Cynthia Schuck: “Species such as chicken, pigs and cows act as intermediate or amplifier hosts where pathogens can evolve and spill over into humans.”

Cynthia Schuck, author of the book Pandemics.

Author of the book Pandemics, Cynthia received her D.Phil. in Zoology (Evolutionary Biology/Animal Cognition) from Oxford University. Her D.Phil was followed by two research fellowships (at Oxford and Brazil), as well as by various research projects for institutions in the UK, USA and Brazil. As a scientist Cynthia published a number of articles on subjects ranging from the evolution of advanced cognition and disease epidemiology to the mathematical modeling of animal distributions based on climate. She currently investigates farm animal health and welfare.

Would a ban on the production, trade and sale of wild animals for consumption protect us from future pandemics?

The production (or hunting), processing and sale of wild animal species for consumption, particularly in conditions of poor welfare, health and sanitation, greatly amplifies the risk that viral strains harbored by wild animals are transmitted to human populations. This was the case, for example, with the SARS epidemic and the Covid-19 pandemic. However, the risks of infectious disease outbreaks are far from restricted to the trade and consumption of wild animals. Intensive animal farming systems, currently where most of the meat is produced in the world, also create ideal conditions for the emergence of highly pathogenic viral strains, along with the ideal conduit for the infection of human beings. This was the case, for example, with the 2009 influenza (swine flu) pandemic, along with the multiple sources of the highly pathogenic bird flu, currently a major threat to global health. In the latter cases, chickens and pigs made the genetic bridge between the wild virus and the virus that finally spread in the human population.

Why does the way we treat animals, such as chickens and pigs, matter for pandemics?

Because species such as chicken, pigs and cows act as intermediate or amplifier hosts where pathogens can evolve and spill over into humans. We all have learned during the Covid-19 pandemic that good health, strong immune systems, social distancing, spending time in the sun, and proper sanitary conditions are all important to prevent and fight infections. The opposite of these conditions are present in most animal production facilities. In these places, large populations of animals are confined at high densities in closed and barren environments. Additionally, high levels of aerial pollutants such as ammonia and fecal dust, which naturally result from the presence of high volumes of animal waste, are often found in these facilities. Not surprisingly, the respiratory function of animals and their first barriers of defense against infection are often compromised. In pig farms, for example, respiratory disease is widespread, with most pigs experiencing some form of lung pathology, including pneumonia, during their lives. Immunosuppression induced by chronic stress (due to confinement, aggression, deprivation of natural behaviors and poor hygiene) is also a reality in these systems. These conditions enable not only the rapid transmission of infections but, more worryingly, allow different viral strains to mix up in the hosts and combine their genetic material, which time and again has led to the emergence of viruses that can also infect humans.

What are the origins of infectious disease outbreaks with pandemic potential in the last century?

The following list describes the likely origin of epidemics and pandemics that emerged in the last century, all of which are associated with the hunting and preparation, or production and sale, of animals for human consumption. Ebola: bats; HIV: primates; SARS: civet cats; SARS-CoV-2 (Covid-19): pangolins; H1N1pdm influenza pandemic: pigs; multiple avian influenza (bird flu) outbreaks: chickens.

What is antibiotic resistance?

Antibiotics act by either killing bacteria directly or reducing their ability to grow and reproduce. However, they can stop working if bacteria find ways of countering their actions, for example by changing their structure so that the antibiotic no longer recognizes them, or by directly neutralizing (e.g. digesting) the antibiotic. These abilities can be acquired fortuitously by mutation, or by the incorporation of genes from other bacteria possessing such abilities. The greater the exposure to antibiotics, the greater the chances that the abilities that confer ‘resistance’ to antibiotics will spread.

What do farmed animals have to do with antibiotic resistance?

Although part of the problem is the overuse of antibiotics by the human population, most antibiotics (over 70% in the world) are not used in humans, but in animals raised in intensive farming systems. In these systems, antimicrobials are widely used not to treat sick animals (which would be justifiable), but prophylactically, to ensure the survival of animals of frail health under the strenuous conditions of factory farms. Not surprisingly, antimicrobial-resistant bacteria have been isolated in several food-producing animals and derived food products.

How does antibiotic resistance spread from animals raised as food to humans?

One way antibiotic-resistant bacteria make their way from these animals to humans is through products of animal origin. For example, the same strains of Escherichia coli ST131, which is responsible for millions of infections each year and has become resistant to multiple drugs, has been found on samples of poultry and pork meat sold in grocery stores and on urine and blood samples from patients diagnosed with extra-intestinal infections. In Hyderabad, India, samples of chicken meat sold at retail stores had been found to be contaminated with a similar superbug. In Brazil, methicillin-resistant Staphylococcus aureus (MRSA) has been identified in milk samples. In that country, bacterial strains resistant to multiple drugs have n also been identified in pork and chicken meat. In China, identical genes conferring antibiotic resistance to E.coli were detected in retail meat samples and human patients. In the United States, 75 percent of the bacteria the Food and Drug Administration found on grocery store meat was antibiotic-resistant. Environmental contamination is another route of transmission. Bacteria can make their way to water bodies and the soil and contaminate other agricultural produce through the use of animal waste as fertilizers.


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