Rabies is an ancient disease that has affected humans and animals for thousands of years. It is caused by the rabies virus, which infects the central nervous system and almost always leads to death if left untreated. Rabies is spread through the saliva of infected animals, usually through bites. Due to its antiquity and high fatality rate, rabies has inspired fear throughout history. But is rabies truly the oldest known virus afflicting humankind? To answer this question, we must examine the history and origins of this deadly pathogen.
A History of Rabies
References to a disease resembling rabies appear in some of the earliest human writings. The Eshnunna Code, written in Babylon circa 1930 BCE, describes symptoms consistent with rabies infection. Ancient Greek and Roman writers such as Homer, Aristotle, and Pliny the Elder describe a disease transmitted by dogs that matches rabies.
The first written record using the term “rabies” comes from the Ebers Papyrus, an Egyptian medical document dated to circa 1500 BCE. This suggests the disease was already established in Africa and the Middle East by the 2nd millennium BCE.
In the Americas, rabies was present when the first colonists arrived. Mexican pottery dating back to 300 BCE depicts a salivating dog that may have rabies. Rabies is described in Aztec writings from the 10th to 16th centuries CE.
From medieval times onward, rabies outbreaks in dogs and wolves were common across Eurasia and Africa. The earliest vaccines were developed in the 19th century after Louis Pasteur identified the rabies virus particle in 1885. Despite control efforts, rabies remains endemic in over 150 countries today. Ancient origins combined with persistence through history make a strong case for rabies as one of the oldest known human viruses.
The Origins of Rabies Virus
To determine if rabies is truly the oldest human virus, we need to explore its evolutionary origins. All extant rabies viruses appear to derive from a common ancestor. But when did this progenitor virus first emerge?
Rabies virus belongs to the Lyssavirus genus, within the Rhabdoviridae viral family. Its nearest relatives include other lyssaviruses that infect bats and livestock today. Analyses of viral RNA sequences indicate that lyssavirus lineages diverged from a common ancestor between 700 and 1500 CE.
This datable ancestry only covers modern lyssaviruses. But rabies-like viruses existed earlier in history. Analysis of viral remnants within ancient dog genomes suggests dogs were infected by rabies-like viruses as far back as 9,000 to 5,400 BCE.
The ultimate origin of rabies virus remains unknown. But evidence shows rabies-like viruses already infected mammals during the Neolithic period. This gives rabies virus ancestry stretching back at least 7,000 years.
How Rabies Virus Compares to Other Ancient Viruses
To determine if rabies is the oldest human virus, we need to compare its antiquity against other viral diseases. Here are some of the other contenders for oldest known human viruses:
Smallpox – Caused by variola virus. Egyptian mummies show evidence of smallpox dating back to at least 1157 BCE. Smallpox may have emerged as early as 10,000 BCE in Africa.
Polio – Caused by poliovirus. While first described in 1789 CE, poliovirus is an enterovirus closely related to other intestinal viruses that likely circulated for thousands of years earlier.
Dengue – Caused by dengue virus. Evidence of illness resembling dengue fever dates back over 2,000 years in Asia and Africa. Dengue evolved from an African primate virus an estimated 500-2000 years ago.
Hepatitis B – Caused by hepatitis B virus. Existed in northern Africa and Asia by 1200 BCE based on DNA evidence. Belongs to an ancient family of primate hepatitis viruses.
HIV – Caused by HIV. Related to primate immunodeficiency viruses in Africa. Current HIV pandemic stems from a simian virus that jumped to humans in the 20th century CE.
Considering the available evidence, rabies matches or exceeds the other contenders in antiquity. Rabies has remained in continuous circulation for at least several millennia. Its ultimate origin may stretch even earlier into mammalian prehistory. So while an absolute date for “oldest” human virus remains elusive, rabies remains a strong candidate due to its ancient roots and persistence through history.
Why Rabies Virus Has Persisted for So Long
What explains rabies’ extraordinary longevity as a human and animal pathogen? Several key attributes enable ongoing survival of this virus:
– Broad mammalian hosts – Rabies infects all warm-blooded mammals, giving it numerous reservoirs. Dogs remain the main reservoir globally, but various wildlife also harbor rabies strains.
– High mutation rate – RNA viruses like rabies have high mutation rates. This enables adaptation to new hosts and evolution of strains that can escape immunity.
– Neuroinvasiveness – Rabies powerfully infects neurons and spreads through peripheral nerves into the central nervous system. This shields it from antibody detection.
– Neurotropism – The virus concentrates in neural tissues, which favors transmission via bites. Concentration in salivary glands promotes shedding into saliva.
– Stealth infection – Rabies causes slow progressive disease, enabling transmission for two weeks or more before symptoms appear.
– Lethality – While rabies kills infected hosts, death occurs after opportunities for transmission. Selective pressure thus builds virulence.
Together, these traits help rabies sustain long epidemics and spillovers between animal reservoirs. They may explain its maintenance across millennia as a resilient human and animal pathogen.
Conclusion
Rabies is certainly among the most ancient of viruses afflicting humankind based on several lines of evidence:
– Historical records date rabies-like disease back over 4,500 years in Africa and Eurasia.
– Evolutionary evidence indicates rabies-related viruses already infected mammals as far back as the Neolithic period.
– Rabies matches or exceeds the antiquity of other notorious viral diseases that have left their mark on human history.
– Key viral attributes enable long-term survival, adaptation, and transmission between animal reservoirs and humans.
While its ultimate origins remain uncertain, rabies virus has proven a tenacious and dangerous companion throughout much of recorded human history. Given its ancient roots and ongoing impact, rabies remains one of the oldest known threats to public and animal health. Understanding its history and evolutionary adaptations can help inform continued efforts to control and eventually eliminate this age-old viral scourge.