THE EFFICACY OF TWO FOOT AND MOUTH DISEASE VACCINATION REGIMENS FOR CALVES OF IMMUNIZED DAMS IN SAUDI DAIRY FARMS

The programme of immunization of calves born to repeatedly vaccinated dams is considered a complex problem in dairy farms in which control of FMD relies predominantly on vaccination. Consequently, depending on the waning of maternally derived antibodies (MDA), a study was undertaken to find out the age and the regimen at which calves could be vaccinated against Foot and Mouth disease (FMD) to elicit maximum immune response. Two vaccination regimens on two groups of colostrum-fed calves each of 16 animals were implemented. The animals were vaccinated with a commercial inactivated FMD vaccine at 4,5 months of ages (regimen I) and 4,5 and 6 months of ages (regimen II). The MDA level and the neutralising antibodies against FMD vaccine were assessed by microneutralization test . M A cou! be detected in all animals of the two groups up to the age of 4 months, some animals had effective MDA up to the age of 210 days. It was found that calves with MDA did not merely fail to respond to primo-vaccination, but also serum titres were depressed: this depression was reverse proportional to the level of pre-existing MDA at the time of primo-vaccination. However, the increase in the mean of the vaccine induced antibody titres at 5 months of age ( one month after the primo-vaccination ) were lower as compared to the increase one month later after the booster dose, as the titres provoked by revaccination in 75% and 81% of calves of regimens I and II would be regarded as protective. MDA were also found to influence secondary humoral immune response as 25% and 19% of calves in regimens I and II at 6 months of age showed decrease in neutralizing antibodies titres. Further seroconversion was observed among all animals of regimen II at 7 months of age (one month after the second booster), while 25% of the animals of regimen I still showed considerable decrease in antibody titres. Based on the above findings, it is recommended that calves could be primo-vaccinated at 4
months postpartum with two booster adiministered not later than 5 and 6 months of ages (regimen II)