Frequency of transmission.

One infected donor animal was introduced into a cage of 3 naïve recipient mice one day after inoculation and housed together until end of experiment. Frequencies are expressed as percentage of transmission (no. positive/total no. naïve animals).

The contact transmission experiment was completed once using only 70 PFU direct inoculation as previously published data [21] revealed similar frequency of transmission and dynamics of infection after contact transmission regardless of direct inoculation dose.

nd = not done.

Individual naïve recipient mice were housed in one of 3 isolation chambers per large transmission cage and were separated from the ten infected donor animals by the indicated distances. 7.6 cm = 3 inches and 15.2 cm = 6 inches. Ten infected animals were placed in transmission cage directly after inoculation for both the 7.6 and 15.2 cm experiments.