Animal-Borne Diseases, Overspill of Pathogens Increased When Humans Practiced Animal Husbandry

According to a recent study, human adoption of animal husbandry was a major factor that caused an increase in pathogen overspill and animal-borne diseases.

Animal-Borne Diseases or Overpill of Pathogens

If not for farm animals, human civilization would not exist. But it seems that many severe diseases throughout human history wouldn't either.

The domestication and care of cattle coincided with the spread of animal-borne illnesses including the plague (Yersinia pestis) along with louse-borne relapsing fever (LBRF), according to newly discovered direct evidence by experts.

The risk of infections spreading from animals to people would have grown as nomadic hunters and gatherers in Eurasia started to settle down into sizable, pastoral groups approximately 12,000 years ago, according to a long-held theory among archaeologists.

405 Billion DNA Sequences

Recent advancements in ancient DNA analysis have allowed experts to examine the hypothesis closely.

Using 405 billion DNA sequences from 1,313 ancient human remains across Eurasia, an international team led by geogeneticist Martin Sikora at the University of Copenhagen uncovered genes of ancient microbes.

This extensive search revealed a 12,500-year timeline for the emergence and spread of human diseases.

Although the results await peer review, a significant portion of these microbes likely originated from environmental sources like animals.

While many human-infecting microbes remained constant over time, zoonotic diseases, transmitted between humans and animals, only appeared around 6,500 years ago.

Notably, plague and LBRF pathogens were undetectable in humans until roughly 6,000 years ago, coinciding with the shift from hunter-gatherer to agricultural societies.

Subsequently, zoonotic microbial DNA consistently appeared in ancient human genomes.

Pitfalls of Animal Husbandry

The rise in zoonotic diseases isn't just due to human-animal contact; population growth led to reduced hygiene and increased pests like rodents and insects.

Historical cases of LBRF outbreaks are linked to poor living conditions.

Sikora's study offers the first evidence of increased zoonotic infections post-agriculture.

Presently, over 60% of new diseases are zoonotic, but early humans encountered these microbes for the first time.

Eurasian steppe societies exposed to zoonotic pathogens early gained an advantage with regular access to meat and dairy.

Detection rates for zoonotic DNA spiked around 5,000 years ago as pastoralists migrated, bringing agriculture with animal husbandry and zoonotic diseases to new regions.

Developing Immunity and Historic Epidemics

The Steppe pastoralists may have developed immunity to zoonotic diseases due to their long-term animal exposure, which they then spread west and east.

This could have fueled genetic changes in Europe, possibly causing epidemic waves of zoonotic diseases.

If true, this migration may have led to significant casualties in Europe, similar to later Indigenous populations during European colonization.

As Eurasian communities grew denser, zoonotic pathogens multiplied, escalating endemic outbreaks into epidemics.

The bacterium behind the plague, found in horses, cattle, and sheep, first caused an epidemic in the Roman Empire around 540 CE.

Genomic analysis indicated Yersinia pestis existed at lower, consistent levels from 5,700 to about 2,700 years ago.

By medieval times, it became a deadly threat.

In Denmark, 11 out of 39 individuals in three medieval cemeteries succumbed to the plague.

In contrast, LBRF peaked around 2,000 years ago during a period of minimal plague activity, likely due to factors such as crowding, poor hygiene, war, migrations, poverty, or famine.

These outbreaks stemmed from shifts in human lifestyle, impacting global health and history.