Thelazia lacrymalis refers to a parasitic eyeworm that primarily affects horses. This nematode can cause various ocular issues, ranging from mild irritation to more severe eye conditions.
Understanding Thelazia lacrymalis and Its Hosts
Thelazia lacrymalis is a type of nematode, or roundworm, that specifically infects the eyes and surrounding tissues. These worms typically reside in the conjunctival sac, tear ducts, and even under the eyelids and nictitating membrane. They are whitish.
While horses are the primary hosts for Thelazia lacrymalis, other mammals, including cattle, pigs, sheep, goats, deer, water buffalo, dromedaries, hares, and various wild carnivores, can also be infected. Human infections, though less common, have also been reported, particularly with other Thelazia species like T. callipaeda, T. californiensis, and T. gulosa.
How the Eyeworm Spreads
The life cycle of Thelazia lacrymalis involves an indirect transmission, relying on an intermediate host: the face fly, such as Musca autumnalis or Musca larvipara. Adult female worms residing in the host’s eye release first-stage larvae (L1) into the ocular secretions, such as tears. These larvae are then ingested by face flies when they feed on the eye secretions of an infected animal.
Inside the fly, the L1 larvae develop through two molts, becoming infective third-stage larvae (L3) over a period of about two to four weeks. These L3 larvae migrate to the fly’s mouthparts. When the infected fly subsequently feeds on the tears of a new host, it deposits the infective L3 larvae onto the surface of the eye. These larvae then develop into sexually mature worms within the new host’s eye, a process that can take approximately 10 to 11 weeks in horses, thus completing the life cycle.
Identifying the Signs of Infection
The presence of Thelazia lacrymalis can lead to a range of clinical signs in infected hosts, particularly horses. Common symptoms include conjunctivitis, which is an inflammation of the conjunctiva, and excessive tearing, also known as lacrimation or epiphora. Owners might also observe ocular discharge, eye irritation, or a sensation of a foreign body in the eye.
In more severe cases, the infection can progress to keratitis, an inflammation of the cornea, which may manifest as corneal cloudiness or opacity. Ulcers on the cornea, or even perforation, can develop in chronic or severe infestations. While infections in North American horses may sometimes be asymptomatic or present with mild signs, European and Asian cases in cattle are often associated with more pronounced symptoms.
Diagnosis, Treatment, and Prevention
Diagnosing Thelazia lacrymalis infection typically involves a combination of clinical observation and direct visualization of the worms. Veterinarians often examine the eye and surrounding tissues for the presence of the whitish, thread-like worms, which can sometimes be seen moving across the eye surface or under the eyelids. Local anesthetic may be used to facilitate this examination and aid in the collection of samples. Microscopic examination of tear samples for larvae or eggs can also be attempted.
Treatment options for Thelazia lacrymalis in horses include manual removal of the worms, often performed using fine forceps or a cotton swab after topical anesthesia. Irrigation of the eye with specific solutions, such as a 0.5% iodine and 0.75% potassium iodide aqueous solution, has also been suggested. Systemic anthelmintics, such as ivermectin, administered orally or via stomach tube at a dosage of 0.2 mg/kg, have been used, though their effectiveness specifically for T. lacrymalis in horses may be limited. Topical medications like 0.03% echothiophate iodide or 0.025% isoflurophate have also shown success.
Preventing Thelazia lacrymalis infection largely focuses on controlling the face fly population, as these flies are the primary vectors. Measures include the use of fly masks on horses, especially during periods of high fly activity, and applying fly repellents, though repellents may only offer short-term protection. Maintaining good hygiene around stables and pastures can help reduce fly breeding sites. Regular veterinary check-ups allow for early detection and intervention, which can prevent the development of more severe ocular issues.