All the recent news regarding the coronavirus (COVID-19) got me wondering about the question: Can plant viruses affect (infect?) humans?
Is this a silly question?
Maybe not, because I was a bit surprised when I did a cursory search on Google Scholar.
It turns out that some virologists have also recently addressed this question (e.g., see Refs 1-4 below).
As outlined by Mandal and Jain (Ref. 2 below), “Numerous viruses infect plants, however, none of them so far is known as pathogen to animal and human beings. Only three families, Bunyaviridae, Rhabdoviridae and Reoviridae contain viruses known to infect plant, animal and human. Philippe Colson and coworkers from France reported in the recent issue of PLoS ONE that Pepper mild mottle virus (PMMoV), a plant virus might infect human beings [see Ref 1 below]. The findings trigger to reevaluate the dogmatic concept that plant viruses are safe to human health even though numerous viruses are consumed through various types of fresh foods and food-products.”
In a study published in 2013, Liu, et al. (see Ref 3 below) approaches our question from the prespective of the Tobacco Mosaic Virus (TMV), to wit: “Tobacco mosaic virus (TMV), the first plant virus discovered, is a single stranded RNA virus. It is present worldwide and is known to infect over 150 different plants, including tobacco, tomatoes, peppers, and cucumbers. Because of its stability at high temperatures, TMV resists tobacco manufacturing processes and can be present in cigarettes, chewing tobacco and cigars for many years. To date there are no efficient chemical treatments that protect plants from virus infection. Similar to bacterial infections, plant viruses are transmissible among plants by direct contact, such as from contaminated farm tools and human hands.”
These researchers provide evidence that smokers have a relatively high level of antibodies (part of their immune responses) to TMV. And because of TMV’s structural similarities to a human virus, they conclude: “Our results showing molecular mimicry between TMV and human TOMM40L raise the question as to whether TMV has a potential role in smokers against Parkinson’s disease development. The potential mechanisms of molecular mimicry between plant viruses and human disease should be further explored.”
Wow! If true, that would indeed be amazing.
Balique, et al. (see Ref 4 below) take a more comprehensive look at our question. From the abstract of their paper: “Phytoviruses are highly prevalent in plants worldwide, including vegetables and fruits. Humans, and more generally animals, are exposed daily to these viruses, among which several are extremely stable. It is currently accepted that a strict separation exists between plant and vertebrate viruses regarding their host range and pathogenicity, and plant viruses are believed to infect only plants. Accordingly, plant viruses are not considered to present potential pathogenicity to humans and other vertebrates.”
But they raise the following questions: “Notwithstanding these beliefs, there are many examples where phytoviruses circulate and propagate in insect vectors. Several issues are raised here that question if plant viruses might further cross the kingdom barrier to cause diseases in humans. Indeed, there is close relatedness between some plant and animal viruses, and almost identical gene repertoires. Moreover, plant viruses can be detected in non-human mammals and humans samples, and there are evidence of immune responses to plant viruses in invertebrates, non-human vertebrates and humans, and of the entry of plant viruses or their genomes into non-human mammal cells and bodies after experimental exposure.”
After reviewing pertinent literature on the subject, they conclude: “Overall, the question raised here is unresolved, and several data prompt the additional extensive study of the interactions between phytoviruses and non-human mammals and humans, and the potential of these viruses to cause diseases in humans.”
So, at the present time, the answer to the question “Can plant viruses infect humans?” is:
Probably not, but, scientifically, the jury is still out…
Indeed, the fact that most plant viruses don’t cause disease in humans has made them potentially useful tools in, for example, human cancer immunotherapy.
As summarized by Shoeb and Hefferon (see Ref. 5 below):“The nonpathogenic nature of plant viral nanoparticles makes them an ideal choice for therapeutic applications such as cancer. Understanding the molecular mechanisms behind the immune response to cancer has facilitated the use of nanotechnology as an effective cancer therapy. Biologically active self-replicating plant virus particles can be introduced to the bloodstream of the human body and used as effective drug delivery vehicles.”
Finally, as an aside, it should also be noted that, although plant viruses themselves may not cause disease in humans, fruits and vegetables may be vectors for the transmission of human pathogenic viruses.
As summarized in a recent review of the subject (see Ref. 6 below): “Important foodborne viruses such as norovirus, hepatitis A and rotavirus are usually “only” contaminating food and are detected on the surface of foodstuffs. However, they are threats to human public health and make up for the majority of cases. In contrast, the meaning of viruses born from within the food such as natural animal and plant viruses is still in many cases unknown.”
1. Colson P, et al. (2010) “Pepper mild mottle virus, a plant virus associated with specific immune responses, fever, abdominal pains, and pruritus in humans.” PLoS ONE, Vol. 5(4):e10041. doi: 10.1371/journal.pone.0010041. (Full Text)
2. Mandal, B. and R. K. Jain (2010) “Can plant virus infect human being?” Indian Journal of Virology, Vol. 21, pp. 92–93. (Full Text).
3. Liu, R., R. A. Vaishnav, A. M. Roberts and R. P. Friedland (2013) “Humans have antibodies against a plant virus: Evidence from Tobacco Mosaic Virus.” PLoS One, 8(4): e60621. (Full Text).
4. Balique, F., H. Lecoq, D. Raoult and P. Colson (2015) “Can plant viruses cross the kingdom border and be pathogenic to humans?” Viruses, Vol. 7, pp. 2074–2098. (Full Text).
5. Shoeb, E. and K. Hefferon (2019) “Future of cancer immunotherapy using plant virus-based nanoparticles.” Future Science OA, Vol. 5, No. 7, (DOI:10.2144/fsoa-2019-0001) (Full Text).
6. Bachofen, C. (2018) “Selected viruses detected on and in our food.” Current Clinical Microbiology Reports, Vol 5, pp. 143–153. (Full Text).