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Quietly lingering on our doorknobs, children's toys, or next gourmet meal, the Norwalk virus patiently waits for the opportunity to horizontally transfer its agonizing gastrointestinal (GI) infection to any unsuspecting man, woman, or child. No one is safe or excluded from the Norwalk virus's extremely contagious nature, however it does have an unnerving predilection for school-aged children, the elderly, the immunocompromised, and the inhabitants of third world countries. The battle to evade the nasty Norwalk is further complicated by the small number of viral particles necessary to inflict a successful GI infection and the absence of an available vaccine. However, all hope to avoid infection is not lost! Through the completion of this reading, the answers of how to understand and undermine the tenacious Norwalk virus will no longer be elusive to you.
To begin shedding light on the Norwalk virus, the most appropriate place to start is with the classification of the virus. To understand viruses, it is important to understand the history of how they were categorized. Previously, viruses were categorized by the disease caused, host, organism, and morphology5. However, not all of these methods of classification are still in use today. The most ubiquitous way to currently classify a virus is using the Baltimore system, which classifies viruses based on their nucleic acid. The Norwalk virus, more accurately denoted as a norovirus (part of the genus Norovirus), is part of the family Calciviridae6 which is classified as a Class IV virus using the Baltimore system5. Being defined as a Class IV virus means that the nucleic acid of noroviruses is single-stranded RNA, and more specifically for the norovirus, it is a plus sense, single-stranded RNA (+ssRNA).
The norovirus genome is 7.6 kb in length and is comprised of three open reading frames (ORFs). The first ORF encodes the nonstructural polyproteins, which include RNA helicase, protease, and polymerase (pol) proteins, the second ORF encodes the major structural capsid proteins, and the third open reading frame encodes a small virion-associated protein6. The norovirus is also a non-enveloped, icosahedral virus1, 2 that is 38 nm is size.
As you acquire more knowledge about the norovirus through this reading, the confidence to overcome your fear about the virus's debilitating GI infection should be bubbling inside you! As you ponder this bubbling feeling (and hopefully it is not in a gaseous bubbling nature because that could be a significant indication a potential norovirus intrusionâ€¦.), transition your mind from classification to replication. While noroviruses are classified as plus sense, single-stranded RNA (+ssRNA) virus that is non-enveloped, they also have the reputation of being extremely arduous to grow in cell culture or small animal models2. Due to this difficulty, the replication process of the virus is largely uncharacterized. However, murine norovirus has been shown to replicate in dendritic cells in vitro and also in macrophages in vivo and in vitro. These models of culturing the murine norovirus will provide the tools to better understand norovirus replication in humans in the future2.
While the specific nature of the norovirus is unknown, the nature of replication of a +ssRNA virus is well understood and it is hypothesized that this is also how the norovirus replicates2. The nucleic acid of the +ssRNA functions as mRNA and is recognized by the host cell's translational machinery. However, the virus does bring along its own RNA dependent RNA polymerase to later transcribe the produced -ssRNA because host cells do not contain this necessary enzyme. The +ssRNA serves two purposes: (1) being translated to make proteins and (2) to use the proteins to make -ssRNA to be transcribed into more +ssRNA which makes viral particles for assembly3.
As we continue to progress in our acquisition of norovirus knowledge, that curious bubbling feeling of confidence should have subsided and been replaced with excitement about being so close to overcoming the mysteries of the norovirus. It hasn't? Oh dear â€¦ well, the best suggestion is to keep reading with vigilance about the symptoms of norovirus infection. Just in case of course! Norovirus is commonly acquiesced via the fecal-oral route, either by consumption of fecally contaminated food or water or by direct person-to-person spread. Environmental and fomite contaminations may also act as a source of infection1. As previously mentioned, it takes a small number (~10-100) of viral particles1 to sufficiently infect an individual, but consumption of one viral particle in drinking water has also been reported to show infection6. Norovirus generally has an incubation period of 24-48 hours and the most common clinical symptoms are acute onset of nausea, vomiting, abdominal cramps (oh no, bubbling!), myalgias, and non-bloody diarrhea6 and up to one-third of persons exposed may develop an asymptomatic infection2. While the majority of clinical symptoms are manageable, immunocompromised individuals, children, and the elderly can have more persistent and severe symptoms including, but not limited to, dehydration, weight loss, renal failure, disseminated intravascular coagulation (DIC), chronic diarrhea for months to years, malnutrition, and even death4.
The onset of norovirus symptoms are caused by damage to the cells in the lining of the small intestine that results in fluid leaking from the cells into the intestine and watery stools3. Norovirus gastroenteritis is a self-limiting disease that lasts 24-60 hours in the majority of cases. However, infected persons may continue to shed norovirus for up to eight weeks with high viral loads (up to 1012 norovirus per gram of feces) after clinical resolution of the symptoms4. There are no long term effects of becoming infected with the norovirus unless the infection is in children, the elderly or immunocompromised individuals because the virus is self-limiting in healthy persons.
The mechanisms that the immune system uses to clear the norovirus infection are unknown due to the inability to culture the virus in a laboratory setting. It has been shown that the immunity granted through an immune response may be strain-specific and lasts only a few months. Also, due to the high genetic variability of noroviruses, individuals are likely to have recurrent infections throughout their lifetimes. Recent evidence also suggests that susceptibility to infection may also be genetically determined1.
By now either your stomach bubbling with "confidence" has stopped, thus leaving you ready to continue on the norovirus knowledge quest or this paper has been haphazardly thrown on to the floor because your hypochondriac demeanor has convinced you that you have become infected with the norovirus and you're currently rushing to the hospital. Preferentially it is not the latter because diagnosing a norovirus infection is not an easy task. The inability to culture norovirus in a laboratory has forced the characterization and classification of norovirus to be accomplished using reverse transcriptase-polymerase chain reaction (RT-PCR), genomic sequencing, and phylogenic analysis6. Currently, state public health laboratories in all fifty states have the capability to test for noroviruses by RT-PCR1. RT-PCR uses reverse transcriptase to transcribe the RNA into complementary DNA that will be amplified using the PCR procedure. RT-PCR detects the norovirus RNA and can be used to test stool and emesis samples, as well as environmental swabs in special studies1. While it is preferential to collect samples during the acute phase of infection (within 48-72 hours after the onset of symptoms), virus can be found in stool samples that are taken as late as two weeks after recovery from clinical symptoms1.
Other methods for diagnosis include direct and immune electron microscopy of fecal samples, enzyme-linked immunosorbent assays, and the detection of specific antibodies in acute- and convalescent-phase blood samples (ELISA) 1. While none of these alternative methods are nearly as specific or as sensitive as using RT-PCR to diagnosis a norovirus infection, in an outbreak setting1, these tests are preferential to rapidly identify norovirus due to short identification time when compared to RT-PCR. While diagnosis of norovirus is possible, there is not specific therapy that exists for norovirus gastroenteritis because it is a self limiting disease and will not cause serious problems if left untreated. The potential therapies for norovirus infection, which are most often used with children, the elderly and immunocompromised patients, consist of replacing fluid losses and correcting electrolyte levels through oral and intravenous fluid administration1. There is no drug or vaccine that exists to treat norovirus infections due to the absence of a robust tissue culture system6.
Now you can relax about your curious stomach bubbling because nothing too serious will happen outside of some uncomfortable stomach problems! So, instead of fixating on the symptoms, focus on the methods used to prevent norovirus transmission. The most important method to prevent horizontal transmission and infection is frequent, thorough hand washing with soap and water for at least 20 seconds (hand sanitizer alone will not prevent infection) 1. The prevention of food borne norovirus disease relies on the provision of safe food and water. A challenge with preventing food borne norovirus is the environmental durability of the norovirus. Noroviruses are able to survive at freezing temperatures and temperatures as high as 60°C. Moreover, noroviruses can survive in up to 10 ppm chlorine, well in excess of levels routinely present in public water systems1. Despite these scary features, simple measures, such as washing your hands, proper handling of cold foods, and paid sick leave, may substantially reduce food borne transmission of noroviruses1.
In the United States, the CDC estimates that more than 21 million cases of acute gastroenteritis each year are due to norovirus infection, and more than 50% of all food borne disease outbreaks can be attributed to noroviruses1. Among these outbreaks, common settings included restaurants and catered meals, nursing homes, schools, and vacation settings or cruise ships1. Noroviruses are the second more common cause of severe gastroenteritis in children less than 5 years of age worldwide. Travelers to developing nations, where greater fecal contamination of food and water supplies may be encountered, are at risk for developing norovirus gastroenteritis (K). These known facts make children, the elderly, immunocompromised individuals, and persons living in third world countries that lack proper water and food sanitation the most susceptible to norovirus infection.
Relived yet? Well, you should be! Your acquisition to thoroughly understand the norovirus is complete. So, step away from the nurse at the hospital and try to remember what you've learned: norovirus has an incubation period of 24-48 hours, there are common symptoms such as nausea, vomiting, and diarrhea, it is a self limiting +ssRNA virus, and it is most threatening to children, the elderly, and the immunocompromised. If you're still afraid of contracting this not-so-threatening virus remember to implement a strict hand washing routine, especially when handling food and water, and stay home if you aren't feeling well one day! Now that you are well versed in the norovirus and all of its topics, breathe a sigh of relief in knowing that the curious bubbling in your stomach is probably nothing more than hunger pains (I would be hungry too after reading such a long paper)!