We have kept domesticated animals for thousands of years. The close contact between humans and animals has altered bodies on both sides.
For example, our gut microbiome, the first line of defence in the immune system, has changed. As we interact with animals, we become exposed to their microbes through our mouths, skin, and breathing.
“A diverse microbiome benefits the immune system. It trains the body to defend itself against a wider range of pathogens,” Professor of Immunology Seppo Meri says.
Scientific research has proved time and again that animals have a positive effect on human mental and physical health. Early childhood contact with domestic animals seems to protect against asthma and allergies. People susceptible to allergies constitute a notable exception, as animals can trigger the onset of an allergy in them.
Unfortunately, the microbes that animals transmit are not always beneficial, and some of them can cause illness. The most dangerous pandemics in human history have often originated in animals.
Jumping to a new host
A zoonosis is an infectious disease that spreads between animals and people. Zoonotic diseases are caused by bacteria, viruses, fungi, and parasites. Viruses create the most pandemics. However, the Black Death, the most infamous pandemic, was caused by the plague bacterium Yersinia pestis.
Some human viruses have passed on from mother to child since the dawn of humanity. Some others have spread from other species hundreds or thousands of years ago, causing humans and viruses to adapt to each other.
Transitions from one species to another still occur. Viruses are at their most dangerous right after jumping to a new host who has not yet developed resistance against them.
“Other species harbour so many viruses that there is plenty of material to draw from. We know very little about what nature still has to offer. Less than one percent of viruses are currently known,” Professor of Zoonotic Virology Olli Vapalahti says.
Spreading between species
Many diseases follow a similar pathway. First, a new microbe spreads and mutates in, for example, birds, rodents, or bats. It is then transmitted to humans either directly or through a domestic animal. Many common childhood diseases today have come to exist this way, including mumps, rubella, and measles. The already eradicated smallpox had a similar background.
In the 1890s, a coronavirus spread from rodents to humans through intermediate hosts, causing a pandemic that swept through the world population. Today, the same virus is rather harmless and among the causes of the common cold.
The origin of the currently raging coronavirus pandemic has not been certified. However, as bats constitute a natural reservoir for coronaviruses, it is probable that this virus also emerged from animals.
To cross the species barrier, pathogens need to infect a new host, replicate in its body, and avoid the immune system, Meri explains.
“The host must have a suitable receptor and conditions for the pathogen, as well as the inability to block it.”
Viruses and immunity: an arms race
Animal species that differ significantly from each other can have similar receptors, causing diseases to spread between them. Such is the case with humans, pigs, and birds – for example, influenza viruses can spread from fowl to swine to humans.
Over millions of years, influenza has spread and evolved in the gut of waterfowl. As birds have warmer bodies than mammals, the virus needs to mutate to replicate in mammals’ upper respiratory tract.
“The arms race between viruses and immunity is an endless cycle,” Vapalahti says.
At times, flu variants become unrecognisable to the human immune system. This may lead to a devastating pandemic, as happened in the case of the 1918 Spanish influenza.
In some cases, contracting the disease or getting vaccinated provides lifelong immunity. However, in some diseases, the virus mutates continuously, which does not allow the immune system to catch up. Influenza viruses require new vaccinations every year.
Diseases as travel companions
As our immunity is constantly evolving, some encounters with pathogens can become less serious. For example, an amoeba may cause a severe stomach bug in tourists, while locals may not develop any symptoms.
The American Indigenous peoples experienced the effects of this phenomenon particularly severely. They had not developed defences against the diseases transmitted by the European conquerors.
While Europeans had lived with sheep, pigs, and cattle for millennia, these animals had not been domesticated in the Americas. The Europeans had gotten ill and developed immunity together with the animals over time. However, as smallpox, measles, and influenza arrived with the Europeans, they wiped out most of the Indigenous population.
Meanwhile, the viruses indigenous to America overpowered the conquerors’ pets. A canine distemper pandemic was sparked when Spanish conquistadors took their dogs with them to South America. The dogs were exposed to the disease through vampire bats.
Even today, the diseases found in wild animals are the most dangerous to humans, as we do not have a shared history with them. Seppo Meri urges people to keep a respectful distance from them.
“Bats, foxes, and hares, for example, transmit pathogens that we have developed no resistance for.”
Bats are famous for spreading the Ebola virus disease. They are also suspected of being the source of the current coronavirus pandemic. In Finland, people should remember to keep their distance from common hares. They can spread tularaemia, a common disease in areas including Central Finland.
Viruses can mutate and spread efficiently in groups of animals, such as a flock of birds, a bat cave, an intensive piggery, or a mink farm. Intensive livestock production is sometimes referred to as a pandemic incubator. Large farms with inadequate hygiene practices can generate new virus variants.
“Compared to pigs kept in backyards, intensive piggeries might be able to better minimize contact with wild animals. However, if a suitable virus enters a large-scale facility through feed, for example, it will spread and multiply as if in a cell culture dish,” Vapalahti says.
A symptomless menace
Most zoonoses do not spread further from infected people. However, if a pathogen can transmit fast from one person to another, it can spark a pandemic. Usually, transmission occurs through airborne particles or droplets.
However, not all pandemics are fast or particularly communicable. An example of this is the HI virus, which originated in chimpanzees. Although its transmission requires unprotected sexual intercourse or direct contact with blood, it has killed tens of millions of people over the past four decades.
A pandemic becomes more likely if the disease has a long incubation period and thus has many asymptomatic carriers. The current SARS-CoV-2 coronavirus tends to cause milder symptoms than the 2002–2003 SARS epidemic, which has enabled it to spread more effectively. With asymptomatic individuals spreading the disease, controlling it becomes difficult.
Diseases spread by insects such as mosquitoes or ticks are not usually referred to as pandemics. However, malaria and yellow fever are major global killers that can be compared to pandemics. They have wiped out villages and towns in Africa and South America.
Viral cooking pots
Several factors predispose the modern world to pandemics. The loss of biodiversity reduces the supply of helpful microbes. Our microbiotas are growing poorer, which weakens the immune system.
As wild habitats become fragmented, causing animals to roam closer to humans, we are exposed to their pathogens more frequently. Species are becoming increasingly involved with each other. Production animals encounter wild animals, and people take exotic animals as pets.
The closer people live to each other, the easier it is for viruses to spread. Cities are cooking pots for new virus combinations. The more people and animals move, the more pathogens travel with them. As the movement becomes faster, the spread of diseases accelerates.
However, we have much better means to fight pandemics today than in past centuries. Understanding the functions of diseases makes it possible to develop vaccines and treatments against them.
“The new RNA vaccines have constituted a major leap forward. If we know what types of viruses animals carry, we can better prepare for pandemics. It is crucial to chart and investigate viruses,” Vapalahti says.
Will common colds return?
In normal times, we should not sterilise the environment excessively, Seppo Meri says. We need a natural microbial flora, and our immune systems must be exposed to stimuli.
The spread of diseases is partly influenced by our culture and attitudes. Coronavirus prevention has integrated face masks, remote working, and hand sanitiser into our everyday lives. Common colds and stomach bugs have become less prevalent.
Due to reduced contact with the pathogens, our immune protection against these diseases has become less effective.
“If preventative measures are lifted after the Covid period has passed, there will be more cases of the common cold. But are we ready for it, or will we continue to wear masks?”
It is interesting to see whether the reduction in colds will affect the emergence of type 1 diabetes, rheumatoid arthritis, multiple sclerosis, and other immunological diseases in the coming years. Some experts have suspected viruses to be an underlying factor in these diseases. A visible reduction in cases would bolster the theory.
The article has been published in Finnish in the 3/2022 issue of the Yliopisto magazine.
Many diseases are transmitted from animals to humans and vice versa. However, doctors do not always think to ask patients whether they have animals, according to Paula Kinnunen, docent of zoonotic microbiology and a specialist at MSD Animal Health.
Most everyday zoonoses are transmitted by domestic animals and pets. Kinnunen and her colleagues carried out a case study on zoonotic diseases caught by veterinarians. The risk of developing a disease also applies to animal owners and anyone involved with animals.
Some of the diseases are life-threatening, including rabies and the Capnocytophaga canimorsus bacterium, which spreads through canine mouths and causes sepsis.
Rabies primarily constitutes a risk with imported animals. When the Finnish Food Authority investigated 170 legally imported dogs whose documents were in order, it found that more than 25 percent lacked protection against rabies. It seems that some vaccination certificates are forged.
“Importing unvaccinated animals from countries where rabies occurs is a ticking time bomb. Without up-to-date vaccinations, rabies is fatal.”
The most reliable choice for a pet is a healthy animal of a healthy breed and from good conditions. It is best for animals living in poor conditions abroad to receive assistance on location. Kinnunen recommends supporting local organisations as a way of helping.
In addition to vaccinations, deworming and parasite control should be kept up to date. People can avoid many infection risks by washing their hands with soap after coming into contact with secretions.
As pets can also spread roundworm, yersinia, salmonella, and campylobacteria, their food bowls should be kept separate from human dishes. Food matters as well.
“Raw meat is not safe even if it is domestic and frozen. Antibiotic-resistant MRSA bacteria were found in 41% of domestic raw food samples.”
Humans too can infect animals with their diseases. For example, SARS-CoV-2 has been transmitted from humans to other species including cats, dogs, minks, ferrets, and deer. In zoos, people have spread the virus to at least lions, tigers, hyenas, coatis, and hippopotamuses.