First-stage larvae of this fly species are commonly deposited in the conjunctival sac of hartebeest and wildebeest. They migrate to the nasal cavity via blood vessels, meninges and subdural spaces in these natural hosts without causing clinical illness. Aberrant infection occurs in domestic animals (unnatural hosts) causing severe ocular and neural disease. This disease is also known as ophthalmonnyiasis, uitpeuloog and gedoelstial myiasis.
Affected horses present with acute-onset intense ocular pain or blepharospasm, lacrimation, severe chemosis and periorbital oedema. These animals are often head-shy because of the intense pain. These symptoms often occur as an outbreak with numerous animals affected simultaneously and are generally in close proximity to wildlife such as blesbok and wildebeest. Numerous small white flecks (larvae) may be visible on the cornea with an intense superficial fluorescein stain uptake.
Gedoelstia spp. deposit first-stage larvae on the eyes of their preferred hosts and these migrate via the cardiovascular system or Dura of the cranial cavity to the paranasal sinuses. Despite the continuous presence of the parasites in their preferred hosts, there appear to be no obvious detrimental clinical manifestations, which points to a unique parasite-host relationship.
Domestic animals are aberrant hosts. Larvae have never been found in their nasal cavities and it is assumed that the larvae are either unable to follow the normal route leading to the sinuses or that there is some preventative reaction in the host. Three main forms of the disease caused by Gedoelstia larvae have been recognised in domestic animals, including the ophthalmic form which is a specific oculovascular myiasis called bulging eye disease or "uitpeuloog".This ranges from a mild inflammation to a very severe exophthalmia with protrusion of the eyeball. The encephalitic form results in a variety of nervous symptoms depending on the damage caused by migrating larvae. The cardiac form can result in death due to heart failure.
These outbreaks of ophthalmomyiasis are of emerging significance as the distribution of wildebeest and blesbok has increased dramatically in the areas were the horses are stabled. Owners of smallholdings increasingly keep wildlife, which seem to adapt well to a semi-urban environment. This may well result in seasonal early summer outbreaks of gedoelstial nnyiasis in domesticated animals. The route of entry for the larvae is ocular, and they use their mouth hooks and body spines as well as pulsatile body movements. Their ultimate goal is to reach the subdural space and then the nasal cavity. A number of alternative routes have been documented and these may result in severe pathology.
Affected eyes are treated with a cypermethrin spray that has proved to be effective in killing the larvae on the cornea and conjunctiva. This treatment does not cause any further damage to the cornea and is a safe and effective means of managing the problem.
Topical broad-spectrum antibiotic drops are also required for treating the fluorescein-positive lesions, which are often very painful. Topical atropine is indicated as well as a short course of oral or systemic anti-inflammatories.
This is an article from Vetnuus February 2014
By Lo-An Odayar BVSc MMed Vet (Ophthal) JHB/Cape Animal Eye Hospital firstname.lastname@example.org