Agent factors
The specific number of organisms required to establish infection
for specific age groups has not been determined. Infection can
occur in calves with a dose of 1.6 3 107 organisms, which would
easily be surpassed in a 2-gram sample of heavily infected feces
(36). However, this number is likely to increase with increasing
age as the resistance to infection of the animal increases (25).
Infection of animals may cause clinical disease, but this is not
necessarily advantageous or essential to the organism. To survive,
MAP only needs to colonize, replicate, and be shed, so that the
rate of recruitment of new bacteria is equal to or greater than
the loss of bacteria from the population. The presence of obvious
clinical disease is not required for the spread of the organism in
the animal population (31). It has been shown that, although
the risk of individual cows being infected is higher on farms
with clinical JD, there are still many herds that are infected yet
display no clinical signs of JD (37).