Apicomplexa, also called sporozoans, are unicellular parasites with nuclei whose reproduction alternates between asexual schizogony and sporozoites resulting from sexual fusion of gametes. As a rule, the alternation is associated with a host change typical of apicomplexa. The best-known representatives of apicomplexa belonging to the eukaryotes are Plasmodia (causative agent of malaria) and Toxoplasma gondii (causative agent of toxoplasmosis).
What are apicomplexa?
Apicomplexa, are unicellular parasites with a nucleus, so they are classified as eukaryotes. They owe their name to the so-called apical complex, which is common to all apicomplexa and enables the parasites to penetrate the membranes of cells to allow their so-called rhoptria, small cell organelles, to empty their lytic enzymes and some kinases into the cytoplasm of the attacked cell. Apicomplexa feed via micropores of their complex enveloping membrane rather than via phagocytosis. Although apicomplexa have lost their cilia and flagella during evolution, they can easily meander and glide. A characteristic of Apicomplexa is their host switching, which is usually associated with a change from asexual to sexual reproduction. In some species, host switching is less spectacular and occurs from vertebrate to vertebrate. A spectacular host switch between the mosquito Anopheles and humans is accomplished by four different species of plasmodia, the causative agents of malaria. In asexual reproduction, division of the nuclei and subsequent cell divisions give rise to up to 4 merozoites from each cell, some of which further develop into male microgametes and female macrogametes. Two gametes each unite after a host change and, after meiosis and further mitoses, form a large number of infectious sporozoites that grow into oocysts.
Occurrence, distribution, and characteristics
In all likelihood, apicomplexa evolved into obligate intracellular or extracellular parasites during evolution. Evolution resulted in the loss of their original cilia or flagella, which they no longer urgently need in their present mode of life, since they no longer need to fan themselves with food and the need for active locomotion is largely eliminated. Plasmodia living intracellularly in erythrocytes are literally surrounded by numerous nutrients in the cytoplasm, which they only have to take up via the micropores of their cell membrane. Most representatives of the Apicomplexa live extracellularly in body cavities of their hosts. In most cases, this is the intestinal tract. Infectious sporozoites, which mark the beginning of an infection and development cycle, are excreted in the feces and can survive an up to two-year “waiting phase” unharmed even in the soil. Sporozoites of Apicomplexa are therefore ubiquitous in almost all climates. The situation is somewhat different for plasmodia living intracellularly in erythrocytes. They depend on host switching from Anopheles mosquitoes to humans or other vertebrates, so that infection can occur exclusively through the proboscis of an infected mosquito, with a few clinical exceptions. The mosquito’s saliva contains sporozoites that settle in the liver tissue where they begin to multiply. Later, they return to the blood and migrate into erythrocytes, where they undergo characteristic further development, depending on the type of plasmodia. Some of the resulting merozoites further differentiate into male microgametes and female macrogametes, which can be ingested by a female Anopheles mosquito and again develop infectious sporozoites in the mosquito in a sexual reproductive process within oocysts. Infection is therefore limited to regions where Anopheles lives. Exceptions are airports and ports, where introduced infected Anopheles mosquitoes may transmit malaria for a short period of time. The most significant apicomplexa are coccidia, Toxoplasma gondii, and the plasmodia described above. Intracellular coccidia predominantly colonize the gastrointestinal tract of many vertebrates and cause coccidiosis, which usually has a mild course with diarrhea and similar symptoms. Toxoplasma gondii, the causative agent of toxoplasmosis, lives intracellularly and prefers cells of the intestinal epithelium.One of the main routes of infection is domestic cats, which can be infected by mice, for example, and can also infect humans in close contact.
Diseases and complaints
As causative agents of coccidial infections in humans, cyclospores, isospores, and cryptospores are primarily relevant. Individuals with weakened immune systems are particularly at increased risk of infection. Coccidiosis is manifested by non-specific symptoms such as severe diarrhea and abdominal cramps, which – if left untreated – can last for several weeks and lead to severe electrolyte loss. Toxoplasma gondii, the causative agent of toxoplasmosis, frequently infects cats, which can transmit the infectious sporozoites to humans in close contact with humans. If the pathogens encounter an intact immune system, there appears to be no danger because there are few to no disease-related symptoms. The infectious sporozoites can infect cells of the phagocytic system and form there via the so-called endodyogeny mother cells with two daughter cells each, which can settle as bradyzoites (also cystozoites) without symptoms in all organs, in the CSF and even in the CNS in so-called pseudocysts. Although the pseudocysts are suppressed by the immune system, they can – even after several years – cause a kind of endogenous infection in a weakened immune system, due to pregnancy, disease, or artificial immunosuppression, without any renewed contact with the pathogens.
