Review Article Zoonotic Ancylostomiasis: an Update of a Continually Neglected Zoonosis
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Am. J. Trop. Med. Hyg., 103(1), 2020, pp. 64–68 doi:10.4269/ajtmh.20-0060 Copyright © 2020 by The American Society of Tropical Medicine and Hygiene Review Article Zoonotic Ancylostomiasis: An Update of a Continually Neglected Zoonosis Katharina Stracke,1,2* Aaron R. Jex,1,3 and Rebecca J. Traub3 1The Walter and Eliza Hall Institute of Medical Research, Melbourne, Australia; 2The Faculty of Medicine, Dentistry and Health Sciences, The University of Melbourne, Melbourne, Australia; 3Faculty for Veterinary and Agricultural Sciences, The University of Melbourne, Melbourne, Australia Abstract. Hookworm infections are classified as the most impactful of the human soil-transmitted helminth (STH) infections, causing a disease burden of ∼4 million disability-adjusted life years, with a global prevalence of 406–480 million infections. Until a decade ago, epidemiological surveys largely assumed Necator americanus and Ancylostoma duo- denale as the relevant human hookworm species implicated as contributing to iron-deficiency anemia. This assumption was based on the indistinguishable morphology of the Ancylostoma spp. eggs in stool and the absence of awareness of a third zoonotic hookworm species, Ancylostoma ceylanicum. The expanded use of molecular diagnostic assays for differentiating hookworm species infections during STH surveys has now implicated A. ceylanicum, a predominant hookworm of dogs in Asia, as the second most common hookworm species infecting humans in Southeast Asia and the Pacific. Despite this, with the exception of sporadic case reports, there is a paucity of data available on the impact of this emerging zoonosis on human health at a population level. This situation also challenges the current paradigm, neces- sitating a One Health approach to hookworm control in populations in which this zoonosis is endemic. Here, we have summarized the available research studies and case reports on human A. ceylanicum infections in Southeast Asia and the Pacific after 2013 using a systematic review approach. We summarized eight research articles and five clinical case studies, highlighting the importance of future in-depth investigation of zoonotic A. ceylanicum infections using sensitive and cost-effective diagnostic tools. Hookworm infections in humans occur predominantly in the true species-specific prevalence and intensity of human countries with low socioeconomic status located in tropical hookworms. and subtropical areas of the world.1 Symptomology is mea- Hookworm infections in humans have largely been attrib- sured in disability-adjusted life years (DALYs), which include uted to Necator americanus and Ancylostoma duodenale.6 the years a human host has lived with the infection (years lost However, contrary to this belief, recent molecular studies have due to disability (YLD)) and the years that are lost because of unequivocally demonstrated that Ancylostoma ceylanicum is early death (years of life lost (YLL)).2 The global infection highly endemic, comprising the second most common spe- prevalence for any hookworm in 2016 was around 450.68 cies of hookworm, after N. americanus, in many parts of million (1,297 million in 1994),3 with a disease burden of Southeast Asia and the Pacific and estimated to infect ∼100 around 1.8 million DALYs in 2015.4,5 Primary symptoms as- million people.21 This situation challenges the current hook- sociated with infection include iron-deficiency anemia caused worm paradigm. Despite the global efforts to meet the WHOs by intestinal blood loss within the small intestine,6,7 which 2020 road map for eliminating morbidity associated with STH particularly impacts pregnant women8 and children.9 For infections, A. ceylanicum is not included as a causal agent of hookworms, as well as other soil-transmitted helminths human hookworm infection and therefore not included within (STHs), disease burden has decreased significantly over the this WHO framework.11,22 This is despite a growing body of last two decades.3 This decrease can be attributed principally evidence to demonstrate that its prevalence in cer- to targeted mass drug administration (preventive chemo- tain restricted areas or countries outcompetes that of A. – therapy) in populations at risk of infection10 12 and integrated duodenale.23 intervention programs targeting improved access to safe Importantly, A. ceylanicum is a zoonosis and transmissible water, sanitation, and hygiene13,14 and development of to humans from animals. This species comprises the pre- molecular high-sensitivity diagnostic methods (improved dominant hookworm of dogs and cats throughout Southeast infection prevalence estimation)15 and socioeconomic de- Asia and the Pacific, which act as reservoirs for human velopment.16 The WHO-recommended and -approved di- infections.21,24,25 This situation contrasts that of agnostic tool for STH detection is the Kato–Katz thick N. americanus and A. duodenale, which are both specificto smear.17 However, this microscopy-based diagnostic method humans. In 2013, a review by Traub highlighted the history of has important limitations: innately low diagnostic sensitivity in the reported distribution and overlooked public health signif- regard to hookworm detection,18 the requirement for rapid icance and the impact of A. ceylanicum infections in humans in sample screening to avoid over-clearance of hookworm eggs Southeast Asia and the Pacific.21 The review hypothesizes following slide preparation,19 and the inability of microscopy- that mass anthelmintic programs may contribute to the re- based diagnostics to identify human hookworms to a species emergence of A. ceylanicum infections in humans unless a level.20 These factors together suggest an underestimation of One Health approach toward its control is undertaken.21 In this review, we aim at summarizing and updating information spanning the 6 years since the last major review by Traub, on the prevalence, distribution, and impacts of A. ceylanicum on * Address correspondence to Katharina Stracke, The Walter and Eliza human health. Hall Institute of Medical Research, 1G, Royal Parade, Parkville 3052, Here, we conducted a systematic review, searching the Australia. E-mail: [email protected] publicly available database Google Scholar on June 14, 2019 64 ZOONOTIC ANCYLOSTOMIASIS 65 with no restrictions in terms of scientific journal or author for proportion of A. ceylanicum infections even further,25 owing to the publications in English only. We limited the output to novel relatively lower egg-shedding intensities of A. ceylanicum– original research articles published from 2013 to 2019 using infected individuals than those infected with N. americanus.41 the following search term in the title: “Ancylostoma ceylani- Despite the increased availability and application of mo- cum.” Publication titles and abstracts of 70 peer-reviewed lecular diagnostic assays,40 there remains a lack of large- articles were screened and relevant articles included in this scale mapping of A. ceylanicum in areas other than Southeast systematic review, as well as two other known case reports. 26,35 Asia and the Pacific, with a clear lack of available data in regard Review articles were excluded, with only original research to infection prevalence and intensity, particularly in human articles included within the context of this mini-review. A list of populations. This lack of knowledge is partly due to the ap- all included publications is found in Tables 1 and 2. plication of microscopy-based diagnostic tools that are not Since 2013, A. ceylanicum infections in humans continue to able to differentiate between hookworm species.42 Apart from be identified across Southeast Asia and the Pacific, including Southeast Asia and the Pacific, A. ceylanicum has also been Thailand, Myanmar, India, Cambodia, and the Solomon Is- reported in 3% of canines in Tanzania.43 To date, the global lands (Table 1, Figure 1). The proportion of A. ceylanicum in- distribution of cases in humans reflects that of its distribu- fections among all hookworm-positive samples was relatively tion in dogs. During the previous 6 years after publication high in some regions (16.7–46.0%). Five of 13 publications of Traub’s 2013 review, the focus of STH research has describe individual cases of returning travelers or personnel been on molecular biological areas such as genomics, tran- from Southeast Asia and the Pacific, highlighting the rele- scriptomics, and diagnostics.44 At the same time, diagnostic vance of this emerging zoonosis to the field of travel medicine tool application is shifting from invasive endoscopy38 to mo- (Table 2). Most of the aforementioned patients had a severely lecular laboratory–based techniques such as semi-nested increased eosinophil count, which is a known immune re- polymerase chain reaction (PCR)-RFLP, quantitative PCR sponse to hookworm infections.33 Symptoms ranged from (qPCR), and multiplexed-tandem qPCR.28,45 Moving toward abdominal pain to weight loss, fever, diarrhea, vomiting, and these advanced molecular biology–based diagnostic tools other generalized symptoms (dizziness, tightness of chest, using PCR systems can prove beneficial in increasing general and sweats). Interestingly, only one of these clinical cases knowledge of hookworm infection prevalence, the species- presented with anemia (Table 2), which is likely due to the specific burden they cause and examining effects of coinfections general health status of travelers from developed countries within hosts.46 Currently, however, novel diagnostic techniques compared with that of the