After obtaining the results of the first histologic evaluation (i.e. the worm was Thelazia species), we treated the dog with topical ivermectin using the injectable formulation (10 mg/ml; 1 drop in the left eye twice
weekly for six weeks).3 We gave the injectable product because its pH was compatible with ophthalmic use. We chose topical therapy over systemic therapy
because this dog was a Border collie mix and we were concerned about systemic toxicity.4 A six-week course of therapy was directed at killing the Thelazia species microfilariae based on their prepatent periods and the susceptibility of the larvae at different stages of development.3 To control the inflammation that would accompany parasite death, we treated the dog with prednisone (5 mg orally b.i.d.).
We prescribed amoxicillin trihydrate-clavulanate potassium (250 mg orally b.i.d.) for 14 days and continued the topical neomycin-polymyxin
B-dexamethasone ointment for a minimum of 14 days.
Figure 3 : Microfilariae (arrows) free within the granulomatous tissue of the mass (hematoxylin-eosin stain; 100X).
The dog returned to California before the definitive diagnosis of onchocerciasis was made, so follow-up examinations were
with the original ophthalmologist. Two weeks after the surgery, the surgical site had healed well, and the owners were instructed
to continue the topical neomycin-polymyxin B-dexamethasone ointment and ivermectin as instructed by us.
When onchocerciasis was definitively diagnosed, melarsomine (2.5 mg/kg every 24 hours for two days) was administered as an
adulticide for this nematode. Subcutaneous ivermectin (50 µg/kg) was added one month later.5
One month after receiving melarsomine therapy, the dog developed a golf ball-sized swelling below the left eye and was treated
with carprofen (2.2 mg/kg b.i.d. for seven days). The swelling resolved three days after beginning the anti-inflammatory therapy.
At follow-up examinations, the dog continued to have moderate eyelid swelling that was suspected to be due to the death of
the worms. However, the dog was fairly comfortable, and no masses recurred.
Figure 4 : The left image shows thick cuticle overlying prominent bands of coelomyarian musculature (large arrowhead) separated
by thin lateral cords (small arrowheads) around two large ovoid uteri containing ova and a small cross section of intestine
(small arrow) (hematoxylin-eosin stain; 100X). The right image shows prominent, regularly spaced cuticular ridges (arrows)
over the external surface of the cuticle, characteristic of Onchocerca species (hematoxylin-eosin stain; 400X).
Onchocercosis is a disease caused by infection with various species of filarial nematodes of the genus Onchocerca. Although more commonly seen in horses and ungulates, it has been reported in dogs worldwide. Onchocerca species have also been incriminated as a factor in the disease in people known as river blindness, which affects more than 17 million people worldwide and in 1993 caused vision loss in 270,000 people.6