For many years, the protozoa were placed within the Kingdom Animalia, but they now represent a subkingdom within the Kingdom Protista. That is, the protists (single celled eukaryotes) now having a separate kingdom of their own. Thomas Cavalier‐Smith and his co‐workers have proposed even more radical re‐arrangements (Ruggiero et al. 2015). They suggest that several phyla that have always been considered protozoa should be moved to form a further new Kingdom, the Chromista. This would include the apicomplexans, the ciliates, dinoflagellates, and the foraminifera (Cavalier‐Smith 2018). We are not adopting their proposals because they are not yet widely accepted but this may change.

In this and the next chapter, we summarise the life cycles and biology of some of the most important parasitic protozoa in human and veterinary medicine. Although they consist of just a single cell, parasitic protozoa come in numerous shapes and sizes. Many of them have complicated life cycles involving two or more taxonomically unrelated hosts with reproduction occurring in both. These complexities often contribute to their success as parasites. They exhibit a vast array of immune‐avoidance mechanisms, and their pathology is often influenced by their interactions with other microorganisms. Parasitic protozoa live in all the organs of our body and cause diseases ranging from benign to rapidly and incurably fatal. They also exhibit every imaginable means of infecting their hosts from simple contamination to sexual and vector‐assisted transmission.

3.2 Phylum Rhizopoda

The Rhizopoda used to be known as the Sarcomastigophora. It is a small phylum consisting of about 200 species and contains the amoebas. Most rhizopods are free‐living or commensal, but it includes important parasitic species such as Entamoeba histolytica. The name ‘Rhizopoda’ translates as ‘root‐like foot’ and refers to the process by which the cytoplasm flows within the cell to form projections of the body wall called pseudopodia (false feet) that are used for both movement and acquiring food. By sending out pseudopodia, they can surround and entrap small food particles within membrane bound vesicles (food vacuoles) in a process called phagocytosis. In some species, their movement is aided by one or more flagellae. Although amoebas are sometimes described as ‘primitive’, ultrastructural, and molecular studies indicate that this is incorrect.

3.2.1 Entamoeba histolytica

Entamoeba histolytica is essentially a human parasite. There are records of infections in other primates (Deere et al. 2019), but it is uncertain whether zoonotic transmission occurs in the wild. Its pathogenicity in other primates is uncertain, but in humans it causes potentially fatal amoebic dysentery. Dysentery is a generic term for a serious inflammatory disorder that affects the intestines and results in intense diarrhoea, pain, and fever. It can arise from various causes and amoebic, bacterial, and viral dysentery occur in both temperate and tropical regions. Dysentery has long been known as a ‘handmaiden of war’, often inflicting more casualties than bullets and bombs. Dysentery epidemics have accompanied nearly every account of war from antiquity to the present day. Wherever large numbers of people (especially if they are malnourished) live in proximity and in squalid conditions, the situation is ripe for an outbreak of dysentery. In recent years, large‐scale migrations, conflicts, and deteriorating economic conditions have led to enhanced levels of amoebic dysentery in some countries; tourists travelling on ‘exotic adventure holidays’ have also occasionally found themselves victim of amoebic dysentery.

Entamoebae and Amoebic Dysentery

In the past, it was often stated that E. histolytica infected about 10% of the world’s population, but that the majority never expressed disease symptoms. The reason so many apparently infected people remained asymptomatic was put down to different strains of the parasite varying in their virulence. It is now clear that three species of morphologically identical Entamoeba are commonly found in our intestines: Entamoeba histolytica, Entamoeba dispar and Entamoeba moshkovskii. These three species are only distinguishable by species‐specific antigens or DNA analysis. Entamoeba histolytica is notoriously pathogenic, whereas E. dispar is generally considered a harmless commensal. However, in hamsters, E. dispar damages the intestinal wall and causes liver abscesses, and there are occasional case reports of pathogenic effects in humans. The pathogenic status of E. moshkovskii remains uncertain. When faecal surveys distinguish between Entamoeba species, E. dispar is always by far the commonest species present and E. moshkovskii is also relatively common (Calegar et al. 2016). Consequently, it is difficult to draw firm conclusions from literature in which the three species are not differentiated. Nevertheless, strain differences occur between populations of E. histolytica, and these reflect the pathology they cause. Many people who are genuinely infected with E. histolytica remain asymptomatic, and whilst sometimes this relates to host factors, there are avirulent strains of the parasite (Escueta‐de Cadiz et al. 2010). A characteristic feature of virulent strains is that they over‐express genes coding for lysine‐rich factors and glutamic‐ and lysine‐rich proteins. The function of these genes, referred to as KRiPs and KERPs, respectively, is unknown but one of them, KERP1, is associated facilitating the adhesion of E. histolytica to red blood cells and causing liver abscesses (Santi‐Rocca

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