**1. Introduction**

The ocular parasitic infections (OPI) are considered significant causes of ocular pathologies worldwide [1]. The common protozoal parasites primarily infecting the ocular tissue(s) are *Acanthamoeba* species and *Toxoplasma gondii* [2–7]. In addition, case studies of eye diseases caused by *Leishmania, Trypanosoma cruzi, Entamoeba histolytica, Hartmannella, Plasmodium falciparum, Microsporidia* and *Giardia lamblia* have been rarely reported [1, 8, 9]. Among the helminths, ocular infections are caused primarily by nematode parasites (*Onchocerca volvulus,*

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*Loa loa, Toxocara canis and Toxocara cati*) [1, 8, 10–12]. In addition, case studies of ocular infections caused by other nematodes (*Angiostrongylus cantonensis, Dirofilaria repens*, *Trichinella spiralis, Thelazia callipaeda, Baylisascaris procyonis, Wuchereria bancrofti* and *B. malayi), c*estodes (*T.soli‐ um cysticercus*, *Echinococcus granulosus,* and *Multiceps multiceps* larvae) and trematodes (*Fasciola hepatica and Schistosoma* species) have been reported from different geographical areas [1, 8, 13–15]. The ectoparasites infecting the eye include larvae of flies [16] (*Oestrus ovis*, *Rhinoestrus purpureus, Dermatobia hominis, Chrysomia bezziana, Lucilia* spp., *Cuterebra, Hypoderma, Cochlio‐ myia, Wohlfahrtia, Gastrophilus*), *Phthirus pubis*, hard and soft ticks (belonging to class Arach‐ nida) [1, 17]. Ocular pentastomiasis caused by the larval stage of Pentastomida, the crustaceanrelated parasites, is reported to cause permanent loss of vision due to the retinal detachment or lens subluxation [18]. Further, with the advent of HIV/AIDS (human immunodeficiency virus/acquired immune deficiency syndrome), few ocular infections have also been reported in HIV-infected patients [19, 20].

Ocular parasitic infections have been widely reported from different geographical areas (Table 1), mainly depending on the endemicity of the parasite(s). The prevalence depends primarily on the geographical distribution of the parasite, socioeconomic environment and immune status of the patient. The common modes of infection are direct contact (blepharoconjunctivitis caused by *Leishmania, Acanthamoeba* keratitis, microsporidial infections, infestation caused by lice and mites) [21–23], through blood stream (Toxoplasma chorioretinitis, retinal involvement in malaria, uveitis caused by *Toxocara)* [1, 23, 24], congenital transmission (Toxoplasmosis) and zoonotic transmission (primarily infectious diseases of animals that can naturally be trans‐ mitted to humans) [25]. In addition, few of the helminths that may lead to ocular infection are transmitted by vectors (onchocerciasis, dirofilariasis and thelaziasis), consumption of conta‐ minated food (sparganosis, trichinellosis) and indirectly from the environment (fascioliasis, ascariasis and echinococcosis).

Adult and/or larval stages of the parasites may reside in human ocular tissues externally or in the ocular globe. The clinical symptoms and signs vary, depending on the etiological agent and the ocular tissue/part involved. However, local defense mechanisms and host immune responses play role in establishing the infection. The pathology in the eye can occur due to direct damage by the infecting pathogen, indirectly by toxic products, immune mediated or ectopic localization by ectoparasites. The clinical diagnosis usually mimics other pathologies due to numerous etiologies both infectious and non-infectious, which can cause conjunctivitis, keratitis, uveitis and endopthalmitis [26]. Thus, a high index of clinician suspicion is required for infective parasite etiology in patients having inflammation in the eye. In addition, eye can be involved in various systemic disorders and thorough ocular examination along with history of travel to the endemic area, risk factors and other associated medical illness that help in establishing the preliminary diagnosis. However, confirmatory diagnosis is usually achieved by direct demonstration of parasite in clinical samples and/or pathological changes observed by either slit lamp or biopsy examination [1, 8, 27, 28]. The antigen and antibody detection in ocular fluids and/or serum usually substantiates the clinical diagnosis in few parasitic infections (Toxoplasmosis, malaria, leishmaniasis, ocular gnathostomiasis, cysticercosis, toxocariasis, echinococcosis) [1, 10, 29, 30]. Molecular techniques including detection of parasite DNA by polymerase chain reaction (PCR) have added new dimensions in the diagnosis and species identification [31–36]. The treatment of choice is mostly surgical excision, while in few infections, medical treatment is usually advised either in conjunction with surgical procedure (onchocerciasis, dirofilariasis [37], cysticercosis [38], echinococcosis [39], myiasis, infections due to ticks and mites) or for inoperable patients. Although surgical excision is usually reserved for worms that are large, it is also recommended for space-occupying lesions of the orbit. Drug resistance is posing problem for the effective medical treatment, thus necessitating the discovery of new antiparasitic drugs [32]. Prevention and control measures differ in various infections and usually include proper health education and awareness of various risk factors. The various experimental animal models for few of the ocular infections have been successfully established to study the pathogenic mechanisms, drug efficacy and local immune responses [40, 41].

Although issues mainly are the timely diagnosis and treatment, yet many challenges need to be considered/addressed.
