LATENT PERIOD

A "latent period" is the lag time between exposure to a disease-causing agent and the onset of the disease the agent causes. In infectious diseases it is often identical to the incubation period, but not always. A disease may have incubated but remains latent, or dormant, within the body. For instance, the latent period between exposure to HIV (human immunodeficiency virus) infection and the onset of AIDS (acquired immunodeficiency syndrome) may be many years, although invasion of the body by HIV does cause a transient primary infection two to three weeks after initial exposure. Transmissable spongiform encephalopathy, caused by infectious prions, may have a latent period of up to twenty years or more.

The distinction between latency and incubation is illustrated by the natural history of the asbestos-induced cancer mesothelioma, where there may have been a single finite exposure many years before the onset of malignancy. Exposure to a single large dose of ionizing radiation, such as the exposure of residents of Hiroshima to the atom bomb explosion of 1945, demonstrates the variation in the latent period between exposure to ionizing radiation and the occurrence of malignancies, as well as the relationship between malignancies and radiation dose. In general, there is an inverse relationship between exposure dose and latent period—a high dose generally results in a short latent period, while a low dose means long latency. Leukemia follows exposure to ionizing radiation with an average latency of about five years, while some other malignancies, such as bone cancer, may have a latent period of twenty or more years.

During the latent period between initial exposure and the onset of clinically detectable disease, pathological processes may be progressing slowly and inexorably, and they may be detectable with suitable screening tests. Timely intervention at this stage may arrest the progress of disease. However, much remains to be discovered about the natural history of many disease processes with long latent periods between exposure and the appearance of overt disease.

John M. Last

(see also: Incubation Period )

latent period The short time that elapses between the reception of a stimulus and the start of the response in an irritable tissue. For a contracting muscle the latent period lasts about 0.02 seconds, during which time calcium ions are released from the sarcoplasmic reticulum.