This article, from the Winter 2021 issue of "Microcosm," is adapted from a May 27, 2021, ASM-organized Reddit "Ask Me Anything" discussion on zoonotic diseases. The full discussion is on this Reddit page.

Zoonotic diseases, those transmitted between humans and animals, account for 75% of new or emerging infectious diseases. The future of public health depends on predicting and preventing spillover events, particularly as interactions with wildlife and domestic animals increase. In this article, several infectious disease experts answer pressing questions related to zoonoses.

The panelists are:

• Dr. David Blehert, Ph.D., Chief, Laboratory Sciences Branch, U.S. Geological Survey National Wildlife Health Center.
• Dr. Greg Gray, M.D., M.P.H., Professor of Medicine and Global Health, Duke University.
• Dr. Barbara Han, Ph.D., Disease Ecologist, Cary Institute of Ecosystem Studies.
• Dr. Tara Smith, Ph.D., Professor, Department of Epidemiology, Kent State University College of Public Health.

Q: Do Humans Have Reservoirs of Viruses that Don't Affect Us (Are not Pathogenic to Humans) but Can Spread to Animals and Cause Disease?

Dr. Greg Gray, M.D., M.P.H.: I think humans likely have viruses that we tolerate well but that may harm other animal species. Certainly, we know of human disease outbreaks (like measles) among non-human primates that have had relatively high mortality.

Dr. Tara Smith, Ph.D.: We can certainly spread viruses to other species, though perhaps we do not function as often as asymptomatic reservoirs as we know other animals, such as rodents or bats, do. We probably gave human rhinovirus C to non-human primates. Other pathogens, like MRSA, can also be spread from humans to animals, including our pets and zoo animals.

Q: How Do Climate Change and Environmental Factors Impact Zoonotic Disease Type and Frequency?

Dr. Smith: There are many ways that climate change can increase the risk of emerging infections and zoonotic disease in humans. Deforestation can lead to diversity loss and force populations to congregate in smaller areas, bringing new species into contact with each other and potentially with humans, which can allow for spillovers from species to species. Warming can allow disease vectors such as mosquitoes and ticks to move into new areas that were previously inhospitable to them. Climate refugees may be forced to move, potentially bringing animals with them into new areas, which again risks spillovers. Land may no longer be farmable, forcing rural individuals into cities that are denser and can lead to outbreaks.

Dr. David Blehert, Ph.D.: One of the results of climate change is that temperatures are warming in northern regions of the world. As this occurs, the population ranges of various cold-sensitive animal species are expanding northward. As the population ranges of these animals expand, they bring along various microbes, including pathogens, that may be new to the regions into which these species are expanding. As the ranges for various wildlife species expand, they also begin to interact with other wildlife species and populations from which they used to be isolated. Thus, changing ecological conditions result in the movement and spread of pathogens among animal species, which could result in increased risk for outbreaks of wildlife disease among previously naïve wildlife species and populations, increased risk for spillover of novel wildlife pathogens to domestic animals, and increased risk for zoonotic transmission.

Q: Large-Scale Animal Operations Are a Common Focus of the Origin or Spread of Zoological Diseases. What Can be Done to Minimize the Risk?

Dr. Gray: The Food Safety and Inspection Service of the U.S. Department of Agriculture (USDA), the Centers for Disease Control and Prevention (CDC) and the Food and Drug Administration all hold food-production industries accountable for zoonotic pathogen contamination in food. Food industries work hard at reducing the risk of food contamination. You really need to tour a modern plant to understand the many safety measures they employ. I think it's very feasible to work with the industry to make livestock farming safer. I also see the benefit of conducting surveillance for novel viruses among livestock workers.

Dr. Smith: It's still tough to know exactly how many spillovers we see from farming, especially "factory farming." Surveillance is notoriously difficult for a lot of reasons. It's tough to get on farms to acquire samples from animals and workers. In the U.S., many farm workers may be undocumented and in precarious positions regarding employment, so it's tough to get them to participate in studies if they don't necessarily trust researchers or worry it may "out" them to authorities. If they do get sick from something they may have acquired on-farm, they may not seek treatment for those same reasons. So we have incomplete knowledge in this area.

Dr. Blehert: The USDA routinely screens poultry farms for avian influenza viruses and removes infected animals from farms to prevent further losses to the poultry industry and prevent zoonotic transmission. Another mechanism to control outbreaks of highly pathogenic avian influenza is to conduct routine surveillance for avian influenza viruses in wild birds. When wild bird surveillance indicates that transmission risks are elevated, poultry producers can be advised to increase biosecurity.

Q: What Are the Arguments for Why We Will Likely Never Get Rid of Influenza?

Dr. Barbara Han, Ph.D.: The historic low in influenza cases last year was brought about because of global dampening in human-to-human transmission. Continuing to reduce human-to-human transmission will keep spillover events from turning into epidemics, but it will not eradicate the spillover events from animals from occurring.

Dr. Gray: I don't see eradication of influenza viruses in the near future. I would argue that avian species, swine, humans and cattle are all reservoirs for influenza A, B, C and D viruses, but swine influenza viruses concern me the most.

Q: What Are the Primary Drivers of Zoonotic Disease?

Dr. Blehert: Zoonotic diseases, or diseases of animals that can also infect humans, can be transmitted between animals and humans by a number of mechanisms, one of which is human interaction with infected domestic animal(s).

Dr. Smith: There are a lot of other ways that diseases can be transmitted from animals to humans. We know they can also come from farmed animals, and diseases such as Ebola, HIV and the original SARS can come from animals hunted/butchered/consumed as wild game.

Dr. Han: Spillover events, where pathogens are transmitted from an animal host into a human, occur frequently, but the vast majority of these don't cause any problems in humans. Each spillover event is not a complete roll of the dice; there is consensus about certain groups of pathogens that are more likely to cause severe or highly transmissible disease in humans.