Microbiology and epidemiology

Influenza is epidemic almost annually in the winter months in many parts of the world and at long intervals the disease occurs in pandemic form, notably in 1889–92, 1918–19, 1957–58, 1968 and 2009. It is estimated that in the pandemic that followed the world war of 1914–18, some 20–30 million people died from the disease in little more than a year, more than were killed in the war itself. Until 1933 it was widely believed that the disease was caused by the bacterium Haemophilus influenzae. The discovery in 1933 that the disease could be transmitted to ferrets by intranasal inoculation of filtered washings from the noses of patients established its viral nature.⁹

There are several types of influenza virus. All are originally bird viruses, some crossing from birds to species such as humans, pigs, horses and seals. Once established in the new species they undergo constant changes in their antigenic makeup, a feature that necessitates the constant manufacture of new vaccines. The strains involved in most serious human infections belong to types A and B, with the former much the more virulent. Outbreaks of infection with influenza A occur most years, with epidemics every 5–15 years. Influenza B also causes epidemics, but less frequently. Influenza C does not appear to cause epidemics. Successive epidemics appear to be decreasing in severity, possibily due to natural selection involving evolutionary changes that favour transmissibility over pathogenicity.¹⁰

Antigenic lability involves changes in the principal surface antigens of the virus, haemagglutinin and neuraminidase. Minor changes (‘antigenic drift’) are seen progressively from season to season. Major changes (‘antigenic shift’) due to acquisition of a new haemagglutinin occur periodically and are responsible for the emergence of new subtypes to which populations have little immunity and which therefore cause epidemics or pandemics. Influenza epidemics generally occur in the winter months and often start in countries south of the equator or in the east, giving northern and western countries time to prepare and distribute appropriate vaccines to those most at risk, notably the infirm. Influenza A viruses are named after their haemagglutinins (H1, H2, etc.), their neuraminidases (N1, N2, N3, etc.), and the place where and year when the strain was first identified. Thus, the 1968 pandemic virus was H3N2 influenza A/Hong Kong/68.

The haemagglutinin molecule enables the virus to attach to host cells prior to infecting them and neuraminidase permits new virus particles to bud from the cell membrane. Although there are considerable differences between the influenza viruses that infect different species, the haemagglutinin molecule’s lability occasionally permits its virus to switch from one host species to another, in which the disease is likely to spread in epidemic form. It appears that all the devastating influenza pandemics of the twentieth century, such as Spanish flu in 1918, Asian flu in 1957 and Hong Kong flu in 1968, were caused by viruses that made such a switch from birds to humans.¹¹

Recent genomic studies of the H1N1 virus responsible for the 1918 pandemic suggest that it was an avian virus that adapted to humans, rather than developing by a reassortment of avian and human viruses, as in the 1957 and 1968 pandemics.¹²

The switch from animals to humans in the ‘wet markets’ of the Far East is also relevant to the severe acute respiratory syndrome (SARS), described on page 163.

In 2004, the H5N1 strain of the influenza A virus devastated flocks of poultry in Asia and was responsible for the deaths of some humans from what was termed ‘Asian bird flu’.^13,14 This strain was not very infective to humans, possibly because the temperature of the human nose (32°C) is too cold for a virus whose natural host is the avian intestine (temperature 40°C).^15,16 However, when the virus succeeded in infecting humans it was particularly virulent for two reasons. First, it was resistant to the antiviral effects of human interferons and tumour necrosis factor-α, a property acquired by a simple mutation in the non-structural gene resulting in a change from aspartate to glutamate at position 92.¹⁷ Second, it preferentially targeted the alveolar rather than the tracheobronchial epithelium.¹⁸

The year 2009 saw swine influenza A/H1N1 virus switch to humans and exhibit the ability to spread from case to case, rapidly assuming pandemic proportions. Pigs are unusual in that their respiratory epithelium has receptors for both avian and human influenza viruses so that they can be infected with these viruses simultaneously, providing the ideal environment for genetic reassortment. This appears to have taken place as the 2009 swine influenza virus shows a novel reassortment of genes derived from swine, avian and human influenza viruses, a feature that is probably responsible for this virus being able to infect both pigs and humans. Unlike seasonal influenza, the disease made its first appearance in the northern hemisphere, apparently originating in Mexico, and its first impact was during the summer months. There was much mild disease but again, in contrast to seasonal influenza, severe respiratory failure was seen in young, previously healthy persons, as is often the case in influenza pandemics. This feature is not well explained but it is possible that the healthy mount a more vigorous immune response that is in some way detrimental to the host. Older people were less susceptible but more likely to die when infected.¹⁹ However, one year later it was apparent that the pandemic had not proved as severe as first thought.

Clinical features

The severity of influenza varies from one epidemic to another and from case to case. In its uncomplicated form it is relatively mild, with fever, coryza, headache and body aches as its main features, and recovery after a few days. When the viral infection is followed by invasion of the lungs by staphylococci, pneumococci, streptococci or Haemophilus influenzae, the condition assumes a much graver form and the fatality rate may rise alarmingly. Infection rates are highest among schoolchildren and decrease with age but death is commonest in infants, the elderly and those with underlying lung or heart disease, and is generally due to complications.

Primary influenzal pneumonia is generally rare but figured prominently in the 1918–19 pandemic. It carries a high case fatality rate and in the 1918–19 pandemic it was notable that mortality was highest in adults aged 25–34, possibly because older people had been exposed earlier to a similar strain of the virus.^20,21 Primary influenzal pneumonia may be fulminant, leading to the death of a previously healthy person within a few hours of the onset of symptoms.²²

Recent years have seen the development of effective antiviral agents such as the drugs oseltamivir (Tamiflu) and zanamivir (Relenza) which inhibit viral neuraminidase (in contrast to anti-influenza vaccines, which target viral haemagglutinin).

Pathology of uncomplicated influenza

The cytopathic effect of influenza virus is seen microscopically in characteristic degenerative changes in the epithelial cells of the bronchial and bronchiolar mucosa. These changes involve all cells of the surface epithelium and often the cells lining the bronchial glands: swelling of the cells, vacuolation of their cytoplasm and degeneration of the nucleus proceed to cell loss and frank necrosis (Fig. 5.1.4). Viral inclusions are not evident but the virus can be identified in tissue sections by immunocytochemistry and in situ hybridisation.^23,24 The deeper tissues show oedema, hyperaemia and a moderate to marked accumulation of lymphocytes; neutrophils are present but account for only a small proportion of the cellular infiltrate.

Figure 5.1.4 Influenza. This cytopathic virus has totally destroyed the bronchial epithelium, predisposing to bacterial superinfection.

Alveolar involvement is unusual but cases of fulminating influenzal viral pneumonia are occasionally encountered, particularly with the 1918 and more recent H5N1 and H1N1 strains.^14,24,25 Autopsy in such cases shows that the lungs are bulky and deeply congested.^14,22,24a Blood-stained, frothy fluid oozes freely from the cut surface. Areas of haemorrhage are present and may be extensive. The mucosa of the bronchial tree is very hyperaemic. Microscopically, the alveoli contain a fibrin-rich oedema fluid, which is often frankly haemorrhagic. Macrophages may be numerous in the exudate and hyaline membranes are often found lining the alveoli. T lymphocytes are often prominent in the alveolar interstitium while focal necrosis of alveolar walls and thrombosis of capillaries are conspicuous features in the parts most severely affected.²⁶

Postmortem examination of two victims of the H5N1 virus showed similar evidence of diffuse alveolar damage but it was also found that viral replication in the respiratory tract had resulted in particularly high levels of cytokines such as interferons and tumour necrosis factor-α, which, with the resultant haemophagocytic syndrome, was considered to be the chief cause of death.²⁷ Generally, however, H5N1 viral pneumonia shows no special features and it may be difficult to distinguish the changes brought about by the H5N1 virus from those caused by viruses such as that responsible for SARS or by factors such as acid aspiration or oxygen toxicity. More specific tests such as viral isolation, in situ hybridisation and reverse transcription-polymerase chain reaction (RT-PCR) are required to confirm H5N1 infection.

Fulminant uncomplicated influenzal pneumonia is often associated with changes in other parts of the body that indicate the occurrence of influenzal viraemia¹⁴: among these are haemorrhagic encephalomyelitis, which is an acute infective condition distinct from postinfluenzal encephalomyelopathy, rhabdomyolysis and placentitis.^28,29

In the healing phase of influenzal pneumonia there is conspicuous swelling of the alveolar lining cells, which proliferate and in places may virtually fill the lumen. The proliferation of alveolar lining cells may be so marked as to produce appearances somewhat resembling a neoplastic state. The changes reach their peak on about the third to fifth day of the disease and then regress, eventually subsiding completely. Also during the phase of recovery, regeneration of the bronchial epithelium may involve squamous metaplasia, but this soon gives place to normal ciliated pseudostratified respiratory tract epithelium.

Bacterial superinfection in influenza

Although pneumonia is the usual cause of death in epidemics of influenza, it is often a secondary bacterial pneumonia that is responsible. Neutrophilic exudates, organising pneumonia and bronchiolitis obliterans are then added to or replace the changes seen in uncomplicated cases.^{22,24,30–33} The impact an influenza epidemic has on respiratory death rates in general is shown in Figure 5.1.5.

Figure 5.1.5 Weekly deaths from respiratory disease in England and Wales 1987–92, showing the impact of an influenza epidemic in 1989/90 on deaths from other respiratory diseases.

(Data supplied by the Lung and Asthma Information Centre.)

Before the discovery of the influenza virus in 1933, the changes that are now known to be due to the viral infection were often confused with those of complicating infections, mainly caused by bacteria. Haemophilus influenzae was first isolated in the 1889–90 pandemic and so named because its discoverer, Pfeiffer, recovered it from a large proportion of cases and mistook it for the cause of influenza. In the 1918–19 pandemic H. influenzae was again found, along with Streptococcus pneumoniae and Staphylococcus aureus.³² With the advent of antibiotics, resistant strains of Staphylococcus aureus emerged and in the 1957–58 pandemic staphylococcal superinfection of the lungs was the major fatal complication of influenza. In the 1968 epidemic Streptococcus pneumoniae was the principal bacterial pathogen in the elderly and Staphylococcus aureus in the young.³⁰

The relationship of the staphylococcus and the influenza virus has been much studied and there is evidence that each promotes the growth of the other.³⁴ Thus, certain staphylococci have a protein in their cell wall that binds to the Fc region of immunoglobulin G. In the presence of anti-influenzal serum this protein enhances staphylococcal binding to cells infected by the influenza virus³⁵ and in this way the staphylococcus takes advantage of the host’s immune reaction to the influenza virus. In turn, the staphylococcus aids entry of the virus into the host’s cell. It does this by secreting a protease that activates a viral surface protein necessary for penetration of the host’s cells³⁶: normally such proteases are in short supply, so limiting the rate at which influenza virus can infect cells and reproduce. The relationship of influenza and Streptococcus pneumoniae infection has been studied in less detail but there is evidence of similar enhancement of bacterial adherence to tracheal epithelium following influenza infection.³⁷

Epidemiology

Respiratory syncytial virus was first isolated in 1956 from an outbreak of coryza in a colony of chimpanzees and its infectivity for Man was shown when one of the investigators of this epizootic illness contracted the disease. It has since been shown that the virus frequently infects the lower respiratory passages of Man, particularly the young.⁴² Specific neutralising antibodies indicative of an earlier infection are found in the serum of almost all children over the age of 5 years in Britain. Most children merely develop a cold but a few suffer from severe bronchiolitis, which in the developing world is an important cause of death in the very young.^42a,b Those that recover are prone to develop recurrent wheeze later in childhood.^42c The immunity infection conveys is incomplete and reinfections may occur throughout life.

Respiratory syncytial virus infection shows a marked seasonal pattern, producing annual epidemics each winter in temperate climates and in the hot rainy season in tropical countries. During an incubation period of 3–6 days the virus replicates in the upper respiratory tract, causing fever, cough and coryza. Spread from the upper to the lower respiratory tract may occur, with consequent bronchiolitis and pneumonia.

Particularly important is the bronchiolitis that respiratory syncytial virus is prone to cause in infants. The conductive airways of small infants are quite narrow and easily blocked by relatively small amounts of inflammatory exudate: because of this, fatal asphyxia is liable to follow respiratory syncytial virus infection. This is particularly the case in those who have airflow obstruction as a consequence of prematurity and its treatment. Several epidemics of acute bronchiolitis due to this virus have been described among infants. These outbreaks are often remarkably focal in distribution, affecting only a comparatively small area or community. Infants with bronchopulmonary dysplasia and those who have congenital heart disease or are immunocompromised are particularly at risk and units specialising in these underlying conditions have to guard against nosocomial spread of infection.^43–45 However, most children with respiratory syncytial virus infection have no predisposing factors and even previously healthy children may suffer fatal infection.^42,46

Pathogenesis

It is notable that infants under six months of age are particularly prone to respiratory syncytial virus infection. Although this is a period when the infant benefits from the presence of maternal antibodies, placental antibody transmission is selective, being better for immunoglobulin G than A. Breast-feeding protects against respiratory syncytial virus infection,⁴⁷ presumably by virtue of breast milk being rich in immunoglobulin A. It has been noted that infants immunized against the virus and subsequently infected naturally, suffer a more severe illness than those not so immunised,⁴⁸ suggesting that the damage is mediated immunologically.^49,50 This is supported by the finding that the typical bronchiolitis is characterised by scanty virus whereas in the rarer pneumonic form of the disease the virus is abundant. This is compatible with the bronchiolitis representing an adverse immune reaction and the pneumonia being the result of direct viral damage to the lungs.⁵¹ The cytokine profile suggests that the reaction involved in the bronchiolitis involves a predominantly type 2 response characterised by high interleukin-10/interleukin-12 and interleukin-4/γ-interferon ratios.⁵² Suspicion has fallen upon the formaldehyde inactivation of the vaccine creating reactive carbonyl groups on the antigen.^52a Passive immunisation, conferred by monthly injection, is free of the hazards induced by active immunisation and is recommended for high-risk babies.⁵³

Histopathology

The virus infects the bronchiolar epithelium and usually leads to its destruction.^54,55 Occasionally cytoplasmic inclusion bodies may be seen in degenerating bronchiolar epithelial cells or the virus may be demonstrated by immunocytochemistry.^1,55 Regeneration involves the proliferation of poorly differentiated cells which form a stratified non-ciliated epithelium. Occasionally micropolypoid epithelial protrusions are evident (Fig. 5.1.7).⁸ The bronchioles are occluded by plugs of mucus, fibrin and epithelial cell debris, and cuffed by an infiltrate of lymphocytes, plasma cells and histiocytes. Except in the immediate vicinity of the bronchioles, alveoli are generally not involved in the inflammatory process. If, however, infection is on a major scale there may be pneumonia with the general features of a viral pneumonia, as described above.⁴⁶ In severe immunodeficiency, respiratory syncytial virus may cause giant cell pneumonia,⁵⁶ a condition that is more often caused by measles virus.

Figure 5.1.7 Respiratory syncytial virus infection in which the patency of the bronchioles is compromised by epithelial proliferation forming micropolypoid intrusions into the lumen. The bronchiolar lumen is further narrowed by a neutrophil exudate in response to secondary bacterial infection.

Clinical features and epidemiology

Measles (rubeola) is highly infectious and most children are infected soon after their maternal antibodies have waned, the peak incidence being between 1 and 5 years of age. After an incubation period of 1–2 weeks the patient develops coryza, cough and fever. The subsequent development of small red spots with a white centre (Koplik’s spots) on the buccal mucosa followed by an erythematous maculopapular rash first involving the face and then the rest of the body facilitates the diagnosis. The infection resolves in about a week, following which the patient enjoys lifelong immunity.

In those parts of the world where measles has been prevalent for centuries the disease is almost invariably mild and, unless complicated by bacterial pneumonia, it has a very low mortality. In contrast, the mortality from measles may be appallingly high in lands to which the virus is newly introduced. When the disease was carried to Fiji from Australia in 1875, almost the whole population contracted it and a quarter of them succumbed. Similar outbreaks have occurred in more recent times, when the infection first reached Greenland for instance.

Measles has been regarded as one of the inevitable infections of childhood but with an effective safe vaccine now available this is no longer necessary (Fig. 5.1.8).⁵⁹ In the developed countries first and more recently in the developing world, immunisation against measles has been promoted vigorously, with spectacular success. Between 2000 and 2007 mortality from measles fell by 74% worldwide and by 89% in Africa, where it was formerly a major cause of death in childhood. The Americas were declared free of endemic measles transmission in 2002 and cases there now occur only as a result of its importation from other countries.^60,61 The disease has not yet been eradicated in the UK, partly because unfounded claims that the vaccine is responsible for autism led to declining vaccine uptake with consequent focal outbreaks of measles.⁶² Other European countries still experiencing large outbreaks include the Ukraine, Switzerland and Austria.

Figure 5.1.8 Annual measles notifications and vaccine coverage in England and Wales 1950–1999.

(Adapted by permission from BMJ Publishing Group Limited.⁵⁶)

Where measles is prevalent, the incidence is usually highest in the early spring, when droplet infections are particularly rife. The epidemics have a remarkably consistent biennial character (see Fig. 5.1.8) and the explanation of this has been the subject of several interesting hypotheses. The one most favoured envisages waning immunity over the succeeding 2 years in those children who had only a subclinical illness in the last epidemic. With the influx of two further entries into infant schools, a new population of susceptible children is formed that is liable to contract overt disease when the seasonal conditions are again favourable for the spread of the virus. In this way a fresh epidemic develops. Overt clinical disease, on the other hand, confers lifelong immunity.

If the lower respiratory tract is infected, the measles virus propagates in the epithelial cells of the main respiratory passages, leading to the destruction of many of the infected cells. In time there is recovery and multiplication of surviving cells, but at the height of the disease the natural defences of the lower respiratory tract are greatly compromised and secondary invading bacteria can successfully establish themselves in the lungs, causing bronchiolitis and pneumonia.

Pneumonia is a rare complication of measles in the western world but is common in malnourished African children, in whom it frequently proves fatal.⁶³ That pneumonic foci may develop in prosperous countries in the course of severe but non-fatal attacks of measles is shown by the demonstration in some such cases of patchy opacities on chest radiography; in almost all these cases the condition resolves rapidly. The cause is usually one of the common bacterial pathogens but it can be another virus taking advantage of the patient’s debility and impaired cellular immunity. Measles virus infection is characterised by both the development of a strong antiviral immune response and abnormalities of immune regulation: there is often a poor skin response to common antigens and helper/suppressor T-cell ratios may be low in both the blood and bronchoalveolar lavage, suggesting that cellular immunity is impaired.⁶⁴ Thus, measles has predisposed to both adenovirus and herpesvirus pneumonia.^65,66 Secondary pulmonary infection is responsible for about half the mortality in measles.⁶⁷ Other causes of death include measles pneumonia and measles encephalitis.

Measles may also be very severe when it affects immunodeficient patients, whether they are suffering from primary immunological defects, acquired diseases such as leukaemia or conditions which require treatment with cytotoxic or immunosuppressant drugs.⁶⁸ Such patients may have unpredictable responses to measles virus. They may have a rash but fail to produce antibodies, or they may fail to develop a rash although infected with the virus. Fatal measles pneumonia in a previously healthy adult is very rare.⁶⁹

Pathology of measles pneumonia^70,71

Death from measles pneumonia occurs typically about 2 weeks after the appearance of the rash. At necropsy, the lungs are heavy and of rubbery consistency, and their cut surface is pale pink. Close examination may show that the small bronchi are cuffed by a greyish zone. Extensive vascular thrombosis has been a feature of some cases.

Microscopically, there are degenerative changes in the epithelium of the bronchi and bronchioles, often accompanied by hyperplasia, particularly in the small airways. As in influenzal pneumonia, squamous metaplasia may occur and mitotic figures may be numerous. Measles pneumonia may take the form of diffuse alveolar damage with hyaline membrane formation (see Fig. 5.1.3), or, more characteristically, multinucleate giant cells may line the alveolar ducts and alveoli (Fig. 5.1.9). Electron microscopy shows that the giant cells are formed from type II alveolar epithelial cells.^71,72 The giant cells contain prominent cytoplasmic and nuclear viral inclusion bodies that are clearly evident in eosin-stained sections. Being epithelial, the pulmonary giant cells are quite different from the Warthin–Finkeldey giant cells that are found in lymphoid tissue throughout the body in measles, particularly in the immediately pre-exanthematous stage.

Figure 5.1.9 Measles giant cell pneumonia. There is a syncytial proliferation of type II pneumocytes containing eosinophilic cytoplasmic and nuclear viral inclusions. This response is typical of measles pneumonia but is occasionally encountered with other forms of viral pneumonia (see Fig. 5.1.6).

As well as the epithelial changes, there is a heavy accumulation of macrophages, lymphocytes and plasma cells in the alveolar walls. This cellular infiltrate extends into the connective tissue surrounding the bronchioles and small bronchi, accounting for the pale cuff that is seen around them on naked-eye examination. Neutrophils are not numerous unless there is a secondary bacterial infection. The appearances are closely comparable to those found in the lungs of dogs that have died of distemper (Carre’s disease), which is also caused by a paramyxovirus.

Differential diagnosis

Measles is the commonest cause of giant cell pneumonia but occasionally other viruses such as parainfluenza, respiratory syncytial and varicella-zoster viruses are responsible, especially in the immunocompromised (see Fig. 5.1.6).^38,56,73 The diagnosis is usually evident clinically or is made serologically but immunohistochemistry can be performed on tissue sections. Hard-metal disease is also characterised by the presence of alveolar epithelial polykaryons but these are more focal than those of measles, lack viral inclusion bodies and are not accompanied by such severe interstitial pneumonia (see Fig.7.1.25, p. 354).

Clinical features

The incubation period ranges from 1 to 10 days, following which there is a prodromal fever, cough and dyspnoea. Less common symptoms include headache, diarrhoea, dizziness, myalgia, chills, nausea, vomiting and rigor.⁹⁰ There is no apparent sex predilection and the age distribution is wide. Common laboratory features include lymphopenia involving both CD4 and CD8 lymphocytes, thrombocytopenia, prolonged thromboplastin time, elevated alanine transminase, lactate dehydrogenase and creatinine kinase. Positive viral recovery rates from urine, nasopharyngeal aspirate and stool specimen have been reported to be 42%, 68% and 97% respectively on day 14 of illness, whereas serological confirmation may take 28 days to reach a detection rate above 90%. However, quantitative measurement of blood SARS coronavirus RNA using real-time RT-PCR techniques has a detection rate of 80% as early as day 1 of hospital admission.⁹¹

Radiographic abnormalities include focal, multifocal or diffuse opacities. Computed tomography is more sensitive, sometimes showing extensive consolidation in patients with normal chest radiographs. However, the radiological features are not specific and need to be correlated with the clinical and histological findings.⁹²

Pathogenesis

Although pulmonary involvement is the dominant clinical manifestation, extrapulmonary features are common and the virus can often be recovered from faeces and urine, indicating that it is widely distributed in the body. The identification of a specific receptor for the virus is relevant to its tissue distribution. The receptor, a metallopeptidase known as angiotensin-converting enzyme 2, is expressed particularly strongly by pulmonary alveolar and small intestinal epithelia and vascular endothelia.^93–95

Pathology

The histology varies according to the duration of illness but the predominant pattern is diffuse alveolar damage.^96–100 Cases of less than 10 days’ duration show air space oedema and hyaline membranes whereas those of longer duration exhibit type II pneumocyte hyperplasia, squamous metaplasia, multinucleated giant cells and acute bronchopneumonia succeeded by intra-alveolar organisation.^26,96 The alveolar pneumocytes may also show striking cytomegaly with granular amphophilic cytoplasm (Fig. 5.1.11), that sometimes contains eosinophilic inclusions akin to the Mallory bodies of alcoholic hepatitis.^96,97 Less common features include haemophagocytosis and thrombosis.^97,99 The virus can be identified by RT-PCR in fresh or formalin-fixed, paraffin-embedded lung tissue. Electron microscopy may reveal the viral particles in the cytoplasm of epithelial cells.^97–99

Figure 5.1.11 Severe acute respiratory syndrome. (A) The features of early diffuse alveolar damage are seen, consisting of extravasation of red blood cells, desquamation, an acute and chronic interstitial inflammatory infiltrate and a few hyaline membranes. (B) Regenerating epithelial cells show nuclear atypia.

Treatment and prognosis

Treatment with corticosteroids, broad-spectrum antibiotics and antiviral agents has been beneficial.^98,102 Interferon-α may also have a role.¹⁰³ However, infection control is as important as pharmacological therapy in this disease.

In the acute phase, SARS is associated with considerable morbidity and mortality, with a global case fatality rate ranging from 7 to 27% (average about 11%). Adverse prognostic factors include advanced age, coexistent disease, high lactose dehydrogenase levels and high initial neutrophil counts. CT data on the extent of the disease are also useful in assessing prognosis.¹⁰⁴ Clinical follow-up of patients who recover has demonstrated residual abnormalities of varying degree, including abnormal lung function and patchy fibrosis.^105,106 Many survivors also experienced transient pituitary dysfunction.¹⁰⁷

Virology and predisposing causes

Herpes simplex virus typically causes mucocutaneous vesiculation, serotype 1 (HSV1) generally affecting the oronasal area and serotype 2 (HSV2) the genital mucosae. As with all herpesviruses, infection is lifelong; the virus remains dormant until immunity weakens, which may be triggered by a variety of factors. In these circumstances either serotype may involve the lower respiratory tract. Respiratory infection by HSV1 is more commonly encountered in adults, the infection spreading from the oropharynx, whereas neonatal respiratory infection is usually due to HSV2 as a component of generalised haematogenous disease.^110,111 Infection of the lower respiratory tract takes the form of tracheobronchitis or pneumonia. Herpes simplex tracheobronchitis is predisposed to by damage to the respiratory epithelium, especially factors that lead to squamous metaplasia, such as endotracheal intubation and burns.^112–116 Risk factors for herpes simplex pneumonia include transplantation,¹¹⁷ cytotoxic chemotherapy and human immunodeficiency virus (HIV) infection¹¹⁸: herpes simplex pneumonia is rare in the immunocompetent.¹¹⁹

Pathology

In herpes simplex tracheobronchitis there is extensive mucosal ulceration and pseudomembrane formation. Viral inclusions are most prominent at the periphery of the ulcers and may be identified in exfoliated cells (Fig. 5.1.12). If they are not well developed an immunostain may establish the diagnosis. Long-standing airway infection leads to luminal narrowing and obstructive features. In the lungs the changes are very similar to those of adenovirus pneumonia, including the presence of Cowdry type B ground-glass intranuclear viral inclusions, although these are more eosinophilic than those of adenovirus. Both adenovirus and herpes simplex pneumonia bear a superficial resemblance to bacterial bronchopneumonia but the similarity is in the distribution of the lesions rather than their character: the bronchioles and centriacinar alveoli are mainly affected but the lesions are characterised by necrosis and the accumulation of nuclear debris rather than exudation of neutrophils. Occasionally herpes simplex infection takes the form of a focal necrotising pneumonia more typical of varicella infection (see below). Alternatively there may be arterial involvement in herpes simplex pneumonia with a necrotising vasculitis affecting small and medium-sized pulmonary arteries.¹²⁰ Sometimes the pneumonia is diffuse: it is suggested that focal disease represents extension of oral mucocutaneous herpesvirus infection down the tracheobronchial tree into the lung whereas diffuse pneumonia is the result of haematogenous spread.¹⁰⁸

Figure 5.1.12 Herpes simplex virus. Bronchial brushings from an ulcer in the lower trachea show multinucleate epithelial cells with glassy nuclear features.

From a patient with oral herpes who had started steroid therapy for asthma.

Virology and epidemiology

Cytomegalovirus is the largest of the herpesviruses and is widespread in most communities, persisting for life, like all herpesviruses. It is transmitted in saliva and blood and by sexual contact and organ transplantation. Seropositivity, taken to indicate carriage of the virus, steadily increases with age. The prevalence of seropositivity in adults is generally over 50% and approaches 100% in homosexual men. However, carriage of the virus does not necessarily equate with disease. The immunocompetent host is unlikely to experience any recognisable clinical effects of cytomegalovirus infection.

Symptomatic cytomegalovirus infection is seen in newborn children infected before birth by virus carried by their mother, and in adults who have undergone organ transplantation or have been infected with HIV. In the newborn the disease presents as an acute fatal infection with jaundice and leukoerythroblastic anaemia.

Cytomegalovirus is a serious pathogen in transplantation recipients,¹²¹ possibly because the virus replicates best in cells that are activated, as in a transplanted organ. The risk is greatest with bone marrow transplantation, intermediate with heart, lung and liver transplantation and lowest with renal transplantation. However, donor and recipient matching for cytomegalovirus status has reduced the incidence of transmission from the donor. Before the introduction of this policy, fatal cytomegalovirus pneumonia or systemic infection was common. Today reactivation of latent infection is a more common problem but it is important to distinguish the mere presence of viral inclusions from pneumonitis. When a lymphoid infiltrate accompanies the viral inclusions it is also important to distinguish an infective pneumonitis from lung allograft rejection: generally, the infiltrate of cytomegalovirus pneumonia lacks the perivascular lymphocyte distribution seen in rejection. As well as causing a pneumonitis that has to be distinguished from rejection, cytomegalovirus may be involved in chronic lung rejection.¹²² It has been speculated that cytomegalovirus could promote allograft rejection by stimulating the production of proinflammatory cytokines or increasing the expression of major histocompatibility complex molecules.

The position of cytomegalovirus in regard to pulmonary disease in acquired immunodeficiency syndrome (AIDS) can also be difficult to determine. Cytomegalovirus inclusions are frequently encountered in AIDS but it is often difficult to determine whether pathological changes are due to the virus or to accompanying bacterial or Pneumocystis infection. Only occasionally is cytomegalovirus the only pathogen identified in severe pneumonia in AIDS patients.¹²³ Some investigators claim that cytomegalovirus contributes to the high mortality from pneumonia in AIDS patients¹²⁴ while others view it merely as a bystander rather than the primary pathogen in these patients.¹²⁵ Sometimes, replication of the virus is unaccompanied by any significant degree of pulmonary inflammation or damage,¹²⁶ indicating a poor host response, which, as in other viral infections, largely involves T lymphocytes, cells that are particularly defective in AIDS. The differing roles of cytomegalovirus in AIDS and transplant recipients have led to the view that the pathological changes are not a direct effect of the virus but an immunopathological condition attributable to the T-cell response to the virus.¹²⁷

Pathological features

The pneumonia may be unilateral or bilateral, and generally involves the lower lobes; advanced lesions may appear as reddish purple nodular areas. Two patterns of pulmonary involvement have been described in bone marrow transplant recipients: a fulminant systemic infection characterised by a miliary pattern of disease and a more insidious disease with a more diffuse distribution in the lungs.¹²¹

Histologically, there is a chronic interstitial pneumonitis and some of the alveolar epithelial cells are enlarged and contain characteristic inclusions. These measure up to as much as 10 µm in diameter and are surrounded by a clear zone inside the nuclear membrane (Fig. 5.1.13). These Cowdry type A intranuclear inclusions have been likened to an owl’s eyes. The inclusions represent clumped chromatin and the clear zone the virus. Cytoplasmic inclusions up to 2 µm in diameter are often also present. Severe cases may show a necrotising pneumonia or tracheobronchitis without the inclusions being well developed, in which case immunocytochemistry or in situ hybridisation may be used to advantage as these techniques show that many more cells are infected than those containing the characteristic inclusions (see Fig. 5.1.13D).^6,7,128 Diffuse alveolar damage is a further pattern of disease that is occasionally seen in cytomegalovirus pneumonia.

Figure 5.1.13 Cytomegalovirus pneumonia. (A) There is a prominent nuclear inclusion in the centre of the field. (B) Electron micrograph of an alveolar epithelial cell infected by cytomegalovirus. Numerous viral particles are evident in both nucleus (above) and cytoplasm (below). As the viral particles leave the nucleus and enter the cytoplasm they acquire a coating derived from the nuclear envelope and consequently enlarge. (C) Low-power electron micrograph of the cell seen in (B), showing that it is greatly enlarged compared with its neighbours. Coated viral particles are evident in the cytoplasm but uncoated particles in the nucleus are too small to be recognised at this magnification. However, characteristic central clumping of the chromatin is evident.

(B and C courtesy of Miss A Dewar, Brompton, UK.)

(D) Immunocytochemistry shows abundant virus in the cytoplasm as well as the nucleus (immunoperoxidase stain).

HIV and AIDS