Challenges to Elimination of gambiense-HAT and Comparison with rhodesiense-HAT
Human African
trypanosomiasis (HAT), also known as sleeping sickness, is caused by two
related parasites of Trypanosoma brucei. The parasites are transmitted
by tsetse flies and has historically posed a major public health burden in
sub-Saharan Africa. The chronic form, T. b. gambiense (gambiense-HAT),
accounts for the vast majority of cases is predominant in the Western and
Central Africa while T. b. rhodesiense is more frequent in Eastern and
Southern Africa. Although these fatal infections (Untreated cases) are in the
decline trend, the elimination of gambiense HAT is still challenging.
Democratic Republic of
the Congo (DRC) has set a goal to eliminate the transmission of gambiense
HAT by 2030. Since the cases are declined in the recent years, several
biological, epidemiological, and operational challenges complicate the path
toward elimination. These challenges relate to the existence of human and
animal reservoirs, limitations in surveillance, and constraints in diagnosis,
treatment, and integrated vector control. One health approach should be
implemented to achieve the goal to eliminate by the target year. This
integrated approach includes the control of human and animal reservoirs,
extensive surveillance strategy, and control strategies on diagnosis, treatment
and vector.
1. Human and
Animal Reservoirs
Human is the primary
reservoir that plays a key role in transmission of gambiense-HAT disease
making a major concern for control strategy. In addition, studies suggest that asymptomatic
human carriers may harbor parasites for extended periods without detectable
parasitemia using standard diagnostics. These individuals can continue
transmission of this infection silently, acting as “cryptic reservoirs”
(Buscher et al. 2018). These infections even challenge for elimination because
they escape the routine screening programs leading undiagnosed.
Likewise, gambiense
HAT has animal reservoirs as well. Animal reservoirs—including pigs, dogs, and
other domestic or wild animals—have been found to carry T. b. gambiense
in some endemic areas. Although their epidemiological significance remains unclear,
experimental and field data indicate that animals could contribute to
transmission cycles, particularly in regions where human cases are declining
(Mehlitz et al., 2019). If animals maintain the parasite independently of
humans, elimination strategies focused solely on human case detection may be
insufficient. The unpredictability of transmission cycles, including human and
potential animal reservoirs, underscores the warning to “expect the unexpected”
(Mehlitz et al., 2019). These hidden reservoirs represent a major obstacle to
achieving transmission chain blockage.
2. Surveillance
Strategies
The surveillance of
entire communities has been highly successful in reducing disease burden during
outbreak and higher number of cases using traditional active screening
campaigns. However, a number cases are dropping, an alternative, sustainable
and effective control strategy is utmost which is logistically challenging. As
a consequence, health care center based screening is an alternative cost
effective option. This method shifts toward passive surveillance, is necessary
but presents difficulties. Lacking proper training for healthcare workers, resources
and facilities at health care centers may miss the recognition of parasites and
diagnosis gambiense HAT, especially when encounter with patients showing
nonspecific early symptoms. Moreover, reduced awareness as cases decline can
lead to underreporting and delayed diagnosis.
New approaches include:
- Targeted surveillance, focusing on high-risk populations and
historical foci
- Serological screening tools (e.g., rapid diagnostic tests) to identify
suspected cases
- Molecular diagnostics, which may detect low-level infections even in
early stage of infection but remain expensive and technically demanding
Additionally, xenomonitoring
(presence of parasites in tsetse flies) may be used as supplementary screening strategy
to detect ongoing transmission (Rotureau et al., 2013). However, implementing
such methods at scale remains challenging. A major issue is maintaining sensitive
surveillance systems in low-prevalence settings.
3. Control
Strategies
Diagnosis
The diagnosis of gambiense-HAT
is based on a two-step process: serological screening followed by
parasitological confirmation. Although the method is effective, it has few
limitations including low sensitivity while detecting low parasitemia
infections. In addition, skilled manpower and well facilitated laboratory
infrastructure are required. The most important challenge is identifying latent
infections or infections with non-specific symptoms. Recent developments, such as oral treatments that may
not require parasitological confirmation, could simplify diagnostic pathways,
but their integration into elimination programs requires careful evaluation,
particularly in the context of evolving control strategies and uncertainties
around transmission dynamics and reservoirs (Mehlitz et al., 2019; Buscher et
al.).
Treatment
The introduction of safer
and simpler drugs improves the treatment significantly. Oral therapies of
antiparasitic drug; fexinidazole allow the effective treatment of a number of outpatients
reducing logistical burdens. However, ensuring accessibility to remote areas,
managing treatment adherence and avoiding drug resistant remain still
challenging. Furthermore, treating asymptomatic carriers—if identified—raises
ethical and operational questions.
Integrated Vector
Control
The tsetse fly is the
sole vector of HAT, making vector control a critical component of elimination
strategies. A multiple approach is necessary to control the vectors that not
only includes tsetse fly biting but also reduce their population. The use of insecticide-treated
targets and traps, aerial spraying and sterile insect technique (SIT) are some
of those methods used for vector control. Recent innovations such as “tiny
targets” have proven highly cost-effective in reducing tsetse populations.
However, sustaining vector control over large and often inaccessible areas is
difficult. Ecological factors, such as tsetse habitat diversity and reinvasion
from untreated areas, complicate efforts (Van den Bossche et al., 2011). The
complicity of transmission dynamics of gambiense HAT also stays major
obstacle on vector control.
The transmission
efficiency of the vector depends on vector competence that is influenced by
complex biological interactions within the fly, including different parasite
development stages (procyclic forms,
metacyclic forms) (Rotureau et al., 2013).
Would the same strategy
work for rhodesiense-HAT?
The above elimination
strategy used for gambiense-HAT may not be directly applicable to T.
b. rhodesiense (rhodesiense-HAT), which differs fundamentally in its
epidemiology. Rhodesiense-HAT is an acute zoonotic disease, primarily infecting
Eastern and Southern Africa that accounts for only 8% of reported cases. The
number of cases are also much less than gambiense-HAT. In addition, wildlife
and livestock serving as the primary reservoirs for rhodesiense-HAT, rather
than humans gambiense-HAT. Transmission occurs at the interface between
wildlife, livestock, and human populations (Auty et al., 2016). Consequently, humans
are incidental hosts, not the main reservoir. Only eliminating human cases does
not interrupt transmission, rather animal reservoirs sustain the parasite
independently and maintain within the animal population. Hence, a slight
different approach should be implemented to eliminate rhodesiense-HAT
transmission.
Control strategies must
therefore focus on integrated approaches, focusing on treatment and proper
management of livestock as cattle are major reservoirs. Vector control in
livestock and wildlife areas are essential although vector control in wildlife
is the most challenging one. Moreover, the physical barrier for human-animal
interfaces should be clearly defined to avoid close interactions.
The concept of a Progressive
Control Pathway for African Animal Trypanosomosis (AAT) highlights the need for
stepwise, multisectoral strategies targeting animal reservoirs (Diall et al.,
2017).
Unlike gambiense-HAT,
where elimination of transmission is theoretically feasible due to the
predominance of human reservoirs, rhodesiense-HAT control is more about reducing
incidence rather than complete elimination. The ecological complexity and
involvement of wildlife make eradication extremely difficult.
Concluding remark
The decline of gambiense-HAT
in the DRC is a landmark public health achievement, yet reaching the 2030 goal
of zero transmission remains a demanding effort. There is a complex set of
hurdles to face: "silent" or cryptic human infections that go
undetected, the looming possibility of animal reservoirs, and the logistical
difficulty of maintaining robust surveillance and treatment in high-risk areas.
Overcoming these barriers will require more than just persistence; it demands
flexible strategies and a new generation of diagnostic and vector control
tools. A one health approach should be adopted that treats human, animal, and
environmental health as one interconnected system where all stakeholders work
together to achieve the goal.
The situation with rhodesiense-HAT,
however, is a different story entirely. Because this form of the disease is
zoonotic—meaning it cycles through animals—human-focused medical care alone
won't be enough. To make a real impact, only treating infected human population
may not work, rather control of infections in wild and domestic animals within
their natural habitats may be the mainstay. Even then, given the current tools
at our disposal, complete elimination remains a distant and difficult target.
Adopted from CDC
