Infectious diseases

Bordetella

Whooping cough is a worldwide infectious disease caused by the bacteria Bordetella pertussis and Bordetella parapertussis. It is only found in humans and is transmitted from person to person in airborne droplets. The bacteria are highly infectious and unprotected close contacts are prone to become infected. Whooping cough can affect people of any age, though incidence is highest in children under five. Many infants who get pertussis are infected by older siblings, parents or caregivers who might not even know they have the disease.

Whoping cough is vaccine-preventable disease that presents a great mortality rate in some countries. For over 40 years wholecell pertussis vaccines have been very effective, preventing about 760,000 deaths annually worldwide. Nevertheless, whooping cough continues to be a high burden since every year 50 million cases of pertussis disease are reported and 300,000 deaths occur, mostly among infants.

Pertussis is an important cause of infant death worldwide and continues to be a public health concern even in countries with high vaccination coverage.

Clinical features: Following an incubation period of 1-2 weeks with no symptoms, whooping cough begins with the catarrhal phase. In the course of 1-2 weeks, patients develop low-grade fever, rhinorrhea and an irritating cough that gradually becomes paroxysmal. The symptoms are similar to those of a common cold with a dry cough and the patient is highly infectious. The subsequent paroxysmal phase, lasting 2-4 weeks, is characterized by sever and spasmodic cough episodes, cyanosis, apnea and vomiting. At the end of the catarrhal phase a leukocytosis with an absolute and relative lymphocytosis frequently begins, reaching its peak at the height of the paroxysmal stage. At this point, the total blood leukocyte levels may resemble those of leukaemia. Finally, the convalescent phase, lasting 1 to 3 weeks, comes with a continuous decline of the cough before the patient returns to normal. Serious complications, sometimes fatal, are bronchopneumonia and acute encephalopathy, the latter being characterized primarily by convulsions and frequently resulting in death or lifelong brain damage. At this stage the disease is very severe in infants, sometimes life-threatening in those less than two months of age, being pneumonia the most common cause of death.

Parapertussis is a similar and milder disease due to Bordetella parapertussis, which occurs with less frequency. Approximately 40% of cases may be asymptomatic. 

Diagnosis: Specific diagnostic tests include culture, direct immunofluorescence, PCR, and detection of serum antibodies. While culture is almost 100% specific but little sensitive, direct immunofluorescence lacks both sensitivity and specificity. Serology is widely used for the diagnosis of pertussis in older vaccinated children, adolescents and adults. Yet, the immune responses against infection and vaccination cannot be distinguished. Due to its sensitivity, specificity and speed, PCR is accepted as a proof of infection in many countries with notification systems. 

Treatment: Although B. pertussis is susceptible to several antibiotics, such as tetracycline, erythromycin, and chloramphenicol, the efficacy of these drugs in patients during the paroxysmal phase is not convincing. Treatment with erythromycin, which is usually considered the antibiotic of choice, will eliminate viable B. pertussis organisms from the respiratory tract.  Although the treatment has no influence on the course of the disease, unless supplied during the catarrhal phase, helps prevent spreading the disease to close contacts.

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