Parainfluenza virus infections in the Cirencester Survey: seasonal and other characteristics.

Parainfluenza viruses were isolated 165 times during 14 years surveillance of the illnesses of a general practice population of around 3700. Type 1 isolations numbered 57, type 2 isolations 22 and type 3 isolations 86, representing annual rates of 33, 13 and 50 infections respectively per 10000 of population. Type 4 parainfluenza virus was not isolated. Three major classes of illness gave the following rates: sore throats (Throats) nine, acute febrile respiratory diseases (FRD) 23, acute non-febrile respiratory diseases (non-FRD) 71. The illnesses caused by the three types isolated were similar. Type 1 infections were most abundant in November and type 2 infections in December, and only 11.4% of these types were isolated in the warm semester April through September. Type 3 infections were seasonally bi-modal, with a winter peak in January and an even greater prevalence (66% of the total) in the warm semester. Type 3 infections in the warmer months and in the later years of the Survey were usually more severe. Type 3 virus may therefore be heterogeneous, one subtype possessing and the other lacking the genetic mechanism of "cold-season' prevalence. Geographical discontinuity between summer and winter isolation strengthens the case for the existence of the two subtypes of type 3 parainfluenza virus. Type 3 infections caused the majority of the infections in very young infants. Type 2 infections were widely distributed at all ages. Females were attacked more often than males: type 1, 68.4%; type 2, 63.6%; type 3, 53.5%. Type 3 infections in males outnumbered those in females up to 60 years of age, whereas female predominance became apparent in types 1 and 2 before 10 years of age. All types were widely and sparsely distributed, areas of prevalence changing from year to year. Recurrences occurred only twice, both with type 3 infections. Six persons suffered both a type 1 and a type 3 infection, and one person suffered both a type 2 and a type 3 infection.