Infectious disease

INTRODUCTION TO INFECTIOUS DISEASES

Lecture 2. 3rd year

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.
--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