INTRODUCTION Angiostrongylus vasorum (Bailet, 1866) (
1), commonly known as „French heartworm“, is a metastrongylid nematode affecting the cardiopulmonary system of dogs and other carnivores. First described over 150 years ago in a dog in France, it has recently become the focus of extensive studies in parasitology, epidemiology, phylogenetics, and other fields. This species is recognized for its significant global expansion, raising growing concern as it poses a potential global issue (
2). Increasing numbers of cases have been reported in dogs, often presenting with severe and variable clinical signs that can sometimes be fatal.
A. vasorum has a complex biological life cycle that involves not only definitive hosts like dogs and other carnivores but also intermediate hosts such as slugs and snails, as well as paratenic hosts like frogs.
This review article updates the range of
Angiostrongylus vasorum, critically examining the parasite‘s genetic diversity, and lists its main morphological and taxonomic distinctions from related species.
HOST AND DISTRIBUTION RANGE OF ANGIOSTRONGYLUS VASORUM
Angiostrongylus vasorum and
A. cantonensis are the most studied species within the genus
Angiostrongylus, significantly contributing to the understanding of this genus.
A. vasorum is primarily associated with canid species, and research has shown that red fox (
Vulpes vulpes) serves as the main reservoir for this parasite in nature (
3). Recent advancements in diagnostic techniques, particularly molecular methods, have revealed a considerable expansion of the host range of
A. vasorum. The following species have been confirmed as definitive hosts: dogs, domestic cats, wild cats, gray wolves, African golden wolves, red foxes, gray foxes, crab-eating foxes, African desert foxes, hoary foxes, golden jackals, coyotes, stoats, badgers, raccoon dogs, European otters, weasels, meerkats, bears, invasive American minks, and red pandas (
2,
3,
4,
5,
6,
7,
8,
9). Furthermore,
A. vasorum migratory larvae have been discovered in wild boar muscles (
10).
Although
A. vasorum is thought to be a widespread species, it can be difficult to identify and forecast the risk factors for its spread (
2). This intricacy is due to the complex developmental cycles in every species of the
Angiostrongylus genus, including intermediate, paratenic, and definitive hosts. Europe, North America, South America, and Africa are all included in the range of
A. vasorum (
3). In Europe, enzootic regions have demonstrated a rise in incidence in several countries, including the UK, Denmark, France (earlier reports), Germany, and Switzerland. According to continuing epidemiological investigations (
11),
Angiostrongylus vasorum prevalence has been shown to increase annually in endemic regions such as Switzerland.
These areas serve as sources from which the parasite continues to spread, contributing to its increasing prevalence. However, it is not always possible to directly correlate the prevalence of the parasite in dogs and foxes. Variability in prevalence has been observed across different regions of Europe, with recent data suggesting that this cardiorespiratory nematode is less common in Eastern Europe compared to Central and Western Europe (
2,
12). There are probably several factors that contribute to the differences in prevalence and distribution between Eastern and other parts of Europe. Not all reasons for this difference can be attributed to biological factors (climate, vector and host distribution, etc.). Based on our observations, there is a significant difference in the number of epidemiological studies and the priorities of researchers who regularly examine carnivore helminths. For example, in personal communication with parasitologists in the Russian Federation, it was noted that there is a tendency to report only on helminths that show a zoonotic character
(Echinococcus,
Trichinella, etc.). Also, the lack of samples in laboratories is an additional factor that may contribute to the scarce data of angiostrongylosis in some countries (this was concluded from communication with colleagues from Montenegro). Finally, a possible scenario is that this parasite is diagnosed in private clinics with the help of now-available serological rapid tests, but such findings remain publicly unpublished.
Although many papers dealt with this species, there are still gaps in understanding its presence and prevalence in Eastern Europe. The most recent information regarding the distribution of
A. vasorum in Europe is summarized in
Table 1 and
Fig. 1.
In their review article, Morgan et al. (
2) examine several hypotheses regarding the biological and social factors that have contributed to the spread of
A. vasorum. These factors include the movement or migration of owned dogs, climate change, the urbanization of fox populations, increased awareness among veterinarians, and changes in the population of intermediate hosts, such as gastropods. Additionally, other potential risk factors for the spread of this parasite may include the migration of wild canids, specifically jackals, as well as the rising number of owned and stray dogs in certain countries (
22). However, it is worth mentioning that epidemiological and clinical studies that investigate the risk factors for spread and distribution can be quite demanding and costly, and the results are not always conclusive. Regardless of the actualization of this problem, in many cases, canine angiostrongylosis (
sensu stricto) remains under or misdiagnosed.
MORPHOLOGICAL DIAGNOSIS: ANGIOSTRONGYLUS VASORUM OR NOT?
In the species
A. vasorum, the male and female adults exhibit significant morphological differences (
Fig. 2). It is visually hard or impossible to establish a valid diagnosis based solely on the characteristics of the females. The basic morphological features of the species are shown in
Fig. 3 and
Fig. 4.
Morphological descriptions of the species reveal differences in the structure of the copulatory bursa in males. These differences, along with certain morphometric variations, the localization of adults within the host – site of infection, and host specificity, are sufficient to differentiate individual species within the genus
Angiostrongylus.
The first taxonomic dilemma concerns classification of the species
A. vasorum. According to the descriptions by Anderson (
96), which were accepted by Ubelaker (
97), the genus
Angiostrongylus is divided into two subgenera:
Parastrongylus and
Angiostrongylus sensu stricto. This proposal is significant and influential for many researchers (
5). The primary morphological criterion for this taxonomic classification is the structure of the lateral rays of copulatory bursa. In the subgenus
Parastrongylus, these rays emerge from a common trunk, while in the subgenus
Angiostrongylus, anteriolateral and the other lateral rays have no common trunk (
98,
99). The defining and latest morphological features of
A. vasorum suggest that its lateral rays share a common trunk (
100). That means that
A. vasorum has morphology typical of the subgenus
Parastrongylus (
101). However, Souza et al. (
101) found that the
A. vasorum present in dogs in Brazil lacks trunk that separates the anteriolateral ray from the mediolateral and posteriolateral rays. In the original drawings and pictures of the male
A. vasorum by Guilhon and Cens (
99), a difference in the structure of the lateral rays is evident, suggesting possible misdescription. Souza et al. (
101) highlights a morphological asymmetry between the right and left lateral rays in
A. vasorum. The original description of the male
A. vasorum from present-day Russia, as noted by Kozlov (
4), indicates that the anteriolateral ray does not share a common trunk with the other two lateral rays supporting original descriptions of the species by Rosen (
98) and Guilhon and Cens (
99). In general, the morphological variation and associated dilemma in the case of
A. vasorum involve whether the anterolateral ray is part of the same common trunk as the mediolateral and posteriolateral rays. Additionally, there is uncertainty regarding how deep the cleft of the anteriolateral ray is within the trunk of the copulatory bursa (
102,
103).
Another significant difference is the length of the spicules. According to Kozlov (
4), the spicules of male
A. vasorum measure 360-400 µm, while Guilhon and Cens (
99) describe them as considerably longer, ranging from 460-500 µm. Additionally, the structure of the gubernaculum differs among sources: Guilhon and Cens (
99) describe it as triangular, whereas Costa et al. (
100) characterize it as more tongue-shaped. The gubernaculum can be difficult to observe, as it is often covered by spicules (
Fig. 5). Differences reported in the foundational literature strongly suggest the presence of distinct intraspecific morphological variations, which may complicate the definitive identification of
A. vasorum.
In a recent reclassification by Cowie (
5), four genera,
Angiostrongylus, Gallegostrongylus, Rodentocaulus, and
Stefanskostrongylus, were accepted, designating
Parastrongylus as a junior synonym of
Angiostrongylus. However, Cowie (
5) focused more on nomenclature than on taxonomy. Another taxonomic issue involves the morphological similarities between
Angiocaulus raillieti and
A. vasorum in South America. The previously described species
A. raillieti, found in dogs and crab-eating foxes in South America, was reclassified as
A. vasorum (
100). This change has not been universally accepted, as
A. raillieti is still mentioned in the literature as a separate species, along with comparisons to other species in the
Angiostrongylus genus (
5,
103,
104,
105). A similar taxonomic issue arose concerning the discovery of
A. vasorum in badgers in Spain and Italy (
60,
86). In the original investigation conducted in Spain, the morphological characteristics of the
A. vasorum specimens collected from badgers were not described. However, subsequent morphometric and molecular analyses confirmed the presence of
A. daskalovi in the same area (
87). Increasing evidence from Europe suggests that
A. daskalovi, rather than
A. vasorum, is found in badgers (
25,
79,
87). Errors in identification of
Angiostrongylus vasorum in mustelids are recognized in earlier reports by Ubelaker (
97). The third taxonomic challenge is more general. In morphological studies, it has been observed that newly described species in rodents and wild carnivores (such as cats) often lack phylogenetic support (
101,
102,
103). Although not yet described in rodents as natural infection,
A. vasorum is highly similar to
A. schmidti,
A. gubernaculatus,
A. morerai,
A. lenzii, and
A. minasensis (
101,
105). The differences between these species are based on body length as well as morphometric characteristics of spicules and bursal rays. In Europe, the taxonomic differences are more pronounced, as sympatric species like
A. chabaudi, A. daskalovi, A. cantonensis and
A. dujardini have been molecularly characterized, revealing clearer morphological distinctions. In original specimens of
A. daskalovi from a badger (shot in 2018 near Srbac, Bosnia and Herzegovina), a sheath (membrane) was observed around the distal part of the spicule, which was not seen in
A. vasorum found in golden jackals and foxes (
22). This spicule sheath gives the appearance that the spicules are expanded at the end in a “fan-shaped” similarity (
Fig. 6). Additionally, the spicules in
A. daskalovi are better developed, wider, and shorter compared to those of
A. vasorum (
79).
Angiostrongylus daskalovi is a significantly larger species compared to
A. vasorum. In this species, the anteriolateral ray shares a common trunk with other lateral rays (
25). The main differences between
A. vasorum and
A. chabaudi are morphometric, but also include a notably more developed cephalic vesicle in
A. chabaudi (
20). The female
A. dujardini exhibits a visible papilla at the end of the tail, which is not present in
A. vasorum (
Fig. 4). In male
A. dujardini, the medio-lateral ray is longer than the other lateral rays, which all originate from a common trunk. Males of
A. cantonensis possess significantly longer spicules (over 1,000 µm) compared to those of
A. vasorum, along with differences in the morphology of the gubernaculum.
A. vasorum has a shorter, thicker, and more triangular gubernaculum, while
A. cantonensis has a filariform gubernaculum. Bursal rays are much shorter in both
A. cantonensis and
A. dujardini compared to
A. vasorum. Despite
A. vasorum being the most researched species, the morphological descriptions of adults in the literature tend to be scarce. Many studies rely on a phrase stating that the morphological diagnosis is made “according to” certain references. Furthermore, molecular studies have often focused solely on L1 larvae of
A. vasorum, with inadequate descriptions of adult forms. Morphological diagnosis based on the examination of L1 larvae can be sufficient in enzootic areas; however, in all other cases, confirmation with additional diagnostic methods is required. Therefore, providing a morphological diagnosis without information on the host, geographic location, and site of infection can be problematic for some parasitologists. In practice, additional challenges arise when analyzing adult specimens, such as lower quality samples (especially from wild animals), a limited number of samples, the absence of male specimens, and laboratory personnel lacking sufficient experience. Even in cases where adult forms are absent from the heart and pulmonary artery, this does not entirely rule out the presence of parasites and it is still necessary to examine the lungs for L1 larvae. Finally, many laboratories that do not specialize in the genus
Angiostrongylus may lack “reference” specimens for comparative morphological analysis (
Table 2).
GENETIC DIVERSITY OF ANGIOSTRONGYLUS VASORUM: THERE IS MORE THAN MEETS THE EYE
It is generally accepted that significant genetic variation exists within natural populations (
106). Nematodes are particularly superabundant, with many species being cosmopolitan. These species (parasites and/or free-living) have adapted to various hosts (parasites) and environmental factors throughout their evolution and coevolution. As a result, they often display notable genetic diversity, which can serve as a resource for adaptation in the face of natural pressure (
107). The primary genetic markers studied in
Angiostrongylus vasorum are 18s (ribosomal RNA), ITS-2 (internal transcribed spacer region 2), and cox1 (cyclooxygenase 1). Differences in the nucleotide sequences of these loci reflect variations in population dynamics, species history, speciation processes, and evolutionary changes. Most investigations into genetic diversity have focused on the species
A. vasorum, aiming to establish connections between individual variants – haplotypes and their geographical areas and host species (
108). However, it appears that this goal may be overly ambitious, primarily because the high level of genetic diversity can be interpreted more from the perspective of parasite population dynamics (expansion, bottleneck effect, population differentiation, etc.). Understanding whether genetic variants of
A. vasorum differ in pathogenic potential remains unknown. In their initial study, Jefferies et al. (
108) analyzed the internal transcribed spacer region 2 (ITS-2) and cyclooxygenase 1 (cox1) genes. They found a significant pairwise nucleotide distance (p-distance) between the ITS-2 sequences from Europe and South America, leading them to propose that
A. vasorum comprises two distinct genotypes/lineages - European and South American variants. Subsequent research indicated that
A. vasorum can be divided into Palearctic/Nearctic and Neotropic clades based on ITS-2 and mitochondrial genes (
109). This suggests that the Neotropic clade essentially represents a South American lineage of
A. vasorum. Different hosts (such as foxes and dogs) sharing identical haplotypes of this species indicate a host-switching effect. However, evidence has demonstrated that populations of this nematode have differentiated in certain Palearctic/Nearctic and Neotropic regions due to geographic isolation and varying microclimatic conditions (
109). Some researchers have proposed that the larger p-distance observed in ITS-2 gene sequences between the European and South American genotypes (2 - 2.3%) may signify the existence of two distinct species:
A. vasorum and
A. raillieti (
103). However, without a detailed comparative analysis of specimens attributed to these species (holotypes), this cannot be conclusively proven. Additionally, there is currently no established “yardstick” p-distance value that can reliably separate species. Blouin (
110) suggests that the intraspecies p-distance (ITS-2) might be around 1%, while interspecies p-distances for the
Angiostrongylus genus usually range from 12.1% to 29.3% (108) (
Table 3).
Another challenge is that it is not always feasible to distinguish species based solely on single gene analysis (SGA). A combination of multiple nuclear and mitochondrial genes is often necessary to achieve higher resolution and more comprehensive phylogenetic data. For instance, greater differences in the p-distance of
cox1 gene sequences have been observed in
A. vasorum, where intraspecies p-distance ranges from 1% to 2%, while interspecies divergence is significantly higher, often between 10% and 20% (
110). Recent data supported that global phylogenetic analysis of
A. vasorum is notably more complex. Lange et al. (
111) identified
A. vasorum larvae in the African giant snail (
Achatina fulica) in Colombia, where the ITS-2 sequence showed 99% identity with the analogous sequences of the European lineage. The inferred phylogenetic tree clearly grouped the ITS-2 sequences of
A. vasorum with sequences from Europe, Canada, and Colombia (
Fig. 7). The authors speculate that both genotypes of this species circulate in South America and that there has likely been an introduction of this nematode from Europe.
We offer the following potential explanations for this finding:
- a single ITS-2 sequence lacks adequate resolution to differentiate intraspecific haplotypic or genotypic variants,
- the genetic diversity of the A. vasorum is greater than previously recognized, and
- the recommendation to categorize the ITS-2 genotypes of A. vasorum based on geographical origin has limited taxonomical significance.
Some studies support some of these observations. Mitochondrial genes are advantageous for investigating evolutionary processes of speciation compared to nuclear genes in veterinary-relevant nematodes (
112). The genetic diversity of the mitochondrial cytochrome c oxidase subunit 1 (cox1) gene is extremely high in restricted geographical areas with high prevalence and parasitic burden within hosts (
113,
114). Comparing two studies conducted in London and Zurich reveals genetic signal for population expansion of
A. vasorum in enzootic regions (as indicated by negative neutrality tests), with shared haplotypes among populations. Although Fst (fixation test) and AMOVA analyses were not performed between these populations, the results suggest the high gene flow and low population differentiation into subdivisions. From an epidemiological perspective, these findings imply that the expansion of a species, underpinned by genetic diversity, is progressive and cannot be halted merely by implementing control measures for this parasite. Additionally,
A. vasorum exhibits significantly greater genetic diversity compared to
A. cantonensis, another species experiencing recent geographical expansion (
9). This high genetic diversity is further supported by the discovery of new ITS-2 variants of
A. vasorum in coyote and black bear in Tennessee, USA (
7). Analysis of ITS-2 sequences indicates that these isolates demonstrate a higher p-distance (95%-96% similarity) compared to the European clade (
Fig. 7), suggesting the presence of a distinct genetic lineage. Studies on the phylogeny of
A. vasorum indicate the potential occurrence of cryptic diversity or speciation (
sensu lato). The p-distance of cox1 sequences from
A. vasorum in Costa Rica was found to be 8.6% compared to homologous sequences from Europe and Brazil (
9). Similar to the isolates from the USA,
A. vasorum from Costa Rica does not fall within the variants originally proposed by Jefferies et al. (
108,
109). The concept of cryptic diversity - or the notion that
A. vasorum contains cryptic variants - requires thorough revision. A limitation of the study by Robleto-Quesada et al. (
9) is that it analyzed only two samples and did not include adult forms of
A. vasorum, which would be necessary for detailed morphological analysis. It is difficult to define cryptic diversity without precise morphological descriptions of the species, supported by statistical validity concerning the number and origin of samples at all phyletic and ecological levels (
115). Further complication is that speciation and diversity in this case are typically inferred from genetic divergence, which is often overemphasized, whereas essential ecological criteria - particularly reproductive isolation - are largely overlooked. A greater divergence in mitochondrial DNA (mtDNA) and other genes may result from the higher population differentiation observed in the species
A. vasorum. This differentiation often arises from spatio-temporal and ecological isolation (host- associated divergence). Although this process can occur independently, it is faster and more ephemeral compared to speciation (
116).
If there is a lack of biological or ecological data on the species and interpretations are solely based on gene data, it is possible that incomplete lineage sorting (ILS) could introduce noise, leading to misleading conclusions about divergence. As shown in the review,
A. vasorum displays significant morphological variations that have unresolved taxonomic implications (e.g.,
A. vasorum and/or
A. railleti). Furthermore, the species exhibits considerable genetic diversity that is expanding in terms of geographical range and host distribution. This indicates that new variants in terms of haplotypes, genotypes, or lineages of this species may emerge, presenting varying p-distances and other indicators of genetic diversity.
CONCLUSION
Angiostrongylus vasorum exhibits substantial morphological and genetic variability, challenging morhological diagnosis and phylogenetic interpretation. Morphological ambiguities have limited impact in general practice but are essential in taxonomic and epidemiological studies. When assessing the genetic diversity of
Angiostrongylus vasorum, it is important to avoid premature conclusions, as population dynamics in this species are known to be highly fluid and complex. We emphasize that identification in non-canid hosts should always be supported by molecular confirmation. The high genetic variability within
A. vasorum may reflect a combination of factors, including reproductive capacity and environmental pressures, rather than host and geographical range alone. Future research should adopt a more integrative approach, incorporating standardized diagnostic protocols and exploring host–parasite coevolution and population genetics.
CONFLICT OF INTEREST
The authors declare that they have no financial or non-financial conflict of interest regarding authorship and publication of this article.
ACKNOWLEDGMENTS
We would like to express our sincere gratitude to Dr. Alexander Khrustalev and Dr. Oleg Andreyanov from the All-Russian Scientific Research Institute for Fundamental and Applied Parasitology of Animals and Plants, Moscow, for kindly providing valuable literature and data from the Russian Federation. We are also grateful to Mr. Milan Rogošić from the Food Safety, Veterinary and Phytosanitary Affairs Authority, Podgorica , for his assistance in supplying data from Montenegro.
AUTHORS' CONTRIBUTIONS
OS wrote the manuscript. DD, AR, DN, TI, AV and IP edited the final version of the manuscript.
10.2478/macvetrev-2025-0029
