Dynamics of Disease Transmission, Community medicine.pptx

Source - Textbook of social & preventive medicine - Park

1. Dynamics of Disease Transmission John Joseph M Roll No 30

2. Communicable diseases are illnesses due to a specific infectious agent arising through transmission or production of the agent from an infected person, animal or reservoir, - To a susceptible host either directly or indirectly - Or to the inanimate environment Communicable diseases contain 3 links in their chain of transmission

3. Source & Reservoir Source The person, animal, object or substance from which an infectious agent passes to the host. e.g. Tetanus - soil Typhoid fever – Urine/feces of patients or contaminated food & water Hookworm infection - Soil Reservoir Any person, animal, arthropod, plant, substance or a combination of these in which the infectious agent lives and multiplies, on which it depends for survival and where it reproduces itself in such a manner that it can be transmitted to a susceptible host e.g. Hookworm infection – man is the reservoir Typhoid fever – case or carrier

4. Homologous reservoir – Applied when another member of same species is affected. e.g. Vibrio cholerae - Man Heterologous reservoir - Applied when infection is derived from a reservoir other than man e.g. Salmonella – Infected Animals/Birds The reservoir may be of 3 types Human Reservoir Animal Reservoir Reservoir in non living things

5. Human reservoir – I . Case Defined as the person in the population or study group identified as having the particular disease, disorder or health condition under investigation. The presence of infection may be -clinical, -subclinical or -latent. These variations in manifestations are referred to as ‘Spectrum of Disease’ or ‘Gradient of Infection’.

6. 1 . Clinical Case – Cases which have recognizable signs and symptoms. They may be; mild or moderate; typical or atypical; severe or fatal. Mild cases are more important as they are ambulant & spread the infection wherever they go, whereas severe ones are confined to beds.

7. 2. Subclinical Case – The disease agent multiplies in the host, but does not show signs and symptoms. The agent is eliminated/shed & contaminates the environment. They do not appear in any statistics & play a dominant role in maintaining the chain of infection. Can be detected only by laboratory tests. Occurs frequently during one’s lifetime and is responsible for the immunity to a variety of diseases.

8. Subclinical infection occurs in most infectious diseases. e.g. Mumps, Polio, Hepatitis – A, B, Japanese Encephalitis, Influenza, Diphtheria. 3. Latent Infection – The infectious agent lies dormant in the host without symptoms & often without demonstrable presence in blood, tissues, and bodily secretions. The host does not shed the infectious agent. e.g. Herpes simplex, Brill – Zinser disease, Ancylostomiasis,

9. Primary Case : First case of a communicable disease introduced into the population. Secondary case : Cases developing from contact with the primary case. Suspect case : Individual/Group having all signs and symptoms, but has not been diagnosed as having the disease I ndex case : First case to come to the attention of the investigator

10. II . Carrier Defined as an infected person or animal that harbors a specific infectious agent in the absence of detectable clinical disease and serves as a potential source of infection for others. Carrier state is due to non elimination of the disease agent due to inadequate treatment or immune response. As a rule, carriers are less infectious than cases, but more dangerous as they escape recognition.

11. Elements in carrier state - Presence of disease agent in the body Absence of signs and symptoms Shedding of the disease agent in the discharges/secretions. Thus acting as a source of infection for other persons

12. Carriers by type Incubatory carrier Carriers those who shed the infectious agent during the incubation period. Usually during the last few days of the incubation period. Capable of infecting others before the onset of illness e.g. Mumps, Measles, Polio, Pertussis, Diphtheria, Influenza Convalescent Carriers Those who continue to shed the disease agent during the period of convalescence Here clinical recovery does not coincide with bacteriological recovery.

13. Healthy Carriers Healthy carriers emerge from subclinical cases. They are victims of subclinical infection who have developed carrier state without suffering from overt disease. Carriers by duration Temporary carrier Carriers who shed the infectious agent for short periods of time Includes incubatory, convalescent and healthy carriers

14. Chronic carrier A chronic carrier is one who excretes the infectious period for indefinite periods. Chronic carriers are more important Sources of infection than cases. Longer the carrier state, greater is the risk to the community. They are known to reintroduce disease into areas which are otherwise free of infection.

15. Carriers by portal of exit Carriers can be classified according to portal of exit of infectious agent Urinary carrier Intestinal carrier Respiratory carriers Others – skin eruption, open wounds, blood

16. Portal of exit and the occupational status of the carrier are important considerations. e.g. A typhoid carrier working in a food establishment/water works Pseudo carrier – Carriers of avirulent organisms. Pseudo carriers are not epidemiologically important.

17. Animal Reservoir The source of infection may sometimes be animals and birds. These can be cases or carriers. The diseases and infections which are transmissible to man from vertebrates are called zoonoses There are over 100 zoonotic diseases which may be conveyed to man The recombination between animal and human viruses can produce new strains of viruses.

18. Best known examples - Rabies - - Yellow fever - Influenza Pigeons can lead to infection with Chlamydia Histoplasmosis is carried globally by birds

19. Reservoir in Non Living Things Inanimate matter can also act as reservoirs of infection e.g. Soil harbors agents causing - Tetanus, - Anthrax - Coccidioidomycosis - Mycetoma

20. Modes of Transmission Communicable diseases are transmitted from the reservoir/source of infection to a susceptible individual in many different ways. As a rule infectious diseases is transmitted by only one route e.g. Typhoid Fever by vehicle transmission, Common cold by direct contact. There can be transmission by several routes also e.g. AIDS, Salmonellosis, Hepatitis B, Brucellosis, Q – fever, Tularemia. Multiple routes enhance survival of the infectious agent.

21. Classification of modes of transmission

22. Direct Transmission Direct Contact Direct & immediate transfer of infectious agents from the reservoir/source to a susceptible individual without an intermediate agency. e.g. skin to skin, skin to mucosa, mucosa to skin as by touch, continued close contact sexual intercourse. Direct contact reduces the duration for which the organism has to survive outside the host & also ensures a large dose of infection. Examples are – STDs - AIDS - Leprosy - Leptospirosis - Skin infections - Eye infections

23. 2 . Droplet Infection This is the direct projection of a spray of droplets of saliva & nasopharyngeal secretions into the surrounding atmosphere, during coughing sneezing, speaking, coughing etc. The droplets can impinge on the conjunctiva, oro – respiratory mucosa, skin and can contain millions of bacteria and viruses. Particles of size >10mm are filtered by the nose, <5mm can reach up to the alveoli. Spread is limited to a distance of 30 – 60 cm b/w source & host Potential for droplet spread in high in close proximity conditions, lack of ventilation. Diseases transmitted – Respiratory infections, Eruptive fevers, meningococcal meningitis

24. 3. Contact with Soil The disease can be acquired by direct exposure of susceptible tissue to the agent residing in soil, compost or decaying matter e.g Hookworm Larvae, Tetanus, Mycosis etc. 4. Inoculation into skin/mucosa The disease agent can be introduced directly into skin/mucosa e.g Rabies by Dog bite, Hepatitis B through contaminated needles and syringes 5.Transplacental transmission Transmission directly from mother to embryo, fetus or baby during pregnancy or childbirth. The disease agent can produce malformations of the embryo by impairing its development e.g. Toxoplasma gondii, Rubella, Cytomegalovirus, Herpes virus, Varicella, Syphilis, AIDS, Coxsackie B virus, Thalidomide, Diethylstilbestrol (teratogens).

25. Indirect Transmission Includes a variety of mechanisms including the traditional 5 Fs – “Flies, Fingers, Fomites, Food, Fluids. For indirect transmission, the infectious agent must - Survive outside the host - Retain its properties of pathogenesis - Retain its virulence

26. 1.Vehicle Borne Transmission Implies transmission of infectious agent through Water, Food, Ice, Blood, Serum, other biological products. Most frequent – water & food The agent may have multiplied/developed in the vehicle or passively transmitted Water transmitted – Acute diarrheas, Cholera, Typhoid, Intestinal Parasites Blood Transmitted – Hepatitis-B, Syphilis, Brucellosis, CMV, Infectious Mononucleosis Organ transplantation can introduce disease agent e.g CMV in Kidney Transplant

27. Epidemiological Features If dose of contamination is heavy , the outbreak may be explosive. E.g Cholera, Hepatitis-A Cases are initially confined to those exposed to the contaminated vehicle Primary cases may be obscured by secondary cases Distance travelled by the agent may be great e.g. Food poisoning Not always possible to isolate the agent in the vehicle e.g. Typhoid bacilli in water When the vehicle is controlled or withdrawn the epidemic subsides e.g. Cholera Common source of infection is traceable

28. 2. Vector Borne Vector – Defined as an arthropod or any living carrier that transports an infectious agent to a susceptible individual Transmission by a vector may be Mechanical – Transmission only Biological – Agent may pass through development/multiplication in the vector

29. Mechanical Transmission Infectious agent is transported by the vector by soiling its feet or proboscis, or by passage through GIT and excretion. There is no development or multiplication in the vector Biological Transmission Infectious agent undergoes replication or development or both and requires an incubation period before transmission Biological transmission is of 3 types 1. Propagative – Agent only multiplies in vector. e.g. Plague bacilli in rat fleas 2. Cyclo propagative – Agent multiplies & changes in form. e.g. Malaria parasites in mosquito 3. Cyclo developmental – Agent undergoes only development, no multiplication e .g. Microfilaria in mosquito

30. Transovarial transmission – Vertical transmission from infected female to progeny in the vector Transstadial transmission – Transmission of disease agent from one stage of life cycle to another e.g Nymph to adult F actors Influencing Transmission by Vectors Host feeding preferences Infectivity Susceptibility Survival rate of vectors Domesticity – Degree of association with man Suitable environmental factors

31. 3. Airborne Droplet Nuclei Airborne transmission by particles in the range 1-10 microns (1-5 can easily be drawn into the lungs) They represent the dried residue of droplets Formed by - Evaporation of coughed/sneezed droplets - Generated by atomizing devices - Accidentally in microbiological laboratories, autopsy rooms, abbatoirs Can remain airborne for long periods, may retain or lose infectivity e.g. Tuberculosis, Influenza, Chickenpox, Measles, Q-Fever

32. B. Dust Larger droplets expelled by cough, sneeze settle down & become part of dust A variety of organisms are found in hospital dust, living rooms During human activity the dust is airborne Airborne dust is primarily inhaled but may settle on milk, food products. Diseases transmitted include – Pneumonia, TB, Coccidioidomycosis, Staphylococcal infection, Q-Fever, Psittacosis Most common in nosocomial infection

33. 4. Fomite Borne Inanimate article or objects contaminated by infectious discharges from a patient and capable of harboring and transferring the infected agent to a healthy person. Includes – Soiled clothes, Towels, Linen, Handkerchiefs, Cups, Spoon, Surgical dressings etc. Diseases transmitted – Diphtheria, Typhoid fever, Bacillary Dysentery, Hepatitis-A, Skin infections

34. 5. Unclean Hands & Fingers Transmission takes place directly (Hand to Mouth) and indirectly e.g. Streptococcal & Staphylococcal infections

35. Susceptible Host Successful Parasitism Involves 4 stages The infectious agent enters the host by a portal of entry e.g. Respiratory tract, alimentary tract, skin etc The organism must reach the appropriate tissue or site of election where it finds the optimum conditions for its multiplication & survival The agent must find a way out (Portal of Exit) to propagate the infection After leaving the host body the organism must survive in the external environment for a sufficient period till a new host is found. In addition a successful disease agent should not cause death of the organism but only produce a low grade immunity so that the host is vulnerable again to the same infection. e.g. Common cold virus

36. Incubation period Defined as the time interval between invasion by the infectious agent and appearance of the first sign or symptom of the disease in question. For non infectious diseases the term ‘Latent period’ is used. Defined as the period from disease initiation to disease detection. During incubation, the infectious agent undergoes multiplication, when a sufficient density is reached equilibrium is disturbed and the disease becomes overt.

37. Factors determining incubation period Generation time of the pathogen Infective dose Portal of entry Individual susceptibility As a rule; during incubation infectious diseases are not communicable. Exceptions are Measles, Chickenpox, Whooping Cough, Hepatitis-A.

38. Length of the incubation period is characteristic of each disease. A minimum period is required for every disease to occur. Incubation period can be short, median or long. - 2-3 days – Staphylococcal food poisoning, Cholera, Bacillary Dysentery, Influenza. - 10d-3 weeks – Typhoid, Chickenpox, Measles, Mumps, COVID-19 - Months-Years – Hepatitis-A,B, Rabies, Leprosy

39. Incubation period in epidemiology Tracing the source of infection and contacts. For diseases with a short incubation period, tracing the source of infection is simple. E.g. food poisoning, bacillary dysentery, typhoid fever For diseases with medium or long incubation period, the case is difficult as so many things will have happened in the time period. Once the source of infection is traced then only the appropriate control measures can be administrated. Period of surveillance. Incubation period is useful in determining the period of surveillance (Quarantine) This period is usually equal to the maximum incubation period of the disease

40. Immunization Knowledge of incubation period helps us to prevent clinical illness by human Igs & antisera. Identification of point source/Propagated epidemics In point source epidemic, all cases occur within one incubation period of the disease In propagated epidemic cases occur later than the known length of the incubation period Prognosis Incubation period can be used to estimate prognosis of a disease. In some diseases shorter the incubation, worse is the prognosis.

41. Serial Interval The gap in time between the onset of primary case and the secondary case. In actual practice the precise incubation period is seldom known But when an outbreak occurs, in a closed group (e.g. a family-smallest group) there is an initial primary case followed by 2-3 cases. By collecting information about a whole series of such onsets, we get a distribution of secondary cases from which we can guess the incubation period.

42. Generation Time Defined as the interval of time between receipt of infection by a host and maximal infectivity of that host. In general generation time is roughly equal to the incubation period.(But not the same) Generation time refers to transmissions of infection The time of max communicability may precede or follow the incubation period. Interval between cases is determined by generation time with person to person transmission of infection

43. Communicable Period Defined as the time during which an infectious agent may be transferred directly or indirectly - - From an infected person to another person - From an infected animal to man - Or from an infected person to an animal, including arthropods. Communicability is variable in different diseases, some are more communicable during incubation than clinical illness. Communicability of some diseases can be reduced by early diagnosis & treatment A measure of communicability is secondary attack rate.

44. Secondary Attack Rate (SAR) Defined as the no. of exposed persons developing the disease within the range of the incubation period, following exposure to primary case. It is given by the formula No. of exposed persons developing disease within incubation period SAR = ______________________________________________ X 100 Total No. of exposed/susceptible contacts T he primary case is excluded from the numerator & denominator

45. The denominator includes all persons exposed to the case, more specifically it is restricted to susceptible contacts if possible to distinguish susceptible persons from the immune. e.g. A family of 6; 2 parents & 4 children. The parents are immune, the children are susceptible to a specific disease say Measles. There occurs a primary case of measles & within two days 2 secondary cases SAR = ( 2/3 ) x 100 = 66.66% SAR is limited to infectious diseases in which primary case is infective for a short period of time measured in days. E.g. Measles, Chickenpox When infective over a long period of time, duration of exposure is an important factor in determining the extent of spread. e.g. Tuberculosis. It is obtained by No. of contacts developing Tuberculosis SAR = ______________________________________________ X 100 No. of person – Duration of exposure

46. Another limitation is identification of susceptibles . It is feasible only where history can be used as a basis for ID. E.g. Measles, Chickenpox. In many other cases susceptibles cannot be identified. E.g Influenza. In such cases SAR is based on all exposed family members An additional advantage of SAR is that vaccines & non vaccines from several families can be added to determine the overall attack rates in the vaccinated & unvaccinated populations.

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