Diseases you never want to get

I recently received a tetanus booster shot. Actually, it was called DTaP, and it was a combination vaccine against tetanus, diphtheria and pertussis. As I recovered from the booster shot (I had quite a sore arm for several days), I started wondering about these diseases. I wanted to know: what, biologically speaking, are they, anyways?

Let’s start with tetanus. Tetanus is caused by a soil-dwelling bacterium named Clostridium tetani. It can enter your body through a deep tissue wound that gets dirt in it (the classic example is stepping on a rusty nail). Other injuries involving dead skin, however, such as frostbite or burns, can also allow the bacterium a way into your system. There is also a form of neonatal tetanus, which can occur when a baby is delivered under unsanitary conditions. Once the bacterium is in the body, it produces a neurotoxin called tetanus toxin, which acts as a poison on your neurons. It stops the ability of the nerves controlling your skeletal muscles to turn themselves off; the risk is that these nerves will be firing signals at the smallest stimulus. This means that a small touch on your skin that would normally be ignored by your nervous system will result in a large, spastic muscle movement. The classic symptom of the disease is lockjaw, though other symptoms include difficulty swallowing, severe muscle spasms, and seizures. The spasms may continue up to 4 weeks, unless death occurs (in approximately 10-20% of the cases).

Sounds dreadful, doesn’t it? So here’s some good news: vaccination against tetanus is very effective. Even if you are behind on your tetanus boosters, there is even a post-exposure tetanus prophylaxis that you will receive if needed. Because of this, tetanus is very rare in the US. There have been fewer than 40 cases per year in the US since 1999. Phew!

How about diphtheria? Diphtheria is caused by Corynebacterium diphtheriae, There are 2 types of diphtheria – one in the upper respiratory tract, and one on the skin. The cutaneous (meaning skin) form of diphtheria is usually a secondary infection of a pre-existing skin wound. It is considered a mild form of the disease. Respiratory diptheria, however, can be very severe. It is spread very easily through respiratory droplets (such as those made by a cough or a sneeze) of an infected individual. The disease manifests as a sore throat, low-grade fever, swollen glands, and weakness. The most prominent symptom, however, is the accumulation of a thick, gray membrane over the throat and tonsils. This membrane can make swallowing and breathing difficult. Once established in the respiratory system, it can also spread to other organs, such as the heart and kidney. One in ten infected individuals will ultimately die from such an infection.

Corynebacterium diphtheriae is scientifically very interesting. It colonizes local tissues of the throat, then produces a toxin called diphtheria toxin, which interferes with a cell’s ability to make protein. This interference is lethal; any such cell affected by the toxin will die. There are actually 3 different substrains of this bacteria, each of which is capable of producing different amounts of toxin. The gravis strain can produce the most toxin, thus it causes the most severe form of the disease. However, toxin production is controlled not only by the genetics of the bacterium, but also the environment of the throat. In particular, inorganic iron prevents the synthesis of the toxin. So if you don’t have much iron in your bloodstream, the bacterium can produce much more toxin.

I know, this sounds horrible, too. But here’s the good news: diphtheria is also very rare in the US, again due to good vaccination protocols. There are fewer than 10 cases reported across the country every year. If someone does become infected, treatment options are pretty good. Immediate hospitalization and treatment allows most patients to recover from the disease.

And for the final disease of the day – pertussis. You may know pertussis better as whooping cough. This is a highly contagious disease of the upper respiratory tract. It is caused by the bacterium Bordetella pertussis, which lives in the mouth, nose and throat of mammals; there is no other known environment in which is bacterium is found. It is pathogenic only in humans. B. pertussis causes this disease because it, too, produces a toxin. Petussis toxin alters a cell’s ability to regulate internal levels of a molecule called cAMP. Elevated cAMP levels in a cell results in it losing fluid and ions. Pertussis toxin also has a very specific effect on cells of your immune system called phagocytes. Phagocytes normally engulf and destroy foreign matter in your body, such as bacteria. However, phagocytes affected by pertussis toxin can no work in this way, and a primary line of defense against the B. pertussis bacterium is lost.

The outcome of infection progresses in several stages. The first stage resembles a simple cold – runny nose, sneezing, low-grade fever and mild cough. The second stage is when the disease is at its worst. The cough becomes very severe. Infected individuals will have uncontrollable fits of coughing, following by a characteristic high-pitched crowing, or “whoop,” as the person breathes in. (This is why it is commonly called whooping cough.) The coughing fits can be so violent that they can cause the person to turn blue from lack of oxygen, or they may vomit once it is over. Between coughing episodes, the person will appear normal; however, they can count on around 20 such fits a day. The third phase of pertussis is the recovery phase, during which symptoms gradually disappear. There are many possible complications from pertussis, including pneumonia, ear infections, dehydration, seizures, encephalopathy (abnormal brain function due to oxygen deprivation) and death. The disease is most severe in young children; older children and adults usually have an easier time of it.

I was very surprised to learn about the frequency of pertussis in the US. Cases reached an all-time low in 1976 (1,010 cases reported); but that number has been slowly rising. According to the CDC, 5000 to 7000 cases of the disease are reported every year. One of the reasons for this is that immunity from infection or vaccination is not life-long; boosters are required to maintain immunity. But because it is so rapidly transmitted, anyone behind on their boosters can get a mild form of the disease, which is then transmitted. It is believed that more of these mild cases are now being recognized, whereas they would have previously been treated as a regular respiratory infection. There is some good news, though – treatment for established cases of pertussis is very good, and the number of reported deaths is very small.

After learning about all of this, I’m really glad that I was recently vaccinated against all of these things! None of them are anything I’d ever want to contract.