Impact of a porcine epidemic diarrhea outbreak on swine productivity in Japan: a retrospective cohort study

YAMANE, Itsuro; YAMAZAKI, Hisanori; ISHIZEKI, Sayoko; WATANABE, Yugo; OKUMURA, Hanako; OKUBO, Mitsuharu; KURE, Katsumasa; HAYAKAWA, Yuiko; FURUKAWA, Makoto; OOI, Munetaka; MIZUKAMI, Yoshihiro; ITO, Mitsugu

Reduced productivity, characterized by PRWM, POWM and PWL, was only observed during April-June 2014, which indicates that in many of the PED-positive herds, the outbreaks were controlled within a short period. This finding agrees with those reported in the United States, in which many PED-infected herds reached their baseline production levels within 12.6 weeks [3] . This may be because the herds rapidly developed immunity and therefore recovered from the severe POWM after the PED outbreak [3] . Alternatively, the owners of the herds targeted in the present study may have taken urgent preventive action, because they used consulting veterinary services. In many of the PED-positive herds, the clinical signs of PED were observed in the nursery and finishing barns, which is consistent with the evidence of higher POWM in the PED-positive herds. This finding is also consistent with that of a previous report [8] , but contradicts to that of another study [7] , in which no mortality was observed in stages other than the suckling stage. These different results may be attributable to differences in the swine management systems used. For example, most of the herds targeted in the present study were farrow-to-finish systems, which are at higher risk of introducing pathogens into the postweaning stages than other production systems. The increased POWM in the PED-positive herds in the present study was similar to that reported in a retrospective cohort study in which pigs that survived PED infection showed a 10% increase in postweaning mortality [1] . The L-PED-positive herds had significantly higher AFI than the PED-negative herds. This may be because herds with larger size required more time and efforts to control PED. The L-PED-positive herds had higher PBA and LMFY than in the PED-negative herds, although these differences were not significant. This indicates that the L-PED-positive herds are more common among larger herds, which usually involve more intensive production systems and more rigorous biosecurity measures. Many participating veterinarians commented that the first PED cases in their regions appeared in herds with excellent biosecurity controls (personal communication), which is inconsistent with the standard theories regarding many other infectious swine diseases, and constitutes a mysterious aspect of the PED outbreaks in Japan.