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Evolutionary Biology of Parasitism
Published in Eric S. Loker, Bruce V. Hofkin, Parasitology, 2023
Eric S. Loker, Bruce V. Hofkin
For parasites with complex life cycles, the genes that optimize fitness in one host may not optimize fitness in another obligate host in the life cycle. In the tick-borne apicomplexan parasite Theileria parva that causes East Coast fever in cattle in Africa, passage of the parasite through ticks results in a dramatic alteration in genetic composition of the parasite, as does passage through cattle. The parasite is continually being shaped by selection imposed by two very different host environments and immune systems. As another example, in Schistosoma mansoni, deliberate selection to optimize fitness in the snail intermediate host resulted in an inverse relationship with the parasite’s fitness in the definitive host (Figure 7.9). Exposing such genetically based constraints has important implications for interpreting the epidemiology of parasites with complex life cycles. Under natural situations, the alternating demands regularly imposed by selection acting in very different host species and in free-living environments likely results in an integrated overall optimization of deployment of genetic resources by the parasite, one very much shaped by the properties of the particular host species involved. Once again, unique aspects of the biology of the particular parasite–host system in question can strongly influence the microevolutionary process, often in distinctive ways that defy generalization.
Immunological Considerations
Published in F. Y. Liew, Vaccination Strategies of Tropical Diseases, 2017
The role of Tc cells against viral infections has been demonstrated in several systems (reviewed in Reference 32). However, their effectiveness against bacteria, protozoa, and helminths is by no means clear. The efficacy of Tc cells against infectious agents is based primarily on the destruction of host cells containing replicating pathogens which when released prematurely could not survive in the host environment or fail to reinfect other host cells. This generally works very well against obligatory intracellular infectious agents such as viruses. However, few protozoa or helminths have such limitations. It is not surprising, therefore, that despite considerable effort, so far there is only one convincing report on the presence of Tc cells in parasitic and bacterial infections, respectively. These are during Theileria parva infection in cattle33 and Listeria monocytogenes infection in mice.34 Even in these instances, the protective role of Tc cell is not certain.
The role of apoptosis in non-mammalian host-parasite relationships
Published in G. F. Wiegertjes, G. Flik, Host-Parasite Interactions, 2004
In 2001 several extensive reviews were published that considered the involvement of apoptosis in the protozoan-host interaction. All of these reviews (e.g. DosReis and Barcinski, 2001; Heussler et al., 2001; Luder et al., 2001), however, consider the extensive research carried out on the role of apoptosis in protozoan infections of mammals, particularly humans. In contrast, very little is known about how the apoptotic process relates to protozoan infections in lower animals. However, a consideration of the protozoan-mammalian interaction may be useful in determining the mechanisms by which the parasite manipulates the apoptotic process. Studies on apoptosis in relation to the interactions between mammals and their protozoan parasites have mainly been associated with intracellular pathogens of medical and veterinary importance. For example, Plasmodium sp., the causative organism of malaria, has a complex life cycle within its vertebrate host which involves intracellular parasitization of both liver parenchyma cells and erythrocytes; Trypanosoma sp. such as T. cruzi, which induces the disease termed ‘Chagas’ disease’ in Latin America has an infective stage in the vertebrate blood which invades various host cells including immunocompetent cells, the macrophages. Indeed, Theileria parva, which is responsible for East Coast fever in cattle, also replicates within lymphocytes. Toxoplasma gondii, an intracellular parasite which infects a range of vertebrate definitive hosts including many mammalian and bird species is a ubiquitous opportunistic pathogen of particular importance in fetuses and immunosuppressed patients, as is Cryptosporidium parvum which causes diarrhoea.
How relevant are in vitro culture models for study of tick-pathogen interactions?
Published in Pathogens and Global Health, 2021
Cristiano Salata, Sara Moutailler, Houssam Attoui, Erich Zweygarth, Lygia Decker, Lesley Bell-Sakyi
The theilerioses are tick-borne diseases of ruminants caused by obligate intracellular protozoa of the genus Theileria, which are responsible for immense losses in domestic livestock. Although these organisms are of great importance in the veterinary field, cultivation in tick cell lines has not been reported. Maturation in vitro of Theileria parva in backless tick explants was compared with that in cultured excised salivary glands derived from already-infected ticks [58]. Backless tick explants and excised salivary glands showed similar numbers of infected acini per infected tick when cultured at 36°C, possibly due to the high temperature [170]. However, after 12 days at 28°C, backless tick explants showed 20–30 times as many infected acini per infected tick as excised salivary glands.