Diphtheria and Pertussis Infections

Study Notes

Diphtheria

Diphtheria is an acute toxic infection caused by Corynebacterium diphtheriae and rarely toxigenic strains of Corynebacterium ulcerans.
C. diphtheriae is an aerobic, nonencapsulated, non-spore-forming, mostly nonmotile, pleomorphic, gram-positive bacillus.
There are four biotypes of C. diphtheriae: mitis, intermedius, belfanti, and gravis, differentiated by colonial morphology, hemolysis, and fermentation reactions.
Diphtheritic toxin production occurs only after acquisition of a lysogenic Corynebacteriophage by either C. diphtheriae or C. ulcerans, which encodes the diphtheritic toxin gene and confers diphtheria-producing potential on these strains.
The toxin is lethal in human beings in an amount of 130μg/kg body weight.

Epidemiology

Transmission occurs through airborne respiratory droplets, direct contact with respiratory secretions of symptomatic individuals, or exudates from infected skin lesions.
Asymptomatic respiratory tract carriage is important in transmission, with 3-5% of healthy individuals carrying toxigenic organisms in endemic areas.
Skin infection and skin carriage are silent reservoirs, and organisms can remain viable in dust or on fomites for up to 6 months.
Transmission through contaminated milk and an infected food handler has been documented.
Children aged 1-5 years are commonly infected, and a herd immunity of 70% is required to prevent epidemics.

Pathogenesis

The local effect of diphtheritic toxin includes paralysis of the palate and hypopharynx and pneumonia.
Systemic effects of toxin absorption include kidney tubule necrosis, hypoglycemia, myocarditis, and/or demyelination of nerves.
Myocarditis typically occurs 10-14 days after infection, while demyelination of nerves occurs 3-7 weeks after infection.

Clinical Manifestations

Clinical manifestations are influenced by the anatomic site of infection, the immune status of the host, and the production and systemic distribution of toxin.
The incubation period is 1-6 days, and classification of diphtheria is based on the location of infection, including nasal, pharyngeal, tonsillar, laryngeal or laryngotracheal, skin, eye, or genitalia.
Nasal diphtheria causes serosanguineous, purulent, erosive rhinitis with membrane formation and is more common among infants.
Tonsillar and pharyngeal diphtheria is characterized by a universal early symptom of sore throat, with fever and dysphagia present in only half of patients.

Diagnosis

Diagnosis is based on clinical features and laboratory tests, including culture from the nose and throat and any other mucocutaneous lesion.
The Elek test is a rapid diagnostic test that takes 16-24 hours, and other diagnostic tests include enzyme immunosorbent assay, PCR for the toxin gene, and hypoglycemia, glycosuria, BUN, or abnormal ECG for liver, kidney, and heart involvement.

Complications

Respiratory tract obstruction by pseudomembranes can occur, and bronchoscopy or intubation and mechanical ventilation may be necessary.
Toxic cardiomyopathy can occur in 10-25% of patients and is responsible for 50-60% of deaths, with tachycardia, prolonged PR interval, and changes in the ST-T wave.
Toxic neuropathy can occur acutely or 2-3 weeks after infection, with symptoms including hypoesthesia, soft palate paralysis, weakness of the posterior pharyngeal, laryngeal, and facial nerves, and symmetric polyneuropathy.

Treatment

Antitoxin is the mainstay of therapy and can neutralize only free toxin, with efficacy diminishing with elapsed time.
Antimicrobial therapy is used to halt toxin production, treat localized infection, and prevent transmission of the organism to contacts.
Elimination of the organism should be documented by negative results of at least two successive cultures of specimens from the nose and throat (or skin) obtained 24 hours apart after completion of therapy.

Prevention

Asymptomatic case contacts should receive antimicrobial prophylaxis, such as erythromycin or a single injection of benzathine penicillin G.
Administration of diphtheria toxoid is indicated at recovery to complete the primary series or booster doses of immunization.

Note: The above study notes are written in detail, focusing on key facts with context, and are divided into headings and subheadings to facilitate understanding and revision.