To and the disease presents even greater challenges

To ‘eradicate’ a disease is to reduce, through deliberate
efforts, the incidence of infection to zero worldwide (Box 1; Keenan et al., 2013). The possibility of
disease eradication was first acknowledged in 1801, when Edward Jenner
developed a vaccine which would provide immunity to smallpox. Yet it took 170
years for smallpox to be certified as ‘eradicated’, and to date, it is the only
human disease to have been successfully eradicated, despite efforts to
eradicate others such as Guinea Worm and Poliomyelitis (CDC, 2008). This
highlights two important points:

(1)  
Worldwide eradication of a disease is not a quick process.

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(2)  
Worldwide eradication of a disease is not an easy process.

 

Amongst the diseases currently targeted for eradication is
Schistosomiasis. In fact, the 65th World Health Assembly set 2025 as
the target date for elimination of Schistosomiasis as a public health problem
(WHO, 2012). However eradication of Schistosomiasis has been the subject of
debate for at least 30 years with little progress, and the disease presents
even greater challenges than did smallpox since there is currently no Schistosomiasis
vaccine . This begs the question: Is worldwide eradication of Schistosomiasis a
realistic possibility?

Three intricately linked concepts, ‘elimination’,
‘eradication’ and ‘control’, are essential to answering this question and are
defined in Box 1.

Box 1

Figure 1

WHAT IS
SCHISTOSOMIASIS?

Schistosomiasis is the second most important parasitic
disease after Malaria (Lewis and Tucker, 2014) and one of 17 Neglected Tropical
Diseases (NTDs) listed by the World Health Organisation (WHO) (Savioli et al., 2017). The disease is caused by
digenetic trematodes of the genus Schistosoma
(Colley et al., 2014). Six
different species of Schistosoma may
cause Schistosomiasis, but the majority of human cases are infected with three
species, namely Schistosoma haematobium,
S. mansoni and S. japonicum (Bergquist et
al., 2017).

WHY ERADICATE THIS
DISEASE?

Schistosomiasis as a substantial health burden

In 2015, the global burden of Schistosomiasis was estimated
at 2.6 million disability-adjusted life years (DALYs) (GBD, 2015 cited in Bergquist
et al., 2017). Reports have indicated
that over 200 million people worldwide are infected (Siqueira et al., 2017) with an additional 779
million at risk of acquiring the disease (Molehin et al., 2016). The disease causes an estimated 200,000 deaths every
year (Lewis and Tucker, 2014).

Consequences of infection include anaemia, stunted growth, impaired
cognition, decreased functional work capacity, liver abnormalities, sub-fertility,
decreased physical fitness and potentially fatal complications such as portal
hypertension, renal failure and bladder cancer (Colley et al., 2014; Burnim, Ivy and King, 2017; Othman and Soliman, 2015).

Schistosomiasis as a substantial economic burden

Schistosomiasis may be considered a disease of poverty
(Figure 2). A World Bank analysis indicated that the majority of the
sub-Saharan population survives on $1.25-2 per day (Adenowo et al., 2015). Coincidentally,
sub-Saharan Africa accounts for over 90% of the world’s cases of
Schistosomiasis (Siqueira et al.,
2017). Although there is no current estimate of the global economic burden of
Schistosomiasis, a study in 1972 indicated that resource loss attributable to
reduced productivity amounted to over US $6 million (Wright, 1972). This figure
excluded the cost of public health programmes, medical care and compensation
for illness, therefore the true economic burden of Schistosomiasis is likely to
be many times higher.

Schistosomiasis as a global threat

Despite decades of integrated control measures with improved
sanitation and hygiene, the disease is spreading into new areas of the globe,
as indicated by recent reports of infection in Europe (Molehin et al., 2016).

IS ERADICATION OF
THIS DISEASE A REALISTIC POSSIBILITY?

While the ultimate achievement of control is eradication,
diseases that are controlled are not always eradicated. Indeed, according to Dowdle
and Cochi, eradication of a disease is only feasible if a “constellation” of
four conditions exists. These are (1) biological feasibility, (2) adequate
public health infrastructure, (3) sufficient funding and (4) substantial
political/societal will (Dowdle and Cochi, 2011).

Biological feasibility  

To address recent concerns about the emergence of resistance
to Praziquantel (PZQ), the drug most commonly used for the treatment of
Schistosomiasis (Siqueira et al.,
2017), efforts have focused on finding alternative strategies to fighting
Schistosomiasis. A number of molluscicides have been developed to help decrease
disease transmission (Duval et al.,
2015), since Schistosoma development
depends on the presence of its intermediate host, freshwater snails of the
genera Biomphalaria, Oncomelania and Bulinus (Pila et al.,
2017; Stothard et al., 2017).
Interestingly, various microbial pathogens, such as Candidatus Paenibacillus glabratella,
have been shown to induce snail mortality, and therefore show potential as
biocontrol agents to reduce snail populations (Duval et al., 2015).

Although a Schistosomiasis vaccine does not currently exist,
evidence in both humans and animal models suggests that developing an effective
vaccine for Schistosomiasis control and elimination is feasible (Molehin et al., 2016). In fact, a few vaccine
candidates are currently under development. S.
haematobium 28-kd Glutathione-S-transferase (Sh28GST), S. mansoni fatty acid binding protein (Sm14) and S. mansoni tetraspanin (Sm-TSP-2) have
entered human clinical trials (Tebeje et
al., 2016). With modern tools and a renewed interest in the elimination of Schistosomiasis,
there is increasing hope that a vaccine will be developed to help eliminate the
disease.

Sufficient funding/Political and societal will

Political and societal will with regard to Schistosomiasis has
increased substantially in the last 15 years. Established in 2002, the
Schistosomiasis Control Initiative (SCI) successfully instigated national
control programmes in Zambia, Mali, Burkina Faso, Niger and Uganda (Adenowo et al., 2015). In 2008, the
Schistosomiasis Consortium for Operational Research and Evaluation (SCORE) was
established, its main focus being on control and elimination of S. haematobium and S. mansoni infections in sub-Saharan Africa (Burnim, Ivy and King,
2017). Most recently, in 2012, the
Global Schistosomiasis Alliance (GSA), the biggest partnership yet, was set up (Savioli
et al., 2017). The GSA is a
partnership of endemic countries, academic and research institutions,
international governmental and non-governmental agencies, foundations,
organisations and private sector companies (Savioli et al., 2017).

Also in 2012, the 65th World Health Assembly
(WHA) adopted resolution WHA65.21 which calls for elimination of
Schistosomiasis through regular treatment of 75% of school-age children in
at-risk areas, through intensification of control programs and through
initiation of elimination campaigns where possible (WHO, 2012).

A number of pharmaceutical manufacturers have pledged to
continue large medicinal donations of PZQ. Amongst them, Merck Serono, a
Science and Technology company, has committed to donate PZQ until
Schistosomiasis is eliminated, increasing its annual production of PZQ tablets
from 25 million to 250 million (Colley et
al., 2014). The cumulative value of all these donations, US $17.8 billion
between 2014 and 2020, represents the greatest growing public health donation (Savioli
et al., 2017).

Public health infrastructure

Public health infrastructure, particularly provision of safe
water and sanitation, is key to any Schistosomiasis control initiative. Indeed,
while potable water reduces the risk of transmission, good sanitation and
hygiene practices reduce the risk of water contamination by parasite eggs found
in human excreta (WHO, 2012).  

Unfortunately, lack of public health infrastructure in
endemic countries is a major obstacle to successful eradication of Schistosomiasis
(WHO, 2012). However, this does not mean that eradication is impossible, as stronger
political and societal support for eradication of Schistosomiasis is likely to
drive further funding in this area.

Elimination is already under way

Schistosomiasis has already been controlled and even
eliminated in previously endemic countries, demonstrating the feasibility and
real possibility of eradication (Figure 2). In China, Schistosomiasis was made
a public health priority in the 1950s, as the disease was seen to be a major
obstacle to rural development (Savioli et
al., 2017). After more than 60 years of dedicated planning and
interventions, the number of infected humans in the country was reduced from
around 12 million to less than 100, 000 (Bergquist et al., 2017). Japan also achieved elimination through long-term,
uninterrupted activities, the last new human infection being reported in 1977
(Bergquist et al., 2017). Other
countries, such as Morocco, are close to elimination of the disease (Bergquist et al., 2017).

Certain countries already stand out as possible targets for
elimination due to the restricted geographic distribution of the disease. For
instance, in Indonesia, the majority of cases of human Schistosomiasis occur in
three, small isolated valleys in Central Sulawesi (Bergquist et al., 2017). The disease is similarly
constrained in Cambodia and Lao People’s Democratic Republic of Lao. Elimination
in these countries seems highly feasible (Bergquist et al.,2017).

CONCLUDING REMARKS

Eradication of Schistosomiasis may not be an immediately realistic
possibility, but eradication certainly is possible.  For the first time, the establishment of the
GSA and the adoption of WHA65.21 is proof that there is gereral, worldwide
consensus regarding the possibility of eradication. Improvements in sanitation
and hygiene, and the development of a vaccine will no doubt accelerate the
process of eradication.