Читать книгу Parasitology - Alan Gunn - Страница 105

On reaching the stomodeal valve, the Leishmania promastigotes secrete a gel‐like substance called promastigote secretory gel, the main component of which is filamentous proteophosphoglycan, and some of them transform into the infective metacyclic promastigote stage (this has a long flagellum and is very active). The gel physically blocks the gut, and this together with the vast numbers of parasites severely compromises the fly’s ability to feed. Further compounding this, the parasites also produce chitinase enzymes that physically damage the peritrophic membrane and stomodeal valve. Because the insect’s ability to ingest food is impaired, it becomes hungry thereby increasing its probing and number of visits to hosts all of which increases the chance of transmission. Physical probing probably does not transfer many parasites but to ingest food the infected fly must first expel some of the promastigote secretory gel. This gel contains numerous infective metacyclic stage parasites, as well as other non‐infective stage(s). The secretory gel also facilitates the establishment of the infection in the vertebrate host, so it has a dual role in both the invertebrate and vertebrate host (Giraud et al. 2019).

The transmission mechanism(s) employed by the Sauroleishmania remain uncertain. Within the sandfly vector, these species tend to remain in the posterior regions of the gut, and therefore, it is unlikely that transmission occurs when the sandfly feeds. Furthermore, sandflies do not usually defaecate whilst feeding, so it is unlikely that transmission resembles that of T. cruzi by triatomid bugs. A third possibility is that the transmission occurs through the lizards consuming infected sandflies.

Much of the work on how Leishmania establishes and develops within its mammalian hosts involves mice as model organisms and those few Leishmania species capable of being cultured in the laboratory. One should therefore be careful of extrapolating from these studies to the likely behaviour of other Leishmania species and infections in other hosts. Mice and humans are both mammals, but one cannot assume that their immune systems react identically to the same infectious agent. Furthermore, leishmaniasis manifests itself in numerous ways. Therefore, different Leishmania species and strains probably exhibit variations in the way they establish themselves and interact with the host immune system. Nevertheless, all species follow a basic pattern of development following their entry into the blood stream that involves morphological and physiological transformations and establishment in the mononuclear phagocytes and in particular the macrophages.