(Thelazia spp.)

Eyeworms are spirurid nematodes that infest the eyes of several mammals, including dogs and cats. They are zoonotic.

Parasite species: Thelazia californiensis, Thelazia callipaeda
Common name: Eyeworms
Hosts: Wild and domestic mammals, including dogs and cats
Pre-patent period: 2 weeks
Location in the host: Conjunctiva and under the lids and nictitating membrane
Distribution: North America, Europe and Asia
Transmission route: By fruit flies (P. variegata) or by muscoid flies (Fannia spp.)
Zoonotic: Yes


Thelazia callipaeda is present in Asia and Europe, whereas T. californiensis is restricted to western North America.

Clinical signs

Thelazia infections in cats are usually asymptomatic. Clinical signs in cats may include blepharospasm and epiphora of the eye.

Figure 1. Thelazia callipaeda adult worms in the eye of a dog (Image credit: Dr. G. D’Amico)


Considering the external location of the eyeworms, the diagnosis can be confirmed by finding the worms during ocular examination (Fig. 1).


Thelazia infections are usually treated by mechanically removing the worms from the eye. An oral formulation containing milbemycin oxime (2 mg/kg) and praziquantel (5 mg/kg) showed therapeutic efficacies of 53.3% and 73.3% after one or two treatments, respectively [1]. The application of the spot-on formulation moxidectin 2.5% and imidacloprid 10% was 100% effective in the treatment of thelaziosis in dogs and may have similar efficacy in cats [2]

Prevention and Control

Control may be achieved by preventing flies from feeding around the eyes of cats. In dogs, the monthly application of a spot-on formulation containing 10% imidacloprid and 2.5% moxidectin was shown to be highly effective in preventing T. callipaeda infection [3], although similar field-studies have not been carried out in cats.

Public health considerations

Both T. californiensis and T. callipaeda are zoonotic.


[1] Motta B, Schnyder M, Basano FS, Nägeli F, Nägeli C, Schiessl B, Mallia E, Lia RP, Dantas-Torres F, Otranto D. Therapeutic efficacy of milbemycin oxime/praziquantel oral formulation (Milbemax®) against Thelazia callipaeda in naturally infested dogs and cats. Parasit Vectors. 2012;5:85.

[2] Otranto D, Colella V, Crescenzo G, Solari Basano F, Nazzari R, Capelli G, Petry G, Schaper R, Pollmeier M, Mallia E, Dantas-Torres F, Lia RP. Efficacy of moxidectin 2.5% and imidacloprid 10% in the treatment of ocular thelaziosis by Thelazia callipaeda in naturally infected dogs. Vet Parasitol. 2016;227:118-121.

[3] Lechat C, Siméon N, Pennant O, Desquilbet L, Chahory S, Le Sueur C, Guillot J. Comparative evaluation of the prophylactic activity of a slow-release insecticide collar and a moxidectin spot-on formulation against Thelazia callipaeda infection in naturally exposed dogs in France. Parasit Vectors. 2015;8:93.