Toxoplasma (syn. Isospora) gondii belongs to the heteroxenous cyst-forming Coccidia, an order of Sporozoa syn. Apicomplexa. The latter are characterized by a complex of organelles consisting of a conoid, polar annulus, rhoptries and micronemes. Cell division takes place inside the mother cell. Thereby, two new apical complexes develop synchronously, close to the dividing nucleus: Endodyogenia (fig. 2.59, page 121). In the case of multiple divisions, paired apical complexes are finally formed after extra- or intranuclear merogonia (division of chromosomes with or without the division of the nucleus, respectively): typical Merogonia and Endopolygenia, respectively.
Life cycle of Toxoplasma gondii (fig.2.60, page 124): the parasite moves between two warmblooded vertebrates. Both acquire the parasite per os, i.e. by feeding: the definitive host (DH), e.g. the cat or felid, as a carnivore via its prey, the intermediate hosts (IHs), e.g. mice, as herbivores by contaminated food. Coprophagous insects mediate as transporting hosts. As soon as the IH falls prey to an immunologically naïve carnivorous DH, it may be e.g. a canid, the latter becomes a facultative secondary IH. Even a cat that has played the part of a DH can later become an IH by picking up sporozoites.
Development in the felid DH: after the consumption of a parasitized IH, several merogonias inside the epithelial cells of the ileum are followed by a single entero-epithelial gamogonia leading to an oocyst (zygotocyst) inside of which the sporogonia takes place at the open air i.e. after excretion of the oocyst by the faeces.
Development in the herbivorous IH: two cycles of extraintestinal merogonias take place. In the first cycle, a rapidly repeating proliferative phase occurs inside macrophages in the lamina propria to produce endozoites, which are also called tachyzoites. Thereafter, the mobile stages (trophozoites) reach the brain, muscles, liver, spleen and eyes via circulating blood cells. During the following cystic phase, numerous cystozoites (also called bradyzoites) are produced. They remain in diapause for many years. Additionally, in the case of a pregnant sheep, trophozoites may be transmitted to its foetus via the placenta to the foetus or via milk to the sucking lamp. In regions where the lynx has been newly released, it hunts lambs in addition to small mammals, indicating that the lynx was probably the original DH in northern temperate zones.
Infections by Toxoplasma gondii unequivocally impair the IHs with respect to their behaviour but are symptomatically negligible. Thus, a nearly unlimited reservoir of inapparently affected warmblooded animals will be created. In the DH, the parasite persists for only a limited time after a primary infection. However, on being faced with cystozoites of T. gondii once again, the DH changes its immune reactions to those of an IH.
Man is affected as an IH after eating pork or, more rarely, beef. As soon as the acute symptoms disappear, permanent neurological alterations (retarded reaction time, risky behaviour) are observed. An infection acquired during childhood sometimes causes central nervous failures.
Human toxoplasmosis is a zoonosis and Man can be regarded as a misplaced host. For the diaplacental infection of the foetus, the mother has to suffer a primary infection. The third trimester of pregnancy is a particularly risky period. Without treatment, the foetus is affected in 50% of cases and alterations of the retina, hydrocephalus and mental disorders are to expect. Tests of the course of serological reactions allows an estimation of individual risk. An automatic test is capable of detecting primary infection during the proliferating phase (2-4 weeks p.i.) by using recombinant antigens of tachyzoites. Chemotherapy does not injure the foetus. In Europe, 70%-80% of the population is expected to be seropositive by the 6th decade of life.
Sarcosporidians parasitize mammals (including seals and whales odontoceti and mystacoceti), birds and reptiles as DH and as IH. They belong to the genus Sarcocystis of about 130 species and to the genera Besnoitia, Frenkelia and Isospora (including Toxoplasma gondii), the cysts of which developing in nervous tissue, muscles and other organs.
Life cycle of Sarcocystis spec..(fig. 2.62, page 128): after the infection of a carnivorous DH (man, dog, cat, etc.), gamogonia starts immediately either enteroepthelially in the intestinal cells or subepithelially in the lamina propria. The course of the infection in the DH runs subacutely or pathogenously or can even be fatal according to the received dosage of endozoites. The subsequent sporogonia mostly occurs in the tissues of the DH. Fully sporulized oocysts (zygotocysts) are delivered with four sporozoites, each which are infective. Herbivores can become infected by eating contaminated food and carnivorous animals feeding on DHs may also become infected. Man is an IH for Sarcocystis lindemanni, the associated DH is unknown. Tissue cysts of Sarcosporidians develop within the IH in different organs according to the genus of the parasite. Once inside a parasitophorous vacuole, metrocytes develop located in the cyst wall and continously create endozoites by multiple endodyogenia. Their masses fill the chambers of the cyst, the latter growing to a size that is macroscopically easy to recognize. The wall of the tissue cyst consists of an inner primary cover produced by the parasite. Inwardly, it produces the basic substance in which the metrocytes are imbedded. Outwardly, the cyst develops characteristic genus-specific and species-specific wall structures (recognizable at the electron microscopic level), thereby increasing its surface area. The primary cover is sometimes surrounded by a double-layered secondary cover, a lammellar syncytium developed by the neighbouring host cells.
The developmental translocation of the sporogonia into the lamina propria of the DH allows Sarcosporidians to colonize almost all warmblooded animals, even those living in the open sea. Hosts, such as seals, penguins and whales, presumably acquired their parasites during their former terrestrial life and retained them despite their transition to a marine life. The relevant transporting hosts today may be pelagial crustaceans and shoal fishes (fig. 2.61, page 127).
The heteroxenous life cycle between two trophically combined warmblooded animals requires specific molecular recognition of the respective host cell (e.g. intestinal epithel cell of the felid versus those of all other warmblooded animals). This stimulates a corresponding developmental programme. The evolutionary selective advantages can be assessed at three levels (Box 2.4, page 120):
1. Genetic level: Nonsexual multiplication of the genome in the nonspecific IH provides an huge substrate for mutations and genetically divergent clones are accumulated and/or lodge together within one host individual. Sexual propagation in the specific DH enables the genetic recombination of novel mutations.
2. Parasite-host interaction (individual level): The lifelong diapause makes the IH a reservoir host. The short timed Infection of the DH accompanied by the changed immune reaction initiates an alternative parasite-host relationship regulating the density of the DH population (see chapt. 8.1.1 Alternativstrategen).
3. Ecological level: The alteration of the behaviour of the IH conditions (see below) it to become prey. Diaplacental and lactogen transmission compensates for the age drift of the parasite. The fitness of the DH is impaired and its progeny is decimated. As a result, both the physical and the reproductive superiority of the predator are down-regulated.
Sarcosporidians stabilize a hunter-prey relationship by equilibrating the ecological balance between the carnivorous DHs and the herbivorous IHs, whereas the parasites control their own propagation (as an example, the balance between lions and gnus is given in fig. 2.58, page 119). In view of the ubiquitous distribution of sarcosporidians, the dynamic ecological balance induced by them and by other Apicomplexa constitutes an outstanding factor for the evolution of each member of that threefold partnership. Thus, ecolocical balancing may be regarded as a general feature of parasitism.