“Contagion” and Emerging Infectious Diseases from an Occupational and Environmental Medicine Perspective.

The past two decades have seen some remarkable emergence of new and deadly pathogens, from SARS to avian influenza H5N1 and the novel 2009 H1N1influenza strain. It is often pointed out that a majority of recent emerging infections are “zoonotic” (transmitted from animals to people) in origin. The movie “Contagion” follows on this theme by depicting a novel virus sweeping the globe. About halfway through the movie, we learn that the virus has genetic sequences from both bats and pigs, implying that at some point “the wrong bat met up with the wrong pig”.

At the end of the movie, there is a montage that hints at the origin of the new lethal virus. Fruit bats in disturbed rain forest are seen flying and having contact with pigs in a large barn. The montage follows the pigs and presumably the virus as the pigs go to market and slaughter and into a restaurant in Asia where blood on the chef’s hands and apron appears to infect the business traveler character played by Gwyneth Paltrow, who then becomes the “index” patient spreading the epidemic from person to person around the globe.

This bat-to-pig-to-human scenario hearkens back to the real emergence of a novel virus called "Nipah virus" from a fruit bat reservoir in Malaysia into pig farms located close to areas of recently logged rain forest (see review by Chua 2003 ) . In 1999, Nipah virus caused a new and fatal encephalitis in Malaysian pig workers and a milder disease in pigs. It is thought that the movement of sick pigs spread the disease to other parts of the country, often infecting other pig workers.

The Nipah outbreak in Malaysia is a reminder about how often it is the people working closely with animals or their products who first experience disease from a viral or bacterial pathogen crossing from animals to humans.

Similarly, with SARS, while we now think that the virus may have emerged from a natural reservoir in bats and spread to humans via intermediate host animals such as civets, the initial animal to human transmission events may have involved workers in animal markets, who have been found to have elevated rates of antibodies to the virus, suggesting that they had become infected through their work with animals (CDC MMWR 2003).

With avian influenza, the majority of human case have been among people performing tasks associated with poultry production, trade, and consumption, such as working on farms, in animal markets, or butchering and preparing poultry for consumption.
And with the 2009 H1N1 virus, while the emergence events remain unknown there is a suspicion that one of the key events was zoonotic transmission from swine influenza viruses to humans working closely with the animals.

So, what if emerging infectious diseases can be seen as a problem of animal workers? It might lead us to pay greater attention to these workers than we do currently.

Agricultural animal workers work worldwide across a large spectrum of activities, from backyard farms to large industrialized facilities, to large and small slaughter facilities to animal markets. They deal on a daily basis with a wide range of hazards, from injuries, to inhalation of dusts, to the possibility of infection from the animals.  Regardless of whether these animal workers are members of the “informal work sector”  (such as workers on family farms), or salaried employees in large mechanized facilities, in general, in contrast to, say, factory workers, there are no regular programs to monitor or foster their health and safety. Their health and safety concerns, including their risk of occupational infections, are ‘off the radar’ of most physicians who practice the specialty “occupational and environmental medicine” (the diagnosis, treatment, and prevention of diseases caused by exposure to hazards at work and in the environment). In addition, due to their often marginal economic status,  they may also not be receiving medical care benefits or have access to regular basic medical care.

In the U.S., many agricultural animal workers are recent immigrants who may be less aware of their health and safety conditions, and due to language and cultural barriers less likely to access such information if it is provided by the employer or others.

What is needed is more awareness of the work these people do and better ways to help them do it in a healthy way that reduces the risk of disease transmission. Ironically, doing so may be also be better for animal health, since we have seen that “zoonotic” pathogens can sometimes move in both directions, from animal to human or from human to animal (‘humanosis’ or ‘reverse zoonosis’). An example is H1N1 influenza, which, as it spread globally, demonstrated many more instances of humans apparently infecting animals (pigs, cats, etc.) than the other way around. So keeping animal workers healthy may help keep animals healthy as well!

In addition to the occupational medicine aspects of emerging zoonotic infections, it is also important to look at the environmental aspects. When Nipah virus emerged in Malaysia, it was related to the way that the farms were impacting the forest ecosystem. And many of the recent episodes of food safety problems seem to be caused by persistence of  pathogens in the environment related to the way that animal waste and human waste are handled.

Perhaps in the future there can be greater emphasis on a “One Health” approach to animal agriculture, one that brings together experts in agriculture, veterinary medicine, environmental health, and human occupational health, and that emphasizes the value of healthy workers and healthy animals, and the need to raise animals in such a way that the environment is enhanced. It would seem that to take our attention off of any of these three issues (the health of animals, animal workers, and the environment), and their interrelationships, in our push as a society to produce greater amounts of animal protein to feed a growing human population is to risk big problems in the long run.

Moving from “Us vs. Them” to “Shared Risk”: Human and Animal Health are inextricably connected

As interest in the One Health initiative increases, one of the first things that crosses the mind of a human health professional learning of the concept is that with increasing contact between humans and animals, there is increasing risk of zoonotic disease: infections crossing from animals to humans. We know that among recently emerging infectious diseases, more than 2/3 are zoonotic in origin, meaning that they originated as animal diseases. And we know from history that many important infectious diseases, from TB to measles, can be traced to increasing contact between humans and domesticated animals in the past several thousand years.
It seems natural, therefore, that a typical attitude of human health care professionals towards animal infectious diseases is an “us vs. them”  perspective, in which the animal is seen as a reservoir for a lurking threat to unsuspecting humans who venture into too close proximity. An example is Hanta Virus Pulmonary syndrome, a rapidly progressive and often fatal disease causes by a virus that is carried asymptomatically by the deer mouse, a frequent inhabitant of sheds and other peri-domestic environments. While seeming not to harm its deer mouse host, the virus, when encountered through breathing airborne virus from mouse droppings, wreaks deadly havoc on infected humans. No wonder that the public health response is to “Seal up, Trap up and Clean up” to keep the animals as far away from humans as possible. 

The down side of the “us vs. them “ approach is that it ignores the role of the environment, and the fact that animal populations, just like human ones, are often dealing with the emergence and spread of infectious diseases related to underlying environmental change. To ignore this fact is to risk missing valuable information about environmental change from the occurrence of these emerging diseases.
As an alternative to the “us vs. them” approach to animal health, we have proposed the consideration of a “shared risk” approach, whereby disease events in animals are viewed  as possible indicators of environmental health hazards that could also be putting humans at risk.  Of course this sharing of risk applies to toxic hazards as well, following the example of the “canary in the coal mine”.  For example, a dog with lead poisoning could provide warning to humans living in the same household. In short, human health professionals should care about disease events in animals since they could be sentinel events for hazards to human health.  In the same way, animal health professionals should consider new disease outbreaks in humans as indicators of possible underlying environmental changes that could affect the health of animals as well.

Using the “shared risk” paradigm, tracking human and animal sentinel events globally could help detect and prevent new disease threats due to changing ecological and environmental conditions.
Readers can find evidence about animal and human sentinel events in the Canary Database.

High altitude cattle and comparative medicine

A recent article in the New York Times is a reminder of the potential for new approaches to comparative medicine outside of the typical research laboratory. For years, veterinary researchers have studied the effect of high altitude on cattle grazing in mountainous areas of New Mexico and elsewhere. These studies have revealed important findings about the chronic effect of hypoxia on the cardiovascular system, including insight into the condition of hypoxic pulmonary hypertension that is an important disease in humans. The researchers have also been able to study the effect of genetic susceptibility on these physiologic responses, and to develop clinical approaches for early diagnosis of altitude illness. The recent research reported in the Times actually builds on studies of high altitude cattle in Colorado and other parts of the Southwestern US that have gone on over the past 100 years. And while we have learned a lot about human pulmonary hypertension (a disease that develops in individuals with sleep apnea and other hypoxic conditions) from “Brisket disease” in cattle, there are also preventive lessons to learn from animals that are evolutionarily adapted to high altitude, such as certain types of llamas, rodents, and yaks. These adaptations can involve not only much thinner blood vessels in the lungs, but also a lack of the response to low oxygen that is seen in cows and persons who develop pulmonary hypertension.  These findings reinforce the fact that there is much to learn from studies of naturally occurring animal populations that can influence both human medicine, veterinary medicine, and our understanding of the effects of environmental factors on health.  

The Need for Evidence

Over the past decade, human medicine has become more “evidence based.” Under this paradigm, recommended clinical policies regarding prevention, diagnosis, and treatment of diseases should be based on explicit evidence of the value of a particular approach. For example, whether or not a patient should be started on a daily aspirin for prevention of heart disease should be based on studies of such an intervention, and a weighing of the reported benefit of the intervention versus both the risk and cost of the intervention. The push for evidence based recommendations has resulted in certain traditional treatment approaches being abandoned and new ones adopted.

When it comes to human-animal medicine issues, including the use of animals as sentinels for human environmental health hazards, the use of animal assisted therapy for certain medical conditions, measures to prevent zoonotic disease in animal workers, and even the potential impact of adopting a “One Health” approach to disease prevention in the developing world, the truth is that there is a relative lack of “high quality”  evidence (such as controlled studies) either supporting or not supporting such interventions. This of course does not mean that such approaches are not worthwhile or do not hold potential for enhanced treatment and diagnosis of disease affecting both humans and animals (there is abundant anecdotal evidence that it may). Rather it means that there is a real need for research to test and rigorously evaluate such approaches. The Canary Database is an attempt to bring together published evidence about animals as sentinels of human environmental health hazards. Users can see “summary screens” that provide preliminary summaries of the strength of evidence that for a particular environmental hazard, particular species of animals may be either more susceptible, show shorter latency between exposure and disease onset, or have greater environmental exposure compared to humans. Advocates of “One Health” should be willing to see the concept tested objectively, and use the results to better hone future efforts to integrate human, animal, and environmental health.  Conducting studies that search for such evidence will better justify the commitment of health care resources in a human-animal medicine direction.

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