Avian influenza – situation in Turkey – update 7: Assessment of the outbreak
30 January 2006
A WHO collaborating laboratory in the United Kingdom has now confirmed 12 of the 21 cases of H5N1 avian influenza previously announced by the Turkish Ministry of Health. All four fatalities are among the 12 confirmed cases.
Samples from the remaining 9 patients, confirmed as H5 positive in the Ankara laboratory, are undergoing further joint investigation by the Ankara and UK laboratories.
Testing for H5N1 infection is technically challenging, particularly under the conditions of an outbreak where large numbers of samples are submitted for testing and rapid results are needed to guide clinical decisions. Additional testing in a WHO collaborating laboratory may produce inconclusive or only weakly positive results. In such cases, clinical data about the patient are used to make a final assessment
Assessment of the outbreak
The outbreak was investigated by international teams coordinated by WHO, including teams of experts drawn from the Global Outbreak Alert and Response Network. Teams in Ankara and Van Province have now completed their work, which has included an overall assessment of the epidemiological situation, the effectiveness of control measures, and the risk of further human cases. Staff departures took place over the weekend. Mechanisms of close collaboration with the Ministry of Health, established during the outbreak, will remain in place.
Epidemiologists in Van investigated several clusters of childhood cases in families from the Dogubayazit district, where the majority of patients, and all fatal cases, resided. Field investigations, including interviews with family members, have found that almost all cases had a documented history of direct exposure to diseased or dead poultry.
The investigation found no clear evidence of human-to-human transmission and no evidence that the virus is now spreading more easily from birds to humans. The vast majority of cases have occurred in children aged 15 years or younger. This age pattern remains puzzling, as adult members in some families were engaged in such high-risk behaviours as the slaughtering of obviously ill birds, yet did not develop infection. This observation further supports the possibility, raised previously during field investigations in Asia, that some as yet unidentified genetic or immunological factor may influence the likelihood of human infection.
Monitoring of patient contacts and of staff at hospitals treating patients found no evidence of infection in these groups, further supporting the conclusion that, in Turkey as elsewhere, the virus is not spreading easily from person to person.
The WHO team found that patients received a high quality of clinical care. The rapid detection of cases, facilitated by high public awareness of the disease, may have contributed to the lower fatality seen in Turkey compared with other countries reporting recent cases.
On 16 January, WHO established a virtual network of clinicians experienced in the management of H5N1 infection and other severe respiratory diseases, allowing Turkish doctors to confer, in real time, with experts elsewhere. Three teleconferences have been held. At present, all evidence, including laboratory and radiological findings, suggests that the disease seen in Turkey is similar to that seen in the Asian outbreaks. In all outbreaks, severe pneumonia and a rapid progression to respiratory failure have been characteristic features in severe cases of infection.
Data on cases in the Turkish outbreak show that patients were hospitalized between 31 December 2005 and 13 January 2006. Dates of symptom onset indicate that all infections were acquired prior to the implementation of control measures. These have included heightened surveillance for poultry outbreaks, culling operations, intensive public information campaigns, contact tracing and prophylactic or post-exposure administration of oseltamivir, and good infection control practices in hospitals managing patients or investigating possible cases.
In poultry, outbreaks of highly pathogenic H5N1 avian influenza are now confirmed or under investigation in numerous provinces across the country. Intensive culling operations are under way; some 1.3 million birds have been culled to date. As in many parts of Asia, the poultry outbreaks in Turkey have involved mostly free-ranging backyard flocks. Control measures under this type of poultry production system are more difficult and time-consuming to implement that those involving outbreaks in large commercial farms. Full control of the disease in birds is likely to remain a goal for some time to come, particularly in view of the recurring risk that migratory birds may introduce the virus.
Experience with this disease over the past two years in Asia has shown that the risk of sporadic human cases persists as long as the virus continues to circulate in birds. For this reason, some additional human cases may occur, but the numbers are expected to be small. This risk will diminish as culling operations further reduce the presence of the virus in its animal reservoir and thus limit opportunities for human infections to occur. High public awareness of risks to avoid reduces these opportunities even further.