We reported relatively high crude incidence rates for liver abscesses in our setting: 35.9/100,000 hospital admissions/year and 5.9/100,000 inhabitants/year. A 10-year study from the United Kingdom (UK) reported overall crude annual incidence rates of liver abscesses of 18.2/100,000 hospital admissions and 2.3/100,000 inhabitants [10]. Although there are published case series on liver abscesses from Saudi Arabia, Qatar, and Iraq, none of these studies reported crude annual incidence rates to set regional benchmarks or allow direct comparison with our data [11,12,13]. Furthermore, we reported an annual incidence rate of 4.9/100,000 inhabitants for PLA. However, it is important to note the significant global variation in PLA annual incidence rate ranging from 17.6/100,000 inhabitants in Taiwan and 3.6/100,000 inhabitants in the United States [2].
The finding that most of our patients with liver abscesses were relatively young at presentation (median age 39 years for ALA and 46 years for PLA) concurs with previous reports from our region [11,12,13]. However, it contrasts with reports from the Western hemisphere in which older age at presentation was documented [2, 10, 14,15,16]. Male preponderance was evident in our series (87.5% for ALA, 75.7% for PLA) and is consistent with similar studies from our region [11,12,13].
A study from Saudi Arabia and the UK found that all patients with ALA were either from or had recently travelled to the Indian subcontinent [11]. Likewise, a study on Israeli travellers found that the vast majority of them had travelled to the Indian subcontinent before the onset of symptoms [17]. Recent data from the United Nations Department of Economic and Social Affairs indicate that migrants comprise a staggering 87.9% of UAE’s total population [18]. Leading countries of origin of these migrants, who are mostly males, are India (39.8%), Bangladesh (12.6%), and Pakistan (11.4%). These facts, together with our observation that ALAs were exclusively found in immigrants from the Indian subcontinent, strongly suggest that ALAs were most likely imported into UAE. Of note, a large study on migrant workers in the Emirate of Sharjah in UAE showed that those from the Indian subcontinent had a significantly higher burden of protozoal and helminths infections [19]. Furthermore, a laboratory-based study by the same research group reported a detection rate of 13.3% of E. histolytica in stool samples using molecular methods; however, the authors provided no information as to the ethnic origins of their study participants [20].
Indirect haemagglutination is a reliable method for the serodiagnosis of ALA. A large study from Kuwait evaluated its diagnostic utility for ALA against the ImmunoTab and enzyme-linked immunosorbent assays and found it to have 99% sensitivity and > 95% specificity [21]. An earlier study from Saudi Arabia reported similar validity estimates in patients with ALA [22].
Nearly two-thirds of our PLA cases had a culture-confirmed monomicrobial infection. This is consistent with the UK findings in which 47% of PLAs were due to a single organism but contrasts with data from Spain in which polymicrobial infections predominated [14, 15]. These differences are most likely a reflection of the limitations of traditional culture methods as it has been shown that molecular techniques such as 16 s ribosomal ribonucleic acid testing can increase the diagnostic yield of culture-negative pus samples by 13% [14]. Furthermore, pyrosequencing has been shown to identify an aetiologic pathogen in all patients with abscesses involving the liver, brain, or pleura and that it was significantly more likely than culture to reveal the polymicrobial nature of these infections [23].
With a high prevalence of 43.2%, this study highlights the importance of K. pneumoniae as a major pathogen for PLA in our setting. A study from Qatar conducted during 2009–2010 reported a 37.5% prevalence for K. pneumoniae among patients with PLA [12]. Another study combining data from Saudi Arabia and the UK during 1995–2005 reported a 23.3% prevalence for Klebsiella species in PLA [11]. However, it did not stratify the results by exact species or country of study [11]. In contrast to these relatively recent studies, an earlier study from Iraq conducted in the 1980s showed that E. coli was the most commonly isolated organism in patients with PLA, and this observation, when contrasted with recent findings, highlights the change in regional trends since that time [13].
The emergence of the hypervirulent strains of K. pneumoniae and their subsequent global spread over the past two decades raises several diagnostic and clinical challenges [24, 25]. For example, phenotypic detection of a hypermucoviscous strain of K. pneumoniae entails performing a string test on an agar plate. In contrast, the detection of genetically encoded virulence factors such as K1 or K2 capsular antigens, mucoviscosity-associated gene A (magA), regulator of mucoid phenotype A gene (rmpA), and aerobactin involves complex molecular methods. However, these tests are not routinely performed in clinical diagnostic laboratories in many settings, including ours, mainly because of the prohibitive cost, labour-intensive nature of testing, and limited local expertise. Furthermore, the phenotyping results of K. pneumoniae may be of uncertain clinical significance as not all infections with serotypes K1 or K2 translate into an invasive disease [25]. This has been the case in our setting as no extra-hepatic complications were observed despite the dominance of K. pneumoniae. Furthermore, only one out of all K. pneumoniae isolates responsible for PLA in 16 patients had evidence of ESBL production, and none were carbapenemase producers. This low prevalence of antimicrobial resistance among our K. pneumoniae isolates is reassuring but should not leave any room for complacency.
Mucoid K. pneumoniae isolates with rmpA and K1 virulence factors were reported in two diabetic patients presenting with PLA in Saudi Arabia – one of them developed endogenous endophthalmitis [26]. Two other K. pneumoniae PLA and endogenous endophthalmitis cases were reported in diabetic patients from Saudi Arabia, but no characterization of virulence factors was carried out [27]. Neither of these studies provided detailed information on the antimicrobial susceptibility profile of K. pneumoniae isolates.
A large study from mainland China identified invasive K. pneumoniae as the leading cause for PLAs and showed that diabetes mellitus, hypertension, and fatty liver disease were the key risk factors [28]. Diabetes mellitus was documented in 32.4% of our PLA patients, 18.8% of PLA patients in Saudi Arabia, and 35.7% in Qatar [11, 12]. Although UAE nationals and non-Arab Asians living in UAE have a similar burden of diabetes mellitus and pre-diabetic state [29], no UAE national was found to have K. pneumoniae liver abscess during the 7-year period of our study. This observation, coupled with the fact that a sizeable proportion of K. pneumoniae PLA were found in Asians from the Indian subcontinent, calls for further research into possible explanations as to why we UAE nationals, who have a similar prevalence of diabetes mellitus, did not contribute any PLAs to our study.
Ultrasonography was used to detect liver abscesses in all of our patients. Furthermore, plain chest radiography was useful in detecting concomitant unilateral pleural effusions in a quarter of cases in our setting and a fifth of Saudi Arabia cases [11]. These findings confirm the utility of these cheap and widely available diagnostic tools in aiding the diagnosis of liver abscesses. Moreover, image-guided drainage, coupled with appropriate antimicrobial therapy, has been an integral part in managing liver abscesses – only one patient in our series required surgical fenestration.
Data on the response to amoebicidal therapy from our region have been mixed. A 2-year prospective study of 12 ALA patients from Iraq responded very well to metronidazole treatment, and none of them required drainage [30]. Furthermore, a case series from Kuwait on 19 ALA patients with sluggish response to amoebicidal therapy reported rapid clinical recovery and complete abscess resolution following adjunctive percutaneous drainage [31]. In our study, all patients with ALA received amoebicidal therapy coupled with percutaneous or surgical drainage. This practice may be explained in part by previous local experience with severe ALA cases [32]. A large study from India on 144 ALA patients showed that nearly 50% of them failed to respond to amoebicidal therapy and required percutaneous drainage [33]. All these findings highlight the need for better evidence to guide ALA management.
Although we acknowledge the limitations of the retrospective design for this study, we would like to point out that it is the first series from the Arabian Peninsula to report crude annual incidence rates for liver abscesses over a well-defined period. Incomplete or absent data, e.g., travel history and short follow-up duration, are examples of the limitations. Furthermore, the small sample size of patients with PLA did not adequately power the multivariate regression model examining potential risk factors associated with K. pneumoniae PLA. Although this study was based at a regional infectious’ disease centre in the Emirate of Abu Dhabi, caution should be exercised when extrapolating our findings to other parts of UAE.
