1.
A
brief
review
is
given
of
the
epizootiology
of
horse
sickness,
the
antigenic
plurality
of
the
virus
strains,
and
the
history
of
the
control
of
the
disease
by
immunization.
2.
It is
often
not possible
to
isolate
virus
directly
in
mice
from
cases
of
horse sickness
in
immunized
horses.
3.
The
usefulness
of
the
ferret,
and
possibly
the
dog,
for
the
isolation
of
virus
from
such
horses
was
demonstrated.
4.
Immunological
studies
on 42
mouse-adapted horsesickness
virus strains
were
conducted.
5.
For
these
studies
an intracerebral
neutralization
test
in
mice
was
used.
Hyperimmune
sera
from
rabbits
were
mainly
used
in
the
tests.
6.
Cross-neutralization
tests
with
rabbit
antisera
were
carried
out
on
the
eight
virus
strains
included
in
the
present
vaccine
issued
from
Onderstepoort
and
16
strains
recently
isolated
from
cases
of
horsesickness
in
immunized
horses.
7.
This
series
of
tests
showed
that
the
virus
strains
could
be
grouped
into seven
immunological
types.
It
was
also
evident
that
some
immunized
horses
become
infected
with
strains
of
the
same
immunological
type
as
the
vaccine
strains.
8.
Using
type
rabbit
antisera
a
further
series
of
neutralization
tests
was
conducted
on
18
other
horsesickness
virus
strains.
These
tests
showed
that
these
strains
also
could
be
grouped
within
the
same
immunological
types.
9.
It
was
shown
that
four
recently
isolated
strains
belonging
to
the
same
immunological
type
were
not
represented
in
the
present
Onderstepoort
vaccine.
10.
A
limited
number
of
neutralization tests
with
ferret antisera
supported
the
antigenic
grouping
as
revealed
by
the
tests
with
rabbit
antisera.
11.
A
serological
study
of
sera
obtained
from
eight
immunized
horses
reacting
to
horsesickness
was
made.
12.
Viruses
isolated
from
each
of
these
horses
were
included
in
the
cross-neutralization
tests
which
showed
that
seven
of
these
viruses
are
of
the
same
immunological
type
as the
vaccine
strains.
13.
High
level
antibody
against
all
the
vaccine
strains
was
shown
to
be
present
in
sera
from
seven
horses.
14.
It
was
concluded
that
these
particular
failures
in
immunity
were
not
due
to
inadequate
immunization
but
were
apparently
the
result
of
slight
antigenic
differences
between
the
infecting
virus
and
the
vaccine
strains.
15.
The
significance
of
the
antigenic
grouping
based
on
the
mouse
neutralization
test
to
immunity
in
equidae
is
discussed.
16.
It
is
believed
that
this
antigenic
grouping
has
a
definite
relationship
to
immunity
in
these
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