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Food borne infections involve the ingestion of pathogens, followed by growth in the host, tissue invasion and/or release of toxins. These food borne microorganisms are major contributors to a wide variety of diseases worldwide due to food and water being a very common and widespread source of infection. Contamination of these vectors can be from a range of pathogenic microorganisms spanning from DNA or RNA viruses, gram-positive/negative bacterium, protozoans and parasitic nematodes. Each of these can lead to food borne illnesses which require differential testing techniques and an assorted knowledge of preventable factors to avoid unnecessary contamination. A diverse understanding for each type of microorganism's mode of replication, survival and growth, which will be discussed in this essay, and is required to be able to prevent food and water sources becoming reservoirs for infection and resulting in prevalent illnesses; typically via the faecal-oral route. Food practices can include hygienic handling, refrigeration, heating and other treatments to maintain quality and also disseminate pathogenic microorganisms and provide inhibitory conditions for specific organisms.
The most common DNA-viruses associated with food borne infections are known as Adenoviruses; these are medium-sized (60-90nm in diameter) icosahedral, nonenveloped viruses, in the Adenoviridae family and are classified into group I under the Baltimore Classification Scheme. They are composed of a nucleocapsid and a double-stranded linear DNA genome and play a major role in food borne illnesses such as gastrointestinal inflammation, but can also cause respiratory, ocular, kidney and neurological infections. They were first isolated from human Pharyngeal tonsils, or adenoids, in which the name Adenovirus is derived from. (Willey. J et al, 2008)
Adenoviridae can be divided into five genera: Mastadenovirus, Aviadenovirus, Atadenovirus, Siadenovirus and Ichtadenoviru when not excluded to only human adenoviruses. The genus Mastadenovirus, of the Adenoviridae family, are known to infect humans by transmission of either; the faecal-oral route with water and food as possible vectors, or a respiratory route. (Damen. M et al, 2008)
There are 51 accepted serotypes of adenoviruses (HAdV-1 to 51) which have been grouped into seven serogroups (Human adenovirus A to F) based on their ability to agglutinate red blood cells (Sarantis. H, 2004).The different serotypes/groups from Table A can be associated with various conditions, serotypes 1, 3, 5, 7 and 31 are occasionally found in stools although enteric adenovirus infection is principally caused from serotypes 40 and 41 within subgroup F (HAdV-F serotypes 40 and 41, refer to Table A). Enzyme immunoassays (EIA) are used for the serological identification of adenovirus infections by detection of the hexagon antigen, a major capsid protein; and Immunoglobulin A, G and M increased responses can be seen in most patients within 2 weeks after infection. (Fratamico. P et al, 2005)
Table A: Adenovirus Serotypes and Groups
12, 18, 31
3, 7, 11, 14, 16, 21, 34, 35, 50
1, 2, 5, 6
8, 9, 10, 13, 15, 17, 19, 20, 22-30, 32, 33, 36-39, 42-49, 51
*Table adapted from http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mmed&part=A3533
Regardless of which host adenoviruses infect, they all share the common morphology of being the largest nonenveloped viruses with unique projection fibers. These "spikes" have globular knobs projecting from the vertices associated with penton bases on the capsid surface. Refer to Image 1: Adenovirus Structure, the image shows the projection fibers extending from each penton base.
Image 1: Adenovirus Structure
*Image applied from Rux.J, Burnett. R (2004) Adenovirus Structure
After the virus is ingested, this structure is used for attachment to the surface of host cells via the binding of the coxsackie-adenovirus receptor (CAR) to the globular knob domain; once attached, they are able to be transported through the membrane by receptor-mediated endocytosis, without envelope fusion being necessary. Clathrin-coated pits in which the adenovirus is attached to pinch off and form coated vesicles carrying the viruses, and then the nucleic acid is inserted into the cytoplasm through the vesicle membrane. (Rux. J, Burnett .R, 2004)
All adenoviruses have linear, double-stranded DNA genomes usually between 28-45kB. This allows the viruses to approximately carry 20-40 genes resulting in a simple virus, which relies heavily on host cell factors for survival and replication. Therefore a terminal 55 kDa protein is associated with each 5' ends of the dsDNA and used as primers to ensure sufficient replication each cycle within the host cell's nucleus; where capsid and virion assembly occurs leading to further infection. (Heise. C, Kirn. D, 2000)
As well as Adenoviruses being observed in outbreaks of gastroenteritis, there can also be smaller round single-stranded DNA viruses measuring 20-26 nm in diameter called Parvoviruses. They are classified into the virus family Parvoviridae and under group II in the Baltimore Classification System. Parvoviruses operate in much the same way as adenoviruses being a naked, icosahedral DNA virus which replicates within the host cell's nucleus and can rarely cause gastroenteritis in humans via the faecal-oral route. Although it has been shown to be possible, reports of viral gastroenteritis being associated with Parvoviruses are much less frequent then Adenoviruses. (Appleton. H, 2000)
Both viral families Parvoviridae and Adenoviridae are causes of viral gastroenteritis from ingestion of contaminated food or water sources. Once the virions have entered the host cells via receptor-mediated endocytosis and integrated their DNA into the host cell's genome, the replication processes of the host are used to synthesise the viral proteins and nucleic acids. The virions then assemble within the nucleus and release with the cell's lysing, causing an acute infection of the gastro-intestinal tract. (Willey. J et al, 2008)
If the virus is able to successfully contaminate vector sources and infect individuals viral gastroenteritis will occur, which is an inflammation of the gastrointestinal tract. It involves both the stomach and the small intestine, leading to a mild, self-limiting illness which can last for 24-48 hours. Symptoms generally occur suddenly and can include abdominal pain, pyrexia, malaise, diarrhoea and/or vomiting. Management generally requires hygienic food handling and proper food and water distribution to limit the source of infection. (Willey. J et al, 2008)
Parvoviruses and adenoviruses have both been observed in outbreaks of food poisoning associated with the consumption of shellfish, such as oysters or clams. These can be major vectors for viral infection since they are filter-feeders which sift large amounts of water through their gills and quite often come into contact with contaminated water due to their preferred habitat of shallow, coastal waters. These polluted waters can contain pathogenic microorganisms such as adenoviruses or parvoviruses and even though human viruses don't replicate within the shellfish, the concentration of them can increase very rapidly, since oysters are often consumed raw this leads to a prominent source of infection. (Appleton. H, 2000)
The most common source of viral gastroenteritis is due to improperly disinfected drinking water. If the water is improperly chlorinated or the distribution system isn't efficient, it can allow viral contamination of adenoviruses or parvoviruses leading to an outbreak of viral gastroenteritis in a widespread community. Improving the distribution of drinking water is a preventable factor by the addition of holding tanks for treated water before being supplied to the consumers. This allows sufficient contact time for free chlorine to disinfect any viral contaminants such as adenovirus or parvovirus, which could cause an acute gastroenteritis infection. (Werber. D et al, 2009)
Food poisoning is a very common illness and almost everybody gets it at some stage in their life. Its severity depends on the microorganism that was involved. There is a vast variety of microorganisms that are involved in food borne infections, including Bacteria, fungi and viruses. However, viruses do not multiply in food, but use it only as a source of transmission to get inside the human body, where they will multiply and infect the host. [i] Such viruses that infect the host via digestion and multiply inside the intestines are called intestinal viruses. The three main infectious RNA viruses that are transmitted though food are poliovirus, hepatitis A and Norwalk agents.
Poliovirus was the first virus to be associated with food-borne viral infections. It belongs to the genus Enterovirus and it is a single-stranded RNA virus with an incubation period of 3-5 days. It is usually characterized by headache, fever, and sore throat, however, there are some rare cases where it advances to a second stage and invades the meninges which cause back pain and headaches. The virus may also, but rarely, spread to the neurons in the spinal chord and that causes cell destruction and paralysis, and if the infection reaches the brain it may cause death. [ii]
Poliovirus enters the body orally, usually after being in contact with an infected person's faeces, especially if that person has not washed their hands after visiting the toilet and has been preparing food. In most food-borne outbreaks of polio, raw milk has been the main source of the transmission of the virus as it is more prone to contamination. However, this problem has diminished as milk is now pasteurized and this limits viral contaminations. Also, there are vaccinations that have been developed to stop the spread of this virus and since then cases have been very limited. [iii]
Another common virus transmitted via food is the infective hepatitis, caused by hepatitis A virus. Hepatitis A virus belongs to the family Picornaviridae and is also a single-stranded RNA genome. It has a longer incubation period of 15 to 45 days and once infected, the host will have lifetime immunity. After ingestion, the virus multiplies in cells of the gut epithelium then travels through the blood to the liver causing it to inflame, then it is passed in the faeces, therefore it is an acute disease. [iv]
Hepatitis A is mainly transmitted by faecal-oral contact and also through contaminated food or water. Molluscan shellfish (oysters and clams) grow in shallow coastal waters that are usually contaminated with sewage and since they filter sea water to extract suspended organic matter, they also concentrate bacteria and viruses from the surrounding environment, therefore they are the most common source of food-borne infections. Undercooked or raw shellfish leads to the Hepatitis A infection. [v] However, it is possible to purify the shellfish by placing them in clean water where they cleanse themselves and get rid of harmful bacteria and viruses. ii
To prevent the transmission of Hepatitis A individuals should always wash their hands thoroughly before eating, during food preparation and after going to the toilet and handling soiled linen (e.g. nappies)i. Besides that ensure that meats and seafood (shellfish in particular) are well cooked. Also, when travelling in countries with poor sanitation, always use bottled water and avoid eating food that has been prepared using contaminated water. Vaccinations are also available to prevent the transmission of the virus.v
Norwalk agents, also known as small, round structural viruses (SRSVs), are nonenveloped single stranded RNA viruses that belong to the family Caliciviridae. After an outbreak of gastrointestinal illness in 1972 in Norwalk, Ohio, Norwalk virus was identified, hence the name. The family name comes from the Latin word for cup or goblet due to the cuplike depressions in the surface of the virion seen through the electron microscope( [vi] )(i). Most caliciviruses are host specific thus infecting one animal species, therefore, it is difficult to study the human calicivirus as it is not possible to grow them in cell culture or infect experimental animals.vi
Raw seafood, mainly oysters, have been noted to be the main source of human infection and similar to the hepatitis A virus, the main reason behind this is that shellfish grow mainly in contaminated waters that attract a number of bacteria and viruses. Other foods that carry the virus include salads, sandwiches and fresh produce and this could be because of the use of contaminated water. i
The incubation period is a lot shorter, lasting between 18 to 48 hours than the Hepatitis A virus and the main symptoms are vomiting, diarrhea and abdominal cramps. Following an infection, immunity is established to the virus, however, unlike the hepatitis A virus, immunity against the Norwalk virus is only for a short period of time, one or few years. [vii]
Again, to prevent the spread of Norwalk virus, hands should be washed thoroughly with water and soap after toilet visits and before preparing and eating food. As well as that, raw fish should be cooked well and raw vegetables should be washed. Avoiding infected persons is also advised to minimize the risk of infection.
In conclusion, there is an incredibly large niche for invasion of microorganisms in food and water sources. The preventable methods discussed can be maintained with an aim to leave these vectors sterile or with minimal presence of microorganisms which would allow ingestion without infection. These various processes can be implemented to inhibit replication, growth or survival of pathogenic microorganisms to allow for safe consumption of food and water with a limited chance for infections to manifest. In most countries strict guidelines are put in place by government agencies and are precisely followed with food and water sterilisation to ensure the safety of populations and reduce risk of contaminated supplies reaching consumers.