Incubation period

Incubation period
n., [ˌɪn kyəˈbeɪ ʃən ˈpɪɹ.i.əd]
Definition: The period between the initial infection and the onset of signs and symptoms

Table of Contents

Incubation Period Definition

The incubation period is the time duration between exposure to the pathogen and the appearance of the disease symptoms. In epidemiology, the incubation period implies the duration taken by the pathogen to multiply to reach the threshold beyond which clinical symptoms start appearing. The incubation period differs for each pathogen; for example, the Influenza virus of the flu, the incubation period is one day whereas the incubation period of the measles virus is around 12 days. Hepatitis virus takes approximately four weeks to exhibit any clinical symptoms. In comparison, the chickenpox incubation period is about 2 to 3 weeks to show clinical symptoms after exposure. Canine Parvovirus of dogs has an incubation period of 3 – 7 days.

In the current context, it is pertinent to know: How long is the incubation period of coronavirus?

The viral incubation period for coronavirus varies between 1-14 days; however, the incubation time is around five days in most patients. Hence, the incubation period COVID is considered to be five days.

The incubation period or incubation phase of a pathogen is an essential parameter in disease epidemiology as it reflects the efficiency of the control measures in controlling the infection. It also helps to map out the strategies to control the spread of infection.

Any pathogenic disease development undergoes four stages of infection. They are as follows:

Stage 1- Incubation period of infection
Stage 2- Prodromal period of infection
Stage 3- Acute period of infection
Stage 4- Convalescence period of infection

As per the time, different stages of infection can be defined, and these stages are characteristic of an epidemiologic infectious disease. The infection in the body is initiated by the entry of the pathogen, which is defined as ‘exposure‘. After the exposure, the pathogen will get transferred to the organ or tissue in the body where the conditions for its survival are optimum, and it can multiply to establish the infection. For example, the liver is the site for the multiplication of the hepatitis virus and the COVID-19 virus multiplies in the lungs. Once the pathogen starts multiplying, the body’s immune system gets activated. Although, at this stage, the clinical symptoms of the disease may not be present. However, the infection may be diagnosed with the help of laboratory diagnostic tests. The disease may be classified as ‘symptomatic‘ and ‘asymptomatic’ at this stage. The time between exposure and infection is known as the ‘latent period.’ In the latency period, the clinical symptoms are not manifested.

So, what is “incubation period”? In this context, the meaning of incubation period is the duration between exposure to the pathogen and the onset of symptoms. This particular period characterizes or defines the ‘incubation period’ (also referred to as the ‘incubation stage of infection’). Hence, in this stage, the manifestation of clinical symptoms occurs, i.e., at the incubation period disease is symptomatic. Each pathogenic organism undergoes the stages of incubation in the host.

The host may transmit infection at any stage of infection; however, it may differ from pathogen to pathogen. So now the question arises, how long are viral infections contagious? Usually, a period of 2 weeks is considered to be contagious. Nevertheless, it varies from pathogen to pathogen.

Different stages of infection have been depicted figuratively in Figure 1 for better understanding.

Figure 1: Different stages of infection as per the time. Image Credit: Europa.eu

Biology definition:
The incubation period is the period between the initial infection and the onset of signs and symptoms. The incubation period is the initial stage of an infectious disease. During this stage, the pathogens are actively dividing and growing in number. The duration of the incubation period may vary depending on various factors, such as the immunocompetence of the host and the virulence of the pathogen. The next stage after the incubation period is the prodromal stage.See also:

Intrinsic and Extrinsic Incubation Period

The terms intrinsic and extrinsic incubation periods are applicable for vector-borne diseases.

The intrinsic incubation period is the duration between pathogen exposure and the appearance of symptoms of the disease in the definitive host.
The extrinsic incubation period is the duration required for the complete development of the pathogen in the intermediate host. For example, it takes 28 days for the malaria parasite’s gametocyte stage to develop into the sporozoite stage in the mosquito body. Hence, 28 days is the extrinsic incubation period of the malarial parasite. The extrinsic incubation period of the malaria parasite is also known as sporogony, which is the duration between ingestion of gametocyte stage via infected blood meal to the stage where sporozoite stage reaches the salivary gland of the mosquito, making it capable of transmitting the infection. The extrinsic incubation period in malaria is affected by various factors like temperature, genetic diversity of the parasite and vector, biotic and abiotic factors. Thus, the incubation period of malaria ranges from 7 to 30 days.

The intrinsic and extrinsic incubation periods can be understood with the help of a diagrammatic illustration of the infection cycle of the Zika virus (Figure 2).

Figure 2: Extrinsic incubation period and intrinsic incubation period of Zika virus. Image Credit: Abushouk et al., 2016.

Determining Factors

The incubation period of a pathogen is dependent upon a number of factors, that includes-

Duration of exposure to the pathogen
Amount or dose of the pathogen on exposure
Route of entry of the pathogen
Vulnerability or predisposition of the host to the pathogen
Rate of the multiplication of pathogen in the host
Host defense system

Examples for Diseases in Humans

The answer to the question ‘what is the average incubation period for a virus’ is pretty dramatic as the virus incubation period may vary from 1 day to year(s). Some of the examples of the viral incubation period of pathogens are as mentioned below:

Incubation period covid- 19 virus: 5 to 6 days
Delta variant incubation period: 2-3 days
Chickenpox incubation period: 2 to 3 weeks
Shingles incubation period: 7 to 10 days
Flu incubation period: 2 days
Common cold incubation period: 1 to 3 days
HIV incubation period: around four weeks or one month
Rabies incubation period: few days to a year
Respiratory Syncytial Virus or RSV incubation period: 2 to 8 days
Salmonella incubation period: 12 to 72 hours
Cough or croup incubation period: 2- 5 days
Stomach flu incubation period or stomach bug incubation period: 24 to 48 hours
Group A Strep pharyngitis or Strep incubation time: The incubation period of Strep throat ranges from 2 to 5 days
E. coli incubation period: 3 to 4 days
Mumps incubation period: 16- 18 days
Pink eye incubation period: 24 to 72 hours
Tetanus incubation period: 3 to 21 days
Roseola incubation period: 5 to 15 days
Ebola incubation period: 2 to 21 days
Pneumonia incubation period: 2 to 5 days
Listeria incubation period: 1 to 2 weeks
Rotavirus incubation period: 2 days
Scabies incubation period:4 to 8 weeks
Ringworm incubation period: 10 to 14 days
Tonsillitis incubation period: 2 to 4 days
Rubella incubation period: 12 to 23 days
The incubation period of hepatitis b: 90 days
The incubation period of tuberculosis: few months to two years
The incubation period of cholera: 2 hours to 5 days
The incubation period of diphtheria: 2 to 5 days

Sexually transmitted diseases or STD incubation period:

Chlamydia incubation period: 7 to 21 days
Herpes incubation period or Genital herpes incubation period: 4 days
Gonorrhea incubation period: 2 to 5 days
Syphilis incubation period: 21 days
Hepatitis A incubation period: 15 to 50 days

Try to answer the quiz below to check what you have learned so far about the incubation period.

References

Lai, C., Yu, R., Wang, M., Xian, W., Zhao, X., Tang, Q., Chen, R., Zhou, X., Li, X., Li, Z., Li, Z., Deng, G., & Wang, F. (2020). Shorter incubation period is associated with severe disease progression in patients with COVID-19. Virulence, 11(1), 1443–1452. https://doi.org/10.1080/21505594.2020.1836894
Ohm, J.R., Baldini, F., Barreaux, P. et al. Rethinking the extrinsic incubation period of malaria parasites. Parasites Vectors 11, 178 (2018). https://doi.org/10.1186/s13071-018-2761-4
Rothman K. J. (1981). Induction and latent periods. American journal of epidemiology, 114(2), 253–259. https://doi.org/10.1093/oxfordjournals.aje.a113189
Virlogeux, V., Fang, V. J., Wu, J. T., Ho, L. M., Peiris, J. S., Leung, G. M., & Cowling, B. J. (2015). Brief Report: Incubation Period Duration and Severity of Clinical Disease Following Severe Acute Respiratory Syndrome Coronavirus Infection. Epidemiology (Cambridge, Mass.), 26(5), 666–669. https://doi.org/10.1097/EDE.0000000000000339