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Memórias do Instituto Oswaldo Cruz
Fundação Oswaldo Cruz, Fiocruz
ISSN: 1678-8060 EISSN: 1678-8060
Vol. 90, Num. 2, 1995, pp. 241-248
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Memorias Instituto Oswaldo Cruz, Vol.90(2): 241-148
mar./apr. 1995
Current Concepts of Snail Control
RF Sturrock
Department of Medical Parasitology, London School of Hygiene
and Tropical Medicine, Gower Street, London WC1E 7HT,
U.K.
Code Number: OC95048
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Text: 45K
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Schistosomiasis control was impossible without effective
tools. Synthetic molluscicides developed in the 1950s
spearheaded community level control. Snail eradication proved
impossible but repeated mollusciciding to manage natural snail
populations could eliminate transmission. Escalating costs,
logistical complexity, its labour-intensive nature and
possible environmental effects caused some concern. The
arrival of safe, effective, single-dose drugs in the 1970s
offered an apparently better alternative but experience
revealed the need for repeated treatments to minimise
reinfection in programmes relying on drugs alone. Combining
treatment with mollusciciding was more successful, but broke
down if mollusciciding was withdrawn to save money. The
provision of sanitation and safe water to prevent transmission
is too expensive in poor rural areas where schistosomiasis is
endemic; rendering ineffective public health education linked
to primary health care. In the tropics, moreover, children
(the key group in maintaining transmission) will always play
in water. Large scale destruction of natural snail habitats
remains impossibly expensive (although proper design could
render many new man-made habitats unsuitable for snails).
Neither biological control agents nor plant molluscicides have
proved satisfactory alternatives to synthetic molluscicides.
Biologists can develop effective strategies for using
synthetic molluscicides in different epidemiological
situations if only, like drugs, their price can be reduced.
Key words: schistosomiasis control - history - molluscicides -
chemotherapy - biological control - strategies
This presentation considers snail control that is, or may be,
used today for the control of schistosomiasis. It focuses
primarily on Biomphalaria spp. that transmit
Schistosoma mansoni, but where necessary I will use
examples of Bulinus and Oncomelania spp.
transmitting S. haematobium and S. japonicum
respectively. First, though, I will summarise briefly what
has been done in the past, what lessons have been learned, and
how the views prevailing today have evolved. Without some
historical perspective, it is easy to misunderstand current
concepts of snail control. There is a real danger that workers
planning control programmes today, whether against the snails
or the parasites, will repeat the same mistakes as their
forbears - although cynics may say that this is the inevitable
lesson of history!
In such a revue as this, it is very easy to resort to military
metaphors, and I well remember Professor BG Peters, my PhD
supervisor, abstracting a lengthy paper with the single
sentence 'Yet again Colonel X likens the control of a
parasitic disease to a military campaign'. Nevertheless,
military terminology is often convenient. Once Leiper (1915)
had worked out the full life-cycle of S. mansoni, it
was quite easy to identify its 'weak' links. Basically, there
are four main targets: (i) kill the worms (in man); (ii)
kill the aquatic snails (including those carrying
intramolluscan parasites); (iii) stop people infecting snails
(by contaminating water bodies); (iv) stop cercariae
infecting man (keep people out of infested waters).
One must have the resources to achieve any combination of
these four objectives. Money is obviously of paramount
importance, and also trained personnel; but neither is of any
value without appropriate weapons. Until well after the Second
World War, those needed for objectives (i) and (ii)
(antimonial drugs and inorganic molluscicides) were
inadequate, and the costs of those for (iii) and (iv)
(sanitation and safe water supplies - which would have
benefits far beyond the control of schistosomiasis), were (and
remain) impossibly expensive, both in poor, rural areas and
even some urban areas where schistosomiasis is endemic.
Similarly, it would be prohibitively expensive and
ecologically questionable to try to make extensive natural
water bodies unsuitable for snails; but much could be done at
the time of construction to prevent new water developments
spreading schistosomiasis, if only engineers would consult
biologists early in the design stage.
Which brings me to two more military precepts: 'know your
enemy' and 'time spent on reconnaissance (i.e. research) is
seldom wasted'. More conventionally, this means identifying
the snails and understanding both their biology and their role
in transmitting schistosomes, before planning any control
programmes. The following examples illustrate three instances
where these precepts were ignored.
Once it was realised that schistosomiasis on Egyptian
and Sudanese irrigation schemes was transmitted by
aquatic snails, the obvious way to kill the snails was
periodically to dry out the canals. Leaving aside the
many logistical and other problems this process
actually involves in large-scale irrigation schemes,
cultivating several different crops simultaneously, the
strategy failed because it ignored two important
biological features evolved by Biomphalaria and
Bulinus spp.: to survive in naturally
unstable, aquatic habitats, both genera can aestivate
(remain alive out of water) for extended periods; and
both have high reproductive potentials, allowing
survivors to explosively repopulate habitats as they
refill with water. A second example is the Egypt-49
Scheme on the Nile Delta, where spring and autumn
mollusciciding in the 1960s did not control schistoso-
miasis to the expected extent. The original strategy
failed to appreciate the speed with which snails
surviving the spring treatment recovered to allow the
resumption of transmission before it was stopped by
high summer temperatures (Gilles et al. 1973). The
third example is the prolonged and expensive effort in
the late 1960s and 70s to molluscicide the major canals
of the Gezira irrigation scheme in the Sudan, first
using gravity dispensers and then later by aerial
spraying (Amin & Fenwick 1977). The effect on
transmission was minimal because by the time the
treated water reached the main transmission sites at
the tail ends of the most minor canals, molluscicide
levels had fallen below concentrations toxic to the
snails.
Some people negatively cite these examples as proof that snail
control is useless against schistosomiasis. I prefer to take a
more positive view: they revealed the need for appropriate
precontrol studies. Certainly we must never under estimate the
role of the snails in maintaining schistosomiasis
transmission. I will end this section on a more optimistic
note. S. japonicum was virtually eliminated from Japan
(WHO 1985, 1993) using molluscicides after lining irrigation
canals with concrete. Similarly, filling snail infested canals
with a mixture of chemicals and the spoil from newly-dug,
parallel canals, controlled S. japonicum in extensive
areas of China (Mao & Shao 1982). Both of these programmes
largely predated the appearance of modern drugs and
molluscicides. However, I am getting ahead of myself.
EVOLUTION OF SCHISTOSOMIASIS CONTROL SINCE 1950
Phase 1 - Snail control (1950-75)
Synthetic molluscicides - In the late 1940s,
the newly established World Health Organization recognised the
need for weapons to fight diseases. The available drugs and
insecticides were thought adequate rapidly to eradicate
malaria, but there were no such effective drugs or
molluscicides for schistosomiasis. WHO collaborated with other
organizations and industry in a massive research programme to
remedy this deficiency. Quite by chance, the first fruits in
the late 1950s and early 1960s were the molluscicides: NaPCP
(sodium pentachlorophenate), then niclosamide (2', 5- dichloro
-4'- nitrosalicylanilide) and finally Frescon ^R (N-
tritylmorpholine). Thereafter, the work on synthetic
molluscicides gradually declined and, despite promising
experimental results with certain organotins, nicotinanalide
and its 3'- and 4'-chloro analogues and, in Japan, compound B-
2 (sodium 2, 5 dichloro-4-bromophenol), no new synthetic
compounds have been developed for commercial production, apart
from Yuramin^R (3,5- dibromo -4 hydroxy -4- nitroazo
benzene) produced in Japan to replace NaPCP. Frescon and
Yuramin production has now ceased, but NaPCP is still used in
some parts of China (Webbe 1991). Niclosamide proved to be
the most versatile and effective of the commercially produced
compounds and is now the molluscicide of choice. It is
produced commercially as a wettable powder, with between 50
and 70% a.i., in Germany (Bayluscide^R), Egypt
(Mollotox^R), Korea and the Peoples Republic of
China.
Plant molluscicides - The use of plant products in the
battle against tropical diseases is well known, e.g. quinine
and artemesin in malaria control; pyrethum for controlling
numerous insect pests and vectors. Not surprisingly, there
have from time to time been reports of plants with
molluscicidal properties (see Mott 1987 for extensive
reviews). Examples are Artemisia maritima, Schwartzia
madagascarensis, Polygonum senegalensis, Euphorbia elegens,
Balanites aegyptiaca and, perhaps the best documented of
all, Phyto-lacca dodencandra (Endod). Infusions and
extracts of different parts of these plants, collected at
various stages of their development, undoubtedly contain
chemicals that are molluscicidal in the laboratory, and some
have even been tested with mixed success in the field.
The main justification for studying plant molluscicides is
that, because they may be grown in countries where
schistosomiasis is endemic, their substitution for synthetic
chemicals will conserve precious foreign exchange. However,
there are two very real problems in developing them. The
first is toxicity. Many were originally considered because
they were known to kill fish and other animals. Since they
will be applied in endemic areas to water which is often the
only supply for humans and their domestic animals, preliminary
and expensive testing is necessary to confirm their safety
before approving their widespread use. Secondly, although
many plants grow in small numbers in the wild, the large-scale
cultivation of, for example, Endod, has proved extremely
difficult (Lugt 1981). The provision of adequate amounts for
continued, routine large-scale use has rarely, if ever, been
achieved. This is not to say that we should not continue to
search for plant molluscicides, but their development from the
detection of activity to full production is no easier than for
a synthetic molluscicide, especially today when there is a far
greater awareness of both toxicological and environmental
risks.
Biological control - Predators, pathogens and
competitors of snails - As with plant molluscicides,
researchers have tried to find biological control agents for
snails (Hairston et al. 1975, McCullough 1981, WHO 1984,
Madsen 1990). Some proposed agents, such as birds, turtles,
fish and crayfish, have been observed eating snails in the
field. Many more, such as leaches, nematodes, rotifers and
ostracods, caused extensive mortality when they accidentally
infested laboratory snail colonies. While most undoubtedly can
exterminate snails in the laboratory, such predators are
rarely effective in the field. Snails form only part of their
diet and they turn to other food sources when snail numbers
drop. So they eat snails only when they are abundant and their
main effect is to limit the size of established snail
populations. In stable habitats this may lead to alternating
predator-prey cycles, but rarely to the elimination of the
snails.
Theoretically, obligate snail pathogens are likely to
be the most effective biological control agents (WHO 1984).
Unfortunately, once all the snails have died so, too, will
the pathogens. They would have to be reintroduced if the site
were reinvaded by the snails. Microsporidia are the only
organisms I know of that might be used in this role. They have
been so difficult to maintain in the laboratory that it is
hard to visualise their use in routine control (Michelson
1963). Facultative pathogens face the same problems
as predators: they may limit but will rarely eradicate
snails.
So far, the most promising control agents have been competitor
snails, and there have been several documented examples of the
'natural' displacement of snails transmitting schistosomes in
the field (Ferguson 1972, Barbosa 1973, 1987, Michelson &
Dubois 1979, Pointier & McCullough 1989). It is not always
easy to dissociate such phenomena from long-term,
environmental changes, sometimes precipitated by human
activities. If the effect is due solely to competition, then
the competitors either modify the snail habitats by
destroying aquatic vegetation which provides food,
refuges and egg laying sites for the target snail (e.g.
Marissa cornuariatus, which may also accidentally eat the
eggs and hatchlings of the target snails); or they deplete
essential ions (e.g. possibly Thiara spp.), or they
compete more vigorously for limited resources (e.g.
Pomacea spp., Helisoma spp., and B.
straminea). However, aquatic snails can survive a wide-
range of physico-chemical conditions. If the optimal
combination of conditions differs for the competitor and
target snails, the two species will eventually segregate into
different niches within the same habitat, with competition
confined to overlapping areas sub-optimal for both species.
The best competitors will be those most closely related to the
target species, sharing virtually identical requirements.
B. straminea displacing B. glabrata in the
Caribbean and parts of northeastern Brazil is especially
interesting: the greater fecundity of B. straminea
gives it a competitive advantage over B. glabrata.
Caribbean and some Brazilian strains of B. straminea
are refractory to the local S. mansoni. Alas,
susceptible strains of B. straminea also occur in
Brasil, and there is a risk that an initially resistant strain
could become susceptible to the local S. mansoni. The
same applies to resistant strains of other Biomphalaria
and Oncomelania spp. (Chu et al. 1982). Much more
needs to be understood about the processes determining
resistance and susceptibility of snails to schistosomes,
possibly at the molecular level (Johnston et al. 1993), before
seriously considering apparently 'resistant' strains for use
as biological control agents.
It is sometimes argued that exotic species (i.e. species
introduced from other geographical areas) will have a greater
chance of success. However, this approach is not without its
dangers and using exotic species is generally discouraged,
unless proper steps have been taken to ensure that they will
not pose other environmental and health problems (WHO 1984,
1993).
Hyperparasites - trematode antagonists - A considerable
amount of research has been conducted on the use of
hyperparasites (other trematode species whose redia actively
seek and devour larval schistosomes within the snail). Most
of the studies used echinostomes in the laboratory, although
there have been some experiments in simulated field sites in
endemic areas. However, there have been very few field studies
to assess this impact on schistosome transmission. In
Thailand, adding 600 million eggs to a pond over seven months
never produced infection rates above 50% of E.
malayensis in Indoplanorbis exustus, i.e.
insufficient to stop S. spindale transmission (Lie et
al.1974).
As with competitor snails, the use of exotic parasites is not
generally encouraged, but augmenting transmission of endemic
parasites does not seem to be a promising alternative. On the
Caribbean island of St Lucia, the prevalence of B.
glabrata infected with Ribeiroia marini, an
excellent hyperparasite in laboratory studies (Huizinga 1973),
often greatly exceeded that of S. mansoni, but at a
time when high prevalences of S. mansoni. persisted in
the human population (Basch & Sturrock 1969).
There may even be dangers in using hyperparasites. Depending
on the relative timing of infection, their presence may
sometimes enhance the susceptibility of snails to
schistosomes, rendering resistant species open to infection
(Lie 1982). A polyploid variant of Bulinus tropicus, a
snail normally totally resistant to all schistosomes, was
found in Kenya naturally infected with the cattle schistosome,
S. bovis, but only in mixed infections with
paramphistomes (Southgate et al. 1986).
Attractive, then, as the idea may appear of setting parasites
to fight one another, there is little evidence so far that it
will work, and it may even involve some unexpected dangers.
The use of synthetic molluscicides - Changing
objectives - Molluscicides spearheaded the attack on
schistosomiasis at a community level from the 1950s well into
the 1970s. There was, though, a progressive shift from the
initial objective of eradication to suppression of snail
populations. This shift paralleled research on snail
population dynamics to allow mollusciciding to capitalise on
the natural seasonality in snail numbers. This research also
revealed an even more restricted transmission season in many
areas. A logical development was to manage snail populations
with molluscicides to suppress transmission, rather than to
eradicate the snails (Sturrock 1989).
Overall delivery strategy: area-wide versus focal control -
Adequate precontrol studies are essential to define the
basic transmission patterns in any given area. These studies
should cover at least one, and better two, seasonal cycles.
Based on their findings, rational decisions can be taken on
the subsequent strategy to be adopted for snail and
schistosome transmission control. There has been some
controversy about the relative merits of area-wide (sometimes
referred to incorrectly as blanket) treatment versus focal
mollusciciding (Webbe 1991).
For densely populated, man-made habitats, such as irrigation
schemes, and some natural water sheds with extensive and
complicated water bodies, it is usually considered that a
relatively small number of well-trained and supervised staff
are best used systematically to molluscicide all potential
transmission sites, in effect all water bodies. Particularly
on extensive irrigation schemes, systematic mollusciciding at
predefined intervals must be integrated with controlled water
management and sound synoptic data (i.e. distribution
information, rotations and discharges) to insure proper
molluscicide coverage. This approach has high chemical costs
and exposes all water bodies to adverse environmental effects,
if any; but it does not require large surveillance teams or
decisions on whether or not to apply treatment.
In contrast, focal mollusciciding relies on accurate
identification of transmission sites, and a system of regular
surveillance to detect snails and allow rational decisions on
whether or not to treat them. Molluscicide costs and any
environmental damage will be minimised, but the approach
requires well trained and supervised surveillance teams,
capable of making the necessary decisions. In many rural
areas with relatively simple water systems, there is no reason
why this approach should not succeed. In particular, it is
eminently suitable for integration into Primary Health Care
programmes involving the lowest echelons of the conventional
health services collaborating with members of the local
community. Both methods, area-wide or focal, were later used
to compliment population chemotherapy.
Phase 2 - Drug control (1975 - present)
The drugs - This is not the place for an extensive
description of the new generation of schistosomicidal drugs
(see Davis 1993). However, because they have profoundly
altered attitudes to the use of molluscicides, we should note
three drugs, in particular, that were developed as the
programme initiated by WHO bore fruit. The organophosphorus
drug metrifonate is safe, cheap and effective against S.
haematobium, but has the disadvantage that it requires
three doses given at fortnightly intervals. Oxamniquine is a
safe, single dose drug effective against S. mansoni.
The last of the trio, praziquantel, a safe, single dose drug
effective against all schistosomes infecting man, came into
general use in the early 1980s.
Programmes using drugs alone - WHO (1980) had already
warned that schistosome control programmes should not rely on
one, but should incorporate a range, of control measures.
Despite this advice, some programmes have used drugs alone,
mainly because of limited resources, but also to allow
uncomplicated comparisons of different drugs (King et al.
1988) or delivery strategies (Butterworth et al. 1991). In
other cases, chemotherapy was incorporated or ethically
necessary in studies of other aspects of schistosomiasis, e.g.
treatment-reinfection studies on immune phenomena with the
long-term goal of developing vaccines (Butterworth et al.
1994).
In Zanzibar, a pilot chemotherapy project reduced overall
prevalence and pathology due to S. haematobium, judged
by a decline in haematuria (Savioli et al. 1989). Since then,
an expanded primary health care programme is producing a
steady decline in haematuria, but transmission continues (WHO
1993). Much the same was found in a comparison of the
efficacy of metrifonate and praziquantel against S.
haematobium on the coast of Kenya (King et al. 1989,
Sturrock et al. 1990). In a series of studies of S.
mansoni on the Kenyan plateau, single and multiple
chemotherapy reduced prevalence and intensity of infection for
varying periods, depending on the local intensity and
transmission patterns. Repeated treatments were essential to
maintain any overall reduction in schistosomal pathology and,
even after as many as six treatments, severe pathology
persisted in some patients. Transmission, though probably
reduced, continued (Butterworth et al. 1989, 1994).
Integrated programmes - In the mid 1970s, using
hycanthone (a forerunner of today's single dose drugs) on the
West Indian island of St Lucia, several successive community
chemotherapy programmes eventually reduced prevalence and
intensity, but failed to stop transmission and reinfection.
Focal mollusciciding was then introduced and transmission was
held to insignificant levels for several years (Jordan
1985).
In Brazil, the 'vertical', national PECE programme was
initiated in the mid 1970s using oxamniquine as the primary
weapon for mass or selective population chemotherapy which
varied according to the endemicity found in pilot surveys in
each community. Additional control measures were used as
appropriate (Almeida Machado 1982). Such an extensive
programme covering 10 million or so people cannot be
summarised in a few sentences but the recently published
findings from the town of Peri-Peri are illuminating (Coura
Filho et al. 1992, Lima Costa et al. 1993). Between 1974 and
1983, annual community treatments of all cases detected
resulted in a steady if somewhat erratic drop in prevalence
and incidence of S. mansoni, accompanied by a decline
in the prevalence of advanced schistosomal disease.
Supplementary measures included improvements in water supplies
and sanitation which were available to 90% or more of the
inhabitants by 1984, and molluscicides used in a surveillance-
treatment programme covering known transmission sites.
Nevertheless, transmission was still active when the main PECE
programme ceased in 1983. The control programme continued in
what was, in fact, a Primary Health Care programme, executed
by the local health authorities advised by 'experts' from the
local Oswaldo Cruz Institute. The various indices of
schistosomiasis control continued to decline but, despite this
success, infected snails were still present in 1987 indicating
continued transmission. Despite similar successes elsewhere
in Brasil, cases with heavy pathology continue to be reported
(Domingues et al. 1993).
In Egypt, a vertical national schistosomiasis programme was
set up in 1977 to control the transmission of S.
haematobium in much of middle Egypt (Mobarak 1982). Based
on the lessons learned from earlier Egyptian bilharzia control
campaigns, this programme combined regular, population based
chemotherapy with systematic, area-wide mollusciciding to
minimise transmission. Between 1977 and 1985, the overall
prevalence of S. haematobium dropped steadily from over
30% to below 9% (Anon 1987). Then, an attempt was made to
reduce costs by switching from area-wide to focal
mollusciciding (Webbe & El Hak 1990). Over the following five
years, prevalence stabilised but reinfection rates in children
indicated substantial continuing transmission (Webbe
1991).
These examples give apparently conflicting evidence about the
value of snail control as an adjunct to chemotherapy against
schistosomiasis. The results from St Lucia and Egypt support
its value, as too, do results from other programmes in north
Africa, islands in the Indian Ocean, and from the Middle East
(WHO 1985, 1993). The findings from Brasil are equivocal as
far as snail control is concerned, and, for that matter, the
provision of water and sanitation. Prolonged, systematic
chemotherapy and mollusciciding eventually reduced
schistosomiasis to levels of minimal public health
significance, but would the process have taken longer without
the supplementary control measures?
To summarise, the accumulated weight of evidence suggests that
drugs alone, applied repeatedly over an extended time period,
can contain or minimise the amount of schistosomal pathology.
Rarely, though, will this inherently expensive process halt
transmission, and when treatment is stopped, all the ground
gained will eventually be lost. Molluscicides represent an
important, well tried and effective tool to supplement
chemotherapy (Sturrock 1989). They can also help to maximise
its cost-effectiveness.
INTEGRATED DRUG AND MOLLUSCICIDAL CONTROL
General concepts - We are all familiar with the
basic schistosome life-cycle but its quantitative aspects are
often overlooked. Most of the millions of eggs produced by
the adult worms in man are wasted, remaining trapped in man's
tissues or producing miracidia which fail to find a snail.
However, the few successful miracidia multiply in an
amplification phase in the snail to produce enough cercariae
to reach man. Again, very few succeed. A conservative
estimate from St Lucian data suggested that a single pair of
worms could produce 30 billion offspring, but only two, one
male and one female, need complete the cycle to maintain a
stable parasite population. Note, too, that drugs used to
kill adult worms do not affect the parasites already in the
snails, and that the amplification process permits successful
offspring of worms surviving treatment to multiply rapidly and
maintain transmission. On the other hand, although it can stop
transmission, effective mollusciciding will have no immediate
effect on the adult worm population: the inevitable
reappearance of snails will soon be followed by a resumption
of transmission.
Neither method alone, even applied for years or possibly
decades, eliminates transmission. A combination of the two
methods offers two advantages, especially in areas of seasonal
transmission. The optimum time for chemotherapy is when the
snail populations are absent and there is no risk of
reinfection. This window of opportunity is often quite short,
especially when the prepatent periods of infection in both
snails and man are taken into account. The latter delays
diagnosis of new infections by 6 to 8 weeks using
parasitological methods (egg detection), or by 4 weeks using
the antigen detection techniques currently being developed.
Unfortunately, in the real world it is hard to predict
precisely when transmission seasons will start or end.
Accurate snail surveillance by field teams is possible, if
costly, but will work only on a relatively small scale when
the logistics of rapidly mounting a drug treatment campaign
will be feasible. A simpler alternative would be to mount a
mollusciciding programme immediately before a preplanned
chemotherapy campaign to avoid immediate reinfection.
It is, of course, naive to hope that a single, combined
molluscicide and drug campaign in the 'attack' phase of a
control programme will eradicate schistosomiasis from a
community. The concept of subsequent 'consolidation' and
'maintenance' phases lasting many years is now well accepted
(WHO 1985, 1993). In recent years this has usually been
interpreted as repeated (targeted) chemotherapy, plus long
term investment in sanitation and/or water supplies, coupled
with health education to improve community participation in
primary health care programmes.
The weakness of this strategy is that these supplementary
control measures have least impact among young children;
probably the most important group maintaining transmission.
In tropical countries, children will invariably continue to
play in and contaminate water bodies, irrespective of any
improved sanitation and water supplies. By the time they are
old enough to understand health education designed to change
their behaviour, they will already have fulfilled their role
in maintaining schistosomiasis in the community. Transmission
control independent of childrens' behaviour is required
between annual chemotherapy campaigns. Molluscicides provide
an obvious and proven solution in many rural and urban
situations.
This brings us to the second benefit given by molluscicides.
By minimising transmission after chemotherapy, they may allow
the interval between successive chemotherapy campaigns to be
extended. It will reduce how often the local population is
'disturbed' - compliance is, in general, inversely related to
how often people are subjected to examination and
treatment.
Costs - Accurate costing of control programmes is
not easy to obtain. People quoting the apparently high costs
recorded for the St Lucia Project (Jordan 1985) often forget
that it was essentially a research programme. More realistic
costs for a fully operational programme are given by Webbe
(1991), and these show that mollusciciding was not excessively
expensive in a large, densely populated irrigation schemes.
But can these costs be translated to small, rural or urban
situations?
The precise costing of combined treatments can only be
determined under operational conditions. However, a
hypothetical case illustrates the potential savings in
areas with seasonal transmission. In a small community of
about 1000 people, annual mass chemotherapy (to avoid
diagnostic costs) for 6 years at $4 per person for the entire
population would cost $24,000 for drugs alone, excluding
additional delivery costs. Targeting treatment immediately
requires diagnostic costs which effectively cancel out most
potential savings from reduced drug usage. Inevitably, during
such a programme, community compliance will diminish,
increasing the risk of reinfection.
In the same community, a few small streams could be treated
two or three times a year with no more than 30 kg of
Bayluscide costing about US $1200 per year ($40 per kg at
present prices). To this must be added labour costs and that
of mollusciciding dispensers. Labour costs would be trivial -
12 man days a year. Regular surveillance of the streams would
require another 40 to 60 man days a year. This labour could
quite easily be recruited and trained locally within a primary
health care programme. Over 6 years, molluscicide would cost
about $7,000.
For a six year programme combining annual mollusciciding and
mass treatment at three yearly intervals, the basic cost would
be $15,200, i.e. $7,200 for molluscicides and $8,000 for
drugs. This gives both a 30% cost reduction, compared with
the basic drug cost of annual chemotherapy, and also the added
advantage of addressing the recurrent problem of
reinfection.
These hypothetical figures are obviously subject to
correction, and any substantial changes in the assumptions
will lead to different conclusions. For example, donor
agencies are actively trying to reduce the price of drugs. If
they were available at less than $1 per treatment, or even
free, the costing picture could change dramatically in favour
of repeated chemotherapy alone, but this would then leave the
problem of reinfection untackled. Is it not time that the
donor agencies made similar efforts to cut the cost of
niclosamide?
Operational requirements - For such an approach to
work, there are several prerequisites. There must be a cadre
of competent field biologists to undertake precontrol studies
and participate in the design and execution phases of any
control programmes, including the training and supervision of
locally recruited field workers. Surely training suitable
biologists is not impossible? It is also critical to ensure
an uninterrupted supply of necessary drugs, molluscicides and
equipment, possibly by bulk purchase through a central,
national agency, once a control programme has been agreed.
Possible problems - Toxicity - It is likely that
there may be some resistance to the use of niclosamide because
of supposed, unwanted environmental effects. Niclosamide is
biodegradable and soon disappears from treated field sites.
There is no doubt that it kills fish, amphibians and various
invertebrates, but these generally mobile species have great
powers of dispersion and soon return from untreated sites.
Fish mortalities can even be turned to advantage as the fish
may be safely eaten: advance warning of mollusciciding can
provide the community with a much appreciated fish supper! In
particular, it should be stressed that niclosamide is not
toxic to man, domestic animals or crops which may come into
contact with treated water.
Formulations - An emulsifiable concentrate of niclosamide
was produced in the 1970s but it is now available only as a
wettable powder. Alas, there is little likelihood of other
formulations being developed commercially, but various
granules can be made locally from the wettable powder to treat
certain difficult habitats. Molluscicide application relies on
equipment, developed primarily for agricultural use, that may
require some adaptation to use in aquatic habitats.
Resistance - A perennial worry is that snails may
eventually develop resistance to molluscicides. So far, there
has been no reliably documented case of this occurring in the
laboratory or after years of continuous use in the field.
CONCLUSION
Webbe (1991), commenting on some current misapprehensions
about molluscicides, recommended that 'The Expert Committee
[of WHO] should give the lie to these fallacious beliefs, in
recognising that mollusciciding is an important adjunct to
chemotherapy and other methods in many endemic situations, if
cost-effective, lasting control is to be achieved.' I am
happy to report at least two positive steps to implement his
recommendation: WHO has published a handbook on the use of
molluscicides (McCullough 1992); and they still recommend
snail control in the most recent report of the Expert
Committee on Schistosomiasis (WHO 1993).
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