Bordetella pertussis (Whooping cough)

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Pertussis is a contagious infection caused by the bacteria Bordetella pertussis, which causes violent coughing spells, called paroxysms, which make it difficult to breathe.
When it is finally possible to breathe in, air is drawn in through partially closed, swollen airways and that creates a whooping noise which gives pertussis its other name, whooping cough.
Bordetella pertussis is a gram negative coccobacilli - meaning that it looks like a short pink rod on a gram stain. It transmits from one person to another through a sneeze or cough, when that happens thousands of bacteria-filled droplets spray out about two meters or six feet away.
These droplets can land in the mouths or noses of nearby people, or get directly inhaled into the lungs. The bacteria can also survive for several days on dry surfaces, so it’s also possible to get the bacteria by touching a surface, like a contaminated doorknob, and then touching your own eyes, nose, or mouth.
Bordetella pertussis releases toxins which are proteins that help the bacteria in various ways to attach to and damage the respiratory epithelial cells.
It starts with three toxins: Filamentous hemagglutinin, pertactin, and agglutinogen - all of which help to anchor Bordetella pertussis to the epithelia where it remains during an infection.
Next there’s the tracheal cytotoxin which paralyzes the cilia that are the little hairy projections on the epithelial cells so they can’t sweep back and forth anymore.
Normally these cilia sweep away mucus and any bacteria stuck in the mucus, so paralyzing the cilia allows pertussis to stay snugly attached to the epithelia.
This also means that mucus starts building up which triggers a violent cough reflex to clear the airway starting up those coughing fits.
Another toxin is pertussis toxin which also helps with anchoring pertussis to the epithelia as well. In addition to this, though, pertussis toxin causes an increase in the absolute lymphocyte level in the blood, specifically an increase in the population of T cells floating around through a few mechanisms.
First of all, Pertussis toxin stimulates T cells to divide, causing them to leave the spleen and thymus and enter circulation, and it also blocks them from leaving the blood and migrating into tissues.
Pertussis toxin also makes the blood vessels in the respiratory tissue more sensitive to histamine, which makes it easier for fluid to seep out of the blood vessels and into airway tissues.
This makes the airways swell up, making it harder for a person to breath, and causes the classic “whooping” sound during a coughing fit.
Finally, there’s a toxin called adenylate cyclase toxin which blocks phagocytes from getting to the site of infection and prevents them from being able to kill the bacteria that they do manage to engulf once they arrive.
As if that wasn’t bad enough, the adenylate cyclase toxin even induces phagocytes to undergo apoptosis - effectively killing themselves.
A pertussis infection begins with the incubation period, which is the time between the bacterium entering the body and the onset of symptoms and it usually lasts about a week.
During this time, Bordetella pertussis is in the respiratory tract, but hasn’t multiplied enough to create a noticeable amount of damage.
Once the bacterial concentration increases, though, damage to the respiratory tract causes symptoms like nasal congestion, cough, and occasionally a low-grade fever.
This is called the catarrhal phase, and it lasts about 2 weeks. At this point, pertussis is very contagious because the presence of lot of bacteria in the respiratory tract makes them easy to aerosolize.
After that, there’s the paroxysmal phase, which lasts another 1 to 6 weeks. Even though the immune system is killing and clearing Bordetella pertussis during this phase, symptoms persist from the damage caused by the bacteria when it was alive and thriving.
The most notable symptom that persist is when a person gets paroxysms that come out like a machine gun burst - an uninterrupted fit of coughing, followed by an inspiratory whooping noise which comes from air sliding past a glottis that’s still partially closed and swollen.
The violent force of these paroxysms can cause vomiting, a collapsed lung, broken ribs, and tiny petechiae in the face as capillaries burst from the pressure.
Rather than having violent coughing fits or making a whooping noise, really young infants often have gasping, cyanosis, apnea, or can have an “apparent life threatening event” called an ALTE during this phase.
Decreased oxygen levels can cause serious problems like seizures, encephalopathy, and even death. There’s also an increased risk of a pneumonia caused by other pathogens that take advantage of the fact that the respiratory tract is damaged.
Finally, there’s the convalescent phase, which lasts between 2 to 3 weeks, during which the cough slowly improves, the paroxysms and whooping fade away, and the airway heals.
It’s best to diagnose pertussis during the catarrhal phase because the antibiotics can be used to kill the bacteria and reduce damage.
Some ways to identify the bacteria are by swabbing the nasopharynx and trying to grow the bacteria in a culture or identifying the DNA of alive or dead bacteria by polymerase chain reaction.
Another option is to use a direct fluorescent antibody which detects Bordetella pertussis antigens. A final option is to look for pertussis serology - looking for a antibody response to pertussis, which is usually detectable after the infection has gone on for a few weeks.
One important predictor of the severity of illness, especially in young infants, is the degree of lymphocytosis. Fortunately, antibodies from pertussis vaccine are a great way to avoid the disease in the first place or lessen the symptoms if it does occur.
Pertussis vaccine is most often given as a part of the DTap vaccine which stands for Diphtheria, Tetanus, and acellular Pertussis.
The vaccine is greater than 90% effective in protecting against pertussis. That’s why pertussis outbreaks usually only occur in countries that don’t have access to the vaccine, in communities with a high rate of unvaccinated individuals, and among the elderly who may have gotten the vaccine a long time ago, but now have waning levels of antibody.
This is why vaccines as well as boosters are needed to maintain protection. The main treatment for pertussis is macrolide antibiotics, like azithromycin, which can be used when bacteria are still alive in the catarrhal or early paroxysmal phase.
It’s also important to prevent pertussis transmission to other susceptible individuals - especially young infants and immunocompromised individuals - and this can be done by isolating the infected person and giving antibiotic prophylaxis to household contacts.
Infants younger than 1 year old are often monitored closely because they have an incomplete immune system and can get very sick and even die from pertussis.
Pregnant women are encouraged to help protect their infects by getting the pertussis vaccine in her third trimester. That’s because the mother’s immune system generates antibodies that cross the placental barrier and offer passive immunity to her baby for months after birth.
All quick recap, a quick recap: Bordetella pertussis releases a number of toxins that damage the respiratory epithelium causing “whooping cough”.
Initially, there’s a highly contagious catarrhal phase with symptoms like cough and congestion, then a paroxysmal phase with coughing fits and whooping, and finally a convalescent phase when everything slowly improves.
In infants, it can cause apnea, ALTEs, and even death.