By Kelly A. Reynolds, MSPH, PhD
If you eat peppers or products containing peppers, you have likely been exposed to (and probably even infected with) the ubiquitous pepper mild mottle virus. Pepper mild mottle viruses (PMMoV), scientifically known as Tobamoviruses, are commonly found in human feces and wastewater. Scientists are studying the potential for using this virus as an indicator, or tracer, of fecal pollution monitoring. Ever in search of a better indicator of water safety, researchers are evaluating the benefits of PMMoV to determine the quality of a wide variety of water sources.
PMMoV: a major plant pathogen
The PMMoV is a major pathogen of pepper crops (including bell, hot and ornamental peppers) resulting in significant crop yield losses each year in the US and globally. The virus can also infect pepper seeds and plants and remain undetected until the vegetables appear with spotty, burnt, shrunken or distorted outer skins. Harvesting and planting practices naturally create small cuts in the pepper plants, leaving the product vulnerable to infection transmission. Humans contribute to the spread via contact with plants and cross-contamination from hands, gloves and clothing. Once a plant is infected, there is no cure for the disease.
Human health risks
In the environment, PMMoV can survive for long periods and are extremely stable, adsorbing to soil particles and thriving in hot, humid conditions. Field prevention methods have focused on methyl bromide fumigation (until its ban circa 2005) and soil supplements to reduce particle adsorption. Despite efforts of environmental control, PMMoV is frequently isolated from the environment. Further, due to routine pepper consumption, most human stools also test positive for the virus; consumption of spicy food is linked to human infection. While the PMMoV is a major hazard for pepper plants, their effect in humans is uncertain. Most appear to be asymptomatic carriers of the virus. One study tested 21 commercial food products containing peppers, 357 adult and 208 children stool samples. Many food products (57 percent; 12/21) were positive and infection was noted in 7.2 percent (22/304) of adults and 0.7 percent of (1/137) children.(1) Positive patients were significantly (p < 0.05) more likely to exhibit fever, abdominal pains and other symptoms. Whether or not symptoms were directly or indirectly related to PMMoV remains in question.
The search for an ideal indicator
Since the late 1800s, we have been searching for an ideal indicator of water quality, since historically, methods were not available to sample directly for the hundreds of pathogens of concern. Even when present, they are in such low concentrations that large volumes of water must be filtered to detect them. Detection methods are usually costly, time consuming and require advanced technical experience. Therefore, the search for an ideal indicator ensued.
Coliform bacteria, including the fecal coliform Escherichia coli, have long been utilized worldwide as a water quality indicator. They are naturally present in the feces of warm-blooded animals and thus are useful for determining if a recent contamination event may have occurred. Criteria for an ideal indicator have been previously defined and should be:(2)
1. universally present in the feces of humans and warm-blooded animals
2. readily detected by simple and inexpensive methods
3. unable to grow in natural waters
4. persistent in water and susceptibility to treatment should be similar to waterborne pathogens
5. present in large numbers
6. only present when pathogens are present
While E. coli meets the majority of the criteria for an ideal indicator, it can grow in natural waters, particularly in temperate or tropical climates and its presence does not always correlate with other pathogens. Many viruses and protozoan pathogens are more resistant to disinfectants than E. coli and survive longer in drinking water supplies. Since viruses do not multiply in the environment and are often more resistant to treatments, they are potentially better indicators than fecal bacteria. Thus, PMMoV is being evaluated for its suitability as a water quality indicator.
Suitability of a PMMoV indicator
PMMoV has been found in various types of water in Vietnam, including 94 percent (n=36) of samples from surface water, farm irrigation water, ponds and rivers and 100 percent of wastewater samples.(3) The virus was also found in 38 percent of groundwater. In comparison to commonly used chemical tracers, including caffeine, PMMoV concentration ratios were unchanged. This means that PMMoV persistence in the environment is similar to other non-biological tracers. In the same study, PMMoV was a conservative tracer for human enteric viruses, which degraded faster and were less frequently detected in surface waters at up to 59 percent. PMMoV was not found in tap or bottled water samples although the caffeine tracer was. Thus, researchers questioned their validity in treated water supplies where biological tracers are typically removed but pharmaceuticals and personal care products may persist.
Another study in Europe found PMMoV in 95 percent of (19/20) human stools but only 17 percent (9/53) of animal (cow, chicken and geese) stools.(4) Since animals contribute to pathogen transmission in the environment, an indicator that does not detect animal feces contamination is problematic. A survey of PMMoV in wastewater in the US found it present 100 percent of the time and in concentrations approaching a million detectable units per mL of raw sewage, and also found good correlations between PMMoV and pathogens in seawater.(5) Unfortunately, they too found a low prevalence in most of the animal feces tested.
A study in Japan focused specifically on surface water sources and found PMMoV in 76 percent (140/184) of the samples, with contamination trends differing geographically but not seasonally.(6) Here again PMMoV was frequently detected in the absence of human enteric viruses and E. coli. Others have suggested that due to its relative resistance to treatment processes, PMMoV may be a suitable indicator for more resistant human enteric viruses, such as the Aichi virus.(7)
Although PMMoV has many suitable indicator qualities, current data suggests it is more appropriate as an indicator of surface waters under the potential impact of sewage. The low detection rate of the virus in most animal stools and its abundance in wastewater-impacted waters at concentrations much higher than human enteric viruses means it can overestimate health risks. On the other hand, the tracer does not persist as long as some of the chemical water contamination tracers. To date, there have been only a few studies comparing PMMoV to contaminants of human health importance and some of those studies had very small sample sizes. In the end, even after 100-plus years of research on the use of E. coli and other microbes and chemicals, we still have not found an indicator that meets all of the criteria for an ideal tracer. The lack of a consistently reliable indicator increases the uncertainty of our recreational and drinking water safety.
(1) Colson, P., Richet, H., Desnues, C., et al. “Pepper mild mottle virus, a plant virus associated with specific immune responses, Fever, abdominal pains, and pruritus in humans.” PLoS One. 2010;5(4):e10041. doi:10.1371/journal.pone.0010041.
(2) Edberg, S.C., Rice, E.W., Karlin, R.J., Allen, M.J. “Escherichia coli: the best biological drinking water indicator for public health protection.” J Appl Microbiol. 2000;88(S1):106S-116S. doi:10.1111/j.1365-2672.2000.tb05338.x.
(3) Kuroda, K., Nakada, N., Hanamoto, S., et al. “Pepper mild mottle virus as an indicator and a tracer of fecal pollution in water environments: Comparative evaluation with wastewater-tracer pharmaceuticals in Hanoi, Vietnam.” Sci Total Environ. 2015;506:287-298. doi:10.1016/j.scitotenv.2014.11.021.
(4) Hamza, I.A., Jurzik, L., Überla, K., Wilhelm, M. “Evaluation of pepper mild mottle virus, human picobirnavirus and Torque teno virus as indicators of fecal contamination in river water.” Water Res. 2011;45(3):1358-1368. doi:10.1016/j.watres.2010.10.021.
(5) Rosario, K., Symonds, E.M., Sinigalliano, C., Stewart, J., Breitbart, M. “Pepper mild mottle virus as an indicator of fecal pollution.” Appl Environ Microbiol. 2009;75(22):7261-7267. doi:10.1128/AEM.00410-09.
(6) Haramoto, E., Kitajima, M., Kishida, N., et al. “Occurrence of pepper mild mottle virus in drinking water sources in Japan.” Appl Environ Microbiol. 2013;79(23):7413-7418. doi:10.1128/AEM.02354-13.
(7) Kitajima, M., Iker, B.C., Pepper, I.L., Gerba, C.P. “Relative abundance and treatment reduction of viruses during wastewater treatment processes—Identification of potential viral indicators.” Sci Total Environ. 2014;488:290-296. doi:10.1016/j.scitotenv.2014.04.087.
About the author
Dr. Kelly A. Reynolds is an Associate Professor at the University of Arizona College of Public Health. She holds a Master of Science Degree in public health (MSPH) from the University of South Florida and a doctorate in microbiology from the University of Arizona. Reynolds is WC&P’s Public Health Editor and a former member of the Technical Review Committee. She can be reached via email at [email protected]