Although it has been hard to find a silver lining in the arrival of HPAI H5N8 and its resultant reassortant offspring, we’ve been fortunate that thus far that no human infections from these new subtypes have been reported. A not entirely unexpected outcome as no human infections have been reported out of South Korea, Japan, or China where the parental H5N8 virus has been in circulation for at least 18 months.
Other Asian HPAI H5 viruses – primarily H5N1 and H5N6 – have caused serious human illness (and death) in more than 800 people worldwide, although given the number of people who have had some contact with likely infected birds, the virus is obviously not easily passed from bird to human.
There has been some serological evidence (see Taiwan: Three Poultry Workers Show H5N2 Antibodies and Avian Flu Antibody Survey In Poultry Workers – Taiwan 2012) that other avian H5 viruses might have the ability to jump to humans, but the data is sparse, and those who tested positive apparently had few, if any symptoms.
Still, the CDC has maintained a cautious stance, knowing that the virus could either reassort again, or pick up amino acid changes, that could make it more likely to infect humans. We'd seen guidance issued by the CDC previously on antivirals and monitoring for those who may have been exposed, but in early June the CDC has published a detailed HAN Advisory for clinicians.
Bird Infections with Highly-Pathogenic Avian Influenza A (H5N2), (H5N8), and (H5N1) Viruses: Recommendations for Human Health Investigations and Response
Distributed via the CDC Health Alert Network
June 2, 2015, 13:00 ET (1:00 PM ET)
Highly-pathogenic avian influenza A H5 viruses have been identified in birds in the United States since December 2014. The purpose of this HAN Advisory is to notify public health workers and clinicians of the potential for human infection with these viruses and to describe CDC recommendations for patient investigation and testing, infection control including the use personal protective equipment, and antiviral treatment and prophylaxis.
Today the EID Journal carries a report on more than 160 people who were exposed to HPAI H5 birds in North America over the winter and spring, and reassuringly finds very few developed any symptomology in the 10 days after exposure, and of those, none of them tested positive (via PCR or Serology) for the virus.
I would note there are some limitations to this study. Prime among them: Not everyone who was exposed was followed up on, and only those who developed symptoms were tested. One hopes a more aggressive serological survey is in the works, but for now this is excellent news.
A link to the study, and some excerpts from the Abstract and Discussion follow:
Volume 21, Number 12—December 2015
Infection Risk for Persons Exposed to Highly Pathogenic Avian Influenza A H5 Virus–Infected Birds, United States, December 2014–March 2015
Author affiliations: Centers for Disease Control and Prevention, Atlanta, Georgia, USA (C.S. Arriola, L. Blanton, K. Kniss, M.Z. Levine, S.C. Trock, L. Finelli, M.A. Jhung); United States Department of Agriculture, Washington, DC, USA (D.I. Nelson); United States Department of Agriculture, Fort Collins, Colorado, USA (T.J. Deliberto)
Newly emerged highly pathogenic avian influenza (HPAI) A H5 viruses have caused outbreaks among birds in the United States. These viruses differ genetically from HPAI H5 viruses that previously caused human illness, most notably in Asia and Africa. To assess the risk for animal-to-human HPAI H5 virus transmission in the United States, we determined the number of persons with self-reported exposure to infected birds, the number with an acute respiratory infection (ARI) during a 10-day postexposure period, and the number with ARI who tested positive for influenza by real-time reverse transcription PCR or serologic testing for each outbreak during December 15, 2014–March 31, 2015. During 60 outbreaks in 13 states, a total of 164 persons were exposed to infected birds. ARI developed in 5 of these persons within 10 days of exposure. H5 influenza virus infection was not identified in any persons with ARI, suggesting a low risk for animal-to-human HPAI H5 virus transmission.
Although this early assessment suggests that the risk of bird-to-human transmission of HPAI H5 viruses in the United States may be low, the CDC recommends vigilance when considering future human exposures to birds that are or may be infected. Similar HPAI H5 viruses, such as Eurasian H5N1 and H5N6 viruses, have caused severe illness and death in humans in Europe, Asia, and Africa (13,26), and these newly identified US HPAI viruses should be regarded as having the potential to cause severe disease in humans until shown otherwise. The best way to prevent human infection with avian influenza A viruses is to avoid unprotected contact with sick or dead infected poultry. Persons who have been exposed to HPAI-infected birds should be monitored for 10 days after last exposure and be tested for influenza as soon as possible after illness onset if respiratory symptoms develop. Exposed persons may also be offered influenza antiviral chemoprophylaxis. Additional guidance on testing, monitoring, and chemoprophylaxis is available at www.cdc.gov/flu/avianflu/guidance-exposed-persons.htm.
HPAI H5 virus outbreaks in US birds will likely continue, and additional reassortment with North American viruses may also occur. Although the risk of virus transmission to humans appears to be low, each exposure incident should be reported immediately and investigated collaboratively by animal and human health partners. A rapid response to any potential human cases of HPAI H5 infection in the United States is critical to prevent further cases, evaluate clinical illness, and assess the ability of these viruses to spread among humans.