394
Differences in Incidence of Encephalomyelitis in Horses
the incidence of the disease in these two age groups has been made. Among
41 foals 22 were of Nonius breeding and 19 of other breeding. Of the 22
foals of Nonius breeding 16, or 72.7 per cent, were affected, whereas only
two out of 19, or 10.5 per cent, of the foals of other breeding were affected.
This difference is highly significant, P being less than .01.
Only four yearlings with Nonius breeding were in the stud but two of
these were affected, while among 27 yearlings of other breeding only three
were affected. Here also the trend in mortality was in the same direction as
was observed in the foals. Thus it is apparent that in these two comparable
age groups the horses with Nonius breeding were more susceptible to this
strain of encephalomyelitis virus.
Of the 30 animals affected seven died. Three died during the course
of the disease and four died during the convalescent period. Of the latter
cases one apparently recovered but was found dead in the pasture the latter
part of September. Of the others, according to the veterinarian in charge,
two died from the effects of the disease and one from septicemia resulting
from bed sores acquired during paralysis. Six of the seven animals that died
were of Nonius breeding, five of these being foals and one a yearling. The
seventh animal, a yearling, was the one found in the pasture in late Sep-
tember.
The distribution of the affected animals in the various pastures both
preceding and during the outbreak of the disease precludes the possibility
of the differences in resistance observed in the various groups being due to
their location on the station. The station comprises an area of approximately
50,000 acres and the various pastures are well scattered over most of this
area. These pastures varied in size from about 400 acres to several thousand
acres. The distribution of animals of the various groups in these pastures
was largely at random so that all groups had similar chances of exposure
to the disease.
During the breeding season from June 1 to July 15, 70 head of mares
(including the purebred Nonius) with 39 foals were rotated in pastures
near the station headquarters. The density of the stocking in these pastures
was greater than in the other pastures but since all mares with foals were
included the opportunities for exposure in the various groups were equal.
Twenty-four of the 30 cases of encephalomyelitis occurred in this group.
The distribution of the cases of encephalomyelitis in the various pastures
gives additional support to the above conclusion. Encephalomyelitis occurred
in each of the several pastures and in most of the pastures animals from two
or more of the different groups were affected.
Fourteen mares of the grade Belgian stock, all sucking foals, were vacci-
nated with brain emulsion vaccine on June 18 and June 26. Of these mares,
one was affected with encephalomyelitis on July 13, this being the first case
of encephalomyelitis in the 1938 epizootic. The foal of this mare likewise
developed encephalomyelitis, as did the foals of three others of these mares.
The incidence of infection in the vaccinated mares (one out of 14) was equally
as high as that observed in the unvaccinated mature stock. Three of the
four foals were from a cross with a Nonius stallion and one from a purebred
Belgian stallion.