Contamination of a specific-pathogen-free rat breeding colony with Human parainfluenza virus type 3

Routine antibody surveillance for Sendai virus in a breeding colony suggested viral invasion into laboratory rats. A more specific haemagglutination-inhibition test implied that the agent was related closely to Human parainfluenza virus type 3 (hPIV3), rather than Sendai virus. To isolate this virus, Vero cells were inoculated with lung homogenates of 30 young animals from the colony. One of the cultures became positive at the second passage by RT-PCR directed to the hPIV3 NP and L genes. Cytopathic effect with cell fusion was observed at the third passage. The HN gene of this virus (KK24) had > 93 % similarity to those of other hPIV3 isolates, suggesting a human origin of KK24. Experimental intranasal inoculation of KK24 into SD rats showed virus replication in the lungs at 3-5 days post-infection (p.i.). Pathological examination of the lungs at day 5 p.i. indicated a moderate detachment, degradation and apoptosis of bronchial epitheliocytes with peribronchial mononuclear infiltrations. At day 7 p.i., these changes became less prominent, and no lesions were apparent at day 10 p.i. or later. The infected rats seroconverted at day 7 p.i. On the contrary, none of the 30 experimentally infected ICR mice showed any pathological lesions in their lungs, despite seroconversion at 7 days p.i. These results suggest that hPIV3 can invade rat colonies and has a moderate and transient pathogenicity in rats. This is the first report of non-experimental hPIV3 infection in laboratory rats, unexpectedly detected by antibody screening for Sendai virus.