According to a Wednesday, October 30th release, USDA personnel isolated H5N1 virus from an asymptomatic hog on a backyard farm in Crook County, OR. Poultry on this mixed livestock operation were diagnosed with H5N1 on October 25th . The farm has been quarantined.
Five hogs on the farm housed in close proximity to chickens and with a common water source were sampled. Only one hog was regarded as psitive to H5N1, presumably from a nasal swab assayed with PCR while two of the hogs yielded negative samples. Results from the other two are pending completion of diagnostic procedures suggesting that the initial results were other than an absolute negative. The significance of demonstrating H5N1 from thee nasal swab of one hog could be questioned as an environmental contaminant. The situation would be different if H5N1 virus were to be demonstrated or cultured from pulmonary tissue or other organs obtained from the hogs that were reportedly euthanized.
USDA NVSL will conduct genomic sequencing of viruses isolated from both poultry and the hog to detect any changes that would suggest infectivity to humans.
The isolation of H5N1 from a hog is regarded as significant since this species has receptors for both avian and mammalian strains and can serve as a “mixing vessel” for strains of influenza viruses creating the prospect of a reassortment event. Viruses undergoing exchange of genes could be infectious to humans and even become contagious as in the 1917 pandemic.
If the whole genome sequencing of the virus does not disclose mutations associated with human infection, it may be assumed that this case is a spontaneous event restricted to the specific, noncommercial operation. The important issue will be to ascertain whether the virus is transmissible among hogs and to evaluate pathogenicity in ferrets and mice. It is presumed that Oregon health officials in cooperation with the CDC will sample residents on the farm that have had contact with both poultry and hogs to determine whether virus can be detected from the upper respiratory tract over the next week and whether these individuals subsequently demonstrate H5 influenza antibody irrespective of the absence of clinical signs.
Given the circumstances of the case thus far, it does not appear to increase risk to either humans or other livestock unless additional cases emerge or if laboratory evaluation of the isolate suggests possible infectivity for humans.
It is hoped that sequences of the chicken and hog isolates will be shared with the scientific community and that USDA and CDC will be transparent and forthcoming in reporting on this case.