Influenza C Virus Infection in Rats

Four-week-old rats (WKA/Hkm strain) were infected intranasally with the Ann Arbor/1/50 strain of influenza C virus and examined for clinical symptoms, virus replication, and serum antibody response. Although the animals showed no definite signs of illness, the virus replicated in the nose, and the hemagglutination-inhibiting (HI) and neutralizing antibodies were produced in their sera. When the inoculum sizes of 10(6.2) and 10(3.2) PFU were used, virus was recovered from nasal homogenates between days 1 and 10, and serum HI antibody became detectable by 10 days after infection. The rats infected with 10(1.2) PFU of the virus continued to shed virus until as late as day 20 without producing serum HI antibody. The amount of virus recovered from the nose was not affected significantly by either sex, age, or strain of the rat except that a slower virus growth was seen in the LE strain. It was also observed that the rats, previously inoculated with 10(3.2) PFU of the virus, showed no virus shedding when reinfected 7 weeks later but produced virus though in low titers when reinfected 50 to 55 weeks later. Virus was also recovered from rats once inoculated with 10(1.2) PFU of the virus when challenged 7 weeks later. Thus repeated infections characteristic of human influenza C can be produced in rats under the restricted conditions.