Actinomycosis pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

Actinomycosis is a chronic pyogenic bacterial infection caused by Actinomyces species. Infection most frequently follows dental work, trauma, surgery, or other medical conditions. When there is break in the mucosa, anywhere from the mouth to the rectum they reach tissues and cause damage. Incubation period of Actinomycosis varies from one to four weeks. But occasionally, it may be as long as several months. Actinomycosis elicits both humoral and cell-mediated immune responses

Pathophysiology

• Actinomycosis is caused by the bacteria Actinomyces. The pathophysiology of Actinomycosis can be described in the following steps.

Transmission

• Actinomyces are part of natural flora of human body, resides in the oral cavity, lower gastrointestinal tract and urogenital tract.

• They are non virulent under normal conditions
• When there is breach in normal mucosal architecture , anywhere from the mouth to the rectum they enter tissues and cause damage.

Types	Site of Infection	Source of infection
Cervicofacial actinomycosis	Neck Jaw Mouth	Dental problems like tooth decay
Thoracic actinomycosis	Lungs Pleura Chest wall Mediastinum	Inhalation of droplets of contaminated fluid Aspiration of oropharyngeal secretions or gastric contents Direct extension of cervicofacial infection into the mediastinum Transdiaphragmatic or retroperitoneal spread from the abdomen Hematogenous spread
Abdominal actinomycosis	Abdomen	Secondary to abdominal infections like appendicitis Accidental swallowing of a foreign body such as chicken bone containing the actinomycetes bacteria Penetrating trauma Perforation of the gut (e.g., the colon or appendix) Surgical manipulation of GI tract
Pelvic actinomycosis	Pelvis	Occurs most commonly in woman as the bacteria passes from the female genitals into the pelvis Long-term use of IUD type of contraceptive
Central nervous system actinomycosis	CNS	Secondary to hematogenous spread from primary infection in the lung, abdomen, or pelvis Direct extension from paranasal sinuses, ears, and cervicofacial regions

Incubation

Incubation period of Actinomycosis varies from one to four weeks. But occasionally, it may be as long as several months. 

Dissemination

Following transmission, lesions spread by direct extension.

Seeding

• Once the endogenous bacteria are introduced into the tissues, they multiply due to low oxygen tension.
• It triggers an inflammatory reaction which results in formation of hard yellow hard granules(sulfur granules).
• These are solidified bacterial filaments with surrounding tissue exudates.
• Abscesses with fibrous walls and pus along with sulfur granules develop.
• It finally drain out through sinuses.

Immune response

Actinomycosis elicits both humoral and cell-mediated immune responses

Genetics

There is no known genetic association to Actinomycosis

Gross Pathology

On gross pathology the following features can be noticed:

• Single or multiple abscesses
• Indurated masses with hard fibrous walls and soft central loculations containing white or yellow pus.
• Granules are seen grossly.
• Sinus tracts extended from abscesses to the skin surface or into organs; vertebral bone, and retroperitoneal tissue.
• Pleural, pericardial, or serosal thickening with infection of the lung, heart, and wall of the bowel.

Microscopic pathology

• Outer zone of granulation around central purulent abscess.
• Positive for sulphur granules in the center of abscess.
• Focal liquefication type of necrosis is seen.
• Lymphocytes and neutrophils forms a layer around the abscess.
• Gram positive organism with branching filaments forming segment-like structures.

This is a low-power photomicrograph of the retroperitoneal abscess. At this magnification, multiple dark-staining foci can be appreciated. These foci are Actinomyces colonies (arrows). These colonies are known as "sulfur granules" because in gross specimens they are visible to the naked eye as yellow grains, thus resembling grains of sulfur.

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References

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