DVM 360 Amid rising human cases of the highly pathogenic avian influenza (HPAI H5N1) in Colorado,1 infection cases in domestic cats have also been diagnosed in the state during 2024. This year, 6 feline cases of HPAI H5N1 have been reported, with 2 of the affected animals being indoor-only cats who had no direct exposure to the virus, according to the Colorado Department of Public Health and Environment (CDPHE).2
Kay Russo, a veterinarian in Colorado, said in an interview with Colorado Public Radio News, that indoor cats may be interacting with mice that bring the virus into a house. She further postulated that perhaps owners were feeding their cats a raw diet—a risk factor for virus infection—or that the cats were actually spending time outdoors, according to the report.3
Meanwhile, 1 of the 6 cases in Colorado was directly linked to a known infected commercial dairy facility and the other 3 cases involved indoor/outdoor cats that hunted mice and/or small birds, while also spending time indoors with their owners.2 Additional risk factors for H5N1 in cats include feeding them raw or discarded milk, having associations with dairy operations, and hunting or consuming rodents or birds.4
Five of the 6 cases exhibited similar clinical signs and disease progression. These 5 felines initially presented with lethargy and loss of appetite, followed by worsening respiratory symptoms in some cases and consistent progressive neurological signs in most, according to state health officials. Several of the cats were also tested for rabies before being diagnosed with an H5N1 infection because the neurological signs the cats were exhibiting were akin to rabies, making both conditions indistinguishable.2
Five of the reported feline cases were located in Northern Colorado, specifically in Larimer, Adams, and Morgan counties. All involved the H5N1 strain, increasing the level of concern, according to Colorado Public Radio News, which cited May Chu, an epidemiologist and clinical professor at the Colorado School of Public Health in Aurora. Chu recommended additional testing be conducted, specifically full genome sequencing of these strains to investigate the cases’ chain of transmission, according to the news report.3