Wednesday, May 31, 2023

WHO DON & Risk Assessment On Two Asymptomatic H5N1 Cases In the UK



Two weeks ago the UK reported the detection of a pair of H5N1 positive nasal/throat swabs collected from two asymptomatic poultry workers, although it wasn't immediately clear whether these were genuine infections, or merely the detection of contaminated `dust' in these worker's respiratory tract.

Viable avian viruses can be spread via dried feces, feathers, and other airborne contaminants that are generated by wet markets and poultry farms (see Zoonoses & Public Health: Aerosol Exposure of Live Bird Market Workers to Viable Influenza A/H5N1 and A/H9N2 Viruses, Cambodia). 

Which means that it would not be unexpected to see H5N1 positive nasal and throat swabs from poultry workers, even if they aren't truly infected. Last year Spain detected viral fragments in the throat swabs of two asymptomatic poultry workers, and a similar case was reported by the United States, but actual evidence of infection was lacking. 

For now, the jury is still out on whether these latest cases from the UK were actually infected. Serology testing over the next few weeks may provide additional clues.  

Yesterday the WHO published their a new DON (Disease Outbreak News) and Risk Assessment on this event, where they deem the risk of spread as low.   I've only posted some excerpts, so follow the link to read it in its entirety. 

Avian Influenza A(H5N1) - United Kingdom of Great Britain and Northern Ireland
30 May 2023

Situation at a glance

In mid-May, the United Kingdom of Great Britain and Northern Ireland reported to the World Health Organization (WHO) the detection of avian influenza A(H5) virus in a poultry worker at a farm in England where poultry was infected with high pathogenicity avian influenza (HPAI) A(H5N1) viruses. Another detection was reported in a second individual performing culling operations on the farm. Both detections were later confirmed by additional testing as A(H5N1). Both cases were asymptomatic and detected as part of an ongoing enhanced surveillance study of asymptomatic workers exposed to poultry infected with avian influenza.

All the workers at this farm and their contacts have been identified; none of the contacts have reported symptoms, and no other influenza cases have been identified. The United Kingdom Health Security Agency (UKHSA) has not detected evidence of human-to-human transmission.

Based on the available information, WHO considers these as sporadic detections of avian influenza viruses among humans with no evidence of person-to-person transmission to date. Thus, the likelihood of international disease spread through humans is considered to be low.

Given the widespread circulation in birds and the constantly evolving nature of influenza viruses, WHO stresses the importance of global surveillance to detect virological, epidemiological and clinical changes associated with circulating influenza viruses which may affect human (or animal) health.

Description of the situation

In late April, the UKHSA was notified by the Animal and Plant Health Protection Agency (APHA) of an outbreak of HPAI (H5N1) on a poultry farm in England, United Kingdom. The human cases were detected through an ongoing enhanced surveillance study of asymptomatic workers exposed to poultry infected with avian influenza.

The UKHSA Rapid Investigation Team were deployed to the farm in early May 2023 to recruit exposed participants for the study. Of the 24 eligible persons, one tested positive for influenza A (with no detection of human seasonal subtypes H1 or H3) on the first sample self-taken at the premises. Two further nasopharyngeal samples collected from the same person tested negative for influenza A by a UKHSA regional laboratory and by the UKHSA national influenza reference laboratory. The participant remained clinically asymptomatic throughout.

An update from the United Kingdom authorities to WHO in mid-May 2023, notified of an additional case from the same farm as influenza A(H5) positive on two separate samples. This second person was a poultry culler exposed to infected birds at the same farm. The poultry culler worked on the farm in early May using personal protective equipment (PPE). The case was clinically assessed and remains asymptomatic. The case was treated with oseltamivir and was negative on respiratory sampling taken on the last day of isolation.

Sequencing later confirmed the virus detected in both individuals as A(H5N1). All samples from these two individuals were negative for seasonal influenza viruses. All other study participants remain well and have tested negative for influenza A on their samples to date. Follow-up of contacts has been completed. The affected farm is one of the first recruited in the ongoing enhanced surveillance study of asymptomatic workers exposed to poultry infected with avian influenza.

Work to determine whether these are infections or not (i.e., could instead be due to transient mucosal contamination of the nose with virus particles) is underway, though it may be difficult to reach a conclusion.


WHO risk assessment

The two reported individuals with influenza A(H5N1) detection in their samples have remained asymptomatic and tested negative for influenza in their most recent samples to date. Their close contacts were asymptomatic and the follow-up period has been completed. 
Both cases were detected as part of an ongoing enhanced surveillance study of asymptomatic workers exposed to poultry infected with avian influenza. In these cases, detections may have resulted from either transient respiratory tract contamination (with no virus replication) or asymptomatic infection. Further testing (e.g., serology) is needed to confirm infection.
Whenever avian influenza viruses are circulating in birds, humans who are exposed to these birds or their environments are at risk of infection.

Sporadic human cases and transient contaminations of humans are rare, but not unexpected in such contexts. Thus far, there is no evidence of person-to-person transmission in this incident.

Although both reported cases were asymptomatic in this instance, previous A(H5N1) infections have resulted in severe infections in humans.

Based on the available information, WHO assesses that the risk for the general population posed by this virus is low, and for occupationally exposed persons it is low to moderate.

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