In recent years, the prevalence of zoonotic diseases, or zoonoses, has been on the rise, prompting warnings from public health officials worldwide. But what exactly are zoonoses, and why is their increasing incidence a cause for concern?
The last few decades have seen a plethora of new zoonotic outbreaks, including Ebola, Mpox, and Avian Influenza.
The Basics
Zoonoses are diseases that can be transmitted from animals to humans and vice versa. The most famous examples are viruses like Ebola, HIV, and SARS-CoV-2 (the virus that causes COVID-19), but other pathogens ranging from bacteria to fungi can also jump between species.
Typically, a zoonosis occurs when humans come into direct contact with infected animals and their bodily fluids or consume contaminated food or water. In many cases, the disease is often limited in its spread, as most zoonotic diseases are adapted to their natural hosts and poorly replicate in humans.
Still, largely due to the sheer amount of contact that occurs between humans and animals means, the global health burden of zoonoses is enormous. Pathogens with nightmarish names like Rabies and Q Fever cause an estimated 2.5 billion illnesses a year—that's right, almost a third of the human population—of which 2.7 million result in death.
Why are zoonoses increasing?
In recent decades, the rate at which zoonotic diseases have emerged has accelerated. The trend holds even after accounting for increases in reporting and is largely driven by human development.
As human populations expand and encroach upon more and more natural habitats, interactions with animals naturally increase, providing more opportunities for disease transmission. Direct habitat destruction to create farmland, urbanization, and the broader effects of manmade climate change not only alters ecosystems, but also the spread of their diseases.
Deforestation, in addition to pushing our planet to the brink, is contributing to the rise of zoonotic diseases.
At the same time, globalization has allowed zoonotic diseases to spread more widely than ever before, leapfrogging borders through international travel and trade. While the plagues of history took years or even decades to circumvent the planet, recent outbreaks like SARS have gone global before their threat was even fully recognized.
To feed the growing global population, food production practices are also increasingly contributing to zoonoses. In addition to the increasing size of farm animal populations, intensive farming methods are putting animals in extremely close contact and allowing different species to interact in ways they rarely do in nature. Compounding the issue, sanitation is extremely poor, making disease outbreaks inevitable.
In addition to providing the grounds for disease spread, farms also enable disease evolution. In the absence of other control measures, drugs are used carelessly and in large amounts to keep animals healthy, which can enable pathogens to develop drug resistance. In these settings, untold germs can be shared and new diseases can evolve, all while having the chance to infect human farm workers, creating the perfect conditions for a zoonosis.
What does this mean?
As zoonoses increase, so will their already enormous health burden. This will have a disproportionate impact on the poorest people in the world, whose increased dependence on livestock exposes them more to zoonotic diseases. At the same time, their limited access to healthcare will only increase the lethality of these diseases. However, the threat of zoonoses is not only limited to those in poverty.
One example of this is antimicrobial resistance (AMR). As discussed earlier, the misuse and overuse of common medication—already bad enough in humans—also occurs in animals on a massive scale, causing new drug-resistant diseases to spill over to us. These diseases have already spread globally and are only becoming more prevalent. If resistance develops against last-resort drugs, as happened with Vancomycin, even the wealthy will have nowhere to turn. The cost is clear: by 2050, more than 10 million people are expected to die from AMR every year.
Additionally, many zoonotic diseases are entirely novel, meaning humans have either little or no immunity to them. As a result, if one takes root and becomes fully adapted to humans, as has happened several times in the last century, a devastating pandemic could ensue.
The chances of this are increasing. We have already been through one Coronavirus pandemic that may have jumped from animals, but that virus was not alone. One estimate puts the number of human infections by SARS-related Coronaviruses at over 60,000 a year in Southeast Asia alone. Other viruses with pandemic potential, such as novel Influenza viruses and Paramyxoviruses, are likely just as prevalent.
A map of emerging zoonotic disease hotspots. Source
To effectively combat the growing threat of zoonotic diseases, concerted efforts far beyond the current status quo are needed. We need better surveillance and early detection in all countries, not just wealthy ones. As of right now, because we have little knowledge on where or when the next big zoonotic threat will emerge, we are fighting zoonoses with one hand tied behind our backs.
However, if robust surveillance systems are established around the world, scientists can monitor both animal and human populations for any signs of illness and respond accordingly. We've already seen with the COVID-19 pandemic that even with the harshest government response, people will still die if we act too late. However, if we could detect zoonotic outbreaks in their infancy, we might have a chance to contain them entirely.
Other tools include vaccination programs for animals, which can stop diseases before they even make the jump to humans. Certain regulations on farming practices and habitat destruction can limit opportunities for animal diseases to transmit. We can also implement education campaigns that raise awareness around public health and set improved standards for sanitation and hygiene practices—the options are endless.
Perhaps most importantly, global health leaders must adopt and pursue a One Health approach, where human, animal, and environmental health considerations are integrated. As diseases increasingly smash past traditional barriers, humanity must respond accordingly.
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