Microbial Behaviour: Herd Immunity - Essay Example

Microbial Behaviour: Herd Immunity INTRODUCTION Herd immunity is a widely used term with reference to microbial behaviour that is variously interpreted by different authors. John & Samuel (2000) define herd immunity as “the proportion of subjects with immunity in a given population”. Herd immunity can be measured in terms of a chosen immune parameter and the number of individuals in population observed to be positive for the parameter refers to herd immunity of that population for the specific immune parameter. A closely related and often confused term is herd effect. It is used to describe the proportion of immune individuals in a population or the specific threshold population of immune individuals that when once achieved in a population, leads to decline in the infection incidence. It thus implies an immunity pattern that prevents a new infection to effect a population of unimmunised individuals as a consequence of the presence of immunised individuals in the same population. Another implication of the phenomenon is that the presence of higher number of immune individuals in a population is inversely proportional to the risk of the susceptible individuals in the population being affected by the infection (Plotkin et al., 2011). Thus herd immunity is the immunisation or infection in human or other organisms while herd effect is the health interventions or immunisations aimed to reduce the transmission of infection from human to human or though vector. Herd immunity is a phenomenon that is one of the determinants of herd effect (John & Samuel, 2000). MASS VACCINATION AND HERD IMMUNITY Medical interventions usually target an individual, dealing with treatment of specific diseases in a particular individual that is not expected to exercise any physiological effect on others. Vaccines contradict this principle. Vaccine, the term is derived from the Latin word for cowpox; variole vaccinae” as used by Edward Jenner in 1796. Vaccines are biological preparations that enhance or help build immunity to a specific disease. Immunity is the ability of the living body to accept the indigenous material or their ‘own’ and reject foreign materials. This ability allows body to protect itself against infections which result as a consequence of presence of the microbes; organisms that are treated as foreign particles. The process of vaccination involves the introduction of live attenuated or inactive microbes in to the living body. These vaccines function by stimulating body’s immune system to produce antibodies specific to the microbe for the disease for which vaccine has been introduced (CDC). HISTORY OF MASS VACCINATION History of vaccination can be traced back to 1796, when Edward Jenner, a doctor of England; found that inoculation of individuals with material from cow pox lesion enables successful development of immunity against small pox. The process does not involve risk of small pox development and hence was safer compared to variolation. The process soon became a successful strategy for prevention of small pox all over the world. Thereafter control of several diseases such as diphtheria, tetanus, yellow fever, pertussis, Haemophilus influenza type b disease, measles, mumps, poliomyelitis, rubella, typhoid and rabies has been possible through the use of vaccines (Plotkin & Plotkin, 2008). MASS VACCINATION AND HERD IMMUNITY With the utility and efficacy of small pox vaccine established, by 1807, vaccination for the same was made compulsory in most European countries. Vaccination acts enforced by vaccination officers ensured the administration of free, universal and compulsory vaccines in United Kingdom with fined imposed on those who opposed them. These were the days of early of mass vaccination which later involved general vaccination days and resulted in considerable decrease in incidences of small pox. However besides the effect of vaccines on those vaccinated; it came in to light that individuals who were not vaccinated were also indirectly protected from the disease due to an overall decrease in disease incidence leading to fall in disease transmission rates. This led to the emergence of the concept of herd immunity. Thus the major goal of mass eradication on one hand is to protect individual from disease and also to raise the herd immunity levels to fulfil the criteria for global eradication of disease at the time of introduction of new vaccine (table 1) (Heymann & Aylward, 2006). Table 1: Mass Vaccination to Accelerate Disease Control (Heymann & Aylward, 2006) Mass Vaccination Category Objective of Mass Vaccination Examples Comment New Vaccine Introduction Rapidly optimize the impact of a new antigen and/or minimize potential side effects associated with its introduction Rubella campaigns One time supplement at time of initiation of routine childhood immunization with a new vaccine Disease eradication Achieve population immunity needed during time limited period to interrupt transmission Polio Eradication programs Essential of coverage required for herd immunity exceeds that of routine immunization coverage or goals Mortality Reduction Accelerate achievement of specific national and international disease control goals Measles, tetanus Continued as temporary strategy while routine immunization is strengthened MATHEMATICAL MODEL OF HERD IMMUNITY Efficacy of vaccination can be quantitatively expressed as vaccine efficiency (VE): Thus a 100% efficient vaccine reduces risk of infection completely and vice versa. However this equation represents levels of individual protection offered by the vaccine. In cases of mass vaccination wherein the phenomenon of herd immunity comes in to play; the measures of efficacy vary. In cases of mass vaccination, if the levels of immunity imparted to a population exceed the levels of herd immunity, the infection is not able to sustain in the population and is therefore eradicated. Thus a minimum threshold level of immunity needs to be ensured through mass vaccination for disease eradication. Herd immunity level prior to mass vaccination is expressed as: Where, R0 is the basic reproductive rate or transmission potential of an infective agent. It refers to the average number of secondary infections produced from one infective particle in a completely susceptible population. S is the proportion of susceptible individuals in the population Thus 1-1/R0 is the threshold value or critical immunisation threshold (qc). Table 2: Typical Values of R0 and qc for Selected Infection (Farrington, 2003) Infectious Diseases R0 qc(%) Measles 10-20 90-95 Whooping Cough 10-20 90-95 Chickenpox 5-10 80-90 Mumps 5-10 80-90 Rubella 4-7 75-85 Diptheria 4-7 75-85 Polio 4-7 75-85 Smallpox 3-5 65-80 If immunity levels can be maintained at or above this level through immunisations at birth or as early as feasible after birth; the disease can be eradicated. If q be the number of individuals in a completely susceptible population immunised at birth; then for eradication of the disease: The typical values of R0 and qc for some of the common diseases are shown in table 2 (Farrington, 2003). The value of R­0 and hence of herd immunity is dependent on the social and behavioural aspects of the population and its demographic characteristics besides the biological characteristics of the infectious agent. Hence significant variations are observed both with respect to various diseases as well as developmental status of the country. For instance the difference in the average age at which children receive immunisation for measles in developed and developing countries is high and hence value of R­0 accordingly for the two are different (Anderson & May, 1990). LIMITATIONS OF MASS VACCINATION PROGRAMS IN ACHIEVING HERD IMMUNITY At the initiation of mass vaccination programs the probability of an individual developing severe disease is lower than the probability of the same developing from a natural infection. As the eradication program reaches its target, this condition gets reversed. This leads to a decline in vaccine in the community prior to complete eradication and thus hinders the accomplishment of herd immunity. This can be overcome by enforcement of vaccinations. However complete eradication of a disease can only be ensured through eradication at global level. Further mass immunisation does reduce the incidence of an infectious disease; however the number of susceptible individuals remains constant, with each incidence of infection leading to one infection irrespective of whether the prior case of susceptibility is lost due to vaccination or infection. Herd immunity reduces rates of transmission; and thus raises the gap in two consecutive epidemic cycles and the average age of infection (Anderson & May, 1990). The latter issue can lead to various levels of risk in different diseases. This can be alarming in certain diseases such as Rubella and mumps. Infants born of women in third semester of pregnancy who have contacted congenital rubella syndrome are at considerable risk. To prevent this girls in UK are vaccinated for Rubella at approximately 12 yrs to ensure that immunity is effective in early years. In USA, both boys and girls are vaccinated at around 2yrs for complete eradication of the disease (Anderson et al., 1987). CONCLUSION Achieving herd immunity is as important as it is difficult because of the multiple factors involved. That global scales of interventions are required to ensure its achievement further complicates the issue. Mathematical models and theoretical considerations cannot effectively account for the multiple social, behavioural and economic factors. Motivating and convincing communities for being a part of the mass eradication campaigns is the primary issue that needs planning and thorough preparation. However considering the wide and permanent impact of herd immunity on public health and living conditions, the limitations of the scale and strategy should not discourage the planner and policy makers. In combination with other methods of control, such as chemotherapy, improvement of nutritional status, hygiene and sanitation, education and awareness; the target of herd immunity and eradication is not impossible to achieve. MICROBES AND NUCLEAR WASTE DISPOSAL INTRODUCTION Nuclear reactions involved in nuclear weapons, nuclear energy generations and scientific investigations are based on two basic reactions of nuclear fission and fusion. Vast amounts of energy are released as a consequence of fission reactions of elements such as Uranium (235U) and Plutonium (239Pu) and fusion reactions of hydrogen. While the former can be produced in controlled conditions and extensively utilized for energy productions; similar set ups for the Controlled generation and utilization of energy from latter remain to be devised. Nuclear energy in recent decades has become an important part of global electricity supply accounting for 17% of the same. They have the added advantage of being the greener alternative to fossil fuel as a source of energy (Purushottam et al., 2000). Besides the release of energy, nuclear fission reactions in reactors and nuclear weapons produce, like any other industrial process, certain by-products or wastes. However; unlike other industrial processes the wastes produced from nuclear reactions or not only highly hazardous and radioactive; but also recalcitrant (Rao, 2001). At this point it is important to define the three terms: waste, radioactive and recalcitrant. Waste can be defined as any material (solid residue, liquid and gaseous effluent) that cannot be used any further. Radioactivity is defined as the spontaneous emission of particles and radiations by certain elements, referred to as radioactive elements. Recalcitrant wastes refer to those that are difficult to manage or do not respond to treatments. The major cause of concern is the radioactive and hence hazardous nature of nuclear wastes which effect man and other organisms through various channels. Though the exact nature of damage to living organisms is dependent on the nature dosage and duration of exposure; severe damages to the cells causing severing of electronic bonds within the cell, as well as damage to the genetic material (DNA) have been reported (Rao, 2001). Considering the hazards to biosphere presented by the nuclear waste it is important to ensure their treatment and safe disposal. The current paper aims to explore the role of microbes in the effective management of nuclear wastes. Management of Radioactive Waste Management of nuclear waste is a challenge to the scientific community that still requires solutions. In absence of a technically feasible and ethically sound strategy available for the degradation of these wastes, it has been the aim of the researchers to find the most suitable technique for the isolation of living organisms and their immediate surroundings from the impact of nuclear wastes. Thus the chief methods used are disposal of nuclear wastes in to geological repositories, seabed burial and ocean dumping etc (Rao, 2001). The physical and chemical treatment approaches involve the production of secondary wastes that require further treatment before they can be safely disposed. Biological treatment of nuclear wastes using microbes has been demonstrated to provide a feasible and effective alternative to chemical and physical treatment options (Tikilili & Chirwa, 2009). Microbes in nuclear waste Initial assumptions that the environment of nuclear wastes would be too harsh for the sustenance of microbial activity were proved to be false both for the low and intermediate level (l/ILW), and high level (HLW) wastes. Increasing evidences are available for the existence of microbes in unlikely environments including those in the vicinity of radioactive wastes. The microbial diversity studies in deep subsurface radionuclide waste has been able to identify microbes that are capable of surviving extreme pH, high salt, oxyanion, metal and radionuclides concentrations and can be utilized for the bioremediation of nuclear wastes. Common bacteria have been identified to belong to Actinobacteria and Firmicutes. The predominant bacteria identified and extensively investigated belong to the phylum Deinococcus-Thermus; which can withstand desiccation as well as ionizing radiations. These bacteria are able to bind to and selectively remove toxic metals and/or radionuclides from nuclear waste deposits (Tikilili & Chirwa, 2009). Microbial Activity In Nuclear Waste Mechanism of action of microbes involves uptake of the waste material through diverse channels such as active transport though cell membrane, entrapment through cellular appendages; and passive mechanisms such as cation exchange, chelation and microprecipitation. The accumulation abilities are subject to cell surface characteristics including polarity of cell wall components, charge distribution within the component macromolecules of cell wall etc. The radionuclides are taken up through ion exchange process, with the cellular biomass providing the channel for exchange (Eccles, 1999). Nuclear waste repositories are further non-homogeneous with acute chemical gradients that are available as reaction fronts with redox reaction front proposed as a significant location with respect to microbial activity. Several models have been proposed on the basis of mass balance or kinetics to account for the maintenance and growth patterns of microbial populations in the nuclear wastes repositories (McKinley et al., 1997). The major source of energy for microbial activity in nuclear wastes is hydrogen that is made available through radiolysis of water, organic matter and corrosion of metal containers. Thus as a part of the nuclear waste disposal process the metal reducing bacteria are introduced into the site of nuclear waste or the contaminated site and the metals are precipitated by these bacteria from the solution. These bacteria are capable of directly reducing the metals from their oxidized soluble form to a reduced precipitate. This is accomplished by the bacteria by accepting electrons from an organic compound and using the metal as the ultimate electron acceptor for their energy generation pathway. Thus these bacteria are able to precipitate uranium, chromium etc from their salts. Certain bacteria also indirectly reduce radioactive metals. The final acceptors are Fe3+, SO42- etc that are converted to Fe2+ and H2S respectively. These reduced molecules then reduce and precipitate the metals from their solutions. Thus these redox bioremediation techniques are effectively used for nuclear waste disposal. The major limiting factor for the use of microbes for nuclear waste disposal is the inability of microbes to withstand the stress of waste environments. In case of nuclear wastes the stresses involve toxicity, radioactivity and water stress. Solution to this problem is available in form of genetically engineered microbes (GEMs) (Leung, 2004). Microbes for Disposal of Nuclear Wastes Discovered in 1956 by Anderson and associates, Deinococcus radiodurans is able to survive 4000 Gray of gamma radiations, which 250 times the dosage that can kill Escherichia coli. The ability of the bacterium to withstand the damaging effect of high dosage of mutagenic ionizing radiations can be attributed to their highly efficient DNA repair mechanism. Several researches in recent decade have targeted the exploration of this unusual ability of Deinococcaceae. Wild type bacteria Deinococcus can reportedly reduce uranium in presence of humic acids under anaerobic conditions. Engineered variants of the bacterium such Deinococcus radiodurans R1 have also been intensively studied for their ability to bioremediate radioactive substances (Appukuttan et al., 2006). Other bacteria that have been proposed to be capable of aiding the process of disposal of nuclear wastes include Shewanella putrefaciens and E. coli. Perdrial et al (2009) presented evidences of utilization of smectite clay by facultative anaerobic bacterium Shewanella putrefaciens and to survive on it exclusively. A recent discovery of mechanism underlying the generation of electricity by Geobacter bacteria, along with simultaneous nuclear waste clearing has been reported. The discovery by a group of researchers at Michigan State University is yet to be further investigated and patented. The significant role of conductive pili of Geobacter has been studies by these researchers who identified nanowires or hair like appendages on the outer surface of bacteria as the chief catalytic agents for uranium reduction. They are responsible for the electrical activity that causes immobilisation of radioactive material in nuclear wastes thereby preventing its leaching in to the groundwater. Further the pili have also been reported to protect the bacteria from the toxic environment (Michigan State University, 2011). CONCLUSION Nuclear wastes are as much a risk to human health and living organisms in general; as nuclear energy is of use to mankind. Where every other form of treatment has failed to provide satisfactory results; microbes and their ability to dispose off radioactive wastes seems to be a feasible alternative. Genetically modified bacteria find significant application in disposal of radioactive nuclear wastes. However, two major considerations involved in their use include the ethics and the cost factor. Further the severity of the hazard and the threat to the biosphere due to accumulation of nuclear waste must remain the ultimate criteria in the decision regarding expenses and utility of producing effective microbes. REFERENCES Read More