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Genetics of Viruses Pathogenic to Animals
of living cells. Some viruses are infective for
susceptible animals in very high dilutions.
They are titratable and the minimum infect-
ive dose can be determined by tests on suit-
able laboratory animals according to standard-
ized technique. When an animal or a living
chick embryo is infected with 1 to 10 minimal
doses, with ensuing fatal results, it is found
that the virus has reproduced itself in relatively
enormous quantities, a small fragment of
tissue yielding billions of infective doses.
Perpetuation of the virus and its multiplica-
tion at each successive animal passage can be
carried forward at will for unnumbered genera-
tions or passages. Animals which have re-
covered from a virus disease or which have
survived experimental infection with a virus
of modified or attenuated activity are more or
less immune to subsequent infection with the
same virus. In such cases the serum may
contain antiviral bodies capable of neutralizing
the activity of the corresponding virus in vivo
and in vitro. A striking feature of many of
the viruses is their power of adaptation to
foreign hosts and changed environment. Evi-
dence, so briefly indicated in the foregoing, of
reproduction, multiplication, adaptation and
the development of anti-viral bodies and
immuno-biological reactions—corresponding to
similar evidence in respect to living bacteria—
forms the basis of belief in the living nature
and particulate state of viruses from animal
sources. But no virus from such sources has
yet been obtained in a pure state or dissociat-
ed from its co-existence with and dependency
upon the complex proteins and molecules of
living tissue cells. Until that has come about
by cultivation in artificial media in the absence
of living matter and more exact criteria are
available for determining the size structure and
physical properties of viruses, their origin,
nature and mode of development lie in obscurity
and are open to doubt and question, and to
new ideas and conceptions.
Certain virus diseases are associated with
the formation of bodies generally referred to
as cell inclusions, or virus bodies, occurring in
the cell cytoplasm or in the cell nucleus or in
both cytoplasm and nucleus. They are more
or less pleomorphic, of various kinds and types,
differently situated within the cell and pecu-
liar to certain types of cell. Among the best
known examples there may be mentioned (1)
negri bodies, occurring within the ganglion
cells of the brain, particularly in Ammon's
horn of animals affected with rabies: (2)
Guarnieri bodies, within the epidermal cells in
lesions of vaccinia and sheep pox; and (3)
Bollinger bodies, within the epidermal cells of
lesions of fowl pox. The nature, significance
and relationship of these and other like inclu-
sion bodies to the specific virus with which they
are associated is a much discussed question.
Their presence or absence may be of much
diagnostic importance
Some authorities are
inclined to believe that the inclusion bodies and
the virus are one and the same, or that the
inclusions are bodies which produce the virus.
Others view the inclusions as degeneration
products of cells attacked by virus. They are
not to be observed in all virus diseases but, in
fact, only in the minority. They are found
mostly in association with neurotropic and
dermotropic viruses; and but rarely and doubt-
fully in typical septicaemic and viscerotropic
virus diseases.
Selective affinity for one of the body tissues
of its host is a striking characteristic of certain
viruses as, for example, the neurotropic viruses
of rabies, equine encephalomyelitis and loup-
ing-ill of sheep, and the dermotropic viruses of
pox diseases. In these the viruses appear to
multiply only in the tissues attacked: the
brain and spinal cord, the skin and mucous
membranes, according to the selective affin-
ities of the respective viruses. On the other
hand, in the virus diseases that occur as acute
generalized infections, such as hogcholera,
South African horse-sickness, infectious equine
anaemia, distemper and influenza, the causat-
ive viruses multiply in the circulating blood
and viscera and are there found in greatest
concentration.
There do not appear to be any significant
differences between virus and bacterial diseases
in the manner of their occurrence and their
wide range and variability in respect to conta-
giousness, infectivity, points of attack, clinical
manifestations, selectivity and immunity. As
regards allergy and hypersensitivity to virus
disease, little, if anything, is known and the
question does not appear to have been explored
owing, probably, to the difficulties in preparing
suitable test reagents.
Ample evidence has been produced to show
that a specific virus disease can be caused by
strains or types of virus immunologically dis¬
tinct. This is well recognized and exempli
fied in foot-and-mouth disease which is caused
by at least three virus strains. in South African
horse-sickness with its six or more virus strains
and in equine encephalomyelitis with its so
called Eastern and Western types or strains.