3.3. In situ hybridization (ISH) of EHP
All 3 major species of farmed penaeid shrimp, P. vannamei, P.
stylirostris and P. monodon, were shown to be infected by EHP
based both on histology and on in situ hybridization (Tourtip
et al., 2009; Tangprasittipap et al., 2013). However, ISH was more
sensitive in detecting the causative agents; as cells with no visible
signs of inclusions or microsporidian spores by histological examination
were found to be positive by ISH. In our study, the cloned
18S rRNA gene fragment was labeled with digoxigenin and used
as a probe for ISH to the infected P. vannamei. The probe reacted
intensely to the basophilic inclusions within the cytoplasm
(Fig. 1B). Our P. stylirostris samples collected from Brunei in 2006
also showed the presence of microsporidium spores within the
hepatopancreas (Fig. 1C), where the tubule epithelial cells had a
positive reaction by ISH (Fig. 1D). In both cases, the hepatopancreas
was the only tissue that reacted to the EHP probe, and the
ISH results were associated with lesions detected by H&E
histology. These results are similar to those described by
Tangprasittipap et al. (2013) with samples of EHP-infected
P. vannamei. The diagnosis of EHP in the samples from Brunei also
show that EHP has been present in shrimp farms in Asia at least
since 2006, earlier than previously thought.
The probe appears to be highly specific. No reaction was seen in
any of the tissues prepared from SPF shrimp (data not shown). We
also tested the EHP probe to a P. monodon affected with cotton
shrimp disease, in which the tail muscle was severely infected with
a Pleistophora-like microsporidium, of which the spores appeared
more eosinophilic (Fig. 1E). The EHP probe did not react to this P.
monodon with cotton shrimp disease (Fig. 1F).
From this cotton shrimp disease shrimp, we cloned a fragment
(1.1 kb) of 18S rRNA gene and labeled with diagoxigenin for ISH.
This probe reacted to the cotton shrimp disease P. monodon (see
the inserted photo in Fig. 1F). The sequence of this 18S rRNA gene
fragment had a 94% identity to a member of the genus Pleistophora,
which appears distant from Enterocytozoon spp. based on the SSU
rRNA gene-phylogeny (Stentiford et al., 2013). This also explains
why the EHP probe did not cross react to the microsporidium of
cotton shrimp disease.