Reverse Zoonosis: The Transfer of Infectious Agents from Humans to Animals: The Case of Covid-19

First published Jan. 1, 2022

Over half of human infectious disease originate in animals. The organism is transferred to humans either directly or through another animal vector. The disease caused by the transfer of an infectious agent from animal to humans is called a “zoonosis”. Rabies is a classical zoonosis with direct transfer from infected animal to humans. Vector-borne human diseases are those transmitted from human to human by blood-feeding insects, notably mosquitoes. Malaria is an example. 

The first coronavirus-induced severe acute respiratory syndrome (SARS) appeared in 2003 in China and spread, principally, to a few neighboring countries, and, later, inexplicably disappeared. It originated in horseshoe bats and was transferred, presumably, through an intermediate animal vector, possibly the civet cat, to humans. The coronavirus that causes Middle Eastern Respiratory Syndrome (MERS), originated in bats and moved to camels and then to humans. It is endemic on the Arabian Peninsula, but cases have been brought to other countries by travelers. It has an extraordinary fatality rate. Although occasional cases continue to occur, it is, currently, an uncommon infection in Arabia. 

“Reverse zoonosis” indicates the transfer of an infectious agent from a human to an animal. If a viral disease, proliferation in that animal gives the virus the opportunity to mutate, which could make it more virulent for humans to whom it could be transferred back by animal contact, theoretically. SARS-CoV-2, the virus that causes covid-19, can be transmitted from humans to dogs and cats. There is no current evidence for passage of the virus from house pets back to humans, however. The virus has since been found in many wild animals, such as white-tailed deer. One hopes that Dasher, Dancer, Prancer, Vixen et alia were not affected during their international visits on the night of December 24th.! This transmission is a threat to species in danger of extinction, such as the great cats. Three snow leopards have died from covid-19 in a Nebraska zoo. Animal-human contact, including from urban monkeys that roam freely in cities in Asia and South America, are a potential site of reverse zoonosis. The virus could be transferred from humans to non-human primates, resulting in a more virulent variant being transferred back to humans (human to monkey to human transmission). Such an event has not been documented. 

Snow Leopard

A veterinary vaccination has been developed and vaccination of zoo animals is moving apace in an effort to protect the endangered species housed there. The San Diego Zoo has been a leader in vaccinating its animals.  Approximately 250 animals, of different species, have received a veterinary covid-19 vaccine at the San Diego Zoo and Safari Park. Mink are farmed for their fur and fat to make oil for a variety of products. Many thousands of farmed mink have died from the covid-19, worldwide. There is evidence of mink-to-human SARS-CoV-2 transmission. Many mink farms in the U.S. have agreed to vaccinate their animals. Vaccinating house pets is not recommended since transmission to humans has not been identified. Veterinary investigation examining species that are likely to have a high frequency of human contact is intensifying.

The SARS-CoV-2 virus spikes that project from its surface are the key and the ACE2 receptor, a feature of animal and human cells, is the doorway into cells. One must open the cellular entryway with the key, the spikes on the virus surface. Once in cells, the virus replicates, bursts forth and enters other cells and causes the disease, covid-19. The ACE2 receptor is a shared part of animal and human cells, important for several biological function. This commonality makes it possible for the virus to invade an animal. It is too soon to say to what degree the shared property, the ACE2 receptor, allowing animal infections, will complicate the control of the human epidemic. Thus far, it has not been identified as a factor.     

The scimitar-shaped horned Oryx is one of many species with an ACE2 receptor much like humans. The oryx is the basis for the myth of the unicorn. In profile it appears to have one curved horn. Indigenous to the Sahara desert, it is adapted to tolerate extreme heat and sand storms. They are an endangered species. 

December 2021