Protozoa Subclass Coccidiomo
This group is distinguished from the previous one by spending the whole of its trophozoite life intracellularly, and by effecting massive reproduction with schizogony. There is only one major order which contains three suborders. These are markedly different from one another.
These are parasites of epithelial cells of invertebrates and vertebrates, though variations do occur.
In this group the young gametocytes associate and during their development produce only a few gametes. This is, as already noted, an efficient fertilization method. The life history is monogenetic in some genera but digenetic in others.
Among the digenetic ones, a variety of blood-sucking arthropod pods including tsetse flies, lice, mites, and ticks act as the insect vector. In Haemogregarina sexual reproduction 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 independently. Many wild birds and mammals pass cysts of coccidia and live normal lives. But poultry suffers heavy losses from outbreaks of this disease.
Eimeria tenella or E. necatrix infections are usually fatal to chickens. Young birds become infected when they ingest cysts passed in the feces 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 hindgut). In intracellular development proceeds and schizogony occurs less than three days after infection.
As many as nine hundred merozoites (3ux lu) may be produced from one sporozoite. Usually, these first-generation merozoites invade other cells and cause hemorrhage 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-gametocytes 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. The 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 fecal material and also to add traces of sulphonamides or a
newer drug to the feed and thus suppress the multiplication of the initial stages.
True digenetic intracellular blood parasites which cause malaria in man and related diseases in other mammals form this uniform group.
The devastation 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 mosquitoes, 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 favorable for the mosquito’s development.
On a worldwide 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 techniques of control developed during and just after that war have been refined and applied on a worldwide 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 anticoagulant by the mosquito prior to sucking up 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.