Routes of entry of Piscirickettsia salmonis in rainbow trout Oncorhynchus mykiss

Abstract

Since 1989, Piscirickettsia salmonis, the causal agent of piscirickettsiosis, has killed millions of farmed salrnonids each year in southern Chile. The portal of entry for the pathogen was investigated by use of selected experimental infections in juvenile rainbow trout (12 g). The methods used were intraperitoneal injection, subcutaneous injection, patch contact on skin, patch contact on gills, intestinal intubation and gastric intubation. Cumulative mortalities at Day 33 post-inoculation were 98, 100, 52, 24, 24, and 2 % , respectively. It was shown that intact skin and gills could be penetrated by P. salmonis. The high mortality obtained in subcutaneously injected fish indicated that s k i injunes could facilitate the invasion of this pathogen. Results suggested that the main entry sites are through the skin and gills and that the oral route may not be the normal method by which P. salmonis initiates infection of salmonids.

Experimental infection of coho salmon Oncorhynchus kisutch by exposure of skin, gills and intestine with Piscirickettsia salmonis

“…Gills were also shown to be also an entry site to Piscirickettsia salmonis in coho salmon, which agrees with the results reported for rainbow trout (Smith et al 1999) and Atlantic salmon (Almendras et al 1997).…”

“…In salmonid fish, besides data on P. salmonis (Smith et al 1999), the results reported by Effendi & Austin (1995) also suggests that undamaged skin may be one of the entry sites of A. salmonicida, but in this case a swabbing method of inoculation was used which could have facilitated bacterial entry. Although it has been suggested that the gills are the primary site of antigen uptake in bath immunization of fish, it was demonstrated that the principal site of uptake in rainbow trout using soluble (Ototake et al 1996) and particulate (Moore et al 1998) antigens is the skin with a lesser role for the gills.…”

“…Among the routes tested, skin proved to be the most efficient entry site of this bacterium, followed by intestine and then gills. In a similar study with rainbow trout Oncorhynchus mykiss, skin was also the most effective portal of entry of P. salmonis (Smith et al 1999) although trout infected by GE exhibited the same cumulative mortalities (24%) as those exposed to the bacteria by IE. In contrast, the higher rate of mortality following IE compared with GE in this study might be explained by the difference in the salmonid species used (coho salmon vs rainbow trout).…”

“…Theoretically these results suggest that ascending colonization of the intestine through the anal opening and/or degluted food contaminated with viable Piscirickettsia salmonis might lead to infection of the fish through the gut epithelium in natural conditions. Nevertheless, these hypothetical conditions are not likely to occur because ascending infections would move against the natural flow of the intestinal contents; on the other hand, oral infections with P. salmonis are effectively inactivated by the stomach environment as was documented in rainbow trout inoculated by intragastric intubation (Smith et al 1999).…”

Comparison of in vitro-cultured and wild-type Perkinsus marinus. II. Dosing methods and host response

“…Instead, they support the findings of Perkins (1976) and Dungan et al (1996) that the external epithelia of the gill, mantle, and labial palps are equally, if not more, important. Many intubated parasites may not have survived the digestive process in the stomach, as was hypothesized to explain the very low apparent virulence of the prokaryotic fish pathogen Piscirickettsia salmonis delivered via intubation into the stomach compared to that delivered by skin, gill patches, or by direct injection (Smith et al 1999). When we provided oysters with Perkinsus marinus in their food or via shell-cavity injection, the outcome was similar: within 3 d of dosing, parasite densities were the same in the gill and mantle as in the digestive gland.…”