Download Epidemiology of Pneumocystis infection in Human

Journal de Mycologie Médicale (2009) 19, 270—275
CONGRESS OF THE FRENCH SOCIETY OF MEDICAL MYCOLOGY — NOVEMBER 29, 2008,
PARIS/CONGRÈS SFMM — 29 NOVEMBRE 2008, PARIS
Epidemiology of Pneumocystis infection in Human§
Épidémiologie de l’infection par Pneumocystis chez l’homme
E.J. Calderón
Instituto de Biomedicina de Sevilla, CIBER de Epidemiologı́a y Salud Pública, Servicio de Medicina Interna,
Virgen del Rocı́o University Hospital, Avda Manuel Siurot s/n, 41013 Seville, Spain
Received 28 June 2009; accepted 29 August 2009
Available online 23 October 2009
KEYWORDS
Pneumocystis;
Epidemiology;
Transmission;
Colonization
MOTS CLÉS
Pneumocystis ;
Épidémiologie ;
Summary Pneumocystis jirovecii (P. jirovecii) is an atypical fungus that causes pneumonia in
immunosuppressed individuals and significant questions about its epidemiology and transmission
remain unanswered. It is widely accepted that animal sources of P. jirovecii can be excluded
because the Pneumocystis organisms that infect mammalian species are characterized by strong,
close host species specificity. Similarly, an environmental reservoir of infection has not been found.
Airborne transmission has been demonstrated in animal models and is assumed among humans.
Highly sensitive PCR-technologies have allowed the detection of low numbers of Pneumocystis
organisms in respiratory samples from colonized individuals who do not have Pneumocystis
pneumonia. Studies have shown that individuals who have underlying Human immunodeficiency
virus (HIV)-infection or other types of immunosuppression and those who are not immunosuppressed but have a chronic lung disease are often colonized by P. jirovecii. Further hypotheses claim
that these groups may play a role in person-to-person transmission and that they may serve as
reservoirs for future Pneumocystis infection in other susceptible individuals. On the other hand,
P. jirovecii DNA was recently identified by molecular techniques in 35% of foetal lung and 5% of
placenta samples from nonimmunodepressed women, who had a miscarriage, evidencing transplacental transmission in humans. Vertical transmission of P. jirovecii in humans could be an
additional route of transmission of this stenoxenic microorganism that would ensure the persistence
of Pneumocystis independent of environmental hazards. However, further studies are needed to
confirm the role of this transmission route in the epidemiology of Pneumocystis infection in humans.
# 2009 Elsevier Masson SAS. All rights reserved.
Résumé Pneumocystis jirovecii (P. jirovecii) est un champignon atypique responsable de
pneumonie chez les sujets immunodéprimés. Des questions significatives sur l’épidémiologie et
la transmission demeurent sans réponse. On accepte en général que les animaux ne constituent
pas une source d’infection pour l’homme car les Pneumocystis des divers mammifères présen-
§
Conférence présentée au symposium « Pneumocystis et pneumocystose 100 ans après », Présidence Patricia Roux et Eduardo Dei-Cas,
faculté de médecine Necker, Paris le 29 novembre 2008.
Adresse e-mail : [email protected].
1156-5233/$ — see front matter # 2009 Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.mycmed.2009.08.001
Epidemiology of Pneumocystis infection in Human
271
Transmission ;
Colonisation
tent une très étroite spécificité d’hôte. Similairement, il n’y a pas d’évidence d’un réservoir
environnemental. La transmission aérienne a été démontrée chez les modèles animaux et l’on
suppose qu’elle existe aussi dans les populations humaines. Des techniques PCR hautement
sensibles ont permis de détecter des faibles taux de Pneumocystis dans des échantillons
pulmonaires de sujets colonisés par le champignon mais sans pneumonie. Des recherches ont
montré que les patients VIH positifs ou immunodéprimés par d’autres causes ainsi que des sujets
non immunodéprimés mais affectés de maladie chronique pulmonaire peuvent être colonisés par
P. jirovecii. Il a été suggéré que ces groupes peuvent jouer un rôle dans la transmission
interhumaine en faisant partie du réservoir et en pouvant transmettre l’infection par voie
respiratoire aux sujets susceptibles. Par ailleurs, l’ADN de P. jirovecii a été identifié dans le
poumon de 35 % des fœtus et dans 5 % des placentas de mères non immunocompromises
développant des fausses couches, ce qui montre l’existence d’une transmission transplacentaire
de Pneumocystis chez l’homme. Ainsi, la transmission verticale de P. jirovecii pourrait représenter une voie additionnelle de transmission pour ce microorganisme sténoxène ; elle assurerait sa
persistance indépendamment des conditions environnementales. Des nouvelles recherches sont
nécessaires pour confirmer cette voie de transmission et clarifier son rôle dans l’épidémiologie
de l’infection humaine par Pneumocystis.
# 2009 Elsevier Masson SAS. Tous droits réservés.
Introduction
presence of P. jirovecii in patients with underlying chronic
diseases such as chronic obstructive pulmonary disease has
been suggested to be a comorbidity factor [7,28].
The strategies used by these organisms to grow and
survive in the context of an intact or debilitated host defenses are largely unknown and limited progress has been made
in understanding its life cycle due in large part to the absence
of a continuous in vitro culture system. Many questions about
P. jirovecii epidemiology and transmission remain unanswered. Until now, human is the only known reservoir host for
P. jirovecii and groups at risk for carriage probably represent
a major species-specific reservoir of infection, which would
allow P. jirovecii propagation [29].
Organisms that are known today as Pneumocystis carinii
(P. carinii) were thought to be life cycle stages of American
trypanosomes in 1909, but 3 years later were identified as a
different genus and species altogether. During the next two
decades, these organisms remained in the realm of medical
curiosities until they were linked to epidemics of plasmacellular interstitial pneumonia that plagued institutionalized
premature and malnourished infants in Europe around World
War II [8,10]. For many years, the entity named ‘‘P. carinii’’
was considered like a unique pulmonary pathogen able to
cause pneumonia in immunosuppresed hosts. Only for the
last years, marked genetic divergence was documented
among the Pneumocystis strains of different mammals and
now it is well-recognized that each form of Pneumocystis
possesses unique genetic characteristics and strict host specificity [1]. The explanation for such strict host specificity
has not been determined and is largely unprecedented in the
fungi. More recently, the form that infects humans has been
renamed Pneumocystis jirovecii (P. jirovecii) and P. carinii is
now the name used to describe specifically the organism that
infects rats [38,41].
The dramatic increase in the incidence of Pneumocystis
Pneumonia (PcP) with the emergence of Human immunodeficiency virus (HIV) pandemia made Pneumocytosis a major
medical and public health problem in the 1980s. During the
1990s, advances in the treatment of HIV reduced the frequency of PcP. Although at the beginning of the 21st century,
the incidence of frank pneumonia caused by these organisms
has decreased in developed countries, the prevalence of AIDSrelated PcP in developing countries remains high and poorly
controlled. Likewise, the number of patients who have an
altered immune system or who are receiving chronic immunosuppressive medications and are thus at a risk for PcP is
rapidly growing [16,27]. But at the beginning of the third
millennium, the interest in Pneumocystis infection goes
beyond PcP because a new spectrum of disease seems to
emerge in immunocompetent hosts and mounting evidence
points to new niches being exploited by these fungi. The
Detection methods
As P. jirovecii cannot be grown in culture from clinical
specimens, laboratory diagnosis of PcP has relied mainly
upon microscopic visualization with conventional cytochemical or immunofluorescence staining of organisms in respiratory samples. These methods are useful when the organism
burden is relatively high but they are insufficient for reliable
detection when there is a small parasite load [18]. Although
some investigators have detected Pneumocystis carriage by
using traditional staining methods, these methods are generally not adequate for detection of Pneumocystis colonization, and researchers have turned to more sensitive
molecular techniques [6,39].
Pneumocystis were first detected without the need to
visualize the organisms through microscopic examination by
the application of polymerase chain reaction (PCR) methods
[51]. Nested PCR protocols soon supplanted standard singlecycle amplification owing to their increased sensitivity. More
recently, real-time PCR has allowed levels of detection
similar to the nested procedures, but is advantageous as it
is performed in a one-step process [2,16]. Several straintyping methods of amplified PCR products have allowed insight
into epidemiology and transmission of P. jirovecii [4]. These
sensitive techniques have enable detection of very low levels
of P. jirovecii, not detectable by conventional histochemical
272
E.J. Calderón
staining, in respiratory samples from individuals in whom it
was not expected. It is thought that detection of P. jirovecii
or its DNA in respiratory samples from individuals without
signs or symptoms of pneumonia represents colonization
with the organism, and accumulating evidence suggests that
these groups of subjects may be important in the person to
person transmission of P. jirovecii. Furthermore, they may
be a major reservoir for future Pneumocystis infection in
other susceptible individuals [29,33].
Pneumocystis colonization in adult human
HIV-positive individuals are the most studied population of
Pneumocystis colonization, although estimates of the colonization rate vary considerably between reports from 10 to
68.8% (Table 1) [11,14,19,21,26,34,50]. The initial observations of a carrier state were obtained when PCR-based assays
were evaluated for the diagnosis of PcP in HIV-positive
patients. As there was an absence of a fulminant infection,
but the presence of P. jirovecii was shown by PCR-based
analyses, multiple laboratories independently arrived at the
conclusion that this large population might serve as carriers
[33].
In an interesting study, the length of carriage following an
episode of AIDS-related PcP ranged from 2 week to
16 months. Importantly, two individuals were noted to have
a change in genotype, which suggests that exogenous reexposure, as opposed to latent infection, is the source of
these organisms [50]. In a group of patients evaluated for PcP
at San Francisco General Hospital, 68.8% of patients without
PcP were colonized with P. jirovecii. No statistically significant differences were noted in demographic factors, CD4+ T
cells, HIV RNA level, history of PcP, or use of PcP prophylaxis
between carriers and non-carriers [14]. In a multicenter AIDS
cohort study, colonization was detected in 46% of autopsy
specimens and only smoking was associated with an increased risk of Pneumocystis colonization [26].
Pneumocystis colonization also occurs in non-HIV immunosuppressed patients [39]. Many non-HIV-positive people
with impaired immunity, such as organ transplant recipients,
patients with malignancies or those with connective tissue
diseases, can develop PcP [16]. In the same way, these
individuals are at risk of Pneumocystis colonization
(Table 1) [13,20,25,31]. More subtle changes in immune
function, such as those associated with pregnancy or chronic
lung diseases (Table 2), have been related with Pneumocystis
colonization as well [6,7,36,47,48].
There are some studies that have detected transient
Pneumocystis colonization in healthy subjects, as such
health care workers in contact with PcP patients [23,44].
There is also a study demonstrated the presence of Pneumocystis DNA among 20% adults that were otherwise healthy,
although these data require independent verification [22].
Pneumocystis colonization in children
Primary exposure to P. jirovecii is common among infants, as
is demonstrated by the increase in anti-Pneumocystis anti-
Table 1 Pneumocystis colonization in adults with immunosuppression.
Colonisation par Pneumocystis chez des adultes immunodéprimés.
Author, year [Reference]
Subjects Sample
no.
Leigh et al., 1993 [19]
70
Rabodonirina et al., 1997 [34]
80
Nevez et al., 1999 [31]
69
Matos et al., 2001 [21]
52
Helweg-Larsen et al.,
2002 [13]
20
Huang et al., 2003 [14]
32
Wakefield et al., 2003 [50]
Maskell et al., 2003 [20]
16
18
Morris et al., 2004 [26]
91
Davis et al., 2008 [11]
172
Mori et al., 2008 [25]
55
Induced sputum/BAL
Diagnostic
method
PCR
Population
HIV Infected at different
levels of CD4
BAL
Nested PCR
HIV infected with
respiratory symptoms
BAL
Heminested PCR Various causes of
immunosuppression
Induced sputum/
Nested PCR
HIV infected with
Oropharyngeal washes
respiratory symptoms
BAL, tracheal aspirate, Nested PCR
Patients with suspected
sputum
pneumonia and receiving
corticosteroids
Induced sputum/BAL
Nested PCR
HIV infected with respiratory
symptoms
BAL
Nested PCR
HIV infected
BAL
Nested PCR
Receiving prednisolone
(> 20 mg/day)
Autopsy lung
Nested PCR
HIV-infected men dying of
non-PcP
Induced sputum/BAL
Nested PCR
HIV infected with
non-Pneumocystis pneumonia
Induced sputum/BAL
PCR
Patients with rheunatoid
arthritis receiving
methotrexate, prednisolone
or tacrolimus
BAL: bronchoalveolar lavage; Modified of Morris A, et al. JID 2008;197:10—7 [29].
Colonization
(%)
10—40
25
14
28.8
60
68.8
43.8
44
46.2
68
10.9
Epidemiology of Pneumocystis infection in Human
273
Table 2 Pneumocystis colonization in subjects with different chronic pulmonary diseases from same geographical area.
Colonisation par Pneumocystis chez des patients porteurs d’affections pulmonaires chroniques.
Author, year
[Reference]
Subjects Sample
no.
Calderon et al.,
1996 [6]
50
Calderon et al.,
51
2007 [7]
Vidal et al., 2006 [48] 37
Vidal et al., 2006 [48] 16
Respaldiza et al.,
88
2005 [36]
Diagnostic method
Sputum
Disease
Microscopic demonstration Chronic bronchitis
with conventional and
immunofluorescence
staining
Sputum
Nested PCR
Chronic obstructive
pulmonary disease
Bronchoalveolar lavage
Nested PCR
Idiopathic interstitial
pneumonias
Bronchoalveolar lavage
Nested PCR
Sarcoidosis
Sputum or Oropharyngeal Nested PCR
Cystic fibrosis
washes
body titers during the first few years of life. Among healthy,
immunocompetent infants in Chile, the seroconversion rate
reached 85% by 20 months of age and Pneumocytis DNA was
found in 32% of infants studied [42]. In a study carried out
among Spanish children, the overall seroprevalence of antiPneumocystis antibody was 73%. Furthermore, there was
evidence of an age related increase in seroprevalence, from
52% at age 6 years to 66% at 10 years and to 80% at age
13 years [35]. These findings suggest the continuous exposition to the pathogen during the first and the second infancy
and it is concordant with an airborne transmission. In this
sense, the first molecular evidence of P. jirovecii transmission from colonized immunocompetent grandparents to their
infant granddaughter has been recently provided [37].
Pneumocystis primary infection has been presumed to be
an asymptomatic or mild nonspecific disease, however
recent reports indicate that it can present clinically as a
self-limiting upper or lower acute respiratory tract infection
[17,40,42]. In a study carried out in Santiago de Chile,
Pneumocystis DNA was detected by nested PCR in specimens
from 45 (51.7%) of 87 infants who died unexpectedly in the
community, but only 15% had pneumonia [46]. The significance of these findings is unclear. Some studies have suggested a likely association between primary Pneumocytis
infection and sudden infant death syndrome but now, this
association is not clear [43,45].
The early age of acquisition of Pneumocystis infection in
different mammals, including humans, led scientists to think
that vertical transmission could be as an additional route of
transmission of Pneumocystis. In humans, transplacental
transmission was first suggested by a few reports of PcP in
neonates published before the AIDS epidemic [3,32]. A few
years ago, a controversial case of vertical transmission of
P. jirovecii was reported: an infection in the lungs of a fetus
of an HIV-positive mother with PcP [30]. However, the study
did not identify the organisms as Pneumocystis, and a subsequent fluorescein-labeled monoclonal antibody test yielded negative results [15]. Therefore, vertical transmission of
P. jirovecii in humans has remained controversial. Only
recently, the first molecular evidence of P. jirovecii transplacental transmission in humans has been provided [24]. In
this study, tissues from human fetuses and placentas from
nonimmunodepressed women, who had a miscarriage, were
evaluated using the exquisitely sensitive detections method
Colonization (%)
10
55
37.8
18.8
28.8
of PCR targeting the mtLSU and DHPS genes. P. jirovecii DNA
was identified in 35% of foetal tissues and 5% of placenta
samples, showing that transplacental transmission can occur
in humans and that it is not a rare event [24].
Conclusions
Although still impaired by the lack of a reliable and reproducible continuous in vitro cultivation system, research into
the natural history of P. jirovecii has advanced owing to the
expansion of molecular-based techniques [33].
Even though early studies described the identification of
Pneumocystis DNA from the air surrounding apple orchards
and the surface of pond water, no Pneumocystis forms were
identified in environmental samples using microscopy, and it
is uncertain whether there is an ecological niche for Pneumocystis outside the mammalian hosts [9,12,49]. Animal
sources for P. jirovecii can be excluded because the Pneumocystis organisms that infect mammalian species are characterized by strong, close host species specificity [1]. So
far, human is the only known reservoir host for P. jirovecii
[29].
Highly sensitive PCR-technologies have allowed the
detection of low numbers of Pneumocystis organisms in
respiratory samples from colonized individuals who do not
have PcP. Studies have shown that individuals who have
underlying HIV-infection or other cause of immunosuppression and those who are not immunosuppressed but have
chronic lung disease may often be colonized by
P. jirovecii [29]. These groups at risk for carriage probably
represent a major species-specific reservoir of infection,
although transient Pneumocystis colonization has been also
identified in healthy subjects that could behave as a sort of
dynamic reservoir for future Pneumocystis infection in other
susceptible individuals [22,23,44].
Recent demonstration of P. jirovecii transplacental transmission may explain the accumulating evidence that the
primary infection is widely acquired very early in the life
and support the view that human infants are a major natural
reservoir for P. jirovecii, since they can remain colonized as
their immune response matures [5,24].
The accumulating evidence suggests that P. jirovecii is a
highly infectious organism with low virulence that takes
advantage of hosts as temporary reservoirs of infection.
274
Probably, any person may be colonized by Pneumocystis at
some stage in his life [10].
P. jirovecii is a highly adapted parasite that most likely
circulates by active airborne horizontal and vertical (transplacental or aerial) transmission mechanisms among human
populations developing mostly mild, though frequent, parasitism in the host lungs [1,12]. The length of carriage and the
possible detrimental effect of colonization in some populations as individuals with chronic lung diseases or infants
remains to be defined.
There is growing evidence that P. jirovecii colonization is
an important part of the organism’s life cycle and could have
significant clinical consequences. Further investigation is
needed to throw light on the role of Pneumocystis colonization in development and transmission of PcP and in other
human diseases.
Acknowledgments
I am indebted to Dr. Eduardo Dei-Cas for his assistance during
the preparation of the manuscript.
This work was developed in the framework of the project
‘‘Pneumocystis Pathogenomics: Unravelling the Colonization-to-Disease Shift’’ into a Coordination Action supported
by the European Commission (ERA-NET PathoGenoMics) and
the Spanish Ministry of Science and Innovation (FISPI080983).
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