Rat coronavirus/sialoadenitis virus

The world today is undergoing a transformation of unprecedented magnitude in a century. The global pandemic of COVID-19 has accelerated these sweeping changes, presenting humanity with challenges and tests unlike anything experienced before—both in our daily lives and in the workplace. Following the outbreak of Severe Acute Respiratory Syndrome Coronavirus (COVID-19), there has been a significant surge in research on zoonotic diseases, both domestically and internationally. Among these, Rat Coronavirus/Sialodacryoadenitis Virus (RCV/SDAV), given its potential as a human pathogen, raises an important question: How much do we truly understand about this virus?

Rat coronavirus (RCV) and Rat Salivary Gland-Derived Virus (SDAV) both belong to the family Coronaviridae and the genus Coronavirus. Coronaviruses are classified into four genera: α, β, γ, and δ. Both rat coronaviruses and human coronaviruses belong to the β-coronavirus group. Although RCV and SDAV share identical morphological and physicochemical characteristics, as well as consistent antigenicity—making them difficult to distinguish virologically and serologically—they exhibit markedly different clinical manifestations when infecting laboratory animals.

Rat Coronavirus Disease (RCV, also known as rat lethal pneumonia) is a highly contagious virus that causes respiratory and lung inflammation in rats. It is widely prevalent among rat populations, with newborn rats being the most susceptible. The virus has a short incubation period, spreads rapidly, and is associated with a high mortality rate. Rat Salivary and Lacrimal Gland Virus (SDAV), on the other hand, is highly infectious and widely distributed. Both RCV and SDAV are listed as pathogens that SPF-grade rats—essential for use in China's national standards for laboratory animals—must be free from. Rats are the primary hosts for infections caused by RCV/SDAV.

Incidence and Mortality Rates

In some laboratory animal housing environments, RCV/SDAV infection is highly prevalent; however, most infected rats do not exhibit obvious clinical symptoms, with only a small number showing mild respiratory disease signs. Some studies indicate that the mortality rate of RCV/SDAV infection can be as low as around 1%, yet in certain cases, it may rise to over 50%. This wide variation is likely due to differences in viral strains, rat breeds, age, sex, and immune status, among other factors.

Mice recovered from RCV infection exhibited poor growth and development. Post-mortem examinations revealed pulmonary congestion, non-purulent interstitial pneumonia, and atelectasis, while adult mice showed latent lung infections with no observable symptoms. SDAV primarily targets the salivary and lacrimal glands, causing acute inflammation especially in the submandibular and Harderian glands. This results in swelling of the salivary glands, leading to a visibly thickened neck that persists for 3 to 7 days, accompanied by reduced appetite and mild weight loss. In female mice, the estrous cycle becomes irregular, and some young pups develop conjunctivitis. Further examination reveals gelatinous edema around the salivary glands and significant swelling of the Harderian glands.

RCV/SDAV spreads via aerosols, direct contact, and contaminated surfaces—but there is no evidence of intrauterine transmission. When animal infections occur and are confirmed through laboratory testing, infected animals can be promptly removed, and the animal facility should remain empty for 5 to 8 weeks afterward. Importantly, RCV/SDAV is highly contagious, making handlers of isolated animals a significant potential source of transmission, as viral particles can even penetrate protective clothing. Meanwhile, asymptomatic or mild cases are particularly challenging to treat and may inadvertently provide an ideal environment for the virus to mutate. Therefore, it’s crucial to maintain impeccable hygiene standards within the animal housing facilities.

Domestic methods for diagnosing viral infections primarily rely on serological tests targeting antibody detection. However, since immunodeficient mice cannot produce an antibody response, antibody-based testing cannot be used with these animals—and instead, PCR methods must be employed.

In recent years, most of the newly emerging infectious diseases in humans have been caused by viruses. Rodents, particularly those belonging to the family Muridae, serve as the primary natural hosts for many pathogenic viruses that infect both humans and other animals. These rodents are widely distributed, boast a vast diversity of species, exhibit remarkable reproductive capabilities, and demonstrate exceptional adaptability. Therefore, gaining a deeper understanding of the viral species carried by rodents is crucial for effectively preventing and controlling future outbreaks of emerging infectious diseases.

References

1. SDAV, the Rat Coronavirus—How Much Do We Know About It in Light of Potential Zoonoses? Bartak M, Słońska A, Bańbura MW, Cymerys J. Viruses. 2021 Oct 4;13(10):1995. doi: 10.3390/v13101995

2. A Novel Potentially Recombinant Rodent Coronavirus with a Polybasic Cleavage Site in the Spike Protein. Li X, Wang L, Liu P, Li H, Huo S, Zong K, Zhu S, Guo Y, Zhang L, Hu B, Lan Y, Chmura A, Wu G, Daszak P, Liu WJ, Gao GF. J Virol. 2021 Oct 27;95(22):e01173-21. doi: 10.1128/JVI.01173-21.

3. *Veterinary Encyclopedia*, China Agricultural Press, Kong Fanyao; Cai Baoxiang

4. Establishment and Application of a Serological Detection Method for Rat Coronavirus/Sialodacryoadenitis Virus