Vaccination against whooping cough is now so widespread that doctors may fail to diagnose the disease. But, in a small baby, it can still be highly dangerous, as Emma Haughton found out.
When our seven-week-old son, Zachary, first got the runny nose and persistent cough that was going round the family, we weren't too concerned. Minor coughs and colds seem to be a feature of life with three small children, especially once the eldest starts school.

After a few days, however, Zachary's cough became more alarming. One moment he appeared entirely well; the next, he was seized by a fit of violent choking, going blue from lack of oxygen and bringing up mouthfuls of sticky white phlegm. These paroxysms became more severe, occurring every half hour or so and lasting up to a minute, and he'd often be sick afterwards. But the GP seemed unconcerned, prescribing antibiotics for what she loosely referred to as a "chest infection", and assuring us that it was fine to go off on holiday as planned.

We spent a week in Suffolk watching Zachary deteriorate. He became quiet and lethargic, his coughing fits longer and more violent. The antibiotics did nothing except add five days of diarrhoea to his misery, and he quickly became dehydrated in the hot July weather. The journey home was peppered with emergency stops when Zachary went lifeless and floppy after a coughing attack. On several occasions we held him upside down and thumped him on the back, thinking he was choking on mucus.

Despite our graphic accounts of his symptoms, two further doctors failed to find anything wrong; one laughed openly when we said we were scared our son might die during one of his coughing fits. Slowly, Zachary did recover, but it took several months, and he dropped to the bottom of his weight range and has remained there ever since: he is now two-and-a-half.

Just as the trauma of the whole experience began to recede, I came across a description of whooping cough in an old child care manual. The penny dropped. Zachary's symptoms were classic, even down to the absence of a whoop, rare in babies. For children under the age of three months, whooping cough can be severe; it might have killed him.

So why did the doctors - who eventually agreed with my diagnosis - get it so badly wrong? Probably because the disease is now comparatively rare, so they aren't familiar with its symptoms; and because mass immunisation has lulled them into a false sense of security.

Whooping cough, or pertussis, is usually fairly mild in adults and older children, especially those who have been vaccinated. It is caused by a bacterium, spread via airborne droplets. After several weeks' incubation, the illness starts with a mild cough, sneezing, runny nose, and fever. The cough becomes more persistent and severe, especially at night, and can provoke vomiting; in small babies it may temporarily cause them to stop breathing. Recurrent vomiting can cause dehydration.

Little can be offered in way of treatment, beyond antibiotics early on, to reduce infectivity and shorten the duration of the illness.

However, more than half of reported cases strike those under three months old, where the disease can be life-threatening. Possible complications include pneumonia, brain damage and collapsed lungs, leading to death in one in 200, and occasionally permanent damage in survivors.

Whooping cough used to be a common disease - up to three-quarters of the population succumbed to it at some point - but immunisation has brought the incidence down from around 100,000 cases a year to a reported 2,000 in 1972. Despite scares in the Seventies and Eighties associating the vaccine with brain damage in children, whooping cough immunisation recently hit record levels of 93 per cent of the population.

But Douglas Jenkinson, a GP from Nottinghamshire, believes that the pertussis vaccine currently given to babies at two, three and four months old is not as effective as most people assume. His 10-year study of 500 cases of whooping cough in his general practice of 10,000, published in the British Medical Journal, led him to conclude that vaccination is usually only fully effective for the first year, and that by the time children reach school, it provides only around 50 per cent protection. For small babies, the most common route of infection is via older siblings who have picked up pertussis at school.Joshua, Zachary's older brother, had ghastly coughing fits for a month. In retrospect, it seems obvious that he was the first to contract the disease.

Jenkinson also believes that whooping cough is far more prevalent than the reported number of cases would suggest, and his suspicions are backed up by research across the Atlantic. A study published in the Journal of the American Medical Association revealed that nearly a quarter of adult US patients complaining of a persistent cough were later found to be suffering from whooping cough. US health officials estimate that only 10 per cent of pertussis cases are ever reported, partly because doctors fail to diagnose it.

Much the same situation seems to be occurring here. The idea that vaccination provides universal protection tends to lull most doctors into a false sense of security, believes Jenkinson, who estimates that he now diagnoses 100 times more cases of whooping cough than the average GP. Faced with a patient complaining of a bad cough, most doctors commonly attribute the symptoms to other chest problems, such as bronchitis, bronchiolitis, and even asthma.

But even when pertussis is suspected, it can be difficult to reach a definitive diagnosis. "Whooping cough is difficult to diagnose, and most GPs have little direct experience of it," says Jenkinson. "Although the `tune' of a typical spasm of whooping cough is unforgettable, the doctor is unlikely to hear a typical spasm in any given patient."

Even when doctors do make the connection, proving it can be difficult. According to Dr Ruth Matthews, head of the Pertussis Reference Laboratory in Manchester, which attempts to diagnose whooping cough from samples, only 60 per cent of cases are successfully cultured. "There are cases of whooping cough which are almost certainly genuine but which culture negative," she says, "but a clinical diagnosis from the symptoms alone is never definite."

The result is that many mini-epidemics pass more or less unnoticed. In our village, we discovered at least two other families whose children had suffered similar symptoms at around the same time that Zachary had been ill, and, like us, had not even had the possibility of pertussis considered. A mother from a nearby town whose school-age twins were probably the source of the infection had been told by her GP that it was "pseudo whooping cough" and not as serious; without a solid diagnosis, the school was unable to warn other parents of a possible outbreak.

The good news is that most children who get whooping cough, even those suffering a severe case, are unlikely to experience any long-term ill effects. Zachary, thankfully, seems none the worse for his early trauma; as his parents, however, we're left with a more lingering legacy - anger at his misdiagnosis, and considerable guilt that we didn't trust our instincts and push harder for help when he was clearly so very ill.