Coronaviruses

Coronavirus infection in mice or rats is the most important and the most common viral problem encountered in laboratory rodent facilities. Coronaviruses in mice, mouse hepatitis virus (MHV), and rats, sialodacryoadenitis virus (SDAV), and rat coronavirus (RCV), are antigenically close but are different and highly species-specific viruses. Clinical symptoms are not a major feature in relation to MHV infection in mice, but it depends on characteristics of the virus and the animal. Some strains are enterotropic, i.e., they infect through the gastrointestinal system, while others are pneumotropic, i.e., they infect through the respiratory system.124 The most common symptom is diarrhea in suckling mice.125 Several organs are affected by the infection, and the virus has a tropism for all tissues, which in connection with its immunomodulating effects makes it a significant modulator of research and an undesired organism in rodent facilities. The liver may be pale with multiple white, yellow, or hemorrhagic foci. The spleen may be enlarged, while the thymus may be reduced in size and there is widespread necrosis of lymphoid tissue. BALB/c and C57BL mice seem to be more sensitive than other mice.126 In immune-deficient mice, such as SCID127 and nude mice,128 infection causes high mortality. Strains such as MHV-2, MHV-3, and MHV-A59 are more virulent than, e.g., MHV-1, MHV-S, MHV-Y, and MHV-Nu. A strain designated MHV-4 has a specific affinity for the nervous tissues.129 Coronavirus infection in rats caused by one of several substrains of SDAV or RCV may often be asymptomatic, but clinical symptoms seem to be more common after corona viral infection in rats than in mice. A mild necrotizing rhinotracheitis develops into interstitial pneumonia with severe necrosis and swelling of salivary and lacrimal glands. In this phase, the ventral neck region of the rats is swollen, and red-brown porphyrin rings are observed around their eyes. Diseased rats fully recover within 5 weeks.130 Corona viruses easily spread in rodent facilities, and the prevalence in a rat colony reaches 100% within 4 weeks.131 Although unstable, coronaviruses may be transported passively between facilities by staff or equipment. Coronaviruses are nonpersisting in immune-competent animals but persisting in immune-deficient and certain transgenic animals. Current screening can easily be performed by serology.132 Diagnosis of acute disease may be attempted by PCR133 or tests for anticoronaviral IgA131 on fecal samples.

Coronaviruses different from murine coronaviruses may infect guinea pigs or rabbits, and until the 1960s, pleural effusion as the result of infection with rabbit coronavirus was a common disease in laboratory rabbits.134 Enteric infection with rabbit coronavirus may still be common.48

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