Measures relating to animal human healthcare interface -
Changing patterns of infectious diseases
Infectious diseases have shaped the evolution and history of animal kingdom especially human and animals. Infectious diseases are dynamic and will always continue to influence the overall activities of human being in all the spheres of their lifestyle. It is very obvious by now that human activity will alter the paths and lexis of infectious diseases. The distribution, incidence, and burden due to infectious diseases are influenced by a range of societal, biological, and physicochemical factors. The pattern of infectious disease has been changing continuously which has been responsible for sustained microbial evolution. These changes include the description of diseases caused by pathogens long present but not previously identified, recognition of apparently new microbes, changes in epidemiology (e.g., changes in distribution, disease incidence, virulence of microbial strains, microbial resistance to drugs) of old pathogens, new disease—disease interactions, and the spread (to human/
animals) of organisms never previously known to be human/animal pathogens.
The understanding of how these factors influence the emergence, re-emergence and spread of infections is of paramount importance in control and eradication of infectious diseases. The changing pattern of infectious disease is typically affected by interaction of multiple factors. The emergence/re-emergence of a disease could be an unintended consequence of many developmental activities which is perceived as progress. Various developmental activities which can be attributed to many ecological changes include: (i) building of a dam; (ii) clearing of lands; (iii) change in landscape, (iv) mass processing and wide distribution of foods and water; (v) medical/veterinary interventions (transfusions of blood/blood products; vaccines; animal feed containing livestock/fish/poultry offal); (vi) mass immunization; (vii) use of drugs and chemotherapeutics; (viii) use of antimicrobial agents; (ix) inland/national/international travel and trade; (x) changing pattern due to land use (intensive cropping, animal husbandry); (xi) indiscriminate use of fertilizers, insecticides, pesticides, hormones in agricultural operations, (xii) use of probiotics and unconventional feed additives in animal feeds and use of hormone injections (oxytocin) for realizing over production; (xiii) extensive mechanization of agricultural operations; (xiv) practice of extensive undefined livestock husbandry systems inappropriate to a particular region, state or a nation; (xv) international/inter-continental movement of migratory birds.
Factors contributing to change in disease pattern
Many factors are contributing to the changing patterns of infectious disease. Those commonly identified are microbial adaptation and change, human demographics and behavior, environmental changes, technology and economic development, breakdown in public health measures and surveillance, and international travel and commerce (Lederberg et al., 1992). How these influence the appearance, reappearance, and spread of infections will become apparent in the discussion of specific disease examples. Typically, multiple factors interact, leading to changes in a disease. The emergence of a disease may be an unintended consequence of what is viewed as progress: the building of a dam, clearing of
lands, mass processing and wide distribution of foods and water, medical interventions (namely, transfusions of blood and blood products, tissue and organ transplantation, cancer chemotherapy), and use of antimicrobial agents. The burden of disease in humans can increase through (i) increased contact between a pathogen (disease-causing agent) and host, (ii) increase in virulence or resistance of the pathogen, (iii) increase in the vulnerability of the host, or (iv) limited access to effective prevention or therapy. A human or a population can be completely or relatively invulnerable to some infections because of (i) immunity (past infection or immunization), (ii) genetic factors, or (iii) a whole range of barriers (such as shoes, screens, good housing) or (iv) interventions (for example, provision of clean water and adequate waste disposal, control of organisms responsible for transferring pathogens between hosts) that prevent contact between human and pathogen. Good nutrition, including adequate intake of micronutrients, can lead to an improved outcome in at least some infections.
Prioritization in National Emergency Disease Eradication Programmes
Much of the discussion has been based on the presumption that an emergency disease outbreak has been detected relatively early and is still only present in one or a few separate pockets. Many countries are not in this fortunate position and have to contend with an epidemic livestock disease that has become well established in the country, and may well have been present for a number of years. In these circumstances, commencing a national disease eradication campaign that covers the whole country at once may be neither practical nor wise. The spreading of resources too thinly over too large an area may result in overall setbacks and frustrations. It may be more effective in the long term to tackle the eradication in a step-by-step progression moving from one region to the next. In this case regions should be defined and selected on the basis that once eradication has been achieved in one region, and the campaign moves on to the next, there can be confidence that the disease will not re- enter the first region. Geographic barriers should be utilized wherever possible. In this respect, archipelago countries are fortunate in that the eradication can take place as an island-hopping campaign. Otherwise, utilization should be made of any epidemiological or livestock production and marketing patterns that tend to make an area a discrete unit in terms of disease spread. Next is the question of prioritization - which region(s) to tackle first. There is merit in selecting the major livestock breeding areas in the country since they are often important source areas for the disease, and livestock movements (and possibly infection) tend to spread centrifugally from there. The other advantage of tackling these areas first is that, when free, they will act as a valuable source of disease-free animals for restocking other areas. Further prioritization should also be based on an understanding of epidemiological factors and livestock production and marketing systems which influence how the disease spreads and to where. A policy could be to follow the spread of the disease, starting regional campaigns at its source and ending where it finishes. In tropical and semi- tropical countries, livestock movements and direct contact among animals are often overwhelmingly the most important method of spread of infection. Therefore a thorough
understanding of livestock movement patterns and routes is often vital for effective prioritization within epidemic disease eradication campaigns.
References
Alexander, D.J. 2000. A review of avian influenza in different bird species. Veterinary Microbiology, 74 (1/2), 3- 13
Anderson R.M., May R.M., 1991. Infectious Diseases of Humans: Dynamics and Control. Oxford University Press, Oxford UK
Ewald.W. Paul. 1993. Emerging Infectious Diseases. Oxford University Press, UK