268
Forms of Babesia Gibsoni (Patton) in the Dog
GENERAL OBSERVATIONS
In the course of studies on the various forms of
the parasite found in the vertebrate host, it was
found that when the infective inoculum was intro-
duced by the intravenous route the average in-
cubation period with 55 dogs was eight days and a
half (range 2 to 29 days), while in 48 dogs which
received the inoculum subcutaneously, the average
was 18.5 days (range 9 to 40 days). This difference
in the period of incubation between the two groups
of animals is perhaps only natural, for in the intra-
venous method the parasites are brought directly
in contact with the circulation of the host, and
this facilitates their establishment in the red
blood cells earlier than by the subcutaneous route.
Another interesting feature noted was that in
the majority of cases the presence of a large num-
ber of parasites in the peripheral circulation was
not attended by a very high temperature. For
instance, about 30 per cent showed frequent para-
sites, 14 per cent numerous parasites and only 2
per cent very rare parasites in their peripheral
circulation when the temperature ranged between
100.8° F. to 104.0°F. On the other hand, when
the animal recorded the highest temperature, its
peripheral blood was found to be negative for
parasites in 25 per cent of cases. Thus, in this
particular behaviour also, this parasite appears
to differ from the other members of the genus
Babesia, in which we find the frequency of the
parasite increasing with the rise of the body
temperature. For example, it has often been
found that a large number of parasites in the peri-
pheral circulation was coincident with the high
thermal reaction of cattle infected with B. bige-
mina and dogs infected with B. canis. It may be
presumed from this characteristic behaviour of
P. gibsoni that, when the organism is undergoing
rapid multiplication in the erythrocytes of the
internal organs, such as the spleen and bone
marrow, there is a severe tissue reaction which
results in a relative increase in the body tempera-
ture in the absence or presence of few parasites in
the peripheral circulation. Moreover, as will be
seen later, the red blood corpuscles, which con-
tain the dividing individuals, apparently become
heavy and therefore are mechanically held up in
their respective places in the internal organs.
The parasites appear in large numbers in the peri-
pheral circulation only when the products of
multiplication individually enter into fresh red
blood corpuscles. However, by this time the
severity of the tissue reaction may have abated
and consequently have caused a relative decrease
in the body temperature.
In general practice, therefore, it is probably
advisable to examine the blood smears when the
animal shows both low and high temperatures.
MORPHOLOGY OF THE PARASITE
As will be seen from later remarks, the parasite
occurs in two distinct forms with a series of inter-
mediate forms, apparently representing its develop-
mental changes. These stages will be described in
what is believed to be their proper sequence,
although, in dealing with such a very minute orga-
nism, it is impossible to dogmatize in this respect.
(a) Ring forms. The common form of the para-
site in the peripheral circulation has the shape of
a signet ring, with the nucleus situated at the
periphery. In smears or sections of the internal
organs, particularly of the spleen and bone marrow,
rounded or oval or spindle-shaped forms are
encountered, with an increased amount of cytop-
lasm and often with their nuclei in a process of
division.
Prior to division the nucleus of a ring or oval
form, measuring 2.2µ to 4µ in diameter, shows a
comparatively heavy chromatin (Plate XX, fig. 28).
Later on, a bud-like structure appears in the proxi-
mity of the mother nucleus (Plate XX, fig. 29).
With further separation of this bud-like structure
from the mother nucleus, it presents a picture
illustrated in figs. 10, 17 and 18 on Plate XIX, and
fig. 33 on Plate XX. Finally, one-half of the
nuclear material thus divided, it is found to travel
towards the other pole. Occasionally, the connect-
ing filament, which stains pink or purple with May -
Grunwald-Giemsa's stain, is seen to persist for
sometime. Figs. 30 to 32 on Plate XX, and fig.
9 on Plate XIX, represent one, two and four
parasites in the process of binary fission. As divi-
sion progresses, the cytoplasm at the equatorial
region constricts and ultimately separates
(Plate XIX, fig. 17). The newly-formed daughter
individuals, as they retract, appear to remain
connected for sometime by means of a fine
strand of cytoplasm (Plate XIX, figs. 15 and 16).
This strand is often seen to be twisted until finally
it gives way. A cytoplasmic strand broken off
in this manner is frequently found to degenerate
or remain scattered in the cytoplasm of the eryth-
rocytes and appears as minute, irregular, blue-
tinged bodies in May-Grunwald-Giemsa prepara-
tions (Plate XIX, figs. 11, 13, 14, 16 and 17
and Plate XX, figs. 33, 34, 35, 38 and 40).
Until the final stage of division is reached, the
nucleus of a newly-formed individual is usually
placed in the centre of the organism, while its
body contains more cytoplasm than is usually
found in the ring forms met within the peripheral
circulation. In other words, the body now exhi-
bits a comparatively condensed and solid appear-
ance (Plate XIX, figs. 11, 12, 13 and 25 and Plate
XX, figs. 34 to 41). The completion of the process
of binary fission is marked by the migration of the
nucleus towards the circumference of the cytoplasm