Reverse zoonosis

A reverse zoonosis, also called zooanthroponosis or anthroponosis, occurs when a pathogen that has its reservoir in humans is transmitted to non-human animals. This is the opposite of a zoonosis, where a pathogen from a non-human animal moves to humans.

The terminology around these concepts has been complicated. Anthroponosis originally referred to pathogens that come from humans, covering both human-to-human and human-to-animal transmission. The word "zoonosis" technically means any disease transferred between animals—human or non-human—without specifying direction, and has sometimes been defined as disease moving from animals to humans and back again. However, because of a human-centered bias in medicine, "zoonosis" is often used like "anthropozoonosis," which specifically means a pathogen reserved in non-human animals that can infect humans. This confusion led scientists to use "anthropozoonosis" and "zooanthroponosis" interchangeably until a 1967 joint meeting of the Food and Agriculture Organization and the World Health Organization recommended that "zoonosis" should describe the bidirectional exchange of infectious pathogens between animals and humans.

Because humans rarely have direct contact with wild animals and often introduce pathogens through "soft contact," the term "sapronotic agents" is needed. Sapronoses (from the Greek for "decaying") are human diseases that can grow and replicate—not just survive or contaminate—in non-living environments like soil, water, decaying plants, animal corpses, excreta, and other substrates. Sapro-zoonoses involve both a live host and a non-animal developmental site, such as organic matter, soil, or plants. Obligate intracellular parasites—like viruses, rickettsiae, chlamydiae, and *Cryptosporidium parvum*—cannot replicate outside cells and rely entirely on entering a cell to use its resources, so they cannot be sapronotic agents.

Categorizing diseases by their supposed source or direction of transmission raises contradictions that cyclical models could resolve. For example, consider diseases spread by arthropod vectors, such as urban yellow fever, dengue, epidemic typhus, tickborne relapsing fever, Zika fever, and malaria. If a human infected with malaria is bitten by a mosquito, which then becomes infected, that is a reverse zoonosis (human to animal). But if that mosquito later bites another human, it could be a zoonosis (animal to human) if the mosquito is seen as the original source, or an anthroponosis (human to human) if the human is seen as the original source. If the mosquito instead infects a non-human primate, it could be a reverse zoonosis (human to animal) if the human is the primary source, or simply a zoonosis (animal to animal) if the mosquito is the primary source.

Similarly, HIV, which originated in simians when humans consumed wild chimpanzee bushmeat, and influenza A viruses, which originated in birds through an antigenic shift, were initially zoonotic transfers from vertebrate animals. But they could now be considered anthroponoses because they can spread between humans.

Typical sapronotic agents are fungal, such as those causing coccidioidomycosis, histoplasmosis, aspergillosis, cryptococcosis, and *Microsporum gypseum*. Some are bacterial, from sporulating *Clostridium* and *Bacillus* species to *Rhodococcus equi*, *Burkholderia pseudomallei*, *Listeria*, *Erysipelothrix*, *Yersinia pseudotuberculosis*, legionellosis, Pontiac fever, and nontuberculous mycobacterioses. Others are amebic, like primary amebic meningoencephalitis. Classification difficulties arise with sporulating bacteria whose infectious spores are produced only after a period of inactive vegetative growth in an abiotic environment—this is still considered sapronosis. However, zoo-sapronoses involving *Listeria*, *Erysipelothrix*, *Yersinia pseudotuberculosis*, *Burkholderia pseudomallei*, and *Rhodococcus equi* can be transmitted by an animal or an abiotic substrate but usually occur via a fecal-oral route between humans and other animals.

Malaria involves the cyclical infection of animals (human and non-human) and *Anopheles* mosquitoes with various *Plasmodium* species. The parasite transfers to the mosquito when it feeds on an infected animal's blood, then undergoes a sporogonic cycle in the mosquito's gut, infecting another animal at the next blood meal. The mosquito does not appear to be harmed. *Plasmodium brasilianum*, normally found in primates, is morphologically similar to *Plasmodium malariae*, which is more common in humans, and it is debated whether they are actually different species. Nonetheless, 12 reports of malaria in remote Yanomami communities in the Venezuelan Amazon were surprisingly caused by a strain of *P. brasilianum* with sequences 100% identical to those found in *Alouatta seniculus* monkeys. This suggests a definite zoonosis and a high possibility of spillback into non-human primate groups as reverse zoonoses.