Control Of Communicable Diseases

FCM 2 Control of Communicable Disease Dr. Biso

4th Shifting 1-29-08 Class no.

COMMUNICABLE DISEASE- an illness due to a specific agent or its toxic products, arising through transmission of that agent or its products from reservoir to susceptible host, either directly from an infected person or animal, or indirectly through the agency of an intermediate plant or animal host, a vector or the inanimate environment. INFECTIOUS DISEASE- a disease of man or animal resulting from an infection. INFECTION- the entry and development or multiplication of an infectious agent in the body of man or animal. CONTAGIOUS DISEASE- implies transmission through direct contact Variations in Severity of Illness (Gradient of infection/ Biologic Gradient) -range of manifestations of illness in the host as a result of infection with an agent Inapparent infection-----------severe clinical illness/death Criteria for severity of an illness: A. Individual 1. High Case Fatality Rate 2. Substantial proportion of the surviving patients are left with sequelae B. Public Health Impact 1. High incidence rate Characteristic or modal severity of diseases Class A- inapparent infection; frequent Eg. Tubercle bacillus Inapparent (A)

0

Mild (B)

Mod (C)

Severe (D)

Fatal (E)

Percentage of infection

100

Class B- Clinical disease frequent; few deaths Eg. Measles virus (A)

(B)

(C)

0

(D)

Percentage of infection

(E)

100

Class C- Infections usually fatal Eg. Rabies virus (D) 0

(E) Percentage of infection

100

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The relation of severity of illness to disease statistics: INFECTION DISEASE

(A)

(B)

(C)

(D)

(E)

Likely to be seen by a Doctor; may be recorded Likely to be Hospitalized & recorded

Infectious Disease Process Infectious Agents-intrinsic properties a. Morphology b. Size c. Chemical character d. Antigenic make up e. Growth requirements (temperature, nutrients, etc.) f. Ability to survive outside a host in a variety of vehicles (eg. Water, milk, soil) g. Viability under different conditions of temperature and humidity h. Spectrum of hosts (animals and arthropods) i. Ability to produce toxins j. Ability to become resistant to antibiotics or other chemicals k. Ability to acquire new genetic information from plasmids or other sub viral particles (bacteria) Host-Parasite (Infectious agent) Interactions Infectivity- ability of an agent to invade and multiply (produce infection) in a host. ID50 – the minimum number of particles or agents required to establish infection in 50% of a group of hosts of the same species. Varies with: agent, route of administration of the agent, source of the agent, and host factors, such as age or race. Example: High infectivity- measles Low infectivity-leprosy Techniques of evaluating infectivity: 1. Studies of the ease or speed with which an agent spread in a population of close contacts (such as household contacts) who become infected (secondary attack rate) 2. Serosurvey after epidemics to determine the proportion of persons recently infected. Pathogenecity- ability to produce clinically apparent illness Factors that may alter pathogenecity: host and environmental factors, dose, route of entrance of infection, and source of the infection. Virulence-proportion of clinical cases resulting in severe clinical manifestations (including sequelae).

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Measure of virulence-CFR Factors that affect virulence: dose, route of infection, host factors such as age or race. Immunogenecity- infection’s ability to produce specific immunity. Factors that can affect immunogenicity: age, nutrition, dose, and virulence of infection. Pathogenetic Mechanisms: 1. Direct tissue invasion, e.g. parasitic diseases such as amoebiasis, giardiasis, and many nematodes, trematodes and cestodes, bacterial meningitides, UTI, pharyngitis or otitis, skin abscesses, and viral infections, such as upper respiratory or gastrointestinal viruses, and encephalitis (rabies, arbovirus encephalitides) 2. Production of a toxin, e.g. tetanus, diptheria, and infections by enterotoxigenic E. coli. 3. Immunologic enhancement or allergic reaction leading to damage of the host, e.g. TB, post streptococcal GN, dengue h. fever. 4. Persistent or latent infection e.g. pharynx- H. influenzae, streptococci; gallbladder- salmonella typhi; urinary tract- E. coli, Serratia, pseudomonas; persistent viral infection- herpes I &II, varicella zoster, CMV, Hep B, measles 5. Enhancement of host susceptibility to drugs or otherwise minimal toxicity, e.g. Reye’s syndrome. 6. Immune suppression, e.g. AIDS Reservoir- defined as the living organisms or inanimats e matter ( such as soil) in which an infectious agent normally lives and multiplies. The reservoir is an essential component of the cycle by which an infectious agent maintains and perpetuates itself. a. Simplest Cycle Human Human Human e.g. most of the viral and bacterial respiratory diseases, most staphylococcal and streptococcal infections, diptheria, venereal diseases, childhood exanthemata, mumps, typhoid fever, amoebiasis, etc. b. human is not an essential part (usual reservoir) of the lifecycle of the agent: Animal Animal Animal Human e.g. bovine tb, anthrax, leptospirosis, rabies

c. Complex cycle- multiple reservoirs and different developmental stages of the agent. E.g. echinococcosis, tapeworm infestations, schistosomiasis, malaria, and vectorborne viral infections.

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Humans as reservoirs: Cases and Carrier Range of reaction to the occurrence of infection: 1. Colonization- e.g the presence of Staphylococcus aureus on the nasal mucosa. Contamination- presence on the surface of the body or on inanimate objects (fomites) of an infectious agent that can serve as a source of infection. 2. Inapparent infection- (covert or subclinical) organisms not only multiply in the host, but also cause a measurable reaction that, however, is not clinically detectable. 3. Infectious disease- when infection leads to clinical (overt) disease with symptoms, physical findings, or both. Carrier- an infected person who does not have apparent clinical disease, but is, nevertheless, a potential source of infection to others. Types of carrier 1. Inapparent throughout

Examples Polio virus, meningococcus, hepa viruses 2. Incubatory Carrier Chicken pox, measles, hepatitis 3. Convalescent C. diptheriae, hepa B, Carrier Salmonella 4. Chronic Carrier S. typhosa, hepa B Mechanisms of transmission of infection: 1. Direct transmission – direct and immediate transfer agent of an infectious agent to a receptive portal of entry through which human infection is established. E.g. touching, kissing, or sexual intercourse droplet spread (distance approx 1 meter). – Host susceptible tissue is exposed to the agent such as by the bite of rabid animal or when it comes in contact with soil or decaying matter in which the agent usually leads a saprophytic existence ( e.g. systemic mycoses) – Transplacental transmission 2. Indirect transmission – vehicleborne – vectorborne – airborne (droplet nuclei and dust) Aspects of person to person spread of disease 1. Generation time- the period between the receipt of infection by a host and the maximal communicability of the host. Incubation period- time interval between the receipt of infection and the onset of illness. 2. Herd Immunity- resistance of a group to invasion and spread of an infectious agent, based on the immunity of a high proportion of individual members of the group.

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3. Secondary Attack Rate- no. of cases of a disease developing during a stated period of time among those members of a closed group who are at risk SAR= no. of new cases in a group-initial cases No. of susceptible persons in a group-initial case Community response to the introduction of an infectious agent: 1. 2. 3. 4.

sporadic endemic epidemic pandemic

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