pathology of infectious diseases of the lower respiratory ... of infectious diseases of the lower...

MINI-SYMPOSIUM: PATHOLOGY OF INFECTIOUS DISEASES e I

Pathology of infectiousdiseases of the lowerrespiratory systemMichael A den Bakker

AbstractSpecific texts on infectious diseases and those in textbooks on pulmonary

pathology commonly approach the subject of infection in the lung from an

etiological microbiological perspective, listing infectious agents by

taxonomy followed by associated pathology. However, in practice one is

faced with a specimen in which a reaction pattern is seen (usually of

inflammatory nature) which may be indicative of a specific type of infec-

tion. In these circumstances an approach leading from reaction pattern to

specific microbiological diagnosis is required. This approach will form the

basis of this review. General aspects are covered without exhaustive

discussion of specific microbiology. For more detailed discussion of

specific entities the reader is referred to the excellent textbooks on

pulmonary pathology and those on pathology of infectious diseases.

The reaction patterns described here are combinations of gross pathology

and cellular reactions. The breakdown of these patterns is somewhat arbi-

trary and artificial. In many cases the pattern will not be absolutely typical

and many overlapping features will be present in individual cases.

Keywords Differential diagnosis; histopathology; infection; pulmonary

Introduction

Infections of the lower respiratory tract are extremely common

and world-wide are the third leading cause of death, a statistic

which is unlikely to change significantly in the next decades. Of

all fatal infections, those of the lower respiratory tract are by far

the most common and including tuberculosis, account for over

half of all infectious deaths world-wide. Many respiratory infec-

tions are self-limited and do not require specific medical care.

More severe respiratory infections may come to medical atten-

tion and are commonly treated empirically with antibiotics, not

requiring a specific (microbiological) diagnosis. More extensive

analysis may be required in patients with impaired immunity or

for patients with a history of travel to destinations in which

unusual pathogens are prevalent. Histology is not a typical first-

line investigative modality employed in diagnosing respiratory

tract infection. However, in some circumstances histology may

provide the final diagnosis, because of superior sensitivity, or

may provide the fastest route to establish a diagnosis. Further-

more, on occasion infections form tumorous masses with a pre-

operative diagnosis of neoplasia which are resected and

submitted for histology.

Michael A den Bakker MD is a Consultant Histopathologist, Maasstad

Hospital, Rotterdam and Department of Pathology, Erasmus MC,

Rotterdam, The Netherlands. Conflicts of interest: none declared.

DIAGNOSTIC HISTOPATHOLOGY 19:2 44

Diagnosing infection through histology can be broadly

simplified to two aspects which are commonly combined in one

diagnostic process. First, a specific infectious agent may be

identified in a sample, or alternatively a specific tissue reaction

may be recognized which is highly suggestive of infection,

prompting further searching for a specific causative agent

(Table 1). Factors which must be taken into account when

attempting to diagnose infection through reaction patterns on

tissue samples are the integrity of the patients’ immune system,

the duration of disease before tissue samples were obtained,

travel history, co-morbidity and treatment that may already have

been given. The usual tissue reaction may be severely influenced

by these factors (Figure 1).

Pathological diagnosis

The standard haematoxylin and eosin (HE) stain is sufficient for

evaluating the tissue reaction pattern. However, identifying

specific micro-organisms (MOs) may require special techniques.

Histological special stains

Classical histochemical stains, such as Gram, ZiehleNeelsen

(ZN) and GrocotteGomori’s methenamine silver stain (GMS),

are performed routinely to identify MOs in tissue sections.

However, it should be borne in mind that many of these stains

were developed for use on direct preparations, such as secretions

and are less sensitive when applied to fixed tissue samples.

Moreover, the application of several of these stains, in particular

those using silver impregnation or decolourization steps,

requires optimal histotechnology and familiarity of the staining

reaction to be of diagnostic use.1

Immunohistochemistry

Antibodies to an increasing number of pathogens are available.

However, apart from a limited number of regularly encountered

MOs for which immunohistochemical identification may be

justified (for instance Cytomegalovirus, Pneumocystis jiroveci)

stocking a battery of rarely used reagents is not efficient.

Nucleic acid based techniques

In situ hybridization offers the advantage of direct visualization

of infectious organisms in a tissue background. Although

generally considered more sensitive than immunohistochem-

istry, routine use is fairly limited and few commercial probes are

available. PCR amplification of nucleic acid targets of MOs is, at

least theoretically, the most sensitive technique to detect an MOs.

The inherent sensitivity is also its main drawback; false positives

and contaminations are realistic pitfalls.

Reaction patterns

Acute granulocytic e neutrophilic (pyogenic, suppurative)

tissue reaction: this pattern is the most common infectious

disease reaction pattern encountered in the lung and may be

caused by various MOs; by far the most common of these are

bacteria (Table 2). Histology of pneumonia with an acute

neutrophilic tissue reaction, in which a viral cause is suspected

by serology or other means, will, with rare exceptions, have been

complicated by secondary bacterial infection. Lung tissue is

seldom submitted for histology to diagnose acute pneumonia.

� 2013 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.mpdhp.2013.01.010

Reaction patterns

Pattern Typical micro-organism Pattern in immunosuppressed

C Acute e neutrophilic (pyogenic)

inflammation

Bacteria Neutrophilic dysfunction, change to

granulomatous pattern

More frequently necrotizing� Bronchocentric e Lobular (bronchopneumonia) Bacteria

� Confluent e Lobar (confluent) Bacteria

� Abscess Bacteria

� Necrotizing Bacteria, virus, fungus More common

C Diffuse alveolar damage Virus No change

C Fibrosing (chronic pneumonia) Bacteria Unknown

C Nodular Fungi, mycobacteria,

parasites

Mycobacterial: less cavitation, severe deficiency:

no granuloma formation. Fungi: more frequently

necrotizing; may be reduced tissue reaction,

increased number of MOs

C Cystic Parasites No change

C Pseudotumour Mycobacteria Typical in severely immunosuppressed

C Specific cell type reaction pattern

� Cytopathic changes Virus More frequently necrotizing, otherwise similar

� Eosinophilic inflammation Parasites (Helminths) No change; increased severity

� Histiocytic Bacterial

� Lymphocytic inflammation Virus No change

C Others

Exudative (PCP) Pneumocystis jiroveci No e change; no reaction in severe

immunosuppression

Vascular proliferation (bacillary angiomatosis) Bartonella rochalimae Typical in immunosuppression

Non-reactive Typical in severe immunosuppression

Table 1


Recognition of this reaction pattern is straightforward with

a neutrophilic infiltrate (variably admixed with other cell types)

in the lung parenchyma. Morphological variations of this pattern

are well established and depend on the virulence of the causative

organism, the integrity of the host’s immunity and effects of

treatment. Classical morphological patterns of acute pneumonia

Figure 1 Necrotizing granulomatous inflammation in chronic granuloma-

tous disease (CGD). Pyogenic bacteria, which ordinarily induce a neutro-

philic inflammatory response, were cultured from this specimen. CGD is

caused by defects in granulocytes which fail to produce an oxidative

burst, important in killing pyogenic bacteria.


are lobular pneumonia (bronchopneumonia) and lobar (or

diffuse) pneumonia.

Bronchopneumonia is acute neutrophilic inflammation cen-

tred on the airway conductive components of the lung with

spillover into adjacent lung parenchyma. This type of pneumonia

commonly follows previous damage to the airway, which may

have been induced by any form of preceding mild airway damage

such as inflammation, immunosuppression or debilitation.

Primary acute inflammation of the conductive airways (bron-

chitis and bronchiolitis) soon extends to the parenchyma proper,

with a neutrophilic infiltrate filling alveoli, the defining feature of

this reaction pattern. Patchy consolidated firm lung tissue is the

macroscopic correlate; the parenchyma breaks up easily when

pressed, in contrast to spongy normal lung tissue, and pus may

be expelled from the tissue. Clearance of the offending MO leads

to restoration of the pulmonary architecture through a fibrotic

stage in which granulation tissue is formed within air spaces

(organizing pneumonia). Scarring may result with loss of lung

capacity. However, progression of the inflammation and infec-

tion may result in confluent areas of involved lung tissue, leading

to a pattern of lobar pneumonia.

Lobar pneumonia is a classical pneumonia pattern, which in

its complete form is currently seldom seen. The sequential,

macroscopically recognized stages (congestion, red hepatization,

grey hepatization and resolution), which were initially described

by Laennec, are modified by improved medical care and in

particular by antibiotic use. In contrast to bronchopneumonia,

lobar pneumonia produces diffuse acute inflammation which is



Common bacteria e acute granulocytic pattern

Common bacteria e granulocytic inflammation

MO Primary pattern, secondary

pattern; MO properties

Specific features

Streptococcus pneumonia (pneumococcus) Lob, Bpn; Gþ co

Non-pneumococcal streptococcus

(Streptococcus; incl. beta haemolytic

streptococcus)

Lob, BPn; Gþ co

Staphylococcus aureus Bp, Lo; Gþ co Abscess common

Klebsiella pneumoniae Bp, Lo; G� ba, Mucoid pneumonia

Haemophilus influenzae Bp, Lo; G� co-ba

Pseudomonas aeruginosa Bp, Lo; G� ba C Bacteraemic form. Immunocompromised

patients, pseudo-vasculitis pattern caused

by bacteria in vessel wall

C Non-bacteraemic form. Chronic disease

patients. May result in chronic (fibrosing)

pattern

Legionella pneumophila Bpn / Lob; G� ba May be associated with DAD pattern; may

have vasculitic component; may result in c

hronic (fibrosing) pattern; may be predominantly

lymphocytic

Mycoplasma pneumonia Bronchocentric necrotizing granulocytic

inflammation (bronchiolitis)

Nocardia spp. Weakly Gþ filamentous ba Abscess common; pathological diagnosis

may be sought; culture takes up to two weeks.

Silver stain most sensitive

Actinomyces Gþ filamentous ba Abscess; sulphur granules may be seen

SplendorieHoeppli phenomenon

Botryomycosis Pseudo-fungal appearance of

compact tangles of bacteria

which may be Gþ or G�

Nodular abscess-forming pattern

Lob ¼ lobar; BPn ¼ bronchopneumonia; Gþ ¼ Gram positive; G� ¼ Gram negative; co ¼ coccus; ba ¼ bacillus.

Table 2


only restricted by anatomical boundaries such as septae and

fissures and produces firm non-aerated, consolidated, lung

tissue. Consolidation results from massive influx of inflammatory

cells, fluid or fibrosis. In addition to the acute reaction pattern

resulting in consolidation, other infections which elicit some

other form of heavy diffuse inflammatory response, such as

histiocytic of fibrotic may cause consolidation.

In addition to the classical lobular and lobar acute pneumonia

patterns, produced by airborne spread of a pathogen, bacteria

may also reach the lungs through the bloodstream and cause less

clear-cut patterns of acute inflammation, which may be patchy or

confluent, but which are not related to the bronchiolar structures

or strictly bounded by anatomical structures.

Necrotizing pneumonia e necrotizing pneumonia as a reac-

tion pattern is an acute pneumonia pattern which overlaps with

the granulocytic reaction pattern and with diffuse alveolar

damage (DAD). The separation of these patterns is based on

complete loss of the underlying tissue architecture owing to

extensive destruction in necrotizing pneumonia, while in DAD

the connective tissue framework is still recognizable. A number

of the bacteria which are commonly identified as causative


agents of acute pneumonia similarly cause necrotizing pneu-

monia, especially Staphylococcus, Streptococcus pneumoniae,

non-pneumococcal Streptococcus, Klebsiella and Pseudomonas.

Viruses, most commonly Adenovirus, Herpes Simplex and Vari-

cella zoster virus cause a bronchocentric necrotizing pneumonia.

Histological recognition of a viral cause of necrotizing pneu-

monia may be accomplished by identification of viral cytopathic

features, such as inclusions. Fungi, parasites and protozoa rarely

cause necrotizing pneumonia except when individuals are

immunodeficient or are exposed to unusually high levels of

infective organisms.

Abscess e an abscess is a specific localized form of granulo-

cytic inflammation with discrete collections of neutrophils

forming pus in a newly formed cavity. Abscesses are typically

produced by bacterial infection and are often sequelae of diffuse

or lobar acute pneumonia. However, abscesses may form

without a background of pneumonia (primary abscess) through

aspiration, in which case enteric anaerobic bacteria are the

typical causative MOs. The differential diagnosis of an abscess

includes other infections which cause cavities or cysts as dis-

cussed below. Bacteria which commonly cause abscesses include




Staphylococcus, Klebsiella, Actinomyces, Nocardia and Rhodo-

coccus equi. The distinction between necrotizing pneumonia and

pneumonia producing an abscess is not always clear-cut.

Necrotizing inflammation may result in local suppuration

without actually forming a pus-filled cavity (Figure 2).

A particular form of abscess is Botryomycosis. As the name

suggests, the appearance is superficially similar to that of

a ‘fungus ball’ such as an Aspergilloma. However, the offending

MOs are bacterial, most commonly Staphylococcus aureus,

although many other Gram positive and Gram negative bacteria

have been described.2 Histology reveals dense colonies of

bacteria, referred to as granules or grains, surrounded by acutely

inflamed tissue. A SplendoreeHoeppli phenomenon consisting of

dense eosinophilic material deposited around the bacterial colo-

nies may be present.

Non-bacterial MOs may also cause abscesses. Viruses do not

cause typically result in an abscess unless complicated by

secondary bacterial infection. Fungal infections may result in

cavities through necrotizing inflammation but this is typically

histiocytic/granulomatous and these are not considered an abscess

(see below). Similarly, of the protozoa and parasites, Echinococcus

and Amoebiasis may result in cystic change. The abscess in

Amoebiasis is an extension of hepatic disease with similar histo-

logical features. Echinococcus may infect with destruction, but,

with little inflammation, it is not considered an abscess; this is

discussed further in the cystic/cavity reaction pattern.

Other organisms producing a neutrophilic response: viral

pneumonia may on occasion produce granulocytic inflammation.

Herpes simplex virus (HSV) may cause a bronchocentric necro-

tizing pneumonia, with a heavy neutrophilic infiltrate. The virus

may be recognized by its cytopathic effect with nuclear inclu-

sions in epithelial cells (see below). Other viruses that may cause

acute pneumonia with granulocytic infiltrates include adenovirus

(Figure 3), which is also typified by intranuclear inclusions,

facilitating its recognition.

Fungi do not generally produce granulocytic inflammation;

the typical response is histiocytic or granulomatous. However,

rarely mainly granulocytic inflammation is present, particularly if

massive exposure occurs. Neither protozoa nor helminths are

associated with granulocytic inflammation, although eosino-

philic pneumonia (see below) is a common reaction pattern in

lungs harbouring parasitic infestation.

Figure 2 Necrotizing pneumonia caused by Nocardia spp. An area of suppura

been likened to Chinese characters, are well demonstrated by sliver staining

identified by modified ZN stains.


Diffuse alveolar damage: as a reaction pattern in infectious

disease diffuse alveolar damage (DAD) may complicate any

severe pneumonia as a secondary pattern. DAD as a primary

pattern is most commonly, although not exclusively, seen in viral

pneumonia (Adenovirus, Herpes Virus, Influenza, CMV), as it

may be seen as a primary reaction in bacterial and fungal

infection with heavy exposure. Patients with compromised

immunity are more likely to develop DAD as a consequence of

viral infection. Parasites seldom cause DAD with the exception of

malaria and toxoplasmosis (in immunosuppressed hosts).3

Pathological identification of a virus as the cause of DAD is

only feasible for the small number of viruses that are typified by

specific cyto-morphological changes, such as multi-nucleation or

inclusions. In cases in which certain viral characteristics are not

seen the pattern is non-specific and other causes have to be

considered.

Fibrosing pattern e chronic pneumonia: recognizing this tissue

reaction as stemming from an infection may be difficult, partic-

ularly if the pattern is that of organizing pneumonia, which has

many potential etiologies. Prototypical chronic pneumonia is

a common finding in cystic fibrosis where repeated bouts of

infection eventually lead to scarring of the lung parenchyma.

MOs which have been implicated in chronic pneumonia include

common causes of acute pulmonary infections such as S. aureus,

Haemophilus influenzae, non-pneumococcal Streptococcus,

Pseudomonas aeruginosa, Klebsiella pneumoniae, Actinomyces,

Legionella and Histoplasma capsulatum (with and without cavity

formation).4,5 Ultimately, unchecked chronic pneumonia may

result in a so-called destroyed lung.

Identifying MOs in the setting of chronic infectious lung

disease by histology is unlikely to be successful; serology and

culture are the preferred methods. The differential diagnosis of

focal fibrosis in the lung will reflect some form of localized

damage to the lung parenchyma, which may include previous

destructive infection.

Nodular pattern: the nodular pattern often is the result of

a localized, frequently granulomatous or fibro-histiocytic, tissue

reaction. Ill-defined nodules resulting from foci of suppurative

inflammation are not considered to fall within this spectrum. The

commonest causes of a granulomatous nodular pattern are

tion almost producing an abscess (a). Filamentous Nocardia, which have

(b). These bacteria are poorly stained by Gram staining but may also be



Figure 3 Necrotizing adenovirus pneumonia. Destruction of lung archi-

tecture with multiple intranuclear inclusions.


Mycobacterium tuberculosis and mycobacteria other than Myco-

bacterium tuberculosis (MOTT) in immunocompetent hosts.

Fungi, in particular yeasts, similarly often result in

granulomatous-fibrotic nodules (Figure 4). Helminth infestation

in the lung may result in nodular fibrosis when either the

organism is not viable in the human host and a fibrotic reaction

ensues around the dead worm or a fibro-granulomatous reaction

to ova forms.

The classical tuberculous necrotizing granuloma is well

known as a pulmonary lesion. However, it should be borne in

mind that mycobacteria may cause other reaction patterns,

including micro-nodular (miliary tuberculosis), cavitating

lesions, fibrotic cavitating lesions and consolidation.6 However,

despite the variability of these macroscopic patterns the micro-

scopic tissue reaction is consistently granulomatous.

Identifying a nodular lesion in the lung as a granuloma, with

or without necrosis, immediately generates a list of differential

diagnoses. In addition to infection other causes of granulomatous

disease must be considered, including sarcoidosis, Wegener’s

disease, ChurgeStrauss syndrome and hypersensitivity pneu-

monia.7,8 Approximately one-quarter of granulomas encountered

Figure 4 A PET positive nodule in left lower lobe in which malignancy was susp

reaction pattern (b, GMS stain).


in the lung will have an identifiable infectious aetiology. Over

half of these cases are tuberculous and slightly fewer than half

are due to fungi.9,10 The probability of an infection causing

a granuloma is higher still if there is necrosis.10,11 Useful over-

views of the pathological approach to granulomatous lung

disease are given by Mukhopadhyay&Gal and El-Zammer&

Katzenstein.7,8

Mycobacteria e mycobacteria in tissue may be identified by

a number of histological staining techniques (ZN, fluorescent

Auramine/AuramineeRhodamine), PCR or immunohistochem-

istry.12 Increasing the chances of identifying mycobacteria can be

accomplished by combining techniques (stains, culture and PCR)

and increasing the amount of tissue that is examined, for

instance by examining additional sections.11

Pathological changes caused by MOTT in immunocompetent

hosts may be indistinguishable from those of M. tuberculosis.

In immunodeficiency, specifically those forms in which cell-

mediated immunity is involved, such as in AIDS or in

patients receiving immunosuppressive drugs, the ability to

form a granuloma is impaired and necrosis is absent. The

immune response fails to control the bacterial proliferation

resulting in engorged macrophages producing a tissue reaction

pattern described as “diffuse histiocytic” (see below). Granu-

lomatous inflammation, with or without necrosis, may also be

caused by fungal infection and parasites, or products thereof,

such as eggs, and rarely by bacteria, which are either restricted

to specific geographical regions or which occur in immunode-

ficient hosts.

Fungi e fungi in tissue are present in two forms, unicellular

yeasts and filamentous moulds (hyphal or mycelial) forms. In

contrast to yeasts of which many species may produce disease in

humans, the number of hyphal forms of fungi is limited. Asper-

gillus species and mucormycosis (zygomycetes) produce hyphae

while Candida species form pseudohyphae, in which the

segments retain a cytoplasmic connection. Yeasts in particular

often produce localized (granulomatous) disease although

numerous other tissue reaction patterns are described ranging

from bronchopneumonic to DAD. Localized fungal necrotizing

granulomas may be almost identical to those produced by

mycobacteria, referred to as ‘histoplasmoma’, ‘coccidioidoma’ or

‘cryptococcoma’ (Figure 4).

ected, was resected (a) and revealed Cryptococcus with a granulomatous



Fungi in pulmonary infection

Fungus

Cryptococcus neoformans Y HE, GMS, PAS World-wide

Histoplasma capsulatum Y GMS, PAS World-wide

Coccidioides immitis Y HE, GMS, PAS Southern USA,

Mexico

Pneumocystis jiroveci Y HE, GMS

Blastomyces dermatitidis Y GMS, PAS ZN Mainly USA,

Ohio and

Mississippi

river valley,

Great lakes,

Canada

Aspergillus spp M HE, GMS, PAS World-wide

Zygomycetes

(mucormycosis)

M HE, GMS World-wide

Y ¼ yeast; M ¼ mould; GMS ¼ GrocotteGomori’s methenamine silver stain.

Table 3


Fungal identification in tissue is facilitated by their propensity to

stain with silver impregnation techniques, such as Grocotte

Gomori’smethenamine silver (GMS) stain.Most fungiwill also stain

in periodic acid Schiff (PAS). However, dead fungimay not stain and

yeast forms (particularly Histoplasma)may be difficult to identify in

necrotic tissue. Inpracticebothstains shouldbeorderedwhen fungal

infection is suspected.Most fungi can also be identified in HE stains.

However, care must be taken not to overinterpret cytoplasmic frag-

ments and small foci of calcification as yeasts or parts of hyphae

(Figure 5). Accurate histological subtyping of fungi is not usually

possible.

When faced with nodular necrotizing inflammation, there are

no definite features, apart from identification of an organism,

which allow a certain diagnosis of infection. An exception may

be the identification of true necrotizing (leucocytoclastic)

vasculitis in the adjacent lung tissue which would be highly

suggestive of Wegener’s disease, or in the appropriate clinical

context, Churg-Strauss syndrome.8,11 To identify fungi in tissue

areas with necrosis should be specifically searched. Relatively

common fungal pathogens which may cause pulmonary disease

are listed in Table 3. Moulds (most commonly non-pigmented

Aspergillus and Mucor) may cause cavitating lesions and

consolidation but nodular granulomatous disease is not typical.

The so-called ‘aspergilloma’ or fungus ball is discussed below in

the section of cystic reaction patterns. For detailed accounts,

uncommon and newly described fungal infections the reader is

referred to a series of recent reviews.13e15

A nodular reaction pattern may also be caused by various

other infectious agents. Although rare, infection with Tropheryma

whippelii produces a micro-nodular peribronchial pattern (see

below) owing to accumulation of foamy histiocytes.16 A micro-

nodular miliary pattern is described as one of the patterns

produced by CMV.5 Healed pulmonary varicella lesions may also

cause rounded circumscribed nodules (coin lesions) (Figure 6).

Parasites, inparticular helminths or their products,mayproduce

nodules in the lung. In Dirofilariasis (Dirofilaria immitis e

Figure 5 Granuloma in which no micro-organisms were identified or

cultured in a pulmonary explant from a patient with cystic fibrosis.

Mycobacterial PCR was negative, small rounded calcifications (calcos-

pherulites) mimicking yeast forms are seen. These must be not be over-

interpreted as fungi. A GMS stain will not mark these structures.


heartworm) the dead nematode embolizes to the lung, causes

infarction and incites a localized fibro-inflammatory reaction,

producing a coin lesion.17 The eggs of Paragonimus (lung fluke),

a trematode acquired through ingestion of undercooked crusta-

ceans and those of schistosomal species, incite a fibro-

granulomatous response, often with an eosinophilic component

(see below).3,18,19

Cysts and cavities: necrotizing inflammation with suppuration,

including abscesses, necrotizing granulomatous disease and

post-infectious residual lesions, such as pneumatoceles could be

viewed as cystic lesions. In this review these entities are dis-

cussed in sections according to the primary inflammatory reac-

tion pattern (Table 4).

The sole entity resulting in cystic change as a primary tissue

reaction in this review is infection by larvae of the canine

Figure 6 Sclerotic nodular scar of varicella infection (incidental finding in

post-mortem lung).



Primary and secondary cystic and cavitating reactionpatterns

Primary reaction pattern/mechanism Micro-organism

Cystic change Echinococcus

Acute necrotizing inflammation Bacteria, virus,

rarely others

Abscess Bacteria, rarely others

Necrotizing granulomatous

inflammation

Mycobacteria, fungi

Colonization of pre-existing cavity Aspergilloma e

fungus ball

Suppuration with pseudomycotic

colonies of bacteria

Botryomycosis

Residual changes, e.g., pneumatocele Variable

Table 4


tapeworm Echinococcus granulosus. After ingestion of the egg

fluid-filled cysts develop with a characteristic three-layered

membrane of which the outer layer is host-derived and fibro-

inflammatory. On the innermost (germinal) layer protoscolices

and daughter cysts form. The middle (hyaline or laminated)

ectocyst layer is histologically distinctive, being composed of

acellular laminated tissue. Rupture of the cyst results in forma-

tion of secondary cysts.20 Establishing the diagnosis in uncom-

plicated cases by imaging is straightforward. Histological

diagnosis rests on identifying either the laminated layer,

daughter cysts or protoscolices (Figure 7).

Pseudo-tumorous: these masses which, in the first instance, do

not appear infectious are mostly spindle cell proliferations in

immunocompromised hosts. The most common cause is myco-

bacterial infection, hence the name mycobacterial spindle cell

pseudotumour. The tumours are composed of a spindle cell

proliferation most likely a combination of histiocytes and (myo)

fibroblasts. In addition to various subspecies of mycobacteria,

Figure 7 Cystic pattern in Echinococcus granulosus infection. An opened cyst

laminated layer which is internally acellular (b). Protoscolices (c) may be ident

and which are highlighted by the ZN stain.


other bacteria and fungi have on occasion been identified in

pulmonary pseudotumours (Figure 5).

Cell type specific reactions:

Viral cytopathic effect e many viruses may cause bronchio-

litis and pneumonia, in addition to the far more common,

generally self-limiting, infections of the upper respiratory tract.21

The common histology of viral infection of the lower respiratory

tract is that of non-specific interstitial lymphocytic inflammation

extending out from bronchioles into the alveolar parenchyma.

Severe necrotizing pneumonia may be seen in immunocompro-

mised individuals and at the extremes of age. Recognizing the

cytopathic effect permits accurate identification of the causative

virus in situations where the gross reaction pattern is not specific

(Figure 8) (Table 5) .There are no special stains that significantly

improve upon HE to identify virus induced cellular changes. In

equivocal cases immunohistochemistry may be employed.

However, its success in HSV and CMV infection is reported to be

limited if viral inclusions are not identified in HE stains.1 Care

must be taken not to overinterpret reactive changes in regener-

ating airway epithelium, in particular in type 2 pneumocytes

which may simulate viral changes (Figure 9).

Eosinophilic infiltration e a pulmonary inflammatory infil-

trate predominantly consisting of eosinophilic granulocytes,

which is often accompanied by peripheral blood eosinophilia, is

most indicative of parasitic infection/infestation. Fungal infec-

tion may likewise be accompanied by an eosinophilic infiltrate

where it may be the dominant feature as in allergic broncho-

pulmonary aspergillosis. Of the parasitic infections, helminthic

infection commonly causes marked eosinophilia, particularly if

the life cycle of the organism includes a lung passage, which is

common for nematode infection (roundworms, hookworms and

microfilariae).3,22 Establishing a diagnosis is accomplished by

either demonstration of (1) ova (in stool samples) for those

nematodes which pass through the lungs and which mature to

adult worms and shed eggs (Ascaris lumbricoides, Ancylostoma

duodenale, Necator americanus) or (2) demonstration of larval

forms in stool samples (Strongyloides stercoralis) or lung tissue

(A. lumbricoides); (3) solely identification of larvae in tissue for

(a) with several transparent daughter cysts is seen lying on the folded

ified in aspirates from hydatid cysts, the hooklets of which are diagnostic



Figure 8 Fatal measles infection in a 14-year old male. One of the six

respiratory viruses which produces specific cytopathic effects permitting

positive identification of viral pneumonia. A multinucleated cell, of

pneumocytic lineage, with bright red intranuclear and sparse cytoplasmic

inclusions is characteristic for measles pneumonia. The overall tissue

reaction pattern in symptomatic measles pneumonia is that of DAD.

Figure 9 Regenerating alveolar epithelium in the organizing stage of DAD.

Large type 2 pneumocytes with prominent nucleoli may be interpreted as

virally infected cells.


nematodes which do not reach full maturity in the human host in

tissue (Toxacara canis, Toxocara cati, agents of visceral larva

migrans). Extreme eosinophilia with markedly elevated IgE

levels is typical of microfilarial infection (Wuchereria bancroftii,

Brugia malayi, Gnathostoma spinigerum). Infection with these

nematodes is geographically restricted to the tropics, hence the

name tropical pulmonary eosinophilia (TPE). However, because

the infection may last many years, with modern day travel cases

may be encountered outside the endemic areas.

The nodular fibro-granulomatous reaction resulting from

infection by the trematodes Paragonimus23 and Schistosoma

(bilharzia) is commonly associated with blood and tissue

eosinophilia, although in bilharzia eosinophilia diminishes over

time. The differential diagnosis of lung tissue eosinophilia

includes non-infective disorders, including the idiopathic chronic

Viral cytopathic effects

Virus Cytopathic effect

Inclusions

Adenovirus Intranuclear, dense amphophilic

Herpes simplex

varicella (HSV)

Intranuclear, basophilic (“smudge cells”),

eosinophilic

Cytomegalovirus

(CMV)

Eosinophilic e basophilic intranuclear (large),

eosinophilic cytoplasmic (small)

Measles Eosinophilic e intranuclear and cytoplasmic

Parainfluenza Cytoplasmic small eosinophilic

Respiratory syncytial

virus

Cytoplasmic eosinophilic

Table 5


eosinophilic pneumonia, idiopathic acute eosinophilic pneu-

monia, ChurgeStrauss syndrome, drugs and toxins, and the

hypereosinophilic syndrome.

Histiocytic e in this reaction pattern histiocytes form the

predominant cell type, present as dense sheets or conglomerates.

A number of infections are characterized by this reaction. These

include rare infections with the bacteria T. whippelii and R. equi

and in immunosuppressed individuals, Mycobacterium avium-

intracellulare complex (MAI). Infection with T. whippelii, an

uncommon disease, may rarely produce pulmonary pathological

changes mimicking those in the more commonly infected intes-

tinal sites.16 In immunocompromised hosts infection by MAI

produces sheets of foamy histiocytes (Figure 10). The PAS

positive bacteria are acid-fast in contrast to T. whippelii.

Pulmonary R. equi infection is almost exclusively seen in

severely immunocompromised individuals.24 Histologically

a localized histiocytic reaction is seen with cavitation and

formation of neutrophilic abscesses. The histiocytic reaction may

Setting

Multi-nucleation

e Paediatric, malnourished, immunocompromised

e(Rare) Paediatric (neonates), immunosuppressed

e Immunosuppressed

þ Paediatric, malnourished, low

socio-economic status

þ Paediatric, elderly, immunosuppressed

þ Paediatric, immunocompromised;

bronchiolitis, interstitial pneumonitis



Figure 10 Mycobacterium avium-intracellulare (MAI) in a patient receiving

immunosuppressive medication. Pulmonary alveoli are stuffed with

a histiocytic infiltrate composed of macrophages with slightly granular

cytoplasm. In contrast to Mycobacterium tuberculosis MAI is identified in

the PAS stain (inset) as well as in ZN and other AFB stains.


develop into malakoplakia where MichaeliseGutmann bodies

are a tell-tale clue which may be identified by von Kossa staining.

Lymphocytic e a predominantly lymphocytic inflammatory

pattern will infrequently be encountered. Self-limiting viral

infections are thought to cause this reaction pattern.

Other patterns: included here are patterns which do not corre-

spond to any of the above patterns. Infection with P. jiroveci

(Pneumocystis jiroveci (PJ) pneumonia e PJP; formerly known as

Pneumocystis Carinii pneumoniaePCP) in immunocompromised

individuals may result in many different inflammatory reactions.

However, the most commonly encountered pattern is one referred

to as exudative. Slightly granular foamymaterial, often referred to

as ‘bubbly’ is deposited in the alveolar air spaces and is usually

combined with mild interstitial inflammation. The appearance is

similar to alveolar lipoproteinosis. The diagnosis is established by

recognition of the fungus (yeast form) which is difficult to identify

in HE stains but which is readily identified in silver stains (GMS).

Other yeastsmay be confusedwith PJ. However, the intra-alveolar

foamy material in the most common presentation is distinctive. In

other reactions patterns distinguishing PJ from other fungi may

more challenging. P. jiroveci in contrast to other yeasts has

condensations of the cyst wall which are distinctive in well-

performed silver stains. Additionally, a reliable antibody is avail-

able which will pick up PJ even in cases with low numbers of

organisms. Differentiating PJP from alveolar lipoproteinosis rests

on identification of the fungus; pulmonary oedema, which may be

considered in the differential diagnosis, lacks the granular-bubbly

aspect. Although the exudative pattern is the most frequently

encountered and well known pathology in PJP, a number of other

patterns have been described, with a backdrop of immunodefi-

ciency. Awareness of these patterns is required to correctly diag-

nose PJP.A granulomatous reaction appears particularly prevalent

and raises a number of differential diagnostic considerations.

A unique reaction pattern caused by infection of HIV þ indi-

viduals by the Bartonella species. A capillary proliferation is

induced with a mixed inflammatory infiltrate containing


neutrophils, simulating granulation tissue (bacillary angioma-

tosis). The MOs are present in clumps and are identified by silver

impregnation techniques (Dieterle, WarthineStarry). Conven-

tional stains, including Gram are less sensitive.‘

Conclusion

The vast majority of pulmonary infections will not require histo-

or cytopathological investigation to establish a diagnosis.

However, in those cases in which infection is suspected, in

particular if the differential diagnosis includes unusual patho-

gens, a tissue diagnosis may be the most effective and possibly

fastest way of establishing a diagnosis. In these difficult cases

a multidisciplinary approach is advocated, even before tissue is

obtained to maximize the diagnostic yield of harvested material.

Conventional histology remains the basic diagnostic modality

coupled with judicious use of special stains and immunohisto-

chemistry. However, in parallel with the increasing application of

molecular techniques in general and onco-pathology in partic-

ular, it is likely that tissue based molecular analysis, such as PCR

and FISH, will play an increasingly important role in the patho-

logical diagnosis of infection. The introduction and application of

molecular techniques for this purpose requires close collabora-

tion between microbiologists and diagnostic histopathologists.A

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