The number of isolates of various viruses detected in public health laboratories all over Japan is available in the Infectious Agents Surveillance Report, Japan, for each year since 1981, the data between 1980 and 1991 being documented in published supplements (7, 8). All annual data are available from the NESID system (14). This NESID system database includes the data from Yamagata described in this study.

Several previous studies have reported that HPIV1 infections have clear outbreaks in autumn, mostly in September and November, either every two years (15–18) p38 MAPK pathway or at irregular intervals (19). In this study, we found no clear seasonality for HPIV1 infections, although HPIV1 infections did appear to be more common in odd-numbered years. In Japan, no source, including the NESID system, has indicated a seasonal pattern in HPIV1 infections (5–8, 14). In comparison to the clear seasonality of HPIV1 and HPIV3 outbreaks, smaller yearly or irregular outbreaks of HPIV2

have reportedly occurred in autumn (15–19). In this study, we recovered many HPIV2 isolates in the autumn-winter season, observing a particular increase in even-numbered years since 2004 in Yamagata, Japan. The NESID system data support this trend: in the years prior to 1986, HPIV2 infections occurred more commonly in even-numbered years, apart from 1981 and 1983 Selleckchem Seliciclib (7, 8, 14). Thus, HPIV2 infections have commonly occurred in the autumn-winter season every two years in Japan, although this seasonality is less clearly observable than that of HPIV3. In this study from 2002 to 2011 in Yamagata, Tangeritin Japan, we found HPIV3 infections to be grouped in clear

yearly seasonal outbreaks, mainly between May and July. The data in the NESID system also show that HPIV3 infections have peaked in the spring-summer season since 1980 (7, 8, 14). Many previous studies have reported that HPIV3 causes yearly outbreaks, mainly in the spring-summer season, around the globe (15–20); the clear seasonality of HPIV3 in Yamagata appears similar to that observed in other areas. It is generally accepted that HPIV3 as well as RSV infections are common in infants and young children, whereas HPIV1 and HPIV2 infections tend to be commoner in older persons (1–3, 15, 19). Knott et al. reported that the age distribution of HPIV3 infections peaks at 6 months–2 years of age, whereas HPIV1 and HPIV2 peak at 2–5 years (15): findings that are similar to our observations in this study. Clinically, fewer of our patients were diagnosed with croup (2.3–8.2%) than was reported by Knott et al. (9–45%) (15). However, both studies supported the contention that HPIV1 and HPIV2 are more strongly associated with croup than is HPIV3, which is in agreement with the trends described in various textbooks (1, 3). This study indicates that the annual isolation frequencies of HPIV1–3 are 1.6–10.