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Dear editor: Pertussis is an infectious disease caused by the Gram-negative bacteria Bordetella pertussis. Communities of the Highlands region of Chiapas have reported outbreaks of pertussis in different periods, but not confirmed. Early identification of B. pertussis infection among children under five years old is important before occurring severe or malignant pertussis, which may lead to either clinical deterioration or death.1 Following Mexican National guidelines to identify suspected cases of pertussis,2 from March 2019 to August 2019, we enrolled 344 children 0-5 years of age admitted to the Hospital de las Culturas located at the Highlands region of Chiapas, Mexico, which serves 22 marginalized municipalities whose main inhabitants are indigenous people. Nasopharyngeal samples from each child was analyzed by culture and Real Time-Polymerase Chain Reaction (RT-PCR) to determine B. pertussis infection. We used a cut-off for white blood cells (WBC) of ≥ 21.60 x 103/µl and lymphocyte count of ≥11.5 x 103/µl as laboratory predictors of pertussis positivity.3
The proportion of positive cases by culture, within the time frame of both the onset of symptoms in the diagnosis by culture (up to four weeks) and RT-PCR (up to six weeks) was analyzed, together with laboratory predictors, using χ2tests. All statistical analyses were carried out using SPSS v. 25.
A total of 38 out of 344 examined children (11%) satisfied the pertussis clinical case definition.2 From the 38 children, 37 were tested by culture giving two positive cases (5.40%, 95%CI= 2.24-13.05) and confirmed by Matrix-Assisted Laser Desorption/Ionization Time-Of-Flight (MALDI-TOF). For RT-PCR, 31 samples were examined resulting in five positive cases (16.12%, 95%CI= 2.42-29.84), including the two positives cases detected by culture. Using both clinical and laboratory predictors, we identified 11 cases satisfying these criteria (36.66%, 95%CI= 18.40-55.0), including the five positives to RT-PCR (table I).3
Table I : Results of diagnostic tests in patients with suspected pertussis infection. Highlands region of Chiapas, Mexico, March 2019 to August 2019
|
Culture (N=37) |
RT-PCR (N= 31) |
Clinical and laboratory predictors* (N= 30) | |
|
Outcome |
N (%), 95%CI |
N (%), 95%CI |
N (%), 95%CI |
|
Positive |
2 (5.40), 2.24-13.05 |
5 (16.12), 2.42- 29.84 |
11 (36.66), 18.36-54.97 |
|
Negative |
35 (94.59), 86.95-102.2 |
26 (83.87), 71.07-97.68 |
19 (63.33), 45.03-81.64 |
Bivariate analysis was performed between the predictor and the clinical variables of interest, using χ2, for categorical ones, and Mann-Whitney U test for continuous variables, at a 95% confidence level. A p≤0.05 was considered as statistically significant. All statistical analysis was carried out in SPSS v. 25.
* Cut-off for white blood cells (WBC) of ≥ 21.60 x 103/µl and lymphocyte count of ≥11.5 x 103/µl.3
Leukocytes and lymphocytes cut-off counts allowed us to identify two patient groups: the first group had higher median leukocyte (42) and lymphocyte (21.9), as well as a median cough duration of 14 days. In the second group, these values were (13.2) for leukocytes and (5.9) for lymphocytes and seven days of cough, without statistical significance (p=0.525) in these indicators. However, the hospitalization days among the first group (eight days) was longer compared to the second group (4.5 days) (p=0.049).
Considering specific white blood cells count thresholds within the guidelines will be crucial for the development of standardized clinical and laboratory selection criteria for identifying pertussis in highly marginalized settings.1
