Ancylostoma duodenale in Humans and Dogs: Zoonotic Hookworm Infection Clinical and Public Health Aspects

1. Introduction

Ancylostoma duodenale is a blood-feeding intestinal nematode belonging to the family Ancylostomatidae and represents one of the principal etiologic agents of human hookworm disease globally [42, 110, 114]. Although classically regarded as an anthroponotic parasite, accumulating molecular evidence demonstrates that A. duodenale can infect canids and that canine populations may serve as reservoirs for human infection in certain epidemiological contexts [113, 115, 132]. This dual-host capacity places A. duodenale within a growing list of zoonotic hookworms that challenge traditional host-specificity paradigms [81, 85, 95]. The present article provides a comprehensive veterinary and public health reference on A. duodenale, emphasizing its biology, diagnostic detection, clinical consequences in dogs, zoonotic transmission dynamics, and control within a One Health framework [1, 20, 31, 37, 63, 116].

2. Taxonomy and Morphology

Ancylostoma duodenale (Dubini, 1843) is classified within the order Strongylida, superfamily Ancylostomatoidea, and family Ancylostomatidae [42, 114]. The subfamily Ancylostomatinae includes several species of medical and veterinary importance, including A. caninum, A. ceylanicum, A. braziliense, A. tubaeforme, and Uncinaria stenocephala [81, 96, 121]. Adult A. duodenale specimens exhibit a characteristic hooked anterior end, a well-developed buccal capsule armed with two pairs of ventral teeth, and a robust muscular esophagus. The male measures approximately 8 to 11 mm in length and possesses a copulatory bursa supported by characteristic rays, while the female measures 10 to 13 mm and exhibits a vulval opening located in the posterior third of the body [42, 114]. Eggs are ellipsoidal, thin-shelled, measure 55 to 75 micrometers by 35 to 45 micrometers, and contain a segmented ovum when freshly voided [96, 143]. Morphological differentiation of A. duodenale eggs from those of A. caninum, A. tubaeforme, and A. braziliense is unreliable by light microscopy alone due to extensive overlap in dimensions and appearance [96, 150].

Table 1. Comparative morphological features of adult hookworms relevant to canine and human infection.

Data synthesized from references [42, 81, 96, 114, 121, 131].

3. Life Cycle and Transmission Dynamics

The life cycle of A. duodenale is direct and soil-mediated, following the general pattern of hookworm development [110, 114]. Adult worms reside attached to the mucosa of the small intestine, where they feed on blood and tissue fluids. Females produce thousands of eggs per day, which are passed in feces into the environment. Under favorable conditions of warmth, moisture, and oxygen, first-stage larvae (L1) hatch within 24 to 48 hours, develop through two molts to become third-stage filariform larvae (L3), which are the infective stage [81, 110]. The L3 larva is sheathed, non-feeding, and capable of surviving for weeks in moist soil. Transmission to the definitive host occurs primarily via percutaneous penetration of L3 larvae through skin, although oral ingestion of L3 can also lead to infection [81, 126]. After penetration, larvae enter the circulatory system, are carried to the pulmonary circulation, break into alveolar spaces, migrate up the trachea, and are swallowed to reach the small intestine, where they molt twice to become adults. The prepatent period for A. duodenale is approximately 5 to 6 weeks. A unique feature of A. duodenale is the capacity for arrested larval development (hypobiosis), which can prolong the prepatent period and complicate epidemiological assessments [81, 85].

In canine hosts, the life cycle proceeds similarly, although the efficiency of larval migration and establishment may differ from that observed in humans [57, 113]. Dogs become infected through skin contact with contaminated soil, ingestion of L3 larvae, or ingestion of paratenic hosts such as rodents. Transplacental and transmammary transmission, well-documented for A. caninum, has not been conclusively demonstrated for A. duodenale in dogs [81, 95].

4. Host Specificity and Zoonotic Potential

Ancylostoma duodenale has been historically classified as an anthroponotic hookworm, with humans considered the primary definitive host [85, 114]. However, molecular surveys have repeatedly detected A. duodenale DNA in dog fecal samples from multiple geographic regions, challenging this strict host restriction [113, 115, 132]. A study in Ecuador detected A. duodenale DNA in 6.3% of dog fecal samples using ITS1 qPCR and sequencing, with phylogenetic analysis showing close clustering between human and canine isolates. Similarly, research in Tamil Nadu, India, identified A. duodenale in 8.4% of human hookworm-positive samples and noted the presence of canine hookworm species (A. caninum, A. ceylanicum) in human stools, suggesting bidirectional cross-transmission. A study in Nigeria reported A. duodenale infection in dogs at substantial prevalence, alongside A. caninum and other helminths. In Ghana, molecular characterization of Ancylostoma species in protected areas identified A. duodenale in canine samples, further supporting the role of dogs as potential reservoirs [2].

The zoonotic significance of canine A. duodenale infection lies in its potential to undermine control programs based solely on human mass drug administration. If dogs maintain a reservoir of A. duodenale, treated human populations may experience rapid reinfection from environmental contamination originating from canine feces [37, 85, 116]. The public health implications are especially pronounced in settings with high dog densities, poor sanitation, and close human-animal contact [1, 18, 20, 34, 54, 63, 71, 79, 84, 90, 98, 124].

5. Clinical Aspects in Canine Hosts

Hookworm infection in dogs is associated with a spectrum of clinical presentations, ranging from subclinical parasitism to life-threatening disease [25, 92, 93]. The primary pathological mechanism is blood loss at the site of mucosal attachment, mediated by the secretion of anticoagulant peptides and zinc-dependent metalloproteases by the adult worm. Ancylostoma duodenale adults can ingest approximately 0.05 to 0.3 mL of blood per worm per day, contributing to iron deficiency anemia in heavy infections [107, 131].

5.1 Clinical Signs

Canine patients with A. duodenale infection may present with the following clinical signs.

• Pale mucous membranes due to anemia, particularly in puppies and young dogs [92, 93].
• Poor body condition, weight loss, and reduced growth rates in juvenile animals [11, 25, 26, 27, 60].
• Dermatitis, especially on the paws and ventral abdomen, resulting from percutaneous larval penetration [64, 87].
• Coughing and respiratory signs during the pulmonary migratory phase.
• Melena or hematochezia reflecting intestinal hemorrhage.
• Eosinophilia observed on complete blood count analysis.

5.2 Laboratory Alterations

Dogs with subclinical Ancylostoma infection demonstrate significant changes in biochemical analytes. Serum protein profiles may show reduced albumin concentrations and elevated globulin fractions, consistent with chronic blood loss and immune stimulation. Canine patients with heavy worm burdens frequently develop hypochromic microcytic anemia, thrombocytosis, and peripheral eosinophilia [92, 93]. Fecal examination typically reveals pleomorphic, thin-shelled eggs that cannot be reliably speciated by morphology alone [96, 150].

6. Diagnostic Approaches

Accurate diagnosis of A. duodenale infection in dogs and humans requires a combination of coprological and molecular techniques, given the morphological overlap with other hookworm species [75, 150]. The diagnostic workflow is summarized in Figure 1.

6.1 Coprological Methods

Conventional fecal flotation and sedimentation techniques remain the most widely used initial screening methods for hookworm detection in veterinary practice [75, 143]. Centrifugal flotation using zinc sulfate or sugar solutions enhances egg recovery compared to passive flotation [75, 143]. The Kato-Katz thick smear technique, commonly employed in human mass drug administration surveys, allows quantification of eggs per gram of feces and provides a measure of infection intensity [128, 139]. However, Kato-Katz sensitivity declines markedly at low infection intensities, and the method cannot differentiate A. duodenale from N. americanus or other Ancylostoma species [128, 133, 139]. Coproculture with larval recovery facilitates species identification based on L3 morphology, but this approach is time-consuming, requires specialized expertise, and may be unavailable in routine diagnostic settings.

6.2 Molecular Diagnostics

Molecular assays provide superior sensitivity and specificity for hookworm species differentiation [3, 32, 104, 112, 150]. Several platforms have been developed and validated.

Table 2. Molecular diagnostic methods for detection and differentiation of Ancylostoma duodenale.

Real-time PCR (qPCR) assays targeting the internal transcribed spacer regions (ITS1 and ITS2) of ribosomal DNA have demonstrated high sensitivity and specificity for A. duodenale detection, with detection limits as low as one copy of target DNA per reaction [3, 128]. Duplex TaqMan qPCR systems enable simultaneous quantification of A. duodenale and N. americanus in a single reaction, facilitating large-scale epidemiological surveys [3]. Loop-mediated isothermal amplification (LAMP) assays offer a rapid, field-compatible alternative to PCR, with visual colorimetric readout and minimal equipment requirements. High-resolution melting (HRM) analysis following PCR amplification provides a post-amplification species differentiation capability without the need for sequencing. Suppression-competition PCR has been developed to minimize unwanted amplicons in metabarcoding workflows applied to fecal samples.

6.3 Genomic and Proteomic Tools

The availability of a chromosome-contiguous A. duodenale reference genome has enabled transcriptomic and proteomic investigations that inform diagnostic target discovery [42, 129]. Single-cell transcriptome atlases of A. ceylanicum provide comparative data that may identify A. duodenale-specific markers. The excretory-secretory proteome of hookworms contains abundant proteins that may serve as antigen targets for immunodiagnostic assays. Bioinformatics-guided genomic mining has identified potential vaccine and diagnostic antigen candidates in the A. duodenale genome. Deep learning-based platforms have been developed for automated recognition of helminth eggs in microscopic images, reducing reliance on operator expertise.

flowchart TD
 A["Fecal sample collected from dog or human"] --> B["Conventional coprology<br/>(flotation, sedimentation, Kato-Katz)"]
 B --> C{"Eggs detected?"}
 C -- "No" --> D["Consider qPCR or LAMP<br/>if clinical suspicion remains"]
 C -- "Yes" --> E["Cannot speciate by morphology alone"]
 E --> F["Molecular analysis required"]
 F --> G["DNA extraction from feces"]
 G --> H["Select molecular method"]
 H --> I["Duplex qPCR<br/>(A. duodenale + N. americanus)"]
 H --> J["HRM analysis"]
 H --> K["Conventional PCR + sequencing"]
 I --> L{"A. duodenale positive?"}
 J --> L
 K --> L
 L -- "Yes" --> M["Confirm with ITS1 or cox1 sequencing"]
 L -- "No" --> N["Consider other species<br/>(A. caninum, A. ceylanicum,<br/>N. americanus, U. stenocephala)"]
 M --> O["Report: A. duodenale confirmed"]
 N --> P["Apply species-specific qPCR panels"]

Figure 1. Diagnostic workflow for Ancylostoma duodenale detection and differentiation in canine and human fecal samples. Adapted from protocols described in references [3, 32, 75, 91, 104, 128, 139, 150].

7. Molecular Epidemiology and Genomic Insights

The application of molecular diagnostics has transformed understanding of hookworm epidemiology by revealing cryptic species distributions and host associations [17, 49, 52, 74, 84, 90, 102, 103, 113, 115, 118, 125, 133]. Ancylostoma duodenale has been documented in human populations across Africa, Asia, the Middle East, southern Europe, and parts of South America [42, 109, 111, 127, 145]. In many settings where A. duodenale and N. americanus occur sympatrically, N. americanus predominates, but A. duodenale may be more prevalent in specific ecoregions, such as the Indian subcontinent and northern Africa [42, 109, 133].

In dogs, A. duodenale prevalence varies widely by geographic location and sampling strategy. Studies from Nigeria, Ecuador, India, and Ghana have reported A. duodenale in dog feces at frequencies ranging from 1.6% to 12.4% of samples tested with molecular methods [2, 113, 115, 132]. A study in the Solomon Islands found A. ceylanicum but not A. duodenale in dogs, illustrating species-specific regional distributions. Surveys in Tanzania and Cambodia have identified zoonotic hookworm species in dog feces, but A. duodenale was not detected, suggesting that its canine reservoir role may be geographically restricted [74, 90].

The chromosome-contiguous reference genome of A. duodenale was assembled from a single archived specimen and provides a platform for investigating host-parasite interactions at the molecular level. Comparative genomic analyses reveal that A. duodenale encodes a suite of genes involved in blood feeding, immune evasion, and tissue invasion, many of which are shared with A. caninum and A. ceylanicum [42, 129]. Genes encoding excretory-secreted proteins are transcriptionally upregulated in response to host immune signals, highlighting the dynamic interplay between parasite and host [10, 57]. The mitochondrial genome of A. duodenale exhibits conserved gene content and organization relative to other Ancylostoma species, with utility for phylogenetic reconstruction and barcoding [120, 121].

8. Anthelmintic Resistance

Anthelmintic resistance in hookworms, particularly in A. caninum, has emerged as a critical concern in canine veterinary practice [41, 48, 76]. Resistance to benzimidazoles, macrocyclic lactones, and pyrantel has been documented in A. caninum populations in the United States and other regions [39, 41, 48, 76]. For A. duodenale, resistance has not been as widely reported, but the potential for cross-species resistance gene flow exists where dogs are co-infected with multiple Ancylostoma species [53, 69].

Benzimidazole resistance in hookworms is associated with single nucleotide polymorphisms in the beta-tubulin isotype-1 gene, particularly at codons 167, 198, and 200. A systematic review of beta-tubulin mutations in hookworms found that resistance-associated alleles are present in A. caninum and A. ceylanicum populations but have not been extensively surveyed in A. duodenale. The presence of benzimidazole-susceptible beta-tubulin alleles in U. stenocephala populations from the Balkans suggests that resistance selection pressure varies by species and region.

Multi-drug resistance in A. caninum has prompted calls for expanded surveillance and the development of alternative therapeutic strategies [41, 48]. High-throughput screening of compound libraries has identified novel anthelmintic candidates, including kinase inhibitors and drug repurposing leads [16, 30, 62, 68]. Emodepside resistance in A. caninum has been linked to variability in potassium channel activity, highlighting the molecular basis of differential drug susceptibility. Mathematical models of eco-evolutionary dynamics predict that resistance spread is influenced by treatment frequency, drug half-life, and the fitness cost of resistance alleles.

9. Public Health and One Health Considerations

The recognition of A. duodenale infection in dogs carries implications for the control of human hookworm disease, particularly in regions where mass drug administration programs are underway. The World Health Organization targets for soil-transmitted helminth control, defined as reducing the prevalence of moderate-to-high intensity infections to below 2%, rely on the assumption that humans are the sole reservoir for A. duodenale and N. americanus. If dogs contribute to environmental contamination with A. duodenale eggs, reinfection of treated human populations may sustain transmission despite high coverage of human deworming [37, 85, 116, 149].

Environmental surveillance for hookworm contamination in soil and public spaces provides a complementary approach to assessing transmission risk [18, 40, 63, 71, 98, 135, 142]. Studies in Malaysia, Mexico, and Brazil have detected hookworm DNA in soil samples from public parks, playgrounds, and household yards, demonstrating the potential for canine-mediated environmental contamination [18, 40, 71, 98, 142]. In Colombia, integrated One Health assessment of humans, dogs, and soil revealed overlapping parasite communities indicative of cross-species transmission. In Gabon, molecular surveillance identified zoonotic soil-transmitted helminth infections in humans, with potential canine involvement [31, 4].

The role of stray and free-roaming dogs as reservoirs for zoonotic parasites is well established [25, 43, 61, 80, 84]. Studies in Ethiopia, Ecuador, Thailand, and Malaysia have documented hookworm prevalence exceeding 50% in stray dog populations, with multiple species including A. caninum, A. ceylanicum, and A. duodenale coexisting in the same hosts [25, 34, 52, 84]. Dog owners' knowledge, attitudes, and practices regarding deworming and sanitation are critical determinants of transmission risk [58, 67]. Participatory epidemiological studies in India and Bangladesh have identified risk factors for canine gastrointestinal helminth infection, including free-roaming behavior, lack of routine deworming, and access to public spaces [11, 12, 26, 27].

Cutaneous larva migrans (CLM) caused by skin-penetrating hookworm larvae is a common zoonotic manifestation in humans, particularly in tropical and subtropical regions [5, 36, 45, 55, 64, 77, 87, 124]. Although A. caninum and A. braziliense are the most frequently implicated agents, A. duodenale larvae can also produce CLM in human skin [36, 64]. The serpiginous, pruritic tracts characteristic of CLM result from the migration of L3 larvae within the epidermis, eliciting a local inflammatory response [5, 36, 45, 64, 87]. Travel-associated CLM accounts for a substantial proportion of imported dermatologic conditions in returning travelers from endemic regions [5, 45, 64, 77].

10. Control and Prevention Strategies

Effective control of A. duodenale in canine populations requires integrated approaches combining chemoprophylaxis, sanitation, and public education [58, 67, 116].

10.1 Anthelmintic Therapy in Dogs

Several anthelmintic agents are effective against adult hookworms in dogs, including benzimidazoles (fenbendazole, albendazole), macrocyclic lactones (moxidectin, milbemycin oxime), and nicotinic acetylcholine receptor agonists (pyrantel pamoate) [33, 38, 73, 83, 86, 94, 138]. Combination products containing multiple active ingredients are widely used for broad-spectrum control [38, 73, 83, 86, 94]. The efficacy of moxidectin combined with pyrantel and lotilaner has been demonstrated for the treatment of larval and immature adult stages of A. caninum, and these regimens are likely effective against A. duodenale [38, 73, 83, 86]. Topical formulations containing fipronil, moxidectin, and praziquantel have been evaluated for control of zoonotic gastrointestinal helminths in naturally infected cats, with extrapolation to dogs in multi-pet households.

10.2 Environmental Management

Routine removal of canine feces from yards, public parks, and communal areas reduces environmental egg contamination and breaks the transmission cycle [18, 40, 58, 67, 98, 135]. Public awareness campaigns stressing responsible pet ownership and proper fecal disposal are essential components of community-based control programs [11, 58, 67]. Soil surveillance for hookworm contamination can guide targeted deworming interventions in high-risk areas [18, 40, 71, 98].

10.3 One Health Integration

Intersectoral collaboration between veterinary and human health authorities is necessary to address hookworm transmission across the human-animal interface [1, 20, 31, 37, 63, 116]. Integrated surveillance programs that simultaneously monitor infection in humans, dogs, and the environment provide the data needed to design effective control strategies [1, 20, 63, 116]. Mass drug administration campaigns for human hookworm control should consider the potential contribution of canine reservoirs to transmission persistence, particularly in settings with high dog densities and poor sanitation [85, 149].

11. Research Priorities

Several knowledge gaps require further investigation to refine the veterinary and public health response to A. duodenale.

• The geographic extent and intensity of canine A. duodenale infection should be systematically mapped using standardized molecular diagnostic protocols [2, 113, 115].
• The contribution of canine A. duodenale infection to human disease burden should be quantified through longitudinal transmission modeling [37, 85].
• The efficacy of currently used anthelmintic regimens against A. duodenale in dogs should be confirmed through controlled clinical trials [38, 73, 86].
• The frequency and distribution of anthelmintic resistance alleles in A. duodenale populations should be monitored using molecular markers [48, 53, 69].
• The role of paratenic hosts and environmental reservoirs in sustaining transmission should be elucidated [63, 81].
• Discovery of the NCBI and the European Bioinformatics Institute (EMBL-EBI) as resources for hookworm genomics should underpin target identification for vaccines and diagnostics [42, 51, 129].

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Disclaimer: This article is for educational and informational purposes only. It is not intended to substitute for professional veterinary advice, diagnosis, treatment, or regulatory guidance. Always consult a licensed veterinarian or qualified specialist regarding animal health, disease diagnosis, and therapeutic decisions.