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 the 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, without 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. The intrinsic incubation period is the time taken by an organism to complete its development in the definitive host. The extrinsic incubation period is the time taken by an organism to complete its development in the 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 the extrinsic incubation period of that parasite. If a female mosquito does not survive longer than the extrinsic incubation period, then she will not be able to transmit any malaria parasites. After a mosquito successfully transfers the parasite to a human body via a bite, the parasite starts developing. The time between the injection of the parasite into the human and the development of the first symptoms of malaria is its intrinsic incubation period.
The specific incubation period for a disease process is the result of multiple factors, including:
- 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, the incubation period is always expressed as 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, 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|
|Ebola||1||21 (95%), 42 (98%)||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|
|Rabies||1||3||months, but may vary from <1 week to >1 year.|
|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.
- Sharara, A. I. (1997). "Chronic hepatitis C". Southern Medical Journal 90 (9): 872–7. doi:10.1097/00007611-199709000-00002. PMID 9305294.
- Cellulitis, kidshealth.org. Accessed 2012-05-28.
- , Accessed 2012-05-28.
- Azman, Andrew S.; Rudolph, Kara E.; Cummings, Derek A.T.; Lessler, Justin (2013). "The incubation period of cholera: A systematic review". Journal of Infection 66 (5): 432–8. doi:10.1016/j.jinf.2012.11.013. PMC 3677557. PMID 23201968.
- Erythema Infectiosum at eMedicine
- Seasonal Influenza (Flu), Centers for Disease Control and Prevention, cdc.gov. Accessed 2012-05-28.
- Lessler, Justin; Reich, Nicholas G; Brookmeyer, Ron; Perl, Trish M; Nelson, Kenrad E; Cummings, Derek AT (2009). "Incubation periods of acute respiratory viral infections: A systematic review". The Lancet Infectious Diseases 9 (5): 291–300. doi:10.1016/S1473-3099(09)70069-6. PMC 4327893. PMID 19393959.
- Common cold, The Mayo Clinic, mayoclinic.com. Accessed 2012-05-28.
- Gubler, D. J. (1998). "Dengue and dengue hemorrhagic fever". Clinical Microbiology Reviews 11 (3): 480–96. PMC 88892. PMID 9665979.
- Are the Ebola outbreaks in Nigeria and Senegal over?, World Health Organization, who.int. Accessed 2014-10-21.
- Roseola Infantum at eMedicine
- Kahn, James O.; Walker, Bruce D. (1998). "Acute Human Immunodeficiency Virus Type 1 Infection". New England Journal of Medicine 339 (1): 33–9. doi:10.1056/NEJM199807023390107. PMID 9647878.
- Macnair, Trisha, Glandular fever, BBC, bbc.co.uk. Accessed 2012-05-28.
- Huillard d'Aignaux, J. N.; Cousens, S. N.; MacCario, J; Costagliola, D; Alpers, M. P.; Smith, P. G.; Alpérovitch, A (2002). "The incubation period of kuru". Epidemiology 13 (4): 402–8. doi:10.1097/00001648-200207000-00007. PMID 12094094.
- Questions and Answers About Marburg Hemorrhagic Fever, Centers for Disease Control and Prevention, cdc.gov. Accessed 2012-05-28.
- Measles, American Osteopathic College of Dermatology, aocd.org. Accessed 2012-05-28.
- Mumps Disease, Questions & Answers, vaccineinformation.org. Accessed 2012-05-28.
- Norovirus, Centers for Disease Control and Prevention, cdc.gov. Accessed 2012-05-28.
- Pertussis, GPnotebook, gpnotebook.co.uk. Accessed 2012-05-28.
- Polio, GPnotebook, gpnotebook.co.uk. Accessed 2012-05-28.
- "WHO - Rabies". who.int.
- Rocky Mountain Spotted Fever, About.com. Accessed 2012-05-28.
- Dermatologic Manifestations of Rubella at eMedicine
- Scarlet Fever at eMedicine
- World Health Organization (WHO), Severe acute respiratory syndrome, www.who.int. Accessed 2012-05-28.
- Smallpox Disease Overview, Centers for Disease Control and Prevention, cdc.gov. Accessed 2012-05-28.
- Tetanus at eMedicine