C HALLENGING D ANIEL C ORNU ’ S THEORY

COMMUNICABLE DISEASES IN GENERAL

The collection of information in case of communicable diseases should follow a definite pattern so that no important aspects are missed. A standard proforma is described below for this purpose.This proforma will be followed for describing communicable diseases in the next four chapters as far as feasible.

IDENTIFICATION OF CASES OF DISEASE

Clinical, laboratory and field investigations are made to identify the cases reported. Field diagnosis may be more important than the laboratory report in some cases, e.g.

Cholera and food poisoning.

INFECTIOUS AGENT

It means the causative agent of disease. Its nature, infectivity, virulence, antigenicity and viability in the environment should be described.

OCCURRENCE

A longitudinal or vertical study of the past occurrence and distribution of the disease is made. Also, the cross-sectional or horizontal extent of the disease is determined by finding the total cases, new and old in the area at any one time, expressed as per thousand population.

RESERVOIR

This refers to the source from where the infection is contacted. Most often, the source is man himself, either as a patient or as a carrier. Common examples are the mild and missed cases in cholera or dysentery, healthy carriers in diphtheria and convalescent carriers in typhoid.

Sometimes the source is an animal reservoir. Dog is the source for rabies and hydatid cyst disease and rat for plague, Weil’s disease, endemic typhus, rat bite fever and salmonellosis. Rarely, the source may be soil, as in case of tetanus, botulism and gas gangrene infections.

MODE OF TRANSMISSION AND SPREAD

This includes the following: the route and mode of exit of an infectious agent from the source, i.e. the diseased person or an animal, the vehicle and mode of travel, the route and mode of entry into the body of a new host.

Mode of exit: The agent comes out of the body of the diseased through:

• Mouth and nose, along with breath and droplets

• Anus, urethra or vagina, along with feces and dis-charges

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PART II: Epidemiological Triad • Skin and mucous membranes (direct contact)

• Blood (through insect bites, injections and trauma).

Mode of travel: The infection may travel or spread:

• Directly or without a vehicle. This may happen through droplets (by close proximity, such as in common cold and influenza) or through skin and mucous membranes (by close bodily contact such as in venereal diseases, leprosy, and ophthalmia neonatorum).

• Indirectly, through a vehicle. The vehicle may be animate or inanimate. Examples of the former are man himself and various arthropods. Examples of inanimate vehicles are air, water, food, dust and fomites.

Mode of entry: The agent enters the new host by the following routes:

• Respiratory passages: The infection enters directly with the droplets on inhalation of the contaminated air or dust.

• Alimentary canal: The infection enters by ingestion of contaminated water and food.

• Skin and mucous membranes: The infection enters directly by body or sex contact with or indirectly through contaminated articles (socks, shoes, kajal stick, etc.) used by the infected person.

• Blood: The agent enters the body through bite of arthropod vectors, through injections using contaminated syringes and needles or through injury caused by nails, thorns or other piercing objects.

Sometimes transplacental spread may occur from the mother to fetus through placenta. Syphilis is a common example.

INCUBATION PERIOD

This should be stated as the average and the range. It helps in the diagnosis of disease, in tracing the source of infection and in determining the quarantine period.

PERIOD OF COMMUNICABILITY

This is the period during which the patient is infective.

It may include the whole or part of the incubation period, the disease period, the convalescence, as also the post-convalescence period if the patient continues to discharge the infective organism. The period of communicability varies in different diseases. Poliomyelitis and infectious hepatitis patients are more infective during incubation period and the early part of disease.

Whooping cough cases are more infective during the first week of the disease and much less so afterwards.

A typhoid case, on the other hand, is infective after a week or 10 days of the disease. The period of communicability is very important because it determines the period of isolation.

SUSCEPTIBILITY AND RESISTANCE

These pertain to host factors that affect proneness to infection. Important factors are age, sex, race, heredity and immune status.

METHODS OF CONTROL

Spread of infection may be controlled by exercising a check at the level of the reservoir, the host or the transmission from reservoir to the host. Of these three, the first is the most difficult to check, especially when man himself is the reservoir. It may be less difficult when the reservoir is an animal.

The most vulnerable factor in the spread of a disease is the route of transmission and all attempts should be made to block this route. An attempt should also be made to clean the environment and to increase host resistance. Unless all the links in the chain are present, disease transmission cannot take place. The best course for control of a communicable disease is to attack the weakest link first, subject to factors like cost, practicability and acceptance by the people. Examples of control measures are treatment of open cases of tuberculosis (preventing exit of agent), mosquito control measures for malaria (preventing vehicle transmission) and wearing shoes in the fields for hookworm (prevention of entry of the agent).

The methods of control are described under the following headings in the standard format of description.⁵

• Preventive measures

• Control of patients, contacts and the immediate environment as per the following six aspects:

– Report to local health authority – Isolation

– Concurrent disinfection – Immunization of contacts – Investigation of contacts – Specific treatment

• Epidemic measures

• Disaster implications

• International measures.

APPROACH TO ACUTE COMMUNICABLE DISEASES The purpose of describing the disease entity under the above nine headings is to be able to apply epide-miologically sound and effective methods for prevention and control of disease. However, in case of acute communicable diseases, it is better to stick to the following routine of specific and standard methods from a practical point of view.

Notification

Cases of an acute infectious disease should be reported promptly to the appropriate health authority. The latter

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CHAPTER 15: General Epidemiology of Communicable Diseases

may consider it necessary to detect all cases through a house to house survey.

Isolation

Already described.

Quarantine

Already described.

Diagnosis

Proper diagnosis is very important for the adoption of control measures and should be made as early as possible from the clinical picture, supported by appropriate laboratory tests, whenever possible.

SPECIFIC TREATMENT

This is the main method of control in some diseases as chemotherapy helps in decreasing the quantum of infection in the community. Examples are leprosy, tuberculosis and malaria.

Disinfection

Described later in this chapter.

Immunization

Personal and mass immunization should be undertaken during the epidemic or even before hand when the epidemic is threatened.

INVESTIGATION OF CONTACTS AND SOURCE OF INFECTION

Contact tracing helps in early detection of mild and missed cases. The source of the infection, if found, should be effectively dealt with to prevent further spread of disease.

HEALTH EDUCATION

This includes the methods of publicity and education, aimed at instructing the people about steps to be taken during an epidemic.

LEGAL AND ADMINISTRATIVE MEASURES

Legal compulsion may be necessary to enforce certain antiepidemic measures such as prohibition of the use of a contaminated well. In olden days, vaccination against smallpox had to be compulsorily enforced in some villages.

LONG-TERM MEASURES

These are measures to be adopted during the interepidemic period, with the aim of forestalling or

delaying future epidemics. Various sanitation and immunization measures belong to this category.

INTERNATIONAL MEASURES

They are applied to prevent the spread of infection from one country to another (e.g. Yellow fever regulations).

DISINFECTION AND DISINFECTANTS

Bacteria, though agents of disease, also form part of the environment of man. Harmful bacteria have to be destroyed at the original source or reservoir which may be man, animal or an inanimate object. Disinfection has already been defined. Disinfectants are substances that destroy harmful microbes. They are meant for application to inanimate objects and are usually ineffective against spores. Substances or processes that destroy bacteria as well as the spores are called sterilizing agents. Antiseptics are bactericidal or bacteriostatic substances suitable for use upon living tissues.

Disinfectants may sometimes be used as antiseptics in low concentrations. On the other hand, chemo-therapeutic drugs and antibiotics are the substances used internally. Chemotherapeutic agents are chemicals which, when taken in, kill the bacteria or inhibit their growth but do not damage the body cells. Examples are sulphonamides, sulphones, antimalarials, antiamebics and anthelminthics. Antibiotics are chemical substances produced by microorganisms that destroy bacteria or inhibit their growth but do not damage body cells.

Examples—Penicillin, streptomycin and tetracyclines.

Disinfectants act through three mechanisms:

1. Coagulation of protoplasm, as in case of heat and metallic salts.

2. Oxidation and burning of protoplasm, as in case of potassium permanganate and halogen compounds.

3. Interference with cell metabolism, as in case of phenol compounds.

PHYSICAL DISINFECTANTS

They are sunlight, including ultraviolet rays, air, heat and ionizing radiation.

Sunlight

It is a natural disinfectant that destroys bacteria in air, water, and fomites. Direct sunlight is more effective than diffuse light. Short wave radiation, such as ultraviolet rays, is more bactericidal than long wave radiation, such as infrared rays. The penetrating power of ultraviolet light is low and it cannot pass through glass. Addition of fluorescent dyes such as eosin or methylene blue enhances the lethal effect of ultraviolet rays through photodynamic action. Ultraviolet light is used for water disinfection in swimming pools.

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