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Equine Encephalomyelitis in India
Incidentally, about this time, American Veterinary opinion was forsak-
ing the time-honoured term " forage poisoning " in horses, in favour of the
term " equine encephalomyelitis ", it having been proved by Mayer, Howitt
and others [1931] that the common meningo-encephalitis of horses is in reality
a specific encephalomyelitis associated with a neurotropic virus. Somewhat
later, it was shown that two types of the disease exist in North America, the
Eastern and Western, each associated with an immunologically distinct
virus.
In India, Mosley, Heane and Shirlaw [1934] proved the existence of equine¬
encephalomyelitis and adduced some evidence to show that the disease bears
a distinct relationship to the form of equine encephalomyelitis described by
Moussu and Marchand in France [1924], a disease which, they said, was pro-
bably imported from America during the World War. Mosley, Heane and
Shirlaw [1934] expressed the opinion, based on pathological findings and ex-
perimental transmission in two horses, that the disease, in India, is probably
due to a neurotropic virus.
This revolutionary concept of the etiology of obscure nervous diseases
of animals was speedily succeeded by difficulties in interpretation of the syn-
drome and pathological feature. Following upon the acceptance of a virus
etiology, more attention came to be paid to pathological differences, while
variations in the type of virus readily became apparent.
Moussu and Marchand claimed that the French form of the disease posses-
sed no radical difference from Borna disease, an opinion which Nicolau and
Galloway promptly refuted on the grounds that, in Borna disease, the lesions
arè essentially confined to the central nervous system and intranuclear inclu-
sions, considered specific of Borna disease, are absent in the disease described
by Moussu and Marchand.
Accepting this opinion, the type of equine encephalomyelitis occurring
in America would appear to be an intermediary form. As in Borna disease,
lesions of the internal organs are either absent or inconspicuous. Intra-
nuclear inclusions are considered not to be a specific diagnostic feature of the
American form, although Hurst, in a study of the Western type describes the
presence of intranuclear inclusions in the nerve cells of the tissues which he
examined.
These differences have pointed to the probability that several types of
neurotropic virus exist, each tending to a general and similar syndrome,
influenced by common epizootic factors. There has been, moreover, a recent
tendency to consider that all neurotropic viruses are not essentially parasitic
on nerve cells. Certain of them, probably, while evincing a predilection for
nerve cells, are not entirely selective, and may invade the tissues of the internal
organs with resultant lesions therein. In other words, certain neurotropic
viruses possess organotropic affinities. More recently, it has been shown that
the viruses of pox and measles and vesicular stomatitis, previously considered
as dermatropic and organotropic, possess considerable neurotropic affinities.
It appears probable that certain of the equine encephalomyelitis viruses are
essentially neurotropic with organotropic affinities, and this probably accounts
for the variations in syndrome and pathological expression in a disease which
is fundamentally basic in type.