Fig. 2. Life cycles of different coccidian tissue-cyst-forming genera, the gamogony (
GA) of which always occurs in the intestinal epithelial cells of final hosts (
FH).
A Isospora (e.g.
I. serini of canaries,
I. lacazei of sparrows): homoxenous (i.e. one-host-type) cycle. Oocysts are excreted unsporulated (B1); after
sporulation (compare C1) they become infectious to other hosts of the same species. Schizogony (
SC) is not restricted to epithelial intestinal cells, but also occurs in the gut wall and extraintestinally.
B Cystoisospora (e.g.
C. felis and
C. rivolta of cats,
C. ohioensis of dogs): heteroxenous cycle. Oocysts are excreted unsporulated (B1). An intermediate host (B2) may facultatively be involved in addition to the typical development (
SC, GA, SP) inside the intestine of the final host. Rodents and/or cattle are infected by oral ingestion of sporulated oocysts. The sporozoites (
SP) enter numerous types of host cells, but remain there unchanged in a
parasitophorous vacuole (
PV) until this cell is eaten by the final host. Since there is no parasitic development inside the intermediate host, it must be considered as a transport or
paratenic host.
C Frenkelia (e.g.
F. clethrionomyobuteonis): heteroxenous cycle with an obligate alternation of final (hawk) and intermediate (bank vole) hosts (like
Sarcocystis). The oocysts are excreted fully sporulated (C1) and must be ingested by the intermediate host. Inside the liver cells (C2) typical schizonts (
S) produce merozoites, which lead to formation of septated tissue cysts (
TC) in brain and cord (C3). The cysts contain dividing merocytes and finally motile cyst merozoites
(bradyzoites). The latter are infectious to the final hosts, where they (when oral ingestion occurs) initiate gamogony inside the gut cells.
D Besnoitia (e.g.
B. wallacei): obligatory two-host cycle. Oocysts are excreted unsporulated by the final host cat (D1); after sporulation they must be ingested by intermediate hosts (rats, mice), where reproduction initially occurs by
endodyogeny in many cells (appearing as
pseudocysts; D2). Later, tissue cysts (D3) are formed inside fibroblasts, leading to cysts that are often macroscopically visible. The latter exclusively contain cyst merozoites (never
metrocytes), which are infectious to final hosts when ingested. After experimental transmission, cyst merozoites of several
Besnoitia species repeat the developmental steps D2 and D3 in other intermediate hosts.
E Hammondia (e.g.
E. hammondi): obligatory alternation of two hosts. Oocysts are excreted unsporulated (E1) by the final host cat; after sporulation they must be ingested by intermediate hosts (mice, rodents), where rapid asexual reproduction occurs in lymphoid cells (E2); this is followed by formation of tissue cysts (
TC) in striated muscles (E3). The cyst merozoites are finally infectious to final hosts (
FH), where they initiate schizogony in epithelial cells (
SC). This species is discussed as strain of
Toxoplasma gondii.
Infection of final hosts (
FH) by their own excreted oocysts is only possible in the genera
Isospora and
Cystoisospora.
FH, final host;
GA, gamogony;
IH, intermediate host;
N, nucleus;
NH, nucleus of the host cell;
PS, pseudocyst;
PV, parasitophorous vacuole;
S,
schizont; SC, schizogony;
SP, sporogony;
SZ,
sporozoite; TC,
tissue cyst (for more details see Table 2 and
Table 5).
