Miscellaneous Protozoa : Sarcocystis species:

Sarcocystis species
(sahr”ko-sis-tis)
Common associated disease and condition
names: Sarcocystis infection.
There are a number of species of parasites that
fall within the group known as Sarcocystis.
Cattle may harbor Sarcocystis hovihominis, also FIGURE 7-8 Sarcocystis species oocyst.
Double layered
sporocyst wall
Two mature
sporocysts
(each contains
4 sporozoites)
Ruptured
sporocysts are
most often
seen singly
or in pairs
“cemented
together”
in clinical
specimens.
Average sporocyst length: 10-18 m
CHAPTER 7 Miscellaneous Protozoa 169
in wet preparations. However, in many cases, the
oocysts have already ruptured and only the sporocysts
are visible on examination of the stool
specimen. The sporocysts may be seen singly or
in pairs that appear to be cemented together.
Routine histologic methods may be used to identify
the Sarcocystis cyst stage, known as the sarcocyst,
from human muscle samples. An in-depth
discussion of these histologic methods is beyond
the scope of this text.
Life Cycle Notes
Although the morphology of the oocysts of Sarcocystis
resembles that of Isospora, the life cycles
of these two genera are different—hence, the
current organism classification. Asexual reproduction
of Sarcocystis occurs in the intermediate
host. Human infection of Sarcocystis species may
be initiated in one of two ways. The first transmission
route occurs when uncooked pig or
cattle meat infected with Sarcocystis sarcocysts is
ingested. Humans are the definitive host. Gametogony
usually occurs in the human intestinal
cells. The development of oocysts and subsequent
release of sporocysts thus follow. This sets
the stage for continuation of the life cycle in a
new intermediate host. The second transmission
route occurs when humans accidentally swallow
oocysts from stool sources of animals other than
cattle or pigs. In this case, the ingested sarcocysts
take up residence in human striated muscle.
Under these circumstances, the human serves as
the intermediate host. It is interesting to note that
Sarcocystis oocysts do not infect the host of their
origin.
Epidemiology
The frequency of Sarcocystis infections is relatively
low, even though its distribution is worldwide.
In addition to its presence in cattle and
pigs, Sarcocystis spp. may also be found in a
variety of wild animals.
Clinical Symptoms
Sarcocystis Infection. There have only been a
few documented symptomatic cases of Sarcocystis
infections in compromised patients. These
persons experienced fever, severe diarrhea, weight
loss, and abdominal pain. It is presumed that
patients suffering from muscle tenderness and
other local symptoms are exhibiting symptoms
caused by Sarcocystis invasion of the striated
muscle.
Treatment
The treatment protocol for infections with Sarcocystis
spp. when humans are the definitive host
is similar to that for Isospora belli. The combined
medications of trimethoprim plus sulfamethoxazole
or pyrimethamine plus sulfadiazine
are typically given to treat these infections. There
is no known specific chemotherapy to treat Sarcocystis
infections of the striated muscle when
humans are the intermediate host.
Prevention and Control
The primary prevention and control measures of
Sarcocystis infections in which humans are the
definitive host consist of adequate cooking of
beef and pork. Prevention of those infections in
which humans are the intermediate host includes
the proper care and disposal of animal stool that
may be potentially infected with Sarcocystis.
Parameter Description
Shape Oval
Appearance Transparent
Number of sporocysts Two
Size of each sporocyst 10-18 ?m long
Contents of each sporocyst Four sausage-shaped
sporozoites
Oocyst cell wall appearance Clear, colorless, double
layered
TABLE 7-4 Sarcocystis spp. Mature
Oocyst: Typical
Characteristics at a Glance*
*In many cases, only single or double sporocysts cemented
together may be visible in stool samples.
170 CHAPTER 7 Miscellaneous Protozoa
Cryptosporidium parvum
(krip”toe-spor-i’dee-um/par-voom)
Common associated disease and condition
names: Cryptosporidosis.
Morphology
Oocysts. Measuring only 4 to 6 ?m, the
roundish Cryptosporidium oocysts are often
confused with yeast (Figs. 7-9 and 7-10; Table
7-5). Although not always visible, the mature
oocyst consists of four small sporozoites
surrounded by a thick cell wall. Contrary to
other members of the sporozoa, such as Isospora,
Cryptosporidium oocysts do not contain
sporocysts. One to six dark granules may also
be seen.
FIGURE 7-9 Cryptosporidium parvum oocyst.
Sporozoites
Dark granule
(may contain
1 to 6 granules)
Average size: 4-6 m
Parameter Description
Size 4-6 ?m
Shape Roundish
Number of sporocysts None
Number of sporozoites Four (small)
Other features Thick cell wall
One to six dark granules
may be visible
TABLE 7-5 Cryptosporidium parvum
Oocyst: Typical
Characteristics at a Glance
Quick Quiz! 7-8
Which genus of parasite is most similar to Sarcocystis
based on morphologic similarities? (Objective 7-11)
A. Isospora
B. Blastocystis
C. Entamoeba
D. Toxoplasma
Quick Quiz! 7-9
How do humans become infected with Sarcocystis?
(Objectives 7-5)
A. Ingestion of uncooked or undercooked beef or
pork
B. Inhalation of oocysts
C. Ingestion of animal fecal contaminated food
D. More than one of the above: ________________
(specify)
Quick Quiz! 7-10
In addition to oocysts, these Sarcocystis morphologic
forms may be seen in human samples: (Objective 7-5)
A. Packets of eggs
B. Single or double sporocysts
C. Clusters of cysts
D. Groups of sporoblasts
FIGURE 7-10 Modified acid-fast stain, ×1000). Arrows
indicate Cryptosporidium oocysts, each containing four
undefined sporozoites. Note dark-staining granules.
CHAPTER 7 Miscellaneous Protozoa 171
Schizonts and Gametocytes. The other morphologic
forms required to complete the life cycle
of Cryptosporidium include schizonts containing
four to eight merozoites, microgametocytes, and
macrogametocytes. The average size of these
forms is a mere 2 to 4 ?m. It is important to note
that these morphologic forms are not routinely
seen in patient samples.
Laboratory Diagnosis
The specimen of choice for the recovery of Cryptosporidium
oocysts is stool. Several methods
have been found to identify these organisms successfully.
The oocysts may be seen using iodine or
modified acid-fast stain. In addition, formalinfixed
smears stained with Giemsa may also yield
the desired oocysts. As noted, it is important to
distinguish yeast (Chapter 12) from true oocysts.
Oocysts have also been detected using the following
methods: the Enterotest, enzyme-linked
immunosorbent assay (ELISA), and indirect
immunofluorescence. Concentration via modified
zinc sulfate flotation or by Sheather’s sugar flotation
have also proven successful, especially when
the treated sample is examined under phase contrast
microscopy. It is important to note that
merozoites and gametocytes are usually only
recovered in intestinal biopsy material.
Life Cycle Notes
Cryptosporidium infection typically occurs following
ingestion of the mature oocyst. Sporozoites
emerge after excystation in the upper
gastrointestinal tract, where they take up residence
in the cell membrane of epithelial cells.
Asexual and sexual multiplication may then
occur. Sporozoites rupture from the resulting
oocysts and are capable of initiating an autoinfection
by invading new epithelial cells. A number
of the resulting oocysts remain intact, pass
through the feces, and serve as the infective stage
for a new host.
It is interesting to note that two forms of
oocysts are believed to be involved in the Cryptosporidium
life cycle. The thin-shelled version is
most likely responsible for autoinfections because
it always seems to rupture while still inside the
host. The thick-shelled oocyst usually remains
intact and is passed out of the body. This form
is believed to initiate autoinfections only
occasionally.
Epidemiology
Cryptosporidium has worldwide distribution. Of
the 20 species known to exist, only C. parvum is
known to infect humans. Infection appears to
primarily occur by water or food contaminated
with infected feces, as well as by person-toperson
transmission. Immunocompromised persons,
such as those infected with the AIDS virus,
are at risk of contracting this parasite. Other
populations potentially at risk include immunocompetent
children in tropical areas, children in
day care centers, animal handlers, and those who
travel abroad.
Clinical Symptoms
Cryptosporidiosis. Otherwise healthy persons
infected with Cryptosporidium typically complain
of diarrhea, which is self-limiting and lasts
approximately 2 weeks. Episodes of diarrhea
lasting 1 to 4 weeks have been reported in some
day care centers. Fever, nausea, vomiting,
weight loss, and abdominal pain may also be
present. When fluid loss is great because of the
diarrhea and/or severe vomiting, this condition
may be fatal, particularly in young children.
Infected immunocompromised individuals,
particularly AIDS patients, usually suffer from
severe diarrhea and one or more of the symptoms
described earlier. Malabsorption may also accompany
infection in these patients. In addition,
infection may migrate to other body areas, such
as the stomach and respiratory tract. A debilitating
condition that leads to death may result in
these patients. Estimated infection rates in AIDS
patients range from 3 to 20% in the United
States and 50 to 60% in Africa and Haiti. Cryptosporidium
infection is considered to be a cause
of morbidity and mortality.