A case report on parainfluenza virus type 4a infection in a 1-year-old boy with biphasic fever.

A 1-year-old boy was infected with parainfluenza virus type 4a (PIV4a) during an influenza epidemic in January 2016. His body temperature was 38.2 °C on day 1 of the illness followed by an intermittent phase of 36.5 °C on days 2 and 3, and it rose again on day 3 and peaked at 39.6 °C on day 4, of which the fever pattern was reminiscent of an influenza case with biphasic fever. However, results of rapid influenza virus (IFV) antigen tests performed at the first clinical visit and during the second fever phase on day 4 were both negative. The PIV4a was isolated from all the nasal aspirate specimens on days 1, 4, and 7. Other common respiratory viruses were negative in all the specimens in the viral isolation trials using the multiplex cell culture system and RT-PCR tests. The fever disappeared within 5 days after the onset without any antibiotic treatment, which strongly suggested the PIV4a as the causative agent of the patient's illness. On the basis of the incubation period required for the appearance of the cytopathic effect (CPE) in the infected cells, from specimen inoculation to the cells, the viral load in the nasal cavity was speculated to be greatest on day 4. His cough started on day 1 and persisted until day 9, and the viral isolation indicated that the shedding of the active virus continued with the coughing even after the termination of fever.