In a recent study posted to the bioRxiv* preprint server, researchers at the Friedrich-Loeffler-Institute, Germany, assess the susceptibility of cats and dogs to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron variant.

Study: Dogs and cats are less susceptible to the omicron variant of concern of SARS-CoV-2 – a field study. Image Credit: Chendongshan / Shutterstock.com

Can SARS-CoV-2 infect animals?

SARS-CoV-2, the causal agent of the coronavirus disease 2019 (COVID-19), is a betacoronavirus belonging to the subgenus Sarbecovirus. In addition to humans, various animal species, including non-human primates, canines, felines, mustelids, several ruminant species, and rodents, can be infected with SARS-CoV-2. However, the susceptibility of these animals to infection may vary based on the viral variant.

Of the animal species theoretically vulnerable to SARS-CoV-2, those that often interact with people, such as dogs (Canis lupus) and cats (Felis catus), are of particular concern. It is currently unknown whether dogs and cats are more resistant to the SARS-CoV-2 Omicron variant.

About the study

In the present study, researchers use serological methods to evaluate dogs and cats residing in households with human SARS-CoV-2-infected patients.

The team recruited human households for the study. Plasma or serum samples were collected between three weeks and three months after the initial SARS-CoV-2 diagnosis in a pet owner.

A total of 290 dogs and 241 cats were enrolled in the study. To cover the period of Omicron BA.4 and BA.5 variant transmission, feline plasma or serum specimens were serologically analyzed in Germany from week 11 to week 23 of 2022 or weeks 31 and 32 of 2022.

During a clinical assessment, a veterinarian collected samples from household cats, which were then forwarded to a clinical diagnostic laboratory to conduct a non-SARS-CoV-2-related examination.

All sera were evaluated using a species-independent surrogate virus neutralization test (sVNT). The sVNT permitted the identification of antibodies elicited against the SARS-CoV-2 wild-type strain and several SARS-CoV-2 variants of concern (VOCs), such as Delta, but excluding Omicron.

The ability to identify and differentiate antibodies against the Omicron and Delta variants was demonstrated by testing serum from Delta-infected goats and BA.1-infected mice. Cat and dog samples acquired in November 2021 were exclusively analyzed using the sVNT's original composition. Samples acquired after December 2021 were analyzed by the sVNT simultaneously using the original receptor-binding domain (RBD) and Omicron-specific RBD.

Study findings

Almost 20 of the 290 dogs whose owners were infected with SARS-CoV-2 reported positive sVNT results. However, there were significant changes between the study period and the viral variants the dogs were exposed to.

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While antibodies elicited against SARS-CoV-2 were discovered in 18 of 36 dogs exposed to an infected human in November 2021, only two of the 254 animals tested positive between December 2021 and April 2022.

In each instance, the positive reaction was registered against the original RBD. When the Omicron RBD was tested, each dog sample tested negative. All positive sVNT test results were verified by the indirect immunofluorescence assay (iIFA), as all sVNT-positive samples also yielded a positive ilFA result.

Serological results of dogs kept in COVID-19 households. In the upper panel, the shares of positive (red) and negative (grey) results are given, and the animals are sorted into the month in which their owner tested SARS-CoV-2 positive. In the lower panel, the values measured in the surrogate virus neutralization test are shown individually for each canine sample. In November 2021, before the omicron variant of concern was detected for the first time in the human population of Germany, the canine sera were tested only against the original RBD. From December 2021 onwards, the samples were tested in parallel using the original and the Omicron RBD, and the results of individual samples are connected by a black line. A horizontal dashed line indicates the cut-off.

A similar tendency was observed with cat samples. Altogether, 26 of the 241 feline serum samples tested positive.

In November 2021, 16 positive-reacting serological samples were obtained from families with COVID-19 patients. Only 10 of the 199 cats that interacted with a SARS-CoV-2-positive owner from December 2021 to March 2022 tested positive for the virus. Five of the ten samples displayed a stronger reactivity to the original RBD, whereas the five reacted more strongly to the Omicron RBD.

The serological data of dogs and cats highlight the significantly lower number of seropositive animals during the Omicron variant prevalence period as compared to the Delta period. This observation indicates a substantial reduction in vulnerability of these animal species to the SARS-CoV-2 Omicron VOC.

During the initial sampling period between weeks 11 and 23 of 2022, 172 cat sera were further evaluated, among which four tested positive by sVNT.

During the second phase, which was conducted between weeks 31 and 32 of 2022, a total of 200 samples were obtained. Seven of these samples tested positive in sVNT.

All sera exhibited a higher reaction to the original RBD than the Omicron RBD, while all positive sVNT test results were again validated by iIFA. Thus, the random sampling confirmed the previous findings of a reduced ratio of seroconversions in response to the Omicron VOC.

Conclusions

The study findings demonstrate that cats were less susceptible to SARS-CoV-2 Omicron infection, with Omicron subvariants possibly exhibiting little or minimal changes. Nevertheless, cats and dogs should be included in future surveillance studies and epidemiological assessments, particularly when new viral variants appear, for which the sensitivity of companion animals is unknown.

*Important notice

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.

Written by

Bhavana Kunkalikar

Bhavana Kunkalikar is a medical writer based in Goa, India. Her academic background is in Pharmaceutical sciences and she holds a Bachelor's degree in Pharmacy. Her educational background allowed her to foster an interest in anatomical and physiological sciences. Her college project work based on ‘The manifestations and causes of sickle cell anemia’ formed the stepping stone to a life-long fascination with human pathophysiology.