Feline strongyloidiasis: An insight into its global prevalence and transmission cycle

The potential cross-transmission of Strongyloides stercoralis between dogs and humans has become an increasing focus of strongyloidiasis research and control programs. However, the role of cats and wild felids in the maintenance and transmission cycles of human and canine strongyloidiasis has received sparse attention. Feline strongyloidiasis epidemiology remain enigmatic. We conducted a systematic review and meta-analysis to assess the global prevalence of Strongyloides spp. in felines and reviewed cross-species infection studies to elucidate the transmission cycle of some feline Strongyloides species. Literature searched from seven databases identified 42 eligible prevalence studies published between 1985 and 2024. Of these, 44 datasets from 40 studies were included in the meta-analysis. Using a random effect model combined with the Rogan-Gladen method, we estimated the pooled global prevalence of Strongyloides spp. in felines at 13.3% (95% CI: 8.3–18.3%), with rates of 12.2% (95% CI: 6.7–17.8%) in domestic cats (Felis catus) and 20.0% (95% CI: 14.9–25.2%) in wild felids. Feline strongyloidiasis was distributed across all six WHO regions, with Africa (49.7%; 95% CI: 40.0–59.3%) and the Western Pacific (46.9%; 95% CI: 42.6–51.1%) showing the highest pooled prevalence. Subgroup analysis revealed a significantly higher prevalence of Strongyloides infection in stray domestic cats (29.2%; 95% CI: 6.3–52.1%) compared to pet cats (9.3%; 95% CI: 3.7–14.9) and shelter cats (4.4; 95% CI: 0–9.0). Historical cross-species transmission studies demonstrated variable susceptibility of cats to human- or canine-derived S. stercoralis. It remains inconclusive whether cats act as a reservoir for S. stercoralis infection in humans or vice versa. Feline strongyloidiasis is a prevalent condition in wild, stray, pet and shelter cats. Much of the available prevalence data does not discriminate to species level, and the role of cross-species transmission in feline S. stercoralis infections remains obscure. Future studies would benefit from utilising molecular genotyping tools to enable species-level phylogenetic differentiation.


Introduction
Strongyloides (order Rhabditida, family Strongyloididae) is a genus of soil-transmitted helminths infecting a variety of terrestrial vertebrates, including humans (Homo sapiens) and two major companion animals of humans, dogs (Canis lupus familiaris) and cats (F.catus) [1,2].This parasite has a unique lifecycle, characterised by alternating parasitic and free-living developmental phases [3,4].The obligate female-only parasitic generation reproduces parthenogenetically within the host intestine.Depending on the species, eggs or hatched rhabditiform larvae (L1) are passed into the environment where they develop further into infective third-stage larvae (iL3s) (homogonic route), or into facultative dioecious free-living adults which undergo sexual reproduction to produce a new generation of iL3s (heterogonic route).The resulting iL3s then invade the host percutaneously and migrate either directly or via the pulmonary route to the intestinal mucosa, maturing into parthenogenetic adult females [3,4].
Strongyloides in cats remains significantly understudied, with its prevalence, transmission dynamics, and public health impact largely unknown.It has been indicated that four species of Strongyloides infect felines, these being Strongyloides felis [11], Strongyloides planiceps [12], Strongyloides tumefaciens [13] and S. stercoralis [14].Contention over the taxonomy of some species persists, despite new insights provided in the molecular-genetic era.
The first Strongyloides species identified in cats was S. felis by Chandler [11] in India.On morphological grounds, Chandler [11] did not exclude the possibility of it being a subspecies of S. stercoralis.Since this initial discovery, S. felis has only been reported twice globally [15,16].Although genotyping data for this species are unavailable, phylogenetic analyses of putative S. felis isolates from Thailand [16] and Myanmar [17], utilising the partial 18S rRNA gene [16] and proteincoding mitochondrial genome [17], respectively, support it being a distinct but evolutionarily closely related species to S. stercoralis of both human and dog origins.
Strongyloides planiceps was originally discovered by Leiper in rusty tiger cats (Prionailurus planiceps) from Malaysia [18].Rogers [12] subsequently described this species in domestic cats, albeit misclassifying it as a new species "Strongyloides cati".Strongyloides planiceps is distinguishable from other feline Strongyloides spp.by the passage of eggs, rather than L1 larvae, in faeces [12].Genotyping research based on partial mitochondrial cytochrome c oxidase subunit I (cox 1) gene indicated that S. planiceps shares a common ancestor with (human and canine derived) S. stercoralis [17,19].While S. planiceps is believed to predominantly occur in wild felines and canines, infrequent reports of this species in domestic cats exist [20][21][22].
Strongyloides tumefaciens was first described by Price and Dikmans [13] in two domestic cats from the southeastern United States of America (USA).This species was designated based on characteristic colonic nodules observed in the infected cats upon necropsy [13].Complete morphological data for this parasite are unavailable and no molecular characterisation has been attempted.
Recently, Wulcan and colleagues [23] observed similar colonic lesions in S. stercoralis-infected cats from St. Kitts.Morphologically, the recovered parasitic female of S. stercoralis resembled those described for S. tumefaciens by Price and Dikmans [13].Phylogenetically, S. stercoralis isolates from St. Kitts cats [23] clustered closely on the cox1 (522 bp) locus with human S. stercoralis isolates from Lao [24] and dog isolates from Japan [25,26] and the USA [27].This study challenged the taxonomic validity of S. tumefaciens.Although numerous reports of S. tumefaciens [28][29][30][31] and S. stercoralis infections in cats exist, none of the studies detailed how species were confirmed.Wulcan et al. [23]'s work represents the first unequivocal documentation of natural S. stercoralis infection in cats.
The role of companion animals in the transmission cycle of human strongyloidiasis remains enigmatic.While much research effort in this regard has been directed towards dogs [2], cats have received sparse attention.Genetic evidence thus far suggests that at least some catderived populations of S. stercoralis are potentially zoonotic [23].It is unknown whether S. stercoralis or other Strongyloides spp.from cats are naturally transmissible to humans, or vice versa.Understanding the role, if any, cats and wild felids play in the transmission and maintenance of strongyloidiasis in both humans and dogs holds significant public health implications.Essentially, within a One-Health context, it may inform whether co-treatment of companion cats is necessary for controlling human and canine infections in endemic communities.In the hope of inspiring more research in this area, we synthesised and reviewed experimental evidence on cross-species transmission of feline Strongyloides species.
Currently available epidemiological data on feline strongyloidiasis are limited and disparate, with the global prevalence and distribution poorly understood.We hereby conducted the first systematic review and meta-analysis of Strongyloides prevalence in feline populations worldwide.

Search strategy and selection criteria
This review followed the Predefined Protocol Items for Systematic Reviews and Meta-Analyses (PRISMA) guideline.Literature search was performed within seven English language databases, including Web of Science, Scopus, PubMed, Embase, Medline, Global Health, and CINAHL.Grey literature was identified through a Google Scholar search and citation searching.The key search terms used were: (Strongyloides OR gastrointestinal helminth OR intestinal parasit* OR endoparasit*) AND (cat OR kitten OR feline OR felids).The search was not limited by language, and the publication timeframe spanned from January 1983 to January 2024.
Inclusion criteria were: 1) Peer-reviewed original research articles; 2) Studies utilising case-control, cohort, or cross-sectional study designs; 3) Articles reporting the prevalence of Strongyloides spp. in felines.Excluded from the review were experimental studies, review articles, case reports, case series, conference proceedings, as well as letters or correspondences.

Data extraction and quality assessment
Two researchers conducted article screening and study selection independently.Data from the included studies were systematically organised into the following categories: authors and publication year, country of the study, host species, host type, specimen examined, diagnostic method employed, diagnostic stage, sample size, number of positive samples, prevalence (%), and identified Strongyloides species.The Joanna Briggs Institute Prevalence Critical Appraisal Tool, conprisng eight items, was used to assess the methodological quality and risk of bias in the included articles (Supplementary File 1).

Statistical analysis
To ensure accurate prevalence estimations, all prevalence data underwent adjustments to accommodate the imperfect sensitivity and specificity of diagnostic tests.True Prevalence (TP) estimates were calculated using the Rogan and Gladen [32] method, as previously described [9,33].Sensitivity and specificity data for each diagnostic test were extracted from the existing literature (Tables 1, 2).When studies reported the presence of larvae-shedding Strongyloides spp. in felines, TP calculations relied on sensitivity and specificity data specific to the detection of S. stercoralis larvae (Table 1).In cases where Strongyloides eggs were identified, potentially representing S. planiceps, prevalence rates were adjusted using relevant diagnostic performance data for Strongyloides egg detection in dietarily comparable hosts, such as primates (Table 2).When multiple diagnostic techniques were employed to assess Strongyloides prevalence, the method with the highest sensitivity was chosen during the calculation of the pooled prevalence to prevent potential overestimation.
Pooled prevalence estimates were calculated using the random effects model, employing the inverse variance method for weighting, and reported with a 95% confidence interval (CI).Heterogeneity among studies was evaluated using the Cochran Q test and the inconsistency index (I 2 ), with I 2 values exceeding 75% indicating high heterogeneity.Subgroup analysis was conducted based on several variables including World Health Organisation (WHO) regions, host species, host types (pet, stray, shelter, and wild), specimen types (faeces or gastrointestinal contents), and Strongyloides species.All statistical analyses were performed using R studio 4.2.0, with a significance level defined as p < 0.05.

Review of cross-infection studies
A literature search was conducted in PubMed and Google Scholar up to January 2024, using the terms "Strongyloides" AND experiment*.There were no language, publication type, or time restrictions.Peerreviewed original studies reporting experimental cross-species transmission of feline Strongyloides species were eligible for inclusion.Review articles, conference proceedings, and correspondence were excluded.Citation searching was employed to identify grey literature.Data from the included studies were extracted based on year(s) of the study, geographic origin of infection, Strongyloides species involved, experimental hosts, mode and intensity of inoculation, diagnostic methods, and prepatent and patent periods of infection.No statistical analysis was performed on the data.

Overview of the studies
A total of 42 studies were included in the review (Table 3; Supplementary File 2).Quality assessment using the eight-item JBI tool revealed that the majority (35/42) demonstrated high methodological quality with a low risk of bias, scoring between 6 and 8 (Supplementary File 1).However, two studies were excluded from the quantitative metaanalysis due to methodological incompleteness and bias.One of the studies [40] examined Strongyloides spp.prevalence in captive wild felids in a zoo, but its small sample size (n = 9) limited its representativeness for the broader host population in the region.The other study [41] lacked sufficient details on sampling and diagnostic methodologies to permit meta-analysis.Consequently, 44 datasets from 40 studies were included in the meta-analysis for pooled Strongyloides prevalence in felines (Fig. 1, Table 3).

Cross-species transmission of feline Strongyloides species
Seven studies, published between 1925 and 1985, were included in this review (Table 5).Human-to-cat [54][55][56][57] or dog-to-cat [58,59] experimental transmission of S. stercoralis was described in six studies conducted in the Americas.While in some experiments, cats were refractory to human or canine strains, in most cases, patent short-lived infections lasting 1-7 weeks were established.Additionally, one study described experimental transmission of S. planiceps from wild canids and weasels to cats [21].While this study demonstrated that wild carnivores could potentially serve as reservoirs for S. planiceps infection in cats, the potential for this transmission to occur naturally and cats' ability to sustain the infection were not explored.

Discussion
We present, to our knowledge, the first systematic review and metaanalysis to assess the global prevalence and distribution of feline strongyloidiasis.The pooled Strongyloides spp.prevalence in felines (13.3%; 95% CI: 8.3-18.3%)and in domestic cats (12.2%; 95% CI: 6.7-17.8)worldwide was markedly higher than the reported S. stercoralis prevalence in humans [9] and canines [10,60].Several factors may contribute to this disparity.It is worth noting that our study included surveillance data for all feline Strongyloides spp.Comparability of prevalence rates among canine, feline, and human hosts may be compromised by the inclusion of oviparous cat Strongyloides spp. in the present analysis.While human and canine strongyloidiasis are overwhelmingly attributable to S. stercoralis based on the faecal passage of Strongyloides rhabditiform larvae [1,2], it is currently uncertain in feline cases whether such larvae represent S. stercoralis or S. felis.Further advanced morphological, or genotypic, characterisation is required to differentiate between these two species.Additionally, unlike dogs, cats bury their faeces and do not tend to defecate in the open, reducing environmental contamination with feline species of Strongyloides.Despite this, infection in cats persists, suggesting the possibility of other yet-to-be identified transmission routes.While transmammary transmission has been proposed as a route for canine S. stercoralis infection [61], this remains unexamined for felines.This and other vertical transmission routes of Strongyloides in cats could be an avenue for future research.
Methodologically, we employed statistical modelling to account for limitations in the diagnostic data from feline studies, an analytical step lacking in comparable meta-analyses for canine strongyloidiasis [10,60].While this may lead to improved estimation, the prevalence of feline strongyloidiasis could still be underestimated.One possible reason is that, for larvae-shedding feline Strongyloides spp., a negative faecal test may not necessarily reflect the absence of disease due to low and intermittent larval output [34].Although isolation of intestinal worms by necropsy is deemed more sensitive [34], it was only performed in 14% (6/44) of the studies.Another contributing factor to underestimation is the skewed representation of studies from the American (27%; 12/44) and European (39%; 17/44) regions.The paucity of surveillance data from low-income regions, such as sub-Saharan Africa, where veterinary services are often limited, inaccessible, or unaffordable [62], may bias the assessment of the true global feline disease burden.
Globally, feline strongyloidiasis was not restricted to tropical and subtropical regions, although prevalence was generally higher in these areas, mirroring patterns observed in human [9] and canine [10,60] strongyloidiasis.Lower income WHO regions, such as Africa (49.7%; 95% CI: 40.0-59.3%)and the Western Pacific (46.9%; 95% CI: 42.6-51.1%),had the highest pooled Strongyloides spp.prevalence in felines, consistent with findings for the nematode in canines [10].Geographical variations in prevalence may be attributable to climatic, environmental, and socio-economic factors.Laboratories and veterinarians in regions with high rates of, or high awareness of Aelurostrongylus abstrusus infection may be more inclined to perform larval recovery methods such as the Baermann technique, incidentally also identifying more infections with larviparous Strongyloides spp.The wide confidence intervals of prevalence rates for some WHO regions, such as the Eastern Mediterranean region (13.8%;0-37.8%) and South-East Asian region (9.7%; 1.5-17.9%),indicate considerable uncertainty in the estimated prevalence.Future surveillance studies with large sample sizes from diverse geographical areas are warranted.
The high level of heterogeneity (I 2 = 97.7%;Q = 1840.32;df = 43; P < 0.0001) observed among the studies in this review prompted a further investigation into host-specific factors influencing the epidemiology of feline strongyloidiasis.In line with findings for other soil-transmitted helminths in felines [63][64][65], stray domestic cats (F.catus) had a significantly higher prevalence of Strongyloides spp.compared to their owned counterparts (pet and shelter cats) in more controlled environments.This difference may be explained by the unrestricted outdoor activities of stray cats, potentially increasing environmental transmission of the parasite, coupled with the absence of anthelmintic therapy or other veterinary care for this population.
This review found that Strongyloides detection in feline surveys predominantly relied on traditional microscopy, with flotation-based methods being the most commonly employed diagnostic approach.Faecal flotation methods are ineffective for Strongyloides larvae recovery [4,34] and thus are unreliable for detecting S. stercoralis, S. felis, and H. Zhao and R.S. Bradbury S. tumefaciens in feline faeces.Its sensitivity for faecal S. planiceps egg detection remains to be tested.While APC is recommended for isolating the larviparous Strongyloides spp.and potentially allows species differentiation by morphologists [34], its utilisation in existing feline studies was very limited (5%; 2/38).
Only two studies in this review provided robust morphological or molecular evidence for species-level Strongyloides identification [16,52].This makes it challenging to examine the prevalence of individual Strongyloides spp. in felines.Given recent phylogenetic evidence suggesting the zoonotic potential of certain cat S. stercoralis strains [23], surveillance of this species and genotypes in felines using highly sensitive molecular tools is necessary from a public health perspective.Egg-shedding Strongyloides spp., potentially representing S. planiceps, were reported in 13 studies, in both wild felids and domestic cats.While experimentally demonstrated [21], S. planiceps's capacity to cause natural infection in F. catus and its veterinary impact remain unclear and could benefit from future research.
Historical cross-infection studies revealed that cats were poorly susceptible to human-or canine-derived S. stercoralis.Intensity of larval inoculation did not have any detectable influence on subsequent duration of infection.In Sandground [55]'s series of experiments, the passage of human-derived S. stercoralis through dogs seemed to enhance its  H. Zhao and R.S. Bradbury cross-infectivity in cats.However, in other studies, direct inoculation with dog strains either failed to induce infection [58] or only resulted in transient infections lasting seven days in cats [59].Interpretation of these findings require caution, as these experiments were conducted before molecular genotyping was available, so results may be confounded by potentially differing felid infectivity of different genotypes.Additionally, variations in inoculation procedures and diagnostic approaches may limit direct comparability between experiments or to natural infections, and there was a sampling bias towards strains originating in North America.Furthermore, studies in dogs have identified that S. stercoralis parasitic female can enter a barren phase in which fertile eggs are not produced, but fecundity may return later under specific conditions [66].Therefore, without investigation by necropsy, it cannot be definitively determined that the absence of larval shedding after a few weeks of infection reflects true host clearance of the infection, or entry of the parasitic females into a senescent phase.The data    wks: weeks; mo: months; NS: not stated; NA: not applicable.
H. Zhao and R.S. Bradbury from existing cross-infection studies suggests that cats are relatively refractory to infection with S. stercoralis from dogs or humans and may not be a significant reservoir for natural S. stercoralis infections in those hosts, but this evidence remains limited and inconclusive.
The meta-analysis has several limitations.Firstly, the sensitivity and specificity data used for TP calculation were mostly derived from studies of human, dog, or non-human primate hosts, which may not be generalisable to feline hosts due to potential variations in hosts' faecal composition.Moreover, these diagnostic performance data are imperfect owning to inconsistent reference standards used across studies.Consequently, the accuracy of the pooled prevalence may be compromised.Secondly, most of the included studies focused on general intestinal parasitism in felines, without specifically targeting Strongyloides.Faecal Strongyloides detection is challenging and requires experienced morphologists, as the larvae demonstrate low and irregular output, making them easily overlooked, while the eggs can be mistaken for those of hookworms.This may lead to potential underestimation of the pooled prevalence.Thirdly, there is a paucity of country-level data, impeding an unbiased assessment of feline strongyloidiasis prevalence in different WHO regions.Notably, the pooled prevalence for the African region relied on data collected in a single country (Kenya), with a sample size of 103 [49].

Conclusion
This systematic review and meta-analysis highlights the importance of ongoing research, control, and surveillance for feline strongyloidiasis globally.The continued reliance on inadequate diagnostic approaches in most feline surveys remains a challenge for evaluating the true disease burden.Furthermore, the role of cross-species transmission in feline, human and canine S. stercoralis infection is not fully understood.To determine whether cats are truly a significant factor in the epidemiology of human or canine strongyloidiasis, molecular taxonomy studies and controlled population treatment experiments may be beneficial.This could involve sampling cats, dogs, and humans from the same communities and comparing population genetics of S. stercoralis from these hosts.Exploring the impact of co-treatment of cats on the infection dynamics in different hosts may also provide insights into the role cats might or might not play in human and canine infections.

Funding
The first author receives an Australian Government Research Training Program Scholarship from James Cook University.The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Declaration of competing interest
None.

Fig. 1 .
Fig. 1.Predefined Protocol Items for Systematic Reviews and Meta-Analyses (PRISMA) flow diagram of the search strategy.

Fig. 2 .
Fig. 2. Studies included in qualitative and quantitative analysis by publication year.

Fig. 3 .
Fig. 3. Forest plot of pooled Strongyloides prevalence in domestic cats and wild felids.

HFig. 4 .
Fig. 4. Forest plot of pooled Strongyloides prevalence in felines globally and by World Health Organisation regions.SEAR, South-East Asian Region; AMR, American Region; WPR, Western Pacific Region; EMR, Eastern Mediterranean Region; EUR, European Region; AFR, African Region.

Table 1
Sensitivity and specificity of different diagnostic techniques for Strongyloides stercoralis larvae detection (reference standards of the reviewed studies were faecal-based techniques only).

Table 3
Main characteristics of studies included in the systematic review.
(continued on next page) H. Zhao and R.S.Bradbury [17]study[17]utilised PCR and partial 18S rRNA sequencing, but only samples positive by APC were molecularly confirmed for Strongyloides.No study employed serological methods for nematode diagnosis.
a Studies excluded from quantitative analysis.b Methods for species confirmation were unspecified.c

Table 4
Subgroup analyses of Strongyloides prevalence in felines.

Table 5
A summary of human-cat or dog-cat cross-species transmission studies from the literature.