Cats under the shadow of the SARS‐CoV‐2 pandemic

Dear Editor,

I have some comments on the recently published paper in Transboundary and Emerging Diseases titled ‘Serological survey of SARS‐CoV‐2 for experimental, domestic, companion and wild animals excludes intermediate hosts of 35 different species of animals’ by Deng JH and others (Deng et al., 2020).

SARS‐CoV‐2 has become pandemic and brought unprecedented challenges to public health (Ward, Li, & Tian, 2020). The natural reservoir of SARS‐CoV‐2 has been extensively explored by different research groups worldwide but the current reservoirs remain to be confirmed. In report of Deng et al, they excluded 35 different species of animals to be the intermediate hosts of SARS‐CoV‐2 including pet/stray cats (87 samples) and tigers (8 samples) (Deng et al., 2020). At the same time, another Chinese research group reported not only that cats could be infected by SARS‐CoV‐2 but also that adolescent cats showed severe histological lesions and died by artificial inoculation of the virus (Shi et al., 2020). Coincidently, the third Chinese research group performed a retrospective serological survey of cats during the pandemic in Wuhan, and reported more than 10% seropositivity of 102 cats using ELISA and virus‐neutralizing antibody tests without reporting any clinical signs in the positive cats (Zhang et al., 2020, a preprint at the moment). Therefore, we should be cautious in explaining the disparity in these results and discussing the role cats play in SARS‐CoV‐2 transmission.

First, the SARS‐CoV‐2 epidemic situations in different cities sampled may contribute to the disparity in the above results. Wuhan, the epicentre of the SARS‐CoV‐2 pandemic, had more than 50,000 confirmed cased with 5.13% mortality, significantly higher than other Chinese cities. Therefore, the cats in Wuhan may have been more frequently exposed to SARS‐CoV‐2 contaminated environments when stray cats fed in garbage or pet cats had close contact with their infected owners. In the report of Deng et al. (2020), they did not mention from which cities or provinces cats were sampled.

Second, different types of ELISA were used by the two research groups. A commercial SARS‐CoV‐2 double‐antigen sandwich ELISA based on virus S1 protein was applied to clinical samples by Deng et al. (2020) after validating with clear background SARS‐CoV‐2‐negative and SARS‐CoV‐2‐positive serum samples (Deng et al., 2020). Another research group developed an in‐house indirect ELISA base on virus receptor‐binding domain (BRD) protein and set arbitrarily the cut‐off as 0.32 of OD₄₅₀ based on 39 SARS‐CoV‐2‐negative sera (Zhang et al., 2020, a preprint at the moment). Later, neutralizing antibodies (>1/20) were detected in 9 out of 15 ELISA‐positive serum samples. The authors did not explain the disparity between ELISA and neutralizing antibody tests. Interestingly, the owners of 3 cats with higher neutralizing antibody titres (1/360, 1/360 and 1/1,080) were confirmed to be SARS‐CoV‐2 patients.

Third, some important information such as age and clinical signs of cats when sampling in these two field serosurveys were missing. In the experimental cat infection of SARS‐CoV‐2, juvenile cats (aged 70 days to 3 months) were more susceptible to the virus and died with severe histological lesions in the nasal and tracheal mucosa epithelium and lungs (Shi et al., 2020). However, no clinical signs of infected cats were described in their study. Coincidently, a cat naturally infected by her owner in Belgium presented with or harsh breathing, vomiting and diarrhoea; this cat was reportedly confirmed to be SARS‐CoV‐2‐positive (The Brussels Times).

Besides domestic cats, one large cat (tiger) was recently reported to be infected with SARS‐CoV‐2 and showed respiratory signs in a zoo in New York City (USDA). It was believed the tiger was infected by a zoo employee who was actively shedding virus. Although lacking some important information in these experimental studies, serosurveys and case reports, it seems we can conclude that cats are able to be infected with SARS‐CoV‐2 via transmission from humans, but the reverse transmission from cats to humans remains unknown.

CONFLICT OF INTEREST

The author declares no conflict of interest relevant to this article.

ETHICAL APPROVAL

Not applicable to this comment.

Supporting information

Table S1

DATA AVAILABILITY STATEMENT

There are no experimental data available in this letter.