Zoonoses: How animal viruses have shaped human history

By Carmen Rodríguez

Madrid, May 9 (EFE).- Zoonotic diseases, infections transmitted from animals to humans, have accompanied humanity since the Neolithic era, leaving behind devastating episodes such as the plague, amid renewed concern following a hantavirus outbreak aboard the cruise ship MV Hondius.

Renowned Spanish virologist  Margarita del Val told EFE that although they are “reasonably frequent,” they usually result from “a series of chance events that do not normally occur.

The current health crisis aboard the cruise ship MV Hondius, linked to a hantavirus outbreak, has once again drawn attention to zoonotic diseases, much as the SARS-CoV-2 pandemic did previously.

According to the World Health Organization, more than 200 types of zoonoses are known.

Del Val, a researcher at the Severo Ochoa Center for Molecular Biology in Madrid, said zoonoses have existed since the Neolithic era, when the rise of agriculture attracted rodents to human settlements.

Ancient origins and modern threats

In Eurasia, the later emergence of livestock farming significantly increased the spread of zoonotic diseases due to close and prolonged contact between humans and animals.

Intensive farming practices further facilitated the possibility of viruses adapting, by chance, to human-to-human transmission.

According to Del Val, roughly two-thirds of all known human infectious diseases have an animal counterpart, meaning that “the vast majority probably originated in animals, even if some have not yet been identified as such.”

Among the deadliest zoonoses in history, she cited the plague that devastated Europe in the 14th century. One of the oldest known examples is smallpox, which affected humans for thousands of years.

She also pointed to more recent zoonotic diseases that have emerged over the past two or three decades.

Hantavirus, for example, was first identified in the United States in the 1990s, although it was a different strain from the one linked to the cruise ship outbreak and was not transmitted between humans.

Other examples include the MERS coronavirus in the Middle East, which is frequently transmitted from camels to humans in close contact with them, Avian influenza, which generally does not spread between humans, the West Nile virus, and the Nipah virus, which she described as “distantly related to measles.”

Several of these pathogens are monitored closely because of their high mortality rates. Although spillover events can occur regularly, health authorities often react quickly enough to interrupt transmission chains, she said.

How pathogens cross into humans

Del Val explained that the conditions allowing a pathogen to jump from animals to humans vary widely, but at least two random events are usually required.

“The first random event is the appearance of a mutation that, although initially offering no advantage to the virus, allows it to replicate in humans,” she said.

The second step occurs when the virus encounters humans and becomes established in a population large and dense enough to sustain person-to-person transmission.

This, she noted, is the logic behind quarantines: reducing the number of susceptible persons to break transmission chains. “If you cannot pass it on to anyone, the chain of transmission ends,” she said.

In many cases, the jump to humans does not happen immediately after the mutation appears.

Often, scientists later discover that the virus had already been circulating among animals for some time before the conditions emerged for widespread human transmission.

Del Val also highlighted the importance of viral load, the amount of virus present in an infected person. If that load does not reach a certain threshold, the person may not be able to transmit the disease.

She said this may explain patterns observed with the West Nile virus in parts of Andalusia, southern Spain.

Reverse zoonoses and climate change

Humans are not merely victims of diseases originating in animals, Del Val noted, as reverse zoonoses, diseases transmitted from humans to animals, also occur.

The coronavirus responsible for COVID-19 has already been transmitted from humans to at least 18 animal species across several continents during the past six years, she said.

She also recalled that early in the pandemic, humans transmitted the virus to farmed minks. “We are not an island; there is a constant exchange of all kinds,” she said.

Climate change may also contribute to the rise of certain zoonotic diseases, particularly those spread by mosquitoes and ticks. Del Val cited the Asian tiger mosquito, which has become established in Mediterranean regions as temperatures rise.

The “One Health” approach

Given the continuing threat posed by zoonotic diseases, Del Val emphasized the importance of the “One Health” concept, which links human, animal, and environmental health.

“The health of the environment affects animals, the health of animals can affect us, and we are also part of that circle,” she said, pointing to interactions involving infectious diseases, food safety and the transmission of toxins and allergens.

“We cannot wait until people end up in intensive care to react. We need to know in advance what is circulating among domestic animals and wildlife, as well as the environmental impact of certain human actions,” Del Val said. EFE

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