Today is our topic of discussion Incubation period
Incubation period
Definition:
It is the time interval from the moment of exposure to an infectious agent until signs and symptoms of the disease appear. Example: IP of chickenpox is 14-16 days.
The IP ranges from very short (few hours to 2-3 days in cases of staphylococcal food poisoning, cholera, influenza etc.) to long. Most infectious diseases have IP of median length (10 days to 3 weeks). Certain infectious diseases are communicable during the incubation period, sch as chicken pox, measles, hepatitis A etc. Factors which determine the incubation period are –
a. Rate of multiplication of the pathogen
b. Infective dose
c. Portal of entry and
d. Susceptibility of the individual
Importance of incubation period:
a. Tracing the source of infection and contacts
b. To determine the period of surveillance or quarantined
c. For passive immunization, as prophylactic measure
d. Identification of point-source or propagated epidemics
e. To determine the prognosis of disease; some diseases if manifested with short IP (e.g. tetanus, rabies) usually have worse prognosis.
Median incubation period:
It is the time required for 50% of the cases to occur following exposure.
Latent period:
It is the period from disease initiation to disease detection. The term ‘latent period’ is applicable to non-infectious diseases, which may be months or years; such as cancer. IHD. mental illness etc.
Prodromal period:
It is the interval between the onset of symptoms of an infectious disease and the appearance of characteristic manifestations. Example: In measles, fever and coryza occur in the first three days followed Koplick spot on buccal mucosa and characteristics skin lesions appear on the fourth day.
Infective Period/Period of communicability:
The period during which an infected person can transmit a pathogen to a susceptible host.
Convalescent period/Convalescence:
It is the gradual recovery of health and strength after illness. It refers to the later stage of an infectious disease or illness when the patient recovers and returns to normal, but may continue to be a source of infection even if feeling better.
Source
The starting point for the occurrence of a communicable disease is the existence of a reservoir or source of infection.
Definition:
The source of infection is defined as, the person, animal, object or substance from which an infectious agent passes or is disseminated to the host.
Reservoir
Definition:
Any person, animal, arthropod, plant, soil, or substance, or a combination of these, in which an infectious agent normally lives and multiplies, on which it depends primarily for survival, and where it reproduces itself in such a manner that it can be transmitted to a susceptible host.
It is the natural habitat of the infectious agent.
Homologous reservoir:
When another member of the same species is the victim. For example, man is the principal reservoir for some enteric pathogens, e.g., Vibrio cholerae.
Heterologous reservoir:
When infection derived from a reservoir other than man; such as, birds and animals infected with salmonella.
Types of reservoir:
Reservoir is three types –
Human reservoir (case & carrier):
There are a number of pathogens that are specifically be adapted to man.
Animal reservoir (animals, birds etc):
Some infective agents that affect man have their reservoir in animals. Man is not an essential part (usual reservoir) of the life cycle of the agent. Example –
I. Bovine tuberculosis – Cow to man
II. Anthrax – Cattle, sheep, goats, horses to man
III. Rabies – Dogs, foxes and other wild animals to man
IV. Brucellosis – Cows, pigs and goats to man
Reservoir in non-living things (soil & inanimate objects):
The disease agents living in soil are fully adapted to live freely in nature.
Biologically, they are equipped to withstand marked environmental changes in temperature and humidity. Example –
I. Clostridium botulinum, etiologic agent for Botulism
II. Clostridium tetani, etiologic agent for Tetanush the
III. Clostridium welchii, etiologic agent for Gas gangrene
Case
Definition:
A case is defined as “a person in the population or study group identified as having the particular disease, health disorder, or condition under investigation”.
Methods of identification of cases:
Clinical, biochemical, laboratory examination.
Spectrum of illness according to gradient of infection:
Not always cases may present as full-blown discase. This variation in the manifestation of disease is known as spectrum of illness. Accordingly cases may be –
Clinical:
The clinical illness may be mild or moderate, typical or atypical, severe or fatal depending upon the gradient of involvement. In our context, mild cases are more important than the severe cases. Because they are ambulant in the society and spread the infection wherever they go, whereas severe cases are usually confined to bed.
Subclinical:
The subclinical cases are referred to as inapparent, covert, missed or abortive cases. They are also important sources of infection. The disease agent may multiply in the host but does not produce signs and symptoms. They can only be identified by laboratory tests.
Latent:
Those who are in the latent infection do not shed the infectious agent and the agent lies dormant within the host without symptoms and often undetectable by the laboratory tests. Examples are herpes zoster, herpes simplex etc.
Epidemiologically, cases may be:
1. Suspected (usually diagnosed at the primary level), which then becomes
2. Probable (when suspected cases are further investigated at the secondary level) and finally becomes→
3. Confirmed by appropriate investigation (usually at the tertiary level under the supervision of the health authority such as by the DGHS-Bangladesh or the specimen may be sent to CDC-USA)
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