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.