''Trypanosoma lewisi'' is a globally distributed parasite of ''

Rattus ''Rattus'' is a genus of muroid rodents, all typically called rats. However, the term rat can also be applied to rodent species outside of this genus. Species and description The best-known ''Rattus'' species are the black rat (''R. rattus'') ...

'' species and other rodents such as mice, and of

kangaroo rat Kangaroo rats, small mostly nocturnal rodents of genus ''Dipodomys'', are native to arid areas of western North America. The common name derives from their bipedal form. They hop in a manner similar to the much larger kangaroo, but developed thi ...

s in America. Among these host species were two endemic species of rats: '' Rattus macleari'' and '' Rattus nativitatis''. Both are now believed to be extinct. It is not very clear whether or not the same parasite infected both species. However, both parasites are very similar. The northern rat flea (''

Nosopsyllus fasciatus ''Nosopsyllus fasciatus'', the northern rat flea, is a species of flea found on domestic rats and house mice. Northern rat fleas are external parasitism, parasites, living by hematophagy off the blood of rodents. ''N. fasciatus'' can bite humans, ...

'') acts as the

vector Vector most often refers to: *Euclidean vector, a quantity with a magnitude and a direction *Vector (epidemiology), an agent that carries and transmits an infectious pathogen into another living organism Vector may also refer to: Mathematic ...

for the parasite, harboring the epimastigote stage in its midgut. The trypomastigote is the stage that is present in the main host, the rodent. The epimastigote form attaches itself to the rectum of the insect using its flagella to burrow through the rectal walls. The parasites also appear in the flea's feces. Ingestion of either the flea or its feces during grooming infects the host rodent with the parasites. ''T. lewisi'' is normally non-pathogenic but is known to have produced fatal infections in rats.

History

Trypanosomes in the blood of rats were first noted and described by Timothy Richards Lewis from Calcutta and the species was named after him. In the 1900s, a parasitologist noticed that '' Rattus macleari'', a species of rat endemic to

Christmas Island Christmas Island, officially the Territory of Christmas Island, is an Australian external territory comprising the island of the same name. It is located in the Indian Ocean, around south of Java and Sumatra and around north-west of the ...

, were becoming sick. The suspected cause was a species of trypanosomes. There was no proof that this was actually correct until scientists from the

American Museum of Natural History The American Museum of Natural History (abbreviated as AMNH) is a natural history museum on the Upper West Side of Manhattan in New York City. In Theodore Roosevelt Park, across the street from Central Park, the museum complex comprises 26 inter ...

deposited some rats that had been collected from Christmas Island as specimens into museums. Scientists argue that ''Trypanosoma lewisi'' is partially or wholly responsible for the subsequent extinction of ''Rattus macleari''. The parasites were transmitted from fleas infesting the then recently introduced black rats (''

Rattus rattus The black rat (''Rattus rattus''), also known as the roof rat, ship rat, or house rat, is a common long-tailed rodent of the stereotypical rat genus ''Rattus'', in the subfamily Murinae. It likely originated in the Indian subcontinent, but is n ...

'').

Research

''T. lewisi'' can be cultured in various media including ''in vivo'' in rat serum and ''in vitro'' in mammalian cell culture media. The parasite can also be grown in mice if the host is supplemented with a controlled diet and

intraperitoneal injection Intraperitoneal injection or IP injection is the injection of a substance into the peritoneum (body cavity). It is more often applied to animals than to humans. In general, it is preferred when large amounts of blood replacement fluids are needed ...

of rat serum. Ablastin, an

antibody An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the ...

that arises during an infection in the host’s body, prevents the parasite from reproducing although they remain in adult form. A research paper suggests that the data on the aftermath of introduction of a ''Trypanosoma lewisi'' to immunologically naïve murine hosts on Christmas Island around 1900 matches reports of complete extinction within the range of 1–9 years. This gives some more information on the first pathogen introduction to a species to have caused species

extinction Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

. Although rare, there were also many cases in which human beings and primates were infected with Trypanosoma lewisi. In a recent study comparing Brazilian isolates in rats and primates, it was found the DNA sequences were the same when considering Trypanosoma lewisi. This further proves the potential of Trypanosoma lewisi's ability to infect human beings, despite being rare in most cases.

Life cycle

A flea bites an infected rodent and ingests its blood. Within six hours, the parasites migrate and reproduce in the

epithelial cell Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellula ...

s of their host flea's stomach. They then go further into the lumen of the stomach and finally move into the insect's rectum. The parasite's metacyclic trypomastigote infects a rat after it eats the host flea or the flea's feces. Once inside the rat's body, the parasite will then begin reproducing epimastigotes in the

blood capillaries A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the body: ...

of the host. After about five days, trypanosomes will begin appearing in the peripheral blood of the host, with the appearance of thick worms. These parasites are usually attacked by ablastin, a trypanocidal

IgG Immunoglobulin G (Ig G) is a type of antibody. Representing approximately 75% of serum antibodies in humans, IgG is the most common type of antibody found in blood circulation. IgG molecules are created and released by plasma B cells. Each IgG ...

antibody produced by their host's immune system beginning 2–4 days postinfection. After a few weeks, the trypanosomes stop growing and disappear from the bloodstream. The rat then develops

immunity Immunity may refer to: Medicine * Immunity (medical), resistance of an organism to infection or disease * ''Immunity'' (journal), a scientific journal published by Cell Press Biology * Immune system Engineering * Radiofrequence immunity desc ...

against re-infection.

History

Research

Life cycle

References

Further reading