Toxocara cati–Induced Ocular Toxocariasis

Toxocara canis and Toxocara cati are nematode members of the family Ascarididae and known to cause toxocariasis in dogs and cats,¹ as well as ocular toxocariasis in humans. Toxocara canis was first successfully recovered from a human eye in 1950² and has been recognized as a major cause of ocular toxocariasis in humans. We report a case in which T cati was possibly involved in the development of ocular toxocariasis.

A 38-year-old woman, with a history of keeping cats, complained of decreased visual acuity in her right eye. Corrected visual acuity was 20/200 OD. We found a yellowish, creamy lesion of two-thirds disc diameter in the inferior region of the macula. (Figure 1). Standard enzyme-linked immunosorbent assay (ELISA) using excretory–secretory products of second-stage larvae of T canis showed that antibody production was marginal. Despite this inconclusive result, we suspected ocular toxocariasis. If T cati was involved, antibody titer could be weak owing to an incomplete cross-reactivity between T canis and T cati. We conducted ELISA of the patient's serum samples using antigens of adult T cati, adult T canis, and second-stage larvae of T canis (Figure 2).

In Figure 2, our case corresponds to patient 2, who showed an extraordinarily high antibody titer against adult T cati as compared with patients 3 and 4. Patients 3 and 4 were diagnosed as having ocular toxocariasis, shown by ELISA using T canis larvae and typical funduscopic features. The patient's several cats also provided evidence of ocular toxocariasis possibly caused by T cati. Though we had been unable to detect the presence of ova of T cati in the soil of cats, an unusually high titer of antibody against T cati prompted us to initiate therapy. Visual acuity in our patient returned to 20/20 OD following corticosteroid treatment.

Clinical diagnosis of ocular toxocariasis depends on serological testing,¹ and ELISA has both high sensitivity and high specificity.³ Standard ELISA tests use the antigen prepared from T canis and show a high cross-reactivity between T canis and T cati,⁴ that is by no means incomplete. In this case, standard ELISA using T canis revealed that antibody titer was marginal. When using an antigen prepared from T cati, antibody titer was exceptionally high; consequently, we reached to a clinical diagnosis of ocular toxocariasis caused by T cati. Uga et al⁵ reported that cats are in closer contact to humans than dogs, underscoring the need to consider T cati in cases of ocular toxocariasis. Ophthalmologists should always be aware that ocular toxocariasis can be caused by T cati and consider conducting serological testing using T cati as a routine examination.

Corresponding author: Hidetoshi Kawashima, MD, Department of Ophthalmology, Jichi Medical School, Yakushiji, Minamikawachi, Kawachi-gun, Tochigi 329-0498, Japan (e-mail: hidemeak@jichi.ac.jp).