Zoonoses, Emerging Infectious Disease and One Health: Challenging Issues at the Interface Between Human and Animal Health
The evolution and emergence of new pathogens continues to affect the health of all life forms, from plants to wildlife, domestic animals and humans, and recent evidence suggests the rate of pathogen emergence and re-emergence is increasing. A recent study estimated that at least 335 new human pathogens have emerged over the last 60 years; the majority of these originate in animals, and their cross-over into humans is attributed to changes in socioeconomic, environmental and ecological factors.1,2 Rising human population density, increased travel, trade, poor biosecurity and inadequate animal health management, growing antimicrobial use, poverty and changing dietary habits are all considered risk factors for pathogen emergence. The recent rise in the number of new events is often ascribed to the long term effects of human activity on the natural environment3,5; thereby coupling disease emergence with major issues such as climate change, species extinction and habitat degradation. These emerging infectious diseases (EIDs) are often associated with significant morbidity and mortality, affecting human and animal populations worldwide.
In addition to the problem of emerging pathogens, it is important to recognise that some 'old' persisting zoonoses continue to impact on the lives of many of the world's poorest people, and domestic animals are an important reservoir. Rabies is still endemic in more than 150 countries and territories, and kills over 50,000 people per year1 - mostly as the result of dog bites. Children under 15 years of age account for 40% of people bitten by suspect rabid animals and many rabies cases would have been prevented by simple measures such as adequate wound cleaning and vaccination. In addition, animal bite wounds may become infected with a wide range of bacterial pathogens including Pasteurella spp., Staphylococcal spp. (including MRSA), Streptococcal spp. and Neisseria spp. and many infections are polymicrobial. Close contact with pets has also been associated with severe bacterial infections including respiratory illness and meningitis associated with pasteurellosis.6,7 This has led to the conclusion that, for some bacterial pathogens, "a lick may be as bad as a bite".6
Other parasitic diseases associated with domestic dogs and cats, such as echinococcosis (hydatid disease), toxoplasmosis and toxocariasis also persist as important zoonoses in many parts of the world. Only Iceland and Greenland are free from hydatid disease, and New Zealand has recently joined Tasmania and Southern Cyprus with provisional freedom after more than a century of concerted effort to control the disease. Other "neglected" zoonoses include vector-borne diseases such as leishmaniasis. In endemic areas Leishmania spp. cause high morbidity and this neglected disease was estimated to cause 0.7 to 1.2 million cases of cutaneous and 200 to 400,000 cases of visceral leishmaniasis.8 L. infantum is the most widespread zoonotic Leishmania spp. causing visceral leishmaniasis, and domestic dogs are the main reservoir.9,10
Recent applications of genetics and molecular evolution have advanced our understanding of the epidemiology and control of emerging pathogens. Improving our understanding of how these pathogens are evolving to adapt to new hosts and changing environments is key to reducing the current unacceptable burden of infectious disease. It is often not acknowledged that most infectious agents are multi-host, and this includes the zoonotic pathogens. Single-host infections specific to humans are relatively rare and it is understood they only began to appear when human populations aggregated in sufficiently large numbers.11 Predicting when, where, and in what form the next 'species jump' and potential pandemic zoonosis will occur is the subject of much research. It has even been speculated that the continually evolving canine parvovirus, which itself was a descendent of the feline panleukopenia virus, FPV, and subsequently jumped to raccoons (and back into cats), may be a prime candidate for such a trophism shift.12
One of the major challenges in dealing with zoonotic disease is furthering our understanding of the relative contribution of different animal reservoirs to the burden of disease, and the transmission pathways involved. New molecular epidemiological approaches, combined with modelling tools, have helped us in this regard. These techniques have been applied to enteric pathogens such as Campylobacter spp. and Salmonella spp., revealing the importance of livestock species and food pathways.13 However, the role of pet animals in the transmission process is still poorly understood. Pets that live in close proximity to humans are more likely to transmit enteric pathogens to their owners14 and pet ownership (particularly dogs and cats) has been identified as a risk factor for human campylobacteriosis particularly among small children,15,16 but the precise transmission pathways for enteric zoonoses such as campylobacteriosis have not been investigated in detail, and the role of contaminated pet food, rather than direct pet-human transmission is often not considered17. Transmission of Campylobacter from pets to children has been described in a small number of cases,18,19 but the likelihood of healthy pets being infective to humans remains controversial.20 In a case-control study of rural children, pet ownership was reported as protective for human gastroenteritis.21
There is increasing recognition of the importance of ecosystem health and resilience in determining the health of humans, domestic animals and wildlife, with new impetus for research and collaboration in this area stimulated by the "One Health" concept and the drawing up of the "Manhattan Principles".2 "One Health" isn't new, but has gained impetus with the emergence of major epidemic and pandemic zoonoses including severe acute respiratory syndrome (SARS), swine 'flu, Nipah virus and bovine spongiform encephalopathy (BSE). The "One Health" paradigm drives the way we think about public health, food safety, zoonoses, conservation and ecosystem health and has helped drive the closer cooperation between the veterinary and medical professions.
The modern veterinary profession is concerned with the health of domestic animals, humans, wildlife and ecosystems. It is also a profession for food safety and security and for sustainability, biodiversity and conservation. The control of infectious disease, and in particular multi-host pathogens, is one of the most important activities of the veterinary profession, and will continue to have far-reaching benefits for all animals, including humans.
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