As part of its regular pandemic risk assessment process, the CDC added the two viruses to its Influenza Risk Assessment Tool (IRAT). Scientists evaluate the viruses based on two factors one on future emergence, looking at factors such as transmission in animal models and genomic analysis, and the other on public health impact, taking into account topics such as population immunity and antiviral treatment options.
One is a 2024 B3.13 genotype virus from California that is similar to those currently circulating in US dairy cattle and causes sporadic human infections, mainly in people who are exposed to sick cows. The other is a 2024 D1.1 genotype virus from Washington that resembles one circulating in wild birds and poultry, with occasional jumps to humans who have poultry exposure. The CDC now has five clade 2.3.4.4b H5N1 viruses on its IRAT list.
Slight decreases from initial US cattle samples, assessments reflect some uncertainty
The CDC said the new assessments weave in new information, including information from human cases. The two newer viruses scored slightly lower on some risk elements, but slightly higher on others. “However, the mean-high and mean-low acceptable score ranges for these viruses overlap, indicating that these viruses remain similar, and their overall risk scores remain ‘moderate’,” the CDC said.
Potential emergence scores for the California and Washington viruses were 5.59 and 5.21, respectively, putting them at the mid-low range of the moderate risk category. For potential public health impact, the scores were 5.91 and 6.0, respectively, which is in the mid-range of moderate risk. Both scores reflect slight decreases compared with the CDC’s assessment of the initial B3.13 genotype virus from Texas isolated earlier in the dairy cow outbreaks.
Experts varied in their estimates for some of the risk elements, reflecting some uncertainty in interpreting the available data, according to the report.
Animal studies shed light on virulence, pig susceptibility
In researcher developments, scientists published new studies that shed more light on risk of currently circulating H5N1 viruses, one in people and the other in pigs.
In the first study, a team from the CDC examined a conjunctival sample isolated from a dairy worker infected with H5N1 in Michigan. They described their findings in a research letter in Emerging Infectious Diseases.
In experiments with ferrets inoculated with the virus, they found that the virus could spread by the airborne route in the animals, causing a moderate infection that was less virulent compared with earlier similar experiments with a virus isolated from a Texas dairy worker.
In the second study, to assess potential H5N1 susceptibility in pigs, scientists with the National Institutes for Allergy and Infectious Diseases (NIAID) experimentally infected eight pigs with a bovine B3.13 H5N1 isolate through different routes that mimicked natural exposure. They published their preprint findings in Nature Portfolio.
Pigs developed subclinical or mild disease and continued to gain weight during recovery. Virus replication mainly occurred in respiratory tissues, with shedding that occurred in the upper airway tract.
To test transmission, they put infected pigs in pens with uninfected animals, finding evidence of pig-to-pig transmission. The group said the susceptibility and occasional transmission they identified are worrisome from a public health perspective, given that pigs are susceptible to both mammalian and avian influenza virus, making them a mixing vessel for new reassortants.
Source : umn.edu