Vaccines: Pubs/SurvManual/Pertussis Chapter 10

Chapter 10: Pertussis
Manual for the Surveillance of Vaccine-Preventable Diseases (4th Edition, 2008)

On This Page:

Disease Description
Background
Importance of Rapid Case Identification
Importance of Surveillance
Disease Reduction Goals
Case Definition
Laboratory Testing
Reporting
Vaccination
Enhancing Surveillance
Case Investigation
Outbreak Control
References

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Kristin Brown, MPH; Pam Cassiday, MS; Maria Lucia Tondella, PhD; Amanda Cohn, MD; Kris Bisgard, DVM, MPH

Disease Description
Pertussis, a cough illness commonly known as whooping cough, is caused by the bacterium Bordetella pertussis. The illness is characterized by a prolonged paroxysmal cough often accompanied by an inspiratory whoop. Disease presentation varies with age and history of previous exposure or vaccination. Young infants can present to a clinic or hospital with apnea and no other disease symptoms. Adults and adolescents with some immunity can exhibit only mild symptoms or have the typical prolonged paroxysmal cough. In all persons, cough can continue for months.

Severe disease is infrequent in healthy, vaccinated persons. Infants, particularly those who have not received a primary vaccination series, are at risk for complications and mortality. Pneumonia is the most common complication in all age groups. Seizures and encephalopathy are rare and generally only reported in young infants. Death is rare and most likely to occur in young, unvaccinated infants, although fatalities are occasionally reported among older children and adults with serious underlying health conditions.1 Pertussis can be either the cause (primary) or contributing (secondary) cause of death.

Three other Bordetella species cause disease in humans: B. parapertussis, B. holmesii, and B. bronchiseptica. B. parapertussis causes a pertussis-like illness that is generally milder than pertussis because the bacteria do not produce pertussis toxin. Co-infection of B. pertussis and B. parapertussis is not unusual. Disease attributable to Bordetella species other than B. pertussis is not reportable to CDC.

Background
In the pre-vaccine era, pertussis was a common childhood disease and a major cause of child and infant mortality in the United States. Routine childhood vaccination led to a reduction in disease incidence from an average of 150 reported cases per 100,000 persons between 1922 and 1940 to 0.5 per 100,000 in 1976.2 The incidence of reported pertussis began increasing in the 1980s, and in 2005, the incidence of reported pertussis was 8.6 per 100,000 persons (CDC, unpublished data). Reasons for this increase are not fully understood, but likely contributing factors include increased awareness of the disease and the increased use of diagnostic tests for adolescents and adults.

From 2001 through 2003, persons older than10 years of age accounted for 56% of reported cases,3 more than double the 24% they accounted for from 1990 to 1993.3, 4

Despite this increase in reported pertussis among adolescents and adults, incidence remained highest among young infants.3 In 2005, most (38 of 39) pertussis-related deaths reported to CDC were among infants aged younger than 6 months, who were too young to have received three doses of DTaP vaccine (CDC, unpublished data).

Importance of Rapid Case Identification
Early diagnosis and treatment might limit disease spread. When pertussis is strongly suspected, attempts to identify and provide prophylaxis to close contacts should proceed without waiting for laboratory confirmation. When suspicion of pertussis is low, the investigation can be delayed until there is laboratory confirmation of the diagnosis. However, prophylaxis of infants and their household contacts should not be delayed because pertussis can be severe and life-threatening to young infants.

Importance of Surveillance
Surveillance data collected through case investigation are used to assess burden of disease and guide policy and development of control strategies. CDC uses surveillance data to monitor national trends in disease and identify populations at risk. Local and state health departments use surveillance data to identify clusters of related cases that might indicate an outbreak.

Surveillance data have also been used to guide vaccination policy development. Data collected through an enhanced surveillance program suggested that infants often acquire pertussis from close contacts and supported recommendations for vaccination of postpartum mothers and adult and adolescent contacts of infants.5–7

Laboratory surveillance to monitor changes in the B. pertussis organism is also important. See Section VII, “Laboratory Testing” for more details.

Disease Reduction Goals
A disease reduction goal of 2,000 indigenous pertussis cases per year in children younger than 7 years of age was proposed as a part of the Healthy People 2010 project.8 In 2005, 7,347 cases were reported in this group (CDC, unpublished data).

Case Definition
The following case definition for pertussis was approved by the Council of State and Territorial Epidemiologists (CSTE) in June 1997.9

Clinical case definition
A cough illness lasting at least 2 weeks with one of the following: paroxysms of coughing, inspiratory “whoop,” or posttussive vomiting; and without other apparent cause (as reported by a healthcare professional).

Laboratory criteria for diagnosis
Isolation of B. pertussis from a clinical specimen
Positive polymerase chain reaction (PCR) assay for B. pertussis DNA
Case classification
Probable: Meets the clinical case definition, is not laboratory confirmed, and is not epidemiologically linked to a laboratory-confirmed case.

Confirmed:

A case of acute cough illness of any duration with a positive culture for B. pertussis
A case that meets the clinical case definition and is confirmed by PCR
A case that meets the clinical definition and is epidemiologically linked directly to a case confirmed by either culture or PCR
Comment: The clinical case definition was designed to increase sensitivity for detecting pertussis cases when confirmatory laboratory testing was not done or was negative. Laboratory tests can be negative even when the patient has pertussis. The clinical case definition is appropriate for endemic or sporadic cases. In outbreak settings, including household exposures, a clinical case can be defined as an acute cough illness lasting 2 weeks or longer without other symptoms. A case definition of cough illness lasting 14 days or longer has demonstrated 84% sensitivity and 63% specificity for detecting culture-positive pertussis in outbreak settings.10

Collection of epidemiologic and clinical data is essential for reporting cases that meet the clinical case definition. Investigators should collect information on paroxysms of cough, whoop, and posttussive vomiting; when that is not possible, information on duration of cough should be collected for each suspected case. When feasible, case investigations initiated shortly after cough onset should include follow-up calls to collect information on cough duration. Follow-up should be done regardless of confirmatory test results so that cases meeting the clinical case definition can be reported. Both probable and confirmed pertussis cases should be reported to the National Notifiable Diseases Surveillance System (NNDSS) by the state health department via the National Electronic Telecommunications System for Surveillance (NETSS) or National Electronic Disease Surveillance System (NEDSS).

Laboratory confirmation of pertussis is important because other pathogens can cause symptoms similar to pertussis. Culture of B. pertussis is the most specific diagnostic test; all patients with cough and a positive B. pertussis culture should be reported as confirmed, even those with cough lasting less than 14 days. PCR is less specific than culture; cases confirmed with only a positive PCR must meet the clinical case definition to be reported as confirmed. To confirm a case by epidemiologic linkage, the case must be directly linked (i.e., a first-generation contact) to a laboratory-confirmed case by either culture or PCR.9



Laboratory Testing
Determining who has pertussis and who does not is often difficult. Whenever possible, a nasopharyngeal swab or aspirate should be obtained from all persons with suspected cases. A properly obtained nasopharyngeal swab or aspirate is essential for optimal results. Health department personnel who are asked to obtain these specimens should receive training and supervision from persons experienced in collection of nasopharyngeal specimens.

Culture
Isolation of B. pertussis by bacterial culture is the standard pertussis diagnostic laboratory test. A positive culture for B. pertussis confirms the diagnosis of pertussis. Culture of the organism is also necessary for antimicrobial susceptibility testing and molecular typing.

Although bacterial culture is specific for diagnosis, it is relatively insensitive. Fastidious growth requirements make B. pertussis difficult to isolate. Isolation of the organism using direct plating is most successful during the catarrhal stage (i.e., first 1–2 weeks of cough). Success in isolating the organism declines if the patient has received prior antibiotic therapy effective against B. pertussis, if specimen collection has been delayed beyond the first 2 weeks of illness, and if the patient has been vaccinated.

All persons with suspected cases of pertussis should have a nasopharyngeal aspirate or swab obtained from the posterior nasopharynx for culture. For B. pertussis, nasopharyngeal aspirates will yield similar or higher rates of recovery than nasopharyngeal swabs;11–14 throat and anterior nasal swabs yield unacceptably low rates of recovery.15 Therefore, specimens should be obtained from the posterior nasopharynx (Figure 1), not the throat. Specimens should be obtained using Dacron® or rayon swabs and should be plated directly onto selective culture medium or placed in transport medium. Regan-Lowe agar or freshly prepared Bordet-Gengou medium is generally used for culture; half-strength Regan-Lowe can be used as the transport medium.

Polymerase chain reaction for B. pertussis DNA
PCR testing of nasopharyngeal swabs or aspirates can be a rapid and sensitive method of diagnosing pertussis.16, 17 Since its inclusion in the case definition in 1997, the proportion of cases confirmed by PCR has increased, and many laboratories now use only PCR to confirm pertussis. As of February 2007, there are no standardized PCR assays for pertussis; assay procedures, as well as sensitivity and specificity can vary greatly between laboratories. CDC recommends that PCR be used alongside culture, rather than as an alternative test. Direct comparison with culture is necessary for validation of PCR tests performed in different laboratories. Even when a laboratory has validated its PCR method, culturing for B. pertussis should continue; this is especially important when an outbreak is suspected, because isolation of the bacterium confirms pertussis (see above). State laboratories should retain the capability to culture pertussis.

Collection methods for PCR are similar to those for culture, and often the same sample can be used for both tests. However, calcium alginate swabs cannot be used to collect nasopharyngeal specimens for PCR.

Figure 1: Proper technique for obtaining a nasopharyngeal specimen for isolation of Bordetella pertussis

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Vaccines: Pubs/SurvManual/Pertussis Chapter 10