One Health

The world’s leading health organizations believe that SARS-CoV-2 (the virus that causes COVID-19) most likely originated in bats, then was transmitted to an intermediate animal host before infecting humans. While there is less certainty in identifying the intermediate animal host, scientists speculate that it could be pangolins, which are prevalent in the wildlife trade route. From the intermediate animal host, SARS-CoV-2 spilled over to Patient Zero in Wuhan, China. Once the virus spread to humans, its asymptomatic nature allowed it to spread easily around the world, resulting in more than 219 million confirmed cases and 4.5 million deaths across the globe.

Scientists have proven that HIV (the virus that causes AIDS) originated in chimpanzees. Scientists have found that the disease was first transmitted to humans when hunters in Kinshasa in the Democratic Republic of Congo (DRC) killed and butchered or ate infected chimpanzees. In the 1960s, an influx of Haitian professionals who were working in the DRC returned to Haiti, bringing the disease with them. From Haiti and the Caribbean, HIV spread to New York City around 1970. International travel from the United States helped the virus spread across the globe. A century after the first reported transmission to humans, 37.6 million people globally are reported to be living with HIV. A reported 34.7 million people have died from AIDS-related illnesses since the beginning of the epidemic.

Sprawling suburban development into forests inhabited by Lyme disease-infected ticks and animal hosts such as squirrels, mice, and deer has resulted in an increase in Lyme disease in humans. The spread of the disease has also been exacerbated by the climate crisis, as ticks thrive in warmer temperatures and humidity. Scientists warn that rising temperatures could boost the number of cases by more than 20% by 2050. If Lyme disease is not detected and treated early, humans can develop chronic Lyme disease, which causes profound, lifelong debilitation.

Ebola virus disease (EVD) is one of the deadliest viral diseases, with an average case fatality rate of around 50%. Discovered in 1976, EVD was linked to two consecutive outbreaks of hemorrhagic fever in what are now the Democratic Republic of Congo and South Sudan. Public health officials believe that the Ebola virus existed long before its discovery in 1976, and that circumstances including population growth, encroachment into forested areas, and direct interaction with wildlife (such as bushmeat consumption) are likely to have contributed to its spread.

Scientists have found that gorillas, chimpanzees, bats, and other mammals are likely to be implicated when the first cases of an EVD outbreak in humans occur. In 1989, EVD was discovered in monkeys imported to the United States from the Philippines for use as animal test subjects in laboratories. Scientists confirmed that the virus spread through the transported monkey population through droplets in the air in the laboratory facility in Virginia. There was a major outbreak in 2013–16 and in 2021 Ebola outbreaks were reported in the Democratic Republic of the Congo and Guinea. There is no vaccine or cure for EVD.

The first known outbreak of the Nipah virus in 1998 was caused by transmission from bats to pigs to humans in Malaysia and Singapore. It is believed that the first known spillover occurred after pigs were infected through ingestion or contact of the urine, feces, or saliva of fruit bats. The fruit bats lived in a forest near a pig farm created after the deforestation of part of the existing bat habitat. Both the nearer proximity of farms to existing forest land and the loss of food resources for bats due to deforestation have led to an increased likelihood contact between bats and domesticated animals. In Malaysia and Singapore, there was a mortality rate of nearly 40%. The 1998 outbreak also caused a substantial economic impact in Malaysia and Singapore as more than one million pigs were killed to control the outbreak.

Today, outbreaks still occur annually in Southeast Asia, with a human mortality rate of nearly 75%. In 2021, there was an outbreak in India, where a 12-year-old boy is believed to have died from Nipah virus after eating tropical fruit which may have been infected with bat saliva or excreta.

Today, there is still no cure nor vaccine for Nipah virus.

Severe acute respiratory syndrome (SARS) originated in China in 2002 from a novel coronavirus and spread to four other countries leading to more than 8,000 reported cases and nearly 800 deaths. While the exact origins of SARS remain unclear more than 15 years later, the SARS virus is believed to have originated in a horseshoe bat in China before either infecting humans directly or through masked palm civets, who then infected humans. Many scientists believe the virus was transmitted to humans at a wild animal market. The last recorded case occurred in 2002. There is no cure or vaccine for SARS.

Marburg Hemorrhagic Fever (HF) was first recognized in 1967 when outbreaks of hemorrhagic fever occurred simultaneously in laboratories in Germany and what is now Serbia. It has a high case fatality rate of 24–90%. The first humans to be infected with the Marburg virus were laboratory workers exposed to imported African green monkeys or their tissues while conducting research. The monkeys are believed to have been infected by fruit bats, who do not show obvious signs of illness when infected with the Marburg virus. Other confirmed outbreaks of Marburg HF have occurred in Uganda, Zimbabwe, the Democratic Republic of the Congo, Kenya, Angola, Guinea, and South Africa. In 2021, a new outbreak was confirmed in a remote forest area in Guinea near the Sierra Leone and Liberian borders where bat colonies are present. This is the first known case of Marburg virus in Guinea and in West Africa. The World Health Organization has recommended minimizing contact between humans and bats, monkeys and apes. There is no vaccine and no special therapeutic drug for victims of Marburg HF.