Researchers tested cats and dogs living in households with COVID-19 positive owners in Texas, US. They found that the pets were also infected in more than 25% of households with a positive case.
The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative pathogen of the coronavirus disease 2019 (COVID-19) pandemic, is believed to have originated in animals. Based on studies of the closest relatives of the virus, its reservoir is believed to be bats, though it may have been transmitted to humans via another yet unknown intermediary animal host.
Several other mammals such as monkeys, cats, and rodents are susceptible to natural or experimental infection. There have been several documented cases around the globe of human-to-animal transmission in companion animals such as cats and dogs. Cats have been shown to be highly susceptible to SARS-CoV-2 infection and can transmit the infection to other cats. Dogs have lower susceptibility, but neutralizing antibodies have been found in cats as well as dogs.
Although SARS-CoV-2 infections in pets have implications for public health, there are few studies of animals living in households. Studies focusing on animals exposed to people with COVID-19 may help understand the probability of infection in animals and if the animals can serve as potential reservoirs for the virus.
Testing pets in COVID-19 positive households
By active surveillance of cats and dogs in homes with positive COVID-19 cases in Texas, US, researchers quantified the prevalence of infections in pets. They reported their results in a paper published in the preprint server bioRxiv*.
The researchers sampled 76 pet dogs and cats from 39 households in Brazos County, Texas. The pets of various breeds ranged in age from three months to ten years for cats and 1.5 months to 18 years for dogs. They collected samples from the pets between three and 27 days after human household members tested positive.
Two cats and one dog out of 17 and 59 cats and dogs, respectively, tested positive for the virus. Gene sequencing confirmed that all four animals had SARS-CoV-2 with no observed mutations in the spike protein compared to the reference sequence Wuhan-Hu-1. Blood samples from 75 animals showed seven cats out of 17, and seven dogs out of 59 had SARS-CoV-2 neutralizing antibodies.
All the animals testing positive or with antibodies were followed up to three times in two months. In two of the four animals testing positive, follow-up indicated a negative test by RT-PCR. However, one cat tested positive even after three weeks, and another cat remained positive nearly four weeks after the first sampling date.
In houses with multiple pets, even if only one pet tested positive initially, with time, all the animals showed neutralizing antibodies. The owners of the animals that tested positive reported the animals were asymptomatic before testing and all the animals were in good health during the follow-ups.
One of the study's limitations is the number of humans in a household that was infected, and their symptoms and duration while interacting with their pets was unknown. Furthermore, pets that were negative the first time were never retested, so later infections could have been missed.
Animals are asymptomatic
The positive test results and the seroprevalence tests indicate that human to animal transmission of SARS-CoV-2 may occur more for cats than dogs. Since all the animals that tested positive were sampled within seven days of their owners testing positive, this might be a factor contributing to the proportion of pets testing positive. Studies have reported that cats can shed the virus for up to seven days.
Four samples from pet furs tested positive for the virus. Two samples were from the same cat, whose respiratory and rectal swabs were also positive. Three weeks later, only the body swab was positive, suggesting self-licking and grooming behavior may have led to contamination. The other two were from dogs, which did not meet the case definition for being positive. The positive fur samples may indicate living in a contaminated environment.
More than 25% of the households had pets with neutralizing antibodies, but the antibody titers fluctuated with time. More studies are needed to understand the dynamics of antibody variations, not only in pets but also in their owners.
All the animals that tested positive had no symptoms, one cat and dog had sneezing and another cat was lethargic. Previous reports have indicated less than half the animals infected with the virus show any clinical signs.
The authors wrote: "Our results, which reveal a much higher proportion of asymptomatic animals, suggest that infected companion animals showing no clinical signs or only mild, transient illness may be more numerous than global reporting currently captures."
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.