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Observations on the epidemiology of porcine parvovirus.

Evidence presented suggests that porcine parvovirus is highly stable and infective. Introduction of virus to susceptible herds results in 100% infection rate within the following 3 months. Active immunity is associated with high persistent levels of haemagglutination-inhibitating (HI) antibody (greater than 256), piglets suckling immune sows acquiring HI titres between 10,000 and 40,000. Loss of passive immunity, measured by HI, occurs in a majority of pigs between 14 and 26 weeks of age (mean 21 weeks), whilst an average of 25% (2-47%) of pigs lose HI titres between 26 and 36 weeks of age. Susceptibility to challenge with virus does not occur until 3-5 weeks following loss of HI titres. In endemically infected herds 98-100% of adult pigs show serological evidence of active immunity. A significant proportion of gilts may not be actively immune to porcine parvovirus at the time of first service, and subsequent infection may occur while these gilts are pregnant.