Parasitology. Alan Gunn

Чтение книги онлайн.

Figure 4.10 Trypanosoma cruzi. Note the characteristic C shape often seen in the trypomastigote stage.

A reduviid bug vector acquires an infection when it feeds on blood containing the trypomastigote stage. Reduviid bugs can take in large blood meals several times their own body weight, and this increases their chances of ingesting a trypomastigote. In some bug populations, over half of them carry a T. cruzi infection. In addition to feeding on blood, reduviid bugs will also attack and feed on one another. Therefore, bugs may acquire an infection by ingesting parasites from one of their brethren. However, this is probably not a common means of infection. When the parasites reach the posterior of the insect’s midgut, they transform into short epimastigotes. These then divide by longitudinal fission to form long epimastigotes. Ultimately, the epimastigotes move to rectum where they transform into the infective short metacyclic trypomastigote stage. The gut of infected bugs can contain huge numbers of parasites but unlike some of the other trypanosome species, T. cruzi does not appear to harm its invertebrate host.

      The course of Chagas disease varies considerably, and there are marked disparities between individuals and geographic localities. This suggests that genetic differences on the parts of both the parasite and the host influence the manifestation of disease. There are two phases of the disease: an acute phase and a chronic phase. Initial parasite invasion may cause an acute infection or symptoms so general that it is not obvious that infection has occurred. The acute phase is characterised by high levels of parasitaemia and in a small percentage of cases proves fatal. There is often an initial localised inflammatory response with swelling of the nearest lymph node. If the infection starts at an insect bite wound, a raised red nodule develops called a ‘chagoma’. If the infection occurs via the eye, it induces a condition called Romaňa’s sign in which the eyelid and preauricular lymph node swell so much that the eye becomes closed. As the acute phase of the disease progresses, the parasites invade all organs of the body. However, the most severe consequences arise from the parasite’s tendency to localise within and destroy heart muscle and cardiac ganglion cells. The pathological mechanisms are uncertain. However, damage to cardiac muscle during chronic Chagas disease may have an autoimmune basis. As he became older, Charles Darwin suffered from chronic ill health, the symptoms of which were consistent with him suffering from Chagas disease. Obviously, it is impossible to confirm this, but in his South American journals he recorded being bitten by reduvidid bugs. If the parasites invade the brain, meningoencephalitis can develop with potentially fatal or long‐term damage as a result. The patient often develops a fever and their liver and spleen become enlarged; they may also suffer from diarrhoea and exhibit evidence of respiratory infection. The acute phase occurs most commonly in children less than 5‐years‐old but unless their heart or nervous tissues are severely damaged, most of them recover even without adequate medical treatment.

4.3 Phylum Chlorophyta

Commonly known as the green algae, the Chlorophyta is a paraphyletic phylum – that is, the species within it derive from several different ancestors. Because most of them contain chloroplasts, they are often referred to as plants. Furthermore, these chloroplasts are similar in appearance to those in multicellular plants, such as wheat and roses, have a very similar physiology, and contain both chlorophyll a and chlorophyll b. However, the Chlorophyta are single‐celled organisms (although some are colonial) and are usually classed within the Protista rather than Plantae – although this remains a source of debate. Several species have close symbiotic relationships with invertebrates. For example, Chlorella spp. lives in association with the cnidarian Hydra viridis, and some sea slugs extract the chloroplasts from their algal food and utilise them as photosynthetic organelles within their own cells. Although various algae grow on the pelage of sloths and the skin of certain lizards, there are few reports of them becoming intracellular symbionts of vertebrates. An instance where it does occur is between the alga Oophila amblystomatis and the egg masses of certain amphibia. This alga enters the embryos of the spotted salamander Ambystoma maculatum and is maternally transmitted (Kerney et al. 2019). The alga utilises nitrogenous waste generated by the host cells and undertakes photosynthesis; the relationship therefore appears to be beneficial to both organisms. The alga Nannochloris eukaryotum will enter an endosymbiotc relationship with human cells under in vitro culture conditions. However, this is mainly of interest for the development of molecular machines (Black et al. 2014) and not something that happens naturally outside the laboratory.

      There are isolated case reports of Chlorella spp. infecting wounds in humans and other mammals. There are also accounts of fatal disseminated infections in sheep that were presumably acquired via the digestive tract after consuming contaminated drinking water (Ramírez‐Romero et al. 2010). These, presumably, represent rare opportunistic infections. Some species of algae lost their chloroplasts during evolution. Amongst these are members of the genus Prototheca, which includes species that parasitize mammals and the genus Helicosporidium that are parasitic in insects.

4.3.1 Genus Prototheca

      Members of this genus are closely related to the well‐known alga Chlorella, but they lack chloroplasts, and most species survive as saprophytes feeding on dead organic matter in a similar manner to free‐living fungi. They are found throughout the world and can be isolated from the soil, slime, sludge, gut contents, faeces, marine and freshwater, swimming pools, and virtually anywhere which has high organic matter content (Kano 2020). Some species are facultative parasites that infect various animal species with consequences that range from mild disease to fatalities. Prototheca wickerhamii and Prototheca zopfii are responsible for most human infections. These are usually associated with patients who are immunocompromised through disease (e.g., HIV infection) or medical treatment (e.g., chemotherapy/ corticosteroid therapy). A new species, Prototheca cutis, was described from a patient in Japan (Satoh et al. 2010) and further species will probably be discovered in the future now that the genus is receiving more attention. In 2018, an outbreak occurred in a cancer chemotherapy unit in India that resulted in 12 patients becoming infected with P. wickerhamii (Khan et al. 2018).

      The algae gain entry to the body via the skin – usually through an existing wound – and cause a localised cutaneous infection. This often manifests as dermatitis with the formation of pustules, ulcers, and erythematous plaques. Occasionally, the infection becomes disseminated throughout the body and causes potentially fatal meningitis (Joerger et al. 2020).

      There are isolated but increasing case reports of dogs and cats suffering from illnesses caused by Prototheca. These often take the form of gastrointestinal infections that cause diarrhoea, but they can become disseminated elsewhere in the body with often fatal results. In cows, P. wickerhamii, P. zopfii, and Prototheca blaschkeae are responsible СКАЧАТЬ