This is not an exhaustive account of all medically important viruses, but offers a very brief descrip- tion of some of the virus families and the viral infections that they result in which may be seen within the healthcare setting. Blood-borne viruses such as hepatitis B, hepatitis C and HIV, and other viral infections such as norovirus, are discussed in Chapters 23 (norovirus) and 24 (blood- borne viruses).
Adenoviridae
These are medium-sized, non-enveloped icosahedral viruses, containing a single piece of double- stranded DNA with 252 capsomeres (Collier et al., 2011b). There are thought to be more than 100 different serotypes, with 51 known to cause infections in humans (Ahmad et al., 2010b), and most individuals have been infected with several species of adenovirus by the time they reach adulthood. Adenoviruses cause acute febrile respiratory illnesses such as pneumonia and bronchiolitis, particularly in young children, as well as gastroenteritis and conjunctivitis. They spread readily by droplet infection, although faecal-oral transmission can also occur.
Herpesviridae
These are double-stranded DNA, enveloped, icosahedral viruses with 162 capsomeres (Ogilvie, 2004). Herpes viruses, of which there are in excess of 120 known species, include, amongst others, cytomegalovirus (see Fact Sheet 9.2 on the companion website), herpes simplex virus (HSV) 1 and 2, varicella zoster virus (VZV) and Epstein–Barr virus (EBV).
Herpes simplex virus: HSV can be shed in saliva and respiratory secretions, and primary infec- tion generally occurs in childhood from contact with an infected older person (e.g. a parent) (Collier et al., 2011c). It produces vesicles, seen as eruptions on the surface of the skin and mucous membranes, typically appearing on the lips as cold sores, and on the genital mucosa (genital herpes).
Picornaviridae
These are small enveloped icosahedrals, with single-stranded DNA and 32 capsomeres. This group consists of more than 70 enteroviruses (Burns, 2004), such as polioviruses, coxsackieviruses, echoviruses and rhinoviruses. Enteroviruses are found in the intestines and excreted in the faeces, causing a variety of illnesses. Infection with poliovirus can result in asymptomatic infection or a mild flu-like illness; the added involvement of the central nervous system gives rise to symptoms of meningitis, or paralysis. Polio is preventable only through immunisation (see http://www.who.int/
topics/poliomyelitis/en/). Infection with coxsackieviruses and echoviruses can result in meningitis, upper respiratory tract infections, gastroenteritis, pleurisy and pericarditis. Rhinoviruses are the culprits responsible for the ‘common cold’.
Hepadnaviridae
These are small enveloped icosahedrals with 180 capsomeres and double-stranded DNA. Hepatitis B virus (HBV) is discussed in detail in Chapter 24.
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Paramyxoviridae
Enveloped, helical, single-stranded RNA. The viruses in this group include the para-influenza virus, which is associated with croup and bronchiolitis in children, and minor upper respiratory tract infections, and the mumps and measles viruses (Collier et al., 2011d). The mumps virus is gener- ally an illness of childhood and causes pain and swelling of the parotid salivary glands. It is spread by droplet infection, with an incubation period of 14–18 days, with individuals considered to be infectious from 12 to 25 days after exposure. Peak incidence is in the winter and spring. Aseptic (viral) meningitis and orchitis are common complications, along with deafness and encephalitis.
Measles: Measles is a serious disease, responsible for one million deaths among children worldwide each year (http://www.hpa.org.uk/Topics/InfectiousDiseases/InfectionsAZ/Measles).
It has an incubation period of 10–12 days and begins with a flu-like prodromal illness, with a hacking cough and conjunctivitis. A widespread macropapular rash usually appears after four days, beginning on the forehead and spreading down the body. Although it is usually a childhood infection, it can affect any age group, and complications includes pneumonia, otitis media (ear infection) and post-measles encephalitis (Collier et al., 2011d). It became a notifiable disease in England and Wales in 1940 (Department of Health, 2011).
Measles–mumps–rubella (MMR) vaccination
MMR can be given at any age, but it is recommended that the first vaccination is given at around 13 months, and the second before the child begins at school to capture those children in whom the first vaccination did not generate a full immune response (http://www.hpa.org.uk/Topics/
InfectiousDiseases/InfectionsAZ/MMR/).
Controversy in recent years over a suspected link between the MMR vaccine and autism and Crohn’s disease has led to a decrease in MMR uptake and subsequent fears of a measles epidemic.
This has now become something of a reality with almost 2000 cases of measles reported in 2012 (reported by Public Health England and the DH as a ‘record high’), plus a further 587 cases in the first quarter of 2013. This increase, which has seen 108 affected individuals admitted to hospital during January – March 2013 (15 with complications) has been attributed to 10–16 years who were not vaccinated during the late 1990s onwards and who were therefore unprotected.
Information on the safety of the vaccine and the importance of vaccination is available on the immunisation pages of the Department of Health website (www.dh.gov.uk).
Fact Box 5.10 MMR vaccine
A single vaccine against measles became available in 1968, but it wasn’t until the combined MMR vaccine was introduced in 1988 that the number of epidemics occurring among children of school age and younger fell dramatically. In April 2013, Public Health England, NHS England and the DH announced a national MMR vaccination ‘catch-up’ campaign to decrease the pool of unprotected children, amid estimates that approximately one third of a million children are currently unvaccinated, and that there are a further one third of a million who are partly vaccinated.
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Togaviridae
The rubella virus (Rubivirus) belongs to the family of viruses known as Togaviridae, which are enveloped viruses with single-stranded DNA. Rubella (also known as German measles) is spread via respiratory secretions. Symptoms include enlargement of the lymph nodes behind the ears, in the neck and axillae, along with a pyrexia, malaise, conjunctivitis and a rash which affects the head and neck initially before spreading to the trunk (Collier et al., 2010e). Although it is generally a mild infection, it can have serious implications for pregnant women, particularly in the first 8–10 weeks of pregnancy when there is a significant risk of severe foetal abnormalities. Rubella is preventable through vaccination, which is given through the combined MMR vaccine.
Caliciviridae
These are spherical, non-enveloped, single-stranded RNA viruses, with 32 cup-shaped depressions on their capsid (Collier et al., 2011f). They are the main causative agents of viral diarrhoea and vomiting. Norovirus is by far the most notorious member of the caliciviruses and is discussed in detail in Chapter 23.
Coronaviridae
These are complex, enveloped, single-stranded RNA viruses, 60–220 nm in diameter. They have a
‘crown-like’ appearance (corona), which creates a halo effect around the virus. They are primarily responsible for causing upper respiratory tract infections, along with viral gastroenteritis, in humans, but they are also pathogens in animals and have been known to mutate and infect new species (McIntosh and Anderson, 2004).
SARS
As mentioned in Chapter 1, severe acute respiratory syndrome (SARS) was the first new disease threat of the twenty-first century, and was caused by a novel coronavirus. Early in November 2002, when the first cases of SARS emerged in Guangdong Province, China, there were reports that some of those affected had been exposed to live wild game animals in markets. Some of these animals were subsequently tested and were found to carry a virus that was very similar to the human SARS coronavirus, most notably the Himalayan palm civet cat (Paguma larvata) (73% of the market traders primarily trading in palm civet cats were found to be seropositive), the raccoon dog (Nyclereutes procyonoides) and the Chinese ferret badger (Melogale moschata) – all regarded as delicacies in southern China. The virus was therefore presumed to be zoonotic in origin, arising from the animal kingdom with the palm civet cat as the most important animal reservoir (Donnelly et al., 2004).
Fact Box 5.11 SARS – virus amplification
The live market setting, offering a diversity of animal species, is believed to have given the virus ample opportunity to amplify and over a period of time jump hosts, eventually infecting humans and resulting in human-to-human transmission. In September 2005, a study from China confirmed that 40% of horseshoe bats near Hong Kong were infected with a coronavirus similar to SARS, raising the question ‘Did the horseshoe bats infect the palm civet cats?’
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Orthomyxoviridae
These are 80–120 nm, helical, enveloped, single-stranded RNA viruses. This group consists of the influenza viruses. There are three serotypes which are based on antigenic differences and differ- ences in genetic makeup, structure, host susceptibility, epidemiology and clinical features. Influ- enza A viruses, while mostly prevalent among humans, also circulate among many mammalian and avian species, and it is this ability to jump the species barrier that makes this serotype the biggest threat in terms of the evolution of new influenza viruses with pandemic potential, as seen with the emergence of H1N1 (2009) ‘swine flu’, which originated in Mexico and spread globally (the pandemic was declared over by the World Health Organization in 2010). While influenza B can cause severe disease in ‘at-risk’ groups, and influenza C causes mild disease throughout the year, neither of these serotypes have been linked to previous pandemics, although that does not rule out the possibility of a new B or C strain evolving that has pandemic potential.
The virus is covered with surface projections or spikes, which consist of two types of antigen – hemoglutinin (HA) antigen and neuraminidase (NA) antigen. HA antigen plays an impor- tant role in initiating infection by enabling the virus to attach to receptor sites on the respiratory cell surface in the host. NA antigens assist fusion of the viral cell envelope with the surface of the host respiratory cell and aid the release of newly formed virus particles from infected cells, which go on to enter and replicate within other cells. Influenza viruses are able to mutate and swap genes, undergoing frequent changes in their surface HA and/or NA antigens which may be minor (antigenic drift) or major (antigenic shift).
Fact Box 5.12 Antigenic drift
Antigenic drift, caused by a single mutation in the viral RNA, occurs constantly among influ- enza A viruses and is responsible for annual flu epidemics, with new strains emerging every year. Generally, with antigenic drift there is some kind of serological relationship between the new and old HA and NA antigens, so although the virus is ‘drifting’ and changing subtly, it does not differ too drastically from previous strains. Some of the population will have immunity to the virus, and vaccination is a preventative measure in those with no immunity or who are in at-risk categories.
Fact Box 5.13 Antigenic shift
Major changes to actual gene segments on the surface antigens lead to the creation of a ‘new’
virus. Immunity will be virtually non-existent. Antigenic shift can occur either as a sudden
‘adaptive’ change during replication of a normal virus, or from genetic exchange between a human and an animal strain of influenza A. With genetic exchange, an animal is co-infected with both a human and an animal strain and serves as a ‘mixing vessel’ for the virus, allowing it to genetically ‘re-assort’ itself and create a new virus capable of causing disease in humans.
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Retroviridae
These are 80–100 nm, complex, enveloped, single-stranded RNA viruses. Retroviruses cause HIV (see Chapter 24 for more details).