Causes

Microsporidia caused by Loma sp.

Horizontal transmission is presumably accomplished by the disintegration of mature gill xenomas or after the death & decomposition of the host & liberation of spores into the water. After they are ingested by a host or by plankton which a fish host later consumes, spores may germinate in the digestive tract & enter the bloodstream. The gill is apparently the primary seat for initial infection of host cells by spores, which proliferate in a single cell forming an xenoma (xenoparasitic cyst). The close association of the lamellar xenomas with the gill pillar system would indicate the parasite reached the gills via the blood.
The direct association of the xenomas with lamellar epithelium & the fact that some pillar cells are phagocytic, both support the hypothesis of direct infection of the gills from the water. The spores are then apparently spread from deteriorating xenomas into the arterial blood, where they auto-infect downstream tissues or are engulfed by endothelial macrophages & develop into xenomas.
The lack of spores in the intestinal tract may eliminate this site as a natural route for horizontal transmission. The presence of xenomas & spores in the kidney & gills would indicate these tissues may be routes for the transmission outside the host.
Acute infections causing mortality are infrequent , & usually only a few fish from a group, show noticeable symptoms of the infection. In such cases these fish are probably asymptomatic carriers. Although there are several species of microsporidians, with demonstrated transovarian transmission, it is presently unknown whether Loma & Pleistophora are among them.

Prognosis

Acute losses have been reported as high as 75%. Survivors of epizootics are carriers, often asymptomatic . If infections are extensive & systemic, the prognosis is grave. If infections are limited to the gills, the condition is more chronic & the prognosis is more favourable . However, the true prognosis for a fish with a seemingly low infection is unknown.

Treatment

Prevention

Avoidance is the most effective means of control. Quarantine & aseptic disposal should be considered when infections occur in cultured fish. Loma infections are probably quite widespread in parts of the United States because of local stocking practises & the fact that the parasite often does not cause overt signs or disease.

When infections are detected, histopathology should be conducted to evaluate morbidity. Low-grade Loma infections may erroneously be considered as "latent with no potential for disease manifestation". The low infection & lack of overt signs, possibly misrepresent the real condition, that the culturist may attribute as "normal mortality", but persisting over a long period of time.

Should an epizootic occur, the infected fish should be buried or incinerated & not stocked in waters, or transferred to other hatcheries or farms.

Contributors

Dr. Hauck, Kent
Fish Pathologist
Utah Dept. of Aquaculture
Salt Lake City  UT.   USA