94 Contagious or Epizootic Abortion in Cows. 
urine. It is also uncertain, and difficult to determine, whether 
the bacilli commonly escape directly in any numbers from the 
genital passages before abortion or parturition, or for how long 
they generally continue to escape after either of these events. 
In these circumstances, one must regard every infected animal 
— of either sex, and whether pregnant or not — as a potential 
disseminator of bacilli. 
Methods of infection . — Experiments have proved that preg- 
nant animals can be infected in a variety of ways. The 
method which appears to be the most effective for causing 
actual abortion is the injection of bacilli into one of the veins, 
but abortion may also follow the experimental introduction of 
bacilli under the skin or into the genital passages, or their 
administration by the mouth. But the important question 
under this head is not the possible experimental methods of 
infecting animals, but the way by which the bacilli enter the 
bodies of animals when the disease is spreading naturally in a 
herd. Until quite recently the generally accepted view was 
that the bacilli, as a rule, if not always, entered by way of the 
genital passages. Opportunity for such admission was supposed 
to be provided while the animals were lying down in the 
cowshed, the vulva then coming into contact with materials 
containing the bacilli in the channel for the urine and faeces. 
It cannot be said that this view ever had much in its favour 
except the fact that various experimenters had found that 
animals could be experimentally infected by the direct intro- 
duction of bacilli into the vagina, and the further fact that the 
vagina furnishes the most direct route to the womb, which is 
the main seat of the disease. In the light of recent researches 
it appears actually doubtful whether cows ever become infected 
in consequence of such casual or accidental admission of the 
bacilli into the genital passages. As soon as experiments had 
shown that animals could be infected by the mouth this had 
to be regarded as a highly probable natural method of infection, 
and a consideration of all the circumstances leaves little or no 
room for doubt that it is the way in which the disease usually 
spreads in a herd. It is quite obvious that, given the existence 
of abortion bacilli in a cowshed, these are much more likely to 
be taken in by healthy animals with their food than to find 
their way directly into the genital passages. Without denying 
the occasional occurrence of infection in the last-mentioned 
way, it appears to be safe to say that the disease is far more 
frequently contracted by the ingestion of bacilli in food or 
water. And if this opinion is justified with regard to the 
dissemination of the disease in cowsheds, there are still stronger 
reasons for holding that direct infection of the genital passages 
cannot occur with any frequency among animals at grass. 
