Incubation period is the time elapsed between exposure to a pathogenic organism, a chemical or radiation, and when symptoms and signs are first apparent. In a typical infectious disease, incubation period signifies the period taken by the multiplying organism to reach a threshold necessary to produce symptoms in host.
While latent or latency period may be synonymous, a distinction is sometimes made between incubation period, the period between infection and clinical onset of the disease, and latent period, the time from infection to infectiousness. Which is shorter depends on the disease. A person may be a carrier of a disease, such as Streptococcus in the throat, with exhibiting any symptoms. Depending on the disease, the person may or may not be contagious during the incubation period.
During clinical latency, an infection is subclinical. With respect to viral infections, in clinical latency the virus is actively replicating. This is in contrast to viral latency, a form of dormancy in which the virus does not replicate. An example of clinical latency is HIV infection. HIV may at first have no symptoms and show no signs of AIDS, despite HIV replicating in the lymphatic system and rapidly accumulating a large viral load. These persons may be infectious.
Intrinsic and extrinsic incubation period
The terms, "intrinsic incubation period" and "extrinsic incubation period" are used in vector-borne diseases. Intrinsic incubation period is the time taken by an organism to complete its development in the definitive host. Extrinsic incubation period is a time taken by an organism to complete its development in intermediate host.
For example, once ingested by a mosquito, malaria parasites must undergo development within the mosquito before they are infectious to humans. The time required for development in the mosquito ranges from 10 to 28 days, depending on the parasite species and the temperature. This is extrinsic incubation period of that parasite. If a mosquito does not survive longer than the extrinsic incubation period, then she will not be able to transmit any malaria parasites. After mosquito successfully transfers parasite inside human body via a bite, the parasite starts developing. The time frame between injection of parasite inside human and development of first symptoms of malaria is its intrinsic incubation period.
Multiple factors are in act to give specific incubation period for a disease process. Some of which are:
- Dose or inoculum of infectious agent
- Route of inoculation
- Rate of replication of infectious agent
- Host susceptibility and immune response
Examples of incubation periods
Due to inter-individual variation, incubation period is always expressed in a range. When possible, it is best to express the mean and the 10th and 90th percentiles, though this information is not always available. The values below are arranged roughly in ascending order by number of days, although in some cases the mean had to be inferred.
For many conditions, incubation periods are longer in adults than they are in children or infants.
|Cellulitis caused by Pasteurella multocida||0||1||days|
|Erythema infectiosum (Fifth disease)||13||18||days|
|HIV||2||3||weeks to months, or longer|
|Infectious mononucleosis (glandular fever)||28||42||days|
|Kuru disease||10.3||13.2||years (mean)|
|Pertussis (whooping cough)||7||14||days|
|Rocky Mountain spotted fever||2||14||days|
|Rubella (German measles)||14||21||days|
- Window period, the time between infection and when lab tests can identify the infection. The window period may be longer or shorter than the incubation period.
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