Disease Control Methods

General Information




The approaches to the control of parasitic diseases are
crucial components of the control strategy. The realization of the various
approaches is dependent upon the use of individual measures the selection of
which will be determined inter alia by expected efficacy, convenience, economy
and acceptability. In most cases a variety of measures will be required
simultaneously. This applies particularly to anthropozoonoses and zooanthroponoses with highly adaptable biological systems. Such diseases are the most
difficult to control, especially if nondomestic animals are involved as reservoirs of infection. Another general
aspect is man's awareness of parasitic diseases affecting humans and livestock,
and the motivation for taking remedial action. If such broad motivation is
lacking among the afflicted population, it is likely that imposed control
programmes will have only limited and ephemeral success. Health education in the widest sense,
encompassing both the health of humans and of domestic animals, should
therefore prepare the ground for a systematic effort against the diseases
affecting the community.


The simplest measures for achieving a set purpose are usually
the best, but in their planning and execution due attention should be paid to
acceptability, compatibility with cultural and religious background, and
technical feasibility. For instance, it would be expecting too much if the
dietary patterns of large populations were to be changed abruptly. Here the
practical solution will consist of rendering the incriminated food safe rather
than banning it. The adoption of particular individual protective measures will
depend on the person's economic status. Wearing shoes will generally protect
against ancylostomiasis, and the use of impregnated
bed nets supports protection against malaria. However, shoes and bed nets have
their price and not everybody may be able to afford them or even be willing to
use them. Selection of the most appropriate measures for disease control
requires therefore a sound appreciation of advantages, limitations and
disadvantages of the methods.


Parasitic diseases encompass a wide range of biological
systems. Hence, the control of these diseases has many facets, implying a host
of different measures the most important of which are detailed in the following
sections.

Water Supplies




Water is one of the most important vehicles of parasitic
diseases. It harbours a number of pathogens which can reach the human or animal
host through transdermal penetration (e.g. Schistosomiasis, Man) or through ingestion
(e.g. dracunculiasis). It is also the medium through
which the larvae of many parasitic species reach molluscan hosts, ultimately to
be transmitted to man or livestock as a food-borne pathogen. Safe water is
therefore an important means of controlling numerous parasitic diseases,
especially helminth infections (in addition to
controlling the transmission of nonparasitic water-borne pathogens). Safe water
should be available for consumption, bathing, washing and leisure activities.
Ideally, piped, treated water should be available for household use. However,
this will not be feasible as yet in most of the vast rural areas in the tropics
and subtropics. Well-maintained deep wells with elevated rims made out of
masonry or concrete for the prevention of contamination will be an acceptable
and feasible alternative in many places. The use of traditional step wells or
ponds should be discouraged, but, if there is no other source, boiling or
sieving water through a fine mesh may render it largely innnocuous. The
installation of a supply of piped, treated water may permit the abolition of
unhealthy water collections. There may be public objection to this if the water
collections are used for producing food (e.g. fish, crabs) or for the
irrigation of crops. However, larger pools can be constructed on a community
basis and maintained in such a way that they do not permit the transmission of
pathogens while fulfilling the purpose of pisciculture and serving as a source
of water for agricultural and household needs.

Excreta Disposal




Most helminth eggs or larvae have to reach water or humid
ground for further development. They achieve this as a result of urination or
defecation into water or onto wet soil. This may be part of a deliberate
pattern, e.g. for the fertilization of family fish ponds in some parts of
eastern Asia. In other instances it is due to
an ingrained behavioural pattern or due to the lack of appropriate facilities
for the safe disposal of excreta, or a lack of incentive for using available
facilities. Health education is probably the most
important remedial factor in such situations. It is not advisable to embark on
a major programme of building latrines before the population is willing to use
them. This applies especially to rural areas in which the population has easy
access to various types of surface water. The acceptability of latrines or of
even better facilities for excreta disposal is usually higher in urban areas
where the installation of sewage treatment will also often prove to be feasible
and cost effective. If the right type of sewage treatment plant is chosen, the
resulting sludge will be biologically safe and usable as fertilizer.

Agricultural Hygiene




The agricultural, pastoral and piscicultural environment is
often intimately associated with the transmission of parasitic diseases.
Agricultural labourers may serve as a source of infective material, especially
if they do not dispose of their excreta in a safe way whilst in the fields.
They are also exposed to a variety of pathogens, particularly in irrigated
areas. In addition to these occupational aspects, the use of unsafe biological
fertilizers (fecal matter) on vegetables will promote the spread of some
parasitoses, e.g. amoebiasis and ascariasis. Another important
feature is the grazing of livestock in wetland areas. Again, health education
and community efforts towards the development of safe grazing areas, e.g.
through drainage, will be required to remedy the situation. Particular
precautions should be taken when using wastewater and excreta in agriculture
and aquaculture. Improperly managed water resource development entails the risk
of the propagation of parasitic diseases. This should be avoided through
appropriate water management.

Personal Hygiene




Apart from the obvious impact of unsafe excreta disposal, the
lack of personal hygiene is a leading cause of infection with a
variety of parasitic pathogens such as Giardia lamblia, Entamoeba histolytica and Enterobius vermicularis. Washing hands
after defecation and before eating would largely reduce the transmission of
these pathogens. The use of water and soap would also impede the transition
from reversible lymphoedema to irreversible elephantiasis in lymphatic filariasis. However, personal
hygiene must go further than water and soap. It should include the seeking of
treatment if there are symptoms of disease, and the avoidance of dangerous
foodstuffs and of situations conducive to the contraction of infections. Health
education will be an important vehicle for imparting the necessary knowledge,
awareness, and habits. This process should start at as early an age as possible
and schools will have to play a major role in this endeavour.

Housing




Siting and type of human habitations are closely related to
the risk of contracting certain parasitic diseases. Houses with cracked
masonry, mud walls and/or earth floors were found to be a particularly suitable
environment for reduviid bugs responsible for the transmission of
Chagas disease. Simple housing improvement was found to reduce or even remove
the risk of infection. Siting of settlements away from mosquito breeding
grounds was an empirical yet highly effective means of protection against
malaria. The siting of settlements at a long distance from irrigation canals
(accompanied by the provision of safe household water) is an effective
preventive measure against schistosomiasis since it will reduce the
frequentation of the canals for washing, bathing and swimming.


Type and standard of housing play a major role in allowing
the entrance and exit of disease-carrying mosquitos. It also determines the
feasibility of mosquito and fly proofing, and the efficacy of ancillary vector control measures such as mosquito coils
and knock-down sprays.

Environmental Management




Some measures of environmental management as a means of disease
control have been known since ancient times. Environmental management was the mainstay of
malaria control before the advent of residual insecticides and synthetic antimalarials. It
is making a comeback due to the limitations of other methods. The applicability
of such measures in the control of parasitic diseases is very wide. The most
important methods belong to environmental sanitation and water management.
Water collections of various types are known to be breeding grounds for
arthropod vectors of disease and the homestead of intermediate hosts of many
helminthic organisms. Unless local economic (piscicultural and agricultural) and
ecological reasons militate against them, filling, levelling and draining
operations (drains, canals, and use of trees) will be appropriate measures. The
same applies to the sanitation of wetlands to be converted into safe land for
agriculture and livestock.


Environmental sanitation, including peridomestic areas and
the safe disposal of waste, is a field in which individual and community
initiative can be used to great advantage, the more so when the necessary
equipment is easily available and cheap (e.g. pick-axes and shovels) or
obtainable on loan from various government departments (e.g. earth-moving
machinery).


Water management applied to water-storage reservoirs (level
management) and irrigation systems (watering and drying cycles) will facilitate
disease control by rendering the areas unsuitable as a habitat of intermediate
hosts or vectors of parasitic diseases. Water management should be an integral
part of design and operation of water impoundments and irrigation schemes.

Control of Vectors and
Intermediate Hosts




The control of vectors and intermediate hosts of parasitic
diseases may, to a large extent, be achieved through environmental sanitation.
However, in some situations the applicability of such measures will be severely
limited, as for example, in the control of Simulium spp., the vectors of
onchocerciasis. Similarly, widespread temporary breeding places occurring
during the rainy seasons in the tropics may pose insurmountable obstacles to
environmental management. Alternative control approaches are therefore
necessary.


At the beginning of the twentieth century mosquito control
was improved by the use of light oils and chemicals such as Paris Green. In
spite of their efficacy the application of these measures has remained quite
limited due to the need for repetitive use and high cost. The introduction of
chemical insecticides has not fundamentally changed the situation. Moreover,
ecological considerations and non-target effects against the aquatic fauna and
flora restrict the widespread repetitive use of insecticides. Chemical larvicides, rapidly biodegradable insecticides
with low non-target toxicity, are still useful in the rapid control of
epidemics of some vector-borne diseases. The same applies to the control of Cyclops spp., the intermediate hosts of Dracunculus medinensis and various
other helminths.


Biological methods for larval control have a
long tradition inasmuch as larvivorous fish, e.g. Gambusia
affinis, have been used since the beginning of the twentieth century.
Although appealing as a natural solution to a natural problem, larvivorous fish
have a limited usefulness since seasonal and shallow breeding places are not
suitable for their maintenance. It may also be difficult to find a local
species of larvivorous fish. The introduction of non-local species may have a
disastrous impact on the local aquatic fauna and interfere seriously with the
production of food fish species.


Bacterial toxins from Bacillus thuringiensis
and Bacillus sphaericus are selectively
directed against mosquito larvae and being used in the control of Culex spp. and Aedes spp. As the microorganisms sink to
the ground they are not suitable for controlling Anopheles spp. (surface feeders). B.thuringiensis
does not reproduce in the breeding places, but B.sphaericus does to some
extent but not sufficiently to relinquish the need for regular retreatment of
the breeding places.


The intradomiciliary application of residual insecticides
such as chlorinated hydrocarbons (DDT), organophosphorus compounds (malathion),
carbamides (propoxur), fenitrothion and synthetic pyrethroids is suitable and
often quite cost-effective for the control of adult endophilic mosquitoes. However, the occurrence of
specific resistance, aided and abetted by the agricultural use of insecticides
of the same chemical groups, the presence of exophilic mosquitoes, increasing
cost of insecticides and labour, and rising ecopolitical constraints have
reduced their usefulness or applicability. Their use is still important,
though, in the control of threatening or manifest epidemics where they are
generally applied on a focal basis. These are situations where the
ultra-low-volume (ULV) dispersal of suitable insecticides, may also show rapid
effect. On the whole, the use of integrated vector control opens better
prospects for an environmentally acceptable control of arthropod-borne
parasitic diseases.


Pyrethroid-impregnated bed nets (deltamethrin or permethrin)
or impregnated curtains and screens bar or reduce the contact between man and
vector. They gained a firm place in malaria control,. especially in areas with
moderate or intensive transmission. Their efficacy is due to a repellent effect
rather than specific insecticidal action, promoting also epidemiologically
desirable vector deviation to animals.


The control of aquatic snails continues to present a serious
problem. Environmental management is the only effective and widely acceptable
procedure for snail control. The available molluscicides are either not
sufficiently effective (e.g. copper sulfate) or they are too toxic for the
non-target fauna, including fish.
Diagnosis




The ability to diagnose the presence of infections is an
important factor in guiding the treatment of individuals, and forms the basis
of epidemiological assessment which should enable the health authorities to
determine the dimensions of the specific human and/or animal health problem. It
is also an essential tool for monitoring the impact of disease control
activities. Macroscopic and/or microscopic diagnosis of parasitic diseases may
be relatively simple and reliable with some parasite species, especially
intestinal helminths, but exceedingly difficult with others, mostly
tissue-dwelling parasites, e.g. certain types of nematodes. Serological methods based on the
detection of specific antibodies usually reflect past or present host-parasite
contact and therefore do not provide proof of current infection. Similarly,
relatively fresh infections may not have given rise to detectable antibodies as
yet and show seronegativity in spite of the living pathogen's presence.
Demonstration of circulating antigens is a more reliable and specific basis for
diagnosing current infections. Rather simple and reliable antigen detection
tests have been developed for several human parasitoses, e.g. infections with P. falciparum or W. bancrofti.
They are based on the detection of highly specific parasite antigens. However,
relatively high costs continue to restrict their use in the framework of
control programmes. The same still applies to tests for the detection of
lactate dehydrogenase from malaria parasites.


Identification of infections by polymerase chain reaction
(PCR) has been developed for numerous parasite species. However, the routine
use of PCR is currently limited to research institutions and to diagnostic
laboratories in prosperous countries. Its cost and operational requirements are
too high to be affordable by most of the tropical countries.


In order to be widely practicable, diagnostic techniques for
the most important human and animal parasitoses must be simple, cheap, and
undemanding in terms of sophisticated equipment, electricity supply and
operator skill. Human and veterinary health services in many parts of the world
still lack the infrastructure required for establishing reliable data on
prevalence and incidence of major parasitic diseases and associated mortality.
This accounts for serious deficiencies in national and international disease
statistics.

Treatment




Effective agents for the treatment of numerous parasitic
diseases are available (see chapters on disease control). Some are reasonably
cheap, such as those for the treatment of intestinal nematode infections of
humans and domestic animals. Other medicaments are expensive, such as
third-line drugs for the treatment of falciparum malaria. High costs may limit their
use or encourage sub-optimal medication, and consequently allow a parasite reservoir to be maintained that will be an
obstacle to the effective control of the disease concerned. There are, however,
a large number of parasitoses for which the therapeutic armamentarium is
grossly deficient, e.g. Chagas disease, kala-azar and liver fluke infections.


With regard to malaria the situation was relatively
satisfactory after the wider introduction of the 4-aminoquinolines in the late
1940s. However, the advent of chloroquine resistance in P.
falciparum has compromised the efficacy of this group of drugs in wide
parts of tropical Asia and South America. In
the hyper- and holoendemic areas of tropical Africa juveniles and adults
continue to derive therapeutic benefit from chloroquine, but young children
whose immunity is not yet sufficiently developed generally require treatment
with alternative drugs. Resistance to the first-line alternative
drugs, namely combinations of sulfonamides with pyrimethamine, already affects
large areas in southeastern Asia and South America, and is rising in parts of
tropical Africa, necessitating the use of
second-line alternative drugs which are considerably more expensive. Resistance
to mefloquine and structurally related quinine occurs in Cambodia, parts of southern Viet Nam and eastern Myanmar
and in some parts of Thailand
bordering on Myanmar and Cambodia. Here
combined treatment with artesunate or artemether with mefloquine still yields
satisfactory results.


On the whole the veterinary health sector has had greater
success in the development of antiparasitic drugs than the human health sector.
This is largely due to a better financial endowment of agricultural and
livestock development and to the more stringent toxicological requirements
governing the registration of medicaments for use in man. The situation is
compounded by the fact that the development of medicaments against human
parasitoses holds little attraction for the pharmaceutical industry since the
main market for such drugs is in poor tropical countries.

Immunization




In many parasitic diseases there is evidence of the natural
development of immunity to the specific pathogen. Such immunity rarely induces
total refractoriness to reinfection, but it will restrict parasite reproduction
or acceptance and induce tolerance to the pathogen. The development of immunity
is quite slow. Considerable efforts have been made in the field of immunization
against parasitic diseases, especially against those caused by protozoa. In bovine babesiosis, attenuated
live Babesia bigemina is being used for inoculation
of livestock. The resulting immunity is satisfactory, but there is still
significant mortality associated with vaccination which is, on balance,
economically acceptable. Such approaches are not feasible in human parasitic
diseases except for agents with low virulence, e.g. Leishmania
major.


Although immunization holds substantial promise in the
control of many parasitic diseases and the progress in gene technology and
polypeptide synthesis is likely to pave the way to economically acceptable
products, there is still a long and arduous way to go before well-tolerated and
reliable vaccination will become a reality in the control of parasitic
diseases.

Clinical Relevance

Diagnostic and therapeutic measures have direct clinical
relevance. Other disease control measures have indirect clinical relevance
inasmuch as they are geared to the reduction of the community's disease burden,
and thus a lessening of the pressure on the health services.