Epidemiologic analysis of respiratory viral infections among Singapore military servicemen in 2016

High population density, in addition to physical and mental stress to adapt to a new environment, has been recognized to contribute to increased susceptibility of military recruits to respiratory infection [1]. To better protect the recruits and reduce loss in training time, a surveillance programme was put in place to monitor the etiological agents responsible for febrile respiratory illnesses (FRIs) among Singapore military recruits since 2009. Data from the surveillance programme showed that Influenza, Coxsackie, Rhinovirus and Adenovirus are the primary causative agents of FRIs in military camps [2, 3]. Mass vaccination of military recruits against pandemic H1N1 (A(H1N1)pdm09) in 2009 demonstrated that such a policy can reduce disease burden among recruits [4]. A subsequent study also showed that routine administration of influenza vaccine to recruits resulted in significant protection against both A(H1N1)pdm09 and influenza B, and thus, was recommended for all military personnel since [5].

While the initial focus of the surveillance programme is on etiological agents that cause febrile illnesses, some recruits were excluded from the programme as they only presented respiratory symptoms without elevation in body temperature (termed as Acute Respiratory Illnesses (ARIs)). Since late 2015, the surveillance programme was expanded to monitor the etiological agents responsible for ARIs cases as well, owing to the paucity of information on causative agents for ARI cases. Using molecular diagnostic technology, the current study aimed to evaluate the contribution of 26 different pathogens to FRI and ARI cases. This report highlights findings on both the FRI and ARI cases captured by the surveillance programme in its first year of ARI survey. Additionally, there is a focus on significant relationship between pathogens presented as co-infections.

In a 2006 study, Influenza A and B viruses were reported as important etiological agents for FRI cases among the recruits in Singapore [3]. Thirty-six percent of FRI cases were positive for Influenza A and B viruses. Numerous outbreaks of Influenza virus in military camps have been reported [6, 7], reviewed in [1]) and one study showed that the attack rate of influenza infection for cadet was four times higher than that observed in high rank officers (31.4% v.s. 7.7%) [6]. To mitigate the burden of Influenza illness among recruits in Singapore, routine influenza vaccine has been given to recruits since 2010. Ho et al. reported that the strategy was effective in reducing the risk of A(H1N1)pdm09 virus and Influenza B infection, but not for Influenza A (H3N2) [5]. Data presented in this study are consistent with Ho et al. [5] -- A(H1N1)pdm09 virus was no longer a significant FRI etiological agent in 2016. Apart from a small scale outbreak of Influenza B infections in early 2016, there were only sporadic Influenza B cases reported for the rest of the year. The outbreak coincided with the prevalence of Influenza B in the community during that period. Similarly, during the H3 outbreak which peaked between Week 28-32, H3N2 Influenza A virus was the dominant circulating strain (82.6%-88.7%) in the community [8]. Although recruits had restricted access to the community during the training period, other military personnel, that are not required to reside in camps at night, could carry the circulating strains from the community back to the camps to start the outbreaks. Given the dramatic reduction in FRI cases caused by Influenza A and B viruses, influenza vaccination should be continue to be given to all recruits immediately after enlistment.

With 136 FRI and 39 ARI cases caused by Adenovirus E in 2016, it has become a significant respiratory pathogen in the training camp. Another similar Adenovirus outbreak in Singapore military training camp was reported in 2005. In the 2005 outbreak, 30 recruits were affected, and body ache, headache and nasal congestion were the most frequently reported symptoms. Adenovirus 11a was subsequently identified as the causative agent of that outbreak [9]. Adenovirus type 4, the only member in the species E, was also detected among military personnel with FRI in another study conducted between 2011 and 2012 [10]. Currently, addition experiments are underway to better characterize the 2016 isolates and whether there are any new mutations in the viral genome. While the majority of adenoviral infections cause mild symptoms and are self-limiting, cases of fatality have been reported in other military forces [11‐13]. In 2011, an outbreak of Adenovirus Type 7 was reported in a Malaysian police training centre, affecting 851 trainees, with 100 being hospitalized and 3 deaths [13]. More recently, HDAdV-55 was found to be responsible for an outbreak among Korean military personnel with 1 case of fatality [11]. Adenovirus B was also found to be responsible for febrile respiratory illness in a military training camp in China [14]. Serological surveillance on US military basic trainees showed that 66% and 73% of trainees were susceptible to Adenovirus Type 4 and 7 [15]. Similar trend was observed by Russell et al. [16]. Viral genetic material and virus could be recovered from various environmental samples, such as lockers, rifles, pillows, highlighting the possibility of transmission among the recruits. [16]. For countermeasures, live oral vaccines against Types 4 and 7 have been given to the US army recruits from 1971 to 1999. Following the cessation of the vaccination programme between 1999 and 2011, there was a resurgence of adenovirus outbreaks [17]. The vaccination programme was re-introduced in 2011 and resulted in a dramatic reduction in cases, demonstrating the effectiveness of vaccination in controlling adenovirus respiratory illnesses among this high risk group [18].

Pertusis is another vaccine-preventable disease and pertussis vaccination has been given to all newborns in Singapore since 1957 [19] and sero-prevalence of pertussis antibodies in the adult population of Singapore (18-45 years) was 97% in 2002 [20]. In 2015, there was a total of 56 local cases of pertussis reported in Singapore, a rise from the 19 cases in 2014. As vaccine-induced antibodies begin to wan significantly 4 years after the last dose [21], a booster could be beneficial for optimal protection of the recruits against this highly contagious bacterial disease. Such benefit has recently been reported from a study involving military conscripts from Finland [22].

For 2016, the most common etiological agents reported in FRI mono-infections were Adenovirus E (136 cases, 27.4%), Rhinovirus (120 cases, 24.1%), Influenza B virus (55 cases, 11%) and Influenza A H3 virus (34 cases, 6.8%). The absolute number of FRI cases caused by Influenza viruses has reduced significantly in 2016, likely owing to the mass vaccination of recruits against seasonal influenza (485 cases in 2006 [3] and 1007 cases (2009-2012) [2] and 91 cases in 2016 (current study)). In contrast, FRI cases caused by Adenovirus have been on the rise –(5 cases in 2006, 100 cases between 2009 and 2012) and 136 cases in 2016 alone). On the other hand, Rhinovirus has been an important etiological agent for FRI cases for the past 7 years (581 cases for 2009-2012 and 162 cases for 2016). While Rhinovirus and Adenovirus are common etiological agents responsible for FRI cases reported in studies from other military recruit camps, some agents are unique only to a particular study. For example, Levy et al. reported that Rhinovirus, Coronavirus 229 and Parainfluenza 4 were the main FRI etiological agents for recruits of the Royal Thai Army basic training classes [23]. No cases for Adenovirus or Influenza A virus were reported during the study period. A high isolation rate of Adenovirus was reported in a study in Korea [24]. However, another study performed in Korea, found that Rhinovirus, Enterovirus and Coronavirus OC43 were commonly found in recruits with respiratory symptom [25]. Using paired samples, Eslami-Nejad et al. showed an increase in N. meningitidis carrier rate when recruits in Iran completed their basic training course [26]. Similarly, bacterial infections caused by B. pertussis, Chlamydia pneumoniae, Mycoplasma pneumoniae could have occurred among the military service members deployed to Afghanistan [27]. Clearly, multiple factors like the study population, the seasonality of the study period, the type of samples collected, the geographical location of the study, the level of herd immunity of the locals and the exposure of novel pathogens in a new operational environment -- all could influence the type and variety of etiological agents to be found and reported in these studies.

FRI cases have been the main focus of our previous studies and they have provided valuable insights on the importance of various pathogens responsible for febrile respiratory illnesses in military recruits in Singapore [2, 3]. They also led to the implementation of effective countermeasures for better protection of the recruits. The current study expanded from the previous studies by including respiratory cases without fever, so as to gain a more comprehensive understanding on the etiological agents circulating among the recruits. Data from this expanded study revealed that while Rhinovirus has been an important etiological agent for FRI cases, it plays an even bigger role for ARI cases. Similarly, most of the cases caused by Parainfluenza 4 were presented as ARI cases. Additionally, the most common ARI agents involved in dual infections were Rhinovirus and H. influenzae, while Adenovirus E and H. influenzae were commonly detected together in FRI cases. In view of the new perspectives and insights into respiratory infections among recruits, gleaned from our findings, a surveillance programme with a wider scope should be continued.

There are limitations to the present study. Firstly, some bacterial pathogens, such as H. influenzae and N. meningitidis, are present as commensals in certain individuals. Detection of Rhinoviruses from asymptomatic individuals has been reported [28], therefore, positive detection in the sample may not indicate that they are the etiological agents responsible for the disease. Similarly, for samples with multiple pathogens detected, it is not feasible to determine the respective contribution of each pathogen to the disease. Further investigations are needed to determine the clinical relevance of co-infections and their management. Secondly, our study was limited to pathogen identification, without consideration of the pathogen load in the sample. Thirdly, the study focused mainly on respiratory symptoms and did not track other serious conditions such as meningitis. Fourthly, due to the large number of samples, pathogen isolation from samples could not be done for further verification of the findings. In addition, the sample size of the control samples was small. Fifthly, since the classification of ARI and FRI is based on a single temperature measurement, we could not rule out the possibility that some ARI cases could have become FRI cases subsequently. This limitation could have led to an under-estimation of the number of FRI cases. Lastly, the study predominantly involved young adult males in a semi-closed military setting, hence the findings reported in this study might not be applicable to the adult general population.

The current study describes the etiological agents responsible for FRI and ARI among the military recruits in the tropical urban setting. By studying both FRI and ARI cases in the same study period, it allowed a more complete picture of the respiratory pathogens circulating in the training camp. It also provides us with an unbiased assessment of the overall impact of a particular pathogen in causing respiratory illnesses. This is exemplified with the observation that Rhinovirus, apart from being an important FRI agent, it plays an even bigger role for ARI cases. With the rise in FRI cases by Adenovirus infection, apart from promoting personal hygiene awareness, it may be worthwhile to evaluate the risk-to-benefit ratio for introducing adenovirus vaccination to the recruits.