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Thelazia callipaeda (Thelaziosis) - Ocular parasitism in dogs and cats with risk of transmission to people: Microscopic exam; Molecular identification (PCR and sequencing).

 

Information 2020-01-22.


Thelazia callipaeda is a parasitic nematode, sometimes referred to as "eye worm" or "Oriental eye worm" for its traditional distribution in East Asia (China, North and South Korea, Japan, Indonesia, Thailand and India), where the infection is endemic This species is the most frequent cause of thelaziosis in humans, dogs and cats. It was first discovered in the eyes of a dog in China in 1910. By the year 2000, more than 250 human cases had been described in the medical literature. In Europe, cases of human thelaziosis have been described in Italy, Germany, France, Spain and, more recently, in Croatia and Serbia.

Adults of all Thelazia species discovered so far inhabit the eyes and associated tissues (such as eyelids and tear ducts) of various mammals and birds, including people. Although 16 species of Thelazia have been described, only three of them have been reported to infest people, and only seven are frequently reported in veterinary contexts. In people, dogs and cats, cases of Thelaziosis have been reported by Thelazia callipaeda (Asia, Europe) and occasionally by T. californiensis and T. gulosa (Western North America). Horses are infested by T. lacrymalis (worldwide) and, less frequently, by T. rhodesii (Africa, Asia, Europe). In cattle, T. gulosa (Asia, Europe, North America), T. rhodesii (Africa, Asia, Europe) and T. skrjabini (Europe, North America) are the main species involved. In camels, infestations of T. leesei have been reported in post-Soviet states and in India.

Thelazia callipaeda, belongs to the Kingdom Animalia, Phylum Nematoda, Order Rhabditida, Family Thelaziidae Family, Genus Thelazia.

Wild and domestic canids are considered the main definitive hosts for Thelazia callipaeda. Thelazia callipaeda infection has been described not only in dogs and cats, but also in red foxes (Vulpes vulpes), gray wolves (Canis lupus), beech martens (Tuesday foina), brown hares (Lepus europaeus) and wild cats (Felis silvestris). In the Iberian Peninsula, thelaziosis caused by T. callipaeda was detected in red foxes and wild rabbits.

In Spain, the first indigenous case of ocular thelaziosis was reported in 2010 in a dog that had spent a few weeks in the La Vera region (Cáceres). After other reported cases of canine Thelaziosis in this region, the geographical area is now considered endemic to canine Thelaziosis. The prevalences registered in dogs of La Vera and bordering areas (provinces of Salamanca, Ávila and Toledo) have been estimated at around 40%. In 2012, the first case of feline thelaziosis was detected in La Vera.

After the parasite spread between domestic and wild carnivores in Europe, the first human cases of this zoonotic disease in Italy and France were described. Although people are competent hosts, they generally act as accidental final hosts in which the third stage larvae can become adults, but without epidemiological effects on the transmission of the parasite. This is considered because people, unlike animals, report their presence and eliminate parasites, wth the interruption of transmission.

The geographical distribution of this infection is related to the fruit fly Phortica variegata (Drosophilidae: Steganinae). which acts as its vector and intermediate host, This fruit fly feeds on fruits and other vegetables in fermentation, although males show a zoophilic behavior. Phortica variegata and Phortica okadai have been described as vectors of T. callipaeda in Europe and China, respectively. Under experimental conditions, both males and females can act as vectors of T. callipaeda, but under natural conditions, it has only been observed that males transmit third stage infectious larvae (L3) to the definitive host in which the helminth is found adult. Although P. variegata is the main candidate vector for T. callipaeda in European countries, it is believed that other species such as Phortica semivirgo are also involved in the transmission of this eyeworm.

Thelazia callipaeda eggs become first stage larvae (L1), in the uterus of the females while they are in the ocular or periocular tissues of the definitive host. The females deposit the larvae, which are still found wrapped in the membranes of the egg, in the tears of the host. Flies feed on tears in the intermediate host, ingest the larvae of T. callipaeda. Once inside the fly, the L1 larvae "hatch" from the egg membrane and penetrate the intestinal wall of the flies and reach the internal cavity (hemocoel). There they remain for 2 days, and then invade the body or testicles of flies. In these tissues of the flies the larvae become third stage larvae (L3). The L3 larva migrates to the fly´s oral cavities and is released into or near a new host mammal when the fly feeds back into the eye of a host. Once in the eye, the eyelid, the lacrimal glands or the lacrimal ducts of the host, the L3 larvae develop through the larval stage L4 and become adults in about 1 month. In these places they can survive for more than a year. In the definitive host, Thelazia has been found in several tissues of the ocular orbit, including within the eyelids, in the lachrymal glands, the lacrimal ducts, in the nictitating membrane or in the eyeball.

Thelazia adults have filiform appearance and whitish coloration. Females are larger than males, measuring between 6 and 17 mm in length and 0.3-0.85 mm in diameter. Both are characterized by having an oral end without lips. They have a cuticle with transverse striations that cover the entire body, which gives them a serrated appearance.

In females, the vulva is located in the anterior region, and inside the uterus of the fertilized females the presence of larvae with a membranous covering can be observed. The distal end of the females, where the cloaca is located, is blunt and straight, unlike the males, which have five pairs of long postchlocal papillae in the ventral position and smaller papillae in the subventral position, and are characterized by a curvature at its caudal end, where the copulatory organ is located.

In animals and people, Thelazia infestation can be asymptomatic, although it often causes symptoms such as watery eyes (epiphora), conjunctivitis, corneal opacity, edema, photophobia, visual impairment or corneal ulcers (ulcerative keratitis). In some cases, the only symptom is the presence of the helminth that hinders the vision of the host. The clinical picture produced is mainly the result of mechanical damage in the conjunctival and corneal epithelium caused by the serrated cuticle of the helminth at the ocular level and by the presence of adult nematodes that inhabit the conjunctival sac.

The diagnosis is made by finding adult helminths in the eye or surrounding tissues, identifying it by morphological examination. Molecular methods (PCR, followed by sequencing of the cytochrome c oxidase 1-cox1-mitochondrial gene, can be used with adults or larvae for species confirmation and identification.

Thelazia is one of the few infections by nematodes that can be treated topically, because they live on the mucous surface. Human cases are treated simply by extracting helminths. Topical treatment with thiabendazole is lethal to helminths in human cases.

In the canids, topical imidacloprid with moxidectin or milbemycin oxime is recommended. The prognosis of canine ocular thelaziosis is excellent, as long as all helminths are removed and there are no complications such as the presence of deep ulcerations. Prevention against canine telaziosis can be done by monthly administration of a combination of milbemycin oxime and afoxolaner. The Merck Veterinary Manual recommends topical treatment of cattle, dogs and cats with organophosphates (such as ecotiopate iodide or isofluorophate) and systemic treatment with anthelmintics (such as ivermectin, levamisole and doramectin).

Because most of the Thelazia species, if not all, are transmitted by flies, the elimination of flies also reduces the spread of thelaziosis.

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