Invited reviewMolecular epidemiology: A multidisciplinary approach to understanding parasitic zoonoses
Introduction
Molecular tools increasingly have become an integral part of studying the epidemiology of infectious agents. Epidemiology, the study of factors determining the occurrence of a disease in a population, aims to describe the health status of populations, to explain the aetiology of disease, to predict disease occurrence and to help control the distribution of disease. Molecular biology provides one of the many diagnostic tools that can be utilised to strengthen understanding of the epidemiology of a disease, both infectious and non-infectious. Molecular epidemiology has recently been defined as “a science that focuses on the contribution of potential genetic and environmental risk factors, identified at the molecular level, to the aetiology, distribution and prevention of disease within families and across populations” (Dorman, 1992) and has emerged through integration of the disciplines of epidemiology and molecular biology. Molecular tools often provide another dimension to the epidemiology of disease that would otherwise be impossible using conventional diagnostic tools. The objectives of molecular epidemiology for the study of infectious diseases include: (i) descriptive and analytical studies to evaluate host/environmental interactions in disease; and (ii) the development of strategies for the control of bacterial, parasitic and viral disorders through molecular diagnosis. Recently, use of molecular tools to help unravel epidemiological relationships has been most helpful for the study of infectious diseases, particularly zoonoses. Emerging zoonotic diseases are those that are newly recognised or newly evolved or that have occurred previously but show an increase in incidence or expansion in geographical, host or vector range (Marano and Pappaioanou, 2004, Katzer et al., 1998). An estimated 62% of known human pathogens are of zoonotic origin and 75% of emerging infectious diseases have been identified as zoonotic in origin (Cleaveland et al., 2001). Moreover, several zoonotic agents could be potentially used as biological weapons. Multiple host species are involved in transmission of zoonoses including wildlife, companion animals, livestock and fish, as well as countless human behavioural risk factors that increase human exposure to zoonotic diseases. An integral part of prevention and control of these emerging zoonotic diseases is based on early detection, epidemiological investigation, multidisciplinary collaboration and the development of advanced diagnosis and surveillance tools, including the use of molecular biological methods (Chomel, 2003) such as PCR and phylogenetic analysis. Molecular tools have been helpful by accurately diagnosing diseases within a population with greater sensitivity and specificity than conventional methods. It has contributed to the increased understanding of the pathogenicity and virulence of aetiological agents, identified genes of the aetiological agent that are responsible for drug resistance, provided information on the population structure, taxonomy, source and transmission dynamics of aetiological agents, identified genes that increase host susceptibility to disease, and has allowed the zoonotic potential of previously unidentified agents to be determined.
Sound application of molecular epidemiological principles requires a working knowledge of both molecular biological and epidemiological methods. This review will attempt to describe the concept of epidemiology and proper study design, describe the appropriate use of current molecular biological tools available for an epidemiological study and provide examples of how the application of molecular tools can help elucidate aspects of the epidemiology and transmission patterns of parasitic zoonoses. The current limitations of applying these tools, including cautions that need to be addressed during their application and interpretation, will also be discussed.
Section snippets
Concept of applied epidemiology
There are numerous facets to the field of epidemiology that can be grouped into several key areas: determination of the frequency and distribution of disease, identification of the factors that influence the occurrence and severity of disease in a population and development of programs for the control and/or prevention of disease.
One of the major tenets of epidemiology is based on the concept that disease is multifactorial in nature. Epidemiological studies are undertaken to identify those
Application of molecular tools in parasite epidemiology
Possibly the greatest contribution has been the application of molecular systematics to improve parasite classification systems and to provide a better framework for studying parasite biology and epidemiology (Monis, 1999). As a consequence, a suite of genetic techniques have been developed that provide faster and more accurate identification of parasites on a species and subspecies level compared with traditional diagnostic techniques such as morphological identification (reviewed by Monis et
Molecular epidemiology—advantages over conventional diagnostic techniques
When evaluating a diagnostic test, a proper definition must be provided for the performance characteristics and appropriateness of utilising the test under the epidemiological circumstances being investigated. For example, the advantages and disadvantages of using one diagnostic test over, or in conjunction with, another should be clearly understood, including assay sensitivity, specificity and predictive values. An important advantage of PCR-based procedures over the more traditional methods
The importance of conducting simultaneous parasite surveys in both humans and animal hosts to unravel zoonotic relationships
In some instances, PCR-based techniques may be the only diagnostic means of unraveling the role of animals as mechanical or zoonotic transmitters of disease. It is imperative that in situations such as these, simultaneous surveys be conducted in both human and animal populations to correlate data on both an epidemiological and molecular level and to rule out the possibilities of geographical variation. The importance of carrying out simultaneous studies is highlighted by a number of examples.
Cautions required during application and interpretation of molecular epidemiological tools when inferring zoonoses
A well designed and robust PCR is an ideal diagnostic tool for epidemiological studies aimed at unraveling zoonotic relationships. To be utilised cost-effectively and efficiently, a PCR-technique must be reproducible between various laboratories, rapid, with a high degree of sensitivity and specificity, and must be capable of achieving this when applied to mixed templates of various biological samples such as faeces, water, blood, tissue, environmental samples and vectors. However, even if this
Conclusion
Application of molecular epidemiological studies requires a sound knowledge of both disciplines of molecular biology and epidemiology. In recent years, molecular tools have been tremendously advantageous in allowing diagnosis and characterisation of aetiological agents from clinical and environmental samples with far greater accuracy than conventional diagnostic tools. This aspect of molecular diagnosis has made sample collection, processing and the identification of the aetiological agent more
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