The organism is considered a commensal under most conditions, although host strain susceptibility differences have been reported. Typically, a carrier state is established in the nasal passages and middle ears. Clinical signs are uncommon, although natural outbreaks of disease have been reported. When present, clinical signs are nonspecific and may include dyspnea, weight loss, hunched posture, and snuffling. Virulence is related to several bacterial components, most prominently pneumolysin, a multifunctional toxin with distinct cytolytic and complement-activating activities.
Infection begins in a bronchopulmonary segment and spreads centrifugally.
The infection spreads from the lung to the pleura, pericardium, and, via septicemic spread, to the rest of the body. The affected lung is first edematous, then becomes consolidated and eventually is cleared of cellular debris. There may be suppurative or fibrinous lesions throughout the respiratory tract and adjacent structures. These most commonly include suppurative rhinitis and otitis media but may also include similar changes in and around the deeper tissues of the respiratory tract. Septicemia may result in suppurative lesion establishment in virtually any organ, with death being a common sequela.