294
Johne's Disease
cannot be stated, and in the present herd no proof of real or complete re-
covery has been obtained, but the issue is often confused by the difficulty
which is experienced, firstly in making a diagnosis at all in this disease and
secondly, after making a positive diagnosis, in obtaining confirmation at
post mortem examination. In this connection it is of interest to note that
although several of the animals shown in Appendix I (Nos. 6, 9, 12, 13, 14,
38 and 68) are recorded as having shown acid-fast organisms in faecal or
rectal smears, these were not recovered later at post mortem, either by micro-
scopical or cultural examination. Serial No. 44 in the same appendix was
artificially infected intravenously with a dose of pure culture of Johne bacilli
and gave 3 positive reactions to the avian tuberculin test, yet on post mortem
examination acid-fast organisms were not recovered.
Another point to be considered is the comparative value of avian tuber-
culin and of Johnin, applied by the double intradermal method, for the diag-
nosis of this disease. As noted above, in this work out of 46 reactors to avian
tuberculin 24 were proved to be positive for Johne's disease, while out of 14
non-reactors, suspected of Johne's infection on other grounds, 9 were found to
be positive. With regard to Johnin there are a number of observations in the
literature. Reference to some of these is given by Minett [1933, 1935] who
has also reported on the value of a synthetic Johnin for the diagnosis of Johne's
disease in cattle. In a series of 53 animals which had reacted to the double
intradermal method of testing, 39 were proved to be infected on post mortem
examination, while there was presumptive evidence of this infection, either
from post mortem examination or from the history of the case, in eleven of the
remaining 14. Repeat tests on a number of the animals in this series showed
much variation in the degree of local response to Johnin, a result which was
attributed to variations in the allergic state. A few other cattle which were
at an advanced stage of the disease did not react.
The vaccination experiments in the Mukteswar herd were started with
the possibility in mind that the disease would spread easily and that in this
way the value of vaccination might be judged. Unfortunately, since the
disease did not spread and the number of calves available for vaccination
was small, no conclusion can be drawn as to the usefulness or otherwise of the
method and the second object of the work in the experimental herd was
therefore not achieved.
The question may be asked finally in what way the conditions of the
present experiment differ from those cases in which the disease seems to
spread without hindrance. The favourable conditions under which the animals
in this experiment were maintained have already been referred to and it
seems probable that under good farming conditions the disease spreads easily
only (a) when there is a high level of infective material within a restricted
space, (b) when the level of host susceptibility is high, as when there is a high
proportion of very young animals. With the present herd an attempt has
been made to estimate the chances at different times of healthy animals acquir-
ing infection by preparing a list of (a) the number of infected animals in January
and July in each year and their status as regards clinical or latent infection
and (b) the number of healthy or presumably healthy contacts and their age
on the same dates. The ratio of (a) to (b) would clearly be a means of judging