Anthroponoses: The New Emerging Face of Zoonoses

By Tayo Fasuan

The definition of zoonoses as diseases acquired by humans from animals recently got expanded at a recent conference of Center for Control and Prevention of Zoonoses (CCPZ) at the University of Ibadan, Nigeria. There, the word ‘anthroponoses’ was strongly re-introduced by the presenters of seminars with zoonotic relevance.

Zoonoses are diseases transmissible from living animals to humans. The source is an infectious animal; interhuman transfer is uncommon. A limited number of zoonotic agents can cause extensive outbreaks; many zoonoses, however, attract the public’s attention because of the high death rate associated with the infections. A number of zoonotic agents are arthropod-borne; others are transmitted by direct contact, alimentary (foodborne and waterborne), or aerogenic (airborne) routes; and some are rodent-borne.

Anthroponoses simply means the reversal of zoonoses, that is, diseases transmitted from humans to humans, and with more discoveries, to animals. Many observers and audience were amused about this concept because in Nigeria, nobody really cared much about animal health like we cared for our own health. But my interest was ignited by that term because of what I had recently discovered in the research I was carrying out, in which the incident of humans transmitting diseases to animals is not only one of the major explanations for what I got, but also the most plausible one.

The issue of zoonoses has over the years degenerated to many levels due to the continue influx of newly discovered diseases and their etiologies into the zoonotic fold. Ebola fever, Lassa fever, West Nile fever, Coccodiomycosis, Samonellosis and host of others have risen along the years and manipulated themselves and the scientists tackling them in order to stay put within the environment.

As if that is not enough, old diseases are reemerging with resistant tendencies, breeding new armies of infectious agents that are now innately unaffected by any antimicrobial agent. Yellow fever, Dengue haemorrhagic fever and Influenza keep rendering preventive and curative effort useless by devising means of sabotaging the mechanism of the drugs produced against them.

While the whole world is still grasping with the above problems and trying to find lasting solutions to them, the very idea that humans can also pass diseases and infections to animals is quite shocking but not unreasonable. Humans have been known to be dead-end hosts to some infections and also the only known reserviors for varieties of aetiological agents like yellow fever virus and dengue virus to mention a few. Also, humans are important hosts in the life-cycles of many parasitic, fungal and bacterial infections. Therefore, the very concept of anthroponoses should not be too strange to people.

But it is.

The concise definition of anthroponoses, or ‘reverse zoonosis’ as one of my senior colleagues and mentor, Dr. Clem Meseko, called it, is diseases that are transmitted from human to a population of uninfected and infection-naive animals, that is animals which have not been infected prior to close interaction with infected human carriers.

Now this is very interestingly new! Because now, according to animal scientists and veterinary experts, humans now pose a risk to animals. Just imagine that: Humans are now threats to animals! If we really think deep about this, we would realised that for a country like Nigeria and other developing countries whose health systems are in limbo, this is a grave situation, as it now poses a hydra-headed problem for us. If we are still incapacitated in preventing human-to-human diseases and zoonoses, how on earth are we going to stop human-to-animal transmission?

It is no longer news that we lack the diagnostic and prognostic techniques to quickly halt the transmission of human-human transmission. For instance, studies have shown that most viral and bacterial infections whose aetiologies are not known but which display symptoms like fever are usually categorised under pyrexia of unknown origins (PUOs), whereas most infections usually include fever as one of its major symptoms.

The concern now is that while humans can at least be vocal about how they are feeling, how would animals like dogs, goats, cats, birds and others make known their conditions. The veterinarians can argue that they would know due to their experiences and their encyclopedia of known animal diseases, but this may prove to be tricky since these infections and diseases in questions are not primarily for animals, but for humans. To tackle this problem, veterinarians would need to quickly upgrade their encyclopedia of animal diseases and their symptoms while trying to pay more attention to human diseases that are potentially transmissible to animals.

A hilarious but potentially dangerous aspect of this issue is the possibility of now having nosocomial infections of animals, where animal handlers in clinical settings can now transmit diseases to the animals while receiving treatments. The many facets of the problem bear no restriction in possibilities!

These scenarios painted above are what prompted questions at another research seminar about the possibilities of transmission of influenza virus from pig handlers to pigs in a large animal farm in Lagos, Nigeria. While the isolates gotten bore little semblance to the notorious swine flu, there are feelings that the recent swine flu outbreak in the world might actually have started from an anthroponosis case. The hypothesis was that it is possible for human to first transmit the virus to animal, and on getting to the animal, mutation of course will happen before being transmitted back to human, only this time, more pathogenic and virulent.

Epidemiologically, the scenario becomes funnier. One of the functions and stipulations of FAO and IOE is to disallow export of infected animals across borders, and on suspecting infection, animals should be quarantined and possibly destroyed to prevent spreading to other animals. So let’s try and put humans in this spot: should infected humans now be quarantined in the future to prevent transmission to animals and possibly killed to prevent further transmission to other humans and animals? Simply put, in the future, should humans now be treated like today animals to prevent transmission of infections to animals? A reincarnation of Animal Farm is likely on my mind.

To make matter more complex, while aetiologies with humans as the only reservior like smallpox virus can be controlled and even eradicated in the long run, the very knowledge that there can be alternative reserviors for human diseases is disturbing. Any infectious disease with alternative reservior as seen in most cases of arboviral infections can not easily be eradicated.

Which takes me back to why anthroponotic issues are of importance to me? Does poliovirus or even the entire enteroviruses have other hosts apart from human? If yes, is human the primary reservior of these viruses or animals are the primary host for them? Time and further studies will provide the answers to these questions.

The future of zoonotic researches looks dangerously exciting, but I guess that is why I am a scientist. Anthroponoses or zoonoses, the solutions to our myriads of infectious diseases lie in our own hands. So let’s keep the war against infectious diseases going because any lapse in concentration can be anticlimatic, not only for us humans now, but for our animals as well.