Pertussis (whooping cough) is a severe and prolonged coughing disease caused by infection of the respiratory tract with Bordetella pertussis. Bordetella pertussis, the main causative agent of whooping cough, was first isolated by Jules Bordet and Octave Gengou in 1906 [1, 2]. In addition to B. pertussis; B. parapertussis and B. holmesii cause illness similar to whooping cough in humans. Bordetella bronchiseptica primarily an animal pathogen can occasionally cause respiratory illness in humans [2, 3].
Globally, there were an estimated 24.1 million pertussis cases and 160,700 deaths from pertussis in children younger than 5 years in 2014, with the African region contributing the largest proportions of 7.8 million (33%) cases and 92,500 (58%) deaths. Of the above, an estimated 5.1 million (21%) pertussis cases and 85,900 (53%) deaths were among infants younger than 1 year [4]. Even in countries where pertussis vaccination is widely available, there is still a significant burden of pertussis disease, particularly in adolescents and adults [3, 4].
Despite the availability of effective vaccines, resurgence of pertussis disease has been observed in several countries [5]. Several explanations have been suggested for this resurgence including vaccine type used (suboptimal vaccines), decreased vaccination coverage, improved laboratory diagnostic tools, increased awareness of the disease and genotype changes in the circulating Bordetella species population [5, 6]. A further possible factor contributing to the resurgence of pertussis is the growth of a susceptible adult population, due to waning immunity, since the limited duration of protection induced by childhood immunization wanes as children reach adolescence [6].
Infection with B. pertussis in infants and young children is frequently characterized by a significant rise in the number of circulating lymphocytes (lymphocytosis) [7]. However, lymphocytosis is rarely seen in adolescents [8] and adults [9] with pertussis. It is caused by B. pertussis-induced blockage of lymphocytes re-entry into lymph nodes from the blood. The definition of leukocytosis (increased number of leukocytes) varied from 9400 to 13,500 cells/µl for infants, although the most common cut-off is 10, 000 cells/µl [7, 9].
Various diseases, such as leukemia, chemicals, drugs or other infections, can also cause leukocytosis [7, 10], but the more well-defined lymphocytosis is not typically observed in these cases as it is in pertussis. In infants suffering from pertussis, hyperleukocytosis (> 100,000/µl) can occur, and although this is a prognosticator of poor outcome [11], it is still unclear whether it is a contributor to fatality or just an associated marker of severe disease. Studies of pertussis in children show absolute lymphocytosis in > 50% of patients, and characteristic small, mature lymphocytes with hyperchromatic, cleaved nuclei may account for as much as 56% of total lymphocytes [12, 13].
Many researches had indicated that pertussis toxin especially during paroxysmal stage is accompanied by lymphocytosis. The pertussis toxin modifies G proteins of lymphocytes via an Adenosine Diphosphate (ADP)-ribosylation activity. Thus, the G protein modification inhibits/impairs lymphocyte entry from blood into lymphoid tissues and responsible for lymphocytosis [7, 8]. However, the exact mechanisms by which this occurs are still unclear. Thus, marked lymphocytosis is partially helpful to guide the diagnosis of pertussis [7, 14, 15]. In Ethiopia, there are no reports which show the haematological changes in patients with pertussis. Therefore, the aim of this study was to assess lymphocytosis in patients with pertussis in the Amhara Regional State, Ethiopia.