When is a zoonosis not a zoonosis?

Zoonosis. One of the current glam terms in virology. At it's simplest, a zoonosis refers to a disease that is transmitted to humans from animals.

But it's possible to pause and think, what actually counts as a zoonosis?

On the WHO website it's classed as "any disease or infection that is naturally transmissible from vertebrate animals to humans and vice-versa".

From Wikipedia "an infectious disease that is transmitted between species (sometimes by a vector) from animals to humans or from humans to animals".

From the medical dictionary....."refers to diseases that can be passed from animals, whether wild or domesticated, to humans".

In all of these definitions the key word is 'disease'. Look it up and disease suggests there's some sort of pathology, with overt symptoms - admittedly this too is wide open for discussion. Taking this forward, if an animal virus infects a human, but doesn't cause disease, can that then be classed as a zoonosis?

Dealing with an outbreak of Hendra virus; passed from bats to horses...and on to the handlers

I was told once that, many years ago, people who mouth pipetted viruses were seropositive for the (livestock) viruses they were working on. If there were symptoms, they weren't sufficiently serious to become part of the story, so in this case we might argue that there was no disease. That fits with the definition - no disease, therefore not a zoonosis - and indeed these viruses are not regarded as being zoonotic.  

Mouth pipetting; no longer a method of choice

An interesting paper which has recently been accepted into the Journal of Virology describing the isolation and characterisation of two novel paramyxoviruses, Achimota virus 1 and Achimota virus 2 (genus Rubulavirus) from bats (Baker et al., 2012). I enjoy a bit of virus discovery and it's nice to see a study done well with a good level of characterisation and epidemiology. The authors collected urine samples from under an Eidolon helvum bat roost and added them to cultured cells in the lab. Using the viruses recovered from the isolations they could then work out the seroprevalence, the results of which showed that there was evidence of the virus infection in E. helvum samples throughout the geographical range of this species. Variations in seropositivity among different age groups, and from year to year also showed that there has been active circulation of the virus. 

Bizarrely, one of the most intriguing things was the title, namely the "potentially-zoonotic" bit. The genus Rubulavirus contains some serious viruses, notably mumps virus, so this is an important point. The authors speculate about the zoonotic possibility based upon serology, where they found 3 people out of 442 to have evidence of prior exposure to the virus. 

But does this constitute a zoonosis? One of the positive samples was from a febrile patient, so perhaps. The authors acknowledge that whether or not these viruses are zoonotic will take some nailing down. As discussed above though, seroconversion against a livestock virus does not lead to the livestock virus becoming classed as zoonotic. So does the finding of 3/422 being seropositive for these viruses mean that they are zoonotic? Is seroconversion really sufficient to class something as zoonotic? 

Baker, K., Todd, S., Marsh, G., Crameri, G., Barr, J., Kamins, A., Peel, A., Yu, M., Hayman, D., Nadjm, B., Mtove, G., Amos, B., Reyburn, H., Nyarko, A., Suu-Ire, R., Murcia, P., Cunningham, A., Wood, J., & Wang, L. (2012). Novel potentially-zoonotic paramyxoviruses from the African straw-colored fruit bat, Eidolon helvum Journal of Virology DOI: 10.1128/JVI.01202-12