Diphtheria

Diphtheria is an infection caused by the bacterium '' Corynebacterium diphtheriae''. Most infections are asymptomatic or have a mild clinical course, but in some outbreaks more than 10% of those diagnosed with the disease may die. Signs and symptoms may vary from mild to severe and usually start two to five days after exposure. Symptoms often come on fairly gradually, beginning with a sore throat and fever. In severe cases, a grey or white patch develops in the throat. This can block the airway and create a barking cough as in croup. The neck may swell in part due to enlarged lymph nodes. A form of diphtheria which involves the skin, eyes or genitals also exists. Complications may include myocarditis, inflammation of nerves, kidney problems, and bleeding problems due to low levels of platelets. Myocarditis may result in an abnormal heart rate and inflammation of the nerves may result in paralysis.

Diphtheria is usually spread between people by direct contact or through the air. It may also be spread by contaminated objects. Some people carry the bacterium without having symptoms, but can still spread the disease to others. The three main types of ''C. diphtheriae'' cause different severities of disease. The symptoms are due to a toxin produced by the bacterium. Diagnosis can often be made based on the appearance of the throat with confirmation by microbiological culture. Previous infection may not protect against infection.

A diphtheria vaccine is effective for prevention and available in a number of formulations. Three or four doses, given along with tetanus vaccine and pertussis vaccine, are recommended during childhood. Further doses of diphtheria–tetanus vaccine are recommended every ten years. Protection can be verified by measuring the antitoxin level in the blood. Diphtheria can be prevented in those exposed as well as treated with the antibiotics erythromycin or benzylpenicillin. A tracheotomy is sometimes needed to open the airway in severe cases.

In 2015, 4,500 cases were officially reported worldwide, down from nearly 100,000 in 1980. About a million cases a year are believed to have occurred before the 1980s. Diphtheria currently occurs most often in sub-Saharan Africa, India, and Indonesia. In 2015, it resulted in 2,100 deaths, down from 8,000 deaths in 1990. In areas where it is still common, children are most affected. It is rare in the developed world due to widespread vaccination but can re-emerge if vaccination rates decrease. In the United States, 57 cases were reported between 1980 and 2004. Death occurs in 5% to 10% of those diagnosed. The disease was first described in the 5th century BC by Hippocrates. The bacterium was identified in 1882 by Edwin Klebs.

Signs and symptoms

The symptoms of diphtheria usually begin two to seven days after infection. They include fever of 38 °C (100.4 °F) or above; chills; fatigue; bluish skin coloration ( cyanosis); sore throat; hoarseness; cough; headache; difficulty swallowing; painful swallowing; difficulty breathing; rapid breathing; foul-smelling and bloodstained nasal discharge; and lymphadenopathy. Within two to three days, diphtheria may destroy healthy tissues in the respiratory system. The dead tissue forms a thick, gray coating that can build up in the throat or nose. This thick gray coating is called a "pseudomembrane". It can cover tissues in the nose, tonsils, voice box, and throat, making it very hard to breathe and swallow. Symptoms can also include cardiac arrhythmias, myocarditis, and cranial and peripheral nerve palsies.

Diphtheritic croup

Laryngeal diphtheria can lead to a characteristic swollen neck and throat, or "bull neck". The swollen throat is often accompanied by a serious respiratory condition, characterized by a brassy or "barking" cough, stridor, hoarseness, and difficulty breathing; and historically referred to variously as "diphtheritic croup", "true croup", or sometimes simply as "croup". Diphtheritic croup is extremely rare in countries where diphtheria vaccination is customary. As a result, the term " croup" nowadays most often refers to an unrelated viral illness that produces similar but milder respiratory symptoms.

Transmission

Human-to-human transmission of diphtheria typically occurs through the air when an infected individual coughs or sneezes. Breathing in particles released from the infected individual leads to infection. Contact with any lesions on the skin can also lead to transmission of diphtheria, but this is uncommon. Indirect infections can occur, as well. If an infected individual touches a surface or object, the bacteria can be left behind and remain viable. Also, some evidence indicates diphtheria has the potential to be zoonotic, but this has yet to be confirmed. ''Corynebacterium ulcerans'' has been found in some animals, which would suggest zoonotic potential.

Mechanism

Diphtheria toxin (DT) is produced only by ''C. diphtheriae'' infected with a certain type of bacteriophage. Toxinogenicity is determined by phage conversion (also called lysogenic conversion); i.e, the ability of the bacterium to make DT changes as a consequence of infection by a particular phage. DT is encoded by the ''tox'' gene. Strains of corynephage are either ''tox''⁺ (e.g., corynephage β) or ''tox''⁻ (e.g., corynephage γ). The ''tox'' gene becomes integrated into the bacterial genome. The chromosome of C. diphtheriae has two different but functionally equivalent bacterial attachment sites (attB) for integration of β prophage into the chromosome.

Diphtheria toxin precursor is a protein of molecular weight 60 kDa. Certain proteases, such as trypsin, selectively cleave DT to generate two peptide chains, amino-terminal fragment A (DT-A) and carboxyl-terminal fragment B (DT-B), which are held together by a disulfide bond. DT-B is a recognition subunit that gains entry of DT into the host cell by binding to the EGF-like domain of heparin-binding EGF-like growth factor on the cell surface. This signals the cell to internalize the toxin within an endosome via receptor-mediated endocytosis. Inside the endosome, DT is split by a trypsin-like protease into DT-A and DT-B. The acidity of the endosome causes DT-B to create pores in the endosome membrane, thereby catalysing the release of DT-A into the cytoplasm.

Fragment A inhibits the synthesis of new proteins in the affected cell by catalyzing ADP-ribosylation of elongation factor EF-2—a protein that is essential to the translation step of protein synthesis. This ADP-ribosylation involves the transfer of an ADP-ribose from NAD+ to a diphthamide (a modified histidine) residue within the EF-2 protein. Since EF-2 is needed for the moving of tRNA from the A-site to the P-site of the ribosome during protein translation, ADP-ribosylation of EF-2 prevents protein synthesis.

ADP-ribosylation of EF-2 is reversed by giving high doses of nicotinamide (a form of vitamin B₃), since this is one of the reaction's end products, and high amounts drive the reaction in the opposite direction.

Diagnosis

The current clinical case definition of diphtheria used by the United States' Centers for Disease Control and Prevention is based on both laboratory and clinical criteria.

Laboratory criteria

* Isolation of ''C. diphtheriae'' from a Gram stain or throat culture from a clinical specimen,
* Histopathologic diagnosis of diphtheria by Albert's stain

Toxin demonstration

* In vivo tests (guinea pig inoculation): Subcutaneous and intracutaneous tests
* In vitro test: Elek's gel precipitation test, detection of tox gene by PCR, ELISA, ICA

Clinical criteria

* Upper respiratory tract illness with sore throat
* Low-grade fever (above is rare)
* An adherent, dense, grey pseudomembrane covering the posterior aspect of the pharynx: in severe cases, it can extend to cover the entire tracheobronchial tree.

Case classification

* Probable: a clinically compatible case that is not laboratory-confirmed and is not epidemiologically linked to a laboratory-confirmed case
* Confirmed: a clinically compatible case that is either laboratory-confirmed or epidemiologically linked to a laboratory-confirmed case

Empirical treatment should generally be started in a patient in whom suspicion of diphtheria is high.

Prevention

Vaccination against diphtheria is commonly done in infants and delivered as a combination vaccine, such as a DPT vaccine (diphtheria, pertussis, tetanus). Pentavalent vaccines, which vaccinate against diphtheria and four other childhood diseases simultaneously, are frequently used in disease prevention programs in developing countries by organizations such as UNICEF.

Treatment

The disease may remain manageable, but in more severe cases, lymph nodes in the neck may swell, and breathing and swallowing are more difficult. People in this stage should seek immediate medical attention, as obstruction in the throat may require intubation or a tracheotomy. Abnormal cardiac rhythms can occur early in the course of the illness or weeks later and can lead to heart failure. Diphtheria can also cause paralysis in the eye, neck, throat, or respiratory muscles. Patients with severe cases are put in a hospital intensive care unit and given diphtheria antitoxin (consisting of antibodies isolated from the serum of horses that have been challenged with diphtheria toxin). Since antitoxin does not neutralize toxin that is already bound to tissues, delaying its administration increases risk of death. Therefore, the decision to administer diphtheria antitoxin is based on clinical diagnosis, and should not await laboratory confirmation.

Antibiotics have not been demonstrated to affect healing of local infection in diphtheria patients treated with antitoxin. Antibiotics are used in patients or carriers to eradicate ''C. diphtheriae'' and prevent its transmission to others. The Centers for Disease Control and Prevention recommends either:
* Metronidazole
* Erythromycin is given (orally or by injection) for 14 days (40 mg/kg per day with a maximum of 2 g/d), or
* Procaine penicillin G is given intramuscularly for 14 days (300,000 U/d for patients weighing <10 kg and 600,000 U/d for those weighing >10 kg); patients with allergies to penicillin G or erythromycin can use rifampin or clindamycin.

In cases that progress beyond a throat infection, diphtheria toxin spreads through the blood and can lead to potentially life-threatening complications that affect other organs, such as the heart and kidneys. Damage to the heart caused by the toxin affects the heart's ability to pump blood or the kidneys' ability to clear wastes. It can also cause nerve damage, eventually leading to paralysis. About 40% to 50% of those left untreated can die.

Epidemiology

Diphtheria is fatal in between 5% and 10% of cases. In children under five years and adults over 40 years, the fatality rate may be as much as 20%. In 2013, it resulted in 3,300 deaths, down from 8,000 deaths in 1990. Better standards of living, mass immunization, improved diagnosis, prompt treatment, and more effective health care have led to a decrease in cases worldwide.

History

In 1613, Spain experienced an epidemic of diphtheria. The year is known as ''El Año de los Garrotillos'' (The Year of Strangulations) in the history of Spain.

In 1735, a diphtheria epidemic swept through New England.

Before 1826, diphtheria was known by different names across the world. In England, it was known as Boulogne sore throat, as it spread from France. In 1826, Pierre Bretonneau gave the disease the name ''diphthérite'' (from Greek διφθέρα, ''diphthera'' 'leather') describing the appearance of pseudomembrane in the throat.

In 1856, Victor Fourgeaud described an epidemic of diphtheria in California.

In 1878, Queen Victoria's daughter Princess Alice and her family became infected with diphtheria, causing two deaths, Princess Marie of Hesse and by Rhine and Princess Alice herself.

In 1883, Edwin Klebs identified the bacterium causing diphtheria and named it Klebs–Loeffler bacterium. The club shape of this bacterium helped Edwin to differentiate it from other bacteria. Over the period of time, it was called ''Microsporon diphtheriticum'', ''Bacillus diphtheriae'', and ''Mycobacterium diphtheriae''. Current nomenclature is ''Corynebacterium diphtheriae''.

Friedrich Loeffler was the first person to cultivate ''C. diphtheriae'' in 1884. He used Koch's postulates