The bacterium Bordetella pertussis is a small pleomorphic gram-negative staining cocco-bacillus. It is the causative organism of whooping cough, a condition also known by the medical term pertussis.
While classic whooping cough is known as a childhood disease, Bordetella pertussis also causes respiratory illness in adolescents and adults. It is possible for other primates to develop infections with this bacterium but man is the reservoir for infection. The spread of infection is by aerosols produced by the coughing of an infected individual.
Whooping cough is a disease of three stages. The first two stages each lasts from one to two weeks while the third may be longer in duration.
The first or colonization stage is also known as the catarrhal phase. The colonization stage involves the bacteria adhering to the ciliated mucosal cells of the upper respiratory tract and multiplying. Two bacterial proteins found on the outer surface of the bacterium mediate this adherence. These proteins are filamentous hemagglutinin (FHA) and cell-bound pertussis toxin (PTx). The symptoms associated with the colonization stage are similar to many other upper respiratory infections. Symptoms such as a low-grade fever, cough, sneezing, nasal congestion and conjunctival suffusion may all be present.
The second stage is the toxemic stage or paroxysmal phase. Soluble toxins produced by the bacteria cause the symptoms associated with this stage. The main symptom of the toxemic stage is severe or paroxysmal coughing with a whoop when air is inspired through the partially blocked airway. Other symptoms include posttussive vomiting and the face turning bright red with the coughing. The intense coughing commonly causes conjunctival hemorrhages and facial petechiae (pin point hemorrhages).
The final or convalescent stage is characterized by a persistent cough. This cough is not as severe as that seen in the toxemic stage.
Older children and adults often do not exhibit the classic symptoms of whooping cough. Instead, their symptoms include bouts of uninterrupted coughing and severe headaches often combined with a feeling of strangulation. The cough may last for longer than ten weeks. Occasionally the cough may lead to broken ribs.
The complications of whooping cough in young infants can be severe. About one percent of infants under the age of two months with Whooping cough can die as a result of the infection. Complications include pneumonia, which affects approximately thirteen percent of children with whooping cough. In a smaller number of cases, the disease affects the central nervous system causing convulsions and encephalopathy. Pneumonia occurs in adults and adolescents.
The bacterium can be isolated from patients in the colonization stage and the early part of the toxemic stage. The samples taken for successful isolation of Bordetella pertussis are nasopharyngeal swabs and secretions. If a patient has been given antibiotic therapy prior to the samples being taken then the cultures may fail to grow the bacterium. Bordetella pertussis grows very poorly on normal blood agar media producing pinpoint colonies after five days incubation. Better growth occurs when using specialist media such as Regan-Lowe or Bordet-Gengou usually taking three to four days four visible colonies to form. Owing to the slow growth rate of Bordetella, cultures are not considered negative until they have had ten days incubation.
Methods such as polymerase chain reaction and direct antigen detection allow the detection of non-viable Bordetella at the later stages of infection or when the patient has been given antibiotics. These methods also allow for a quicker laboratory confirmed diagnosis.
Treatment of whooping cough in the colonization stage can ameliorate the infection. Antibiotics given in the toxemic stage do not benefit the patient but they may prevent them passing the infection to contacts. Antibiotics should also be given to close contacts of cases of whooping cough to prevent them developing the infection. The antibiotics of choice are the macrolide antibiotics erythromycin, clarithromycin and azithromycin. In children younger than two months these antibiotics have been implicated in the development of infantile hypertrophic pyloric stenosis. As the risk to such young infants of death from whooping cough is great the use of azithromycin is still recommended. Where hypersensitivity to the macrolide antibiotics is known and the patient is older than two months then trimethoprim-sulfamethoxazole can be used as an alternative.
A vaccine against Bordetella pertussis was developed in the 1930’s. Since its introduction, there has been a 99% decrease in the global incidence of whooping cough. The original vaccine was prepared from bacterial cells. This vaccine is now being replaced with an acellular vaccine produced by genetic engineering.
Bordetella is part of the triple vaccine given to infants. This vaccine also protects them against tetanus and diphtheria. Four doses of the triple vaccine are given to ensure protection of infants. These doses are administered at the ages of two, four, six and twelve to eighteen months. Recently the some manufacturers of the vaccine have added another bacterium to its formula. This bacterium is Haemophilus influenzae b, which can cause childhood meningitis.
The immunity to Bordetella pertussis whether obtained by vaccination or infection wanes with time. It starts to drop after about five years and is not detectable after eleven years. For this reason booster vaccinations are recommended at 7 and 18 years of age. The older vaccination is particularly recommended to students attending university. While not fully protected vaccinated individuals do experience a less severe form of the illness than their unvaccinated peers.
There have been concerns expressed over the safety of childhood vaccinations. About twenty percent of children may experience mild side effects with the triple vaccine. One in a thousand can have more severe side effects such as convulsions. About one in one hundred fifty thousand children may develop severe brain damage as a side effect of vaccination.
Recently there has been an increase in the number of whooping cough cases reported worldwide. This increase has also shown a cyclical pattern of infection with infection rates peaking every two to five years. It is possible that the take up of vaccinations has decreased leading to a decrease in the population’s immunity to Bordetella.
