Dirofilaria species are globally recognized zoonotic pathogens that can sporadically infect humans. The adult Dirofilaria nematode reproduces sexually in vertebrate hosts, primarily dogs and cats, and occasionally humans. Microfilariae (first-stage larvae) are ingested by intermediate hosts (e.g., mosquitoes or fleas) during blood feeding. Inside the vector, the larvae develop and mature into the infective third-stage larvae, which are transmitted to vertebrate hosts during subsequent blood meals [9]. The most common species infecting humans include D. immitis (a parasite of dogs), D. tenuis (a parasite of raccoons), and D. repens (a parasite of cats and dogs) [3]. Interactions between these pathogenic species and the endosymbiont bacterium Wolbachia may increase the likelihood of human infections [5, 10]. Risk factors for human infection include contact with wildlife, particularly cats and dogs, as well as environmental factors such as tropical climates, which enhance the feeding and breeding cycles of mosquito vectors, thereby increasing the pool of available filarial larvae [11]. However, in some cases, no clear risk factors can be identified from the patient’s history [6, 7]. In this case, the patient had prolonged exposure (approximately 3 months) to dogs on a village farm with the presence of fresh water and a high-humidity climate. These environmental conditions may have enhanced the feeding and breeding cycles of potential vector species (e.g., mosquitoes), thereby increasing the pool of available filarial larvae and the risk of disease transmission [12, 13].

Dirofilariasis can affect various parts of the human body, including the lungs, subcutaneous tissue, eyes and adnexa, heart, and testicles [3]. Ocular dirofilariasis is rare and can involve any structure of the eyes and adnexa [7]. The disease may mimic an abscess or soft tissue mass in ocular adnexa, necessitating definitive diagnosis by excision of the lesions and pathological evaluation [14]. In a review by Camacho et al. the most common sites of ocular dirofilariasis involvement were found to be the subconjunctiva, eyelid, orbit, anterior chamber, vitreous, and chorioretinal areas, respectively [6]. Similarly, in Iran, most ocular dirofilariasis cases have been reported with subconjunctival involvement, typically caused by D. immitis and D. repens [15,16,17,18]. In most cases, the disease does not lead to serious complications, but there are reports where untreated cases have progressed to rapid-onset orbital cellulitis or extensive chorioretinal damage, resulting in significant vision impairment [14]. Our patient, consistent with most reports, presented with subconjunctival involvement by D. immitis, the most commonly reported site of ocular involvement.

Previous studies have shown that only adult or degenerated immature juvenile nematodes infect humans, with microfilariae rarely reproducing in human hosts [3]. Camacho et al. reported a case involving an adult nematode behind the subconjunctival tissue [6]. Kalogeropoulos et al. and Mahesh et al. documented cases with immature nematodes in the eye and adnexa [7, 14]. In our case, we detected sections of a degenerated immature female Dirofilaria larva in the subconjunctival tissue. The infection triggered an inflammatory granulomatous reaction, characterized by infiltrating neutrophils, eosinophils, and foreign body giant cells. Chronic infection may further lead to granulomatous responses with calcification or abscess formation [6]. Similar pathological findings were observed in our case.

While eosinophilia and elevated inflammatory markers may suggest a parasitic infection and provide clues for residual disease after surgical excision, routine laboratory investigations are not typically recommended. Previous studies have indicated that systemic eosinophilia is observed in only up to 20% of humans infected by Dirofilaria [3, 6, 19].

A definitive diagnosis of dirofilariasis can only be made through surgical removal of the lesion followed by pathological assessment [7]. Identifying the parasite requires histological and morphological examination of the worm, with attention to its large muscle cells, wide lateral chords, and thick laminated cuticles. Species identification is achieved by examining the mature worm under a microscope. When only histopathological specimens are available, features such as the size and characteristics of the body wall—including cuticle thickness, structure, ridges, the number of lateral chords, and muscle cell type—can aid in differentiating the species in cases of deep tissue involvement [3, 6, 10, 20, 21]. PCR-based DNA analysis can also provide an accurate diagnosis when standard morphological assessment is not feasible due to inadequate worm conservation [20].

Prompt and complete surgical excision of the nematode is considered the treatment of choice and is usually curative [6, 7]. Due to the solitary nature of Dirofilaria and its lack of reproductive activity in humans, antihelminthic drugs are generally ineffective [22]. Therefore, systemic therapy with antinematodal drugs is not routinely required, though some reports have employed these drugs, including ivermectin, following surgical excision to reduce the chance of recurrence, particularly when residual disease or microfilaremia is suspected [3, 6]. Furthermore, oral ivermectin is recommended in cases of suspected systemic infection [18]. In our case, we treated the patient with ivermectin for two weeks following excision of the subconjunctival lesionto reduce the chance of recurrence.

In conclusion, ocular dirofilariasis is a rare condition with diverse and atypical presentations across various ocular and adnexal structures. Prompt and complete excision of the lesion, accompanied by meticulous histopathological evaluation, is essential for guiding appropriate management and ensuring favorable outcomes. Minimizing risk factors, such as avoiding exposure to wildlife and refraining from swimming in untreated freshwater, may help reduce the likelihood of infection.