Human African Trypanosomiasis: Sleeping Sickness

Human African Trypanosomiasis, commonly known as sleeping sickness is a highly prevalent parasitic disease in sub-Saharan Africa. The organism responsible for this infectious disease is the Trypanosoma brucei, which claims hold to humans and other animals, as its primary hosts. Since Trypanosoma brucei are parasitic, their environment is their host. The most common carrier is the African native tsetse fly, which is abundant in western and central regions of the continent (Kagbadouno et al. 2012). Cases of sleeping sickness have also been found scattered throughout the eastern and southern portions of Africa, but it is less common in these areas.

Tsetse flies typically reside in African forests and woodlands. They make up about a quarter of the continent’s landscape, have a seasonal climate, and have enough precipitation to support evergreen growth (Fayolle et al. 2014). These regions claim home to a diverse number of flora and fauna species, many of which are African natives. There are nearly 20,000 different species of plants, over 8000 of which are trees and

the most abundant of these trees is the Miombo (Fayolle et al. 2014). Other animals that reside in this habitat are antelopes, lions, rhinos, giraffes, a large quantity of elephants, and cattle. Woodlands also provide a variety of services such as wildlife protections, shade, purifying and regulating water flow, soil erosion prevention, and timber supply (Fayolle et al. 2014).
The tsetse fly’s habitat is very dependent and susceptible to change by the climate, land covers, and the availability of food. In the past forty years, mass migration has occurred from populous central Africa to more rural and uninhabited lands due to the increase of sleeping sickness cases (Kagbadouno et al. 2012). This migration may be the reason why more cases are being observed in eastern and southern Africa. Tsetse flies are beginning to migrate to food sources and they are carrying Trypanosoma brucei along with them.
Generally, tsetse flies reside in the African woodlands but they may travel a short distance to open grasslands if they catch the scent of a potential host (Vale et al. 2013). In this case, one would assume that most human/fly interactions would occur in the wilderness; however, many attacks occur when tsetse flies approach or enter a building (Vale et al. 2013).
An experiment was conducted using an easily accessed house to tsetse flies under a variety of conditions.

A fly sample was captured twice a day at noon and sunset to determine if any of these conditions were relevant. The conditions under observation consisted of human presence, wood smoke from a fire inside or outside, open windows, and simulated cattle or human odor (Vale et al. 2013). The total catches of flies were unaffected by all of these conditions. However, the rise in ambient temperatures had a significant increase in fly abundance. During the hottest months of the year, the amount of fly catches increased from 5%-13% (Vale et al. 2013). Another interesting finding was that 62% of the flies caught were female (Vale et al. 2013), the sex of tsetse flies that less commonly attack humans. Some of the flies captured were old enough to be potential vectors of sleeping sickness (Vale et al.

2013).
Surprisingly, the most common Tsetse fly catching baits used in the woodlands (human odor, human presence, wood smoke, and ox odor) had zero affect on fly abundance. The results, however, do confirm that buildings provide a distinct venue for the transmission of sleeping sickness, and the risk of the disease may be heightened in warmer climates (Vale et al. 2013). Warmer temperatures result in Tsetse flies trying to seek refuge in man-made shelters; therefore, increasing the possibility of human/fly contact.

Life history/reproduction:
Trypanosoma brucei is a single celled parasite that requires two separate hosts to live and reproduce. This parasites lifecycle is a series of complex steps that allow it to continuously travel its way through an insect vector, the tsetse fly, and a mammalian host. While enjoying a blood meal, an infected tsetse fly injects metacyclic trypomastigotes into a human’s skin from the fly’s very own salivary glands. The parasite then travels through the lymphatic system or blood vessels to enter the blood stream of the human where it is then carried to different sites throughout the body (Achcar et al. 2014). Once transformed into bloodstream trypomastigotes, the parasite multiplies by binary fission in various body fluids -blood, lymph, and spinal fluid- (Achcar et al. 2014). Binary fission is a form of asexual reproduction that occurs three times throughout the life cycle of Trypanosome brucei and is a process of cell division resulting in two daughter cells. Binary fission can maintain a rapid rate and can quickly produce a large population size inside a host.
The disease then spreads when a tsetse fly drinks the blood of an infected human. Once a fly consumes the parasite, it takes about three weeks for the entire life cycle process to complete itself. Inside the fly’s midgut, bloodstream trypomastigotes transform into procyclic trypomastigotes and then multiply through binary fission (Achcar et al. 2014). The procyclic trypomastigotes then leave the midgut, transform into epimastigotes and migrate to the salivary glands of the fly (Achcar et al. 2014). In the salivary glands, epimastigotes transform into metacyclic trypomastigotes and once again undergo binary fission to multiply (Achcar et al. 2014). The metacyclic form of trypanosomes meet pre adaptation requirements for life in a mammal’s bloodstream allowing the parasite to change hosts (Portman and Gull 2014). Once this newly infected tsetse fly bites a human, the whole process repeats itself.