Thelazia californiensis

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Thelazia californiensis Price, 1930

(Figure 4-48)

ETYMOLOGY: Thelazia for Dr. Thelaz; californiensis for the location from where the specimens were collected.


HISTORY: Allerton (1929) reported on a Thelazia spp. from the eye of a dog collected in the area of Los Angeles, CA. The worms in this report were identified by Dr. Ackert as Thelaziacallipaeda. In 1930, Price recognized the worm as a new species and gave it the name Thelaziacaliforniensis. Most reports have been made from dogs.

GEOGRAPHIC LOCATION: Parmelee et al. (1956) presented the geographical distribution of this parasite in California. Douglas (1939) reported that the cat could serve as a host of this parasite. Cases from humans in Utah have also been reported (Doezie et al., 1996)

LOCATION IN HOST: These worms are parasites of the orbits of the eye, being found on the conjunctiva and under the lids and nictitating membrane.

PARASITE IDENTIFICATION: These are white to cream-colored worms. The males are 12 to 13 mm long with dissimilar spicules, the longer being 1.5 to 1.7 mm long and the shorter being 0.15 to 0.19 mm long. The females are 15 to 17 mm long. The vulva of the female is located behind the esophageal intestinal junction and is 0.8 to 1 mm from the anterior end (Fig. 4-48). The eggs in the uterus measure 52 μm by 29 μm. The species differes from Thelaziacallipaeda in that in the latter, the vulva is located anterior to the junction of the esophagus and intestine.

LIFE CYCLE: Burnett et al (1957) reported on the life cycle of Thelaziacaliforniensis. Sheathed larvae from worms were fed to various species of flies, and developmental stages of the worm were recovered from species of the muscoid fly, Fanniacanicularis. These authors also collected similar larval forms from Fannia benjamini collected near San Bernadino, CA, and felt that they represented natural infections of the flies with this parasite. When the fly feeds on fluids around the eye, the larvae which have migrated to the mouthparts are deposited, and they enter the eye to begin development in their vertebrate host. Weinmann et al. (1974) have shown that the vector of Thelaziacaliforniensis is actually a new species of Fannia. This fly was described as Fanniathelaziae by Turner (1976).


TREATMENT: Treatment would most typically be by the application of local anesthetic and the careful removal of the worms from the eye.

EPIZOOTIOLOGY: There is very little information on prevalence of this parasite and no information as to whether it is transmitted seasonally.

HAZARDS TO OTHER ANIMALS: Other known hosts include the dog, coyote, bear, jack rabbits, and silver foxes (Burnett and Wagner, 1958) . These animals would be infected from larvae deposited by flies, so their would be no direct transmission between cats and other animals without flies being present.

HAZARD TO HUMANS: There have been a number of human cases of infection with this worm published. Cases in humans have come from the Sierra Nevada and Siskiou mountains, the Mojave Desert, and from the Rocky Mountains in Utah. Lee and Parmelee (1958) reported on 6 human cases, including summarizing the three earlier cases rteported by Kofoid and Williams (1935), Hosford et al. (1942), and Friedmann (1951). Knierim and Jack (1975) reported on a case in a pathology resident who recalled contact with a small fly or gnat while riding on a motorcycle. Kirschner et al. (1990) reported a case from a woman who reported symptoms beginning two weeks after getting a small fly in the eye while hiking in the Sierra Nevada foothills. Doezie et al (1996) reported on two cases from the eyes of patients in the Rocky Mountains of Utah.

CONTROL/PREVENTION: Control would be by preventing the flies from feeding around the eyes of cats. There is no information as to whether monthly ivermectin for cats would prevent infections with this worm; however, because of the external site of development, it is likely that this would not be effective.


Allerton 1929. Worm parasites on conjunctiva in dog. N Amer Vet 10:56.

Burnett HS, Parmelee WE, Lee RD, Wagner ED. 1957. Observations on the life cycle of Thelaziacaliforniensis Price, 1930. J Parasitol 43:433

Burnett HS, Wagner ED. 1958. Two new definitive hosts for the eye worm, Thelaziacaliforniensis Price, 1930. J Parasitol 44:502.

Doezie A.M., Kucius R.W., Aldeen W., hale D., Sith D.R., Mamalis N. 1996. Thelazia californiensis conjuntival infestation. Ophthal Surg Lasers 27:716-719.

Douglas JR. 1939. The domestic cat, a new host for Thelaziacaliforniensis Price, 1930 (Nematoda: Thelaziidae). Proc Helm Soc Wash 6:104.

Freidman M. 1951. Thelaziasis der Conjunctiva, ein Nachtrat. Ophthalmologica 122:252-254.

Kofoid CA, Williams OL. 1935. The nematode Thelaziacaliforniensis as a parasite of the eye of man in California. Arch Ophthalmol 13:176-180.

Kofoid CA, Williams OL, Veale NC. 1937. Thelaziacaliforniensis, a nematode eye worm of dog and man, with a review of the Thelazias of domestic animals. Univ Calif Publ Zool 41:225-234.

Hosford GN, Stewart MA, Sugarman EI. 1942. Eye worm (Thelaziacaliforniensis) infection in man. Arch Ophthalmol 27:1165-1170.

Lee RD, Parmlee WE. 1958. Thelaziasis in man. Am J Trop Med Hyg 7:427-428.

Parmelee WE, Lee RD, Wagner ED, Burnett HS. 1956. A survey of Thelazia californienesis, a mammalian eye worm, with new locality records. JAVMA 129:325-327.

Price EW. 1930. A new nematode parasitic in the eyes of dogs in the United States. J Parasitol 17:112-113.

Turner WJ. 1976. Fanniathelaziae, a new species of eye-frequenting fly of the benjamini group from California and description of F. conspicua female (Diptera: Muscidae). Pan-Pacific Entomol 52:234-241.

Weinmann CJ, Anderson JR, Rubtzoff P, Connolly G, Longhurst WM. 1974. Eyeworms and face flies in California. Cal Agric 28:4-5


Figure 4-48.Thelazia sp. View of the anterior end showing the anterior of the esophagus, the striations in the cuticle, and the anteriorly projecting uterus.

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