The Epidemiology of Bacterial Infections

2 Epidemiology ‘The study of factors affecting the health and illness of populations’ The study of: - the occurrence and distribution of diseases in populations -factors that influence disease occurrence Who has what? When and where? Why and how?

3 Epidemiology is Being a disease detective Knowing how much disease there is  E.g. surveillance Identifying risk factors for diseases Suggesting hypotheses for new causal mechanisms  Hypothesis generation Working to find the best means to minimise risks and control diseases

4 John Snow ( ) London physician Involved in cholera outbreak of 1854 Germ theory unproven Miasma theory prevalent

5 Miasma theory Miasma - bad or malodorous air as a cause of disease Pleasant or strong smelling agents were thought to be protective Public health efforts concentrated on finding the source of bad smells

6 ‘First epidemiologist’ Snow pinpointed source of cholera outbreak to contaminated water pump By mapping cases Snow traced the source to a water pump on Broad Street Disproved miasma theory of cholera spread London cholera outbreak 1854

7 Germ or contagion theory Developed in early 1800’s  Infection spread by random contacts between infected and susceptible people (or animals) 1860’s: Louis Pasteur demonstrated existence of pathogenic organisms 1880’s: Robert Koch isolated and cultured Vibrio cholerae.

8 Introduction Bacterial Diseases: 1. Originate from the environment 2. Originate from the animals normal bacterial flora 3. Transmitted from animal to animal of the same species 4. Transmitted from animals of one species to animals of another species 5. Hospital Acquired Infection Where do the organisms originate that cause clinical infection? Sources, Reservoirs What are their routes of transmission?

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10 Concepts in the epidemiology of infectious disease Reservoir of infection: the infectious agent normally lives and where it may multiply or survive - animal (e.g. in brucellosis) or the inanimate environment (e.g. in tetanus). Source of infection:- infection may be derived from the patient's own microflora (endogenous), or an animal to human (zoonosis) or an environmental source (exogenous). The source of an exogenous infection may sometimes be different from its reservoir.  For example, in an outbreak of human listeriosis in Canada, the reservoir of infection was a flock of sheep, from which manure was used as fertilizer on a cabbage field. Contaminated cabbages from the field were used to make coleslaw which become the source of infection for human subjects When the source of infection is inanimate, e.g. food, water or fomites, it is termed the vehicle of infection.

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12 Mode of transmission Food-, drink- or water-borne infection Direct or indirect contact: This includes spread from patients, carriers or the environment to animals or persons who are contacts. Within this category possible routes include: faeces-to-hand-to-mouth spread, sexual transmission (venereal), skin or mucous membrane contact (e.g. wound infection). Percutaneous infection: This includes: insect-borne transmission via the bite of an infected insect either directly from saliva, or indirectly from insect faeces contaminating the bite wound direct transmission through intact skin or broken skin (e.g. leptospirosis). Air-borne infection: Infectious organisms may be inhaled as droplets (e.g. tuberculosis); aerosols, dust (e.g. ornithosis); spores (e.g. anthrax). Transplacental infection

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14 1. Transmission by means of the Environment Some bacteria survive in the environment In certain circumstances, they or their products may contact the host animal and cause disease Occasionally this disease may spread to more animals directly or indirectly Survival may depend upon conditions (Leptospira dry up rapidly, but survive for 6-12 weeks in water) Survival may depend upon spore formation (anthrax)

15 2. Transmission from the bacterial flora Best examples are wound infections (skin organisms infect wound) Umbilical infection in the neonate Infection prior to colostrum consumption Colonisation of lesions or existing disease Umbilical lesion, Tetanus

16 3. Disease transmitted from animal to animal of the same species Normal method of spread for many diseases Transmission may be direct Transmission may be indirect by means of the environment or vectors Animals infected with the bacteria which do not have clinical signs yet shed the organism (may be part of normal flora or in the incubation period of disease)

17 Routes of Direct Transmission Vertical (Through the placenta or egg) At birth from a carrier parent  By contact  By aerosol In food or water from faeces or secretions By venereal transmission Vertical Transmission, M.synoviae Horizontal transmission- Brchyspira

18 Indirect transmission by means of vectors Less common than with viruses Passive vectors (Flies, Birds, Rodents, Veterinary surgeons) Vectors which are also reservoir hosts Ticks are Vectors in Lyme Disease

19 4. Bacteria transmitted from animals of one species to animals of another species Host of origin may have active disease Host of origin may be asymptomatic Disease may cycle in host species of origin and form a reservoir for the other species Bordetella in dogsBordetella in human

20 5. Hospital Acquired Infection

21 Evidence for the methods of spread of disease Experiment (Infection with various doses and by various routes) Knowledge of the properties of the organism Formation of a hypothesis Field testing of that hypothesis

22 Spread of Infection in Major Diseases Bovine Brucellosis Bovine tuberculosis Salmonellosis For all diseases consider spread between animals, survival and spread in farm environment, spread beyond farm, spread to man

23 Spread of Infection in Bovine Brucellosis Most bacteria spread in secretions of reproductive tract Some shed in milk Some present in carcase or offal Vertical From mother to products of conception Horizontal By Direct contact (Cattle, Man) By ingestion of Milk (Calves, Man) Venereal (Cattle) Indirect from fomites

24 Spread of Infection in Bovine Brucellosis – Farm spread Direct contact spreads from farm to farm Cattle movement Vectors carry placentas/infection from farm to farm Semen Reservoir hosts occasional Milk, meat and offal

25 Human spread

26 Spread of Infection in Bovine Tuberculosis – Human spread Vertical Horizontal  By contact  By aerosol  By ingestion in milk Directly or indirectly from reservoir hosts Consumption in meat (man) Infected Tuberculous Milk

27 Spread of Infection in Salmonellosis Organisms shed in faeces Present in contaminated water, soil and dust Organisms present in eggs (salmonella enteritidis) Shed in reproductive tract secretions Present in meat, milk and eggs Present in rodents and wild birds Infected Ova, S. enteritidis

28 Spread of Infection in Salmonellosis Vertical transmission (Eggs, foetuses) Horizonal transmission  Contact (Direct or Indirect)  Venereal  Ingestion of contaminated food or water  Aerosol