The outbreak of a new and worrying kind of influenza in Mexico may not be as severe as it first looked, with many mild cases that were not immediately noticed, U.S. health experts reported on Friday.

Health workers are rushing to get a fuller picture of the outbreak of H1N1 swine flu, which the World Health Organization fears could tip into a pandemic.

Mexico, the worst-hit country, has reported 176 deaths from the new strain of the H1N1 virus. Cases have been reported around the world, mostly among travelers from Mexico, but they have been mild and most people recovered so far with little or no treatment.

The only death outside Mexico was a Mexican toddler visiting Texas.

Experts have been struggling to explain why so many deaths in Mexico and nowhere else, but the CDC report suggests there is a simple explanation - there are many cases in Mexico; most are mild, and just the bad ones have been seen so far.

“The clinical spectrum of swine-origin influenza A (H1N1) virus illness is not yet well characterized in Mexico. However, evidence suggests that swine-origin influenza A (H1N1) virus transmission is widespread and that less severe (uncomplicated) illness is common,” the CDC said in a special report.

“To date, case-finding in Mexico has focused on patients seeking care in hospitals, and the selection of cases for laboratory testing has focused on patients with more severe disease. Therefore, a large number of undetected cases of illness might exist in persons seeking care in primary-care settings or not seeking care at all.”

But more information is needed, the CDC team stressed. The CDC has sent test kits to Mexico that will help get a better picture of who is sick. It is difficult to know without special tests because respiratory diseases are so common and many infection look like flu.

“Additional investigations are needed urgently to evaluate the full clinical spectrum of disease in Mexico, the proportion of patients who have severe illness, and the extent of disease transmission,” the CDC said.

Even garden-variety flu can be deadly, with seasonal influenza killing an estimated 250,000 to 500,000 people globally every year.

WASHINGTON (Reuters)