In less than two decades, perspectives on the relationship of disease emergence in humans and domestic animals has changed dramatically. Toward the end of the 20th century, the underlying causes of emerging infectious disease were primarily attributed to aberrations in human behaviour. Methods of control and prevention were all too often centred on changing human behaviour, those things characterized by human carelessness, human excesses, human ignorance or human habits of conquest or leisure.
Humans were looked at as the engines of microbial opportunism. The conquest of disease was based on control of the human factors that contribute to emergence. Little attention was paid to the pivotal role animals play as zoonotic agents in the origin and transmission of emerging diseases and the biological niches that microorganisms are all too capable of exploiting. That was a mistake.
A comprehensive literature review conducted by the centre of Tropical Veterinary medicine, University of Edinburgh identified 1,415 species of infectious organisms known to be pathogenic to humans. Sixty-one per cent of them are zoonotic or capable of being transmitted between human and animals. One hundred seventy-five human pathogens fit the definition as causes of new or emerging disease. Of those, 75 per cent are zoonotic. Nearly one-half are viruses and vectors such as mosquitos, ticks and midges are often involved in transmission. Overall, emerging diseases are twice as likely to be associated with zoonotic pathogens.
The importance of the zoonotic nature of disease is often found in how diseases are initially established in human hosts rather than the severity of the disease that follows. Human immunodeficiency virus (HIV), for instance, originated in monkeys, jumped the species barrier and was then transmitted person to person, eventually infecting over 30 million people. With HIV-AIDS fully engaged as a human problem its origin as zoonosis is often forgotten.
The role of swine and birds as mixing vessels for influenza viruses like influenza A H5N1 is well known, but it’s the potential of spread from human to human that incites fear of pandemics. Dengue fever initially a viral disease of nonhuman primates in Africa and its mosquito vector are now established in over 100 countries, and put three billion people at risk every year.
Emerging disease in free-living wild animals that in turn hold zoonotic risk for humans have often had origins in the spillover of disease from domestic herds. Brucellosis and tuberculosis in Wood Buffalo National Park bison, and brucellosis in Yellowstone National Park elk and bison started in cattle. The “spill-back” potential into domestic herds now free of disease and the difficulty posed in trying to eliminate these stubborn diseases from free-roaming populations highlight human intervention gone astray.
It’s customary for humans to analyze the phenomenon of emerging disease as a means of rationalizing investment in control and prevention. Today, the rigmarole is called “risk assessment,” and risks there are plenty. Understanding both animal and human factors is essential in comprehending disease risk. The role of animals in the transmission of emerging diseases is unequivocal, but human behaviour can also be a decisive determinant. Changes in human behaviour can either be a powerful defence against disease, or a catalyst for the evolution of new pathogens.
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Human behaviour contributes to disease emergence, first and foremost, by enhancing the opportunity for microbes to encounter humans and animals. In 2011, world population surpassed seven billion; upwards of 100 billion animals served their need for protein. As the population density of any species increases, so do the chances that a successful and often devastating pathogen will appear. Add things like armed conflict, natural disasters, substandard education and poor sanitation and the human capacity to respond is overwhelmed. The cycle of underdevelopment and indescribable poverty of many nations are sustained. As an example, the indiscriminate disposal of human and animal waste has probably been the greatest single contributor to emergence and re-emergence of diseases.
The helter-skelter movement of people, animals, and animal products across the globe at warp speed only increases the risk of encounters with disease microbes. West Nile virus infected crow rookeries in lower Manhattan and ultimately migratory waterfowl of North America. HIV/AIDS spread along trucking routes out of central Africa. Cholera jumped by ship from Bangladesh to Peru. Influenza constantly changed and played global hopscotch. And BSE wreaked havoc in Canada.
The mix of new technology and a burgeoning population pushes humans into new habitats. Climate change adds to the mix. The invasion increases opportunities for inadvertent exposure to disease and the likelihood for cross-species adaptation of organisms. Technology also permits distribution of fresh and processed foods over vast geographic areas. Animal- and human-derived organisms like salmonella, E. coli, and listeria can now affect people far from the source of production.
Modern communication, another product of technology, provides an unparalleled opportunity to influence human behaviour. For instance, communication is the engine for mass markets. Communication generates fashions and fads, plus the desire to travel. Many close encounters between humans, animals and microbes are motivated by modern communications.
The management of zoonotic pathogens poses challenges outside the scope of traditional medical system and demands much closer collaboration between medical and veterinary scientists. Disease control in the future must consider both animals and people. The concept of “One Health” is no longer just academic fodder.
Dr. Ron Clarke prepares this column on behalf of the Western Canadian Association of Bovine Practitioners.