Age-dependent susceptibility to porcine circovirus type 2 infections is likely associated with declining levels of maternal antibodies

Objectives: To investigate the relationships between age, porcine circovirus type 2 (PCV2) viremia, and pathologic lesions by evaluating the age of porcine circovirus associated disease- (PCVAD-) affected pigs submitted to Iowa State University Veterinary Diagnostic Laboratory between 2003 and 2010 and by experimentally infecting pigs of different ages. Materials and methods: A total of 3565 PCVAD cases were selected and ages of PCVAD-affected pigs summarized. Sixty-two pigs were randomly assigned to three groups and inoculated with PCV2 at 2 weeks (AGE-2; n = 21), 7 weeks (AGE-7; n = 20), or 12 weeks of age (AGE-12; n = 21). A portion of the pigs in each group arbitrarily selected by the farm manager were vaccinated with an adjuvanted vaccine (Mycoplasma hyopneu-moniae) at 1 week of age. All pigs were euthanized 14 days post inoculation (DPI). Lesions and viremia were compared among groups. Results: Among PCVAD cases received from 2003 to 2010,47.6% were 12 to 16 weeks of age. AGE-12 pigs had significantly lower levels of PCV2 antibody compared with AGE-2 and AGE-7 pigs at the time of inoculation, and AGE-12 pigs had significantly higher levels of PCV2 viremia at 7 and 14 DPI. Mycoplasma byopneumoniae- vaccinated pigs in the AGE-12 group had higher levels of PCV2 viremia compare to nonvaccinated pigs; however, this effect was not seen in younger pigs. Implications: Under field conditions, 12 to 16-week-old pigs are most commonly affected by PCVAD. Under experimental: conditions, 12-week-old pigs are more susceptible to PCV2 infection than are 2- and 7-week-old pigs.