Researchers have found that human influenza is common in tropical and sub-tropical countries, but is often mis-diagnosed. Researchers say many cases of severe illness could be prevented if the most vulnerable populations were vaccinated.
The flu can cause serious illness, especially in the elderly and those with weakened immune systems.
In the United States and Europe, and other countries, influenza is a well-recognized ailment, and people who are most vulnerable are urged to get vaccinated each year to prevent the flu, or, at least, to help them recover faster, if they become sick.
But experts say influenza has not been considered a health problem in tropical climates because there is no cold season, when flu is common.
Malik Peiris, a microbiologist at the University of Hong Kong, says researchers have been trying to determine if there is a pattern to flu cases in tropical and sub-tropical countries.
"In Hong Kong, it happens in the spring, and, sometimes, in the summer, and, in Singapore, it happens all year 'round," he said. "So, the exact determinant of this seasonality is not well understood, yet."
Using a complex model, Peiris and colleagues analyzed records of 100,000 patients in Hong Kong between 1996 and 2000, using the country's computerized hospital system. The researchers wanted to try to determine which hospitalizations may have been due to influenza, although they were recorded as other, often related, ailments.
The researchers' results mirrored hospitalizations attributed to flu in the United States, but, in Hong Kong, they were not diagnosed as influenza. The researchers theorize that that is because there is not an influenza season, as there is in countries with cold climates.
The investigators found hospital admissions with respiratory and other symptoms that could be linked to flu were highest among elderly in Hong Kong, as well as in the United States, but more children were admitted in Hong Kong.
Peiris says that, at various times, investigators found a nearly 12 percent spike in hospital admissions in Hong Kong for respiratory illnesses, a 1.5 percent increase in admissions for stroke, a nearly two percent increase in admissions for heart events and a 3.5 percent hike in hospitalizations for diabetes.
"So the diagnosis when the patient gets to hospital is not a diagnosis of influenza, it's a diagnosis of complications of influenza," he said. "So, people do not really appreciate that this whole process was really started by influenza."
As a result, Peiris says, many people in countries with no cold season are not being vaccinated against the flu, which might keep them from getting sick and possibly dying.
The study on influenza in Hong Kong is published in the public access, on-line journal, Public Library of Medicine.