H3N2 Canine Influenza Infects Eight Shelter Cats in Indiana

Eight shelter cats in northwest Indiana have tested positive for H3N2 canine influenza — the first such confirmed outbreak in the United States, said experts at the University of Wisconsin (UW) School of Veterinary Medicine.

“We have had previous reports from [South] Korea that H3N2 is capable of infecting cats, so this news is not unexpected,” said Dr. Sandra Newbury, clinical assistant professor and director of the UW Shelter Medicine Program, in an interview. “It has also been suggested that the virus can experimentally infect other species. Concern about cross-species transmission is common with influenza viruses, but we don’t believe these cat infections portend an increased risk of additional species being affected with H3N2 canine influenza.”

The H3N2 outbreak started among dogs at the shelter, Dr. Newbury said. Cats there had no direct contact with the dogs, and staff cleaned cat rooms before cleaning kennels. Nonetheless, several cats developed rhinorrhea, nasal congestion, malaise, lip smacking, and hyper-salivation. In response, the shelter quickly isolated the cats and submitted nasal swab specimens to UW, which detected H3N2 with a polymerase chain reaction (PCR) assay targeting the matrix gene of the virus, confirmed Dr. Kathy Toohey-Kurth, virology section head at UW Veterinary Diagnostic Laboratory.

Sequential nasal swabs from individual cats also showed rising viral loads over time, suggesting that H3N2 can replicate in cats. All the affected cats are recovering with supportive care, and the shelter remains closed until all infections are cleared, said Dr. Newbury. Studies of other influenza viruses indicate a typical incubation period of 2-3 days, followed by 5-7 days of clinical disease, and up to 2 total weeks of viral shedding.

There are two strains of canine influenza, H3N2 and H3N8. The H3N8 strain was first identified in 2004, when greyhounds at a Florida racetrack developed cough, nasal discharge, fever, and in some cases, fatal pneumonia. Genetic sequencing indicated that H3N8 originated in horses and that its entire genome had transferred to dogs — a rare event in adaptive viral evolution. H3N8 can infect cats experimentally, but is not known to do so naturally.

In contrast, the H3N2 strain appears to have originated in birds in Asia and spread to dogs through direct contact, potentially at live poultry markets, according to scientists. This strain was first confirmed at kennels in South Korea, and has since circulated widely among dogs there, in China and in Thailand. Furthermore, tests conducted in South Korea indicated that cats had acquired H3N2 outside the laboratory.

In March 2015, scientists at UW and Cornell University reported that a large canine influenza outbreak in the Chicago area was caused by H3N2, not H3N8, as had previously been assumed. Genetic sequencing showed a nearly identical match to the H3N2 strain circulating in Asia. In response, UW partnered with shelters in the area to test dogs and cats with influenza symptoms.

The current H3N2 outbreak is the first to be identified in cats after a year of active surveillance, Dr. Newbury said. “We are not currently seeing positives or unusual illness in cats in other shelters,” she added. “We hope infections in cats continue to be very rare.”

There are currently no vaccines for H3N2 canine influenza. Vaccines against H3N8 canine influenza are available from Zoetis and Merck Animal Health, but their ability to cross-protect against H3N2 infection is unknown, and they are not approved for use in cats. For now, supportive care and standard infection control measures are best. “Influenza is not a difficult virus to kill in the environment,” said Dr. Newbury. “In general, special decontamination procedures are not required.”

The UW School of Veterinary Medicine cautions that influenza viruses usually persist in the environment for about 24 to 48 hours, and are very effectively inactivated with soap and water. UW also recommends these steps to prevent the spread of canine influenza:

• Vaccinate dogs when possible, keeping in mind that H3N8 may still be circulating, and that it is unknown whether the H3N8 vaccine protects against H3N2 infection
• Limit oronasal and other increased-risk contact between dogs
• Wash hands and change clothes immediately after exposure to sick animals and before handling pets at home
• House dogs and cats with respiratory illness separately from other animals.

Dr. Amy Karon earned her doctorate in veterinary medicine and master’s degrees in public health and journalism from the University of Wisconsin-Madison. She was an infectious disease epidemiologist and “disease detective” (EIS officer) with the Centers for Disease Control and Prevention before becoming a full-time medical writer. She lives in the San Francisco Bay area, where she volunteers for the local Humane Society.