Comparative Parasitology 68(2) 2001 - Peru State College

260 COMPARATIVE PARASITOLOGY, 68(2), JULY 2001
Table 1. Percentage of encysted (EN) and excysted (EX) metacercariae (M) of Echinostoma caproni from
16 mice, each fed 400 cysts.
Time
postinfection
Group* (hr)
A
B
C
D
E
1
2
3
4
24
M
EN
EX
EN
EX
EN
EX
EN
EX
EN
EX
Stomach
0.9
1.0
0.3
0.8
0
0.5
0
0.2
0
0
1
1.5
1.9
0.5
2.4
0
2.6
0
2.3
0
1.6
Segments of the small intestine
2
1.3
1.6
0.5
1.8
0.1
2.9
0
2.6
0
2.7
3
0.7
2.5
0.4
1.3
0
3.1
0
2.9
0
3.3
* Groups A and B each with 2 mice; Groups C, D, and E each with 4 mice.
ICR mice (Hosier and Fried, 1991). Preliminary
studies on 2 mice each fed 100 metacercarial
cysts and necropsied at 1 and 2 hr p.i. showed
metacercarial recoveries (combined data of excysted
and encysted metacercariae) of about
10% at necropsy. The preliminary work showed
the inherent difficulties in recovering these small
organisms (excysted metacercariae measuring
about 250 (xm in length and encysted metacercariae
about 150 (Jim in diameter) from the intestinal
tract. Moreover, the color, size, and motility
of the villi made it difficult to distinguish
them from excysted metacercariae. Empty cysts
were also seen at necropsy but were not counted.
On the basis of our experiences with the preliminary
study, we increased the cyst inoculum to
400 per host in the study reported herein.
Groups of 2 mice each were necropsied at 1
and 2 hr p.i. (Groups A and B, respectively; Table
1), and groups of 4 mice each were necropsied
at 3, 4, and 24 hr p.i. (Groups C, D, and E,
respectively; Table 1). The numbers of encysted
and excysted metacercariae in the stomach, in 5
intestinal segments of equal length (approximately
10 cm each), beginning at the pylorus
and ending at the ileocecal valve, and in the
combined cecum—large intestine were counted.
Empty cysts were seen but not counted in hosts
necropsied at 1 and 2 hr p.i. The numbers were
converted to percentages and the information is
presented in Table 1. In Group A, the greatest
percentage of encysted metacercariae was in
segment 1 of the small intestine, and excysted
metacercariae were recovered as far posteriad as
segment 4 of the small intestine. Most of the
4
0.4
0.5
0.5
1.7
0.1
2.0
0
1.7
0
2.2
Copyright © 2011, The Helminthological Society of Washington
5
0.4
0
0.2
0.6
0.1
0.3
0
0.4
0
0.5
Cecumlarge
intestine
0.2
0
0.1
0
0.1
0
0
0
0
0
Total
5.4
7.5
2.5
8.6
0.4
11.4
0
10.1
0
10.3
excysted metacercariae recovered at 1 hr p.i.
were located in segment 3. Some excysted metacercariae
were in the stomach at 1 hr and were
alive and active. These organisms either had excysted
in the stomach or, possibly, could have
excysted in the small intestine and migrated anteriad
to the stomach. In Group A, the finding
of most excysted metacercariae in segments 1,
2, and 3 of the small intestine (duodenum-jejunum
region) provides support for claims that in
vivo excystation takes place in the anterior part
of the small intestine. This finding supports the
statement of Simonsen et al. (1989) referenced
above.
With time, the ratio of encysted to excysted
metacercariae declined (see last column in Table
1), and by 4 hr p.i., encysted metacercariae were
not found. These findings suggest that by 4 hr
p.i. most of the encysted metacercariae had excysted
or were voided. The idea of metacercariae
being voided by 4 hr p.i. is consistent with
the fact that the usual transit time for ingested
food in the mouse digestive tract is 4 hr (Barrachina
et al., 1997). Fecal examinations to determine
the possible presence of excysted or encysted
metacercariae in the stool were not made.
About 75% of the excysted metacercariae in
Groups C, D, and E were located in segments 1,
2, and 3. Hence, newly excysted juveniles, up to
at least 24 hr p.i., are more dispersed in the gut
than are older worms. Manger and Fried (1993)
showed that by day 2 p.i. more than 90% juvenile
E. caproni were localized in segment 3 (the
jejunum), and by day 4 and beyond, worms
tended to migrate even more posteriad, with