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An intracellular protozoon was discovered in the epithelium of young rainbow trout (Salmo gairdneri) exposed for as short a time as 1 hr to water known to contain infective stages of Myxosoma cerebralis. Light- and electron-microscopic examination of this tissue revealed what appeared to be a proliferative stage (presumptive schizont) of a sporozoon; other possible stages in the life cycle were also observed. The relationship of this unidentified protozoon of M. cerebralis remains unresolved.

Three evaluative systems, immunodiffusion, fluorescent antibody (FA), and electron microscopy (EM), were used to follow the morphogenesis of Marek's disease virus in inoculated chickens. Of the three, EM and FA were the most sensitive in detecting early stages of infection. Virus particles were found in skin biopsy specimens as early as 12 days post inoculation. Immature naked particles appeared first in the nucleus; later particles were enveloped in the cytoplasm and enclosed in cytoplasmic inclusion bodies. No evidence for continued virus replication was seen in feather follicles after an initial burst of heavy virus production, which lasted several weeks. Residual virus, however, was found occasionally in cytoplasmic inclusion bodies within keratinized material near the feathers. This was believed to contribute to the long-term shedding of infectious virus into the environment.

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