Other Approaches to Control - Control of Protozoan Parasites

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Chapter: Pharmaceutical Microbiology : Protozoa

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.



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


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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