Appendix 21
Foot-and-Mouth disease virus transmission between individually housed calves A. Dekker, A. Bouma and M.C.M. de Jong
Introduction Foot-and-Mouth disease (FMD) virus was most likely introduced in the Netherlands by calves imported from Ireland via an FMDV-contaminated resting point in Mayenne, France. The calves were sent to a mixed veal-calf/dairy-goat farm and housed individually (Figure 1). The first clinical signs of FMD were reported in the goats, three weeks after arrival of the calves (index case 2001/03). At culling of the farm, serum samples were taken from all calves (74) and 100 goats on that farm. From the 74 calves 4 were positive for antibodies against FMDV, whereas 87 of the 100 goats sampled were serological positive. The serological positive calves were not found in an isolated part of the stable (Figure 1). Based on the information given in Figure 1, it seemed that these calves were able to have contact with other calves. 23 calves 25 calves 2+
15 calves 2 +
10 calves Storage room
Milking parlour (goats)
Milk tank
Figure 1: schematic drawing of the stables where the calves where housed on outbreak NET/2001/03
Since only a small percentage of the calves was serologically positive, it was questioned whether the hypothesised route via Mayenne was likely or even possible. After all, FMD is known to be very contagious and the spread of FMD virus among calves appeared to be only limited. The calves on the index farm (NET/2001/03) were initially housed individually. As for every contagious disease, the contact structure between infectious and susceptible animals is important and in part determines whether an infection will spread. In theory, infection of animals housed in a row will stop spreading, if transmission relies on direct contact between animals, because the chance of transmission will always be smaller than 1. However, we had no information on the reduction of transmission of FMD virus if such a housing system is used. In a previous challenge experiment using tethered calves no transmission from infected calves (10 vaccinated and 2 controls) to 5 non-immune contact calves was observed when the 174
