The genus Thelazia (Spirurida, Thelaziidae) is group of nematodes that infect eye of various kinds of mammals and birds [1,2]. The adult worm is parasitized at the conjunctival sac and its adjacent regions of host, and induce several symptoms such as conjunctivitis, epiphora, purulent ocular discharge, keratitis and corneal ulcers so called thelaziasis or thelaziosis [2]. One of the species, Thelazia callipaeda Railliet and Henry, 1910 is comparatively well-known parasite among the group. Likely with its common name, “the oriental eyeworm”, it is distributed wide ranges of East Asian countries with some European countries which are area that the species was introduced and have been inducing the thelaziasis [3].

In addition to dog (Canis familiaris), raccoon dog (Nyctereutes procyonoides) and American red fox (Vulpes fulva), several kinds of mammals such as cat (Felis catus), European rabbit (Oryctolagus cuniculus), rodents, monkey, and human are known to act as a definitive host of T. callipaeda [2]. Current studies also added wolf (C. lupus), red fox (Vulpes vulpes), European wildcat (F. silvestris), beech marten (Martes foina), sable (M. zibellina), European badger (Meles meles), and European hare (Lepus europaeus) to the list of definitive hosts from European countries [4–7]. In the most recent papers, additional wildlife, i.e., raccoon (Procyon lotor), Japanese raccoon dog (N. viverrinus), Japanese red fox (V. vulpes japonica), masked palm civet (Paguma larvata), Japanese badger (M. anakuma), and Japanese black bear (Ursus thibetanus japonicus) were newly added to the host list [8,9]. Although the understanding of sylvatic life cycle is commonly recognized as one of the important factors for parasitic control, it is often neglected. In particular, information on the infection status and host range of T. callipaeda remains scarce in Korea, despite the fact that the host diversity of the parasite has been comparatively well investigated in adjacent countries [2,8,9].

The aim of the present study is to introduce 2 cases of thelaziasis which found in both of captive and free-living domestic wildlife and to identify the causative parasite through morphological analysis.

Korea is considered to be an endemic region and 1 of the original distribution areas of this nematode. However, studies on the parasitic nematodes are limited to approximately 40 human infections, along with 5 papers on canine infections [12–17]. In Korea, the parasitic infection has generally been neglected unless there are unusual or novel cases, as demonstrated by previous authors [12,17], who suggested that infections have been underestimated. It has yet to be evaluated whether dogs are the primary reservoir hosts, as previously recorded, or if there are indeed wild animals involved in transmission in the natural environment.

In the present study, we described 2 cases of thelaziasis identified in wild animals. The infections were found in 2 different species, i.e., a raccoon dog and a leopard cat marking the first reported cases of wild animals in Korea. As noted in previous cases, the animals in this study did not exhibit significant clinical issues, with only minor symptoms such as blood congestion, epiphora and ocular discharge being observed [17]. Although both infections were detected at the same wildlife rescue center, considering the timing of detection and the history of animals, the 2 cases are considered to have occurred independently. Notably, a case represents a nosocomial infection, while the other is presumed to have been acquired in the wild prior to rescue. The raccoon dog, whose infection was detected in August 2017, had been reared in an enclosure at the wildlife center since its rescue in 2011, making it likely that this infection was acquired within the facility. In contrast, the leopard cat was rescued from a location over 15 kilometers from the center where the raccoon dog was reared, and its infection was detected on the day of rescue, suggesting it was infected in wild.

Considering environmental factors, it is suspected that Thelazia infections in the 2 wild animals were transmitted through arthropod vectors that had obtained larvae from animals raised by humans near the locations [18–20]. The raccoon dog’s enclosure was an outdoor pen with an open roof, surrounded by nearby residential areas and mountains. Similarly, the leopard cat was rescued from a plain that included rice field and farmlands, with various residential houses in the vicinity. Although we were unable to investigate the presence of the parasitic infections in animals such as dogs raised in these residential areas, previous cases of Thelazia infections in local dogs suggest a plausible source of infection [13,15–17]. Since the first record of thelaziasis in Korea, and up until recent cases, veterinarians who examined the same wildlife species had never found the parasites (personal communication), suggesting that the prevalence of Thelazia infection in wild animals may be very low. Given that the parasite thrives in environments where arthropod vectors are more likely to access their hosts in immobile conditions, such as tethered dogs, it is likely that animals raised in human settlements serve as the primary reservoir hosts rather than wild animals. According to a study on the experimental culturing of the species under laboratory conditions, arthropod hosts mostly harbor only a larva per individual, with a maximum observed number of 5 [20]. Considering the publication, in the present cases which 5 and 44 parasites were detected, it is likely that both the raccoon dog and the leopard cat were repeatedly exposed to these flies. Given their histories, it is evident that both animals had limited mobility, either being confined to enclosures or unable to move due to pelvis fractures.

This study might contributed to further understanding of the life cycle and ecology of T. callipaeda in Korea, particularly by confirming, for the first time, infections in wild animals, which had not been previously documented. Although this report is limited to 2 cases, making it difficult to ascertain the actual infection status and distribution of T. callipaeda in wild animals in the nature environment, it has at least demonstrated that infections are indeed occurring. In previous studies [2,4–9], the final host range of T. callipaeda was found to be quite broad, and considering this, it is plausible that these parasites could also be found in other Korean wildlife such as rabbits, martens, badgers, wild boars and bears. Given the susceptibility of this parasite, there is also a high likelihood that foxes, which have been the focus of recent restoration efforts, could be exposed to these parasites in the wild, just as one of the foxes have been exposed to Sarcocystis pilosa in a previous case [21]. Therefore, veterinarians working with wildlife should be aware of the potential for parasitic infections in animals susceptible to T. callipaeda. Although veterinarians mentioned not having encountered this parasite before, this may be due to a lack of attention to parasitic infections and the difficulty of conducting eye examination in wild animals without anesthesia, leading to an underestimation of the infection. Therefore, we must pay special attention to the zoonosis like thelaziasis and should try to maintain good hygienic conditions for the outside facility.

In conclusion, we reported the first case of T. callipaeda infection in Korea, found in wild animals with limited mobility. The raccoon dog and leopard cat are the first wildlife definitive hosts in Korea, and in both cases, factors that limited their immobility were described. This has filled a significant gap in the understanding of the ecology and has provided knowledge that can be applied to the prevention of T. callipaeda infections. However, many aspects of T. callipaeda in Korea remain unexplored. Important questions that have been studies in adjacent countries, such as the species of intermediate hosts that transmit T. callipaeda and genetic diversity of Thelazia are still unknown in Korea [20,22,23]. Future research should address these gaps to provide more practical and effective response to T. callipaeda infections, which remain a neglected disease in Korea.