Leishmania mexicana

     Today, I helped a friend attempt to catch woodrats (Neotoma micropus) and sandflies. She is working on her thesis....she's on the hunt for a particular parasite.  This parasite isn't a big problem in this area, but it is a problem in other areas. She's looking to see if it is here...and for good reason. In 2007, dermatologists in Dallas, Texas diagnosed NINE cases of cutaneous leishmaniasis...but we will get to that later. First things first, taxonomy!

Taxonomy

L. mexicana Promastigote

     This parasite is a protist withing the excavate clade. Leishmania in the phylum Euglenozoa....which contains the commonly known free-living Euglena; an elegant example of a flagellate often found in pond scum.  Leishmania belongs in class Kinetoplastida and order Trypanosomatida alongside other trypanosomes such as Trypanosoma cruzi (which causes Chagas' disease) and Trypanosoma gambiense (which causes sleeping sickness).  The genus, Leishmania, has many pathogenic species. The Leishmania donovani species complex causes the most terrible form of this disease, which is known as visceral leishmaniasis (a.k.a. "Kala-Azar"). Leishmania braziliensis causes mucocutaneous leishmaniasis. Many other species cause cutaneous leishmaniasis, but only one is found in the new world....this is Leishmania mexicana.

L. mexicana Amastigote (in green)

Life Cycle
     The life cycle of this organism involves two body forms. An infected female sandfly takes a blood meal from an uninfected human. During this feeding, Leishmania mexicana promastigotes make their way into their new host's blood stream. As the body's immune system goes to work, macrophages ingest the promastigotes. While inside of the macrophages, the promastigotes transform into an amastigote form (the green ovoid organism in the picture on the right). The amastigotes reproduce within the macrophages and other cells. Then, along comes an innocent, uninfected female sandfly hungry for a delicious blood meal. As the sandfly sucks the blood of the infected human, she ingests the infected macrophages. Once inside of the sandfly, the amastigotes burst from the macrophages and transform into the promastigote stage within the midgut of the sandfly. Then the promastigotes migrate to the proboscis of the fly, where they await for the fly to have its next blood meal...completing the life cycle.

Cutaneous and Diffused Cutaneous Leishmaniasis
     These forms of leishmaniasis are less dangerous than visceral forms. The cutaneous form manifests as ulcers at the site where the sandfly bites its host. In this form, the amastigotes don't spread to other areas. The ulcers show up anywhere from a few days to a few months post-infection. This form usually heals on its own given time. 

     The diffused form of cutaneous leishmaniasis only manifests after the amastigotes cutaneously spread due to defective immune cells (such as T-cells). This can cause sores and ulcers all over the body that do not respond well to drugs. Penstostam is a drug that is often used in the US to treat this form of leishmaniasis. Penstostam works by inhibiting ATP synthesis.  There is ongoing research into vaccine development that works against the promastigote life stage.
     These diseases are typically only found in Central and South America. However, there have been cases reported right here in Texas. Cases began appearing in South Texas years ago, but it wasn't until the last few years that it was reported further north. In 2007, nine different cases were reported from around the Dallas area. Researchers believe that sandflies infect woodrats as they search for blood meals. These woodrats then become what we call a "reservoir". This means that they are able to serve as a sort of storage space for the propagation of the Leishmania mexicana protozoans without a human host. This keeps the protists circulating in the sandflies, making them potential disease vectors.

Moral of the Story
     There is strong evidence to suggest that if sandflies and woodrats are present, so could be present this parasite. People in rural areas are more at risk despite the sandflies' recent movements into more urban areas. Using insect repellants will reduce the risk of being bitten, so keep that in mind the next time you head out to the boonies! :p