92
Incidence of Equine Strangles in India
January. In Sargodha there were no cases up to
14th February. Strangles started in Sargodha in
the middle of February after a period of severe
cold wet weather ; this is always the case, as it
reduces natural resistance of the young stock
Strangles is still going on in Sargodha and has
been kept up, by successive batches of stock re-
ceived up to the middle of May. Major F. J.
Andrews, R.A.V.C., in a letter dated 8 February
1935 states ' As is usual during the winter months
here (Sargodha), the heavy rain was followed by
a number of cases of Strangles in nearly all paddocks
of susceptible stock '*. Major W. E. Barry, R.A.V.C.
in a report dated the 14 June 1938 states ' There
were few deaths up to the end of April (at Sar-
godha) ; up to this time the disease was not very
severe, probably due to the favourable weather,
the winter being mild with little rain '. During a
visit to the Punjab in May 1943, I obtained the
following information. Major M. F. Keightley,
Remount Department (commanding at Mona)
said that ' cases are most severe after the Christ-
mas rains of December and January. The dis-
ease tends to begin after a heavy downpour,
followed by three or four cloudy days with cold
wind'. At Sargodha, the veterinary officer and
two risaldars gave independent evidence of similar
nature and added that the warm dry weather
coming at the end of March and in April tends to be
followed by a decline in the number of cases.
At both depots, the storms are local, sharp, and
frequent from January on. Rain is also very
localized at Coleyana and Renala, but the amount is
rather less in winter than that at the depots.
The problem of why diseases have a seasonal
trend is undoubtedly complex. In the case of
respiratory disease, two factors at least may be
operative on the host, (a) extremes of air tempe-
rature and humidity or sudden changes in these
elements, and (b) a transient and perhaps seasonal
defect in the mechanism of immunity
It is common knowledge that a more or less
sudden cooling of the body through exposure to
low air temperature is liable to disturb the func-
tion of the respiratory tract, especially when the
effects of cold are accentuated by wetting of the
body surface through rain and by the action of
wind. In man, a similar result may be brought
about during the hot dry season by chills induced
by excessive radiation or evaporation ; cold in the
head being by no means uncommon through this
cause, when the shade temperature level is over
100°F. Exposure to cold will not only initiate
clinical attacks in the more susceptible animals
but will also raise the general carrier rate. In
consequence, the amount and diffusion of in-
fective material are increased, so that fresh impe-
tus is given to the outbreak and it may continue
for some time, possibly in milder form, in spite of
an improvement in the weather.
Again, in connection with this first factor, Win-
slow, Herrington and Nelbach [1942] have shown
that substantial drying of the nose and throat
occurs at air temperatures of say 70° or 80°F. if
the relative humidities at these temperatures are
less than 54 and 39 respectively. Boyd and Johns.
ton [1940] have pointed out that a fine adjustment
in the water content of the respiratory tract
(trachea and lungs) is required to maintain this in
health. They showed, in fact, with rats that the
water content of the lungs tends to be remarkably
constant. During the autumn, however, when the
animals were exposed to the first cold spell, there
was a significant increase in water content, and
this was followed by an equally significant drying
of the tract when the hot-water system in their
living rooms was turned on. Further, Cralley
[1942] has shown in experiments on rabbits that,
when the animals are made to inhale very large
numbers of bacteria through the medium of drop-
let nuclei, only a very small percentage reaches the
lungs. Under ' normal ' conditions (air tempera-
ture 26°C. and relative humidity 45 per cent), 80
per cent of the organisms that do reach the lungs
are removed within the first hour and over 90 per
cent within three hours. When the animals are
first subjected to extremes of temperature and
humidity (38°C. and 30 per cent humidity, 4°C.
and 80 per cent humidity) or first to one extreme
and then to another, the rate of removal is tem-
porarily but definitely depressed.
As to the second factor, there is reason to sup-
pose that the animal body undergoes an important
modification in susceptibility, which may be sea-
sonal. Wilson [1930] has shown that in mice
there occur transient fluctuations in their resist-
ance to Bact. aertrycke. These variations in re-
sistance, however, were not definitely seasonal
and the author applies the term 'fluctuating
immunity' to those ' variations in resistance
which occur from time to time and which are due
to alterations in the physiological behaviour of the
animal dependent on changing environmental
conditions '. The nature of the change in question
has excited the attention of investigators and is
but vaguely understood. It may have an external
origin, e.g., it may be dietetic and concerned
with the lack of fresh green fodder at cer-
tain seasons. Webster & Burn (loc. cit.), for
instance, noted a higher proportion of Pasteurella
carriers among rabbits on an ' incomplete' diet
than in ones on a ' complete ' diet. As is the case
with ' chills ', there would result a larger amount
of infective material available to the population
* The italics are introduced by me, because this state-
ment points to some uniformly acting cause