++
Cryptosporidium species
are tiny (2–6 μm), obligate intracellular
parasites related to other coccidian protozoan, including Toxoplasma,
Cyclospora, Isospora, Plasmodium, Eimeria, and Sarcocystis.Cryptosporidium species
primarily infect the gastrointestinal tract of a variety of vertebrate
hosts, including humans.1 Host range is largely
a function of species, as a given species of parasite most efficiently
maintains infection within a few species of hosts.
++
Over 20 species of Cryptosporidium have been
described, but there is some debate as to the exact number of species.2 Two
species, C hominis and C parvum, account
for nearly all of the disease caused by Cryptosporidium species
in humans and are the species of public health significance. C
parvum was formerly divided into two genotypes (genotypes
I and II), but current classification separates genotype I into
a separate species known as C hominis.3 This
distinction is important, as humans are essentially the only known
reservoir of C hominis, and the species is more
efficient at causing disease in humans. Furthermore, studies that
distinguish between the two species in cohorts of infected patients
generally find a higher prevalence of C hominis; however,
the exact prevalence of either species depends on the principal
mode of spread (human-to-human vs. zoonotic), sanitation, and living
conditions.6-10
++
Cryptosporidium completes its life cycle within
a single host1,11(Fig. 345-1).
Infection occurs after ingesting the sporulated, thick-walled oocysts.
Excystation occurs in the small intestine after exposure to bile
salts and pancreatic enzymes, releasing four sporozoites. These sporozoites
penetrate a surface epithelial cell in the intestinal mucosa and
form an intracellular parasitophorous vacuole. They then differentiate
into uninuclear trophozoites, which undergo asexual replication (merogony)
to form type I meronts. The type I meront can then autoinfect other
surface epithelial cells or differentiate into a type II meront. The
type II meront then undergoes gametogomy, producing
both microgametocytes and macrogametocytes. These gametocytes fertilize
to produce oocysts. The life cycle is complete when the oocysts
undergo sporogomy, resulting in infectious sporozoites within the
oocysts. Approximately 80% of the oocysts produced in this
fashion are environmentally resistant, thick-walled cysts that are
excreted in the feces. The remaining 20% are thin-walled
cysts that undergo another autoinfective stage. The autoinfectious
stages are important features in the parasite’s life cycle
and accounts for persistent and occasionally severe disease, even
when a low inoculum of cysts are ingested.
++
Cryptosporidia are ubiquitous in the environment and are found
in several sources of untreated surface water. During the AIDS epidemic, cryptosporidia
were recognized as a significant enteric pathogen, causing severe
disease in patients with advanced AIDS. Currently, studies show
that the incidence of AIDS-associated cryptosporidiosis has declined
with the advent of highly active antiretroviral therapy (HAART).12,13
++
The prevalence of disease is higher in developing countries due
to less sanitary conditions that promote fecal-oral transmission
and the lack of safe water sources. The seroprevalence in developing
countries is as high as 75% by age 4, in contrast to the
United States, where seroprevalence is approximately 15%.14 Cryptosporidiosis
is more common in children than adults. Daycare attendance is a
risk factor, and outbreaks have been reported in childcare settings,
likely through secondary person-to-person transmission.15,16Travelers to developing countries are at particular risk.
++
The principal mode of transmission is fecal-oral, through either
a waterborne or person-to-person route. The median infectious dose has
been estimated at 132 oocysts, although disease can occur with ingestion
of as few as 10 oocysts.17 Infectious dose tends
to be lower with C hominis.5 Infected
individuals may shed oocysts for up to 5 weeks after an acute diarrheal
episode.
++
The oocyst is particularly resistant to disinfectants, including
chlorine, bleach, ethanol, and many other hospital, industrial,
and household chemicals.1,11,17 However, 6% hydrogen
peroxide is an effective cysticide. The resistance to disinfectants
coupled with the low infectious dose makes the organism a public
health threat and a potential bioterrorism agent; furthermore, it
allows for effective waterborne transmission. Cryptosporidium is
resistant to standard chlorine concentrations in public water systems,
and it often escapes filtration systems. The largest waterborne outbreak
occurred in 1993 in Milwaukee, Wisconsin, with approximately 400,000
people infected and 69 deaths.18 As is the case
with most diarrheal illnesses, morbidity and productivity loss substantially
outpaced mortality.19
++
Airborne transmission has been suggested but not definitively demonstrated.
Isolated pulmonary cryptosporidiosis has been described in a case
report,20 although it nearly always occurs in severely
immunosuppressed patients with concomitant gastrointestinal disease.
Occupational exposure occurs to C parvum occurs
in farmers and animal handlers, particularly those exposed to cattle.
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In the immunocompetent host, Cryptosporidium primarily
infects the proximal small bowel. The exact mechanism whereby Cryptosporidium causes
diarrhea is unknown. The organism disrupts epithelial tight junctions
in several in vitro epithelial cell models,21 resulting
in a loss of barrier function. Experiments have failed to reveal
any exotoxin or virulence factor produced by Cryptosporidium, although
the organism clearly causes malabsorption. Pathological findings
include loss of intestinal epithelium, villous atrophy with loss
of epithelial architecture and surface area, and infiltration of the
lamina propria with mononuclear and polymorphonuclear cells.11 Various
stages of the parasite can be seen immediately below the brush border
of epithelial cells in biopsy samples.
++
The mechanisms of recovery and immunity to cryptosporidiosis
has not been fully elucidated. An intact cell-mediated immune system with
CD4 lymphocytes and interferon is crucial for recovery from disease.
Animal models support a pivotal role of interferon gamma, with IFN-γ knock-out
models developing severe infections that proceed to chronic forms.22–24 Humoral
immunity is not completely protective but does decrease the severity
of illness and shedding of oocysts. Secretory antibodies are beneficial,
as is hyperimmune colostrum.25
+++
Clinical Manifestations
++
Symptoms of cryptosporidiosis begin between 2 to 10 days (average
7 days) after becoming infected with the parasite. The most common
symptom is watery diarrhea, which varies among individuals with
some being entirely asymptomatic. Symptoms generally persist for
1 to 2 weeks. Occasionally, patients recover and then experience
a recurrence of symptoms before the illness ends.
++
Severe cryptosporidiosis occurs in immunosuppressed patients,
including those with advanced AIDS, cancer, hypogammaglobulinemia,
severe combined immunodeficiency, and bone marrow and solid organ transplants.
Such patients may have severe, life-threatening illness with intractable
diarrhea and severe volume loss. Symptoms often last for months
to years; wasting, hypovolemia, and weight loss is common. In general,
the severity of the diarrheal illness correlates with the severity
of immunosuppression.
++
Children with underlying malnutrition tend to have prolonged
episodes of disease with growth shortfalls.26,27 Multiple
diarrheal episodes in early childhood have been associated with
decreased height-for-age scores and decreased cognition and language
skills, particularly semantic fluency.26-28
++
In immunocompromised patients, fluid loss can be massive and
can exceed 10 to 20 liters per day. Parenteral hydration is usually
required to maintain euvolemia in the immunosuppressed host. Other
clinical findings include fever, nausea, vomiting, and crampy abdominal pain.
Myalgia, malaise, headache, and other flulike symptoms have also
been reported.
++
Extraintestinal infection with Cryptosporidium is
rare and usually occurs only in severely immunocompromised hosts.
In particular, advanced AIDS is associated with biliary cryptosporidiosis,
which results in AIDS cholangiopathy.29 This is
due to the synergistic interaction of HIV and Cryptosporidium infection,
whereby the HIV Tat-1 protein enhances Cryptosporidium-induced
apoptosis in cholangiocytes.30 Biliary cryptosporidiosis
manifests as acalculous cholecystitis, sclerosing cholangitis, and
hepatitis. The clinical presentation includes fever, upper-right
quadrant pain, jaundice, nausea, vomiting, diarrhea, hyperbilirubinemia,
and elevated liver enzymes. Radiographically, the disease resembles sclerosing
cholangitis. Pancreatic cryptosporidiosis may also occur.
++
Respiratory cryptosporidiosis is a rare event and nearly always
requires concomitant intestinal disease.20 It manifests
as cough, shortness of breath, wheezing, croup, and hoarseness.
Diagnosis is made through isolation of oocysts from sputum or tracheal
aspirates.
++
The diagnosis of intestinal cryptosporidiosis is established
by identifying the characteristic oocysts in the stool. The oocysts
may be visualized using a modified acid-fast stain, although this
method lacks sensitivity.31 The diagnosis is almost
never made on routine ova and parasite stool examination. Even with
acid-fast staining, the potential for missing this and other coccidian parasites
is high. Concentration techniques with Sheather’s sugar solution
or zinc sulfate improve the recovery of the parasite. Enzyme-linked
immunosorbent assays (ELISA) are commercially available but lack
specificity. The current diagnostic standard is monoclonal immunofluorescent
staining to visualize the parasite in stool preparations. Trying
to detect the organism in contaminated water is both expensive and
time-consuming, and it requires filtering large volumes of water
through 1-μm resolution filters
with subsequent filter microscopic examination.
++
PCR-based molecular techniques for detecting oocysts in stool
and water have been developed with increased sensitivity and more
rapid turnaround times.32,33Finally, immunomagnetic
beads have been developed to allow rapid identification of oocysts
in stool and water.34 Unfortunately, the latter
two molecular techniques are not widely commercially available for
diagnostic use; thus, most laboratories prefer immunofluorescence, ELISA,
and/or modified acid-fast stains.
++
Extraintestinal cryptosporidiosis remains an elusive diagnosis
and requires demonstration of the parasite from an extraintestinal source.
This is usually accomplished via biopsy, with the parasite appearing
on light microscopy with hematoxylin-eosin stains or electron microscopy. Pulmonary cryptosporidiosis
may be diagnosed by finding oocysts in tracheal secretions; however,
in the absence of pulmonary biopsy, contamination from swallowed
GI secretions remains a possibility. However, extraintestinal disease
is unlikely in the absence of more easily documented intestinal
infection.
++
Until recently, there were no good treatment options for cryptosporidiosis.
Three placebo-controlled studies have demonstrated that nitazoxanide
(> 12 years of age, 500 mg, 4–11 years of age, 200 mg,
1–3 years of age 100 mg, all twice daily for 3 days) leads
to more rapid cessation of diarrhea, and more frequent eradication
of the organism compared to placebo in HIV-negative adults and children.35,36 In
severely malnourished children with chronic cryptosporidiosis, cure
rates were lower but many resolved with retreatment.
++
In patients with HIV higher doses, or more prolonged therapy
(2 weeks) was superior to placebo among those with CD4+ T-cell counts
greater than 50/mm3, but was not better than placebo
in those with lower CD4+ counts.37
++
Paromomycin and azithromycin also in vitro activity against the parasite.
Paromomycin (1 g bid) in combination with azithromycin (600 mg qd)
has been shown to be effective in case series that largely originate
from the pre-HAART AIDS era.20,38 Hyperimmune bovine
colostrum reduces clinical symptoms and shedding of oocysts, but
this is considered experimental therapy.39
++
The cornerstone of therapy in immunosuppressed hosts is restoring immune
function. In patients with AIDS, this is accomplished via HAART
therapy, which serves two roles: (1) the restoration of CD4 count
allows the host to respond to and eliminate infection, and (2) some
HAART drugs, particularly protease inhibitors, may have some in
vitro activity against the parasite.38,40 The clinical relevance
of the latter point is unknown. In bone marrow and solid organ transplants,
decreasing immune suppression, if at all tolerated, greatly enhances
cure rates.
++
Rehydration is also crucial, as large volume fluid loss is a
frequent complication of disease. This often requires parenteral
fluid supplementation. Peptidomimetic agents (octreotide and vapreotide) and
antimotility agents (loperamide, opiates, and atropine) may help
to control diarrheal symptoms.
++
In cases of biliary cryptosporidiosis in severely immunocompromised
individuals, drug therapy has been effective. Endoscopic decompression
of the biliary system and stenting is helpful in cases of sclerosing
cholangitis.
++
Decreasing exposure to oocysts is crucial for immunosuppressed
populations and is sensible for immunocompetent populations. Exposure largely
occurs through a waterborne source, so all potentially contaminated
water must be treated before consumption. Unfortunately, standard
chlorination has no effect on oocysts, and even iodine-based water-purification
tablets are modestly effective at best. The most reliable way to
purify surface water sources involves boiling it for at least 1
minute, which thoroughly inactivates most pathogens, including Cryptosporidium. Boiling,
however, is labor-intensive and time-consuming and not often done. Filters
with resolutions of less than 1 μm are theoretically
effective but are easily clogged and may be bypassed if damaged.
In general, immunosuppressed persons should be advised to avoid
all untreated surface water sources and potentially risky activities
such as camping. Bottled water from a reliable source
offers a safe alternative, although its transport may be cumbersome.
++
Recreational water has been linked to public outbreaks.41 Since
standard chlorination is ineffective, public pools may become potentially
contaminated through one infected person, particularly a child with
diarrhea. For this reason, the CDC recommends advising all patrons
to avoid swimming during a symptomatic diarrheal illness and for
2 weeks afterward due to shedding of oocysts. While ozonation and
ultraviolet treatment are effective cysticidal agents, these methods are
highly dependent on water circulation rates; thus, a contaminated
pool would remain potentially contaminated for up to 24 hours.41
++
Vaccine development is currently under way but is hampered by
the lack of an obvious antigenic target. The parasite has an abbreviated genome,
often relying on host enzymes to survive, complicating vaccine development
and drug targeting. Furthermore, antigens that result in high antibody
titers are not necessarily protective in animal models. The successful
vaccine will likely require both the proper mix of antigens and
adjuvants and delivery to result in the proper combination of cell-mediated
and humoral immunity to the appropriate antigens.