Question

Review the common disease pattern: incubation , prodromal period. Period of illness, decline and convalescence. Give one example.

Answer 1

Stages of Disease Development

The question is to review the stages/phases of disease development with help of an example.

Communicable diseases have shown a common pattern of progression during its causation. Even though the severity and duration of each communicable disease may vary, they all show a common pattern in its progression.

Three factors are considered vital in causation of communicable disease, termed the ‘epidemiological triad’. The overall characteristics of the same disease may vary from person to person due to some predisposing factors and interactions exhibited by the (i) agent causing the disease, (ii) individual characteristics of the host and (iii) factors related to the environment (both internal and external environment).

At first, the infectious agent/pathogen invades the host, undergoes changes through the series of phases/stages eventually leading to the growth and multiplication of agent and thus producing the disease characteristics. The main stages/phases involved are as follows;

Stage 1: Incubation Period

It is defined as the period from the exposure/entry of infectious agent/microorganisms in the host till the exhibition of first signs and symptoms. The incubation period can of very short duration (minutes/hours) in certain diseases and may be very long (20 to 30 years) in some other diseases.

An example for a disease with short incubation period is exposure with bacillus cereus (30 mins to 6 hours). Other common diseases with short incubation periods are with adenovirus, tetani etc (2 days/24 hours).

An example for a disease with the longest incubation period is Creutzfeldt-Jakob disease.

Stage 2: Prodromal Period

In this stage the infectious agent/pathogen starts to multiply and increases in number and inherent immune system gets activated and starts reacting to it. At this stage the generalized symptoms like fever, malaise, loss of appetite are commonly exhibited by majority of infectious diseases. In some cases, the particular disease specific manifestations may only be seen along with the generalized symptoms.

For example, in Lyme disease, along with the general symptoms like fever, malaise, sore throat, chills and muscle pain the disease specific characteristics such as skin rashes and meningeal irritation (in the form of headache) manifests.

Stage 3: Period of illness/Acute Stage

The infectious agent undergoes active replication resulting in an exponential increase in the pathogen population. In this stage, the signs and symptoms become severe and obvious. The specific characteristic features of particular infectious disease are manifested as the pathogens starts releasing toxins/viruses starts to shred in blood and body fluids and may involve multiple body parts/body systems at the same time. Mostly the disease appropriate diagnostic (signs, symptoms, laboratory observations) are made in this period.

For example, in the case of tuberculosis there will be established features of (i) pulmonary TB (productive cough, fever, weight loss, weakness, night sweats etc), (ii) tuberculosis pleuritis and (iii) extra pulmonary TB in some cases.

Stage 4: Period of decline

The pathogens/infections agent after reaching its peak of manifestation starts to decline and decreases in number. As the pathogen population starts to fall, the damaged tissue begins to repair and signs of symptoms of illness begin to decline. There may be manifestations in some cases, but may not be severe and strong as in the acute phase. Some caution is warranted in this period as the clients are susceptible to a secondary wave of infection if the disease process has weakened/affected their immune system severely.

For example, in the case of Lyme disease most people enter the decline phase without treatment and some require antibiotic protocols for entering the decline phase.    

Stage 5: Convalescence

In this stage the signs and symptoms completely disappear, and the host strength is regained. The patient is said to be in recovery period, as he resumes to his normal functions. The damaged tissues are repaired to their pre-infectious state and the infection is now completely resolved.   

For example, a TB affected patient test negative in consecutive sputum samples and chest X-rays shows complete recovery of pulmonary complex. The patient gradually gradual tolerates all activities as in the pre-disease period.

Explanation of disease stages with example of Hepatitis A

Hepatitis A is a viral liver infection caused by Hepatitis A virus (HAV).

Epidemiology

Ø Agent – HAV

Ø Host – Humans are the only natural host.

Ø Environment – Ingestion of contaminated food and water and person to person contact.

• The incubation period of the infection is 14-28 days.
• The acute stage is 10-12 days of initial contact, the virus is present in the blood and is excreted via the faecal -oral route.
• The period of decline is within 7 -10 days after the onset of the symptoms the virus excretion is found to be decreased.
• The period of convalescence is usually within 3 – 4 weeks after the initial exposure with HAV. Almost majority of the clients with HAV infection recovers fully as the symptoms get mild or relieve within a few weeks.