“He’s not whooping,” replied the GP. “He is just coughing and anyway he was vaccinated as a child.”
“It’s still probably whooping cough” replied the Microbiologist stubbornly. “Antibiotics aren’t going to help.”
“Tell that to the patient” grumbled the GP.
“Tell you what. Send us a serum blood sample and we’ll test for pertussis antibodies. Perhaps showing him that he has whooping cough will reassure him that the cough will eventually settle down even though it may take months!”
The GP eventually agreed, but it was clear they weren’t convinced, and they wondered why they bothered asking for advice in the first place.
So, what is pertussis or whooping cough?
Pertussis, also known as whooping cough, is a highly contagious bacterial infection of the upper respiratory tract caused by Bordetella pertussis. The name pertussis was given to the infection by Thomas Sydenham; pertussis means “intense cough” in Latin.
Pertussis is transmitted via droplets from the upper respiratory tract, usually from coughing. The incubation is from 1 to 3 weeks but is usually 7 to 10 days. Up to one third of close contacts of a case will be infected; this is a very high secondary infection rate. So the patient’s wife might actually be better off in a different bedroom!
How does pertussis present?
Pertussis usually starts with the catarrhal phase as a non-specific upper respiratory tract infection similar to the common cold although pertussis can be asymptomatic.
After the catarrhal phase comes the paroxysmal phase with the classical feature of pertussis being intense bouts of paroxysmal coughing followed by an inspiratory “whoop” of air as the infected person “catches their breath”. The person sounds and looks like they might actually stop breathing until they take that gasp of air.
By far and away the most scary presentation of pertussis in the paroxysmal phase is in young infants (<6 months old) who don’t cough or whoop but instead suddenly become apnoeic (stop breathing), cyanotic (low oxygen in the blood makes them turn navy blue) and bradycardic (slow heart rate). This is a terrifying thing to see and one of the most frightening I have had to deal with as a doctor. I have seen babies drop their heart rates to <10bpm and their oxygen saturation to <10%... really scary! The scariest bit is that there is little you can do to help, except wait for them to breathe again or try intubating and ventilating them to see if that puts more oxygen into their blood (which takes skill and experience not to mention specialist equipment and a steady hand!).
After the paroxysmal phase the patient enters the convalescent phase where the frequency and severity of coughing slowly starts to reduce. Perhaps the most common presentation of pertussis is a chronic cough, often for many months. Numerous studies have shown that 25-50% of chronic cough (>2 weeks) in children and adults is actually due to pertussis.
Overall the symptoms tend to last for up 3 months. The Ancient Chinese called pertussis “the cough of 100 days” and that describes the chronicity of pertussis perfectly.
What are the complications of pertussis?
Other than the chronic (and rather irritating) nature of the cough in pertussis there are a number of other possible complications too, including:
- Otitis media
- Severe cough leading to rib fractures, abdominal wall hernia, brain haemorrhages and cerebrovascular accidents (strokes)
The mortality from pertussis depends on age; <6 months has a 1.5% mortality whereas in adults and adolescents it is <0.01%.
How is pertussis diagnosed?
Diagnosing pertussis in the laboratory is tricky. The correct specimen type is required as well as the correct culture media.
Patients suspected of having pertussis should have a pernasal swab taken for culture. This is a “special swab” with a flexible metal shaft not a normal bacterial swab. The tip of a pernasal swab is calcium alginate. Normal swabs are cotton containing fatty acids which are toxic to B. pertussis and therefore stop it growing; don’t use a normal swab!
Bordetella pertussis can also be identified using PCR, BUT this uses a “polyester” or “rayon” swab as the calcium alginate in the normal pernasal swab is inhibitory to PCR! [Really no wonder they wrong swabs are sent!!] In the UK PCR is usually reserved for children <6 months of age due to cost versus clinical significance; children <6 months old have the most severe disease and the highest risk of death.
Once the correct swab has arrived at the lab, B. pertussis requires culture for 7-10 days on specialist agar, either on the classic Borget-Gengou media or the more modern Regan-Lowe agar which has a longer shelf life. These media have lots of extra nutrients in the agar to help the B. pertussis to grow, as well as a cephalosporin antibiotic to stop other respiratory tract flora from out-growing the B. pertussis.
There is also a serology test done on serum blood for B. pertussis however this will not help with diagnosing acute pertussis infection as it shows past exposure only.
How is pertussis treated?
If it is a bacterial infection, then why are no antibiotics given? Pertussis is rarely diagnosed quickly enough for antibiotics to make any difference. If diagnosed before 3 weeks then antibiotics may reduce the duration and severity of coughing. However, after this time, the symptoms will be unaffected as the coughing is usually due to post infectious inflammation and damage; the patient needs “time to heal” before the symptoms will go away.
Antibiotics can still be given between 3-6 weeks to patients at high risk of transmitting the infection to more vulnerable people including pregnant women, healthcare workers and those who work with infants.
The main method for preventing pertussis is with vaccination. This is part of the routine childhood immunisation schedule in many countries, including the UK. Vaccination usually occurs at 2, 3 and 4 months of age with a booster before they go to school.
Whilst it might sound odd, we are not actually immunising against pertussis but rather to provide “herd immunity”. If enough people in the population are immunised it is hard for the bacterium to circulate and so young infants who haven’t yet been vaccinated but who are most at risk and vulnerable from pertussis are actually protected.
But I’ve been vaccinated!
There are two types of pertussis vaccine, cellular and acellular. Acellular pertussis is the most commonly used as it causes less localised side effects such as pain and swelling, however neither vaccine gives long term immunity. Both vaccines last for 5-10 years so children are susceptible to infection again when they reach school age, hence why they need a booster. Booster vaccines every 5-10 years are not normally given later in life as adults although can get re-infected the infection is usually mild in adults.
The pertussis vaccine was introduced in the 1950s and helped reduce the incidence of infection. However, during the 1970s there was a “vaccine scare” as some poor studies suggested a link between the pertussis vaccine and brain damage. My mum still likes to tell me the story of how she had to decide whether to have me, my brother and my sister vaccinated at this time… apparently the media were presenting a one sided view of the vaccine story back then as well. The scare was shown to be unjustified and fortunately the vaccine coverage increased again, but not before lots of unvaccinated children developed pertussis and some died.
Notifications of pertussis compared to vaccine coverage of children by their 2nd birthday in England
Incidence of laboratory confirmed pertussis in England
“Ummm…” …all of that extra work! The Microbiologist thought, it was now the Microbiologist’s turn to start to question why they bothered giving the advice…. Some things just need reassurance and time to heel.