Blindness and ocular diseases represent one of the most traumatic events for human patients as they have the potential to severely impair both their quality of life and their psychological equilibrium. Although it is highly unusual, blindness has always been of great interest in human medicine. For example, the evaluation of the emotional and quality of life impacts in patients with some diseases causing blindness (e.g., macular degeneration) gave results similar to those found in diseases such as AIDS, chronic obstructive pulmonary disease, cardiac disorders and leukemia [1]. In addition, blindness has profound human and socio-economic consequences with high costs for the individual, and society, linked to lost productivity and rehabilitation estimated at $42 USD billion per year in 2000, and predicted to reach as high as $110 USD billion per year in 2020 [2].

There are many causes of blindness and those induced by parasitic agents (i.e., Protozoa, Helminths and Diptera) are of major public health concern in developed and developing countries. For example, eye disease caused by river blindness (Onchocerca volvulus), affects more than 17.7 million people inducing visual impairment and blindness elicited by microfilariae that migrate to the eyes after being released by female adult worms in the subcutaneous tissues [3]. Several parasites localize in human eyes as an effect of a specific neurotropism (e.g., Toxoplasma gondii in the foetuses), larval migration (e.g., ascarids, Dirofilaria spp., Trichinella spp.) and, in a few cases, as a primary localization being released directly into the eyes (e.g., Thelazia callipaeda eyeworm and some oestrid fly larvae causing myiasis) [4].

Helminths at the adult and/or larval stages may infect human ocular tissues externally (i.e., eyelids, conjunctiva sacs, subconjunctiva, and lachrymal glands) or the ocular globe (i.e., optical nerve, intravitreous retina, anterior and posterior chamber). Several parasitic helminths adapted a tropism for animal eyes and related tissues when migrating throughout the host body mainly during their immature stages. This is the case of ascarids and strongylids, causing ocular larva migrans, filarioid species, and larvae of Trichinella, as well of trematode and cestode parasites. Nonetheless, human ocular infestations by zoonotic helminths may also be caused by the parasitic adult stages as in the case of thelaziids (eye worm infestation) and filarioid species including those belonging to the genera Dirofilaria and Onchocerca[8–10]. Thus, ocular localization of helminths is mainly caused by aberrant migration in host tissues and, only in one case (i.e., T. callipaeda), by direct inoculation into the eyes. What is equally unclear is the route that most follow to gain entry into the eye. It is supposed that some migrate along and follow the optic nerve but others may enter the bloodstream and be carried to the eye in that manner; however, it is not known if these are the preferred or aberrant routes, or even which is the most common route that helminths follow to reach the eye. Once in the eye, larvae likely find it to be a more protected site from host immune responses, but it is not clear that a directed migration into the eye had occurred.

Over the last decade, parasitological knowledge has been considerably refined and enhanced by the use of sophisticated technologies and molecular tools, and by the interdisciplinary approach in many fields of the human and veterinary medical sciences. Increasing awareness of physicians on previously poorly known diseases likely is an important part of this process. Reports of several of these infections of the eye have been increasing, but whether this is due to a higher awareness or increasing rates of infection is unclear. Possible reasons for the highest number of reports may include changing epidemiological patterns in the natural definitive hosts, leading to increased exposure of humans, and new geographic range because of spreading into new areas. Three key examples, namely infections by Angiostrongylus cantonensis, Thelazia callipaeda and Onchocerca spp., are discussed below.

Angiostrongylus

Metastrongylids encompass a large group of nematodes (Strongylida, Metastrongyloidae) infecting organs and tissues of different vertebrates [125]. A. cantonensis, also known as rat lungworm, is a well recognized zoonotic infection and, as such, is the primary cause of eosinophilic meningitis in Southeast Asia. The infection has spread widely to many other areas of the world, including the Caribbean and Americas [126–130]. The parasite also enters the eye with some frequency. In a review of 484 cases of eosinophilic meningitis, Punyagupta and colleagues [131] noted that 47 (16%) of the cases had reported ocular involvement, and in 7 cases an actively motile worm (most probably A. cantonensis) was visualized and removed from the anterior chamber of vitreous of the eye [132, 133]. Human ocular infections by larval rat lungworm have been reported in several countries in Southeast Asia [134–143] and, they likely will continue to be reported wherever the parasite occurs, including in new geographical areas such as the Caribbean [144]. Ocular lesions by A. cantonensis may either occur alone or may accompany other symptoms such as meningitis [142, 145]. Often these long and slender worms reach considerable size in the eye, and are up to a centimeter or more in length [141]. The female worm has a distinctive helical pattern of dark intestine intertwined with light coloured reproductive tubes; male worms have a copulatory bursa and very long (> 1 mm) spicules. These features make it fairly simple to recognize and identify a large worm removed from the eye as A. cantonensis.

Infections are acquired through the ingestion of infected intermediate snail or slug hosts, or a variety of paratenic hosts such as amphibians, reptiles, and some crustaceans that have become infected by ingestion of snails and/or slugs. Migrating larvae in the human host make their way to the CNS, and occasionally into the eye, possibly along the optic nerve. Caution in handling and not eating raw or poorly cooked intermediate or paratenic hosts should prevent most human infections. There is some evidence that people can also be infected by ingestion of produce that has been contaminated with slime trails of snails or slugs into which infective larvae have been shed. Washing produce may help reduce the risk of infection but probably does not remove all risk in endemic areas with high level of transmission. Although in a number of cases, worms are successfully removed, ocular disease caused by larval A. cantonensis may vary from blurred vision due to intraretinal haemorrhage [144] to severe optic neuritis due to the presence of the nematode in the vitreous cavity [143]. Other signs include visual disturbances and impairments, extraocular muscular paralysis and a wide range of ocular inflammatory conditions [141, 146, 147]. Diagnosis is based on the identification of a (usually single) living worm, in any eye localization (e.g., anterior chamber, vitreous cavity, and subretinal space) (Figure 8). The treatment regimen relies on the surgical removal or laser therapy, accompanied by oral benzimidazole (e.g., mebendazole) and corticosteroids in the case of inflammatory manifestatios such as retinitis or optic neuritis [147]. Ocular damage caused by A. cantonensis can be severe and may be permanent, thus in some patients the outcome is poor and depends on the initial visual acuity [141, 147].

Thelaziasis

Thelazia callipaeda (Spirurida, Thelaziidae) represents a good example of HIE that is both spreading and new to the scientific community in western countries. Along with Thelazia californiensis that has been reported to infect humans occasionally in the United States [148], T. callipaeda is the only helminth transmitted by secretophagous flies directly into the orbit of humans [149]. This nematode primarily affects the eyes of domestic dogs and cats and wild carnivores (e.g., foxes, wolves, beech martens and wild cats) (See additional file 4: Thelazia callipaeda infecting the eye of a dog) [150]. Since its first description at the beginning of the previous century, this nematode has been known as the "oriental eye-worm" for its distribution in the former Soviet Union [151] and the Asian continent, including China [10], Korea [152], Japan [153], Indonesia [154], Thailand [155], Taiwan [156] and India [157]. Human thelaziasis may cause mild to severe clinical signs (including lachrymation, epiphora, conjunctivitis, keratitis and/or even corneal ulcers) [125]. The worm is transmitted by various secretophagous flies which feed on lachrymal secretions of infected animals and/or humans, thus ingesting Thelazia 1st stage larvae and, after obligate development in the fly, depositing 3rd stage larvae directly back into the orbit. The competence of drosophilid flies of the genus Phortica (Diptera, Drosophilidae) as vectors of T. callipaeda has recently been elucidated under both laboratory and natural conditions [158, 159]. We now recognize that T. callipaeda infection is widespread throughout Italy with infection prevalence as high as 60% in dogs from some municipalities (Figure 9) [160] and also in southwestern France (Dordogne area) [161, 162] and Switzerland [163]. In addition, four cases of human thelaziasis have been diagnosed in patients coming from an area of north-western Italy and south-eastern France [164]. Infected patients present with exudative conjunctivitis, follicular hypertrophy of the conjunctiva, foreign body sensation, excessive lachrymation, itchiness, congestion, hypersensitivity to light and keratitis, depending on the number of nematodes present in the eye [10]. Children and the elderly seem to be at higher risk. Thelazia worms are generally removed intact from the eye, and there are several morphological features that assist in identifying them from other worms that might occur in the orbit, including filaria such as Loa or Dirofilaria. The morphological identification of T. callipaeda has been reviewed [165]. The adult worms measure from 5 to 20 mm in length by 250 - 800 μm in diameter (males are smaller than females). They have a distinct buccal capsule and the cuticle has typical, regularly spaced distinct transverse striations giving the cuticle a ridged appearance (Figure 10). In addition, adult females of T. callipaeda are characterized by the position of the vulva located anterior to the oesophagus-intestinal junction, and the males possess five pairs of postcloacal papillae. Poor living conditions and low socio-economic standards seem to be risk factors for acquiring infection, and better hygiene would probably contribute to prevention. The adults and larvae of T. callipaeda can be removed mechanically by rinsing the conjunctival sac with sterile physiological saline whereas adults can also be isolated with forceps or cotton swabs [10].

Additional File 4: Thelazia callipaeda infecting the eye of a dog in Basilicata region (southern Italy). This video shows Thelazia callipaeda nematodes floating in the eye of an infected dog in an endemic area of Italy. Conjunctivitis and lacrymation were the main symptoms observed. In the second part, numerous T. callipaeda specimens have been collected by an ocular swab. (MP4 1 MB)

Onchocerca

As previously noted, the vast majority of filarioid infections of the eye occur on the conjunctiva, and are caused by species of Dirofilaria. However, there is an increasing number of reports of zoonotic Onchocerca infections, and several of these have been either within the eye or associated with the conjunctiva or connective tissue of the orbit (See additional file 5: Onchocerca sp. infecting the anterior chamber of a human patient). Of the 15 clinical cases reported to date [166, 167], five have been associated with the eye; 3 involved the conjunctiva and 2 involved the cornea. These have been reported from Crimea, the United States, Albania, Hungary, and Turkey [168–172]. The species causing infections of the eye have tentatively been attributed to Onchocerca gutturosa or Onchocerca cervicalis[169, 173], Onchocerca reticulata[170], Onchocerca spp. [171], and, Onchocerca lupi[172]. This last species, O. lupi, is of particular interest because it affects dogs and it induces acute or chronic ocular disease characterized by conjunctivitis, photophobia, lacrimation, ocular discharge and exophthalmia [166]. In most cases, zoonotic Onchocerca spp. are encased in a nodular granuloma, are resected and sectioned, and the worms identified morphologically (Figure 11a). In section of Onchocerca the distinctive muscle anatomy, composed of few, low, poorly developed cells, and the characteristic structures of the cuticle are often apparent, including the circular ridges and inner cuticular striae, making the identification straightforward. The distances between the prominent, undulated annular ridges and the number of transverse striae in the internal layer represent the morphological characters for differentiating filarioids belonging to the Onchocerca genus (Figure 11a). The apparent increase in number and range of zoonotic Onchocerca infections including those affecting the eye, is noteworthy but difficult to fully explain. Recent cases in both the US and Europe highlight this trend. Case reports of canine ocular onchocerciasis by O. lupi[166] have also increased in Europe, including in Greece, Portugal, Germany, Hungary, and Switzerland [174–177]. The number of cases of canine ocular onchocerciasis have also increased in the United States but the species of parasite in the United States has not been established [178–180]. The role played by dogs as reservoir of this zoonotic agent deserves to be investigated further to establish both the primary definitive hosts as well as the vectors that serve to transmit the infection naturally and to humans.

Additional File 5: Onchocerca sp. infecting the anterior eye chamber of a human patient. This video shows the occurrence of Onchocerca sp. in the anterior chamber of a patient from Colorado, United States. The nematode was surgically removed, extracted alive and identified as Onchocerca. The patient recovered without complications after the surgery. A video from the case presented in Burr WE, Brown MF, Eberhard ML: Zoonotic Onchocerca (Nematoda: Filarioidea) in the cornea of a Colorado resident. Ophthalmology 105:1494-1497, 1998. Video courtesy of Dr. W.E. Burr. (MP4 483 KB)

On the other hand, when HIE are enclosed in a granuloma or within the subconjunctiva, the identification is more difficult. Furthermore, in certain situations, removal may increase the risk to the patient, as in the case of the cystic forms of Echinococcus spp. which could induce anaphylactic immunoreactions when disturbed. Indeed, although rare, the localization of Echinococcus spp. cysts in the eye are always cause of severe disease, thus the careful surgical removal of the cysts is the only option.

Where appropriate tests exist, serological diagnosis can often contribute to a definitive diagnosis of infection, such as in the case of some ascarids for which serological testing using a sensitive, specific enzyme immunoassay (EIA or ELISA) is available. Serological testing is available for baylisascariasis and can be very helpful in identifying and confirming infection, and, like for toxocariasis, in conducting serosurveys to document the degree of exposure in different populations. Unfortunately, individuals may not mount a measurable immune response during the early phases of acute infection and serologic testing will not provide conclusive evidence to help guide treatment, hence the need for aggressive presumptive treatment in cases with solid exposure history [46]. Serologic assays can be very helpful to confirm infections caused by Gnathostoma, especially in cases where no larva or tissue is available to examine. Conversely, seropositivity to spargana in IFAT or ELISA tests always needs to be confirmed by histological examination [104, 105].

Unfortunately, surgery is often the only effective treatment for many HIE (e.g., ocular sparganosis, A. cantonensis) and this is one of the reasons why these infections represent a traumatic event for the patients and treatment is not a particularly cost-effective manner in which to manage the infection. In some cases, such as the typical zoonotic filarial infection, only a single worm is present and the surgical removal is both therapeutic and curative. In other instances, most notably OLM or larval tapeworms, there is some likelihood that additional larval stages may exist and chemotherapy may be indicated with corticosteroids in the case of inflammatory conditions such as retinitis or optic neuritis [147].

It should be noted that the use of photocoaggulation and laser ablation continue to prove useful in a number of cases infection with HIE, e.g., ascarid larvae, Alaria mesocercaria, and often result in improved visual outcome while at the same time destroying the invading helminth in situ[122].

Ophthalmologists and physicians often lack an in- depth knowledge of parasites, rendering it difficult for them to correctly address the etiological identification, treatments and control strategies for many HIE. In addition, the scientific information on HIE available in the international literature is scarce and limited to single case reports in which a clear comparative differentiation among helminth infections is not considered. The main limitation for correctly identifying the etiological agent is that often helminths are not removed, or they are seriously damaged during the surgical procedures thus rendering an accurate morphological identification difficult, if not impossible. The microscopic identification of helminths at the species level often relies on the examination of key morphological characters, not all of which are present on any given specimen or not recognized by the person making the examination, sometimes resulting in an incorrect diagnosis. Accurate identification is crucial to understanding both the source of infection and environmental risks, as well as prescribing correct treatment options.

There are several cases in the literature in which helminths were erroneously identified; for example, cases of Trichinella sp. in the vitreous of a woman and Toxocara sp. in the retina of a man both from Germany, and a case Angiostrongylus sp. recovered from the anterior chamber of a man from Sri Lanka were all incorrectly identified as filaria (reviewed in [63]). Twenty-eight cases of human dirofilariasis from the Old World were erroneously attributed to D. immitis, subsequently reviewed and correctly attributed to D. repens[181]. Recently, the helminth causing a case of human intraocular infestation in Japan was erroneously identified as T. callipaeda although the picture published in the article portrayed a filarioid [182]. In the same article, the authors stated that the life cycle of T. callipaeda remains unclear and discussed the possibility of human infection through the skin or by drinking untreated water. This somewhat implausible hypothesis was already dispelled in the late 1990s [183]. The scant attention of medical researchers towards human thelaziasis may also be attributable to the difficulties in its clinical diagnosis and differentiation from allergic conjunctivitis, particularly when small numbers of adult or larval stages are present in affected patients. More recently, the advent of molecular biological techniques has largely supplemented and enhanced knowledge of parasitologists in areas such as systematics (taxonomy and phylogeny), population genetics and molecular identification, diagnosis and control of some HIE [184]. Indeed, the advent of PCR made it possible to study damaged and incomplete specimens, or fragments of specimens encysted in tissues which otherwise would not be morphologically identifiable [185]. The importance of molecular identification and barcoding approach (by the specific PCR-amplification of the cox 1 and 12S genes) for the rapid identification of specimens has been emphasized, including for either recognized or yet unknown species. Recently, an integrated DNA barcoding of cox 1 and 12S markers and morphology approaches was shown to be a powerful tool for the taxonomical identification of many filarioid species even if small nematode fragments were available [184]. In addition, the delineation of Molecular Operational Taxonomical Units (MOUTS) was useful to infer potential new species [184].

Basic parasitological research in this field is often fragmentary due to the fact that experimental human infections are rarely done, and the retrieval of helminths from the patients' eyes may be an infrequent occurrence during the ophthalmologic examination. For a number of these helminths, poor experimental models exist, or, if good models exist, the infections generally do not affect the eye in the same way that occurs in aberrant human infections. Thus, scientific knowledge in this field, as well the information on helminth migration patterns is limited, and often has been gained from studies of the same parasites in other animal models. All the above concerns need to be addressed through basic and applied research. For example, many nematode species have not yet been described and even those that are known are often poorly studied such that there is a lack of basic information on the helminth fauna of wild animals (e.g., O. lupi). This is particularly true, but not restricted to, regions of the world, such as the Brazilian Amazon forest, where there is wide biodiversity and a large amount of animal and plant species yet to be described [186]. Consequently, species identification of some groups of HIE, such as filarial nematodes, can be difficult if not impossible. Another example of insufficient information is represented by the unknown risk of zoonotic infection, such as other species of Baylisascaris (in addition to B. procyonis), that may be considered as potential zoonotic agents [46]. For example Baylisascaris transfuga, infecting bears worldwide [45, 187] has been reported to produce visceral, neural, or OLM syndromes in mice [188–190], gerbils [45, 191], and guinea pigs [192]. In addition, cases of fatal neurological diseases have been reported in a colony of Japanese macaques (Macaca fuscata fuscata) housed with American black bears in a safari-zoo in Japan [193]. However, the zoonotic role of this parasite for humans has never been demonstrated. Since bears are frequently kept in zoos and game parks and often have high prevalence of the infection in the population (up to 50-100% of bears harbour this parasite) studies on the zoonotic capacity of this parasitic species would be pivotal for a better understanding of the public health risk [194]. Overall, a better understanding of the biology of a number of HIE is crucial for addressing their prevention.

Better awareness among physicians (including ophthalmologists) in the field of parasitology and more active collaboration with parasitologists would be very helpful in proper diagnosis, control and prevention of HIE. This would also allow a better knowledge of the potential risks for being infected by an HIE agent in a given area as well as exposure when travelling in endemic areas. Physicians and ophthalmologists need increased awareness about the existence of a range of zoonotic helminths other than those natural parasites of humans that might be expected to be found in patients' eyes.

Unfortunately, there is a lack of knowledge about many parasites in the local fauna and limited basic research studies are carried out. Monitoring and periodic surveillance for the infections of both domestic and wild animals is important to provide a better understanding of what potential pathogens exist locally, and to prevent the HIE. This is the case with B. procyonis which is an emerging infection in raccoons in the southeastern United States, an area traditionally considered to be at low risk [195, 196]. An increasing appreciation of onchocerciasis in domestic and wild animals in Europe and the United States is needed to accurately understand what species exist, what the natural definitive host is and, ultimately, what the risks for human infection are. Veterinarians, physicians, and public health officials all share the need to be alert to the possibility of zoonotic infections inside and outside of traditional high-risk areas. Lastly, we need a better understanding of why some parasites migrate to and occasionally enter the eye, especially given that none of these helminths typically resides in or around the eye.

Despite scientific advances and new methods for treating helminth infections in the human eye, therapies available to patients are somewhat limited and can only be applied in specific cases. This will lead to improvements in the clinical outcome in some cases, but for the foreseeable future, a number of these HIE have complex clinical presentations that still hold potential for serious outcome, including blindness or death, such as in the case of B. procyonis infections, where, despite treatment, neurological outcome is dismal in the overwhelming majority of documented cases [46]. However, many cases of these zoonotic helminth infections are preventable by relatively simple measures of improved health and sanitation conditions and awareness on the part of both public and health care providers. Risks for toxocariasis and baylisascariasis could be significantly reduced through better hygiene and reduction of the amount of animal waste in areas where people, especially children, might come in contact with it. For the foodborne zoonoses, such as angiostrongyliasis, gnathostomiasis and others, proper handling and preparation of foods would minimize the risk of infection. For the vector-borne zoonotic infections, control and prevention is likely going to be much harder, as it involves not only the control of the infection in the definitive animal host, but a concerted control of vectors, which is often outside the control of any individual but almost always done at the community or regional level.