Common Foodborne Pathogens - X

Noroviruses Risk Profile 

Norovirus (NoV), or the winter vomiting bug, is the most common cause of gastroenteritis, which is characterized by non-bloody diarrhea, vomiting, and stomach pain. Noroviruses are environmentally hardy organisms that can be transmitted by food, water, and also can be easily transmitted through person-to-person contact or contact through environmental surfaces. The norovirus infection can infect the same person several times because there are many different types of noroviruses. Thus infection with one type of norovirus may not protect against other types, which is possible to develop immunity to (protection against) specific types and, it is not known exactly how long immunity lasts. Nonetheless, whether you are susceptible to norovirus infection or not is also determined in part by an individual’s genes.
 
NoV is a genetically diverse group of single-stranded positive-sense RNA, non-enveloped viruses belonging to the family Caliciviridae, which can be genetically classified into at least seven different genogroups (GI, GII, GIII, GIV, GV, GVI, and GVII), and further divided into different genetic clusters or genotypes. The strains are known to cause disease in humans, which exist primarily in genetic clusters within genogroups I, II, and IV, whereas the viruses belonging to the other genogroups have been shown to infect other animals such as cattle, swine, and mice. Norovirus in genogroups GI and GII alone can be divided into at least 15 genetic clusters. Thus, a genetic cluster of NoV is defined as strains that have at least 80% homology to a reference strain’s amino acid sequence. 
 
Sources 

The major source of NoV outbreaks have been associated with the drinking of contaminated water, including municipal water, well water, stream water, commercial ice, lake water, and swimming pool or recreational surface-water exposure, as well as floodwater. The second most implicated source was salad ingredients, fruit, and oysters are the foods most often implicated in norovirus outbreaks. However, any ready-to-eat food that has been handled by an ill food worker may be contaminated. Molluscan shellfish, particularly oysters, have been commonly identified in NoV-related gastroenteritis outbreaks worldwide. However, this represents a different etiology that does not necessarily involve a contaminated food worker. The rapid spread of secondary infections is particularly evident in areas where a large population is enclosed within a static environment, such as in institutions, college campuses, schools, military operations, hotels, restaurants, recreational camps, hospitals, nursing homes, day-care facilities, and cruise ships, and after natural disasters, such as hurricanes and earthquakes. Nonetheless, nearly 29% of all NoV foodborne outbreaks from 1997-2004 could be attributed to food purchased or served at a restaurant or delicatessen.
 
Disease 
Norovirus infections occur more commonly during winter months, which is often occur in outbreaks, especially among those living in close quarters, which are a leading cause of foodborne infection in the United States. 
 

Mortality

The infections approximately account for 26% of hospitalizations and 11% of deaths associated with food consumption. 

 

Infective dose

The infective dose is considered very low, which is estimated to be as low as 1 to 10 viral particles. The viral particles are excreted at high levels as high as 1 x 10¹² million viral particles of 1g feces by both symptomatic and asymptomatic people. 

Onset

Mild, brief symptoms usually develop between 24 and 48 hours after contaminated food or water is consumed, but onset times within 12 hours of exposure have been reported.

 

Complications

Norovirus infection is self-limiting, which can be very debilitating as a result of the high rate of vomiting, where recovery is usually complete without evidence of long-term effects. The most common complication is dehydration, particularly among the very young, the elderly, and patients with underlying medical conditions, where no specific therapy exists for viral gastroenteritis or NoV infection. Treatment for NoV infection is consisted primarily of oral rehydration and, if needed, intravenous replacement of electrolytes. There is no antiviral medication or vaccine is available, and antibiotics are not effective for treating NoV infection. Prevention involves proper hand washing and disinfection of contaminated surfaces, and alcohol-based hand sanitizers can be used as an alternative, but they are less effective than hand washing. There is no vaccine or specific treatment for norovirus, where management involves supportive care, such as drinking sufficient fluids or intravenous fluids.

 

Symptoms

Explosive, projectile vomiting usually is the first sign of infection and is often used to characterize the infection. Symptoms usually present as acute-onset explosive often vomiting, watery, non-bloody diarrhea with abdominal cramps, and nausea, headache, low-grade fever, chills, and muscle aches may also occur. Thus, the severity of symptoms appears to be higher in hospitalized patients, immunocompromised people, and elderly people, compared with younger adults and other groups. Nonetheless, there is about 30% of people infected with NoV display no gastrointestinal illness or associated symptoms, though they are still excreted high levels of virus in their stool, and such distinctive groups of people are considered to be silent shedders of NoV. 

Duration

Symptoms generally persist for 12 to 60 hours, with a mean period of 24 to 48 hours, where most patients report feeling better within 1 to 2 days. However, immunocompromised or elderly people and hospitalized patients may retain symptoms as vomiting and diarrhea continue for a while, and generally resolve within 72 to 96 hours, where the non-specific symptoms, such as headache, thirst, and vertigo, could persist up to 19 days.

 

Route of entry

Though the virus is usually spread by the fecal-oral route, foodborne norovirus infections have been epidemiologically linked into three distinctive segments:  

Cases associated with the consumption of ready-to-eat (RTE) foods contaminated by food workers; 

Environmental contamination of produce; 

Consumption of molluscan shellfish harvested from contaminated water. 

In rare cases, transmission can occur through vomit and is often associated with improper sanitation controls or their application. Secondary transmission following the foodborne illness is common, due to the high levels of virus that are excreted. 

 

Pathway

Norovirus infection causes gastroenteritis, an inflammation of the stomach and the small and large intestines, but the precise pathogenic pathway of infection is unknown. 

 
Frequency 

Norovirus infections annually result in about 685 million cases of disease and 200,000 deaths globally, which is common both in the developed and developing worlds. Infants under the age of five are most often affected, which results in about 50,000 deaths in the developing world. According to the Centers for Disease Control and Prevention (CDC) estimates that noroviruses cause 5.5 million infections annually in the United States with an estimated range of 3.2 million to 8.3 million cases of foodborne infections, which accounts for 58% of all foodborne illnesses, including approximately about 0.03% (14,663) require hospitalization, and less than 0.1% of these illnesses results in death (149). 
 
Diagnosis 
Clinical diagnosis, without the diagnostic tests used to identify NoV-associated infections, include the following four criteria: 1) vomiting in more than 50% of affected persons in an outbreak; 2) a mean (or median) incubation period of 24 to 48 hours; 3) a mean (or median) duration of illness of 12 to 60 hours, and 4) lack of identification of a bacterial pathogen in stool culture. The clinical diagnosis of NoV infection is carried out using analytical tests on serum, stool, and in some instances, vomitus. Diagnosis also can be achieved by examining blood serum samples for a rise in virus-specific serum antibody titers, measured by enzyme immunoassay (i.e., ELISA or EIA). However, this method has had only a 55% level of accuracy when compared with a reverse transcription-polymerase chain reaction (RT-PCR) approach. The applicability of these assays is also limited by the requirement to collect stool specimens from acute or convalescent patients for accurate determination. Examination of stool specimens for norovirus can be performed by microscopy through direct electron microscopy or immunoelectron microscopy to visualize viral capsids, which requires high densities (generally >106 /g). Thus, RT-PCR is the preferred method of diagnosis since it is significantly more sensitive than microscopy; does not require a large, expensive electron microscope with highly skilled personnel; and has the ability to rapidly differentiate genogroups, which could be instrumental in follow-up epidemiologic investigations, to determine the route and distribution of NoV in the community. 
 
Target Populations 

NoV may impact people of any age, but it is more prevalent among the elderly and children under the age of 5. According to the research data, there is a genetic predisposition to acquiring an infection that is dependent on the patient’s blood type (ABO phenotype). Nonetheless, the previous infection of Norovirus does not provide long-term immunity, and reinfection by the same strain can occur several months after the initial infection. On the other hand, the rapid spread of secondary infections through congested areas where a large population is enclosed within a static environment, such as in institutions, college campuses, schools, military operations, hotels, restaurants, recreational camps, hospitals, nursing homes, day-care facilities, and cruise ships, or after natural disasters, such as hurricanes and earthquakes. 
 
Food Analysis 

Assays using RT-PCR technology for NoV detection and quantitation are commercially available, but quantitative RT-PCR (qRT-PCR) is the most sensitive method for NoV detection in food extracts, which is an improvement over conventional RT-PCR due to its increased specificity and sensitivity. NoV has been successfully detected and isolated from oysters, irrigation, and groundwater, as well as deli meats.
 
 
Reference:
FDA Bad Bug Book, Foodborne Pathogenic Microorganisms and Natural Toxins. Second Edition. 2013
Preventive Controls for Human Foods. 2016
www.cdc.gov