The spore-formers (Class Sporozoa)
This class, like many others making up the phylum, is a dumping ground for probably unrelated families and orders. Its characters are based on the fact that all are well-adapted parasites. To this end, reproductive efficiency is exaggerated and structural complexity reduced. The same stages, though not the names, are found in trypanosomatids, some foraminifers, trematodes, and some cestodes, and are typical of alternation of generations in the lower plants. In the Sporozoa, as in the other cases in which it occurs, alternation of generation serves to increase the number of organisms by asexual reproduction.
Exchange and recombination of genetic material is achieved through occasional sexual reproduction. The life-history of a sporozoan species may be in a single host (monogenetic) or in two hosts (di-genetic). The names given to the various reproductive stages do not indicate where there is a change of host.
The life-cycle is as follows: sporozoites which are usually long and motile infect a host and change into the feeding form (trophozoite) which grows until it is able to undertake multiple fission (schizont stage); when fission is complete the daughter cells are liberated. Merozoites are often similar to sporozoites and can either become trophozoites or they can become sporonts which give rise to gametocytes; the latter may be large (macrogametocyte) or small (microgametocyte) and in turn change into macrogametes and micro-gametes respectively.
After fusion of the gametes, the zygote enlarges and develops a protective wall, the oocyst; inside the oocyst repeated division takes place to produce sporozoites. When the sporozoites are ripe the oocyst wall breaks down and the sporozoites are liberated, ready to start a new cycle.
Species of Gregarina are common parasites
Species of Gregarina are common parasites within the gut of invertebrates, especially annelids and insects. The cephaline gregarines have a large knob at the anterior end of the cell. A single cell is seen on the right of the photograph. At the onset of the sexual phase of the life-cycle, pairs of cells come together and are attached. This permanent pairing is called syzygy. Two such pairs are visible. One of each pair forms female gametes and the other forms male gametes.
In this group are placed various parasites with reproductive similarities. The trophozoite is large and feeds in the lumen of the host’s gut or between cells. Genetic exchange is achieved by a union of two sporonts (mature trophozoites) prior to gametogenesis. This union and the formation of gametocysts ensures that gametes can find each other. When gametes develop they do so in a gametocyst and fuse in pairs.
These are considered to be primitive gregarines because some species display schizogony, which is lost in the eugragarines. The trophozoites of Selenidium are found in the gut wall of various marine annelid worms and are about 80μ x 25μ. The elongate body has numerous longitudinal myonemes and the nucleus is situated anteriorly. The extreme anterior carries a knob-like process that serves to attach the parasite to its host’s cells. Very little is known about these common parasites.
The parasites of the two suborders making up this order are morphologically very different but neither displays asexual reproduction by means of schizogony. Numerical increase occurs by sporog-ony, that is, the development of tight sporozoites from each zygote, and also by the production of several to numerous gametes from each gametocyte.
Cephalines (Suborder Cephalina)
Many insects have these highly developed parasites as gut parasites. Species of Dummies and Tenebrio are often heavily infected. There are many species of the genus Gregarina. The body of the trophozoite (cephalont) which may be up to 400μ long, has a marked knob (epimerite) on the anterior tip and the remaining pan is two-jointed. The epimerite is lost when the trophozoite is mature and is no longer anchored by it to intestinal cells.
This stage (sporont) moves slowly in the gut contents and unites with another sporont. Whether these are symbionts or parasites and harm their hosts is not clear. In some species of Dermestes the proportion of beetles infected can be variable. No difference in health or reproductive capacity of the hosts is noted. Sometimes a host benefits from a symbiont (as in Trichogympluz) and sometimes it may be weakened, as with poorly-adjusted parasites. That neither advantage nor disadvantage is known in this instance indicates a condition of apparent neutrality. Transmission between hosts is by contamination of food with gametocysts containing mature sporocysts.
Acephalines (Suborder Acephalina)
As the name suggests these gregarines are without either epimerite or two-jointed body. The mature trophozoite (sporont) is large by comparison with other stages but seldom exceeds 200μ long. A typical genus is Monogsth which has its life-history in the seminal vesicles of earthworms. The sporont is elongate and has a small distention of the anterior which is used as an anchoring device. The life-history is similar to that of the cephalines and need not be repeated here.
A characteristic feature is the invasion ofa sperm mother cell of the host by a sporo-zoite at a time when the nuclei have just divided. The young trophozoite is intracellular and takes up a central position surrounded by the host’s sperm nuclei. The latter continues to develop and at a later stage the trophozoite is seen in sections or smears to be covered with sperm tails. Having grown as large as possible in this cell the parasite escapes and continues to feed between the cells of the seminal vesicles. Sporogony is started when two sporonts come together and form a single gametocyst around themselves. Infection is believed to be by the ingestion of soil infected with sporocysts.
This group is distinguished from the previous one by spending the whole of its trophozoite life intracellularly, and by effecting massive reproduc-tion with schizogony. There is only one major order which contains three suborders. These are mark-edly different from one another.
These are parasites of epithelial cells of inverte-brates and vertebrates, though variations do occur.
Suborder Adeleidea In this group the young gametocytes associate and during their development produce only a few gametes. This is, as already noted, an efficient fertilisation method. The life-history is monogcnetic in some genera but digenetic in others. Among the digenetic ones a variety of blood-sucking arthro-pods including tsetse flies, lice, mites and ticks act as the insect vector. In Haemogregarina sexual reproduc-tion occurs in leeches and asexual reproduction in turtles. All classes of vertebrates can serve as hosts for the different species. The parasites seem to be well adjusted to their hosts as these appear to remain healthy even though infected.
The parasites causing `coccidiosis’ in chickens illustrate the severity of maladjusted parasitism. They are distinguished from the previous suborder by having gametocytes that develop indepen-dently. Many wild birds and mammals pass cysts of coccidia and live normal lives. But poultry suffer heavy losses from outbreaks of this disease.
Eimeria tenella or E. matrix infections are usually fatal to chickens. Young birds become infected when they ingest cysts passed in the faeces of infected animals. The cysts hatch and liberate sporozoites which invade the wall of the caecum (one of a pair of blind sacs from the hind gut). Intracellular development proceeds and schizogony occurs in less than three days after infection. As many as nine hundred merozoites (3μ x 1µ) may be produced from one sporozoite.
Usually, these first-generation merozoites invade other cells and cause haemorrhage in the caeca. Second-generation merozoites appear at about six days and the bird is very sick. On the seventh and eighth days the second-generation merozoites have developed independently into macro- and micro-gametocysts and fusion of gametes occurs. The microgametes are liberated from the microgametocyte and swarm towards the macrogametes.
Only one macrogamete forms each macrogametocyte, which after fusion with a microgamete becomes an oocyst with a resistant wall. Development of four sporoblasts continues within the oocyst, each sporoblast forming its own protective wall. Two sporozoites form in each sporoblast. The birds die at 8-10 days unless kept under ideal conditions. If they survive the first attack some immunity may develop but death can occur at any age. It is normal practice in many countries to keep young chickens on wire grids to cut down the ingestion rate of faecal material and also to add traces of sulphonamids or a newer drug to the feed and thus suppress multiplica-tion of the initial stages.
Haemosporidians (Suborder Haemosporidia)
True digenetic intracellular blood parasites which cause malaria in man and related diseases in other mammals form this uniform group. The devasta-tion to human populations by malaria is even today very large. In the recent past, before drugs were used and the method of transmission known, this disease covered much of the world between 50′ north and south of the equator, and was one of the major causes of a stable population number despite man’s high reproductive potential. Only two hundred years ago ague (malaria) was prevalent in the marshy lands of Norfolk, England. At that time the climate was more continental, giving very hot summers and cold winters.
The disease was thus able to develop in the vector, anopheline mos-quitoes, in the summer and so spread from host to host, while it overwintered in man. The climate has changed now and the cooler summers are not favourable for the mosquito’s development. On a world-wide basis, over the years 1945 to 1960, there were 100 million cases per year, a reduction to one third of the annual number of cases reported before World War II. New tech-niques of control developed during and just after that war have been refined and applied on a world-wide scale by many nations with the aid of the World Health Organisation. Even so, as late as 1962, malaria was reported to disable more people than any other disease.
In the United States, malaria was formerly extensive geographically, reaching even into New England. The great bulk of cases, however, lay in the South. A survey made in 1916 and 1917, of over 31,000 people in Mississippi, determined that almost half of the population had detectable malaria parasites in the blood, a history of attacks of malaria fever during the twelve months prior to the survey or both. Even as late as 1938 in the United States, it was estimated that there were three to four million cases annually. Today, malaria is almost non-existent in the United States. Mosquitoes do not become infective until seven to twenty days after taking an infected blood meal.
The time-lapse depends on temperature: lower temperature prolongs development to twenty days. An infection in man is initiated by sporozoites which are inoculated together with anticoagulants by the mosquito prior to sucking up the blood. The sporozoites migrate into the main circulation and enter or are ingested by endothelial cells of the liver. Here each one divides repeatedly to form a large multinucleate pre-erythrocytic form. When mature, at about six days, this liberates a thousand or more merozoites which can infect red blood cells.
Thereafter the schizogony cycle is mostly in the red cells and occurs repeatedly. The bouts of fever, which occur every two or three days depending on the species, coincide with the liberation of merozoites from blood cells. Some of these merozoites enter a long tissue phase similar to the pre-erythrocytic forms in the liver cells (this is the exo-erythrocytic cycle) while others form gametocytes and continue to circulate in the blood. Gametocytes are taken up in a blood meal and pass into the stomach of the mosquito. The liberation of microgametes and fusion takes place.
The motile zygote invades the gut wall and develops an oocyst. This divides repeatedly and forms thousands of sporozoites which are freed when the oocyst ruptures. The sporozoites then migrate to the salivary glands for injection just prior to the next blood meal. Relapses after self- or drug-cure of malaria are due to the exo-erythrocytic forms again enter-ing active schizogony and infecting the red cells. Other haemosporidian include Haentoprokus (found in birds and reptiles) and Babesia, a species of which causes Texas cattle fever. Haemoproteus is carried by blood-sucking flies, Babesia by ticks.