The
Eimeria life cycle has an
exogenous phase, during which the oocysts are excreted into the environment, and an
endogenous phase, where parasite development occurs in the host intestine. During the endogenous phase, several rounds of
schizogony (asexual reproduction) take place, after which the
sexual differentiation of
gametes and
fertilisation occurs. Parasite transmission occurs via the
fecal-oral route. Infections are common in farming environments where many animals are confined in a small space. Once released, the unsporulated oocysts undergo
meiosis upon contact with oxygen and moisture. This process is known as
sporulation and the oocysts take approximately 2 to 7 days to become infectious. The sporulated oocyst is said to be
tetrasporic meaning it contains four sporocysts, while each sporocyst is
dizoic, i.e. it contains two sporozoites.
Sporozoites The motile
sporozoites invade the
enterocytes of small intestine, and migrate to their respective sites of development. Invasion is mediated through specialised membrane-bound structures on the surface of the parasite that release secretions. This results in the recognition of, and attachment to
host cell receptors. This process is known as
gliding motility, which is conserved across all species of
Apicomplexa. Membrane glyconjugates have been proposed as potential host cell receptors for
Eimeria species. After invasion, the sporozoites develop into
trophozoites, then into
schizonts, where they undergo several rounds of
asexual reproduction. This results in many
nuclei developing within the schizont. Each nucleus develops into a
merozoite. Invasion requires the formation of a
moving junction between parasite and host
cell membranes. In
E. tenella, this involves parasite
micronemes and
rhoptry proteins including RON2, RON5 and AMA-2. It is unlikely that the host cell is completely passive in the invasion process, although evidence of host physical forces that assist in mediating parasite entry remains controversial.
Merozoites When
schizonts rupture, merozoites are released, which either go on to re-infect more
enterocytes or develop into either male or female
gametes via the process of
gametogenesis. These gametes fuse to form an oocyst, which is then released in its non-infectious, unsporulated form through the faeces of the host. Merozoite invasion also requires the formation of a moving junction, however the
proteins involved in this process differs from those on sporozoites.
Rhoptry proteins AMA-1 and RON4 are found exclusively on merozoites. There is also a greater diversity of
variant surface antigens found on the surface of merozoites. It is hypothesised that this may be due to the fact that merozoites are short-lived and a greater
antigen repertoire would permit faster binding and invasion. == Taxonomy ==