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By Tan Ee Lyn
HONG KONG, Feb 1 (Reuters) - Experts have called for closer study of less lethal strains of the H5N1 bird flu virus because they might be more likely candidates to spark an influenza pandemic.
Scientists have identified at least four major variants, or "genotypes", of the H5N1 virus since it made its first-known jump to humans in Hong Kong in 1997.
Topping the list is the Z-genotype, which has been found in northern China, Indonesia, Indochina, parts of Africa, Europe and the Middle East, and killed more than half the people it was known to have infected.
Less well known is the V-genotype that turned up in South Korea and Japan in late 2003. This variant infected at least 9 South Koreans who took part in poultry culling in late 2003 and early 2004 to stop outbreaks in the country. None of them suffered any serious symptoms and all recovered.
Another poultry worker was infected after the disease hit poultry farms in South Korea in November but he, too, did not become seriously ill.
Scientists warned against dismissing these less virulent H5N1 strains since they bear more likeness to viruses that have killed millions of people in the past.
Julian Tang, microbiology assistant professor at the Chinese University of Hong Kong, said what made past pandemics deadly was not high mortality rates but the efficiency with which they spread among people. If massive populations were struck down, even a low mortality rate would culminate in devastating death tolls.
"All previous influenza pandemics have shown a relatively low mortality (of less than 3 percent, including the 1918-19 pandemic). The total number of people who died is large because a lot more were infected, but the majority — or over 97 percent — survived," Tang told Reuters.
The Spanish flu of 1918-19 killed about 50 million people worldwide, but the real number will never be known.
"There is evidence that even this virus underwent some adaptation to humans before becoming pandemic, which may be what we are seeing in these more mild or asymptomatic human cases of H5N1 in South Korea," Tang said.
HOSTS MUST LIVE
"This makes sense from an evolutionary viewpoint. If a virus is so lethal that it kills 50 percent of its hosts, then it will not transmit very far to infect many other people, and it will die out relatively quickly. It is less likely to become a worldwide pandemic," Tang said.
The H5N1 virus has flared up in recent weeks, spreading through poultry flocks in Japan, Vietnam and Thailand, killing six people in Indonesia and claiming its first human life in Nigeria.
Although it remains a bird disease, it is known to have infected 270 people since late 2003, killing 164 of them.
Experts fear the virus could kill millions once it learns how to pass efficiently among people.
Lo Wing-lok, an infectious disease expert in Hong Kong, called for closer study of all variants of H5N1.
"How likely will genotypes V and Z cause serious human infection? I believe we should have some ideas as to how virulent the two subtypes are," Lo told Reuters.
Tang believes a less virulent strain, rather than the highly pathogenic genotype Z, would more likely be a candidate that could cause a pandemic.
"If H5N1 influenza really does become the next pandemic influenza virus, then it needs to become more human-adapted to efficiently transmit between humans, and not to kill them off too quickly, to enable infected individuals to transmit the virus to someone else," Tang said.