How SARS, HIV, Other Animal & Environmental Pathogens Are Spreading to Human Populations.
Question: What do these diseases have in common?
- Ebola virus.
- Zika virus.
Answer: They didn’t stay put.
These viruses and many others almost certainly originated among animals (e.g., bats or nonhuman primates) and then spilled over into humans, sometimes through an intermediate carrier, such as insects. They’re called zoonotic diseases, that is, infectious diseases that are transmitted between species. But the transmission doesn’t occur solely in one direction, that is, animal-to-human. Zoonotic diseases also happens when a disease jumps from humans to animals. In the fall of 2020, Denmark slaughtered millions of minks, who probably caught COVID-19 from their handlers, or the deaths of three snow leopards in November 2021 at the Lincoln Children’s Zoo in Nebraska, due to complications of SARS-CoV-2.
The jump of animal and environmental pathogens into the human is an ongoing biological “long game” dating back thousands of years and has led to historical epidemics such as smallpox, cholera and bubonic plague. Rabies is one of the oldest described infectious diseases.
Because of the many variables involved in spillover events, complex life cycles of pathogens, modes of transmission, and random interactions between hosts, the process is a rare and imperfect one. However, with enough opportunities for pathogens to adapt to new hosts or evolve to sustain transmission in more suitable hosts spillover will occur. Experts believe that the rate of novel infectious disease emergence has increased in recent history.
Viruses find a way
HIV offers an example of a virus adapting to transmission within a new host. The evolutionary ancestor of HIV is Simian Immunodeficiency Virus (SIV), which has spread in primates for tens of thousands of years. Multiple spillover events of SIV occurred, but until the early 20th century, the virus was unable to transmit to a human host. Then handling of bushmeat of an infected primate and blood-to-blood contact with the handler was believed to have resulted in the spillover of a SIV variant that was able to sustain itself in a novel host – humans -- which led to the HIV pandemic.
The family of viruses known as “coronaviruses,” like SARS-CoV-2, are particularly poised to cause spillover events, because they seem to have overcome many of the hurdles that usually limit a virus to a single species. Also, coronaviruses are RNA viruses, meaning they can jump species via evolution more quickly than other virus families, given their ability to recombine and acquire point mutations.
Even more concerning, spillover events are almost always invisible. Without a robust public health and healthcare infrastructure, such as advanced molecular epidemiology techniques, emerging pathogens and strains or variants of known pathogens can go unnoticed for long periods, causing delays in treatment and prevention.
The good news is that experts have identified a number of factors that can predict where these viruses are more likely to emerge. These factors include the type of virus, population (both animal and human), degree of urbanization, demand for animal protein, travel and connectivity between population centers, habitat loss, climate change, and increased interactions among people and animals. Developing countries -- with their growing human populations, decreasing animal habitats, – are most at risk from spillover events
The 2003 outbreak of Severe Acute Respiratory Syndrome, or SARS, was the first severe and readily transmissible new disease emerge in the 21st century. Caused by a previously unrecognized coronavirus associated with the animal markets of southern China, the disease spread across the world via major air routes, spreading to 29 countries on five continents. Later evidence pointed to bats as the origin of the virus.
This discovery encouraged further investigations into a variety of wildlife species, which revealed a plethora of new viruses carried by bats, rodents and other species of wildlife. The SARS outbreak demonstrated that:
a previously unknown pathogen could emerge from a wildlife source at any time and in any place and, without warning, threaten the health, well-being and economies of all societies.
there was a clear need for countries to develop the capacity to maintain an effective alert and response system to detect and quickly react to outbreaks of international concern.
responding to pandemic threats requires global cooperation and global participation.
SARS – and soon thereafter, the highly pathogenic avian influenza H5N1 – brought new energy to a worldwide movement known as “One Health,” which explores and addresses the link between human and animal health and the threats that diseases pose to food supplies and economies.
Because of SARS and H5N1, the United Nations Secretary General appointed a UN Systems Coordinator for Avian and Animal Influenza (UNSIC) and formed a collaboration with a number of international and national organizations, including the World Health Organization (WHO), Food and Agriculture Organization (FAO), World Organization for Animal Health (OIE), United Nations Children’s Fund (UNICEF), and World Bank and various national heath ministries, to develop the International Ministerial Conferences on Avian and Pandemic Influenza (IMCAPI).
Many regions and nations, particularly developing nations, have embraced the principles of One Health and are applying them to the extent they can. Nations as diverse as the United Kingdom, France, Sweden, Laos, Thailand, Mongolia, Canada and Tanzania, to name a few, have set up One Health entities within or supported by government.
Zoonotic viruses have caused many of the world’s epidemics and pandemics. To prevent the next one, we need to know: How and why does a wildlife virus become zoonotic? Which viruses pose the greatest threat? How can we change our behavior to prevent disease outbreaks and protect wildlife essential to our ecosystem and life on earth? Scientists have developed a website app called SpillOver for policy makers, scientists and the general public to assess the likelihood that a wildlife virus will spill over and spread in humans.
Will this knowledge affect healthcare delivery on a day-to-day basis? The fact is, the veterinarian community has embraced One Health, but the medical community has been much slower to engage, despite support from organizations such as the American Medical Association, Public Health England and WHO. Engaging the medical community more fully in the future may require the incorporation of One Health into the medical school curricula so that medical students see it as an essential component in the context of public health and infectious diseases.