X-cell parasites: an emerging threat to marine fish (completed)

X-cell parasites are of much interest because they are disease causing agents of many groups of fish, yet we know almost nothing about their distribution, lifecycle, and mode and frequency of infection.

Illustration: Pseudobranchial pseudotumours in Atlantic cod Gadus morhua from Iceland. a) Five cod from the same year-class; two uninfected fish (top) and three infected fish (bottom). b) Pseudobranchs have a gill-red colour in healthy fish and are enlarged and have a creamy-whitish appearance when infected (c) (black arrows). Scale bars a = 4 cm, b & c visible. From Freeman et al. (2011)

About the project

The term X-cell was first used in 1969 to refer to distinctive cells found in epidermal tumours of flatfish. Subsequently, cells with a similar appearance have been found in over 25 species of fish.  Many of these are commercially important, and are also farmed, which greatly increases the incidence of disease and its transmission. X-cell diseases are still mostly unknown, so we currently have a) no way of knowing how important they really are, b) which so far uncharacterised diseases are caused by X-cells, and c) no understanding of how to best manage aquaculture systems to minimise X-cell related disease. Currently the impact of X-cells is likely to be highly underestimated, as many of its effects are attributed to other, or unknown causes, or not easily recognised.

X-cell disease forms in three main fish tissues, depending on the species infected: large epidermal tumours in the skin epidermis, fins, head, and inner opercular region, pseudotumours in the pseudobranchial tissue, and swollen gill filament lesions.

In addition to the commercial interest, X-cell parasites are of much interest because 1) they are disease causing agents of many groups of fish, yet we know almost nothing about their distribution, lifecycle, and mode and frequency of infection, and 2) they occupy a key branching position in the tree of life, being related to many parasites of a wide range of animals, some of which are of significant commercial importance. They will teach us a lot about the evolution of parasitism in alveolates. Therefore, the objectives of the project is to close these knowledge gaps, which are of high ecological and evolutionary importance.


  • To determine the ecological and geographical distribution of X-cell lineages, and their phylogenetic relationships.
  • Use FISH microscopy and immuno-staining to elucidate the X-cell lifecycle.
  • Use comparative genomics methods to identify how this pathogen infects and interacts with host and what are the genetic characteristics associated with the process of infection.
  • Develop biotechnological tools for detection of X-cells in fish and in the environment for early warning systems.


The University of Malaya

The Natural History Museum (London)

The Norwegian Veterinary Instiute



Tags: X-cell, Fish, Fish Farming, Fisheries Management and Ecology, Parasites
Published Apr. 19, 2017 12:48 PM - Last modified Oct. 15, 2019 12:52 PM


  • Kamran Shalchian-Tabrizi
  • Dag Klaveness
  • Dr. Mark Freeman, University of Malaya
  • Dr. David Bass, Natural History Muesum (London)
  • Anders Jørgensen, Norwegian Veterinary Institute
Detailed list of participants