San Mateo County cat diagnosed with bird flu as CDC deletes evidence on possible feline-human transmission

San Mateo County public health officials announced Thursday that they’d discovered a pet cat in Half Moon Bay infected with H5N1 bird flu. That comes just two days after they reported finding the virus in a backyard poultry flock in Redwood City.

The reports came on the heels of a momentary Centers for Disease Control and Prevention release of a table showing that an adolescent had potentially been infected with the virus by a pet cat. According to the New York Times, the information — which was released in the agency’s first Morbidity and Mortality Weekly Report since President Trump came to office, and which largely focused on California’s wildfires — was available “briefly” before disappearing.

The CDC did not respond to questions about the data.

Seema Lakdawala, a microbiologist at Emory University in Atlanta, said it would be “premature to interpret the data in the table without reading the full report” — a sentiment echoed by several other scientists contacted by the Los Angeles Times to review the chart.

According to screenshots of a data table made available by the Washington Post, there appear to have been two households impacted by the virus, though there were no specifics to identify where these infections occurred.

In Household 1, there was initially one cat who became ill. Four days later, that cat died and tested positive for H5N1. On that same day, another cat in the house also got sick. Two days after that — on day six of the infection in the household —an adolescent living in the house began to show symptoms. That child tested negative for the disease, as did a symptomless adult and a symptomless adolescent in the house.

In Household 2 — which appears to be connected with Household 1, though the specifics are vague — an adult dairy farm worker began showing signs of illness one week after the first cat in Household 1 started exhibiting symptoms. This person was not tested for the virus and was “lost to follow-up,” according to the CDC graphic. Two days later, a cat living in Household 2 began to show symptoms. The next day, the cat died and tested positive for H5N1. A second cat in Household 2 tested negative for the disease.

The CDC table — since scrubbed from the site — doesn’t provide any information related to how the initial cat in Household 1 was infected.

Lakdawala noted that while none of the people and a few of the cats did not test positive for the virus, “sometimes influenza nasal tests are negative but there may be virus replicating elsewhere.” Indeed, since the H5N1 outbreak began, researchers have noted that swabs taken from one place in a person or cat’s body, such as the nasal passage, can be negative, while taken from another part, such as the back of the throat, can test positive.

Asked if the case took place in California, where 36 of the 40 cases of H5N1 connected to dairy have occurred, a spokeswoman for the state’s Department of Public Health said her department “can only speak to California cases” and that the state’s current status on bird flu is up to date. She said department officials were “not aware of any confirmed human cases of bird flu in California associated with exposure to domestic cats.”

Regardless, the report from San Mateo is concerning — adding to the growing tally of household cats infected with H5N1.

H5N1 has been detected in more than a dozen California cats, including in Contra Costa, Fresno, Los Angeles, Santa Barbara, San Bernadino, San Mateo counties and Tulare as well as scores across at least 17 other states, including Colorado, Idaho, Iowa, Kansas, Louisiana, Michigan, Minnesota, Montana, Nebraska, New Mexico, Ohio, Oklahoma, Oregon, South Dakota, Texas, Utah and Wyoming.

According to a statement from San Mateo County health officials, the cat was a stray that had been taken in by a family in Half Moon Bay. A county press release states that when the cat began showing symptoms, family members took it to a veterinarian for examination and testing. Lab results confirmed bird flu and the cat was “euthanized due to its condition.”

The county health officials stated they did not know how the cat was infected and did not describe the cat’s symptoms. There’s also currently no confirmation about when family took in the cat, nor what condition it was in at the time of adoption.

Typically, symptoms of bird flu in cats include a loss of appetite, lethargy and fever, along with neurologic signs, such as repetitively moving around in circles or “circling,” tremors, seizures or blindness. Other symptoms include severe depression; discharge from eyes or nose; rapid, shallow breathing, difficulty breathing; and sneezing or coughing. Some cats will die.

The San Mateo County statement also referred readers to a CDC web page stating that “while it’s unlikely that you would get sick with bird flu through direct contact with your infected pet, it is possible.”

The report of the infected cat in San Mateo came two days after the county reported a small outbreak in a backyard poultry flock in Redwood City — though there were no human cases associated with that event.

In the statement, the county advised residents who keep poultry flocks to watch of signs of bird flu and to take appropriate measures — such as washing hands before and after handling birds, and disinfecting footwear to reduce the risk of spreading bird flu.