Comparison of transmission of FMDV in groups of vaccinated and non-vaccinated calves

To quantify the effect of a single vaccination on FMDV transmission in cattle, we performed two experiments with 26 calves each. In 12 groups of 4 calves each, two calves were inoculated intranasally with O/NET FMDV and two calves were contact exposed to the inoculated calves. And 2 groups of 2 calves were only vaccinated to serve as vaccine control group. With the results of clinical inspection, virus isolation and RT-PCR in heparinised blood and mouth swab and sputum samples and the results from the NS ELISA, we determined whether contactinfections occurred. With the final size of the infection we estimated the maximum likelihood estimator MLE for the reproduction ratio R. We found that a single vaccination led to a Rv < 1. This was a significant reduction of virus transmission when compared to the Rc in non-vaccinated groups of calves. Introduction An outbreak of foot-and-mouth disease occurred in The Netherlands in 2001 (Bouma et al., 2001). The capacity for pre-emptive culling and destruction appeared to be insufficient and therefore emergency vaccination was applied. Although in cattle it is known that vaccination prevents clinical signs of FMD, little is known about the effect on transmission (Donaldson et al., 1989). Experiments with pigs showed that emergency vaccination reduces transmission of FMD virus (Salt et al., 1998). Extrapolation of results from pigs to cattle may not be valid, because difference in susceptibility and clinical appearance are known (Kitching, 2002; Kitching and Alexandersen, 2002; Kitching amd Hughes, 2002). Therefore the important question remains, whether single vaccination can reduce FMDV transmission sufficiently in cattle to stop an epidemic (R < 1). The objective of the study was to quantify the effect of vaccination on the virus transmission within groups of calves. Materials, methods and animals For our experiments we used 12 groups of 4 calves each; of which 6 groups were vaccinated. In each group of 4 calves, 2 calves were intranasally inoculated (days post challenge = DPC 0) with 1500 CID50 (cow infectious dose 50%) of the first cattle passage of the FMD field isolate O/NET2001. Transmission of FMDV to contact exposed calves was observed by recording the clinical signs and by virus isolation and RT-PCR on OPF (oro pharyngeal fluid from mouth swabs) and plasma (from heparinised blood samples). Antibody response to the FMDV infection was detected by a virus neutralization assay on serum samples. At the end of the experiment (DPC 29-30-31), probang sputum samples and white blood cells were collected for virus isolation and RT-PCR. Virus isolation on secondary ovine kidney cells was performed to measure virus titres in plasma, OPF and sputum samples Terpstra et al., 1990). The sera were tested in virus neutralisation assay and NS-ELISA (Dekker and Terpstra, 1996). The calves were classified as S (=susceptible), I (= infectious) and R (= recovered) at the beginning and the end of the experiment. In both groups, a contact-exposed calf was classified as infectious at the end of the experiment when it showed clinical signs or when virus isolation or RT-PCR was positive for FMDV. The inoculated calves were all classified infectious by definition. Transmission was quantified with the maximum likelihood estimator (MLE) for the reproduction ratio (R) in the S-I-R model. Results Clinical signs In the vaccinated groups one inoculated calf had a small lesion in its mouth. Neither the other inoculated nor the contact-exposed vaccinated calves showed clinical signs. In the non-vaccinated groups, vesicles in the mouth, on the coronary band or in the interdigital spaces were observed in 10 out of 12 inoculated calves, where 7 out of 12 contact-exposed calves showed clinical signs. The signs were very mild and did not have effect on behaviour or feed intake.