F. C. MINETT
79
logical data from Table I (except those for ' dust
storms ') and the incidence figures for the corres-
ponding period are displayed graphically. Finally
in Figs. 7 to 12 local meteorological data for daily
maximum and minimum temperature and rainfall
are plotted, beside daily case incidence. Natural-
ly, it would have been of value to examine figures
for breeding farms situated in other provinces with
a different climate, but such figures are not avail-
able.
ANALYSIS OF DATA
Examination of Figs. 1 to 6
Inspection of Figs. 1 and 2 shows that at the
remount depots the attack rate rises sharply
from September October when newly purchased
young stock begin to arrive and reaches a peak
in April to fall abruptly during May and June,
and the rate then remains low till September.
On the Punjab stud farms (Figs. 3 to 5) the
curve is not so simple. There is agreement,
however, in that the attack rate declines in
July-August. At Coleyana and Renala the curve
shows a winter rise and a sharp secondary rise
between April and June. At Probynabad the
rise expected in the colder months is delayed
till February-March, while the secondary rise in
May-June is still present. At Ahmednagar (Fig.
6), which has to be considered separately, because
it is situated in an entirely different part of the
country, and has a different and more equable
climate, the incidence rises during the drier winter
months and is much lower from April to October.
The incidence may now be examined in relation
to the meteorological factors.
Rainfall. On the stud farms at least the declin-
ing incidence curve of July and August is associa-
ted with a rising rainfall. The relation to rainfall
is particularly evident at Ahmednagar. Clearly
it appears that monsoon conditions, even when
the total rainfall is light, are not favourable to the
spread of strangles.
Relative humidity (8 a.m. reading). Here there
is no clear connection. As we have seen, the attack
rate is low during the summer monsoon when the
relative humidity is, of course, fairly high. But
in the winter, when the relative humidity is still
comparatively high, the disease may reach the
dimensions of a steady outbreak, while cases may
still be appearing as late as April—May when the
humidity is low.
Temperature. In all the charts for the Punjab
stations, except that for Probynabad, there
is distinct evidence that a declining air tem-
perature is associated with an autumn rise in
the number of case admissions. At the stud
farms a smaller rise is noted in the pre-monsoon
days of summer, when the maximum temperature
is very high. Here it may be mentioned that at
the depots this summer incidence rise would not
be expected because by that time the number of
susceptible stock is small.
Wind velocity and direction. On the stud farms
velocity per se appears to bear no relationship
to strangles morbidity, since at the time of year,
when the velocity is higher, the incidence first
rises and then falls. As regards direction, the
data (not included in this paper) show that at
Sargodha and Montgomery the prevailing breeze
is northerly (N., N.E., or N.W.) from December or
January until April or May, and that during the
rest of the year it is southerly (S., S.E., or S.W.).
Thus, during the winter season, when air tempera-
tures are low and especially when there is rain, the
northerly breeze, though not of high velocity, is
likely to be severely felt by exposed animals.
During the monsoon, the southerly wind, though of
higher velocity, is unlikely to have an ill-effect,
since the temperature is then higher.
Dust storms. From Table I it may be seen that
at the Punjab stations storms are most frequent in
the period May—August and especially in June.
It seems possible that the weather phenomena res-
ponsible for these storms and especially the hot and
dry southerly wind at this season may be con-
cerned with the increased number of cases on the
Montgomery stud farms in May and June.
Years of high and low incidence on stud farms.
Reference to Fig. 1 of the previous paper,
giving case admissions at the stud farms by years,
will show that in some years the disease is very
prevalent, and in others absent or nearly so. It
was of interest to see if this periodicity might be
ascribed to weather conditions. For this purpose,
eight years were taken, in which the attack rate in
the Punjab studs was 42 per cent or more of the
young stock population and compared with eight
years in which the rate did not exceed 2.4 per
cent. For each of these 16 years, the following
meteorological elements were examined: rainfall
for the year, rainfalls for February—June,
July—September and October—January, and the
mean relative humidity and mean maximum
temperature for these three seasons. One might
perhaps have anticipated that, during the years
when there was little or no disease, the rainfall
figures at least might have been above the average,
but no variation of this sort was found. It seems,
therefore, that the variation in incidence from one
year to another on the stud farms must be attri-
buted to some other factor or factors than climate.
There is no indication that the variation is due to a
differing density in the population of young
horses. No doubt, the simple explanation is that
a bad outbreak in a particular year leaves most of
the population immune, since years of high inci-
dence at Probynabad and Coleyana are usually
succeeded by three or four years in which the
incidence is low.