Thelazia

Thelazia is a genus of parasitic nematodes that cause the condition known as thelaziosis in domestic and wild animals. Belonging to the order Spirurida and the family Thelaziidae, these worms are significant veterinary pests that target the ocular tissues of livestock, particularly cattle, causing substantial welfare and economic issues.

The primary hosts for these worms include cattle, horses, sheep, and occasionally companion animals. Adult worms live within the conjunctival sac, beneath the nictitating membrane, and in the lacrimal ducts. Their presence induces chronic inflammation and causes severe discomfort to the infected animal.

The lifecycle of Thelazia depends on an intermediate host, typically flies from the genus Musca. These flies feed on the tear secretions of infected animals, ingesting the first-stage larvae. The larvae then undergo development within the fly, reaching an infective stage before being deposited onto the eye of a new host when the fly feeds again.

The pathogenic effect of the infestation is characterized by lacrimation (tearing), conjunctivitis, photophobia, and edema of the eyelids. If left untreated, the damage can progress to corneal ulcers, opacities, and potential blindness, which negatively impacts the growth rates and productivity of the affected herd.

Effective management and prevention strategies involve several key actions:

• Administration of anthelmintic medication to eliminate existing worm burdens.
• Vector control measures to minimize the population of flies, such as insecticide applications and dung management.
• Use of ear tags or pour-on treatments with insecticidal properties to deter flies from the ocular region.
• Regular clinical observation of livestock herds, especially during the warm months when fly activity is highest.