DAO 46:31-40 (2001)  -  doi:10.3354/dao046031

Evidence that infectious stages of Tetracapsula bryosalmonae for rainbow trout Oncorhynchus mykiss are present throughout the year

M. Gay1,*, B. Okamura2, P. de Kinkelin1,**

1Institut National de la Recherche Agronomique (INRA), Unité de Virologie et d¹Immunologie Moléculaires, Pathologie Infectieuse et Immunité des Poissons, 78352 Jouy en Josas Cedex, France
2School of Animal and Microbial Sciences, University of Reading, Whiteknights, PO Box 228, Reading RG6 6AJ, United Kingdom
*Present address: Institut Français pour la Recherche et l¹Exploration de la Mer (IFREMER), Laboratoire de Pathologie et Génétique des Invertébrés Marins, Ronce les Bains, BP 133, 17390 La Tremblade, France **Corresponding author. E-mail:

ABSTRACT: Proliferative kidney disease (PKD) is a hyperplastic condition of the lymphoid tissue of salmonids infected with the spores of Tetracapsula bryosalmonae, a myxozoan parasite formerly designated PKX, which has recently been described as a parasite of several species of bryozoans. The occurrence of PKD is generally associated with seasonal increase in water temperature, with research indicating that transmission of the disease does not occur below 12 to 13°C. This suggested that the infectious stages are absent from about November to March/April. Here we document the transmission of PKD at water temperatures and seasons previously considered to be non permissive for PKD infection. The exposure of naive rainbow trout Oncorhynchus mykiss (Walbaum) to PKD-infected water ranging from 8 to 13°C during the Autumn, Winter and early Spring, resulted in the infection of kidney interstitium once the trout were transferred to 16°C. In addition, cohabitation studies were conducted with the bryozoan host Fredericella sultana collected from a river at times of low seasonal temperatures because this bryozoan species overwinters as living colonies. Cohabitation of trout with colonies of F. sultana in parasite-free city water at 16°C, also led to renal lymphoid tissue infection with the parasite and even to nephromegaly. Our results provide evidence that the infectious stages of T. bryosalmonae for rainbow trout were present in the water throughout the entire year and that the impact of temperature on the development of PKD is primarily a result of the kinetics of Tetracapsula multiplication in bryozoan and fish hosts.

KEY WORDS: Trout · Freshwater bryozoan · Parasite · Proliferative disease · Myxosporean · PKX · Tetracapsula · Temperature

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