I'm not sure how this event has flown under the radar for so long, but overnight a report posted on the University of Nebraska's Veterinary Diagnostic Center (NVDC) - dated February 24, 2023 - was widely reported across the internet detailing the infection of 2 domestic cats with HPAI H5N1 in their state.
Despite their January detection, they have yet to appear on the USDA's list of mammalian infections with H5N1 (last Updated March 30th), which only cites 3 findings from Nebraska:
The the following brief notice appears to have been posted on the NVDC website 6 weeks ago.
All this comes on the heels of recent notifications of a domestic dog in Canada, and a domestic cat in Wyoming, having contracted the virus via contact with wild birds. Similarly, a domestic cat in France was reported to have been infected in December.
Excerpts (reformatted for readability) from the full (2-page) report written by by Dr. Sarah Sillman, DVM, PhD, dip. ACVP follow:
In January, an outdoor, adult, domestic longhaired cat presented to the University of Nebraska Veterinary Diagnostic Center for postmortem examination after a history of rapid decline with clinical signs of anorexia, recumbency, anisocoria, pyrexia, seizures, tremors, nystagmus, loss of proprioception, and hyperesthesia. Gross necropsy revealed only a few visible changes to the organs including pulmonary congestion and edema, mild pericardial transudative effusion, and a subtle darkening of areas of the cerebrocortical grey matter.
Complete histopathology examination revealed necrotizing lesions in the kidney, liver, adrenal gland, and pancreas; encephalitis with patches of extensive neuronal degeneration and necrosis, particularly in the cerebral cortex; and edema, vessel congestion, and mild inflammation in the lung and epicardium. The lesions were recognized as suspicious for highly pathogenic avian influenza virus infection, which was confirmed with molecular diagnostics. The PCR Ct value for avian influenza in the brain of this cat was remarkably low (12), indicating a very large amount of virus in the brain, as consistent with an acute infection. Highly pathogenic avian influenza Eurasian strain H5N1 was verified by molecular assay at National Veterinary Services Laboratories (NVSL).
Three other outdoor domestic cats of this household were noted at risk, and one of them developed clinical signs shortly after the first affected cat. This cat was described as somnolent and had episodes of walking in circles (circling). The cat was responsive to stimuli and seemed to eat and drink normally. It lived 10 days with neurologic impairment, when the cat suddenly became laterally recumbent with continual tremors, necessitating euthanasia. Gross necropsy documented major lesions only in the brain.
There was excessive bloody cerebrospinal fluid from the subarachnoid space and obvious areas of malacia and hemorrhage in the cerebral cortex. On histopathology of affected brain, there was extensive loss of neurons with severe vacuolation and collapse of the supporting parenchyma, accompanied by reactive changes including astrogliosis and lymphocytic perivascular cuffs.
Avian influenza was detected in the brain, although the PCR Ct value was higher (30), a consequence of the infection being chronic and/or less initial virus burden. Eurasian strain H5N1 was verified at NVSL. Acute hemorrhage from the damaged cortex leading to increased intracranial pressure could have caused the neurological crisis in this second cat. The remaining two cats in the household have been reportedly normal and nasal swabs collected from them did not detect avian influenza virus.
While the risks to humans and their pets are still considered low, this is yet another reminder that the risks posed by avian flu are real (see the CDC's guidance in Bird Flu in Pets and Other Animals).
It is somewhat encouraging that 2 of the 4 household cats escaped infection, but clade 22.214.171.124b H5 viruses have evolved into dozens of distinct genotypes over the past couple of years, and some are likely more transmissible (and more virulent) than others.
UK Reports 2 Dolphins With HPAI H5N1 & EID Report On Infected Harbor Porpoise in Sweden
PLoS Pathogens: Evolution of Highly Pathogenic H5N1 Influenza A Virus in the Central Nervous System of Ferrets
Preprint: Pathology Of HPAI H5N1 clade 126.96.36.199b in Wild Terrestrial Mammals in the United States in 2022
In 2015 that we got our first detailed look (with MRI imaging and extensive histological analysis) of a fatal Neurotropic H5N1 Infection in a Nurse in Canada who recently returned from a visit to China (albeit due to clade 188.8.131.52).
The authors of that study warned:
These reports suggest the H5N1 virus is becoming more neurologically virulent and adapting to mammals. Despite the trend in virulence, the mode of influenza virus transmission remains elusive to date. It is unclear how our patient acquired the H5N1 influenza virus because she did not have any known contact with animals or poultry.