A reverse zoonosis, also known as a zooanthroponosis () or anthroponosis, is a pathogen reservoired in humans that is capable of being transmitted to non-human animals. This is in contrast to zoonosis, the transmission of a pathogen borne by nonhuman animals to humans.
Terminology Anthroponosis refers to pathogens sourced from humans and can include human to non-human animal transmission but also human to human transmission. The term zoonosis technically refers to disease transferred between any animal and another animal, human or non-human, without discretion, and also been defined as disease transmitted from animals to humans and vice versa. Yet because of human-centered medical biases, zoonosis tends to be used in the same manner as anthropozoonosis which specifically refers to pathogens reservoired in non-human animals that are transmissible to humans.
Additional confusion due to frequency of scientists using "anthropozoonosis" and "zooanthroponosis" interchangeably was resolved during a 1967 Joint Food and Agriculture and World Health Organization committee meeting that recommended the use of "zoonosis" to describe the bidirectional interchange of infectious pathogens between animals and humans.
Furthermore, because humans are rarely in direct contact with wild animals and introduce pathogens through "soft contact", the term "sapronotic agents" must be introduced. Sapronoses (Greek "decaying") refers to human diseases that harbor the capacity to grow and replicate (not just survive or contaminate) in abiotic environments such as soil, water, decaying plants, animal corpses, excreta, and other substrata. Additionally, sapro-zoonoses can be characterized as having both a live host and a non-animal developmental site of organic matter, soil, or plants. Obligate intracellular parasites that cannot replicate outside of cells and are entirely reproductively reliant on entering the cell to use intracellular resources such as viruses, rickettsiae, chlamydiae, and Cryptosporidium parvum cannot be sapronotic agents.
Etymological pitfalls Categorizing of disease into epidemiologic classes by the infection's supposed source or the direction of transmission raises a number of contradictions that could be resolved by the use of cyclical models. See the following scenarios:
Zoonosis vs reverse zoonosis vs anthroponosis In the case of diseases transferred from arthropod vectors such as urban yellow fever, dengue, epidemic typhus, tickborne relapsing fever, zika fever, and malaria, the differentiation between terms becomes ever more hazy. For example, a human infected with malaria is bitten by a mosquito that is subsequently infected as well. This is a case of reverse zoonosis (human to animal). However, the newly infected mosquito then infects another human. This could be a case of zoonosis (animal to human) if the mosquito is considered the original source, or anthroponosis (human to human) if the human is considered the original source. If this infected mosquito instead infected a non-human primate, it could be considered a case of reverse zoonosis/zooanthroponosis (human to animal) if the human is considered the primary source, or simply zoonosis (animal to animal) if the mosquito is considered the primary source.
Zoonosis vs anthroponosis Similarly, HIV originating in simians (crossover due to humans consuming wild chimpanzee bushmeat) and influenza A viruses originating in avians (crossover due to an antigenic shift) could have initially been considered a zoonotic transference as the infections first came from vertebrate animals, but could currently be regarded as an anthroponosis because of its potential to transfer between human to human.
Sapronosis vs sapro-zoonosis Typical examples of sapronotic agents are fungal such as coccidioidomycosis, histoplasmosis, aspergillosis, cryptococcosis, Microsporum gypseum. Some can be bacterial from the sporulating clostridium and bacillus to Rhodococcus equi, Burkholderia pseudomallei, Listeria, Erysipelothrix, Yersinia pseudotuberculosis, legionellosis, Pontiac fever, and nontuberculous mycobacterioses. Other sapronotic agents are amebic as in primary amebic meningoencephalitis. Yet again, difficulties in classification arise in the case of sporulating bacteria whose infectious spores are only produced after a significant period of inactive vegetative growth within an abiotic environment, yet this is still considered a case of sapronoses. However, cases of zoo-sapronoses involving Listeria, Erysipelothrix, Yersinia pseudotuberculosis, Burkholderia pseudomallei, and Rhodococcus equi can be transferred by an animal or an abiotic substrate but usually occur via a fecal-oral route between humans and other animals.
Cases with modes of transmission
Arthropod vectors
Malaria Malaria involves the cyclical infection of animals (human and non-human) and mosquitoes from the genus Anopheles with a number of Plasmodium species. The Plasmodium parasite is transferred to the mosquito as it feeds on the blood of the infected animal whereupon it begins a sporogenic cycle in the gut of the mosquito that will infect another animal at the next blood meal. There does not seem to be any deleterious effects to the mosquito as a result of the parasitic infection. The Plasmodium brasilianum parasite normally found in primates is morphologically similar to the malarial inducing Plasmodium malariae that is more commonly found in humans and it is contested as to whether the two are actually different species. Nevertheless, 12 reports of malaria in the remotely located indigenous Yanomami communities of the Venezuelan Amazon arose where it was surprisingly found to be caused by a strain of P. brasilianum with 100% identical to sequences found in Alouatta seniculus monkeys. This suggests a definite zoonosis and high possibility of spillback back into non-human primate bands as reverse zoonoses.
African sleeping sickness Trypanosoma brucei gambiense (T. b. gambiense) is a species of African trypanosomes which are protozoan hemoflagellates responsible for trypanosomiasis (more commonly known as African sleeping sickness) in humans and other animals. The protozoa are transferred via Tsetse flies where they multiply and can be transferred to yet another animal host during the fly's blood meal feeding. Outbreaks of sleeping sickness in certain human communities have been eliminated but only temporarily as constant re-introduction from unknown sources statistically suggests the presence of a non-human reservoir where spillback of the pathogen is maintained in a sylvatic cycle and re-introduced into the urban cycle. The presence of T. b. gambiense has been found separately in humans and livestock. This spurred a molecular study comparing serum reactivity of pigs, goats, and cows to human serum where notable similarities in all samples but especially in pig samples. Combined, these findings implicate a reverse zoonotic human to animal transmission.
Arboviruses
Yellow fever viruses, Dengue fever viruses, and Zika viruses are of the Flavivirus genera and Chikungunya virus is of the Alphavirus genera. All of them are considered arboviruses denoting their ability to be transmitted through arthropod vectors. Sylvatic transmission cycles for arboviruses within non-human primate communities have the potential to spillover into an urban cycle within humans where humans could be dead-end hosts in scenarios where further intermingling is eliminated but much more probable is a reemergence of these viruses into either cycle due to spillback. Apparently the maintenance of an arboviral urban cycle between humans requires a rare or understudied conjunction of factors to occur. One of the following situations occurs: An infected human in an urban environment feeds a sylvatic (typically remotely located) mosquito such as Haemogogus (which has a relatively long lifespan compared to other mosquitoes and can transmit the virus for extended periods) that infects another human or non-human animal that will serve as a reservoir. An urban Aedes (more commonly found in urban areas feeds and transmits the virus to another human or non-human animal that will serve as a reservoir. Sufficient numbers of sylvatic vector mosquito and the animal reservoir inhabit the same ecologic niche in close contact to promote and sustain the zoonotic cycle of the virus. The animal reservoir of the virus maintains a suitable virus level in the blood to allow the infection of a vector mosquito. A bridge-vector mosquito such as Aedes albopictus, which can survive in an urban area and spread to rural, semi-rural, and forest areas could carry the virus to a sylvatic environment.
Zika fever: The Zika virus is caused by the single stranded RNA Flavivirus that uses the Aedes mosquito as a vector to infect other human and animal hosts. A 2015 zika virus strain isolated from a human in Brazil was used to infect pregnant rhesus macaques intravenously and intraamniotically. Both the dams and the placentas were infected with Zika positive tissue samples being recorded for up to 105 days. This confirms a reverse zoonotic transference potential between humans and non-human primates. Yellow fever: Yellow fever virus also transmitted by the bite of an infected Aedes or Haemagogus species of mosquitoes that feed off an infected animal. The historical course of the American slave trade is a prime example of introduction of a pathogen to create a completely new sylvatic cycle. Previous hypotheses of a "New World YFV" were laid to rest in a 2007 study that examined rates of nucleotide substitution and divergence to determine that yellow fever was introduced into the Americas approximately 400 years ago from West Africa. It was also around the 17th century that yellow fever was documented by Europeans complicit in slave trafficking. The actual mode of introduction could have played out in a number of scenarios whether a viremic Old World human, infected Old World mosquito, eggs laid by an Old World infected mosquito, or all three were transported to the Americas seeing that yellow fever transmission was not uncommon on sailing vessels. Amidst more recent yellow fever outbreaks in southeastern Brazil, the spillback potential was highly indicated. Molecular comparisons of non-human primate outbreak strains proved to be more closely related to human strains than strains derived from other non-human primates thus suggesting a continuing reverse zoonosis. Chikungunya: The Chikungunya virus is a single stranded RNA alphavirus typically transmitted by the Aedes mosquitoes to another animal host. There is no evidence to suggest a barrier to Chikungunya switching hosts between humans and non-human primates because it has no preferences in any given primate species. It has a high potential to spill-over or spill-back into sylvatic cycles as was the case with the similar arbovirus that was imported to the Americas during the slave trade. Studies have proven chikungunya's potential to orally infect sylvatic types of mosquitoes including Haemagogus leucocelaenus and Aedes terrens. Moreover, in a serologic survey carried out in non-human primates of urban and peri-urban areas of Bahia State, 11 animals showed chikungunya neutralizing antibodies.