were more likely to suggest that the subjects in the video had asthma than were doctors in India.

"Our approach to asthma was based on idea that physicians were uniformly diagnosing asthma," Van Sickle said. "But this isn't true."

In order to get a better idea of the true prevalence of asthma, we need to develop new ways of easily assessing asthma in large groups of people, Van Sickle said.

"We have to match the epidemiological instruments with what's going on in the clinic," he added.

Impact on the hygiene hypothesis

An interesting question is whether finding asthma's true prevalence could change researchers' ideas about the hygiene hypothesis. This hypothesis says that cleanliness in developed countries has increased the prevalence of allergies and asthma in those areas. But what if asthma is actually more common in developing countries?

Findings from other researchers suggest that there still is something to the hygiene hypothesis, Van Sickle said. "There's obviously something going on," he said.

Previous work has shown that when people migrate from developing countries to more developed ones, their rates of asthma increase, said Kathleen Barnes, an allergy and asthma researcher at Johns Hopkins School of Medicine in Baltimore, who also spoke at the meeting.

But Barnes said it is still difficult to compare asthma's prevalence in developed and developing countries, because the way in which physicians diagnose the condition may differ.

"It's very difficult to make head-to-head comparisons in these two very different environments of the world," she said.

Pass it on: The prevalence of asthma in developing countries may be underestimated. If it is, a widely held theory about asthma's causes may lose importance.

Follow MyHealthNewsDaily staff writer Rachael Rettner on Twitter @RachaelRettner.