Loma branchialis

From Fish
Numerous xenomas of Loma in the gills of Alaska pollack
Fresh spores of Loma branchialis

Loma branchialis and L. salmonae are microsporidian parasites that infect marine and freshwater fish, including as Pacific salmon, Atlantic cod (Gadus morhua), Alaska pollock (Theragra chalcogramma) and haddock (Melanogrammus aeglefinus).

This parasite mainly attaches to the gills but may also affect the heart and liver in heavily infected fish. The parasite infects endothelial cells, predominately within the gills, forming white cyst-like structures, referred to as xenomas[1].

Life cycle

Docker et al. (1997) found a high prevalence of the infection in the ovaries of sexually mature salmon, and suggested that transnlission from adults to progeny may occur. An appealing hypothesis is that fish first become infected as aelvins by spores deposited with eggs. Infected fish may maintain the infection subclinically as they mature during their seawater phase. Shortly before beginning their return migration, and when entering warmer water, fish may begin to develop clinical signs of L. salrnonae. Conversely, they may recover some months after exposure, and become re-infected either in fresh water or seawater when they undergo sexual maturation[2].

A number of spores are produced inside the xenomas. An ellipsoidal spore is 4.8 mm in mean length and 2.3 mm in mean width.

Clinical signs

Many spherical xenomas (a complex formed by a host cell and a parasite) are observed in the gill.

Heavily infected fish exhibit a growth retardation, a low condition factor, a low hematocrit value, finally resulting in mortality (Khan, 2005). Since this parasite is not infectious to human, it is harmless in food hygiene.


Diagnosis is based on determining the presence of Loma spp spores by wet-mount of xenomas. The sample should be smeared and stained by Uvitex 2B followed by a fluorescent microscopic observation. The stained spores emit blue fluorescence under UV radiation.

This parasite has been reported from gadoid fishes in the North Sea and the North Atlantic Ocean. Pathogenicity of Loma branchialis has been recently highlighted because disease outbreak occurred in cultured Atlantic cod[3].


  1. Shaw RW, Kent ML, Adamson ML (1998) Modes of transmission of Loma salmonae (Microsporidia). Dis Aquat Org 33:151-156
  2. Docker MF, Devlin RH. Richard J, Kent IML (1997) Sensitive and specific polymerase chain reaction assay for detection of Loma salmonae (Microsporea). Dis Aquat Org 29:41-48
  3. Khan, R. A. (2005) Prevalence and influence of Loma branchialis (Microspora) on growth and mortality in Atlantic cod (Gadus morhua) in coastal Newfoundland. J Parasitol 91:1230-1232.