The findings of this study suggested that there was a spring peak (late April) in TB cases detected among migrants entering Kuwait from high TB burden countries. The estimated amplitude of seasonal variation in the number of TB cases was 20.4% from the annual mean. The observed amplitude of seasonal variation was substantial enough to be explained by monthly variations in the number of TB cases detected among these migrants. In the context of TB diagnosis/case notification rates, few studies have reported variable peaks of seasonality from different parts of the world. For example, in the pre-antibiotic era, TB related mortality was higher in the late winter and early spring than at any other time of the year [
25]. Relatively few recent studies have shown a similar seasonal pattern in TB notification rates [
13,
26,
27]. Also, peak seasonality has been reported during summer in UK [
14], and Hong Kong [
15], during summer and autumn in Spain [
12], Japan [
16], and April to June in India [
17]. Exactly, why TB diagnosis/case notification rates might vary by season is unknown and the specific contributions of a variety of climatic and meteorological seasonal changes are uncertain. However, it has been suggested that winter indoor crowding in poorly ventilated settings could lead to increased
M. tuberculosis transmission, which then manifests itself 3–6 months later [
13]. As noted above, the preclinical period, from exposure to clinical onset, may be of several weeks, which may in part, explain variable seasonal peaks in different geographic regions [
12,
13,
16].
In this study, all of the detected TB cases were adults, and most of the TB cases in adults are considered to be the results of reactivation of latent
M. tuberculosis infection. In the absence of HIV infection or immunosuppressive therapy, such TB cases resulting from re-activation of latent
M. tuberculosis infection are attributed to poor nutrition and low socio-economic status. Although the exact mechanism of this re-activation of
M. tuberculosis infection remains unclear [
28], yet cell mediated immunity in
M. tuberculosis infection seems to play role because of circannual variation in lymphocyte subsets. The seasonal changes in the absolute numbers and ratios of T helper and T suppressor cells could possibly alter cell mediated immunity that controls host response to
M. tuberculosis infection. Nevertheless, the factors that regulate the seasonal changes in T cell subset numbers or function remain unknown [
29]. It has been argued however, that in winter and spring, the viral infections like flu, are more frequent and cause immunological deficiency leading to re-activation of
M. tuberculosis infection [
12]. Furthermore, a probable link between impaired host immunological defence due to vitamin D deficiency and the re-activation of latent
M. tuberculosis infection has been hypothesized [
30‐
32]. The principal source of vitamin D is ultraviolet radiation from sunlight, and that plasma concentrations of vitamin D have a striking seasonal variation with peak levels after the summer and lowest levels in the spring [
31,
33]. Also, a significant trend of increasing TB risk with increased vitamin D deficiency due to low frequency of meat or fish consumption among vegetarian Asians has been reported [
28]. Since most of the migrants entered in Kuwait during the study period came from South Asia. Therefore, period for the peak seasonality in late April in this study corroborated the findings of reported seasonality in April – June in northern India [
17]. Furthermore, the researchers of the aforementioned study have suggested that winter transmission of
M. tuberculosis due to increased indoor activity and/or vitamin D deficiency leading to re-activation of latent infection may have been the bases for the observed TB seasonality in northern India [
17,
34]. Perhaps similar biological phenomena may have been associated with significant seasonality in late April among migrants in our study. Also, it may well be that the migrants from India have a very high proportion of TB and arrives mainly in winter months while migrants from Sri Lanka and other low burden countries have a low proportion of TB and arrive mainly in summer. Such workers' recruitment pattern may have potentially contributed in seasonal variation in pulmonary TB in migrants entering Kuwait. However, we did not have season-specific recruitment data nested within the countries of origin of these migrants to support this contention. Nonetheless, exact biological basis for period of peak seasonality during April in this population needs further investigations.
Limitations of the study
Limitation of this analysis included, firstly, the data deficiency on demographic characteristics of migrants e.g. age and gender. The variation in amplitude of seasonal fluctuation in TB notification rates by age has been reported [
15]. As noted above, we did not have data on exact birth dates of migrants to compute age-specific amplitudes of seasonal variation in our study. However, all the migrants were adults aged 18 or more and relatively small variation in seasonal amplitudes for higher age groups was reported [
15]. Furthermore, no difference in TB seasonal pattern by gender was observed previously [
15,
17]. Secondly, we did not have data on various tests results on migrants for comparative presentation other than the final diagnosis for each migrant as TB positive or TB negative which we believe was done with a sufficient level of accuracy. Thirdly, some level of misclassification of TB cases or non-TB migrants might have occurred as intra-observer and inter-observer variations in radiological assessment of TB cases have been reported [
35,
36]. However, this misclassification presumably was minimal because of serial application of battery of screening and confirmatory tests on our study population. Also, for this task, Kuwait Public Health Authority ensured to employ experienced radiologists who are known to have the highest level of agreement in chest radiograph reading compared to observers of other specialities [
35]. Finally, the monthly grouped data precluded a more sensitive day-by-day assessment of TB seasonality in this population.