What is pertussis?
- Here are three generations of a family. Let's say that one day the seven year old develops a little cough, which soon turns into something much worse. I have an audio clip of the cough, and I want you to pay special attention to the noise the boy makes when he catches his breath. (coughing sounds) So this boy is definitely having a coughing fit that's causing him to cough out the entire air content of his lungs. But did you hear the noise he makes when he is finally able to take his breath? It's this sort of whooping noise. And, well, this is the signature noise that gives the disease whooping cough its name. Whooping cough is also called pertussis, because it's caused by an infection from a type of bacteria called Bordetella pertussis. If we were to look at pertussis under a microscope, we would see these rod-shaped bacteria like I'm drawing here. And the bacteria infects the respiratory tract, and releases toxins which cause damage and inflammation to the airways. And this damage causes mucus to build until a violent cough reflex is triggered. And when breathing in after one of these fits the air is travelling rapidly though swollen airways and this is what generates that whooping noise. Now pertussis is highly contagious because it's transferred person to person through mucus droplets containing live bacteria that's coughed or sneezed into the air. So let's say that mom is close by, within three feet of her son, she might breathe in mucus droplets and become infected. Or droplets can be passed along by shared surfaces, like a toy shared with this boy's sister. She may pick up the bacteria on her hands and then rub her eyes and nose, and introduce the bacteria into her body. And this is why coughing and sneezing into the elbow instead of the hand, and lots of hand washing is so important. It's also why healthcare workers treating the family will wear gowns, gloves, and masks to minimize cross-contamination between their patients. But there's a particular stage during the infection where the disease is most contagious. So let's get a timeline going to explore that, and we'll start with the initial exposure to the bacteria. Typically, there's about a week long incubation period where the bacteria is in the respiratory tract, and it's dividing and causing it's damage, but there are no clinical symptoms present yet. None of the clinical symptoms present for about 12 weeks. So I'm going to draw out the entire clinical timeline to 12 weeks, but actually, the 12 weeks is divided into three stages. The first stage is on the onset of the clinical symptoms to about two weeks. It has this fancy name called the catarrhal stage. And the word cattarhal literally means inflammation of the respiratory tract with increased secretions. So this stage is appropriately named because the main clinical symptom that you see here is a runny nose with sneezing a small cough, and a low grade fever. The symptoms make it hard to distinguish from a common cold, but unfortunately, it's this stage that pertussis is most contagious, because the bacteria hasn't been killed by the immune system yet, and so it's dividing to high numbers, so each cough, and each sneeze expels live bacteria into the air. So the onset of symptoms, an infected person should really stay away from others until at least 21 days. By then, the body has been able to generate antibodies against pertussis, which tag it as a foreign invader needing to be killed by the immune system. During the next two to eight weeks, known as the paroxysmal stage, most of the bacteria is dead, so the disease isn't as contagious, but the damage and the inflammation caused by the bacteria in the weeks leading up till now, can cause extreme coughing fits and whooping noises like we talked about earlier. And actually, that's exactly how this stage gets its name. A paroxysm is a fit or a re-occurrence of a symptom. Here, it's coughing, and these coughing fits are absolutely exhausting. And they are frequently followed by vomiting and other complications from the forcefulness of the cough. But finally, the convalescent stage is reached at weeks eight to 12. Convalescence literally means recovery from an illness, and that's exactly what the body is doing during this time. The infection has been cleared from the body, and the body now needs to heal from the respiratory tract infection and damage. Over time, the frequency and the intensity of the coughing will go away, but it takes a really long time, and it's the reason that some cultures call pertussis the 100 day cough. Even though I've clearly laid out these three clinical stages, in reality the stages can be difficult to decipher depending on a few modifying factors. And a modifying factor would be anything that changes the length or the severity of the disease. So, for example, age. Let's say a two month old is infected. Infants may not have the energy after a coughing fit to breathe in strongly and get air rapidly in past swollen, inflamed airways, so an infant may not whoop after a coughing fit. Adults may have coughing fits, but they may not whoop after, because their airways are larger, and can handle being swollen a bit without drastically reducing airflow. Another modifying factor is immunization status. And let's say that grandma had a pertussis vaccine in the last 10 years. Her body's primed for a pertussis infection, so if she becomes exposed, her body can fight it faster, and in a shorter amount of time. And the damage and the inflammation may not progress to the point where she has a whooping cough, or any cough at all. For the same reason of having the body primed, previous exposure to pertussis can also be a modifying factor. So, for example, if grandpa recently had a pertussis infection, his body's primed to fight a re-exposure for a second time, and just like grandma, his coughing fits, whooping and vomiting and other complications can be milder, if they occur at all.