Page 74 - Medical Parasitology_ A Textbook ( PDFDrive )

Sarcocystis 67

Habitat

When human is the intermediate host, sarcocysts are found in the striated muscles.
When human is the definitive host, oocysts are found in stools.

Morphology

The sporocysts of S. hominis measure 13.1 to 17.0 by 7.7 to 10.8 μm. Sporocysts of
S. suihominis measure 11.6 to 13.9 by 10.1 to 10.8 μm. Four sporozoites can be
found within each sporocyst.

Life Cycle (Fig. 7.8)

(1) Oocysts (fully developed) and sporocysts are shed in human faeces, which con-
taminate grass and are ingested by cows and pigs. In the intermediate hosts, the
sporocyst ruptures and releases sporozoites which invade the intestinal mucosa and
reaches the vascular endothelial walls, where they undergo schizogony producing
merozoites. These are carried to muscle fibres and develop into sarcocysts. Cattle is
the intermediate host for S. hominis and pig is the intermediate host for S. suihomi-
nis. (2) Human infection is acquired by ingesting raw or undercooked beef or pork
containing sarcocyst. (3–5) When sarcocyst is ingested by human (the definitive
host), the merozoites are released in the intestine, and enter intestinal cells where
they develop into male and female gametes. (6) After fertilization, the zygote devel-
ops into an oocyst containing 2 sporocysts, each having 4 sporozoites. (7) These
oocysts are shed in faeces and are ingested by intermediate host.
Human is the intermediate host in S. lindemanni; the definitive host is unknown.
Human acquires infection by ingestion of oocysts. Sarcocysts develop in the striated
muscles of human.

Pathogenesis and Clinical Features

Intestinal sarcocystosis is usually mild. Patients may have nausea, abdominal pain
and diarrhoea. Muscular sarcocystosis is usually asymptomatic but in some cases,
the early phase of infection may cause fever, eosinophilia, muscle pain, weakness or
myositis.

Diagnosis

1. Intestinal sarcocystosis
(a) Microscopic examination
To detect oocysts or sporocysts in faecal samples.