24
resistent, for the cause may not be a true one. It is evident that there may be different
determining influences at work in different cases; we prove this by our coarse experi-
mental methods even, for in one instance the disease is conveyed by puncture of the skin
and injection of blood into the subcutaneous tissue (a very unnatural determining
influence unless the disease is ordinarily conveyed by mosquito punctures or leech
bites), and in another instance a large number of parasites taken in with food generate
the disorder, reproducing it in probably an exaggerated copy of the natural ingestion
of spirilloids with food and water. From this we learn that, although we are right in
considering the parasite the sole actual and immediate cause, we would err if we stated
that any one specified determining influence is at work in all cases. Some cases may be
due to food, others to water, and others to air introduced into the respiratory organs.
We have no evidence opposed to the view that mosquito and leech bites and visits of
flies to wounds are in some cases the determining influences by which spirilloids are
brought into the system. Several of these influences may be simultaneously operative
on an animal and so the actual way in which the attack was brought on in any particular
case obscured; this was observable in the case of the dog " Motee." She received
blood subcutaneously and ate flesh of animals which died from the fever; it is, accord-
ingly, uncertain from which source she obtained her relapsing fever.
16. To trace out a cause of attack we must decide when it operated, that is, how
long it exerted its influence before it induced the attack, and when the attack actually
commenced. Unfortunately these points are both doubtful in the disease which I have
been studying; the period of incubation varies with the channel through which the
contagium enters the system and with the condition of the contagium at time of admini-
stration. My experiments amply prove this: incubation was longer after gastric inges-
tion than after subcutaneous injection of the virus, and longer still when blood with
no parasites detectable was used instead of that in which they were numerous and
active. Incubation in the dog was much longer than in the mule, and in the pony (and
especially the monkey) it was very short. Again, in examining the outbreak as a
whole, we labour under the very serious difficulty that we do not generally know when
the cases commenced; this is due to imperfect diagnosis, owing to causes which I have
elsewhere indicated. It is singularly unfortunate because it compels me to consider
cases in relation to possible causes only by their date of fatality. This is manifestly a
much less satisfactory datum than the date of attack, for the duration of cases varies
very much. I have been obliged, for want of a more satisfactory method, to strike an
average duration for cases and deduct it from the date of death, and in this way get a
possible date of attack.
17. The fact that we have no positive evidence that the spirilloids leave the blood
of affected mules is of much importance, as it may be associated with the contagious-
ness and curability of relapsing fever in man as contrasted with the strong evidence
of non-contagiousness and non-curability of the mule disease. In man the influence
of contagion can be traced in the manner of spread of the disease into unaffected
districts, but the evidence which we have to hand about the mule disease is strongly
opposed to the theory of its communicability by contagion; no contagious influence
can be traced in its manner of spread among mules in one stable or working together.
In the Somersetshire lines, D troop mules were often intermingled with those of A,
B, and C troops, and had to pass through their lines daily, but, until their movement
into the other stables, not one D troop mule was affected. Further, lame and non-febrile
sick animals have stood in lines containing many fever patients and did not become
affected although many of them had open surgical wounds at the time. They have
been returned to uninvaded lines from the sick lines on recovery and have not carried
the disease. There are two incidents of the outbreaks which deserve mention here.
While the disease was in the Rangoon depôt lines three ponies were started (on 28th
September 1884) from there to Tonghoo. One died on the way at Shwaygyin (cause
unknown), another at Tonghoo shortly after arrival, the third remains alive. One
mule succumbed at Tonghoo previously to their arrival and one in the third month
after their arrival, so, and, especially, considering the obscurity of cause of death in
two ponies, we have not here material for tracing the Tonghoo outbreak (which took
place seven months later) to contagion from Rangoon. Twenty-four mules were sent
from the depot to Thayetmyo on 21st December 1883 ; of these, one died at Letpadan