3. Results
Microsporidia were isolated from a fairy shrimp B. thailandensis
sample from a laboratory stock at the Department of Fisheries, Faculty
of Agriculture and Natural Resources, Rajamangala University of
Technology Tawan-Ok, Chon Buri province, Thailand. Screening of this
fairy shrimp stock showed an infection rate of more than 50% in five
days. Infected fairy shrimps were easily distinguished from normal
specimens (Fig. 1) by the presence of a white zone in the muscle. The
clinical signs of microsporidiosis in fairy shrimp are shown in Fig. 2.
The first phenotype appeared as white tubular masses in the muscle
tissue. Early infection appeared as a small cylinder about 0.5 mm along
in the muscles. Advanced infection appeared as large tubular masses,
often up to 5 mm in length (Fig. 2). Spores had an ellipsoid shape
(Fig. 3). The number of sporonts within a sporophorous vesicle varied
with a maximum of 12 observed in wet mount (Fig. 4).
Histological observation showed the presence of the microsporidia
parasites in large clusters within muscle fibers (Fig. 5). Close contact
between the sporophorous vesicle and degenerating muscle fibers
was observed, but there was no evidence of xenoma or a host response
to the parasite (Fig. 6).
Examination of semi-thin (Figs. 7 and 8) and ultra-thin sections
(Figs. 9–13) revealed that the length of the spores was about 2.829–
3.269 (3.086) µm and the width was about 1.686–2.857 (1.850) µm. The
spore coat consisted of an electron dense exospores of 0.048–0.062
(0.055) µm in thickness, an electron lucent endospore of 0.109–0.131
(0.122) µm in thickness and a plasma membrane. The sporoplasm
contained a single nucleus, the posterior vacuole and ribosomes. The
polar filament is attached to the anterior end of the spore by an
anchoring disc and is divided into two regions: the manubroid or
straight portion, and the posterior region forming a variable number
of coils around the sporoplasm (Weiss, 2001). The polar filament has
12–13 coils, arranged in a single row at the posterior end of the spore.
The coils were 0.074–0.078 (0.076) µm in diameter (Fig. 13).
Optical and electron microscopic investigations of infected fairy
shrimps showed that gross pathology, morphology, histopathology
and ultrastructure of the microsporidia are similar to those described
earlier for Pleistophora spp. (Lom and Nilsen, 2003; Dyková, 2006).