Background
Human adenoviruses (HAdVs) are double stranded non-enveloped DNA viruses which cause a wide range of illnesses including acute respiratory infections (ARIs), gastroenteritis, conjunctivitis, cystitis, and meningoencephalitis. HAdV infection accounts for 5–7% of respiratory illnesses in children and infants [
1,
2]. HAdV-associated acute lower respiratory tract infections (ALRTIs) may be severe and lead to long term respiratory sequelae, such as bronchiolitis obliterans or bronchiectasis [
3]. HAdVs were an important cause of acute respiratory tract infections in Taiwan [
4]. Studies have shown that the most susceptible populations were children < 5 years old, close-quartered populations such as schools and military training camps, and immunocompromised individuals [
5]. Currently, there are more than 60 serotypes of HAdVs, classified into 7 species (A-G). Molecular typing of HAdVs is based on PCR-based direct sequencing of the HAdV hexon gene which contains 7 hypervariable regions [
6]. Different serotypes were associated with different clinical manifestations and various degrees of disease severity [
7,
8]. Serotypes 3, 4, 7 and 21 have been reported to be associated with severe disease, and HAdV7 was related to post-infectious pulmonary sequelae, such as bronchiolitis obliterans [
9‐
11]. Molecular genotyping of HAdV was used to analyze the dynamic changes in epidemiology and the association with disease manifestations [
7,
8].
There was a significant increase in the incidence of HAdV infection in southern Taiwan between 1999 and 2002, with three outbreaks of adenovirus respiratory infection between November 1999 and December 2001. HAdV3 and HAdV7 were the major serotypes in the first outbreak, while HAdV4 was the major serotype in the second and third outbreak [
7,
12,
13]. Species B, especially HAdV3 has been reported to be the predominant respiratory HAdV in Taiwan over the past decade, and has been identified in half of all children hospitalized with respiratory infections [
14]. Other outbreaks recorded by the nationwide surveillance system included an outbreak in 2004–2005 which was predominantly caused by HAdV3 [
7,
15,
16], and an outbreak which occurred between March and October 2011 by HAdV3 and HAdV7 [
17,
18].
The most recent HAdV outbreak in Taiwan was in 2014. However, currently available molecular epidemiology data from this outbreak have not been completely analyzed. In this study, we conducted a molecular characterization of HAdV from the 2014 epidemic, and compared the demographics, clinical characteristics, and risk factors of hospitalization and pneumonia in patients between the 2011 and 2014 epidemics.
Discussion
In this study, we compared the demographics, clinical characteristics, and risk factors of hospitalization and pneumonia in patients from the 2011 and 2014 adenovirus epidemics in southern Taiwan. Genotype 2, 3 and 7 accounted for the majority of infections in the 2011 epidemic. Although genotype 3 and 7 sustained to contribute to the 2014 epidemic, these genotypes comprised < 50% of cases in the 2014 epidemic. The surge of other genotypes, including 1, 4 and 6 constituted the other half of genotypes found in the 2014 epidemic. In addition to genotype, there were also differences in the clinical characteristics, laboratory values, and hospitalization between the two epidemics. Our analysis showed that the 2011 epidemic was mainly caused by HAdV3 and HAdV7, with a higher prevalence of HAdV3 compared to HAdV7 (64.7% vs. 16.7%). This finding was consistent with other epidemiological reports, showing co-circulation of HAdV3 and HAdV7 in the 2011 epidemic [
17‐
19,
21]. HAdV3 has been the most prevalent genotype in Taiwan since 1980, and also contributed to 2004–2005 epidemic [
7,
13,
15]. HAdV3 was also the most prevalent genotype in nearby countries. Lee et al. had reported a pandemic outbreak of acute respiratory infections caused by HAdV3 in 2010 in Korea, which preceded our 2011 epidemic [
22]. In China, Gao and colleagues also reported an epidemiology study of HAdV outbreaks among children in Aug 2010 and Jul 2011, in which HAdV3 (70.1%) was the leading genotype, followed by HdAV7 [
23]. Previous reports showed that HAdV-7 was rarely detected in Taiwan until 1999–2000. However, by the 1999 outbreak, HAdV-7 had emerged as the predominant genotype (45%), even over the HAdV-3 (36%) [
13]. Since then, HAdV-7 has been consistently detectable through epidemiological surveys, but has accounted for a much smaller proportion (ranging from 0.7 to 1.2%) [
7,
15]. The re-emergence of HAdV-7 during 2011 epidemic in Taiwan paralleled the re-emergence of HAdV-7 in southern China, suggesting possible viral spreading through a nearby country [
23,
24]. Although HAV55 and HAdV11 were also reported to be other important emergent genotypes in immunocompetent adults with severe pneumonia in China, we did not identify any HAdV55 or HAdV11 in this study [
25,
26]. Continuous epidemiological surveillance is still needed to monitor dynamic changes of HAdV serotypes.
The clinical presentations varied among different HAdV serotypes. Lin et al. reported that HAdV3 more likely to cause upper respiratory tract infections, while HAdV7 tends to cause lower respiratory tract infections and a more severe clinical course, including longer duration of fever and hospital stay, and need for intensive care [
27]. Many other reports also demonstrated that infection with HAdV7 was more likely to be associated pneumonia [
11,
17]. On the contrary, we found that the 2014 epidemic had a higher rate of HAdV7 compared to the 2011 epidemic (21.0% vs. 16.7%), but had a lower rate of pneumonia (1.6% vs. 3.3%). Also, when we pooled these two epidemics into analysis, we found that infection with genotype 7 was not statistically associated with pneumonia. HAdV4 caused an epidemic outbreak of acute respiratory disease (ARD) in military recruits in United States and may also result in severe pneumonia [
28,
29]. HAdV1 was also reported to cause severe pneumonia in immunocompetent patients in French intensive care unit [
30]. Our study showed that although HAdV1 and HAdV4 were two of the emerging genotypes in the 2014 epidemic, they were not associated with severe lower respiratory tract infection (four pneumonia cases caused by HAdV1; one pneumonia case caused by HAdV4). It is therefore possible that there may have some genomic change within the same HAdV genotype, resulting in different invasiveness and virulence. Sub-serotype or phylogenetic analysis is needed for further study of serotype-specific manifestation.
As well as viral genomic characteristics, host susceptibility is a major risk factor for severe HAdV infection. The neurological disease, chronic lung disease, and airway anomalies were reported to be related to HAdV pneumonia [
19,
21]. Neurological disease was particularly associated with severe pneumonia caused by HAdV3 and 7 during the 2011 epidemic [
17]. Risk factors of hospitalization, including neurologic, respiratory, and metabolic abnormalities, were revealed to be associated with more severe HAdV disease requiring hospitalization [
31]. Although we did not sub-analyze the underlying disease by disease categories, underlying disease for HAdV associated hospitalization in both epidemics was demonstrated.
Our data showed that in both epidemics, hospitalized patients had higher WBC counts and CRP levels compared with non-hospitalized patients. Leukocytosis and elevated CRP levels are common in HAdV infection, even without superimposed bacterial infection [
8]. However, leukocytosis of HAdV infection could be genotype-specific, and also related to the stage of clinical infection. Lin et al. had reported that leukocytosis (WBC > 15,000/mm3) was more common in HAdV2 infection, whereas leukopenia (WBC < 5000/mm3) was more common in HAdV7 infection [
27]. Leukocytosis seems to occur in the early course of infection, whereas leukopenia and thrombocytopenia were shown to correlate with progression [
17,
32]. This could also explain some reports showing leukocytosis as the major laboratory finding in HAdV infection, while other studies reported leukopenia [
17,
19,
21]. In our study, we demonstrated that elevation of CRP values was an independent predictor of hospitalization in both epidemics. Previous data suggested that serum CRP levels were a better predictor of bacterial infection in febrile children compared to WBC, ANC or ABC counts [
33]. Elevated CRP levels in children with HAdV infection in the absence of secondary bacterial infection indicated that HAdV triggered an inflammatory host response similar to that of a bacterial infection [
34]. However, the elevation of CRP could be serotype-specific, since children infected with HAdV3 had higher CRP values compared to children infected with other genotypes [
15].
HAdV pneumonia is the most common manifestation of severe HAdV infection in immunocompetent patients, but accounts for less 5% of all HAdV infection [
35]. In our analysis, pneumonia patients had higher WBC counts and CRP levels than non-pneumonia patients. This was consistent with a previous study, which showed that leukocytosis was seen in one-fourth of the patients and a moderately elevated CRP > 40 mg/L was seen in nearly two-thirds of the patients [
35].
As discussed by Chen et al. study [
36], seven species of adenovirus (A–G), including more than 79 genotypes, have been defined using a new characteristic based on genomics [
37]. Specific genotypes are often associated with particular clinical manifestations [
38,
39]. HAdV species B contains several types related to acute respiratory diseases (ARD), can be further classified into two subtypes B1 and B2 based on their tissue tropism. HAdV types 3 and 7 of species B are most commonly detected in patients with respiratory infections [
40,
41]. Since 2006, there are the outbreaks and severe respiratory disease worldwide, which were caused by HAdV14p1 and HAdV55 of species B [
42,
43]. These two strains have the potential to extensively disseminate and cause severe epidemics due to the lack of herd immunity and their ability to cause more severe ARD than other adenoviruses [
44]. In the present study, we showed that the 2011 and 2014 HAdV epidemics in Taiwan were caused by different HAdV genotypes. In addition, hospitalized patients had higher WBC and CRP levels than non-hospitalized patients. There is a need for a licensed HAdV vaccine for the general population, as the only approved live oral vaccine (comprising HAdV types 4 and 7) has only been used in the US military [
45,
46].
There are some limitations in this study. First, molecular typing was done in only 24 and 30% of all HAdV isolates, in 2011 and 2014 respectively through random sampling. But, the number of viral isolates chosen for further genotype in each epidemic is relative large compared with all other studies conducted in Taiwan during the same period of time. Also, our genotype distribution result is compatible with other epidemiological studies in Taiwan in 2011 [
19]. Second, in our study, we only enrolled cases with HAdV isolated from clinical culture. Although viral culture had been replaced by nucleic acid detection using multiplex PCR in many developed countries, it is more expensive and not yet covered by Taiwan’s health insurance system.
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