The Tough and Deadly Journey from Animal to Human

The increase in zoonotic diseases reflects the complex relationship humans have with animals.

Reading Time: 4 minutes

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By Lillian Zou

Since the beginning of human life on Earth, we have learned to coexist with animals in order to survive. They served as prey to hunt and predators to hunt with. Along the way, our bond with animals deepened from centuries of sharing common environments and resources. In the lab, animal subjects are integral to breakthroughs in research. Additionally, animals with proper training help people with disabilities through their daily routine via animal-assisted therapy. They can simply act as lifelong companions and friends as well. However, our relationship with animals is a complicated one. With monkeypox making daily headlines, we are inclined to take a second look at this situation again and re-evaluate how healthy our relationship really is.

Zoonotic diseases, also known as zoonoses (singular: zoonosis), are pathogens that are naturally transmissible from animals to humans. Some popular examples are rabies, which comes from getting bitten by a rabid animal; anthrax, which comes from cattle; and avian flu, which comes from contact with birds. More recently, the viral outbreak of monkeypox that was declared a global health emergency by the World Health Organization (WHO) was first identified in a colony of monkeys native to West Africa used for research in Copenhagen, Denmark. While monkeypox outbreaks were most common in Africa, the disease traveled via rodents on shipments to Texas, thus causing outbreaks in the US. A key piece of advice public health agencies have given to prevent contamination is to avoid infected animals, especially mammals. As shown in the monkeypox outbreak, animals have played a significant role in carrying and transmitting zoonotic diseases.

A pathogen’s journey from an animal to human involves a series of steps that ensure its survival and replication in the body of its new host. For one, there are many ways the pathogen can enter the human body, not all of which involve direct contact. In the case of COVID-19, the virus came into contact with humans through a wet market, a place where cramped and unclean conditions led to different fluids and residues of infected animals mixing with each other, eventually forming breeding grounds for viral bacteria. During the Black Death, the bubonic plague found in rats was carried by fleas that came in contact with humans, making this a vector-borne disease.

Getting into contact with the human body is one task, but successfully entering the body and surviving the new environment is a whole other battle. In general, it is quite rare for animal viruses to infect humans due to the differences in structures and microbial environments. Diseases such as Ebola and COVID-19, which were found in bats, did not affect them as much as they did humans. In order for viruses to invade a host cell and replicate, they must possess the correct protein to attach to the receptors on the new cell’s surface. However, the same virus that infected the animal body may not possess the properties to effectively bind to the surface of a human cell. For instance, in order for the Simian immunodeficiency virus (SIV) found in nonhuman primates to transmit to humans and become the Human immunodeficiency virus (HIV), the virus had to undergo several genetic mutations. For thousands of years, humans were actually immune to SIV, which allowed them to have the upper hand in evolution. A crucial part of this immunity lies in the structural complexity in the binding site of the human cell which prevented the SIV protein to tightly bind. However, over time, a few SIV variants managed to overcome the biological hurdles in the human body, eventually leading to the emergence of HIV, which has affected tens of millions of people since the first case.

Limiting the prevalence of zoonotic diseases calls for re-evaluating and improving our relationship with animals. One part of this is taking our own steps to prevent the transmission of viruses. This includes basic hygiene and taking preventative measures against animal bites and scratches. To avoid food-borne zoonotic disease, it is just as important to handle store-bought food safely by washing them properly and keeping the cooking utensils and prepping area clean. The second part involves a more industrial approach, which concerns administering proper regulations and safety procedures when handling domestic animals and food production and marketing.

Currently, in the U.S., the Food and Drug Administration (FDA) provides guidelines and information on handling animals and addressing food safety in products and the places where the products are produced. Since the FDA only caters to the US, other countries may have different regulations regarding food safety and handling animals. In some cases, their regulations may not be enough to guarantee complete safety, which emphasizes the importance in handling imported production with expert care.

Though zoonotic diseases start from animals, we should take responsibility in controlling their transmission by working to improve human-animal interactions. Global phenomena such as climate change and the rapid urbanization in recent years don’t just account for the dramatic changes occurring in weather patterns and human populations—they’ve also led to an increase in infectious diseases that were once confined to warmer climates. Additionally, deforestation for the expansion of cities has led to misplaced animals and increased human-animal interaction.

Combined, our modern society has fostered an environment where our interactions with animals have not been completely beneficial. While our relationships with animals may not be perfect, we can take steps individually and as communities to form a better one.