NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
62
The eventual fate of the zygote is to break up into sporozoits, though owing to
certain conditions shortly to be described the nature of this process often becomes
obscured. In a previous note (185) I have described the formation of certain
bodies which I called "sporoblasts " from fission of a single parent form and
another cycle characterised by large " sporulating" or " rosette " forms.
Further investigation has shewn that the appearances seen were due merely to
variations in the method of breaking up of the zygote.
In some cases the zygote breaks up in a very regular fashion, numbers of
sporoblasts being arranged in a rosette (Pl. II, fig. 24). This may go on to the
formation of sporozoits in which case an immense number of small forms are
arranged as in Pl. II, fig. 26.
More commonly the zygote forms a loose group of very characteristic
"sporoblasts" (Pl. II, fig. 29) and these later form " sporozoits." The
sporoblasts are extremely characteristic of hereditary development in the larva,
and of stage to stage development in the tissues of the gorged nymph. Either
by their own movements or by division of the cell in which they lie they come
to be disseminated through the embryonic tissue cells, but whether their growth
continues after separation from the zygote is uncertain. The chromatin of the
newly formed sporoblasts consists of a single or of two or more masses with
filamentous extensions such as are seen in piroplasma in the blood. These
extensions ultimately point to various portions of the periphery where new
chromatin masses are formed. Fission then takes place and the sporoblast
becomes a number of sporozoits.
In the salivary glands of nymphs some comparatively large, very regular
pear-shaped forms occur possessing the reticular extension. Whether these are
sporozoits or whether they are again liable to division I have not been able with
certainty to ascertain, but the latter appears probable since the ultimate products
of division have only a single chromatin mass. Whatever their nature they
are capable of giving rise to infection in the dog. On the whole it is perhaps
at present wise not to be too dogmatic regarding the significance of the imma-
ture products of the division of the zygote, it being certain that the ultimate
products are the bodies here termed sporozoits.
Sporozoits.
These are small bodies about the size and having the general appear-
ance of some of the forms of piroplasma in the blood.* They may be pear-
shaped or circular, and under some circumstances they are seen with what
appear to be pseudopodia extended (Pl. III, fig. 8). The protoplasm stains a
* In smears of the contents of nymphs it is easy to mistake these for blood forms if, as often
happens, they become disseminated through the specimen. ( Vide Pl. II, figs. 26, 27, and Pl. III, fig. 8.)
63
pale blue and often has one or two non-refractile vacuoles. The chromatin
mass is single without any extension. It is situated most frequently more or
less centrally, but may be at or near the periphery. Star-shaped or otherwise
irregular forms have a similar structure.
The number of forms resulting from a single oocyst is very considerable,
but for reasons to be given later they are rarely found all in the same situation,
but infiltrate as it were the tissues (Pl. III, figs. 4 and 5).
Relation of the Parasite to the Tissues.
In the hereditarily infected larva sporoblasts are found distributed among
the cells, but the exact relation which the parasite bears to the tissues has not
yet been worked out, both the egg and the larva, being difficult to manipulate
satisfactorily. Fortunately much more has been ascertained regarding the
passage of the parasite from nymph to adult.
In the nymph we have seen that parasites taken in develop into club-shaped
bodies. These after leaving the gut penetrate the tissues in all directions and
enter cells. The cells chosen are as a rule cells of the embryonic tissue, which
is by this time forming everywhere in the body of the gorged nymph, and which
I have referred to as forming within the cuticle of the nymph a rough model of
the future adult tick. Not infrequently, however, the cells of the gut itself
appear to be invaded and the parasite proceeds to develop in this position.
A peculiarity of the processes of cell invasion is the small amount of change
set up in the victimised cell. Even when a parasite has almost replaced the
protoplasm of a cell with swarms of young parasites the external contour and
general appearance of the cell is retained. Such cells may quite well be mis-
taken by one unfamiliar with the appearances for normal tissue cells containing
granules of chromatin-like substance such as some normal cells in the tick do,
but they have only to be ruptured in the preparation of the specimens to demon-
strate at once the parasitic nature of their contents.
We have previously seen that any given cell of the embryonic tissue at the
base of the limbs stands a chance of becoming the ancestor of cells of one or
more salivary acini, and the presence in such a cell of a zygote does not prevent
it from carrying out its appointed task. The result is that so soon as the
sporoblasts are formed they become separated by the repeated cell division and
when in their turn the sporozoits are formed these are disseminated throughout a
number of cells some or all of which end by being functionally active salivary
cells.
Though this is the history of a large number of the parasites there are
undoubtedly a great many club-shaped bodies which in the first place select cells,
not the progenitors of salivary tissue. Zygotes and the products of these have
64
been seen in the embryonic cells of the cuticle, in tracheal and in fat cells. A
very common situation for swarms of sporoblasts or sporozoits is certain cells
lying about the muscle bundles (Pl. III, fig. 13). It seems impossible to believe
that all these parasites perish and it is probable that by some means they
eventually reach the salivary gland as do those of the malarial parasite in the
case of the mosquito. For some time after the adult tick has hatched there is
still salivary tissue in process of development, and the parasite must have ample
time to reach this before the tick leaves its host. That both sporoblasts and
sporozoits have the power of independently invading cells is shewn by the
frequency with which appearances like those shewn in Pl. III, figs. 11 and 12,
are met with.
SUMMARY AND CONCLUSIONS.
In order to make quite clear what has just gone before it may be advantage-
ous to summarise the main outlines of the course of development in the tick.
(1) In R. sanguineus there are two means by which infection is transmitted.
(a) Hereditarily through the egg. A method shewn both by experimental
infection of dogs and observation of the parasite in the tick.
(b) Stage to stage infection. Not yet proved by experimental infection
but practically certain from observations upon the parasite.
(2) In both methods of infection the parasite goes through the same cycle
of development becoming in turn a club-shaped body and then a zygote which
breaks up into "sporoblasts" and these again into "sporozoits."
(3) In hereditary infection club-shaped bodies originating each from a single
parasite penetrate the ova either in the ovary or in their passage down the oviduct
and in the yelk become zygotes. In the larva the zygotes have broken up into
sporoblasts which are found disseminated in the tissues and in the nymphs the
sporozoits have accumulated in large numbers in the salivary glands.
(4) In infection from nymph to adult the club-shaped bodies after being
formed in the gut of the nymph penetrate cells of the embryonic tissue which will
eventually form the adult, and embedded in the cells of this they become zygotes.
The sporozoits derived from these zygotes may find themselves without any
action on their part in salivary cells or they may be situated elsewhere, in which
case they probably reach the salivary cells by their own movements, possibly
aided by the circulation of fluids in the tick.
The details of development strongly suggest a cycle of a sexual nature, and if
this be the case the sexual cycle of piroplasma has many points in common with
the sexual cycle of the malarial parasites and proteosoma. This has been already
pre-supposed by the nomenclature employed which it seems reasonable to use
until further research either confirms or shews it to be untenable.
65
The greatest difference between the development and that of the malarial
parasite occurs in the peculiar dissemination of the sporoblasts and sporozoits
and the fact that the ookinete (?) comes to rest not in the gut wall but in the tissues.
That the malarial zygote has no true wall of its own has been already supposed
by Grassi and in this the zygote of piroplasma would bear it a resemblance. The
separation of sporoblasts by the growth of the embryonic tissue and possibly by
the movements of the sporoblasts themselves and the infiltrating action of piro-
plasma have so far as I know no parallel in malaria or other of the pathogenic
protozoa. The sporozoits except that they have not the filiform shape of those
of malarial parasites seem to correspond exactly to these and their eventual
location in the salivary acini is exactly parallel to conditions in malaria. Though
it is clear much has still to be done in following out details there can be no
question that in the main the mystery surrounding the passage of piroplasma
through the tick has been solved.
67
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S c h a u d i n n , 1 9 0 4 . —
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Translation in Jour. of Trop. Medicine, Vol. VII, 1904.
1 1 .
S t a r c o v i c i , 1 8 9 3 . —
B e m e r k u n g e n ü b e r d e n d u r c h B a b e s e n t d e c k t e n B l u t p a r a s i t e n
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68.
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22
69
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71
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