In the present study, elevated serum IgG titers were observed in 4.6% of the patients with coughing for more than two weeks. However, several studies carried out in developed countries have so far indicated that pertussis is among one of the common most causes of frequent and violent cough (with an incidence rate of 2.9-32% in adults), but physicians are unfortunately often unaware of this disease [18,21,22].
In this respect, Ghorbani et al. (2016) investigated the epidemiological characteristics of pertussis among Iranian suspected patients and further reported that 6.6% of the cases had shown positive test results similar to results obtained in the present study [23].
The results of another research by Ghatasloo and Mohammadzadeh-Asl (2017) had further demonstrated that the overall positivity of Bordetella pertussis in serum samples from Asians, from 2000 to 2015, was by 36%, but the given value in Iran was 38.4%. This incidence percentage was higher than that reported in the present study [24]. Similarly, in another study by Rabiee et al. (2015), pertussis occurrence had been reported to be 20.44% among patients who referred to Tehran Diagnostic Medical Transplant Laboratory [1].
Similar studies had been correspondingly conducted in other countries on a global scale, including that by Pimentel et al. (2015). Consistent with our study, they investigated 192 patients suspected of pertussis using the culture method and PCR in Brazil and estimated the incidence rate of 21%. In contrast to our findings, the investigation of Lee et al. (2018), examining clinical nasopharyngeal and blood samples of 686 patients with cough, had further shown that the incidence of Bordetella pertussis attained 13.6% in seven distinct places in the United States between 2007 and 2011 [10], which was higher than that of the present study. However, these results had been in agreement with the findings of the studies by Nirada Siriyakorn et al. (2016) in Thailand (18.4%) [15], Sonmez et al. (2016) in Turkey (9.7%) [14], as well as Kayina et al. (2015) in Uganda (20%) [25].
Additionally, in the study by Yeng et al. (2017), it had been estimated that 3.5% of children under 5 years of age residing in Southeast Asia in 2014 had been totally affected with pertussis, which led to mortality in approximately 0.67% of the cases [26]. This finding was lower than that reported in the present study. Also, in another 5-year investigation by Masseria et al. (2017) in the United States, the pertussis incidence rate in children under 12 months of age attained 117.7 per 100,000 people/years wherein the highest incidence rate had been observed in 3-month-old infants (247.7 per 100,000 people/years) [27]. It seems that the discrepancy in incidence rates reported in different studies might be associated with epidemiology of the disease in each region, variety of samples, time of the study (different seasons), and age groups.
Another analysis performed in this study was the relationship between age and pertussis incidence rate, but it did not show any statistical significance. Regarding the prevalence of different age groups, most of the cases were observed in the second age group (21-30 years old, 15.8%) followed by age group 3 (31-40 years old, 6.1%). Studies conducted in different countries had further reported different incidence rates according to age groups. With reference to a study in Turkey, the pertussis incidence rate in individuals aged 13-19 years was the highest [28], however in the survey by Skoff et al. (2018), the highest incidence rate was reported among infants (75.3 cases per 100,000 people) [29]. On the other hand, some studies had addressed the peak incidence rates in the age group over 60 years [30,31].
It is of note that adolescents and young adults with lower levels of protective antibodies against PT may experience pertussis recurrence. There are several reasons for coughing in a vaccinated population including inadequate vaccination, changes in vaccine quality, types of vaccines, immunosuppression following initial vaccination, demographic changes, and adaptation of bacteria and their acquired vaccine-driven resistance, misdiagnosis, as well as lack of reports due to absence of specific symptoms of pertussis and subsequently its transmission to vulnerable individuals. Despite milder manifestations of the disease in older children and adults, some scientists argue that pertussis can be multifaceted in adults [30-32]. Thus, the possible reasons for the difference in the peak incidence rate of pertussis in various age groups can be attributed to variations in sample size, immune response dynamics, and community transmission.
The gender difference was also among other factors investigated in this study. In this regard, the pertussis incidence rate was higher in males than that in females, although this difference was not noticeable. Consistent with our results, a cross-sectional investigation conducted at Shahid Beheshti Hospital in the city of Zanjan, Iran, had also found that the number of patients with a positive culture for pertussis was higher in men compared to women (26.3% vs. 12.1%) [33]. South Korean scientists also conducted a study to assess epidemiological characteristics of pertussis in adults and reported that test results (viz. culture method and PCR) in 6.9% of the patients were positive and the ratio of women to men obtained 356 to 134 [34]. However, these findings were in conflict with those in the present study, but they supported the results reported by Ghorbani et al. (2016) [23]. One possible reason for such a discrepancy was the recruitment of different age groups of populations in these studies.
Although pertussis incidence rate in individuals living in urban areas was higher than that of subjects residing in rural districts, no significant difference was noticed. These slight variations might be attributed to patients’ access to health care centers, earlier diagnosis, level of vaccination coverage, population density, and more transmission routes of the disease. The study by Ghorbani et al. (2016), determining the epidemiological pattern of pertussis in Iran between 2011 and 2013, similarly reported that there is no significant difference between rural and urban populations [23], which was in accordance with our findings.
In total, no emphasis had been laid on gender and residency area in the literature. It seems that most differences associated with these two variables might be related to extent of immunization.
Occupation and family size were another two variables in this study. Pertussis occurrence was higher among employees and military men compared with other occupations in the present study, and it was demonstrated to be more common in families of 1 to 3; however, no statistically significant difference was found between these variables and prevalence of the disease. This finding was also reproduced in the studies of Tarlo et al. (2016) and Lytras et al. (2015). The given studies had suggested that factors such as excessive exposure to dust, gases, or smokes, and cigarette smoke could be accompanied by an increased occurrence of cough or sputum secretion, although chronic cough could be often based on non-occupational types [35, 36]. While many surveys addressed no association between family size and pertussis incidence rate, exposure to family members suffering from a cough or sharing beds with them play a remarkable role in the development of pertussis manifestations [25].
Among the most noticeable limitations in the current study was the fact that no PCR was performed on the patients suspected with pertussis, and thus could lead to false-positive in serologically confirmed cases in laboratories. The second limitation of the study was the difficult recruitment of the patients for sample collection at day 21 (day 0 and day 21), which was resolved through frequent follow-up and exclusion of the reluctant patients for 2nd sampling.