Pharyngeal diphtheria is an infection of the throat caused by toxin-producing strains of Corynebacterium diphtheriae, characterized by a tough, grayish membrane that forms across the tonsils, soft palate, and surrounding tissue. Although vaccination has driven case counts down dramatically worldwide, the disease still kills roughly one in twenty people it strikes in outbreak settings, usually through heart damage or suffocation when the membrane blocks the airway. The biology behind pharyngeal diphtheria is grimmer and more layered than the brief mention it gets in most immunization pamphlets.
What Happens in the Throat
The infection begins when C. diphtheriae colonizes the mucous membranes of the pharynx. The bacteria themselves rarely invade deeper tissue. Instead, toxigenic strains release diphtheria toxin locally, which destroys the cells lining the throat. Dead cells, fibrin, white blood cells, and bacterial debris mat together into a tough, leathery layer known as a pseudomembrane. This membrane can extend across the tonsils, the uvula, the soft palate, and into the larynx or nasal cavities.1The Journal of Infectious Diseases. The Pathology of Diphtheria It adheres tightly to the underlying tissue and bleeds if you try to peel it off, which is one of the classic clinical signs distinguishing it from other causes of sore throat.
The pseudomembrane is the source of two immediate dangers. First, it can grow large enough to physically obstruct the airway. Second, it serves as a toxin factory: while the bacteria sit in this necrotic mat, they continue pumping diphtheria toxin into the bloodstream, where it travels to the heart, nerves, and kidneys. The local disease in the throat, in other words, is only half the story. The systemic poisoning is what usually kills.
How the Toxin Damages Cells
Diphtheria toxin shuts down protein production inside human cells. It does this by chemically modifying a molecule called elongation factor 2, which cells need to build new proteins. The toxin attaches a chemical tag to this factor, permanently disabling it.2PubMed Central. ADP-ribosylation of translation elongation factor 2 by diphtheria toxin in yeast inhibits translation and cell separation Because a single molecule of the toxin can inactivate elongation factor 2 one copy at a time, cycling through the cell’s supply, even tiny amounts of toxin can be lethal to a cell. The reaction proceeds through a direct interaction between the toxin fragment and its two substrates inside the cell.3PubMed. The mechanism of ADP-ribosylation of elongation factor 2 catalyzed by fragment A from diphtheria toxin
This mechanism matters practically because it explains why antitoxin treatment is time-sensitive. Antitoxin neutralizes toxin that is still circulating in the blood, but once the toxin has entered a cell and begun its work, antitoxin cannot reach it. Every hour of delay allows more toxin to lock onto more cells irreversibly.
Airway Obstruction
The most immediately life-threatening complication of pharyngeal diphtheria is suffocation. As the pseudomembrane extends into the larynx and below the vocal cords, the airway narrows. Patients develop a barking cough, stridor (a high-pitched noise on breathing in), and increasing difficulty getting air. In one reported case in the United Kingdom, a patient’s laryngoscopy revealed thick purulent discharge throughout the nasal cavity, throat, and the area above the vocal cords, along with swelling of the tissue below the cords. She deteriorated to the point of near-cardiac arrest before emergency intubation could be performed.4PubMed Central. A Case of Life-Threatening Airway Obstruction Caused by Acute Diphtheria Infection in the United Kingdom
Children are especially vulnerable because their airways are narrower to begin with. A seven-year-old boy admitted after six days of fever and sore throat had already developed visible breathing distress with the chest wall pulling inward on each breath. Despite a tracheostomy and mechanical ventilation, he died the following day.5Forensic Science, Medicine, and Pathology. Diphtheria and lethal upper airway obstruction These cases illustrate why clinicians in endemic areas are trained to think about securing the airway early rather than waiting for a confirmed diagnosis.
Heart Damage From the Toxin
Death from pharyngeal diphtheria is most commonly caused by diphtheritic myocarditis, meaning inflammation of the heart muscle triggered by the circulating toxin. This can present as heart failure, dangerously abnormal heart rhythms, or cardiogenic shock, where the heart simply cannot pump enough blood to sustain the body.6PubMed Central. Diphtheritic myocarditis: a case report, with toxinmediated complications and multi-organ involvement Myocarditis tends to appear during the second or third week of illness, sometimes just when the throat seems to be improving. The disconnect between local recovery and systemic worsening catches families off guard: a patient whose sore throat is healing may suddenly collapse from a cardiac arrhythmia.
The severity of myocarditis correlates roughly with the extent of the original pharyngeal disease. Patients who developed a large membrane covering both tonsils and extending to the palate, or who had the characteristic “bull neck” swelling from massive lymph node enlargement, face the highest cardiac risk. This is another reason treatment guidelines assign the largest antitoxin doses to patients with extensive membrane or neck swelling.
Neurological Complications
Nerve damage from diphtheria toxin follows a somewhat predictable timeline. The earliest neurological sign, often appearing within the first few weeks, is palatal palsy: the soft palate stops working properly, giving the voice a nasal quality and causing liquids to come back up through the nose during swallowing. In a prospective study of children with pharyngeal diphtheria who developed neurological complications, isolated palatal palsy was the most common finding, present in about two-thirds of affected children.7PubMed Central. Prospective Study of Diphtheria for Neurological Complications
As weeks pass, the toxin can damage cranial nerves controlling swallowing and the voice box, leading to difficulty swallowing, drooling, and aspiration of food into the lungs. In severe forms studied in adult patients, involvement of the ninth and tenth cranial nerves between weeks three and five caused not only trouble swallowing but also respiratory failure and choking episodes as fluid entered the trachea.8JAMA Neurology. Diphtheritic Polyneuropathy: Clinical Analysis of Severe Forms Later still, limb weakness can develop, sometimes resembling Guillain-Barré syndrome. Other complications reported in case series include facial nerve palsy, paralysis of the diaphragm, and paralysis of the eye muscles.9PubMed Central. Spectrum of Neurological Outcomes in Diphtheria: A Case Series
The encouraging part is that diphtheritic neuropathy is usually reversible. In the prospective pediatric study, all twenty-eight children with neurological complications recovered completely.10PubMed Central. Prospective Study of Diphtheria for Neurological Complications Recovery can take weeks to months, though, and patients with diaphragmatic palsy may need mechanical ventilation in the interim.
Diagnosis and Why It Gets Missed
Pharyngeal diphtheria is easy to miss in countries where clinicians have never seen a case. Early symptoms look like any severe sore throat: fever, malaise, pain on swallowing. The pseudomembrane itself can be mistaken for the exudate of streptococcal tonsillitis or infectious mononucleosis. One published case involved an eight-year-old partially vaccinated boy initially treated for bacterial tonsillitis who turned out to have both diphtheria and mononucleosis simultaneously, a combination so uncommon that the literature review found it rarely reported.11PubMed Central. A Case of Diphtheria and Infectious Mononucleosis Co-Infection in a Partially Vaccinated Boy
Laboratory confirmation relies on culturing the organism and then testing whether it produces toxin. The gold-standard test for toxin production is the Elek test, an immunoprecipitation assay that detects diphtheria toxin from cultured bacteria.12PubMed Central. Detection of diphtheria toxin production by toxigenic corynebacteria using an optimized Elek test The problem is that culture and toxin testing take time, often several days, and treatment cannot wait for results. In practice, clinicians in outbreak settings start antitoxin and antibiotics based on clinical suspicion alone: a membrane that bleeds when dislodged, a “bull neck,” or a nasal voice with regurgitation of liquids through the nose should trigger immediate treatment.
Treatment With Antitoxin and Antibiotics
The two pillars of treatment are diphtheria antitoxin and antibiotics, and they serve different purposes. Antitoxin neutralizes circulating toxin before it enters cells, while antibiotics kill the bacteria to stop further toxin production and end the patient’s infectiousness.
Diphtheria antitoxin is derived from horses immunized against the toxin. It has been in use for over 130 years, making it one of the oldest biological therapies still in clinical practice. Dosing follows the clinical severity of the case rather than the patient’s age or weight. WHO and CDC guidelines recommend roughly 20,000 international units for mild pharyngeal disease with a limited membrane, 40,000 IU for more extensive membrane or nasopharyngeal involvement of less than 48 hours’ duration, and 60,000 to 80,000 IU for patients with bull-neck swelling, severe disease, or any case that has been symptomatic for more than two days.13PubMed Central. Diphtheria Antitoxin Administration, Outcomes, and Safety: Response to a Diphtheria Outbreak in Cox’s Bazar, Bangladesh The entire dose is given in a single injection, preferably intravenous. Repeat dosing is avoided because the equine-origin serum can trigger allergic reactions, including anaphylaxis and serum sickness, on subsequent exposure.14Memórias do Instituto Oswaldo Cruz. Diphtheria antitoxin treatment: from pioneer to neglected
For antibiotics, penicillin and erythromycin are the standard choices. A clinical trial comparing the two found no difference in how quickly the pseudomembrane cleared or how quickly bacteria were eliminated from the throat, but fever resolved faster with penicillin (a median of about 27 hours versus 46 hours for erythromycin).15PubMed. Penicillin vs. erythromycin in the treatment of diphtheria Erythromycin remains important for treating carriers and for patients allergic to penicillin. However, short courses of erythromycin can leave a significant carrier problem: in one study, all carriers became culture-negative during a six-day course, but about one in five relapsed to carrier status within two weeks of stopping the drug.16PubMed Central. Diphtheria carriers and the effect of erythromycin therapy Longer antibiotic courses are now standard to reduce this relapse rate.
The Carrier Problem
One of the trickiest aspects of controlling pharyngeal diphtheria is the carrier state. People can harbor C. diphtheriae in their throats without developing any symptoms, and vaccination does not prevent this. A study of a diphtheria outbreak found that of 104 people with culture-confirmed diphtheria infection, only 15 were symptomatic; the other 89 were carriers. Crucially, there was no statistical difference in the risk of becoming colonized between fully immunized and unimmunized individuals. What vaccination did protect against was symptomatic disease: unvaccinated people were thirty times more likely to develop actual illness than those fully vaccinated.17American Journal of Diseases of Children. Diphtheria Immunization: Effect Upon Carriers and the Control of Outbreaks
This distinction has practical consequences. It means that even in a well-vaccinated population, the organism can circulate silently. Outbreak control depends not just on vaccination but on identifying, isolating, and treating carriers with antibiotics. It also means that travelers returning from endemic areas can carry the bacteria even if they themselves never get sick.
Waning Immunity and Outbreaks
Vaccine-induced protection against diphtheria does not last forever. A systematic analysis of antibody decline found that after the standard three-dose infant series, protective antibody levels drop below a commonly used threshold within about ten years. Adding a fourth dose extends that to roughly 22 years, and a fifth dose pushes it to about 58 years before antibodies wane to very low levels.18PubMed Central. Waning rate of immunity and duration of protective immunity against diphtheria toxoid as a function of age and number of doses This is why many countries recommend booster doses every ten years through adulthood. In practice, many adults skip these boosters, leaving pockets of susceptibility even in high-income countries.
Outbreaks today cluster in settings where vaccination coverage has collapsed. Yemen’s outbreak beginning in 2017, amid ongoing conflict, saw nearly 1,300 probable cases with a case fatality rate of about 5.6%. Roughly two-thirds of patients were children under fifteen, and almost half had never been vaccinated. Districts experiencing active conflict had an eleven-fold higher risk of outbreak.19PubMed Central. Diphtheria outbreak in Yemen: the impact of conflict on a fragile health system Somalia declared a national diphtheria outbreak in August 2025, driven by the same combination of displacement, fragile infrastructure, and gaps in immunization.20PubMed Central. Diphtheria outbreak in Somalia: a weekly sitrep on the recent health crisis-2025 These outbreaks are not historical echoes; they are happening now.
Antitoxin Supply Is Dangerously Thin
A less visible crisis runs alongside the outbreaks themselves. Diphtheria antitoxin, the single most important treatment, is in chronic short supply worldwide. Because the disease is rare in wealthy nations, demand is low and unpredictable, which discourages manufacturers from investing in production. A recent assessment of global procurement practices found that prices and availability of antitoxin vary widely, and both manufacturers and procurement agencies struggle to predict demand. Substantial concerns were raised about the inability to secure enough antitoxin to respond to increasing outbreaks.21PubMed Central. Diphtheria Antitoxin Production and Procurement Practices and Challenges In several recent outbreaks, treatment centers ran out of antitoxin entirely, forcing clinicians to rely on antibiotics alone, a vastly inferior strategy for preventing toxin-mediated complications.
Cutaneous Diphtheria as a Hidden Reservoir
Pharyngeal diphtheria does not exist in isolation from cutaneous (skin) diphtheria. The same organism can infect wounds and skin lesions, producing sores that look like ordinary impetigo. What makes this relevant to pharyngeal disease is that skin infections can seed throat carriage in contacts even more efficiently than respiratory cases do. During one school-based outbreak, investigators found that classroom contacts of children with skin infections were more likely to be carrying the organism in their throats than were classroom contacts of children with respiratory diphtheria.22The Journal of Infectious Diseases. The Role of Cutaneous Diphtheria Infections in a Diphtheria Epidemic The likely reason is that skin lesions shed bacteria into the environment more freely than respiratory infections do.
This cross-talk means that cutaneous diphtheria, often dismissed as a minor skin infection, can spark outbreaks of the far more dangerous pharyngeal form. Public health authorities have stressed that because cutaneous diphtheria causes outbreaks of both skin and throat disease, early diagnosis and high vaccination coverage are essential.23PubMed Central. Imported cutaneous diphtheria, United Kingdom Travelers and military personnel returning from endemic regions with unexplained skin ulcers should be tested, not only for their own sake but to prevent onward transmission to the throat of household contacts.
Nontoxigenic Strains and the Zoonotic Angle
Not all strains of C. diphtheriae produce toxin. Nontoxigenic strains lack the gene for diphtheria toxin, so they cannot cause the classic toxin-mediated disease. They can, however, cause pharyngitis, endocarditis, and bloodstream infections. In England and Wales, confirmed isolates of nontoxigenic C. diphtheriae increased substantially between the mid-1980s and mid-1990s. Ribotyping of these isolates showed that a single strain, found exclusively in the throat and not previously identified among toxigenic strains, accounted for the majority.24PubMed Central. Nontoxigenic corynebacterium diphtheriae: an emerging pathogen in England and Wales? These nontoxigenic infections are generally less severe, but they matter for surveillance because nontoxigenic strains can, in theory, acquire the toxin gene from a bacteriophage and convert to toxin producers.
A separate wrinkle comes from a related species, Corynebacterium ulcerans, which can produce diphtheria toxin and cause a clinically identical pharyngeal illness in humans. Unlike C. diphtheriae, which is primarily a human pathogen, C. ulcerans circulates in animals, particularly dogs and cats. A toxigenic strain was isolated from a hunting dog in Japan, and the species is recognized as a zoonotic pathogen capable of transmission from pets to owners.25PubMed. Toxigenic Corynebacterium ulcerans isolated from a hunting dog and its diphtheria toxin antibody titer In high-income countries where classical diphtheria from C. diphtheriae has nearly vanished, C. ulcerans infections have made up a growing share of diphtheria-like cases. Pet owners with unexplained pharyngeal membranes, especially if they have close contact with animals showing nasal or skin lesions, should have cultures that look specifically for this organism.