Parasitology. Alan Gunn

infections is uncertain.

3.2.7 Genus Acanthamoeba

The genus Acanthamoeba contains over 20 species, and they are amongst the most common free‐living soil amoebae. They also live in various freshwater habitats, and several are opportunistic pathogens. There are two life cycle stages: the active feeding and dividing trophozoite stage and the cyst stage (Figure 3.5). Opportunistic infections arise through skin wounds and possibly via breathing in the cysts. Infections also possibly arise through swimming in infected water. Immunocompromised individuals are at particular risk of infection.

      The most serious consequences of infection arise when the amoebae disseminate from their initial entry site via the blood stream and reach the brain. This can result in granulomatous amoebic encephalitis that has a high fatality rate. More commonly, Acanthamoeba causes keratitis (inflammation of the cornea) (Roozbahani et al. 2018) when it invades the surface of our cornea. This usually occurs following trauma to the eye and/or if we use contaminated contact lenses. Soft contact lenses are particularly likely to harbour the parasite and transfer it to the wearer’s cornea. The infection is extremely painful and if not successfully treated may lead to the loss of the eye. Acanthamoebae are extremely common organisms in the environment, so it is surprising that infections are not reported more frequently than they are. Presumably, this is at least in part owing to an effective host immune response.

      Acanthamoeba castellanii is the species most associated with keratitis. It harbours numerous other microorganisms including bacteria such as L. pneumophila, Chlamydia pneumoniiae and Pseudomonas spp., yeast such as Cryptococcus neoformans and various Mimiviridae. The extent to which the amoeba transmits these infections is uncertain, but the associations result in the acquisition and maintenance of microbial virulence factors (Guimaraes et al. 2016). Cryptococcus neoformans is not pathogenic in healthy individuals, but it causes potentially fatal meningitis in those who are immunocompromised. Many species of amoebae harbour mimiviridae although the effect they have on their hosts is uncertain. There are isolated case reports of mimiviruses in association with pneumonia (e.g., Saadi et al. 2013). However, the role of mimiviruses in human disease remains controversial (Abrahão et al. 2018).

Figure 3.5 Life cycle of Acanthamoeba spp. These amoebae are normally free‐living in ponds, lakes, and soil. Both the trophozoite (T) and the cyst stage (C) are probably infectious and enter via skin wounds or through the lining of the nose. Infections of the eye also occur. Fatal disease occurs when they invade the brain. Drawings not to scale.

3.3 Phylum Metamonada

      This is a large group of anaerobic flagellate protozoa of uncertain composition and most probably derived from various ancestors (i.e., it is polyphyletic). They lack mitochondria although this is almost certainly a derived characteristic rather than them never having possessed them during their evolution. They have groups of four flagellae and/or basal bodies that are often arranged in association with their nucleus to a form a structure called the karyomastigont.

3.3.1 Order Diplomonadida

      Most members of the Diplomonadida are parasites, the best known of which belong to the genus Giardia. A characteristic feature of diplomonads is the presence of two haploid nuclei each of which is associated with four basal bodies and flagellae. They lack Golgi apparatus and mitochondria, but organelles called mitosomes, that probably represent relict mitochondria, exist in some species. Mitosomes lack DNA and do not undertake oxidative phosphorylation. They probably manufacture iron–sulphur proteins that undertake essential tasks in the cytosol (Tachezy 2019). In common with many other protozoa, the taxonomic position of the diplomonads is under constant revision.

      3.3.1.1 Genus Giardia

      The taxonomy of Giardia spp. is an interesting illustration of the insights provided by molecular phylogenetics. Despite the morphological similarities between Giardia spp. and other parasitic flagellates (e.g., Trichomonas spp., Trypanosoma spp.), the giardias represent much more primitive organisms. Within the genus Giardia, allocating isolates into species groups is very difficult. The first attempts were based on apparent host specificity since cysts are morphologically identical and attempts at in vitro culture to produce trophozoites was often unsuccessful. Subsequently, improvements in culture methods and microscope resolution enabled the examination of trophozoites. On this basis, there are six species of Giardia, but only one of them infects humans. For historical reasons, the species infecting humans is called Giardia duodenalis in Europe and Australia, the North Americans favour Giardia lamblia, while some authors refer to it as Giardia intestinalis. These species names are synonyms, and there is no ‘correct’ one; the only important issue is that locally, clinicians and scientists use the same name.

      Numerous mammal species harbour Giardia spp. Organisation of the Giardia into groups and strains within those groups is based predominantly on genetic sequences (Thompson and Ash 2019). Unfortunately, many of the published studies involve characterisation and genetic sequencing of small numbers of isolates – in some cases only one parasite from one animal. This is a serious limitation, and the area of Giardia taxonomy clearly requires further work.

       3.3.1.1.1 Giardia duodenalis

      Giardia duodenalis is one of the commonest human parasites and has prevalences of 4–43% in low‐income countries and 1–7% in high‐income countries. It has the distinction of being the first parasitic protozoa to be described. This happened in 1681 when the pioneering microscopist Antoine van Leeuwenhoek observed it in a sample of his own diarrhoea. Currently, scientists divide G. duodenalis into eight genetic assemblages (A–H) but only two of these, A and B, infect us. In addition to humans, assemblages A and B both parasitise wild and domestic mammals. In some parts of America, the implication of beavers as reservoirs of infection has resulted in giardiasis gaining the popular moniker of ‘beaver fever’ (Tsui et al. 2018). However, the extent to which G. duodenalis is a zoonotic infection cycling between humans and other mammals is uncertain.

The trophozoite stage of G. duodenalis is pear‐shaped 12–15 μm in length and has four pairs of flagellae (Figure 3.6). Its ventral surface has a concave profile on which there are two depressions referred to as ‘adhesive discs’ or ‘suckers’ although they have a supportive function rather than being contractile. A pair of flagella located within the ‘ventral groove’ work as a ‘pump’ that removes fluid from underneath the adhesive discs and may facilitate the removal of nutrients from the underlying host mucosa. The oval‐shaped cyst stage is 8–12 μm in size and initially contains two nuclei but once they are mature, four nuclei are present along with several prominent axonemes (microtubules that constitute the core of the flagella): the flagellae and adhesive discs are broken down and stored as fragments during the cyst stage. Infected people shed huge numbers of cysts in their faeces – possibly as high as 1 × 10⁸ viable cysts per gram of faeces. Cyst shedding is intermittent, and laboratory confirmation of the diagnosis often requires the patient to produce several faecal samples. Transmission usually occurs through consuming the cysts in food and water, or through touching contaminated surfaces and then transferring the cysts to one’s mouth.