The early success in malaria control can be attributed to the use of professional spray teams who treated the inside of huts with DDT, without any direct involvement of the infected population.
OTHER APPROACHES TO CONTROL
The early success in
malaria control can be attributed to the use of professional spray teams who
treated the inside of huts with DDT, without any direct involvement of the
infected population. However, the problem with pesticides like DDT is that they
lack specificity, and as application is not always well directed there is often
destruction of a wide range of insects, which may have undesirable side
effects. Further problems include the accumulation of pesticide residues in the
food chain and pesticide resistance in the target organism. Window screens and
bed nets do prevent mosquito bites, however, and there has been a lot of
interest in using bed nets impregnated with insecticide. Environmental control
was a major strategy used before the development of modern insecticides. A good
example of this is mosquito control through the removal of breeding sites by
drainage, land reclamation projects, removal of vegetation overhanging water,
speeding up water flow in canals, and periodic drainage and drying out of
canals. Life cycle forms that enter the water system, such as cysts and oocysts
of Giardia and Cryptosporidium, can present a major public health problem. These forms are often resistant to common
disinfection methods and require physical removal from waters. Cysts and
oocysts can be destroyed by use of proper sewage treatments such as anaerobic
digestion, but these systems require regular maintenance in order to remain
effective.
Biological control is an
active but developing area. Genetic control of insect vectors, particularly the
use of irradiated sterile males, has been widely publicized, and the release of
chemically sterile males has been attempted to control anopheline mosquitoes.
Other similar methods include the release of closely related species within the
environment in order to produce sterile hybrids. Genetically modified
mosquitoes are currently being developed that are resistant to Plasmodium infection, and larvivorous
fish have also been employed for mosquito control; other organisms considered
for the same purpose include bacteria, fungi, nematodes and predatory insects.
One of the best studied agents is the
bacterium Bacillus thuringiensis; the
spore or the isolated toxin from this
species can be used as a very effective and specific insecticide.
Where exposure to
infection is likely to occur, killing the parasite as it enters the host is a
sensible approach to control. There are two options available, chemoprophylaxis
or vaccination. Unfortunately long term chemotherapy can have adverse side
effects, and in the absence of symptoms members of the atrisk population may fail
to take the treatment. Vaccination would seem to be the ideal method of
parasite control, as lifelong resistance may result from just a single
treatment. Despite a huge amount of effort, the only successful parasite
vaccines are those for the control of veterinary parasites. However, there has
been significant success with the development of recombinant vaccines for the
control of malaria. The recent development of DNA vaccines may be of use in the
control of parasites. In this method the DNA encoding an important parasite
protein is injected into host cells and the foreign ‘vaccinating’ protein is
synthesized in or on the surface of the cell. This intracellular foreign
protein enters the cell’s major histocompatibility complex (MHC) class 1
pathway resulting in a cell mediated immune response. In contrast, a protein
that is extracellular enters the MHC class 2 pathway, which results primarily
in an antibody or humoral response.
Despite advances in
technology, few commercially available parasite vaccines exist and there are
none for use in humans. Several vaccines are being evaluated in clinical trials
and most of these are against malaria. Information about these can be found at
the Malaria Vaccine Initiative website (www.malariavaccine.org). In addition
there are vaccines being evaluated against other parasites, for example a Leishmania vaccine co-administered with
BCG (bacillus of Calmette and Guérin) is under trial. Recently, DNA and viral
vector-based vaccines have been tested in clinical trials including one utilizing
an adenovirus human serotype 35 based vector encoding the malarial circum-sporozoite
protein.
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