Preventive Methods
Disease prevention implies activities designed to protect patients or other members of the public from actual or potential health threats and their harmful consequences.
It is a branch of healthcare which focuses on the prevention of disease, both in individuals and communities, rather than cure.
It involves a number of branches of science and medicine that are intertwined in their contributions to disease transmission. Several methods are used to prevent diseases but we will concentrate on modern methods using emerging biotechnology methods.
1. Biosynthetic vaccine
As already described, the objective of vaccine development over the years has been to identify the important antigens responsible for protection and to produce them in the purest form.
Until recently, vaccines have been made from attenuated living organisms or inactivated organisms. Recently, however, advantages of biotechnology have made recombinant DNA technology (rDNA, genetic engineering) available to help produce defined antigens, or antigenic determinants, on a large scale and in a cost-effective manner.
The isolation of these antigenic determinants from the surface of infectious agents represents the first step in trying to produce a more specific antigen.
Since these determinants occur in repeated subunits and their production is controlled by specific genes in the nucleus of the organism, these genes may be used to produce antigenic determinants.
By isolating the specific gene (DNA) that encodes for the surface antigenic determinant, and by using a plasmid method to insert this gene as a bacteria, yeast, or mammalian cell, the gene recombines with the cell’s own genes to produce the antigenic determinant along with other cellular products.
The antigenic determinant may be isolated and used as an immunogen. This immunogen will be recognised by the immune system as being foreign and will stimulate the production of antibodies or a cellular response that will protect the animal or prepare the animal’s immune system for future infection with the infectious agent.
Antigenic determinants can be produced by growing the cells on a large scale and collecting arid purifying the antigen as it is expressed. The antigen may have improved characteristics compared to the antigen derived from the whole organism.
These characteristics are purity, safety, and stability. Also, the risk of having the vaccine contaminated with infectious material used in production of the whole organism is reduced. All of these characteristics help in developing improved vaccines.
2. Chemical Synthesis of Vaccines
Another method of producing antigenic determinants is chemical synthesis. Most antigenic determinants are proteins composed of chains of amino acids. Individual amino acids may be linked together in a linear form to mimic antigenic determinants. So, if the amino acid sequence of the native antigenic determinant is known, it can be made synthetically.
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Biotechnology has evolved methods to determine the amino acid sequence of antigenic sites by isolating the gene that encodes for it. The gene is composed of DNA that contains the genetic code in its nucleotide sequence.
This nucleotide sequence can be determined and translated into an amino acid sequence (the bases, adenine, thymine, guanine and cytosine code in triplet- combination for each amino acid).
Thus, an amino acid sequence for a surface protein may be derived from the nucleotide sequence of its gene. Only a small part of this surface protein may be required to produce an immunogen.
The peptide can be made by sequentially adding amino acids. Forty or 50 amino acids may be joined together in a linear sequence forming a peptide by using an amino acid synthesizer controlled by a computer program.
The peptide is removed from the resin and may be coupled to a carrier protein or polymerized to increase its size. These forms of the antigenic determinant have been found to be active in inducing humoral and cellular immune responses.
The advantage of chemically synthesized peptides over biosynthesized peptides is that the chemical process is more precise and reduces the variability found in a biological process.
This precision leads to further improvement in purity. There also is no chance that an infectious agent or foreign nucleic acid will find its way into a chemically synthesized product.
3. Parasite-based Control
1. Gene Therapy
Gene therapy is a procedure that is used to treat genetic and other related diseases. The technique is based on the introduction of copies of a “healthy” gene from one cell or organism into the body cell of the same or another organism. The disease is controlled if the introduced gene(s) work normally.
This is called somatic gene therapy because it introduces the gene into a somatic or body cell. Any cells that could divide to form sperms or eggs will not have genes introduced into them through somatic gene therapy.
Somatic gene therapy is intended solely to eliminate the clinical consequences of the disease in an individual and is therefore not inherited. Future generations are, therefore, not affected by the therapy because the inserted gene is not passed down the hereditary line.
This contrasts with the Germ line gene therapy, involving the insertions of a healthy gene into the fertilized egg of an animal that has a specific genetic defect. This has been performed successfully in several animal studies. The new gene is obtained in every cell in the body including reproductive cells.
There are three overwhelming technical problems that are preventing consideration of this technique for the use in human beings. “The first is that scientists have no way of diagnosing genetic disorders in the fertilized egg.
Secondly, the procedure is most often used to insert genes into fertilized eggs – injection with a microscopically guided glass needle – has a high failure rate and thirdly, the problem is lack of control over where the gene is inserted into the embryo’s genetic machinery.
Procedure for gene therapy is to first identify cells affected by faulty genes through a painstaking diagnosis using symptoms and signs as a first line lead. The affected cell is called the target cell.
When faulty genes are suspected, the first step is to compare their functions with those of the ‘healthy genes’. Once fault is confirmed detailed investigation of how it affects the chemical reactions within a cell is undertaken.
Estimates are made to determine if the reactions could be reversed by drug therapy or not. Where a reaction is reversible by drug therapy, and an effective drug is known, this option is taken because it is cheaper and simpler.
In some cases, it could lead to the development of new drugs or new line of treatment. Where it is obvious that drug therapy is not an option or where it has failed, gene therapy is considered and carried out where this technology is available.
Human sufferings, due to inherited diseases, have been reduced more by the use of genetic diagnosis than any other medical technology. It is important to identity the gene responsible for a genetic disease before beginning to consider gene therapy.
2. Genetic Engineering Drugs
The war against infectious agents has produced a powerful arsenal of therapeutics, but treatment with drugs can sometimes exacerbate the problem. By killing all but the drug-resistant strains, infectious agents that are least susceptible to drugs survive to infect again.
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They become the dominant variety in the microbe population, a present-day example of natural selection in action. This leads to an ever-present concern that drugs can be rendered useless when the microbial world employs the survival-of-the-fittest strategy of evolution.
And frequently used drugs contribute to their own demise by strengthening the resistance of many enemies.
Drug-resistant pathogens whether parasites, bacteria, or viruses can no longer be effectively treated with common anti- infective drugs.
A healthy future for the world’s population will depend on engineering new strategies to overcome multiple drug resistances. Over the past two decades, many genetically-engineered drugs have been developed and approved for the treatment of patients.
Typically, these drugs are characterized by a high and specific activity in the presence of optimal safety. They include hormones, enzymes, growth and coagulation factors, antibodies as well as vaccines.
All these proteins are generated using recombinant DNA technology. An expression vector with the gene encoding for the protein of interest is introduced into an appropriate microorganism or cell line.
The biochemical machinery of the host cell then translates the genetic information into the corresponding protein. Large scale production of the recombinant drugs uses biotechnological processes. The genetically- modified organisms are grown in bioreactors from which the desired protein is finally isolated and purified.
One major challenge in this endeavor will be to understand more fully how drug resistance comes about, how it evolves, and how it spreads. Furthermore, the system for finding and developing new drugs must itself evolve and entirely new approaches to fighting pathogens may be needed also.
Genetic engineering drugs have also provided the opportunity for the development of personalised medicine. This is the process of combining genetic information with clinical data to optimally tailor drugs and doses to meet the unique needs of an individual patient.
Doctors have long known that people differ in susceptibility to disease and response to medicines. But, with little guidance for understanding and adjusting to individual differences, treatments developed have generally been standardized for the many, rather than the few or the individual.
It is also known that though individuals contain about 20, 000 genes encoding three billion letters of information, the specific information that gives an individual his/her distinct characteristics is encoded in less than 1% of the genes. Emerging genetic engineering techniques can now be used to identify these special genes in each individual and tailor treatments for that individual along these lines.
Ultimately, the personalization of medicine should have enormous benefits. It will make disease (and even the risk of disease) evident much earlier, when it can be treated more successfully or prevented altogether.
It could reduce medical costs by identifying cases where expensive treatments are unnecessary or futile. It will reduce trial-and- error treatments and ensure that optimum doses of medicine are applied sooner.
Most optimistically, personalized medicine could provide the path for curing cancer, by showing why some people contract cancer and others do not, or how some cancer patients survive when others do not.
4. Vector-based Control
1. Sterile Insect Technique (SIT)
Sterile insect technique (SIT) is a biological control method that uses sterile male insects to reduce the reproductive rate of a species of target insect. The technology involves a deliberate genetic manipulation of male insects of a target insect, e.g. mosquito to make their males sterile.
It is effective in many insect species because the female only mates once during her lifetime. She carries her mate’s genetic material with her for the rest of her life and may lay several batches of eggs, but in many cases, she only receives genetic material from a male a single time during her life.
If the genetic material she receives from the male fails to produce offspring, then the female will be unable to lay eggs that hatch into young insects. This technique works well with Screw flies (Cochliomyia hominivorax), an ecto-parasite of mammals, for an example.
The Screw fly lays its eggs into the open wound of a large animal like a cow, goat, or sometimes even a human. When the eggs hatch the larvae feed on the flesh of the host animal inflicting pain and injury that sometimes could be fatal. Tsetse fly that causes sleeping sickness in parts of Africa, and the Mediterranean fruit fly, a pest of citrus crops has also been controlled through the sterilisation technique.
The most widely used method of SIT is ionising radiation using gamma isotopic sources (such as cobalt-60 or caesium-137), high-energy electrons or X-rays. Other methods of sterilisation include elevated temperature, chemical liquids or gases although these are still not well established.
Sterilisation by ionising radiation might weaken the newly sterilised insects, if doses are not correctly applied, making them less able to compete with wild males.
However, Insect irradiation is safe and reliable when established safety and quality-assurance guidelines are followed. The key processing parameter is absorbed dose, which must be tightly controlled to ensure that treated insects are sufficiently sterile in their reproductive cells and yet able to compete for mates with wild insects.
To that end, accurate dosimetry (measurement of absorbed dose) is critical. Irradiation data generated since the 1950s, covering over 300 arthropod species, indicate that the dose needed for sterilisation of arthropods varies from less than 5 Gy for blaberid cockroaches to 300 Gy or more for some arctiid and pyralid moths.
Factors such as oxygen level, and insect age and stage during irradiation, and many others, influence both the absorbed dose required for sterilisation and the viability of irradiated insects. Consideration of these factors in the design of irradiation protocols can help to find a balance between the sterility and competitiveness of insects produced for programmes that release sterile insects.
Many programmes apply “precautionary” radiation doses to increase the security margin of sterilisation, but this overdosing often lowers competitiveness to the point where the overall induced sterility in the wild population is reduced significantly.
Recent development in science has made it possible to apply SIT to the control of mosquitoes, the vector of malaria and many other infectious diseases. It was initially thought that this method cannot be used on mosquitoes because when they lay eggs the eggs harden too quickly making it difficult to apply traditional sterilisation treatment.
Recently, genetic engineering method was used to delay the hardening of the eggs by altering the genetic material of the mosquitoes. This variation of sterile insect technology does not use radiation to sterilize insects, but genetic modification or genetic radiation. This is also known as recombinant DNA technology.
It works by adding a Dominant Lethal gene to the mosquitoes. The DNA in such genes can be suppressed while the mosquitoes are being bred in the lab, but once released in the wild, become active.
The gene either causes any mosquito that carries it to die before they are able to reproduce, or make them unable to function as a host to the malaria parasite. This is an exciting new technology that might be used to drastically reduce the number of yearly malaria infections.
Insect sterilisation technology does have its drawbacks. Repeated treatments are often required for the method to be effective. It is more expensive than ordinary pesticides and many insects must be bred in factories and released into the wild.
It can sometimes be difficult to separate the insect sexes for sterilisation. Also, the technology is species specific, so while there are twenty two species of Tsetse fly living in Africa, sterile males would have to be produced for each different species.
Furthermore, when radiation is used it can affect the health of the male insect, causing it to be less likely to mate. This reduces the effectiveness of the effort.
Despite the drawbacks, sterile insect technology is a promising tool to fight insect infestations and insects spreading diseases around the world. It has the benefit of not using chemicals that affect the environment or any species other than the target species.
Besides, recent results have greatly improved the fitness of genetically-modified insects compared to wild populations with new approaches such as the post-integration elimination of transposon sequences, stabilising any insertion in genetically-modified insects.
Encouraging results, suggest that SIT alters some metabolism processes that also affect the viability of offsprings from released parent insect in the wild. Recent studies on vector symbionts would also bring a new angle in vector control capabilities, while complete DNA sequencing of some arthropods could point out ways to block the deadly impact on animal and human populations.
These new potential approaches will improve the levels of control or even in some cases would eradicate vector species and consequently the vector-borne diseases they transmit.
5. Microbe-based Control
The role of microbial populations in the control of insects of medical and veterinary importance has expanded considerably with the discovery and development of new microbial control agents and genetic improvement in bacterial and viral pathogens, and improvements in formulation, application options and compatibility with other interventions.
Several species of bacteria, viruses, fungi, protozoans and nematodes are now used as agents of control either naturally or genetically-modified. The most widely used of all microbial control agents is Bacillus thuringiensis. The isolation within the past two decades of new strains that are larvicidal for certain Diptera and Coleoptera has increased the utility of the bacterium considerably.
Further improvements in efficacy and broadening of its host range are in progress with the isolation of strains with new toxins and the manipulation of B. thuringiensis genes that encode toxin production using both recombinant and nonrecombinant methods.
Genetic manipulation of these genes has also enabled their incorporation into crop plants. The development and commercial availability of entomopathogenic nematodes in the families Steinernematidae and Heterorhabditidae expands the options for the control of insects, especially those with soil inhabiting stages.
The results of natural epizootics of fungi and viruses often make the need for additional interventions unnecessary. Recent understanding of the genetics of Baculovirus and subsequent gene manipulation has increased their virulence and utility.
It is now possible to produce this virus using insect cell culture technology. Hopefully, this will not only make it more affordable but also easily available. Fungi continue to offer the only control options using entomopathogens against plant sucking insects.
Although fungi have great potential for development as microbial control agents, only a few have been used on an operational scale. Potential for development of resistance is also high in this technology and might limit its use.
Since microbes very effective as control agents and have minimal environmental impact, they are ideal components of integrated pest management. However, if they are used merely as replacements for chemical pesticides, then eventually these agents will face some of the same fate as the chemicals they replace, particularly with respect to resistance.
Another approach for reducing disease transmission by arthropods is to genetically modify symbiotic bacteria of arthropod vectors to prevent the arthropods from transmitting human pathogens. By this approach, the arthropod is not transformed rather the symbiotic bacteria that it harbors are changed genetically.
Such arthropods are called para- transgenic. This approach is based on the assumption that
1) Many arthropods (especially those that throughout their entire developmental cycle feed on restricted food sources such as blood, cellulose, phloem, stored grains) harbor bacterial symbionts;
2) in some cases, these symbionts can be cultured and genetically transformed to express a gene whose product kills a pathogen that the arthropod transmits; 3) normal arthropod symbionts can be replaced with genetically-modified symbionts, resulting in a population of arthropod vectors that can no longer transmit disease.
While not applicable to all groups of arthropods, this approach has been successful in the control of arthropod species that transmit Chagas disease.
6. Genetically-modified Microbial Pesticides
Synthetic chemical insecticides provide many benefits to food production and human health, but they also pose some hazards. In many instances, alternative methods of insect management offer adequate levels of pest control and pose fewer hazards.
One such alternative is the use of microbial insecticides, also known as genetically-modified microbial insecticides. Genetically-modified microbial pesticides are either bacteria, fungi, viruses, protozoa, algae or their products whose DNA has been modified to express pesticidal properties.
The modified microorganism generally performs as a pesticide’s active ingredient. Microbial insecticides are especially valuable because their toxicity to non-target animals and humans is extremely low.
Compared to other commonly used insecticides, they are safe for both the pesticide user and consumers of treated crops. Microbial insecticides also are known as biological pathogens, and biological control agents.
Microbial insecticides are comprised of microscopic living organisms (viruses, bacteria, fungi, protozoa, or nematodes) or the toxins produced by these organisms.
They are formulated to be applied as conventional insecticidal sprays, dusts, liquid drenches, liquid concentrates, wettable powders, or granules.
Each product’s specific properties determine the ways in which it can be used most effectively.
Advantages of Microbial Insecticides
Individual products differ in important ways, but the following list of beneficial characteristics applies to microbial insecticides in general:
The organisms used in microbial insecticides are essentially nontoxic and nonpathogenic to wildlife, humans, and other organisms not closely related to the target pest. The safety offered by microbial insecticides is their greatest strength.
The toxic action of microbial insecticides is often specific to a single group or species of insects and this specificity means that most microbial insecticides do not directly affect beneficial insects (including predators or parasites of pests) in treated areas.
If necessary, most microbial insecticides can be used in conjunction with synthetic chemical insecticides because in most cases the microbial product is not deactivated or damaged by residues of conventional insecticides.
Because their residues present no hazards to humans or other animals, microbial insecticides can be applied even when a crop is almost ready for harvest.
In some cases, the pathogenic microorganisms can become established in a pest population or its habitat and provide control during subsequent pest generations or seasons.
Disadvantages of Microbial Insecticides
The limitations or disadvantages listed below do not prevent the successful use of microbial insecticides. Understanding how these limitations affect specific microorganisms will help users to choose effective products and take necessary steps to achieve successful results:
Because a single microbial insecticide is toxic to only a specific species or group of insects, each application may control only a portion of the pests present in a field, garden, or lawn. If other types of pests are present in the treated area, they will survive and may continue to cause damage.
Conventional insecticides are subject to similar limitations because they too are not equally effective against all pests. Nonetheless, the negative aspect of selectivity is often more noticeable for microbials.
Heat, desiccation (drying out), or exposure to ultraviolet radiation reduces the effectiveness of several types of microbial insecticides. Consequently, proper timing and application procedures are especially important for some products.
Special formulation and storage procedures are necessary for some microbial pesticides. Although these procedures may complicate the production and distribution of certain products, storage requirements do not seriously limit the handling of microbial insecticides that are widely available. (Store all pesticides, including microbial insecticides, according to label directions).
Because several microbial insecticides are pest-specific, the potential market for these products may be limited. Their development, registration, and production costs cannot be spread over a wide range of pest control sales.
Consequently, some products are not widely available or are relatively expensive (several insect viruses, for example).
In summary, biotechnology methods involved the direct use of microorganisms or transformation of their genetic materials to enable them performs more efficiently as a control agent.
While some methods such as development of biosynthetic vaccine and chemically synthetic vaccines are used for prevention, others such as gene therapy, genetic engineering drugs, sterile insect technique, and microbe- based control and genetically-modified microbial pesticides are targeting parasites or the vectors of disease transmission.
Generally, these biotechnology methods have been found useful, effective and very promising although there are still some teething challenges yet to be addressed.
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Biotechnology methods can be applied in the prevention and control of diseases. While preventive methods aim at preventing the establishment of disease, control methods target either the parasites or vectors of disease transmission.
The preventive methods are based on the development of biosynthetic and chemically based vaccines. Vaccines are inoculations of dead microbes that help the body immune system develop resistance against future infection of the same disease.
Genetically-modified microbial pesticides are pesticides that use bacteria, fungi, viruses, protozoa, algae or their products whose DNA has been modified to express pesticidal properties as active ingredients rather than the normal organic or inorganic compounds. These have been tested and found very effective.
Several brands are widely marketed and represents one of the most widely used and accepted products of biotechnology. In the next unit, you will learn about how these genetic engineering methods can be applied to food safely and hygiene.