Trypanosomiasis vaccine
The Trypanosomiasis vaccine refers to efforts to create a vaccine for African trypanosomiasis, also known as African sleeping sickness, a serious vector-borne disease endemic to sub-Saharan Africa. This disease is caused by protozoan parasites transmitted primarily through the bite of the tsetse fly, posing a significant public health challenge. Despite extensive research, developing an effective vaccine has proven difficult due to the parasites' ability to evade the immune system by frequently changing their surface proteins, known as variable surface glycoproteins (VSGs).
Recent advances in identifying invariant surface proteins and utilizing DNA vaccine technology offer some hope for future vaccine development. Currently, prototypes are being constructed and tested in animal models, aiming to provide a preventive measure against this disease, which affects rural populations reliant on agriculture and animal husbandry. The disease manifests in two forms, with T. b. gambiense being the most prevalent. As the situation stands, while public health initiatives have reduced the number of reported cases, many remain unregistered, underscoring the need for an effective vaccine to significantly control this potentially fatal condition.
Trypanosomiasis vaccine
Definition
African trypanosomiasis, also known as African sleeping sickness, is a potentially fatal, vector-borne parasitic disease of sub-Saharan Africa. Protozoan parasites cause trypanosomiasis. Human disease is transmitted through the bite of a tsetse fly. Despite many years of effort, no effective vaccine has been developed to prevent trypanosomiasis in humans.
Immunization
Trypanosomiasis vaccine development has been thwarted by the organism’s ability to evade its human host’s immune system. It does this by changing its surface glycoprotein. These variable surface glycoproteins (VSGs) switch spontaneously and by immune pressure. The rapid switching of VSGs has resulted in more than one thousand unique VSG types. Because of this constantly changing surface coat, the development of any vaccine has been thought unlikely. However, more recent identification of invariant surface proteins, and DNA (deoxyribonucleic acid) vaccine technology, promise hope for a future vaccine. Vaccine prototypes are being constructed and tested against controls in animal models.
Pathology
African trypanosomiasis occurs in thirty-six countries in sub-Saharan countries. There are two forms of the infection. Trypanosoma brucei gambiense, which causes disease in western and central parts of Africa, is the most common form; it causes about 95 percent of all cases. T. b. rhodesiense causes infections that occur in eastern and southern Africa. The bite of the tsetse fly is often painful and results in a chancre. The bite is followed by fever, headache, muscle ache, itching, and lymphadenopathy within one to four weeks.
T. b. gambiense infection may occur months or years before secondary disease symptoms emerge. T. b. rhodesiense develops more rapidly toward secondary infection. Symptoms of secondary infection are caused by invasion of the central nervous system and include confusion, personality changes, loss of coordination, slurred speech, and seizure. Death can occur if the infection is left untreated. Disturbance of the sleep cycle results in somnolence, and in the final stages of infection, a comatose state develops, giving rise to the common name sleeping sickness.
Pathogenicity
Rural populations of sub-Saharan Africa who are involved in agriculture, fishing, and herding are at greatest risk. Infected tsetse fly bites are the most common mode of transmission, but infection can also be spread from a pregnant woman to her fetus across the placenta; the infection also can be spread through blood transfusion. Domestic and wild animals may serve as a reservoir for the disease.
During the first stage of infection, called the hemolympahtic stage, symptoms are caused by hemolysis. During the second stage of disease, called the neurologic or meningoencepahlitic stage, the parasite crosses the blood-brain barrier to infect the central nervous system, eventually resulting in coma and death if not treated.
Impact
Public health efforts to control trypanosomiasis have led to a drop in the number of reported cases to a historic low in 2018 of under 1,000 cases, according to the World Health Organization (WHO) in 2023. The disease remained below that threshold in 2022. An epidemic has not occurred since the late 1990s. Many cases still go unreported, so the actual number of cases is probably closer to thirty thousand.
Treatment of trypanosomiasis is complex. Once the disease crosses the blood-brain barrier, drugs become more toxic and difficult to administer. The development of an effective vaccine would help significantly in controlling this potentially fatal disease.
Bibliography
Carvalho, Joanna, et al. “Developing a Vaccine for African Trypanosomiasis: Only Wishful Thinking or a Definite Possibility?” BMC Proceedings 2, suppl. 1 (September 23, 2008): 9.
Centers for Disease Control and Prevention. “Trypanosomiasis, African.” Available at http://www.dpd.cdc.gov/dpdx/html/trypanosomiasisafrican.htm.
Jong, Elaine C., and Russell McMullen, eds. Travel and Tropical Medicine Manual. 4th ed. Philadelphia: Saunders/Elsevier, 2008.
Plotkin, Stanley A., Walter A. Orenstein, and Paul A. Offit. Vaccines. 5th ed. Philadelphia: Saunders/Elsevier, 2008.
"Trypanosomiasis, Human African (Sleeping Sickness)." World Health Organization (WHO), 2 May 2023, https://www.who.int/news-room/fact-sheets/detail/trypanosomiasis-human-african-(sleeping-sickness). Accessed 4 Feb. 2025.