A serological study on infection patterns, control and persistence of classical swine fever in infected farms in the Philippines.

In 3 farrow-to-finish farms with enzootic classical swine fever (CSF) all sick pigs were culled and an intensive vaccination program was introduced. Boars and cows were vaccinated every 6 months and piglets at the age of 6 and 8 weeks. The infection was monitored on clinical grounds and by means of serological examination of the farms. Serological results reflected well the clinical situation in the farms. High titers and high percentage of animals with positive titers were observed in infected farms, compared to lower titers in a clinically "free" farm, which were interpreted as titers due to vaccination. Although clinical improvement was noticed, the rigid vaccination regime was not able to control CSF in none of the farms. In all cases, a small percentage of pigs in the finishing stage was still affected. Since these pigs seemed not to have responded to vaccination at 6 and 8 weeks of age, the programme was questioned. Additionally, very high titers were recorded in sows from infected farms, which would automatically extend the period of passive protection in offspring and postpone the age at which piglets could be vaccinated. More intensive programmes, which include also vaccination beyond the age of 8 weeks, were suggested to bring enzootic infections under control.