Thelazia callipaeda
Encyclopedia
Thelazia callipaeda is a parasitic nematode
, and the most common cause of "thelaziasis
" (or "eyeworm" infestation) in humans, dogs and cats. It was first discovered in the eyes of a dog in China in 1910. By 2000, over 250 human cases had been reported in the medical literature.
of T. callipaeda include the Wolf (Canis lupus), Raccoon dog
(Nyctereutes procyonoides), Red Fox
(Vulpes fulva), and European Rabbit
(Oryctolagus cuniculus). This species has been found in China, France, Germany, India, Indonesia, Italy, Japan, Korea, the Netherlands, Russia, Switzerland, Taiwan and Thailand.
Two intermediate hosts
have been identified so far: Amiota (Phortica) variegata (Diptera: Drosophilidae) in Europe, and Phortica okadai in China, which feed on tears of humans and carnivores. Some data suggests that only the males of A. (P.) variegata carry Thelazia callipaeda larvae. This is noteworthy because in all other known cases of blood-feeding flies which transmit parasites, the parasites are carried by the females.
while the female is in the tissues in and around the eye of the definitive host
. The female deposits these larvae, which are still enclosed in the egg membranes, in the tears (lacrymal secretions) of the host. When a tear-feeding fly (intermediate host
) feeds, it ingests the T. callipaeda larvae. Once inside the fly, the L1 larvae "hatch" from the egg membrane and penetrate the gut wall. They remain in the hemocoel (the fly's circulatory system) for 2 days, and then invade either the fat body or testes of the flies. In these tissues, the larvae develop into third stage larvae (L3). The L3 migrates to the head of the fly, and is released in or near the eye of a new host mammal when the fly feeds again. Once in the eye, eyelid, tear glands, or tear ducts of the mammalian host, the L3 larvae develop through the L4 larval stage and into adults in about 1 month. The seasonal timing of L1 and L4 larvae in the lacrymal (tear) secretions of naturally infested dogs in Italy was found to coincide with the activity of the fly vectors.
, excessive watering (lacrimation), visual impairment, and ulcers or scarring of the cornea. In some cases, the only symptom is the worm obscuring the host's vision as a "floater
".
Diagnosis is made by finding the adult worms in the eye or surrounding tissues. Human cases are treated by simply removing the worms. In canines, topical imidacloprid
with moxidectin
, or milbemycin oxime
(Interceptor) have been recommended.
Nematode
The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...
, and the most common cause of "thelaziasis
Thelaziasis
Thelaziasis is the term for infestation with parasitic nematodes of the genus Thelazia. The adults of all Thelazia species discovered so far inhabit the eyes and associated tissues of various mammal and bird hosts, including humans...
" (or "eyeworm" infestation) in humans, dogs and cats. It was first discovered in the eyes of a dog in China in 1910. By 2000, over 250 human cases had been reported in the medical literature.
Hosts
In addition to humans, cats and dogs, definitive hostsParasitic life cycles
Parasite life cycles can take a variety of forms, all involving the exploitation of one or more hosts. Those that must infect more than one host species to complete their life cycles are said to have complex or indirect life cycles, while those that infect a single species have direct life...
of T. callipaeda include the Wolf (Canis lupus), Raccoon dog
Raccoon Dog
The raccoon dog , also known as the magnut or tanuki, is a canid indigenous to east Asia. It is the only extant species in the genus Nyctereutes...
(Nyctereutes procyonoides), Red Fox
Red Fox
The red fox is the largest of the true foxes, as well as being the most geographically spread member of the Carnivora, being distributed across the entire northern hemisphere from the Arctic Circle to North Africa, Central America, and the steppes of Asia...
(Vulpes fulva), and European Rabbit
European Rabbit
The European Rabbit or Common Rabbit is a species of rabbit native to south west Europe and north west Africa . It has been widely introduced elsewhere often with devastating effects on local biodiversity...
(Oryctolagus cuniculus). This species has been found in China, France, Germany, India, Indonesia, Italy, Japan, Korea, the Netherlands, Russia, Switzerland, Taiwan and Thailand.
Two intermediate hosts
Parasitic life cycles
Parasite life cycles can take a variety of forms, all involving the exploitation of one or more hosts. Those that must infect more than one host species to complete their life cycles are said to have complex or indirect life cycles, while those that infect a single species have direct life...
have been identified so far: Amiota (Phortica) variegata (Diptera: Drosophilidae) in Europe, and Phortica okadai in China, which feed on tears of humans and carnivores. Some data suggests that only the males of A. (P.) variegata carry Thelazia callipaeda larvae. This is noteworthy because in all other known cases of blood-feeding flies which transmit parasites, the parasites are carried by the females.
Life cycle
The eggs of Thelazia callipaeda develop into first stage larvae (L1), in uteroIn utero
In utero is a Latin term literally meaning "in the womb". In biology, the phrase describes the state of an embryo or fetus. In legal contexts, the phrase is used to refer to unborn children. Under common law, unborn children are still considered to exist for property transfer purposes.-See also:*...
while the female is in the tissues in and around the eye of the definitive host
Parasitic life cycles
Parasite life cycles can take a variety of forms, all involving the exploitation of one or more hosts. Those that must infect more than one host species to complete their life cycles are said to have complex or indirect life cycles, while those that infect a single species have direct life...
. The female deposits these larvae, which are still enclosed in the egg membranes, in the tears (lacrymal secretions) of the host. When a tear-feeding fly (intermediate host
Parasitic life cycles
Parasite life cycles can take a variety of forms, all involving the exploitation of one or more hosts. Those that must infect more than one host species to complete their life cycles are said to have complex or indirect life cycles, while those that infect a single species have direct life...
) feeds, it ingests the T. callipaeda larvae. Once inside the fly, the L1 larvae "hatch" from the egg membrane and penetrate the gut wall. They remain in the hemocoel (the fly's circulatory system) for 2 days, and then invade either the fat body or testes of the flies. In these tissues, the larvae develop into third stage larvae (L3). The L3 migrates to the head of the fly, and is released in or near the eye of a new host mammal when the fly feeds again. Once in the eye, eyelid, tear glands, or tear ducts of the mammalian host, the L3 larvae develop through the L4 larval stage and into adults in about 1 month. The seasonal timing of L1 and L4 larvae in the lacrymal (tear) secretions of naturally infested dogs in Italy was found to coincide with the activity of the fly vectors.
Symptoms, diagnosis and treatment
Symptoms of T. callipaeda infestation include conjunctivitisConjunctivitis
Conjunctivitis refers to inflammation of the conjunctiva...
, excessive watering (lacrimation), visual impairment, and ulcers or scarring of the cornea. In some cases, the only symptom is the worm obscuring the host's vision as a "floater
Floater
Floaters are deposits of various size, shape, consistency, refractive index, and motility within the eye's vitreous humour, which is normally transparent. At young age the vitreous is perfectly transparent, but during life imperfections gradually develop. The common type of floater, which is...
".
Diagnosis is made by finding the adult worms in the eye or surrounding tissues. Human cases are treated by simply removing the worms. In canines, topical imidacloprid
Imidacloprid
Imidacloprid is a nicotine-based, systemic insecticide, which acts as a neurotoxin and belongs to a class of chemicals called the neonicotinoids. Although it is now off patent, the primary manufacturer of this chemical is Bayer CropScience,...
with moxidectin
Moxidectin
Moxidectin is a parasiticide used for the prevention and control of heartworm and intestinal worms, and can be found in treatments prescribed for animals such as dogs, cats, horses, cattle and sheep. It is also used for the prevention of red mite in caged birds...
, or milbemycin oxime
Milbemycin oxime
Milbemycin oxime is a veterinary drug from the group of milbemycins, used as a broad spectrum antiparasitic. It is active against worms , insects and mites ....
(Interceptor) have been recommended.