Patterns of Infectious Disease

Infective disease can manifest in many different ways. These depend on the properties of the organsim(s) involved, the patient and the way the two meet.

Three general patterns can be discerned:

1. Acute pyogenic infections:

Generally rapid growing organisms
Interaction with innate immune system and acute inflammation predominate.
Blood neutrophil count increases
Where immune damage occurs it is often "post-infective"

2. Chronic (granulomatous):

Bacterial growth rate often moderate or slow
Organisms often survive and grow intracellularly
Immune damage occurs with infection - predominantly cell-mediated

3. Toxin-mediated disease:

Pathology often distant from site of bacterial growth.
Protective immunity may be mediated by anti-toxin antibodies alone
Disease may be fully reproduced by administering the toxin alone

1 & 3 often occur together.

Examples:

1. Acute Pyogenic Infections

Generally rapid growing organisms
Interaction with innate immune system and acute inflammation predominate.
Blood neutrophil count increases
Where immune damage occurs it is often "post-infective"

Examples already seen:

all S. aureus infections except group 4 (toxin mediated) and endocaditis.
E. coli urinary tract infections

New examples:

Streptococcus pneumoniae infections (Gram positive coccus) - lobar pneumonia and meningitis
Acute pyogenic with immune damage: Streptococcus pyogenes (Gram positive coccus) - acute pharyngitis followed by nephritis or rheumatic fever

2. Chronic Granulomatous Disease

Bacterial growth rate often moderate or slow
Organisms often survive and grow intracellularly
Immune damage occurs with infection - predominantly cell-mediated

Example: Tuberculosis caused by Mycobacterium tuberculosis (Mtb):

Wax containing cell wall covering an conventional gram positive structure
Makes cells very resistant to attack and to stains
Acid fast stain required
Organism lives and grows inside macrophages
Attack on the infected cells by cell-mediated immunity leads to granuloma formation
Balance between bacterial growth and the immune response can lead to very long periods between the time of infection and overt disease

Stage:	Acute pyogenic:	Chronic granulomatous:
Encounter	Often with symptomatic patient or after trauma.	Often impossible to pin down.
Entry/ Establishment	Obvious if associated with trauma. Adhesins and entry mechanisms well-studied.	Not well understood but uptake into macrophages a common feature.
Spread	Often associated with damage.	Usually imperceptible.
Multiplication	Rapid.	Slow.
Damage	Acute inflammation, toxins.	Cell mediated immunity.
Outcome	Easy to treat with antibiotics.	Difficult.