Cryptosporidium

Letters in Applied Microbiology ISSN 0266-8254
UNDER THE MICROSCOPE
Cryptosporidium
O. Sunnotel1, C.J. Lowery1, J.E. Moore2, J.S.G. Dooley1, L. Xiao3, B.C. Millar2,
P.J. Rooney2 and W.J. Snelling1
1 Centre for Molecular Biosciences, School of Biomedical Sciences, University of Ulster, Coleraine, Co., Londonderry, UK
2 Department of Bacteriology, Northern Ireland Public Health Laboratory, Belfast City Hospital, Belfast, UK
3 Division of Parasitic Diseases, National Centres for Infectious Diseases, Centres for Disease Control and Prevention, Chamblee, GA, USA
Keywords
Cryptosporidium, detection and public health,
pathogenesis, transmission.
Correspondence
William J. Snelling, Centre for Molecular
Biosciences, School of Biomedical Sciences,
University of Ulster, Coleraine, Co.,
Londonderry BT52 1SA, UK. E-mail:
[email protected]
Abstract
This review discusses characteristics of the genus Cryptosporidium and addresses
the pathogenesis, reservoirs, public health significance and current applications
for the detection and typing of this important pathogen. By increasing knowledge in key areas of Cryptosporidium research such as aetiology, epidemiology,
transmission and host interactions, the numbers of cases of human cryptosporidiosis should be reduced.
2006/0213: received 15 February 2006,
revised 9 March 2006 and accepted 14 March
2006
doi:10.1111/j.1472-765X.2006.01936.x
Introduction
Almost 75 years after it was discovered in mice by Tyzzer
in 1907, interest in studying the intracellular protozoan
parasite Cryptosporidium increased greatly during the
1980s. This was the result of increasing veterinary attention and the recognition of the important role of this
parasite for humans with the newly described acquired
immunodeficiency syndrome (AIDS) (Casemore et al.
1985). Cryptosporidium is currently placed in the family
Cryptosporiidae, within the phylum Apicomplexa (Arrowood 1997). Members of Cryptosporiidae have the common feature of four naked sporozoites (Fig. 1a), which
are contained within a thick walled oocyst and do not
contain sporocysts (Blackman and Bannister 2001). There
are currently 16 recognized species of Cryptosporidium,
which have been isolated from a large variety of hosts in
all five groups of vertebrates, including humans (Table 1)
(Xiao et al. 2004a). Recent studies have shown that wildlife infected with host-adapted Cryptosporidium spp.
appear to have little potential of infecting humans and
domestic animals (Xiao et al. 2004b; Zhou et al. 2004a,
2004b). Cryptosporidiosis is responsible for significant
neonatal morbidity in farmed livestock and causes weight
loss and growth retardation, leading to large economic
losses (McDonald 2000). The zoonotic Cryptosporidium
parvum and anthroponotic Cryptosporidium hominis parasites are the major cause of human cryptosporidiosis,
although other species including Cryptosporidium meleagridis, Cryptosporidium felis, Cryptosporidium canis, Cryptosporidium suis, Cryptosporidium muris and two cervine
genotypes of Cryptosporidium have been associated with
human gastroenteritis (Table 1) (Xiao and Ryan 2004;
Caccio et al. 2005). To what extent the recently recognized C. hominis species differs from C. parvum regarding
invasion mechanisms and pathogenesis remains unclear
(Hashim et al. 2006).
Public health significance
Cryptosporidium causes a significant health risk to both
humans and livestock (Fayer et al. 1997). It causes selflimited watery diarrhoea in immunocompetent subjects,
but has far more devastating effects on immunocompromised patients and in some cases can be life-threatening
due to dehydration caused by chronic diarrhoea (Caccio
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Journal compilation ª 2006 The Society for Applied Microbiology, Letters in Applied Microbiology 43 (2006) 7–16
7
Cryptosporidium and public health
(A)
O. Sunnotel et al.
(B)
Figure 1 The structure of Cryptosporidium and its life-cycle stages. (A) Longitudinal section through a sporozoite showing the distribution of
internal organelles (Tetley et al. 1998). The apical complex containing the micronemes (mn) and rhoptry (r) was at the tapering anterior of the cell
(labelled ac) with the nucleus (n) and adjacent crystalloid bodies (cb) at the posterior, more rounded end. Dense granules (dg) occurred predominantly in the centre portion of the cell. The putative plastid-like organelle (p) and extended nuclear membrane region (nme) are also indicated.
Scale bar 0 ± 0Æ5 lm. (B) The stages of Cryptosporidium life-cycle (Smith et al. 2005). Upon ingestion by the host, sporozoites are released and
adhere directly intestinal epithelial cells of the host. Cell invasion by sporozoite is followed by intracellular development to trophozoite. Trophozoite undergo schizogony to form schizonts. Asexual replication occurs by re-infection of merozoites, rey type I schizont. Development of type II
from type I schizont is the initial step of the asexual reproductive cycle. Type II merozoites are released and re-infect neighbouring cells were they
develop into microgametocytes (male) or macrogametocytes (female). The macrogametocyte is fertilized by released microgametes and matures
into a zygote, which undergoes further development into an oocyst. Two types of oocysts are released: (A) thick-walled oocysts, which are
excreted in the faeces, or (B) thin walled oocysts for endogenous re-infection (auto-infection).
2005; Chen et al. 2005). Globally, Cryptosporidium is
responsible for the majority of gastrointestinal parasitic
infections (Doganci et al. 2002). Over the last two decades increasing numbers of cryptosporidiosis (in particular water related) outbreaks have been recorded in
developed countries (Craun et al. 2005). Cryptosporidium
is a waterborne pathogen and can survive for months in a
8
latent form outside hosts, as its oocysts retain their infectivity for several months in both salt and fresh water
(Fayer et al. 1998). In 1993, the largest Cryptosporidium
outbreak was registered in Milwaukee, Wisconsin, USA
where 403 000 people were infected through contaminated drinking water (MacKenzie et al. 1994). This outbreak was caused by C. hominis (Peng et al. 1997), and
ª 2006 The Authors
Journal compilation ª 2006 The Society for Applied Microbiology, Letters in Applied Microbiology 43 (2006) 7–16
O. Sunnotel et al.
Table 1 Recorded species of Cryptosporidium, their size and major hosts (Egyed
et al. 2003; Xiao et al. 2004a; Smith et al.
2005)
Cryptosporidium and public health
Cryptosporidium species
C. andersoni
C. baileyi
Size (lm)
5Æ5 · 7Æ4
4Æ6 · 6Æ2
Host
Location
Bovines
Birds
Abomasum
Cloaca, bursa,
respiratory tract
Small intestine
Small intestine
Proventriculus
Small intestine
Intestine
Stomach
Stomach
Intestine
Intestinal and
cloacal mucosa
Stomach
Small intestine
Small intestine
Small intestine
Intestine
C.
C.
C.
C.
C.
C.
C.
C.
C.
canis
felis
galli
hominis
meleagridis
molnari
muris
parvum
saurophilum
5Æ0
4Æ5
8Æ0–8Æ5
4Æ5
4Æ5–5Æ0
4Æ7
5Æ6
4Æ5
4Æ2–5Æ2
·
·
·
·
·
·
·
·
·
4Æ7
5Æ0
6Æ2–6Æ4
5Æ5
4Æ6–5Æ2
4Æ5
7Æ4
5Æ5
4Æ4–5Æ6
Canids, human
Felids, human
Birds
Human
Birds, human
Fish
Rodents, human
Ruminants, human
Lizards, snake
C.
C.
C.
C.
C.
serpentis
suis
wrairi
bovis
scophithalmi
4Æ8–5Æ6
5Æ1
4Æ0–5Æ0
4Æ2–4Æ8
3Æ0–4Æ7
·
·
·
·
·
5Æ6–6Æ6
4Æ4
4Æ8–5Æ6
4Æ8–5Æ4
3Æ7–5Æ0
Snakes, lizards
Pigs, human
Guinea pigs
Ruminants
Fish
the total cost of outbreak-associated illness was estimated
at $96Æ2 million; $31Æ7 million in medical costs and
$64Æ6 million in productivity losses (Corso et al. 2003).
Cryptosporidium has also been associated with treated
water in swimming and wading pools (Craun et al. 2005).
More recently, foodstuffs have been identified as an
emerging aetiological source of Cryptosporidium (Millar
et al. 2002). The usage of surface water for irrigation can
indirectly cause human infection via the consumption of
contaminated fresh produce (Robertson and Gjerde
2001a, 2001b; Armon et al. 2002; Thurston-Enriquez et al.
2002). There have a limited number of studies performed
examining the occurrence of the parasite on vegetables,
including lettuce. Occurrence data is very dependent on
the laboratory isolation technique employed and has ranged from 1Æ2% to 14Æ5% (Table 2). However, numbers of
oocysts on horticultural produce has been consistently
been reported as being low. In turn, the low numbers of
oocysts on leaves of such plants presents a diagnostic
dilemma, in its successful recovery and subsequent labor-
atory detection. Several laboratory detection systems have
been proposed for the detection of this parasite in lettuce.
Of these, two methods, namely the method of Robertson
and Gjerde (2001b) and Ripabelli et al. (2004) have been
employed with increased efficiency, where the method of
Ripabelli et al. (2004) has been the first published method
to combine a nested-PCR approach into the detection
protocol. More recently, molecular surveillance studies of
lettuce at the Northern Ireland Public Health Laboratory
has shown the occurrence of this parasite in lettuce to be
4% (M. Shige Matsu, unpublished data).
Shellfish have the ability to filter large amounts of
water and concentrate oocysts within their gills (GomezCouso et al. 2006). Thus, the consumption of raw or
undercooked shellfish could be a health risk. Unlike other
food pathogens such as Salmonella, Cryptosporidium does
not multiply in foods. However, Cryptosporidium can
retain viability, and therefore infectivity, under moist and
cool conditions for several months (Dawson et al. 2004).
Cryptosporidium oocysts lose infectivity when undergoing
Table 2 Summary of previous reports on the occurrence of Cryptosporidium in lettuce
Country
Comments
Reference
Costa Rica
Cilantro leaves 5Æ2% (4/8), Cilantro roots 8Æ7% (7/80), lettuce 2Æ5%
Radish (1Æ2%), carrot (1Æ2%), tomato (1Æ2%), cucumber (1Æ2%)
14Æ5% of vegetables examined contained C. parvum oocysts
19/475 (4%) fruits and vegetables examined positive – five lettuces,
14 mung bean sprouts oocyst density low (three oocysts per 100 g food)
Lettuce, parsley, cilantro, blackberries all positive at least once (2%) for Cryptosporidium sp.
Mung bean sprout samples (8%) positive for Cryptosporidium sp.,
although parasite concentration in positive samples were low,
between one and three oocysts per 50 g sprouts
Monge and Arias (1996)
Monge and Chinchilla (1996)
Ortega et al. (1997)
Robertson and Gjerde (2001a)
Peru
Norway
Costa Rica
Norway
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Journal compilation ª 2006 The Society for Applied Microbiology, Letters in Applied Microbiology 43 (2006) 7–16
Calvo et al. (2004)
Robertson et al. (2002b)
9
Cryptosporidium and public health
O. Sunnotel et al.
heat treatment above 65C, thus pasteurized and sterilized
foods should pose no health risk (Garcia et al. 2006).
However, these treated foods could still become contaminated during handling after heat treatment (Garcia et al.
2006).
Water treatment
Removal or inactivation of Cryptosporidium oocysts can
be accomplished by conventional water treatment technology, which includes flocculation, coagulation, sedimentation, filtration and chlorination (Betancourt and
Rose 2004). Conventional treatment for removal of
oocysts consist of:
i Coagulation/flocculation: combination of coagulant pretreatment and subsequent flocculation produces large
floc-aggregates containing colloidal and agglomerated
impurities. Clarification/filtration processes remove the
floc-aggregates (Hsu and Yeh 2003).
ii Sedimentation: water is clarified by settlement of flocparticles prior to filtration. This is the first barrier for
protozoan removal in conventional water treatment (Betancourt and Rose 2004).
iii Filtration: filters are the last step in removal of the
protozoan, where oocysts are captured and collected,
when passing through a filter bed (Betancourt and Rose
2004).
Cryptosporidium disinfection can also be accomplished
using ozone or exposure to UV and conventional chlorination is ineffective (Betancourt and Rose 2004). Modernized treatment plants separate oocysts from water by
more efficient micro- or ultrafiltration (Hill et al. 2005).
Implementation of multiple barriers to safeguard drinking
water is recommended as pathogen loads vary during the
season, with peak loads during early spring and late
autumn (Ferguson et al. 2003).
Pathogenesis
Cryptosporidium requires a host for survival and reproduction (Mansfield and Gajadhar 2004; Kim et al. 2005).
All species of Cryptosporidium undergo endogenous development, culminating in the production of an encysted
stage discharged in the faeces of their host (Fayer et al.
2000). Cryptosporidium has a complex life cycle including
both sexual and asexual reproductive stages (Fig. 1a,b)
(Smith et al. 2005; Thompson et al. 2005). Upon ingestion, excystation of viable oocysts, (4–6 lm for intestinal
species and gastric species are slightly larger), is triggered
(Fig. 1b).
Sporozoites contain secretary apical organelles (rhoptries, micronemes and dense granules), each with distinct
structures and functions necessary for host cell invasion
10
(Fig. 1a) (Blackman and Bannister 2001). Dense granules
secrete their contents across the zoite’s surface and stimulate the modification of host cell during invasion (Blackman and Bannister 2001; Huang et al. 2004). Apical
discharge of rhoptries and micronemes proteins provide
anchor sites for host cell adhesion and gliding motility
and a direct cytopathic effect has not yet been observed
(Blackman and Bannister 2001). The mechanisms of host
cell penetration and inclusion are poorly understood.
However, Cryptosporidium is known to have the ability to
re-arrange host cell cytoskeleton using actin-polymerizing
factors (Elliot et al. 2001) and is likely to be induced by
the parasite (Sibley 2004). The parasite nestles itself
within a parasitophorous vacuole, which is connected to
the host cell by a parasitophorous vacuole membrane
(Tzipori and Ward 2002). Here, it is protected from the
hostile gut environment and is supplied by the host cell
with energy and nutrients through a feeder organelle,
which is unique among apicomplexans (Tzipori and
Ward 2002). Recently, gregarine-like stages have been described in Cryptosporidium andersoni and C. parvum,
which undergo multiplication through syzgy, a sexual
reproduction process involving the end-to-end fusion of
two or more parasites (Hijjawi et al. 2002; Rosales et al.
2005). If this is verified by more extensive studies, it
would have major implications in our understanding of
the Cryptosporidium biology, genetics and transmission.
Although Cryptosporidium has been extensively studied
over the last decades, not much is known about host
interactions and pathogenesis. Host cell apoptosis can be
influenced by intra- and extracellular parasite stages, with
inhibition or promotion observed, indicating a parasitedriven regulation of host cell gene expression (Mele et al.
2004). Exposure to Cryptosporidium sporozoite antigens
triggers an immune response in immunocompetent mammals (Riggs 2002; Singh et al. 2005). Host cell immune
defences are both antibody and in particular T-cell mediated (Riggs 2002; Singh et al. 2005). Elevated excretion of
interleukins and interferon-gamma (IFN) has been
observed during the initial stages of infection (Riggs 2002;
Singh et al. 2005). Robinson et al. (2001) showed an IFNg independent mucosal response by increased levels of IL15, suggesting activation of the innate immune response.
Detection
Cryptosporidium requires the direct analysis of faecal samples for detection, as detection by cultivation is not possible. Overall, the choice of technique for the
characterization and detection of Cryptosporidium
depends on the sensitivity, level of differentiation, turnaround time and equipment available. Morphological
analysis and various immunoassays can reveal the
ª 2006 The Authors
Journal compilation ª 2006 The Society for Applied Microbiology, Letters in Applied Microbiology 43 (2006) 7–16
O. Sunnotel et al.
presence of Cryptosporidium, but molecular techniques
are generally required for differentiation and subtyping.
Microscopy is still commonly used to identify Cryptosporidium from environmental and clinical samples. Various
modified acid-fast stains, such as the modified Ziehl
Neelsen stain are commonly used, especially in developing
countries (Henriksen and Pholenz 1981) (Table 3). However, they have low sensitivity and also stain other
protozoan parasites such as Isospora and Cyclospora.
Immunofluorescence and enzyme immunoassays can be
used for the more selective and sensitive detection of
Cryptosporidium (Table 3) (Arrowood 1997).
PCR-based genotyping techniques are routinely used in
research laboratories, but rarely in clinical laboratories
and a whole range of genomic targets have been used
(Sulaiman et al. 1999). PCR may target structural or
housekeeping genes such as the 18S rRNA gene or Cryptosporidium-specific genes such as the Cryptosporidium
oocyst cell wall protein, thrombospondin-related adhesive
proteins 1 and 2 and 60 kDa glycoprotein (Strong and
Nelson 2000; Jiang and Xiao 2003; Trotz-Williams et al.
2005) (Table 3). Expansions/contractions and point
mutations of the microsatellite and minisatellite repeats
within Cryptosporidium species offer ideal template targets
for subtyping (Feng et al. 2000; Alves et al. 2003; Mallon
et al. 2003; Widmer et al. 2004). The presence of these
simple sequence repeats can serve as genetic markers
and can discriminate between different C. parvum and
C. hominis isolates (Table 3) (Alves et al. 2003; Widmer
et al. 2004). Currently, most detection and genotyping
methods utilize the 18S rRNA sequence of Cryptosporidium, because extensive sequence data are available for
most of the Cryptosporidium species and the gene has a
high (five) copy number. Unlike PCR tools based on
most other genes, genotyping tools targeting the 18S
rRNA gene can detect Cryptosporidium species that are
genetically distant from C. parvum and C. hominis, but
nevertheless can infect humans, such as C. canis, C. felis,
C. muris and C. suis (Jiang and Xiao 2003). Whilst showing similar sensitivity to nested-PCR assays (Table 3), the
detection of Cryptosporidium using real-time systems
greatly reduces analysis time and has great potential for
the rapid diagnosis of the parasites in clinical samples
(Limor et al. 2002; Tanriverdi et al. 2002). Hybridization
probes, which overlap an 18S rRNA polymorphic region,
have been used to differentiate five human-pathogenic
Cryptosporidium species, in a single real-time PCR by
melting curve analysis (Limor et al. 2002).
Oocyst viability
The detection methods described in the previous section
cannot distinguish between viable or non-viable oocysts.
Cryptosporidium and public health
Because only viable oocysts are a concern for public
health services, techniques have been developed to distinguish between viable and non-viable oocysts. Microscopically 4¢,6¢-diamidino-2-phenylindole and propidium
iodide are used to determine oocyst viability (Smith and
Hayes 1997; Nichols et al. 2004). The usefulness of vital
dyes is limited as they severely overestimate the infectivity
of oocysts (Bukhari et al. 2000). More recently developed
viability assays are based on the detection of rRNA by
fluorescent in situ hybridization (FISH), which use
labelled complimentary oligonucleotide probes (Lemos
et al. 2005). However, the long rRNA half-life may result
in viability overestimation using FISH or any other methods targeting rRNA (Fontaine and Guillot 2003; Smith
et al. 2004). The detection and quantification of Cryptosporidium using RT-PCR provides the enumeration of low
numbers of viable oocysts (Widmer et al. 1999; Gobet
and Toze 2001a, 2001b; Fontaine and Guillot 2002).
However, their potential as routine diagnostic tools have
yet to be fully defined. In addition, mRNA and rRNA
remain stable for an extended period of time, even after
oocyst death, which may lead to overestimations of viability (Gobet and Toze 2001a, 2001b). A direct comparison
of multiple viability assays has shown good correlations
between mouse inoculation and cell culture assays, but
poor correlation between the two involving FISH or RTPCR methods (Jenkins et al. 2002). Baeumner et al.
(2001) and Thompson et al. (2003) describe potentially
useful applications of the nucleic acid sequenced based
amplification technique to enumerate viable oocysts from
water samples (Table 3).
Future prospects
Although research over the last two decades has dramatically increased our knowledge on Cryptosporidium, key
questions about host–parasite interaction and invasion,
transmission, life cycle and epidemiology still remain
unclear (Smith et al. 2005). The prevalence of Cryptosporidium outbreaks in humans demonstrates the importance
of this parasite in public health (Craun et al. 2005). While
the organism has been clearly shown to be associated with
foodstuffs, the impact of Cryptosporidium contamination
in raw and cooked produce on public health has yet to
be fully defined. Continued monitoring for the presence
of oocysts in produce is recommended, especially shellfish
and ready-to-eat vegetables (Garcia et al. 2006; GomezCouso et al. 2006).
The high occurrence of Cryptosporidium in surface
water sources underlines the need for frequent monitoring of the parasite in drinking water (Frost et al. 2002).
The high cost of waterborne disease outbreaks should be
considered in decisions regarding water utility improve-
ª 2006 The Authors
Journal compilation ª 2006 The Society for Applied Microbiology, Letters in Applied Microbiology 43 (2006) 7–16
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12
Staining of cryptosporidia by a
modified Ziehl Neelsen or
other technique
Anti-Cryptosporidium-specific
fluorescent antibody stain
Cryptosporidium antigen
capture ELISA
Cryptosporidium antigen capture
colorimetric assays
In situ hybridization using
fluorescent-labelled complimentary
DNA oligonucleotide probes
Two-step PCR using two sets of
primers. Common gene targets are
gp60, hsp70, 18S rRNA, COWP
and TRAP (C1 and C2)
Restriction analysis of PCR product
after amplification of genomic DNA
Real-time detection of DNA.
Using hybridization probes
PCR amplification of microsatellites,
which serve as polymorphic
markers
Real-time detection of mRNA
NASBA is an isothermal amplification
method, which uses single-stranded
RNA as template, single-stranded
complementary RNA being
amplified in the course of the reaction
Modified acid fast stain
EIA
Immunochromatography
PCR-RFLP
Real-time PCR
Microsatellite analysis
NASBA
Five oocysts
Depends on gene
copy number
Requires further
investigation
One oocyst
One oocyst
One oocyst
Less sensitive than IFA
10 000–50 000
oocysts per g of
faeces
Less sensitive than IFA
Ten-fold lower than
IFA
Sensitivity
+Highly sensitive and specific detection
of Cryptosporidium
)Sequencing or RFLP analysis is required
to identify species/genotypes
)Relative long procedure
+Can distinguish between most of the
Cryptospordium species and genotypes
)Relative long procedure compared with
real-time PCR
+Rapid, highly sensitive and specific detection
+Could be used as routine in-line detection
+Can be used for species differentiation
)Expensive equipment
+Can distinguish between isolates of
same species
+Does not require further analysis
+Rapid, highly sensitive technique
+Can potentially be used as viability assay
)Currently not robust enough to be
used as routine diagnostic tool
+Potential ability to detect viable-only
oocyst by amplification of RNA samples
+Does not require a thermocycler
)Requires further development for use
as diagnostic tool
+Simple and can be used for range of
other parasites
)Nonspecific
+Highly specific
+With morphological verification
)Mo viability assessment
+Specific and rapid
)Detect antigens of developmental stages
)Without morphological verification
+Rapid
)Prone to QA/QC problems
+Highly specific
+Can also be used for viability assay
Advantages/disadvantages
Thompson et al. (2003)
Widmer et al. (1999), Gobet and
Toze (2001a, 2001b), and
Fontaine and Guillot (2002)
Alves et al. (2003)
Widmer et al. (2004)
Limor et al. (2002) and
Tanriverdi et al. (2002)
Xiao et al. (2004a, 2004b, 2004c)
and Coupe et al. (2005)
Xiao et al. (2004a, 2004b, 2004c)
Lemos et al. (2005)
Johnston et al. (2003)
Johnston et al. (2003)
Weber et al. (1991) and
Webster et al. (1996)
Weber et al. (1991)
References
+, advantage; ), disadvantage; COWP, Cryptosporidium oocyst cell wall protein; TRAP, thrombospondin-related adhesive protein; IFA, immunofluorescent staining; EIA, enzyme immunoassay;
FISH, fluorescent in situ hybridization; NASBA, nucleic acid sequence-based amplification.
Real-time RT-PCR
Nested PCR
FISH
IFA
Principle
Technique
Table 3 Description of common methods used for the detection of Cryptosporidium spp.*
Cryptosporidium and public health
O. Sunnotel et al.
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Journal compilation ª 2006 The Society for Applied Microbiology, Letters in Applied Microbiology 43 (2006) 7–16
O. Sunnotel et al.
ments and the construction of treatment plants (MorganRyan et al. 2002). The integration of watershed and
source water management and protection, scientific management of agricultural discharge and run-off, pathogen
or indicator organism monitoring for source and treated
water, outbreak and waterborne disease surveillance is
needed to reduce waterborne transmission of human
cryptosporidiosis (Ferguson et al. 2003). Water treatment
facilities employing second-line treatment practices such
as UV irradiation and ozone treatment can alleviate the
danger imposed by Cryptosporidium contaminated water
(Keegan et al. 2003). However, outbreaks of waterborne
illness can still occur in developed countries, because of
malfunction or mismanagement of water treatment facilities (Ferguson et al. 2003). Better education and
increased awareness of cryptosporidiosis by the general
public and pool operators could potentially reduce the
number and impact of swimming pool and other recreational water-related outbreaks (Robertson et al. 2002a).
Improved detection methods with the ability to differentiate species can be useful in the assessment of infection
and identification of sources of contamination. These will
also provide vital data on the levels of disease burden due
to zoonotic transmission (Fayer 2004). The roles of
humans, livestock and wildlife in the transmission of
Cryptosporidium remain largely unclear for many areas.
The continued monitoring (using appropriate molecular
methods) of Cryptosporidium in surface water, livestock,
wild life and humans will increase our knowledge of
infection patterns and transmission of Cryptosporidium
(Fayer 2004).
To date, no drugs are licensed for the treatment of
cryptosporidiosis in immunocompromised persons,
although nitazoxanide has been approved for the treatment of paediatric cryptosporidiosis and adult giardiasis
in the USA (Smith et al. 2004; Cohen 2005). Currently,
the most effective therapeutic and prophylactic treatment
of cryptosporidiosis in AIDS patients is the highly active
antiretroviral therapy. It is, however, largely not available
in developing countries. To promote the development of
new therapeutic and prophylactic treatment, further
research is required to understand parasite metabolism,
the invasion process and interactions between host and
parasite (Smith et al. 2005). Database mining, gene
expression and host–parasite interaction studies are essential to understand gene function, which can possibly
reveal novel drug targets (Smith et al. 2005), potentially
lowering case numbers of cryptosporidiosis.
Acknowledgements
OS was funded by a University of Ulster Vice-Chancellor’s Scholarship. This work was partly funded by Safe-
Cryptosporidium and public health
food – the Food Safety Promotion Board, through a
Laboratory Links Project grant.
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