[Correlation between the nucleic acid load of Bordetella pertussis and clinical features and severity of illness in infants and young children with wooping cough].

To study the correlation between the level of Bordetella pertussis nucleic acid and clinical features of the disease in infants and young children and to investigate the risk factors for the development of severe pertussis. Using retrospective research methods, children aged 1 month-3 years who came to Hunan Children's Hospital from August 2023 to February 2024 and were diagnosed with pertussis for analysis. According to the logarithmic value of BP-DNA (log₁₀ copies/ml), 35 cases were divided into the low load group, 78 cases were divided into the medium load group and 94 cases were divided into the high load group; 54 cases were divided into the severe whooping cough group and 153 cases were divided into the general group according to the severity of the disease; the clinical characteristics and laboratory data of the groups were compared, and the risk factors for the occurrence of severe whooping cough were analyzed at the same time. The ROC was used to evaluate the predictive efficacy of BP-DNA and WBC count for the development of severe pertussis. The results showed that in the high-dose group, the WBC count(22.59×10⁹/L), L/N ratio(3.31), and hospitalization days(9.0 d) were significantly higher than those in the medium-dose group and low-dose group (F=6.309, 2.825, 15.149, all P<0.05). The hospitalization rate (100%), combined infection rate (64.96%), incidence of severe whooping cough (31.9%), pyrexia rate (29.8%), and corticosteroid use rate (57.4%) were also significantly higher than the other two groups (χ²=25.977, 9.163, 9.371, 8.299, 20.332, all P<0.05), and the complete immunity rate (9.6%) was significantly lower than the other two groups (χ²=11.632, P<0.05). Compared with the group of common whooping cough, the proportion of children under 1 year old (100%, χ²=9.581), the BP-DNA load (6.56 log₁₀ copies/ml, Z=4.004), the WBC count(31.34×10^9/L, t=7.513), the PCT level(0.07 ng/ml, Z=2.626), the IL-6 level (6.65 ng/ml, Z=4.336), the combined infection rate (88.9%, χ²=36.536), the incidence of wheezing or dyspnea (55.6%, χ²=42.972), the rate of no improvement of symptoms with macrolides prior to the visit (77.8%, χ²=26.266), and the incidence of fever (55.6%, χ²=42.972) were all significantly higher;the complete immunity rate was significantly lower (5.6%, χ²=9.581) in the severe whooping cough group, the differences were all statistically significant(all P<0.05).The result of logistic regression analysis showed severe elevation of BP-DNA, high leukocyte count, co-infection, wheezing or shortness of breath, pyrexia and no improvement of symptoms with macrolides before the treatment were the risk factors for the development of severe pertussis and the logistic regressive model predicts a sensitivity and specificity of 0.83 and 0.90 for severe whooping cough, respectively. The sensitivity of BP-DNA>1.91×10⁶ copies/ml, WBC count >19.97×10⁹/L and the binominal combined test to predict the occurrence of severe pertussis were 0.87, 0.61 and 0.80, and the specificity were 0.43, 0.86 and 0.73, respectively. In conclusion, nucleic acid load in infants with pertussis correlated with clinical characteristics such as the active immunity status, fever, co-infections and hospitalisation and days in hospital. Children with high nucleic acid load, high white blood cell counts, co-infections, fever and no improvement of symptoms with macrolides prior to seeing a doctor were more likely to develop the severe pertussis. When BP-DNA >1.91×10⁶ copies/ml or WBC counts>19.97×10⁹/L, they have the highest predictive efficacy for severe pertussis respectively, and combined detection is better.