The Epidemiology of Bacterial Infections

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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?

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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

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John Snow (1813-1858)

London physician

Involved in cholera outbreak of 1854

Germ theory unproven

Miasma theory prevalent

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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

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‘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

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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.

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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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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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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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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)

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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

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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 vectorsAnimals 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)

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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

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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

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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 dogs Bordetella in human

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5. Hospital Acquired Infection

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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

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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

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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 conceptionHorizontalBy Direct contact (Cattle, Man) By ingestion of Milk (Calves, Man)Venereal (Cattle)Indirect from fomites

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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

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Human spread

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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

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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

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Spread of Infection in Salmonellosis

Vertical transmission (Eggs, foetuses)Horizonal transmission Contact (Direct or Indirect) Venereal Ingestion of contaminated food or water Aerosol