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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. 165-168
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Memorias Instituto Oswaldo Cruz, Vol. 90(2):165-168
mar./apr. 1995
Molluscicide Control of Snail Vectors of Schistosomiasis
Cecilia Pereira de Souza
Centro de Pesquisas "Rene Rachou"- FIOCRUZ, Caixa Postal 1743,
30190-002 Belo Horizonte, MG, Brasil
Code Number: OC95035
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A review of the methodology recommended by the World
Health Organization for the use of molluscicides for the
control of snail vectors of schistosomiasis is presented.
Discussion of the principle molluscicides used, their
advantages and disadvantages, the techniques and equipment
required for their application and evaluation of effect as
well as the biological control of snails is included.
Key words: schistosomiasis mansoni - control - snail vectors -
molluscicide
Schistosomiasis or Bilharziasis is a complex of helminthic
infections that affect man. Some 200 million individuals in 74
tropical countries are infected (Iarotski & Davis 1981, WHO
1985, Sleigh & Mott 1986).
Five species of Schistosoma, S. japonicum, S. mansoni, S.
haematobium, S. intercalatum and S. mekongi which
are taxonomicaly and epidemiologicaly distinct, utilize one
or more species of intermediate snail host parasitise thousand
of individuals.The first three species listed are of greatest
medical importance and the latter two have a restricted
distribution in regions of Africa and around the Mekong river
in South East Asia respectively.
Snail control by means of molluscicides is today considered an
auxiliary method within integrated control of schistosomiasis
(Mc Cullough 1992).
The Expert Committee of the WHO (1985) listed three phases in
the control of schistosomiasis: (1) Planning:
collection of epidemiological data, organization of a national
plan of action and allocation of recourses for the program;
(2) Intervention-attack: a period of active
intervention operations are intense and continually evaluated.
This phase results in a rapid reduction of prevalence; (3)
Maintenance: maintenance is then required in many
situations.
THE OBJECTIVES OF SNAIL CONTROL BY MOLLUSCICIDE
APPLICATION
The inclusion of the definition of objectives in control
programs is essential. According to Mc Cullough (1992) the
principle objectives are: (a) to contribute, preferentially in
combination with chemotherapy and other measures, to the
significant reduction of the transmission of schistosomiasis
through the destruction intermediate host populations,
principally infected snails, in habitats selected during peak
transmission. The reduction recommended by Mc Cullough (1992)
is 95% and this level should be maintained during the period
of peak transmission; (b) the destruction of the snails in a
number of breeding sites that contribute significantly in
increasing the population density in neighboring foci; (c)
prevention of transmission in tourist areas;(d) to achieve a
community involvement in the activities associated with
transmission control; (e) to drastically reduce the intensity
of transmission in sites used for high risk occupations such
as fishing and agriculture; (f) to prevent the introduction of
new potential vectors; (g) in certain types of habitats to
totally eliminate snails in order to prevent the risk of
transmission; (h) to avoid whenever possible the establishment
of new breeding colonies in new irrigation systems.
ADVANTAGES AND DISADVANTAGES OF THE USE OF
MOLLUSCICIDES
In general the advantages of the selective use of
molluscicides in control operations are: (a) interruption of
transmission; (b) the desirable but not essential involvement
of the community; (c) reasonable efficiency and cost; (d)
simple equipment that can also be used in the control of other
vectors; (e) although good supervision is essential the
methods of application are general simple and do not require
specialized operational schemes; (f) the selection of foci
where application is required can usually be based on the
patterns of water usage of the population; (g) low toxicity
for man and other animals; (h) health education programs are
used to reinforce the results achieved.
The disadvantages are: (a) the need for repeated applications
since snail eradication is difficult; (b) the implementation
and evaluation of control is time consuming; (c) the effect
on schistosomal morbidity, even when snail control are
efficient and in the absence of chemotherapy, is delayed; (d)
the technical capacity required to decide appropriate
application procedures in view of the great variation in
transmission sites.
CHARACTERISTICS OF A GOOD MOLLUSCICIDE
The perfect molluscicide does not yet exist. A list of the
desired characteristics in a molluscicide has been given by
the WHO (1965). The minimum requirements are: (a) toxicity
for snails at low concentrations; (b) absence of toxicity for
mammals, neither presenting acute or chronic problems of
toxicity; (c) lack of adverse effects if it enters the food
chain; (d) stable in storage for at least 18 months.
In addition to these characteristics, low cost, proven
efficacy, specificity for snails, low toxicity for other
organisms, a variety of formulations and easy measurement of
concentrations in breeding sites are desirable.
CURRENTLY USED MOLLUSCICIDES
There are a series of compounds with molluscicides action that
are used in the control of schistosomiasis. Between 1946 and
1955 some 7,000 chemical products were tested as potential
molluscicides (Ritchie 1973). Amongst these,
pentachlorophenol (NAPCP) was identified as being promising
however, this was subsequently discarded due to its toxicity
for other organisms and is only currently used in China.
Compounds containing lead and tin are highly active but are
also toxic for various organisms. In Japan, Yurimin (3,5-
dibromo-4 hydroxy-4-nitroazobenzene) replaced NaPCP but its
fabrication was stopped after only a few years of use. The
same happened with Frescon (N-tritylmorpholine), one of the
most active molluscicides for adult snails but which was not
active against eggs. Copper was also widely used although in
the presence of organic material, elevated pH and certain
solids in the water it lost activity. In Japan, a compound
named B-2 (sodium 25, dichloro-4-bromophenol) was tested
against the amphibian snail Oncomelania nosophora (Ka-
jihara et al. 1979). In the People's Republic of China, one of
the most effective molluscicides Fluoroacetamide and its
analogs bromocetamide and chloracetamide were identified.
These compounds have high molluscicides activity and low
toxicity for fish, are soluble in water, stable and easily
applied
Tin compounds, particularly tributyltin oxide, were shown to
be highly active but are not used due to their toxicity for
aquatic fauna.
At present, the only viable molluscicide in terms of efficacy
and complete evaluation is Bayluscide (Mc Cullough 1992). The
usual formulation of Bayluscide powder is 70% active material
and in the form of emulsifiable concentrate 25% active
material. Both are highly effective against adult snails and
egg masses. In practice, a concentration of 0.6-img/l is
recommended and a time of exposure of 8 hr (WHO 1973), or
0.33 mg/l for 24 hr (Barnish & Prentice 1981). The effect
persists for 8 hr after application. A 25% suspension can be
mixed with diesel oil at a proportion of 8.5 parts to 1.5
parts of oil. Bayluscide is lethal for fish. There is no
evidence of resistance by the snails to the compound.
Some compounds containing tin, copper lead and niclosamide
have been used impregnated in latex or other support materials
for slow release into ditches, pools or streams containing
snails. Tributyltin oxide was found to be highly effective in
this form exhibiting toxicity for more than six months (Souza
& Paulini 1969).
MOLLUSCICIDES OF VEGETABLE ORIGIN
The study of plants exhibiting snail toxicity has been
encouraged with the aim of finding alternatives for use in the
fight against snail vectors. The World Health Organization has
published reviews of this work listing plants with recognized
molluscicides activity (Marston & Hostettman 1985, Kloos & Mc
Cullough 1987, Mott 1987). These plants and compounds with
molluscicide activity were also reviewed by Mc Cullough
(1992). These molluscicides of vegetable origin, however,
exhibit low toxicity for snails embryos.
MOLLUSCICIDE APPLICATION
Molluscicide application is the most important method of
aquatic snail elimination particularly of the genera
Bulinus and Biomphalaria. Snails of the
Oncomelania genus, hosts for S. japonicum, are
more difficult to destroy using molluscicides as they are
amphibious and environmental alterations are more
effective.
Types of habitat of aquatic snails: (a) Natural:
represented by shallow water with slow or moderate current and
strong solar illumination; (b) Opportunistic: occur in
certain appropriate areas where there is sufficient vegetation
and which are due to the spread of snails by the current as on
the margins of streams and rivers; (c) Artificial:
represented by irrigation ditches, tanks and furrows of low
current gradient which are continuations of streams, lakes or
reservoirs used for pisisculture, horticulture or
agriculture.
They are responsible for the high prevalence of
schistosomiasis in endemic areas and are thus called epidemic
habitats (Freitas 1968).
The type of habitat and the various types of aquatic plants
found in each, in addition to other variables, affect the
application and dispersion of molluscicides. The methods of
application in still or running water are different.
When transmission is seasonal a minimum of three molluscicide
applications per year are recommended: (a) the first
immediately after the first rains; (b) the second around six
months later; (c) the third in the dry season.
The time of molluscicides application may be determined by
rainfall, temperature, water usage and the snail population
density or may be based on chemotherapy programs. The decision
should be made after a period of observation of the conditions
in each geographical region.
Normally three people are required for molluscicides
application: a technical field supervisor and two
assistants.
The calculation of the quantity of product to be used in each
breeding site is based on the volume of water and the rate of
flow. Rate of flow is obtained by the following formula:
flow = velocity x width x 0.85 = m^3/second.
Volume is calculated by:
volume = width x depth x length = m^3.
Equipment used for molluscicides application: (a) in
running water: containers of 20, 60 or 120 liters fitted with
a tap, a spray pump or watering can; (b) in still water: spray
pump or watering can.
Evaluation is made 24 hr after application by direct
observation of the snails in the breeding site.
List of material used in the field: tape measure,
string, long handled scoop, counter, paper, thermometer, stop
watch (or watch), screen cages for placing sentinel snails,
steak, screen, molluscicides, polystyrene, equipment for
application, container for dilution of the molluscicides,
funnel, overalls, alcohol, gloves and mask.
Mc Cullough (1992) has produced tables of calculations of the
quantity of Bayluscide to be used according to the volume of
water in breeding sites with still or running water.
EVALUATION OF MOLLUSCICIDE APPLICATION
For the most effective evaluation a number of snails captured
in the breeding site to be treated should be placed in cages
made of nylon screens or metal and put both in the area to be
treated as well as in another treatment free area in order to
act as controls for mortality. Twenty four hours after
treatment mortality in the treated area should be high and nil
or very low in the control area.
If cages containing snails are not used, the population
density should be measured one week before treatment and one
week after treatment using a long handled scoop for collection
or alternatively wooden or metal pincers for a standardized
time.
The amphibian snails of the genus Oncomelania are
collected with pincers and the density calculated on the basis
of the number of snails collected per person per minute (Mc
Cullough 1992). During collection before treatment the risk of
exposure to Schistosoma cercariae should be avoided.
In the future, new strategies will be necessary for using
molluscicides formulated for slow release, amongst other
modifications, as well as the development of molluscicides of
vegetable origin in endemic regions (Mc Cullough 1992).
BIOLOGICAL CONTROL
An alternative method used in the control of snail vectors is
the use of predatory organisms or competitors which can
control the expansion of the snail population and eventually
eliminate the snails from the breeding site.
Biological control has been undertaken principally with
snails such as Pomacea haustrum (Milward de Andrade
1972) in Brazil and Marisa cornuarietis (Ruiz-Tiben et
al. 1969) in Puerto Rico. In the northeast of Brazil a B.
straminea strain resistant to S. mansoni has been
used to combat B. glabrata (Barbosa et al. 1981).
Another snail used as a competitor is Helisoma duryi
(Abdallah & Nars 1973). A number of fish species such as
Tilapia melanopleura, Astronotus ocellatus, have been
used to control snails (Milward de Andrade & Antunes 1969,
Motta & Gouvea 1971, Feitosa & Milward de Andrade1986) and
aquatic birds such as ducks (Michelson 1957), chelonian
(Coelho et al. 1975) have also been employed as snail
predators. In addition, a number of other types of predators
such as mosquito larvae and other diverse insects have also
been described (Berg 1964).
In the laboratory a small leeche, Helobdella trise-
rialis lineata and ostracods crustacia have been
found to be good snail predators (Sohn & Hornicker 1972,
Guimar es et al. 1983). In the field, however, these animals
are found in snail breeding sites in an ecological equilibrium
with the snails. Some aquatic plants such as Characeae
have been used to combat snails vectors (Renno 1958). The
pathological action of bacteria such as Bacillus pinotti
against B. glabrata has also been studied (Texera
& Vicente Scorza 1954). Follow up studies were not able to
confirm the action of the latter two possibilities.
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Copyright 1995 Fundacao Oswaldo Cruz (Fiocruz)
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