Diplostomum baeri infection

Ubels, John L., DeJong, Randall J., Hoolsema, Brittany, Wurzberger, Amy, Nguyen, Thuy-Tien, Blankespoor, Harvey D. & Blankespoor, Curtis L., 2018, Impairment of retinal function in yellow perch (Perca flavescens) by Diplostomum baeri metacercariae, International Journal for Parasitology: Parasites and Wildlife 7 (2), pp. 171-179 : 175-176

4.3. Pathology associated with Diplostomum baeri infection

Most studies of eye infection by Diplostomum spp . report metacercariae in the lens and vitreous humor. In the present study the only metacercaria that was observed in the vitreous humor of a yellow perch is seen in Fig. 4D , and is possibly an artifact of dissection.

There are only a few reports of retinal infection that illustrate location and pathology. Marcogliese et al. (2001a) observed Diplostomum sp. in the retina of yellow perch, and Höglund and Thulin (1992) reported that metacercariae are in the choroid of the European perch, but provided no histologic evidence. Lester and Huizinga (1977) published photomicrographs of metacercaria between the pigment epithelium and photoreceptors of yellow perch. Shariff et al. (1980) and Heckmann and Ching (1987) published single images of metacercaria behind the retinas of cutthroat trout ( Salmo clarki ) and rainbow trout ( Oncorhynchus mykiss ), respectively. A very recent report by Padros et al. (2018, published while this communication was in revision) shows that Diplostomum sp. is located between the pigment and photoreceptors of arctic charr ( Savelinus alpinus ) causing extensive damage to both the pigment epithelial cells and photoreceptor outer segments.

In the yellow perch, although Diplostomum bae ri were occasionally observed in contact with the photoreceptors ( Fig. 4E ), we interpret the location of the metacercariae to be primarily within the choroidal layer. This location is of advantage to the parasite in that this would provide an abundant supply of oxygen and nutrients. As discussed in more detail below, the presence of metacercariae in the choroid is expected to be a distinct disadvantage to the fish due to impairment of oxygen delivery to the retina.

Thinning and loss of the pigment epithelium was observed whether metacercaria were behind this tissue in the choroid or within it. This is of profound importance for retinal function because of the importance of pigment epithelial cells in support of retinal function. Spent photoreceptor outer segment discs are phagocytosed by the pigment epithelium, and these cells are essential in the vitamin A cycle which supplies the chromophore, cis-retinal, to the photoreceptors ( Bok, 1990; Saari, 2016). As such, damage to the pigment epithelium will adversely affect the ability of the retina to respond to light. Of note, in both perch ( Fig. 4C ) and arctic charr ( Palmieri et al., 1977) damage to photoreceptors is greatest in areas where the pigment epithelium is lost.

As discussed above, Diplostomum sp. is commonly located in the lens. This provides the metacercariae with a rich source of protein and protects them from the immune system of the fish. In contrast, location in or near the choroid, while providing nutrients, would be expected to render the metacercariae highly vulnerable to attack by the immune system. It is therefore of interest that no inflammatory cells are observed in the choroid of perch in the present study or in infected arctic charr ( Palmieri et al., 1977). In contrast to other Diplostomum spp ., Diplostomum baeri and the species that infects arctic charr, which was not identified, have apparently adapted in ways that render them unrecognizable or protect them from attack by the immune system.

Figures

Fig. 4. Paraffin sections of yellow perch (Perca flavescens) retinas and choroid infected with Diplostomum baeri, stained with H&E. A. Metacercariae in the choroid layer with thinning of the pigment epithelium. Retina in the unifected region on the far left of the image is in proximity to the rete mirabile. B. Pocket of metacercariae with melanin debris due to extensive damage to the choroid and pigment epithelium. C. Large pocket of metacercariae with melanin debris and loss of the pigment epithelium and photoreceptor layer (arrow). D. Uninfected region of the same eye shown in image C with normal retina, pigment epithelium and optic nerve. This was the only metacercaria seen in the vitreous humor of a perch on a histological section. E. Presence of the metacercaria increases the diffusion distance for O2 from the rete mirabile to the retina. Note damage to the pigment epithelium in locations of contact with the metacercaria. F. Wright- Giemsa stain of the same region shown in image E. No inflammatory cells were detected. Bar = 100 μm. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)