broadly speaking, and also Halteridium) leave one corpuscle after a short time, wander about free in the plasma, and then seek out another; and this may be repeated until the parasite is ready for schizogony, which generally occurs in the corpuscle.
As in the case of Trypanosomes (q.v.), normally—that is to say, when in an accustomed, tolerant host, and under natural conditions—Haemosporidia are non-pathogenic and do not give rise to any ill-effects in the animals harbouring them. When, however, the parasites gain an entry into the blood of man or other unadapted animals,[1] they produce, as is well known, harmful and often very serious effects. There are three recognized types of malarial fever, each caused by a distinct form and characterized by the mode of manifestation. Two, the so-called benign fevers, are intermittent; namely, tertian and quartan fever, in which the fever recurs every second and third day respectively. This is due to the fact that schizogony takes different lengths of time in the two cases, 48 hours in the one, 72 in the other; the height of the fever-period coincides with the break-down of the corpuscle at the completion of the process, and the liberation of great numbers of merozoites in the blood. The third type is the dangerous aestivo-autumnal or pernicious malaria, in which the fever is irregular or continuous during long periods.
A very general symptom is anaemia, which is sometimes present to a marked extent, when it may lead to a fatal termination. This is the result of the very considerable destruction of the blood-corpuscles which takes place, the haemoglobin of which is absorbed by the parasites as nutriment. A universal feature connected with this mode of nutrition is the production, in the cytoplasm of the parasite, of a brown pigment, termed melanin; this does not represent reserve material, but is an excreted by-product derived from the haemoglobin. These pigment-grains are at length liberated into the blood-stream and become deposited in the various organs, spleen, liver, kidneys, brain, causing pronounced pigmentation.
Another type of fever, more acute and more generally fatal, is that produced by forms belonging to the genus Piroplasma, in cattle, dogs, horses and other domestic animals in different regions of the globe; and recently Wilson and Chowning have stated that the “spotted fever of the Rockies” is a human piroplasmosis caused by P. hominis. The disease of cattle is known variously as Texas-fever, Tristeza, Red-water, Southern cattle-fever, &c. In this type of illness the endogenous multiplication of the parasites is very great and rapid, and brings about an enormous diminution in the number of healthy red blood corpuscles. Their sudden destruction results in the liberation of large quantities of haemoglobin in the plasma, which turns deep-red in colour; and hence haemoglobinuria, which occurs only rarely in malaria, is a constant symptom in piroplasmosis.
The parasite of pernicious malaria, here termed Laverania
malariae, will serve very well as a type of the general life-cycle
(fig. 1). Slight differences shown by the other malarial parasites
(Plasmodium) will be mentioned in passing, but the
main divergences which other Haemosporidian types
Example
of the life-history.
exhibit are best considered separately. With the bite
of an infected mosquito, the minute sickle-like sporozoites
are injected into the blood. They rapidly penetrate into
the blood corpuscles, in which they appear as small irregular,
more or less amoeboid trophozoites. A vacuole next arises in
the cytoplasm, which increases greatly in size, and gives rise to
the well-known, much discussed ring-form of the parasite, in
which it resembles a signet-ring, the nucleus forming a little
thickening to one side. Some authorities (e.g. Argutinsky) have
regarded this structure as being really a greatly distended
vesicular nucleus, and, to a large extent, indeed, an artifact,
resulting from imperfect fixation; but Schaudinn considers it is
a true vacuole, and explains it on the ground of the rapid nutrition
and growth. Later on this vacuole disappears, and the grains
of pigment make their appearance. The trophozoite is now
large and full-grown, and has become rounded and ready for
schizogony. The nucleus of the schizont divides several times
(more or less directly, by simple or multiple fission) to form a
number of daughter-nuclei, which take up a regular position
near the periphery. Around these the cytoplasm becomes segmented,
giving rise to the well-known corps en rosace. Eventually
the merozoites, in the form of little round uninuclear bodies,
are liberated from the now broken-down corpuscle, leaving behind
a certain amount of residual cytoplasm containing the pigment
grains. Besides the difference in the time taken by the complete
process of schizogony in the various species (see above), there are
distinctions in the composition of the rosettes. Thus, in Laverania,
the number of merozoites formed is very variable; in
Plasmodium vivax (the tertian parasite) there are only few (9 to 12)
merozoites, but in P. malariae (the quartan form) they are more
numerous, from 12 to 24. The liberated merozoites proceed to
infect fresh blood corpuscles and a new endogenous cycle is
started.
After asexual multiplication has gone on for some time, sexual forms become developed. According to Schaudinn, the stimulus which determines the production of gametocytes instead of schizonts is the reaction of the host (at the height of a fever period) upon the parasites. A young trophozoite which is becoming a gametocyte is distinguished from one which gives rise to a schizont by its much slower rate of growth, and the absence of any vacuoles in its cytoplasm. The gametocytes themselves are characterized by their peculiar shape, like that of a sausage, whence they are very generally known as “crescents.” Male and female gametocytes are distinguished (roughly) by the arrangement of the pigment-grains; in the former, they are fairly evenly scattered throughout the cytoplasm, but in the megagametocytes the pigment tends to be aggregated centrally, around the nucleus. As they become full-grown and mature, however, the gametocytes lose their crescentic form and assume that of an oval, and finally of a sphere. At the same time, they are set free from the remains of the blood corpuscle. The spherical stage is practically the limit of development in the Vertebrate host, although, sometimes, the nucleus of the microgametocyte may proceed to division. The “crescents” of the pernicious parasite afford a very important diagnostic difference from the gametocytes of both species of Plasmodium, which have the ordinary, rounded shape of the schizonts. In the case of the latter, points such as their slower growth, their less amoeboid character, and their size furnish the means of distinction.
When a gnat or mosquito sucks blood, all phases of the parasite in the peripheral circulation at that point may succeed in passing into the insect. If this occurs all trophic and schizogonic phases are forthwith digested, and the survival of the sexual phases depends entirely upon whether the insect is a gnat or mosquito. Only in the latter case can further development of the gametocytes go on; in other words, only the genus Anopheles, and not the genus Culex, furnishes specific hosts for the malarial parasites. This is a biological fact of considerable importance in connexion with the prophylactic measures against malaria. In the stomach of an Anopheles, the gametocytes quickly proceed to gamete-formation. The nucleus of the microgametocyte divides up, and the daughter-nuclei pass to the periphery. The surface of the body grows out into long, whip-like processes, of which there are usually 6 to 8 (probably the typical number is 8); each is very motile, in this respect strongly resembling a flagellum. This phase may also develop in drawn blood, which has, of course, become suddenly cooled by the exposure; and it seems evident that it is the change in temperature, from the warm to the cold-blooded host, which brings about the development of the actual sexual elements. Earlier observers regarded the phase just described as representing another parasite altogether, of a Flagellate nature—whence the well-known term, Polymitus-form; and even more recent workers, such as Labbé who connected it with the malarial parasite,
- ↑ For an interesting account of the biological relations between parasites and their hosts, and the penalty Man pays for his roving propensities, the reader should see Lankester’s article in the Quarterly Review, July 1904.
