Although parasitic eye diseases in poultry are less common than bacterial or viral eye diseases, they are caused by a wide variety of pathogens. The resulting lesions range from mild conjunctivitis to blindness and even death, and their harm should not be overlooked. Reported causative agents of parasitic eye diseases in poultry include protozoa (such as Toxoplasma, Cryptosporidium, Leucocytozoon, Sarcocystis, and Plasmodium), nematodes (Oxyspirura spp.), and trematodes (Philophthalmus spp.). Various pathogens can colonize structures in or around the eye and cause complex clinical syndromes.
Toxoplasmosis
Among reports of toxoplasmosis in poultry, chickens are the most frequently reported, but the host range extends well beyond them. Japanese quail (Dubey et al., 1994a), woodcock (Dubey et al., 1993b), rock partridges and chukar partridges (Dubey et al., 1995; Sedlák et al., 2000) have all been shown to be susceptible. By contrast, pheasants (Dubey et al., 1994b), turkeys (Dubey et al., 1993a; Sedlák et al., 2000), guinea fowl, and American partridges (Sedlák et al., 2000) show relative resistance. Notably, toxoplasmosis has also been reported in domestic ducklings aged 15–20 days, with clinical signs including eyelid adhesion and neurological signs, and a mortality rate as high as 50% (Bochringer et al., 1962). The ocular signs of toxoplasmosis in chickens are distinctive: conjunctival edema, serous discharge, corneal opacity, and even blindness; grossly visible yellow necrotic foci may be present in the optic nerve and optic chiasm. Histologically, necrotic foci coexist with multinucleated giant cell granulomatous inflammation in the optic nerve and optic chiasm, with extensive lymphocytic infiltration and small numbers of plasma cells and macrophages. Toxoplasma cysts may be scattered in the iris, ciliary body, choroid, retina, pecten, and optic nerve, as well as in the periorbital connective tissue and muscles. Some cases may result in permanent blindness due to iridocyclitis and secondary cataracts.
Cryptosporidiosis
Cryptosporidiosis is an extremely common infection in chickens and turkeys, caused by coccidian protozoa of the genus Cryptosporidium that parasitize the respiratory and digestive tracts. Although ocular involvement has been reported infrequently, conjunctival colonization does occur in chickens, whereas turkeys are resistant to ocular cryptosporidiosis (Lindsay et al., 1967). Cryptosporidium colonization of the conjunctiva in domestic geese, detected by in situ hybridization, is considered quite common, but is often asymptomatic, and oocyst detection rates vary considerably among studies. However, Cryptosporidium-associated conjunctivitis has been clearly documented in pheasants, ducks, quail, and peafowl. Most cases present with ocular and nasal discharge; bilateral corneal opacity and conjunctival folds extending beyond the eyelid margins may also be seen in peafowl chicks. The microscopic lesions span a broad spectrum: mild cases show slight subepithelial heterophilic infiltration, while severe cases show hyperplasia and thickening of the conjunctival epithelium, with diffuse infiltration of the lamina propria by numerous heterophils. The basophilic spherical bodies attached to the conjunctival epithelial surface (diameter 3—5 μm) are the parasites. Following infection with Cryptosporidium baileyi, red grouse may lose the ability to fly, in addition to developing blepharitis and infraorbital sinusitis, highlighting the cumulative effects of multisystem damage caused by concurrent respiratory infections.
Leukocytozoonosis and malaria
North American waterfowl leukocytozoonosis caused by Leucocytozoon simondi is most common in early autumn. Its megaloschizonts (up to 250 μm) are found not only in the spleen, liver, heart, and brain, but also in the eyes of chickens and ducks. These large schizonts may occur in the cornea, iris, ciliary body, choroid, optic nerve, pecten, conjunctiva, and periocular connective tissue. Their presence, whether or not accompanied by surrounding inflammation, indicates a potential risk of ocular involvement. Plasmodium has been reported in association with severe palpebral conjunctivitis in chicks aged 12—14 days; the disease is also more readily induced when the diet lacks pantothenic acid and biotin, revealing an association between nutritional status and infection severity. Sarcocysts are commonly found in the extraocular muscles of ducks, with or without an accompanying inflammatory response; Sarcocystis falciformis, meanwhile, is more common in parrots. In addition, Haemoproteus may be associated with severe myositis of the periocular muscles in quail, but its pathogenicity remains to be further verified.
Oxyspirurosis (eyeworm disease)
Among worm pathogens, nematodes of the genus *Oxyspirura* can infect more than 100 species of wild and domestic poultry. *Oxyspirura mansoni* and *Oxyspirura petrowi* have been confirmed to infect chickens, turkeys, pheasants, guinea fowl, grouse, and quail. These nematodes parasitize beneath the eyelids and the third eyelid, in the conjunctival sac, and in the nasolacrimal duct, and can further affect the lacrimal and Harderian glands (potentially causing atrophy), the cornea (inducing keratitis), and even the orbit. Clinical signs include ocular irritation, photophobia, and tearing; severe cases may progress to purulent endophthalmitis and blindness, with secondary bacterial infections often aggravating the condition. *Oxyspirura petrowi* is especially common in wild quail in the Central High Plains of the United States and is considered one potential factor in the decline of quail populations in the region. *Oxyspirura mansoni* uses cockroaches as intermediate hosts, while turkeys, ducks, grouse, guinea fowl, peafowl, and pigeons are its definitive hosts. This transmission cycle provides an opportunity for control.
Ocular trematodiasis
Trematodes of the family Philophthalmidae may be found in the eyes of chickens, peafowl, ducks, and geese, and are more common in Asian countries. *Philophthalmus gralli* caused conjunctivitis in an 5-month-old ostrich in Florida, USA; the affected ostrich stood with its eyes closed and continued to lose weight (Greve and Harrison, 1980). Most flukes are located between the nictitating membrane and the outer eyelid, while a few lie directly beneath the nictitating membrane, rubbing against the cornea and conjunctiva. Mechanical irritation and inflammation caused by the parasites result in severe tearing and a burning sensation, followed by blepharitis, uveitis, and sinus involvement. Topical antibiotics combined with corticosteroids may temporarily relieve symptoms, but recurrence is common after treatment is discontinued. As in other birds (such as ostriches; Mukaratirwa et al., 2008), systemic anthelmintics are often ineffective because the flukes are firmly attached to the ocular surface and are not adequately exposed to the bloodstream. Only manual removal of the parasites produces rapid results; long-term control requires managing the breeding of gastropod intermediate hosts.
In summary, the spectrum of ocular parasitic diseases in poultry is broad, encompassing pathogens from multiple taxonomic groups and a wide range of clinical presentations. Diagnosis requires the combined consideration of clinical findings, microscopy, and epidemiological clues. In poultry production, understanding common local pathogens and the ecology of their intermediate hosts, and developing targeted monitoring and intervention strategies, are key to reducing economic losses and safeguarding animal welfare.