Early study results show cats, ferrets susceptible to COVID-19

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The origin of the novel coronavirus (SARS-CoV-2), the causative agent of COVID-19 disease, is not yet completely understood. It is believed to have originated in bats. Given its zoonotic nature, questions have arisen about whether animals in close contact with humans can become infected with the virus.

Over the past month, there have been several reports of positive COVID-19 test results in animals that were in contact with COVID-19–positive human patients. The first and second reports of infected dogs came from Hong Kong in early and mid-March, respectively. Neither dog showed clinical signs associated with COVID-19. Questions about the implications of these positive test results remain unanswered.

Clinical signs that could be attributed to the virus, including vomiting, diarrhea and respiratory difficulty, were reported in a Belgian cat that tested positive in late March; another cat in Hong Kong reportedly tested positive for the virus at the end of March but did not show clinical signs. Most recently, a tiger at the Bronx Zoo that was sick with respiratory signs tested positive after exposure to an infected zookeeper. Six other tigers and lions in the zoo were reported to be showing respiratory signs as well but were not tested.

All infected animals have recovered or are recovering, except for the initial positive dog in Hong Kong. That dog, a Pomeranian, was 17 years old and had significant underlying health problems; no necropsy was performed to determine the precise cause of death.

Idexx Laboratories reported on March 13 that it had tested thousands of canine and feline samples for the virus using a proprietary SARS-CoV-2 (COVID-19) RealPCR test and 100% of the results were negative. WSAVA, the AVMA and the CDC have emphasized repeatedly that there is currently no evidence that pets can spread SARS-CoV-2 to humans.

A recent study performed at the Harbin Veterinary Research Institute (HVRI) of the Chinese Academy of Agricultural Sciences investigated the susceptibility of several domestic animal species to SARS-CoV-2 infection as well as transmission between members of these species. A pre-print of the study, which has not yet been peer reviewed, was released on March 30.

Despite the lack of peer evaluation, the study does offer preliminary data that may help guide the direction of future research as veterinarians, physicians and scientists around the world work to understand the virus, develop treatments and vaccination, and gain control of the pandemic.

Study design

The investigators examined the behavior of the virus in ferrets, cats, dogs, pigs, ducks and chickens. Two strains of SARS-CoV-2 were used in the ferret experiments. The first was isolated from the Huanan Seafood Wholesale Market in Wuhan, and the other was isolated from a human patient in China. For all other species, the viral strain isolated from the human patient was used.

In a series of experiments, the authors investigated:

• Viral replication after intranasal inoculation in ferrets (n = 10), cats (n = 6) and dogs (n = 5), and after intratracheal inoculation in ferrets (n = 8)
• Transmission between cats. Subadult (8 months old, n = 3) and juvenile (70-100 days old, n = 3) cats were inoculated intranasally and housed individually. A naïve cat of similar age was housed next to each inoculated cat. There were no control pairs.
• Susceptibility in pigs, chickens and ducks (n = 5 for all species)
• Transmission in dogs, pigs, chickens and ducks. Five animals of each species were inoculated intranasally and group housed with naïve counterparts of the same species (dogs n = 2, other species n = 3). There were no control groups.

Nasal or oropharyngeal swabs, and fecal or rectal swabs were collected for analysis periodically during the experiments. These samples were evaluated for the presence of viral RNA using quantitative PCR as well as for infectious virus using virus titration in Vero E6 cells. Tissue samples were collected after euthanasia for evaluation of viral RNA, infectious virus and histopathology. Serum samples were evaluated by enzyme-linked immunosorbent assay for antibodies to SARS-CoV-2.

What they found

Ferrets

Ten ferrets were inoculated intranasally, and a subset (n = 4) was euthanized four days post inoculation for tissue sample collection. Both viral RNA and infectious virus were detected in tissues from the upper respiratory tract (nasal turbinate, soft palate and tonsils) of all ferrets. Viral RNA and infectious virus were not detected in any other tissues.

Nasal washes and rectal swabs were evaluated from the remaining ferrets (n = 6) every other day starting on day 2 and ending on day 10. Nasal washes from all ferrets showed the presence of both viral RNA and infectious virus 2 to 8 days after intranasal inoculation. Viral RNA, but not infectious virus, was present in the rectal swabs from some of these ferrets.

Two ferrets became symptomatic (fever and inappetence) 10 to 12 days post inoculation and were euthanized. Pathology was found in the lungs of both ferrets, but viral RNA was not found in the pulmonary tissue of either animal. Antibodies to SARS-CoV-2 were detected in all six ferrets at 13 days (n = 2) and 20 days (n = 4) after intranasal inoculation.

When eight ferrets were inoculated intratracheally with the virus, viral RNA was found only in tissues of the upper airways on days 2, 4 and 8 post inoculation. No viral RNA was detected in the tissue of ferrets euthanized on day 14.

Cats

Subadult cats (8 months old, n = 5) were inoculated intranasally. Two cats were euthanized on day 6. Viral RNA and infectious virus were found in the nasal turbinates, soft palate and tonsils of both cats. The trachea of one cat contained both viral RNA and infectious virus, while the small intestine of the other cat contained viral RNA only.

The remaining three cats were each housed individually and placed next to uninfected cats to study the potential for cat-to-cat transmission. Fecal samples were collected from all cats during the study because nasal washes were not possible. Viral RNA was detected in the feces of all infected cats by day 5 post inoculation and in one exposed cat on day 3 post inoculation.

After euthanasia, viral RNA was detected on the soft palate and tonsils of all three inoculated cats and the soft palate, tonsils, nasal turbinates and trachea of the exposed cat. Antibodies were detected in the serum of the three inoculated cats and the exposed cat. Neither viral RNA nor antibodies were detected in the remaining two exposed cats.

Dogs and livestock

Beagles (3 months old, n = 5), specific-pathogen-free (SPF) Landrace and Large White pigs (n = 5, 40 days old), SPF White Leghorn chickens (4 weeks old, n = 5), and SPF Shaoxin ducks (4 weeks old, n = 5) were inoculated intranasally. Each inoculated group of animals was housed with naïve counterparts (dogs n = 2, livestock n = 3) to evaluate for transmission.

Viral RNA was detected on rectal swabs of three of the inoculated dogs, but no infectious virus was found. Neither viral RNA nor infectious virus was detected in samples from the naïve dogs. Antibodies were present in two of the inoculated dogs and in neither of the naïve dogs.

In the pigs, chickens and ducks, no viral RNA, infectious virus or antibodies were detected in any of the experimental subjects.

Study conclusions and limitations

Based on the study results, the authors formed the following conclusions:

• Ferrets and cats appear highly susceptible to infection with the novel coronavirus.
• SARS-CoV-2 can replicate in the upper respiratory tract of ferrets for up to 8 days.
• Respiratory droplet transmission of the virus between cats is possible.
• Dogs appear to have low susceptibility to the virus, and intraspecies transmission is not likely.
• Pigs, ducks and chickens do not appear to be susceptible to the virus.
• There is minimal discussion of the data presented for juvenile cats, making it difficult to draw conclusions, although the authors comment that they believe the virus replicates better in younger cats.

Caution should be taken when extrapolating the conclusions of this study to wider populations of domestic animals. The data represent small sample sizes for each species with infections occurring under experimental conditions. Inoculations were performed with high viral loads. Minimal information is provided on clinical signs in infected animals, which makes it impossible to draw conclusions on the progression or severity of disease in these animals. Finally, no control groups were used in the transmission studies. As noted previously, the manuscript is currently awaiting peer review.

Implications and future directions

Many questions remain as to the virus’ behavior in animals. What is the degree of clinical illness? Can transmission occur between members of the same species? Can infected animals transmit the virus to humans?

At this time, the CDC recommends that infected people limit contact with pets until more is understood about the virus. The results of this study, combined with recent reports of the virus in cats and a tiger, suggest that precautions may be especially important for infected cat owners. It is important to note that even though it appears that cats can become infected with the virus, there is no evidence that the virus can be transmitted from a domestic cat to a human.

Ultimately, more research is needed to understand the effects of SARS-CoV-2 on companion animals. The results of this study provide preliminary information regarding susceptibility that may direct further research to focus on the species that appear to be most susceptible to the virus: ferrets and cats. Additionally, the authors suggest that ferrets may serve as an appropriate experimental model for testing of antiviral drugs and vaccinations against the novel coronavirus.

Dr. Boatright, a 2013 graduate of the University of Pennsylvania, is an associate veterinarian, freelance speaker and author in western Pennsylvania. She is actively involved in the AVMA House of Delegates as well as her local and state veterinary medical associations. She is a former national officer of the Veterinary Business Management Association.