What causes diphtheria?

Diphtheria is an infectious disease caused by bacterial microorganisms known as Corynebacterium diphtheriae .Other Corynebacterium species can be responsible, but this is rare.

Some strains of this bacterium produce a toxin. Infection with these toxigenic diphtheria bacteria is what leads to the most serious complications of diphtheria. The bacteria are toxigenic because they themselves are infected by a certain type of virus.

Diphtheria is prevented by early-years vaccination.

The toxin that is released:

§ Inhibits the production of proteins by cells

§ Destroys the tissue at the site of the infection

§ Leads to membrane formation - which leads to the telltale sign at the back of the throat

§ Gets taken up into the bloodstream and distributed around the body's tissues

§ Causes myocarditis (inflammation of the heart) and neuritis (nerve damage)

§ Can cause low platelet counts (thrombocytopenia) and produce protein in the urine (proteinuria).

Diphtheria is an infection spread only among humans - people are the only known reservoir for it. It is contagious via direct physical contact with:

§ Droplets breathed out into the air (as an aerosol, in the same way as the spread of flu)

§ Secretions from the nose and throat (again, similar to other upper respiratory infections)

§ Infected skin lesions

§ Intermediary objects such as bedding or clothes (rarely).

The spread from an infected patient to a susceptible new person can be to any mucous membrane, but the toxic infection typically attacks the nasopharynx (the lining of the nose and throat).

Signs and symptoms of diphtheria

Specific signs and symptoms of diphtheria depend on the particular strain of bacteria involved, and the site of the body affected.

One type of diphtheria, more common in the tropics, causes skin ulcers rather than respiratory infection. These atypical cases are usually less serious than the classic cases, first described by Hippocrates, that lead to severe illness and sometimes death.

Early features of the infection, occurring prior to the appearance of the nasopharyngeal pseudomembrane, include:

§ Fever (though not usually a very high temperature), malaise and weakness

§ Enlarged lymph nodes at the front of the neck (popularly referred to 'swollen glands')

§ Swelling of soft tissue in the neck (giving a 'bull neck' appearance)

§ High heart rate (tachycardia, disproportionately high for the level of fever).

Children with a pharyngeal diphtheria infection are more likely to have the following early features:

§ Nausea and vomiting

§ Chills, headache and fever.

After a person is first infected with the bacteria there is an average incubation period of 5 days before early signs and symptoms appear.

This 'prodromal period' lasts between 12 and 24 hours and, if the bacteria colonizing the nasopharynx are toxic, a pseudomembrane forms over the following 2-3 days, leading to:

§ Sore throat

§ Difficulty swallowing (dysphagia)

§ Possible obstruction that causes difficulty breathing (shortness of breath - dyspnea).

Complications of diphtheria

Further illness, and the most severe and potentially life-threatening stage of the disease, results from the absorption of the toxin into the bloodstream, which then goes on to damage other vital tissues, as follows.

Diagnosis and tests for diphtheria

There are definitive tests for diagnosing a case of diphtheria, so if symptoms and history cause a suspicion of the infection, it is relatively straightforward to confirm the diagnosis.

Classic cases of diphtheria cause sore throat and swollen lymph nodes in the neck, and the formation of a characteristic 'pseudomembrane' over the back of the throat.

Doctors should be suspicious when they see the characteristic membrane, or patients have unexplained pharyngitis, swollen lymph nodes in the neck, and low-grade fever.

Tissue samples taken from a patient with suspected diphtheria can be used to isolate the bacteria, which are then cultured for identification and tested for toxicity:

§ Clinical specimens are taken from the nose or nasopharynx, and throat

§ All suspected cases and their close contacts are tested

§ If possible, swabs are also taken from under the pseudomembrane, or removed from the membrane itself.

Once the diphtheria bacterium has been isolated, it is 'biotyped' to elucidate the particular strain, which will point to its toxicity, as outlined above.

Toxigenicity is further tested using the Elek test, which determines whether the organisms produce the diphtheria toxin; if positive, it triggers the diagnosis of diphtheria.

The tests may not be readily available, and so doctors may need to rely on a specialist laboratory.