The ‘100-day cough’ that adults often miss

Whooping cough, medically known as pertussis, is a highly contagious bacterial infection that affects the airways. It gets its common name from the “whoop” sound that some infected children make when they take a deep, gasping breath after a severe coughing fit.

The infection is caused by the bacterium bordetella pertussis, with research suggesting that as few as 140 bacterial cells may be enough to cause infection. The bacterium spreads through infected droplets, which are released when an infected person coughs or sneezes. These droplets can then settle on the lining of another person’s nose, throat or airways.

Doctors usually describe whooping cough in three stages. The first is the catarrhal phase, when symptoms resemble a cold. In this stage, many of the symptoms are similar to other respiratory infections. A person may have a stuffy or runny nose, a low-grade fever and a mild, occasional cough that gradually becomes more severe. This phase typically lasts one to two weeks.

The second stage is the paroxysmal (sudden and intense) phase. During this stage, people experience repeated bouts of uncontrolled coughing. The classic whooping sound is common in infants, but it is often absent in adults. This may be partly because adults have more control over their cough reflex and partly because a child’s larynx, or voice box, is shaped differently from an adult’s. A child’s larynx sits higher in the neck, is funnel-shaped rather than cylindrical and narrower, and has softer cartilage.

The cough can last so long that pertussis is sometimes called the “100-day cough”. It can also cause exhaustion and vomiting, which is one of the most common symptoms reported in adults. One study found that the average duration of coughing was 54 days. The final stage is convalescence, when coughing episodes become less frequent and less severe.

Because pertussis is caused by bacteria, antibiotics can be helpful, especially in the early stages of infection. They can reduce how long a person remains infectious and may reduce the severity of illness if given early enough. The infectious period usually ends 48 hours after starting appropriate antibiotics, or 21 days after the onset of coughing if treatment is not given.

Vaccination remains important because it reduces the risk of severe disease. However, vaccination does not always prevent infection, and protection can weaken over time. This means vaccinated people can still catch pertussis, although symptoms are often milder.

Pertussis cases tend to rise and fall in three to five-year cycles, with increases reported in England and other countries. Possible factors include pandemic disruption to vaccination, mutations in bordetella pertussis, waning immunity and vaccine differences.

Many countries have moved from whole-cell pertussis vaccines to acellular vaccines. A whole-cell vaccine contains killed bacterial cells, while an acellular vaccine contains selected proteins from the bacterium rather than the whole organism. Many countries have shifted to acellular vaccines because they tend to cause fewer side effects. However, they may also provide a shorter period of effective immune protection.

For many adults, whooping cough is unpleasant but manageable. For others, it can be serious. Secondary complications occur in nearly 30% of infected adults. One of the most common is pneumonia, an infection that inflames the air sacs in the lungs.

Pneumonia can develop because bordetella pertussis can directly damage the airway lining, partly by paralysing the cilia, tiny hair-like structures that clear mucus, dust and germs. Damaged cilia make the lungs less able to clear harmful bacteria, including those that cause pneumonia.

The repeated force of coughing can also damage the body. Severe coughing may cause vomiting, exhaustion, disturbed sleep and urinary incontinence. Nearly one-third of women over the age of 50 report urinary incontinence associated with pertussis infection.

Severe coughing can also lead to rib fractures, especially in older people or those with weaker bones. Repeated strain usually affects the fifth to tenth ribs. In rare cases, lung tissue can tear, causing a pneumothorax: air escapes between the lung and chest wall, causing part or all of the lung to collapse. The larynx and vocal cords can also become damaged or dysfunctional from repeated coughing.

Very rarely, severe coughing has been linked to spinal fractures, damage to the discs between vertebrae, organ herniation between the ribs and under the skin, and arterial tears that may interrupt blood supply and lead to stroke.

Some people are at higher risk of secondary complications, including those with respiratory conditions, obesity or weakened immune systems. People who smoke or have asthma may have a longer cough, a higher risk of sinus infection and more disturbed sleep.

Delayed diagnosis can be a problem because early whooping cough overlaps with many other respiratory infections, including respiratory syncytial virus, or RSV, flu and COVID. RSV is a common virus that usually causes cold-like symptoms, but it can be more serious in babies, older adults and people with underlying health conditions. One study has estimated that the misdiagnosis rate may be as high as 95% in some cases.

Although early symptoms can look similar, there are clues that may help distinguish whooping cough from other infections. Once the cough develops, whooping cough is often worse at night and can be severe enough to cause vomiting. RSV more commonly causes shortness of breath and wheezing rather than prolonged coughing bouts. Whooping cough also usually causes a low-grade fever or no fever, while RSV, particularly in high-risk adults, can cause a high fever. Timing can help too. RSV is most prevalent from late autumn to early spring, peaking in December and January.

Vaccines are available for both conditions. But antibiotics only work for whooping cough because it is bacterial rather than viral, and they are most useful when given early enough.

The main warning sign is a cough that comes in severe bouts, lasts for weeks, gets worse at night or causes vomiting. Anyone with these symptoms should seek medical advice, especially if they live with or care for babies, pregnant women, older adults or people with weakened immune systems.