December 2002: Volume 44, Number 12
Emerging Issues: Suspected Viral Pathogens of the Future
by Kelly A. Reynolds, MSPH, Ph.D.
Here’s some food for thought. In approximately half of all waterborne disease outbreaks, a causative agent -- or culpable microorganism -- is never found.
For some outbreaks, illnesses are recognized too late for proper sampling and investigation. For many outbreaks, however, no causative agent is identified even after careful, thorough and timely survey and testing. The latter situation leads researchers to believe there’s still a large portion of waterborne disease occurring for which the causative agent is either undetectable or unrecognized. Recent history has proven that, even with our advanced scientific methodologies of pathogen detection, microbial agents of significant morbidity and mortality -- i.e., causing disease or death -- continue to emerge (i.e., Cryptosporidium, Helicobacter pylori). Therefore, by looking into the future, scientists strive to predict the inevitable emergence of the next waterborne pathogen.
A virus’ elusive nature
Based on the difficulties in detecting viral pathogens, and the epidemiology and symptoms of disease in cases on unknown etiology, it’s thought a significant portion of unidentified agents in waterborne outbreaks is due to viral agents. Viruses tend to survive longer in the environment than most enteric -- or intestinal -- bacteria, including the bacterial indicators used to evaluate water quality, i.e., coliforms. Survival depends on numerous physical, chemical and microbial factors, although it’s feasible for viruses to survive for days in aquatic environments and longer when associated with fecal matter or other protective material. In general, viruses have a very low infectious dose, meaning as few as one to 10 organisms can initiate infection and disease. Their prolonged survival and small size make them especially adept at movement through aquatic and soil environments. Methods for isolation of viruses from large volumes of water have poor recovery efficiency and research is under way to improve methods for monitoring viruses in the environment.
Viruses are obligate intracellular parasites, meaning they need a host cell to multiply and initiate disease. They don’t grow in the environment or outside of a host cell, a condition that makes them particularly difficult to culture in the laboratory. Although many enteric viruses grow in laboratory cell cultures, such as poliovirus and hepatitis A virus, others aren’t yet culturable. Detection of non-culturable agents may be accomplished by microscopy or serology -- studying properties based on reactions with various serums. If genetically characterized, agents may be detected via molecular methodologies (i.e., human caliciviruses). Still, other viruses are vaguely recognized as potential human pathogens and require further evaluation. These viruses include torovirus, coronavirus, picobirnavirus, picotrirnavirus and pestivirus. Could one of these agents be responsible for a portion of the waterborne outbreaks of unknown etiology, or disease origin? What populations, if any, are affected? With little or no information about the incidence, transmission, survival and/or transport of these recently recognized pathogens, such questions are difficult to answer.
Are they a threat?
Little is known about the torovi-ruses, coronaviruses, picobirnaviruses, picotrirnaviruses and pestiviruses and very little emphasis has been placed on these suspected waterborne pathogens in regard to research dollars or recognition from public health and regulatory agencies. What we do know is summarized below.
Coronaviruses: Coronaviruses are well known causes of the common cold in humans (responsible for up to 30 percent of human infections) and respiratory infections in pigs, cats, dogs, mice and chickens. They’re also the cause of diarrhea in animals and have been found in the feces of humans with diarrhea, using high-powered microscopy. Coronaviruses are easily identified based on their distinctive corona-like (halo) appearance. Although first identified in 1965, little is known about their role in human disease. Adding to the uncertainty is the fact that coronaviruses have been detected in the feces of apparently healthy individuals. Higher prevalence rates have been observed in lower socioeconomic populations and in those with poor personal hygiene.
Toroviruses: All toroviruses identified so far cause enteric infections and are thought to be transmitted by the fecal-oral route. Toroviruses have long been recognized as pathogens of cattle and horses but have only recently been associated with gastroenteritis in children and adults. In a study of over 2,500 children throughout the United States with diarrhea, the majority were infected with rotavirus (32 percent) while toroviruses are the third most common agent (3 percent).1 Toroviruses were the second most commonly isolated agent in hospitalized cases and have been associated with persistent symptoms.
Picobirnaviruses, picotrirnaviruses: Picobirnaviruses are also known pathogens of animals, including chickens and rabbits, and have been detected in fecal specimens from humans in developing nations and immunocompromised populations in the United States. Picobirnaviruses were found in 14.6 percent of HIV positive patients with diarrhea but not in the general population.2 Others have found similar prevalence rates in patients with or without symptoms of intestinal disorder.3 Most recently, 35 percent of children with diarrhea in Canada tested positive for the viruses, compared to 14.5 percent of healthy children.4 A potential co-infectant in persons suffering from cryptosporidiosis, picobirnaviruses is a possible public health concern. Like picobirnaviruses, picotrirnaviruses were originally detected in the stools of HIV-infected patients; however, they differ from the picobirnaviruses in size, genomic composition and general morphology. Picotrirnaviruses are known to infect animals, in particular mammals. This group of viruses is probably the most obscure, with almost no data available.
Pestiviruses: Pestiviruses are known to infect cows, pigs and sheep wherein they can cause diarrhea, congenital defects and reproductive failures. They’ve also been identified in 23 percent of Arizona Indian children under the age of 2 with symptoms of diarrhea, compared to only 3 percent in healthy populations.5 Even higher prevalence rates (30-50 percent) were found in studies of children and adults from Arizona, Maryland and Peru.6 These viruses are relatively common contaminants of laboratory cell cultures and have raised concern over their presence in reagents used for human vaccine development.7
While none of the aforementioned viruses have been documented as the cause of a waterborne disease, they’ve been implicated as causative or cohabiting agents in patients with gastroenteritis. Little is known about their prevalence in drinking water and wastewater, or their susceptibility to conventional water treatment processes. Can we assume that all viruses -- those currently identified and those yet to be discovered -- are respondent to approved treatment methods? How can we account for the differing susceptibilities of immunocompromised individuals? The latter question is becoming more and more relevant as the immunocompromised population steadily grows (now up to 25 percent of the U.S. population) because of age or other factors.
Based on the limited number of microbes being looked at for research and/or regulatory control -- as well as continued use of bacterial indicators that aren’t useful for predicting risks of viral or protozoan pathogens as measures of water quality -- the public may be left largely unprotected against waterborne viral agents (see www.asmusa.org/pasrc/pdfs/waterreport.pdf).
Only time will tell if toroviruses, coronaviruses, picobirnaviruses, picotrirnaviruses, pestiviruses, or some other unidentified agent has an impact on human health. Is it necessary, however, to wait for the next major pathogen to emerge? Or, should the public take a more proactive approach in “preventative medicine” -- also known in water treatment as the multi-barrier approach?
1. Waters, V., et al., “Etiology of community-acquired pediatric viral diarrhea,” Pediatrics and Infectious Disease, 19:843-848, 2000.
2. Giordano, M.O., et al., “Diarrhea and enteric emerging viruses in HIV-infected patients,” AIDS Research and Human Retroviruses, 15: 1427-32, 1999.
3. Gallimore, C.I., et al., “Detection and characterization of bisegmented double-stranded RNA viruses (picobirnaviruses) in human faecal specimens,” Journal of Medical Virology, 45: 135-40, 1995.
4. Jamieson, F.B., et al., “Human torovirus: a new nosocomial gastrointestinal pathogen,” Journal of Infectious Diseases, 178: 1263-1269, 1998.
5. Yolken, R., et al., “Infantile gastroenteritis associated with excretion of pestivirus antigens,” Lancet, I:517-519, 1989.
6. Yolken, R., et al., “Pestiviruses: major etiological agents of gastroenteritis in human infants and children?” Clinical Research, 36:80A, 1988.
7. Giangaspero, M., et al., “Genotypes of pestivirus RNA detected in live virus vaccines for human use,” Journal of Veterinary Medical Science, 63: 723-733, 2001.
About the author
Dr. Kelly A. Reynolds is a research scientist at the University of Arizona with a focus on development of rapid methods for detecting human pathogenic viruses in drinking water. She holds a master of science degree in public health (MSPH) from the University of South Florida and doctorate in microbiology from the University of Arizona. Reynolds also has been a member of the WC&P Technical Review Committee since 1997.