Zoonosis: the key to the latest health alerts

Monkeypox is the latest installment in an emerging threat: infectious diseases jumping from animals to humans. These represent an increasingly recurrent global public health problem. Overpopulation, greater mobility, the destruction of ecosystems and the trade in species are some of the causes that explain its rise.

Humans have interacted with animals since the origins of our species. This relationship has brought us benefits, such as being able to better cultivate the fields or eat healthily, but also harm such as zoonoses, caused by harmful germs carried by animals and which can be transmitted to humans.

The last chapter on these pathologies is carried out by monkeypox, a viral zoonosis endemic to Africa which, for the past few weeks, has accumulated 257 cases outside this continent, of which 98 have been reported in Spain.

According to a report by the Food and Agriculture Organization of the United Nations (FAO), 75% of infectious pathologies are of animal origin. These viruses, bacteria, parasites and fungi cause minor and serious problems and can even lead to death. Additionally, they spread through direct contact or through food, water, or the environment.

More than 200 zoonotic diseases have already been identified, and they are increasing and spreading faster and faster. Fortunately, some of them can be prevented entirely by methods such as vaccination.

zoonosis, smallpox, ebola, bird flu, COVID-19, biodiversity, fires, deforestation, wildlifeThe WHO concluded that the civet carried the SARS-CoV-1 virus. /AdobeStock


Once upon a time… zoonoses

Throughout history, these infectious pathologies have influenced human beings and everything seems to indicate that they will continue to do so on a planetary scale. Early civilizations in the Middle East, such as Egypt and Mesopotamia, already documented the existence of rabies. This ancient zoonosis is caused by a virus of the family Rhabdoviridae and is transmitted by bites or scratches from an infected animal.

The most effective way to combat it is the vaccination of domestic dogs, since they are the main culprits in its spread. Currently, rabies is concentrated in poor and vulnerable populations in Asia and Africa.

Far more deadly was the Black Death, which killed 50 million people and caused great concern among the population in the 14th century. The cause of this well-known massacre was the bacteria Yersinia pestisthat inhabits small mammals and the fleas that parasitize them.

Its rapid spread was driven by frequent contact with rats and fleas and the precarious living conditions that existed in the Middle Ages. Today, the plague continues to affect nearly 3,000 people worldwide and is considered endemic in several countries such as Madagascar, the Democratic Republic of Congo and Peru.

In 1986, the first cases of bovine spongiform encephalopathy (BSE) were detected in the UK. “Mad cow disease” is the accumulation of the protein let’s pray in nervous tissue and is transmitted through the consumption of contaminated meat.

According to the World Organization for Animal Health (OIE), the risk of infection arises when food is contaminated with organic material from other herbivores. After obtaining conclusive data and identifying the causes of BSE, the cows were no longer fed the feed that caused this zoonotic episode.

On the verge of entering the new millennium, in 1997, we encountered avian flu. This new pathology of animal origin is caused by subtypes of the virus Influenza A which affect birds, although some of its strains can occasionally infect humans and other mammals.

From 2004 to 2006, the virus spread among poultry from Asia to Europe, and the World Health Organization (WHO) warned that bird flu had the potential to become a pandemic. Although there are still no mutations in the virus that facilitate transmission between people, news of infections in humans with new strains continues to emerge.

Since the end of 2019, humanity has been living with a new virus, SARS-CoV-2, which was the first major pandemic of the 21st century. More than two years after the first outbreak, we are still trying to find out which animal was responsible for transmitting this virus to humans and whether there was an intermediate transmitter. The main suspect so far? The bat.

To this must be added, in 2022, the epidemic of the monkeypox virus (monkeypox). Although this is not the first time he has left Africa, he has now reached several countries in Europe. It affects the general population and is transmitted in any setting involving close contact, not necessarily sexual. Complete sequencing is now complete.

What is the cause of the proliferation of these diseases?

Zoonotic diseases do not happen by chance, but many factors contribute to their proliferation, which influence both the pathogens (germs) and their hosts (humans and animals).

The changes or mutations that both undergo to adapt to the environment are called biological factors. In general, the simpler an organism, the faster it changes, giving rise to several variants of the same species. Something we see up close with SARS-CoV-2.

Physical factors, such as climate or weather, determine the survival (or multiplication, if any) of the pathogen outside the original host. In other words, these elements offer viruses, bacteria, parasites and fungi various possibilities of transmission to other species.

The alteration of ecosystems is included in the ecological factors, which also include deforestation, natural disasters or intensive agriculture. Elisa Pérez, veterinary virologist at the Animal Health Research Center (INIA-CSIC), explains that “the loss of biodiversity seriously affects the balance of ecosystems. Predator-prey systems are disrupted, some species suffer from lack of food and shelter, etc. All this weakens the immune system of animals and increases the risk of new viruses or variants appearing.”.

The expert in eco-epidemiological studies and professor at the University of Barcelona (UB), Jordi Serra, adds: “We have mainly dedicated ourselves to combating the most visible and immediate biodiversity losses such as fires or deforestation. The problem is that we forget that the dynamics between microorganisms also change. These alterations are not immediate and are more difficult to notice, but they also play an important role in episodes of zoonoses.”.

This phenomenon has been aggravated by air travel, allowing infectious agents to reach any part of the world within 24 hours. This has been the case with viruses like Ebola or West Nile.

Ricard Parés, president of the Consell del Col·legi Oficial de Veterinaris de Barcelona (CCVC), illustrates this with the situation in Ukraine: “There are still wild animals that have rabies there that can transmit it. Once humanitarian aid has been provided to these people, which is the main thing, we have to see if they bring pets with them. These could reintroduce diseases already controlled in territories where vaccination is not compulsory”.

On the other hand, intensive exploitation, whether agricultural, livestock or fish farming, is also another element to be taken into account. In today’s world, local animals and new or exotic species are exploited. In this sense, wet markets stand out, ideal breeding grounds for the emergence of pathologies such as avian flu and covid-19.

Unpredictable situations, vigilance required

Should we then identify and monitor a series of animals potentially dangerous to our health? “It would be a mistake –says Parés–, because these situations are unpredictable. At the domestic level, the range of what is considered a pet is opening up. Vietnamese pigs or reptiles such as iguanas or turtles are a good example. On the other hand, in the wild world, there is also more interaction through tourism activities, such as safaris in Africa. There is no need to generate an alarm, these are simply factors that must be taken into account”.

The trade in exotic animals, whether legal or illegal, also influences zoonotic disease episodes. By moving them to different parts of their habitat, often thousands of miles away, the infectious diseases they might suffer from travel with them.

This happened in 2003 with the first recorded outbreak of monkeypox outside of Africa, in the United States. People who contracted the infection were infected by their pets, some dogs from meadow. These rodents, in the pet store, were in contact with mammals from Ghana, which transmitted the virus to them.

One Health: health depends on everything and everyone

Given the large number of factors that influence the appearance and spread of zoonoses, it is not surprising that attempts to protect our health are increasingly multidisciplinary and collaborative. In this sense, in recent years the concept of One Health, which recognizes that the health of people is closely linked to that of animals, plants and the environment. Professionals in these and other fields need to communicate and collaborate to deal with new threats.

Adelaida Sarukhan, science writer on emerging viruses at the Barcelona Institute for Global Health (ISGlobal), explains that health must be designed globally, not just from the global north (global north, in Spanish) as before.

To build global health, we must collaborate to generate and share quality data. It is essential that adequate resources are dedicated to helping low- and middle-income countries produce and analyze this data. The other cornerstone – continues Sarukhan – is to strengthen health systems, especially primary systems. The pandemic has shown us how a health problem in one corner of the planet is a community problem”.

The key: vigilance

Working together can help predict the risk of human infection. Sarukhan comments that the most important action to identify a zoonotic outbreak and prevent epidemics or pandemics is surveillance.

It is believed that there are approximately 300,000 unknown viruses, in mammals alone, capable of being transmitted to humans. Identifying and investigating them can cost between one and five billion dollars, which is nothing compared to the human, social and economic cost of a pandemic.“, To add.

Citizens can help with very simple things: do not feed wild animals, such as wild boars, which may carry hepatitis E.; or avoid leaving waste outside containers, as this attracts them. To avoid the proliferation of mosquitoes, which can be transmitters, try not to have containers with water at home. And last but not least, wash your hands“, explains Jordi Serra, researcher at IRBio.

As human populations grow and expand, people increase their contact with new animals and diseases. “We need citizens who are informed and aware that their daily actions have an impact on nature and health“, concludes Elisa Pérez.


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