Critical Pertussis Illness in Children, A Multicenter Prospective Cohort Study

Introduction

Pertussis outbreaks and fatalities persist even in areas of high immunization coverage. Critical pertussis is defined as pertussis disease that results in pediatric intensive care unit (PICU) admission or death. Critical pertussis occurs primarily in early infancy, and effective protection of this age group by direct immunization has been problematic. Alternate strategies including adult immunization and “cocooning” where neonates are discharged into fully immunized households have been utilized in several developed countries (1–3). In the United States and other developed countries, pertussis critical illness continues to occur(4).

Data presently available about the course of critical pertussis illness have come from retrospective studies of smaller cohorts (5–7). While some markers of illness severity have been suggested (8–10); they have not been assessed in larger prospective series that might distinguish children with critical pertussis illness and at elevated risk of fatal outcomes. Characterization of the acute course of critical pertussis in United States PICUs is the principal aim of the prospective cohort study underway in the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) Collaborative Pediatric Critical Care Research Network (CPCCRN). We describe interim findings and an analysis of data from 127 children with confirmed pertussis who either died or spent at least twenty four hours in the PICU especially focused on findings that appear to distinguish survivors from non-survivors.

Methods

The NICHD Critical Pertussis Study is an ongoing prospective cohort study at the eight CPCCRN affiliated PICUs and 17 additional PICUs across the United States (11). The protocol is available at http://cpccrn.org/documents/PertussisProtocolVersion4.0003Aug10_000.pdf. Approximately 33,000 PICU admissions were screened annually for evidence of critical pertussis by passive and enhanced surveillance. All children from 0 to 18 years of age with laboratory confirmed (polymerase chain reaction and/or positive culture) pertussis were eligible for enrollment if they had a PICU stay of at least twenty-four hours, or died (12, 13). Enrollment began in June 2008. All patients with complete data available on August 28, 2011 were included in the analysis.

Data were collected through daily chart abstraction, parental and staff interviews. Clinical, demographic, and laboratory data were collected at admission to the PICU, and daily until discharge from the PICU, 28 days or death. Parental telephone interviews and chart abstraction was used to collect data on patients identified with pertussis after death (n=2). Data points recorded included presenting symptoms, immunization status, chronic illness, and critical care treatments. A white blood cell (WBC) value obtained within 48 hours of admission was used for analysis. When multiple WBC counts were obtained in this time interval, the highest value was used. Sites reporting patients with pulmonary hypertension were queried regarding method of diagnosis. Echocardiograms were obtained at the discretion of the clinicians. In this analysis, patients were considered to have undergone leukoreduction if at least one exchange transfusion or leukopheresis session was reported.

Results of neurodevelopmental status at PICU discharge were recorded. In addition, health and quality of life measures using Infant and Toddler Quality of Life Questionnaire were obtained 6 and 12 months after hospital discharge. Developmental assessment for those patients who were less than twelve months old at enrollment was done twelve months after PICU discharge using the Mullen Scales of Early Learning. Analysis of neurodevelopmental and quality of life data is ongoing and not reported here. All data were transmitted to the NICHD-supported CPCCRN Data Coordinating Center at the University of Utah for data quality monitoring and analysis. The protocol was approved by the IRB at each participating institution and written parental consent was obtained prior to enrollment. CPCCRN and outside site investigators are uniformly trained in study requirements.

Results

During the initial study period, 143 eligible cases were identified. Three families could not be reached to obtain consent and 8 parents/caregivers refused enrollment. Two families withdrew from the study before baseline data were obtained, and three patients were still hospitalized and their data incomplete at the time of analysis. Complete datasets for interim analysis were therefore available for 127 patients.

Discussion

In this interim analysis, we report data from 127 children with a median age of 49 days who had critical pertussis. We observed a higher mortality rate (9%) than the average across the CPCCRN PICUs (4%). We report an important association between mortality, echocardiographic evidence of pulmonary hypertension and leukocytosis. Female patients did not have a higher rate of critical pertussis illness nor pertussis related death as been suggested in other reports (10). While higher mortality rates have been reported for Hispanic children (10, 15), our data suggest that the higher number of deaths is due to higher number of Hispanic cases. Children with WBC greater than 50,000 × 10⁹/L had nearly a ten times higher risk of death. This is consistent with earlier reports in smaller cohorts (8, 9).

Pulmonary hypertension was associated with elevated WBC levels, the need for mechanical ventilation and death. Seventy-five per cent of patients who died had evidence of pulmonary hypertension by echocardiography. Pulmonary hypertension has been implicated in a report of fatal pertussis (15). This association is larger than that reported in other childhood diseases related to toxin-induced sepsis or forms of acute respiratory failure (16, 17). The pathophysiologic descriptions most commonly used to explain development of pulmonary hypertension are 1) acute vasoconstriction from endothelial dysfunction and/or toxin related mechanisms or 2) reduction of pulmonary vascular cross sectional area from obstructive leukosequestration (18, 19). Clinical studies that might discriminate between these mechanisms are not available.

The efficacy of available therapies including nitric oxide and extracorporeal membrane oxygenation to treat pulmonary hypertension in pertussis is uncertain. No single therapy offered a clear therapeutic advantage in this observational study. The association of high WBC with pulmonary hypertension in this and earlier reports (9), is consistent with autopsy reports showing WBC aggregates in lung microvasculature (18, 19). These findings support the hypothesis that leukosequestration may be important in the etiology of pulmonary hypertension and/or death. This suggests a possible therapeutic target. A single center reported that 80% (4 of 5) of patients with critical pertussis, pulmonary hypertension, and leukocytosis treated with leukoreduction therapy survived (20). In contrast, in our study cohort, a survival benefit for those undergoing leukoreduction was not apparent. The small number of children studied to date is not sufficient to recommend leukoreduction therapy as an intervention to reduce WBC count and pulmonary artery pressure.

Descriptions of the nature of Bordetella pertussis strains circulating in the post vaccine era indicate that changes in regulon-directed virulence have occurred (21). Understanding of the significance and prevalence of these changes and how this change might relate to the resurgence in pertussis in developed countries is incomplete, and contemporary research efforts are ongoing. The importance of change in the organism itself and the onset of disease in developmentally anergic infant hosts may emerge from these studies. B pertussis has a large genome and induces complex effects on organ and cell function. The role of pertussis (PT) and Cya-A (adenylate cyclase) toxins is uncertain in the pathophysiology in human infants, although the molecular characteristics of the substances have been described (22). Once thought to represent a fatal pneumonia, it seems likely that multiple organ systems and tissues are affected in children seriously ill with this disease.

Immune cell activation is a complex and tightly regulated event in sepsis syndromes. Resident alveolar macrophages are reported to be the primary innate immune cell effectors in the initial inflammatory response to B. pertussis. Macrophages express a series of pathogen recognition receptors, notably Toll-like receptors (TLR), capable of recognizing highly conserved sequences expressed by pathogens termed pathogen- or microbial-associated molecular patterns (23). B. pertussis has lipopolysaccharide as a constituent of its outer membrane. This lipid moiety binds to TLR4 receptors, triggering a cascade of events including activation of the NF-kB transcription and mutagen-activated protein kinesis pathways (24). Activation of these pathways drives transcriptional expression of pro-inflammatory cytokines (e.g. TNF-α, IL-1β, Interferon-γ, CXCL8/IL-8 and others) (25). This TLR-initiated immune signaling cascade results in activation of T_H1 (including TNF-α and Interferon-γ) and T_H17 (IL-17) responses, both of which have been shown to be necessary for clearance of B. pertussis infection and development of immunity (26–30). The relevance of immune signaling function or specific perturbation in young infants with B. pertussis remains understudied.

Autopsy series of infants dying from critical pertussis illness in the US and UK have reported that B. pertussis organisms persist (18) and that secondary infections are not uncommon (19). If humans respond to B. pertussis infection in a similar fashion as that reported in experimental models, failure to mount an adequate T_H1 and T_H17 response may contribute to course severity (31). In recent experimental reports, the adenylate cyclase toxin of B. pertussis increased IL-10 production, reducing TH1 and TH17 response, and diminishing pathogen clearance (32–37). Although not yet studied in human B. pertussis infection, increased levels of IL-10 have been associated with blunting of the T_H1 response, delayed pathogen clearance, and nosocomial infections in critically ill infants and children with other sepsis syndromes (38). Additionally, exploratory work suggests that pertussis vaccine may increase susceptibility to infection from non-pertussis organisms in children in Guinea Bissau (39). Further research is needed to clarify how altered function of T_H1 and T_H17 immune responsiveness in young infants with critical pertussis might contribute to course severity and/or persistence of the organism.

There are several important limitations of this study. First, patients were not systematically screened by echocardiogram for the presence of pulmonary hypertension nor was there rigorous definition of echocardiographic evidence or severity of pulmonary hypertension established a priori. Thus, the incidence of pulmonary hypertension may be underestimated and the association with mortality overestimated. Likewise, screening, diagnosis, and treatment for co-infections were based on clinician discretion. Also, the use of critical care therapies including antibiotics with activity against B. pertussis, nitric oxide, leukoreduction, and mechanical ventilation was not standardized across sites in this observational study and limits the ability to assess efficacy. Finally, details regarding the extent of organ dysfunction are not included in this data set and should be included in future studies of the pathophysiologic basis underlying the most severe illness and causes of death in critical pertussis.

Conclusion

In this prospective cohort study of critical pertussis, elevated WBC and pulmonary hypertension were both associated with increased mortality risk. Most critical pertussis occurs in children too young to benefit from direct immunization. Studies to further characterize the course and immune biology of critical pertussis illness are needed to identify the risks and benefits of leukoreduction strategies, as well as other potential life-saving therapies. Furthermore, given that the vast majority of the world’s children live in circumstances where advanced pediatric critical care services are unavailable, insight gained from contemporary pediatric critical care in the United States must provide readily assessed hematologic and hemodynamic data that can enhance our understanding of why B. pertussis still kills between 200,000 and 300,000 children annually, the vast majority developing world (40).