Deadly diseases that hopped across species
Arunima Sen Gupta
Bacteria and viruses that are deadly to one type of creature can evolve quickly to infect another. While the new coronavirus SARS-CoV-2 (which causes COVID-19) is the latest example, a host of infectious and deadly diseases have hopped from animals to humans and even from humans to animals. The cross-species infection can originate on farms or markets, where conditions foster mixing of pathogens, giving them opportunities to swap genes and gear up to infect (and sometimes kill) previously foreign hosts. Or the transfer can occur from such seemingly benign activities as letting a performance monkey on some street corner climb on your head. Diseases passed from animals to humans are called zoonoses. There are more than three dozen we can catch directly through touch and more than four dozen that result from bites. But disease-carrying parasites are not picky about hosts. Human diseases can decimate animal populations, too, from such well-meaning activities as ecotourism.
Novel coronavirus: The novel coronavirus that causes the disease COVID-19 was first identified at the end of December 2019 in Wuhan, China, where officials suspect the source was somehow linked to a seafood market there. Genetic analyses of the virus suggest it originated in bats. However, because no bats were sold at the seafood market at the outbreak’s epicenter, scientists think an as-yet-unidentified animal acted as a go-between in transmitting the coronavirus to humans. This ‘intermediate’ animal could be the pangolin, an endangered, ant-eating mammal, according to a handful of studies of the virus.
Influenza pandemics: The 1918 influenza pandemic swept the world within months, killing an estimated 50 million people — more than any other illness in recorded history for the short time frame involved. The H1N1 influenza virus that infected more than one-third of the globe had an avian origin. First identified in the United States by military personnel in the spring of 1918, the virus killed an estimated 675,000 Americans, according to the Centers for Disease Control and Prevention (CDC). Unlike some flu strains that mainly kill the elderly and those with compromised immune systems, the 1918 strain hit young adults hardest, as the older population seemed to have some immunity built up from a past H1N1 virus. Worldwide, the virus killed between 151,700 and 575,400 individuals. That virus appears to have originated in pig herds, with a so-called re-assortment of influenza viruses — when the viruses swap genetic information — occurring naturally in North American and Eurasian pig herds.
Bubonic plague: Nothing beats the 14th-century Black Death (also called Bubonic Plague) for sheer global impact of a single disease outbreak and bringing civilization to its knees. It is the epitome of plague. Corpses piled in the streets from Europe to Egypt and across Asia. Some 75 million people died — at a time when there were only about 360 million living on Earth. Death came in a matter of days, and it was excruciatingly painful. Plague is a bacterial disease caused by Yersinia pestis. It is carried by rodents and even cats, and hops to humans through bites from infected fleas (often rat fleas). The disease becomes most deadly to us when transmitted between people, as became the case in the 1300s. The plague of the 14th-century resulted after the rare bacteria that had been dormant for centuries in Asia’s Gobi Desert, awaked in the 1320s and piggybacked along trade routes from China, through the rest of Asia and eventually to Italy in 1347, then later to Russia.
HIV/AIDS: HIV, the virus that causes AIDS, has been traced to a type of chimpanzee in Central Africa. The chimp version of this disease (simian immunodeficiency virus, or SIV) was likely passed to humans when they hunted these animals for meat, getting exposed to their infected blood. Once they were exposed, the virus mutated into HIV. Studies suggest the virus may have jumped to humans as far back as the 1800s, the CDC reports. HIV destroys the immune system, opening the door to a host of deadly infections or cancers.
Mind control: The bizarre parasite Toxoplasma gondii may infect the brains of about 2 billion people worldwide. Some studies have suggested the parasite may contribute to schizophrenia. However, its primary hosts are house cats, in which the microbe reproduces sexually inside the feline's gut. Cats left to roam are more prone to picking it up. You can get it from cat feces. The bug is also found in many other mammals, too (where it reproduces asexually). The parasite eggs then get carried inside a cat’s feces, where humans can pick them up when infected poop gets aerosolized (as it would during litter-scooping). Once T. gondii enters its human host, it hides out in body areas lacking immune defense, and these include the brain, heart and skeletal muscle tissue. T. gondii is sometimes called a ‘mind control’ parasite because rodents infected with it seem to forget their fear of cats and in turn be drawn to the smell of cat urine. That makes them easy prey for cats and an easy route of transmission for T. gondii. Most humans infected with the parasite will have no noticeable symptoms.
Ebola: Ebola virus disease, which is caused by one of five strains of the Ebola virus, is a widespread threat to gorillas and chimps in Central Africa. The disease may have spread to humans from infected bats or infected non-human primates. It was first identified in 1976 near the Ebola River in what is now called the Democratic Republic of the Congo. People can catch four strains of the virus through contact with infected blood or bodily fluids from an animal carrying the virus. That person can then spread the virus to others through close contact. The average fatality rate for this virus is 50% though it has varied from 25% to 90% in different situations, the WHO reports.
Lyme disease: Nobody likes to find a tick head-first on their body, lapping up a juicy blood meal. But even worse than the tick factor is the disease that some ticks carry and can transmit during their gorging. Black-legged ticks can transmit bacteria that cause Lyme disease to humans. The disease is typically caused by Borrelia burgdorferi, though sometimes another Borrelia species, called B. mayonii is the culprit. Symptoms typically include fever, headache, tiredness and a distinct ring-like skin rash called erythema migrans. If left untreated, it can spread to a person’s joints, heart and even the nervous system.
Humans infect chimps and gorillas: Humans can deliver pathogens to our animal brethren as well. For instance, scientists have speculated that chimps at Gombe Stream National Park in Tanzania contracted polio from humans, according to Fabian Leendertz, a wildlife epidemiologist at the Robert Koch-Institute and Max Planck Institute for Evolutionary Anthropology in Germany. Gorillas and chimpanzees in West Africa have been killed by outbreaks of anthrax (caused by the bacterium Bacillus anthracis), which might have originated from cattle herded by humans, although Leendertz said these events may have been caused by anthrax existing naturally in the forests. In 2009, exposure to humans may have led to an outbreak of the respiratory disease human metapneumovirus infection in captive chimpanzees at Lincoln Park Zoo in Chicago. A 9-year-old male chimp named Kipper died from the infection, the Chicago Tribune reported at the time.