Sources and Spread of Infection

Sources and spread of infection Some definitions: Epidemiology is the study of the determinants and distribution of disease. Incidence is the number of new specified cases in a specified period in a specified population. Prevalence is the number of cases of a specified disease in a population at a particular point in time.

Sources and spread of infection Carriage Carrier/case ratio depends on the immunity and general health and status of the population and the virulence of the organism. C/C ratio is low in typhoid or dysentery where cases hugely outnumber carriers. C/C ratio is high say, for Staphyloccus where carriers greatly outnumber cases

Sources and spread of infection Endemic Disease. A disease is endemic when it is consistently present in the community. This depends on: The population of susceptible individuals. (depending on immunity – ‘herd immunity’) The relative ease of transmission The size and physical location of the population

Sources and spread of infection If the Herd Immunity falls to say below say 50-75% this facilitates greater spread and results in a paroxysmal increase in the incidence of the disease. This is an epidemic. (Cf enzootic and epizootic) A pandemic disease is one affecting and extended area eg. a continent.

Sources and spread of infection Epidemiology of Infectious Disease Factors involved: a.Microbiology/Pathology b.Drugs c.Immunology d.Public and Social Health e.Statistics

Sources and spread of infection Virulence – The ability to invade and spread Pathogenicity – the ability to cause disease Infectivity – the ability to invade and establish at a primary focus. Not all organisms can do all three.

Sources and spread of infection Pathogenicity is achieved by a variety of methods but principally by the production of toxins. Exotoxins are formed by the organism and released into the environment or at the site of infection eg. Clostridium botulinum, Corynebacterium diphtheriae or the enterotoxin produced by Vibrio cholerae

Sources and spread of infection Endotoxins formed by organisms (mostly Gram-negative bacilli) within the organism and released on degeneration of the organism. Generally membranes must be destroyed or disrupted for infection to be established and this is usually achieved by exotoxins

Sources and spread of infection Pathogenesis Depends on: Toxigenicity InvasivenessToxins Capsules Aggressins Organotropism

Sources and spread of infection Pathogenicity = Transmissibility + Infectivity + Virulence

Sources and spread of infection Establishment of an infection depends on: Numbers of organisms (inoculum) therefore virulence and infectivity. Pathogenicity. Host susceptibility. The host parasite relationship.

Sources and spread of infection MicrobesSusceptible Host Other Hosts

Sources and spread of infection Microbial factors: Pathogenicity. Ability to survive or multiply in a given inanimate environment. Infecting dose Effective portal of entry Means of dispersal Carriers

Sources and spread of infection Poverty Overcrowding Hypothermia Poor personal Hygiene Immigration and population movements War and strife Drug addiction Alcoholism Smoking Malnutrition In the community the factors that put the host at risk are:

Sources and spread of infection In individuals more specific factors may enhance susceptibility to infection: Disorders such as : Diabetes Hormonal upset Haematological eg Leukaemia or drug induced neutropenia

Sources and spread of infection Poor general resistance due to Immunological Deficiency : Neoplasia Pregnancy Primary infections Extremes of age Malnutrition HIV Iatrogenic immunosuppression

Sources and spread of infection Poor local resistance due to: Underlying renal disease Foreign bodies Imperfect blood supply (dead tissue, blood clot or diabetes) Operations Instrumentation and anaesthesia

Sources and spread of infection A reservoir of infection is where organisms can survive and multiply – normal habitat. These include: Active human carriers Human carriers of causative organism Animal cases of disease or carriers The inanimate environment A source may be a reservoir but a reservoir is not always the source.

Sources and spread of infection Exogenous modes of spread Direct Contact Ingestion Inoculation(sharps and bites) Inhalation Inanimate environment – fomites, showers, water & air, soil, (pathology specimens)

Sources and spread of infection Direct contact Hands clothing, sexual contact Indirect contact Fomites Ingestion Faecal-oral route

Sources and spread of infection Inoculation Arthropod borne blood infections – parasites multiply in, and spread by the vector Mosquito - eg. malaria and yellow fever Flea - plague Louse – typhus Tick – Rocky Mountain spotted fever. Lyme disease. Mite – scrub typhus Tsetse fly - trypanosomiasis

Sources and spread of infection Inhalation: Explosive coughing and sneezing colds and flu Organisms in the air eg.C. diphtheriae Streptococci Staphylococci M. tuberculosis Smallpox virus Legionella

Sources and spread of infection From soil Tetanus Gas gangrene Madura foot

Sources and spread of infection Examples of Zoonoses BrucellosisTB Rabies Psittacosis Plague Salmonella Leptospirosis (seldom man to animal)

Sources and spread of infection Vertical spread of infection - Mother to child eg. Viruses or Listeria. Horizontal spread of infection eg. STD

Sources and spread of infection Factors affecting spread: Geography Season/climate Presence of vector (arthropod etc) Presence of suitable host