1
Third stage
Medicine
Lec-2
د
.
جاسم
محمد
1/1/2014
INTRODUCTION TO INFECTIOUS DISEASES
Sources of infecting MO
1. Human reservoirs (sources)
Infection may originate from the patient himself (endogenous), usually from skin,
nasopharynx or bowel, From outside sources i. e from other human (exogenous), often
another person who may be either suffering from an infection or carrying a pathogenic
microorganism.
Carriers are usually healthy and may harbour the organism in the throat (e. g. diphtheria or
meningococci), bowel (salmonella) or blood (hepatitis B or cytomegalovirus).
2. Animal reservoirs (zoonoses) or sources
Examples are:
contaminated meat/poultry (food poisoning organisms, e.g. Campylobacter, Salmonella and
botulism)
Milk from infected animals (tuberculosis, brucellosis)
body fluids of animals, e.g. saliva (rabies), urine (leptospirosis, Lassa fever)
bird faeces/secretions (psittacosis).
3. Environmental reservoirs (sources)
examples
Legionella in air-conditioning or domestic water pipes,
enteropathogens (typhoid, cholera, and hepatitis A) in water supplies.
The soil (spores of clostridia tetanus or anthrax).
METHODS OF TRANSMISSION OF INFECTION FROM ITS SOURCES:
1. From Human sourse
Faecal/oral, like helminthic infection, Salmonella, Campylobacter, Giardiasis,
hepatitis A, cholera, dysentery
Direct contact Skin organisms, e.g. staphylococci, streptococci
Sexually transmitted infections.
Aerosol/droplet spread: Respiratory secretions Common upper respiratory infections,
influenza, Childhood exanthems, e.g. chickenpox, measles Tuberculosis
Water aerosol: Legionella--
Sharps injury/needlestick e.g.Blood-borne viruses, e.g. hepatitis B and C, HIV
2. Animal (zoonoses
Direct contact: Anthrax
Faecal/oral (ingestion): Salmonella,Campylobacter, Tapeworms, Toxoplasma,
Toxocar, Lassa fever, Bovine tuberculosis, Listeria, brucellosis
Aerosol/droplet: Psittacosis, Lassa fever
Penetration of skin: Rabies,leptospirosis.
2
Arthropod: Yellow fever
3. From Environment:
Direct/skin penetration: Tetanus, wound botulism, gas gangrene Wound diphtheria ,
Hookworm
Ingestion: Toxoplasma, Toxocara, Listeria and Giardiasis
MICROBIOLOGICAL INVESTIGATION OF INFECTION
1. DIRECT DEMONSTRATION
A. Microscopic examination of biological fluids or tissue slides with appropriate
stains. For example, Gram staining of CSF to demonstrate Gram-negative diplococci
of meningococcal meningitis, or Ziehl-Neelson staining of acid-fast bacilli in sputum in
tuberculosis or in skin slit-smears in leprosy infections
B. In vitro culture.
e. g. culturing causative organisms, in urinary tract infections and pneumonias.
But long incubation (6-8 weeks) may be required with slow-growing bacteria (e.g.
tuberculosis and brucellosis).
An automated radiometric enables early detection of bacterial growth including
tuberculosis and allows evaluation of antibiotic sensitivity
C. Animal inoculation. Inoculating susceptible animals with infected material is now
rarely used in diagnosis.
2. MOLECULAR DIAGNOSTIC METHODS
The polymerase chain reaction (PCR) has become the standard and most sensitive for
the detection of pathogens in body fluids.
Two major disadvantages prevent its universal acceptance:
1. Expensive 2. False positive results.
3. IMMUNODIAGNOSIS
Enzyme-linked immunosorbent assay (ELISA)
Rapid immunochromatographic test
Western blot (immunoblot) test
Immunofluorescence (direct and indirect)
Complement fixation test
Agglutination test Direct agglutination , Indirect (passive) agglutination test , Latex
agglutination , Haemagglutination test ,
Immunodiffusion