What Is Pertussis?

Pertussis, also known as whooping cough, is a highly contagious infection of the respiratory tract caused by the bacterium Bordetella pertussis. It is transmitted only by humans through close respiratory contact with someone who is infected, according to the Centers for Disease Control and Prevention (CDC).

Pertussis affects about 50 million people annually and causes an estimated 350,000 deaths worldwide each year.

The number of pertussis cases in the United States peaked in the 1920s to 1930s, before a vaccine became available. At that time, the illness affected more than 250,000 people each year and caused up to 9,000 deaths annually. The prevalence of pertussis declined through 1976, with only 1,010 cases reported that year. Pertussis cases usually occur cyclically, with peaks occurring at three- to four-year intervals, the CDC says.

Since 1977, pertussis cases have been on the rise especially among pre-teens and adolescents ages 10 to 19 years and in infants younger than 5 months old. In 2002, for instance, the CDC reported 8,296 cases of pertussis, compared with 4,600 cases in 1994.

Adults are a major source of transmission of pertussis to infants and children. Pertussis in adults is often misdiagnosed as bronchitis, because symptoms in adults are milder than in children. Many adults were vaccinated decades ago and, although their immunity has decreased somewhat, they still have enough immunity to reduce the severity of the illness.

It usually takes a week to 10 days after a child has been exposed to the bacteria for symptoms to occur, says the CDC. The symptoms at first are similar to a cold: sneezing, runny nose, fever and a mild cough. After one to two weeks, however, the coughing becomes worse. The violent spells of coughing may be followed by difficulty in breathing. Symptoms progress to severe coughing spells that can go on for a minute, followed by long, deep breath or "whoop." This stage of the disease last one to six weeks, but can continue past that, according to the CDC.

Common complications of pertussis include otitis media   (ear infections), pneumonia, periods of not breathing (apnea), acute encephalopathy and difficulties getting enough nutrients because of vomiting. Some of these complications can be fatal. Pertussis causes 10 to 15 deaths a year in the United States, according to the CDC. Infants under the age of 12 months have more serious illness from pertussis and are more likely to have complications and be hospitalized than older people.

Pertussis can be prevented through the vaccine DTaP, a combination vaccine that also protects against diphtheria and tetanus. The DTaP is given in five doses, at 2, 4, 6, 15 to 18 months and 4 to 6 years. Because of potential side effects of the original whole-cell version of the vaccine (DPT), an acellular form has been given since the early 1990s.

Immunity to pertussis begins to wear off as a child enters puberty. In May 2005, the U.S. Food and Drug Administration approved a booster vaccine called Boostrix for pre-teens and adolescents ages 10 to 18 years old. A second vaccine, Adacel, was approved in June 2005 for teens and adults, ages 11 to 64. These vaccines contain a reduced amount of diphtheria toxoid to avoid side effects. The vaccine, often designated as dTpa, will prevent immunity from waning in adolescents and adults.