China´s recent spike in human H5N1 avian influenza cases appears to lack the hallmark of nearby poultry outbreaks, a development that some public health officials worry could signal asymptomatic infections in birds.

Veterinary experts, however, suggest the pattern could point to surveillance gaps or the consequences of routine vaccination.

China has reported four human cases so far this year, three of them fatal. According to World Health Organization (WHO) reports:

• The 16-year-old boy from Hunan province who died yesterday had been exposed to sick and dead poultry.
• Investigators found that a 19-year-old girl from Beijing who died on Jan 5 had contact with poultry before she got sick, but they did not say if the birds were ill.
• Authorities are still investigating the virus source in the other two cases, a 2-year-old girl from Shanxi province who is in critical condition and a 27-year-old woman from Shandong province who died on Jan 17.

The country´s agriculture ministry said on Jan 18 after the 2-year-old´s infection was confirmed that no H5N1 outbreaks have been detected in Shanxi or Hunan provinces, the British Broadcasting Corporation (BBC) reported yesterday.

York Chow, Hong Kong´s secretary for food and health, has called on China to release more epidemiologic information on the recent human infections and said that an apparent lack of poultry outbreak reports against the backdrop of human cases raises questions about a possible change in the virus or that asymptomatic H5N1-infected chickens might be contributing to the spread of the virus.

Chinese officials have said they have found no evidence that the virus has mutated to allow easier human-to-human transmission, according to media reports.

Monitoring billions of birds
This isn´t the first time that health officials have voiced their suspicions about asymptomatic poultry infections in China. In 2006, Zhong Nanshan, director of the Guangzhou Institute of Respiratory Diseases, suggested that two victims might have caught the virus from chickens that were carrying it asymptomatically (see link to CIDRAP News story, below).

Avian influenza experts say the size and nature of China´s poultry population creates a difficult surveillance task. Jan Slingenbergh, a senior animal health officer for the United Nations Food and Agriculture Organization (FAO) told CIDRAP News that China has roughly 4.6 billion chickens, 700 million ducks, and 300 million geese that are distributed somewhat unevenly throughout the country.

He said the ducks gravitate toward the double-crop rice growing areas in southern and southeastern China, which are thought to be the main risk areas because the H5N1 virus keeps circulating in ducks. Geese head toward single-crop rice growing areas in the less rainy northeastern and extreme western part of China, Slingenbergh added.

Meanwhile, he said chicken are kept everywhere people live, particularly in urban areas and coastal ports.

Slingenbergh links the low level of poultry outbreak reports to China´s poultry vaccination policy. "The entire national flock is kept under a rigid vaccination blanket amounting to 11 billion applications per annum," he said.

He said he doubts that the H5N1 in China is evolving toward a low-pathogenic virus. "Vaccination creates a rather sparse geospatial mosaic of susceptibles, which may even enhance the pathogenicity level," he said, adding that evidence from Vietnam, where most Chinese viruses spread to, suggests that the virulence increased between 2002 and 2007 when measured by infecting and gauging shedding in young mallards.

Vincent Martin, a senior technical adviser in the FAO´s Beijing office, told CIDRAP News that Chinese officials obtain a lot of samples from farms and live bird markets each year to monitor asymptomatic H5N1 infections among the birds. "Regularly, they find the virus but do not detect any outbreak in the surrounding areas," he said.

However, a combination of factors makes detecting the virus difficult, Martin said. Several strains of the virus are circulating in China, and ducks can excrete the virus without showing symptoms or only exhibiting mild ones.

In addition, suboptimal vaccination can mask the symptoms without stopping viral shedding.

More intensive surveillance and monitoring efforts are needed in China to detect new outbreaks and identify viral circulation that is going unnoticed, Martin said, "to avoid a situation where humans serve as sentinels and reveal infection in birds."

"The concern is, therefore, that the current surveillance is unable to provide a complete picture of the [high-pathogenic avian influenza] epidemiological situation in domestic birds and should be strengthened and improved in order to meet the challenge we are currently facing," he said.

Three Chinese government ministries yesterday issued a joint order for local health, agriculture, and commerce offices to work together to improve surveillance and management of the country´s live poultry markets, Xinhua, China´s state news agency reported today. The government urged local offices to close live poultry markets in urban areas, if possible, or disinfect the markets daily if they can´t be shuttered. The offices were also ordered to conduct daily surveillance and reporting and collaborate when they detect the H5N1 virus.

The role of poultry vaccination
Les Sims, from Australia´s Asia-Pacific Veterinary Information Services and a consultant to the FAO, said though humans are once again acting as sentinels for infections in poultry, so far there is no evidence to support asymptomatic disease as the reason for absence of reported poultry outbreaks in China.

"Vaccination will alter the clinical appearance of disease if the flock is infected, but on a flock basis, some disease will be detected. Infection is not silent," he said. He added that infected vaccinated flocks, for example, have lower mortality rates with fewer birds showing classical symptoms of the disease.

"If vaccines are used, veterinary and medical authorities have to accept that one of the signals they used to rely on for detecting infection in poultry—high mortality—needs to be modified," Sims said.

Infected poultry can still shed small amounts of the virus, even when the vaccine is a good match and the birds are vaccinated properly, he said. "This has always been the case and so can´t explain the current situation in China."

The issue of less severe infections in vaccinated poultry is creating negative sentiments about the measure, Sims said, but he added that China has maintained a close match between the circulating strains and the vaccine antigen, which greatly diminishes the viral load in poultry.

"The benefits of vaccination in reducing viral load need to be considered and balanced against the changes in disease appearance that will occur if a vaccinated flock is infected," he said. "The situation in China would almost certainly be much worse if vaccination was not used."

Sims said he´s not surprised that some poultry infections go undetected, given the size and make-up of China´s poultry population, along with the modified appearance of the disease in vaccinated poultry. He suspects, though, that under-reporting of the disease might be one factor that keeps the number of outbreak reports low. Farmers who raise poultry for their livelihood have little incentive to report the disease.

A seasonal surge in poultry and human H5N1 cases in the winter isn´t unexpected, he said. "Winter peaks have been seen previously and are probably linked to the increased trade in poultry for various festivals and enhanced viral survival due to cooler conditions," Sims said.