Sunday, September 20, 2020

Disease Post: Diphtheria

This is a disease that I have always found interesting and horrifying.

Image credit: CDC/Sarah Bailey Cutchin

The disease diphtheria is caused by a bacteria called Corynebacterium diphtheriae. The part that causes the most problems is the toxin that the bacteria produces (diphtheria toxin). However, not all C. diphtheriae actually have the toxin gene (but this is still infectious and dangerous without the toxin). There are also other Corynebacterium species that can carry the diphtheria toxin, but they cause infections in animals (CDC).

The bacteria is a gram positive (it has a peptidoglycan cell wall) bacillus (rod-shaped bacteria). It is an aerobic bacteria, meaning that it requires oxygen to live.

How is it spread?
There are two different types of diphtheria infection: cutaneous and respiratory.

The cutaneous infection can occur when the bacteria gets in the skin. This can happen if a person comes into physical contact with someone else who has a cutaneous infection, which causes infectious ulcers on the skin. It is also possible to catch it from a surface that has been touched by someone who is infected.

The respiratory infection is the most worrisome type of diphtheria infection. People catch this from inhaling the bacteria via respiratory droplets in the air.

C. diphtheriae is present worldwide. Humans are the only known reservoir, but vaccine non-compliance and the presence of asymptomatic carriers keeps the bacteria around.

The cutaneous infection is less deadly. The bacteria colonizes the skin and gets deeper into the skin through breaks or cuts. A papule develops and then becomes an ulcer that is usually chronic and slow or non-healing. There can be redness, pain, and swelling according to the Mayo Clinic. The ulcer may be covered with a grayish membrane and is infectious. According to the CDC, this form rarely results in more serious disease.
Diphtheria ulcer on a patient's leg. Image credit: CDC
The respiratory infection is the one most people talk and worry about. Symptoms usually start within two to five days of exposure and generally begins with a sore throat, malaise, and low-grade fever (WHO, CDC). The toxin (more on that shortly) causes dead tissue to build up in the throat, eventually forming a "pseudomembrane." According to the CDC, "[The pseudomemebrane] can cover tissues in the nose, tonsils, voice box, and throat, making it very hard to breathe and swallow." As you can imagine, difficulty breathing can lead to many other issues. According to the CDC, without treatment up to half of patients will die of the disease. Even with treatment, the CDC states that one in ten will still die of the disease. And just for the gross out factor: the pseudomembrane can start to slough off and further block your airway. Yuck!
Child with a swollen neck due to diphtheria. Image credit: CDC.
The toxin, diphtheria toxin, causes the most problems. The toxin inhibits protein synthesis (keeps cells from making proteins, which are needed for just about everything that happens in your body). "If the toxin gets into the blood stream, it can cause heart, nerve, and kidney damage," according to the CDC. Some of this damage can be long-lasting, even after the infection is cleared.

Prevention and Treatment:
The best prevention for this disease is the vaccine. Keeping up immunization and getting booster shots are the most important ways to keep diphtheria infections to a minimum. The vaccine is a toxoid vaccine, meaning it is actually a vaccine for the toxin, not to the bacteria itself. It is often included with the vaccine for tetanus and/or pertussis: Tdap, DTaP,  DT, and Td vaccines.

For treatment, antibiotics like penicillin or erythromycin can help get rid of the bacteria. But it is important to neutralize the toxin. There is a diphtheria antitoxin available for this and sometimes people will get injections made from the blood of people who have cleared the infection in order to stimulate antibody production to the toxin and bacteria.

So that's C. diphtheriae! I always liked to use diphtheria as a supporting argument for vaccination. Along the lines of: "Do you know what happens to you when you get diphtheria? The skin in your throat sloughs off and then you die."

Medical Microbiology sixth edition by Patrick R. Murray, Ken S. Rosenthal, and Michael A. Pfaller

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