Yesterday the CDC released an updated Influenza Risk Assessment Tool (IRAT) list of 16 novel flu subtypes/strains that currently circulate in non-human hosts, but that pose a potential threat to human health. This latest update - the first since October 2017 - adds 2 new viruses to the watch list; both are North American H7N9 viruses.
Despite sharing the same HA/NA appellation as their more famous (and far more dangerous) Chinese counterpart - these North American H7N9 viruses are considered to currently pose a very low threat to human health.Demonstrating how quickly the novel fluscape changes, of the 16 viruses currently being tracked, 13 have been either emerged or have been added to the list since 2011.
The IRAT list currently contains 14 avian flu viruses, 1 Canine flu (H3N2) and 1 swine-variant flu virus (H3N2v). Despite its length, this doesn't cover all of the potential pandemic flu viruses out there, only the ones of greatest concern.The two new additions emerged in a multi-state outbreak in 2017 (see map below), which saw both LPAI and HPAI versions of a new North American H7N9 virus emerge.
The two new additions to the IRAT are described below:
H7N9: Low Pathogenic North American avian [A/chicken/Tennessee/17-007431-3/2017]
Surveillance conducted in March 2017 during the investigation of a highly pathogenic avian influenza (HPAI) A(H7N9) virus in commercial poultry in Tennessee revealed the contemporaneous presence of North American lineage low pathogenic avian influenza (LPAI) A(H7N9) virus in commercial and backyard poultry flocks in Tennessee and three other states. The outbreak in poultry appeared limited with no further detections in subsequent surveillance. There were no reports of human cases associated with this virus.
Summary: A risk assessment this North American lineage LPAI A(H7N9) virus was conducted in October 2017. The overall IRAT risk assessment score for this virus falls into the low risk category (< 4). The summary average risk score for the virus to achieve sustained human-to-human transmission was in the low risk category (score 3.1). The average risk score for the virus to significantly impact public health if it were to achieve sustained human-to-human transmission was between the low to low-moderate range (score 3.5). For a full report, click here[228 KB, 4 pages].
H7N9: High Pathogenic North American avian [A/chicken/Tennessee/17-007147-2/2017]
In March 2017, the U.S. Department of Agriculture (USDA) reported the detection of a highly pathogenic avian influenza (HPAI) A(H7N9) virus in 2 commercial poultry flocks in Tennessee. Full genome sequence analysis indicated that all eight gene segments of the virus were of North American wild bird lineage and genetically distinct from the lineage of influenza A(H7N9) viruses infecting poultry and humans in China since 2013. The outbreak investigation revealed that a related North American low pathogenic avian influenza A(H7N9) was circulating in poultry prior to the detection of the HPAI A(H7N9). There were no reports of human cases associated with this virus.
Summary: A risk assessment this North American lineage HPAI A(H7N9) virus was conducted in October 2017. The overall IRAT risk assessment score for this virus falls into the low risk category (< 4). The summary average risk score for the virus to achieve sustained human-to-human transmission is in the low risk category (2.8). The summary average risk score for the virus to significantly impact public health if it were to achieve sustained human-to-human transmission was also in the low risk category (3.5). For a full report, click here[225 KB, 4 pages].
The full list of IRAT viruses (listed from highest risk to lowest) appears below:
The CDC uses two sets of criteria to evaluate novel viruses. One to estimate a virus's potential for sustained human-to-human transmission, and another to gauge it's potential for significant impact on public health.
While both of these new viruses fall very near the bottom of the list in terms of expected impact should they acquire human-to-human transmissibility, last October in EID Journal: Mammalian Pathogenesis & Transmission of Avian H7N9 Viruses - Tennessee 2017 we saw the results of lab experiments with these viruses.
While only mildly pathogenic in ferrets, with limited transmission observed in only 1 of 3 LPAI (and none of the HPAI) exposed animals - both viruses did replicate to a high titer in human bronchial epithelial cell lines.Making them both viruses worth keeping track of.
I'm a little surprised that the avian H7N2 virus which infected 400+ cats and a veterinarian in NYC in late 2016 hasn't made the list yet, but its day may yet come (see Influenza & Other Resp. Viruses.: Airborne & Fomite Detection of Avian H7N2 - NYC 2016).For more background on the CDC's IRAT, you may wish to revisit CDC EID Journal: All About IRAT).