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Understanding where a lethal disease originates is essential for its proper diagnosis and treatment. Although the sources of many illnesses are still debated, several deadly outbreaks have strong evidence pointing to animal origins. These events often involve a spillover, where a pathogen transfers from one species to another. Sadly, in these instances, humans became the new hosts.
10. AIDS - Linked to Cameroonian Chimpanzees
The history of AIDS is vast, with numerous attempts to pinpoint its origin, most notably through the case of the first 'patient zero,' Gaetan Dugas. Initially blamed, Dugas was a Canadian flight attendant mistakenly labeled as Patient Zero due to a misinterpretation—the 'O' stood for 'out of California,' not zero. Although Dugas was wrongly accused of spreading HIV and AIDS in the 1980s, the virus had existed in humans long before his time.
In reality, AIDS entered the human population through a spillover event involving a chimpanzee in southern Cameroon in the early 1900s. The prevailing theory suggests that a hunter, while venturing into the jungle, killed a chimpanzee carrying the virus. During the butchering process, the hunter likely contracted the disease through a cut or an open wound. As a zoonotic disease (transmissible from animals to humans), SIV transformed into HIV upon entering the human host.
9. Ebola - Linked to African Fruit Bats
Initially, the crab-eating macaque was believed to be the source of the latest Ebola outbreak in Africa. However, further research revealed that these primates are merely 'accidental hosts.' The true origin has been traced to African fruit bats, with scientists identifying both the initial human carrier and the exact location where the infection first occurred.
Initially, it was unclear how Emile Ouamouno, the two-year-old first infected in the latest Ebola outbreak, contracted the virus. However, it is now believed that the toddler was exposed to infected bats while playing near a large, hollow tree in Meliandou village. The tree, which has since been burned down, housed thousands of bats, and Ouamouno likely contracted the disease through contact with the bats or their fecal matter inside the tree.
8. African Sleeping Sickness - Transmitted by Tsetse Flies
Human African Trypanosomiasis, commonly known as African sleeping sickness, has seen numerous widespread outbreaks. The disease's prevalence has risen steadily since the 1970s, when environmental concerns led to the ban of DDT as an insecticide. The tsetse fly, carrying the parasite responsible for the illness, causes symptoms such as fever, rash, extreme fatigue, and swelling. According to the WHO, the disease affects approximately 30,000 people annually, with symptoms progressing to coma and death if untreated.
African sleeping sickness is thought to have played a significant role in human evolution, making complete eradication unlikely. However, controlling the disease is feasible, and recent advancements, such as the development of a 'repellent collar' to combat tsetse flies, show promise in preventing infections. Given the tsetse fly's presence in over 37 countries, this innovation is crucial for preventing future outbreaks.
7. SARS - Linked to Chinese Horseshoe Bats
Initially, civet cats were believed to be the primary hosts of severe acute respiratory syndrome (SARS). However, a 2013 study uncovered that Chinese horseshoe bats were the true carriers. While some theories suggested an intermediate host, such as animals sold in Chinese markets, the study confirmed that bats could transmit SARS directly to humans without any intermediary.
Researchers discovered that civet cats were not the original source of SARS after observing that these cats only became infected after exposure to Chinese wet markets. This indicated another host was responsible for the transmission. The SARS outbreak caused by these bats in the early 2000s (2002–2003) is regarded as one of the most critical public health crises in modern history, sparking widespread global alarm.
6. Hendra - Linked to Australian Flying Foxes
The first Hendra outbreak was reported in 1994 in Australia, proving fatal to humans but even more devastating to horses. Vic Rail, a renowned Australian horse trainer, and 14 of his horses contracted Hendra in 1994, succumbing to the illness within days. Since then, only seven human cases have been reported (four fatal), but the disease continues to pose a significant threat to horses.
Research has identified Australia’s flying fox population as the source of Hendra transmission. These megabats, part of the Megachiroptera suborder, are the largest bats globally, with wingspans reaching up to 1.5 meters (5 ft). While there is no direct transmission from flying foxes to humans, all human cases have resulted from contact with infected horses, suggesting that horses act as amplifiers for the Hendra virus.
5. Crimean-Congo Hemorrhagic Fever - Transmitted by Ixodid Ticks
Crimean-Congo hemorrhagic fever (CCHF) shares symptoms with Ebola and Marburg, boasting a fatality rate as high as 40%. The first documented outbreak took place in 1944, impacting soldiers and farmers on the Crimean Peninsula. Although contact with infected livestock can spread the disease, CCHF is primarily transmitted by ticks, and no vaccine currently exists.
Several CCHF outbreaks have occurred, with the most recent in Uganda in 2013. A farmer from Baroma village in Atece Parish was hospitalized with CCHF-like symptoms, followed by others who died exhibiting similar signs. It remains unclear whether the infection came from ixodid ticks or infected livestock. While no vaccine is available yet, a pre-clinical vaccine offers hope for treatment and preventive measures.
4. Machupo Virus - Linked to Bolivian Field Mice
The Machupo virus, also known as Bolivian hemorrhagic fever, is closely related to Ebola. The first cases emerged in Bolivia in 1959, with subsequent outbreaks in northern Bolivia in 1971 and 1994. The virus has a fatality rate of up to 30%. Interestingly, the widespread use of DDT in Bolivia to combat malaria may have indirectly contributed to Machupo outbreaks by decimating the feline population, allowing mice to thrive. These mice act as the virus reservoir and transmit it to humans.
The Machupo virus is carried by Bolivian field mice, with viral particles present in their urine, feces, and saliva. Researchers believe the virus spreads most effectively when dried mouse urine becomes airborne and is inhaled by nearby individuals.
3. MERS - Linked to Egyptian Tomb Bats
Middle Eastern respiratory syndrome (MERS) is a relatively new disease, primarily confined to regions around the Arabian Peninsula. Although widespread outbreaks have not yet occurred, concerns remain that MERS could spread rapidly, similar to the SARS epidemic. Like SARS, the virus is believed to have originated from bats—specifically, the Egyptian tomb bat.
While the Egyptian tomb bat is identified as the source of MERS, researchers suggest the virus is not transmitted directly from bats to humans but through an intermediate host. Various animals, including camels, sheep, goats, and cats, have been tested, with evidence pointing to multiple potential hosts that could facilitate the transmission of MERS to humans.
2. Lassa Fever - Linked to Multimammate Rats
Lassa fever, similar to other deadly viruses on this list, is endemic to West Africa. It was first identified in 1969 after two missionary nurses died from the virus in Nigeria. Like Machupo, the virus is carried by rodents—specifically, the multimammate rat. These rats transmit the virus to humans, often through dried urine that becomes airborne when disturbed. This is especially concerning due to the rats' rapid breeding and their habit of nesting in homes where food is stored.
Lassa fever outbreaks are an annual occurrence in West Africa, infecting up to 500,000 people and causing as many as 20,000 deaths each year. The most recent outbreak in Nigeria happened shortly after the country declared Ebola containment, overwhelming healthcare workers already strained by the previous crisis.
1. Marburg - Linked to African Fruit Bats
Marburg virus exhibits symptoms strikingly similar to Ebola, including severe and often fatal hemorrhagic fever. While monkeys are sometimes blamed, the true natural hosts are believed to be fruit bats. Humans typically contract the virus when handling bats during food preparation, as bats are considered a delicacy in regions frequently affected by Marburg.
Research continues to explore whether other animals act as reservoir hosts, but monkeys are not among them. Primates, including monkeys, suffer high mortality rates when infected with Marburg. Given the vast populations of African fruit bats, the potential for a major Marburg outbreak remains significant.
