Eyeworms in Pigs: Parasites That Cause Tearing and Eye Irritation

Eyeworms are parasitic nematodes that live on the surface of the eye or in nearby tissues such as the conjunctival sac and tear ducts. In veterinary medicine, these parasites are usually discussed under thelaziasis, a condition caused by Thelazia species. In large animals, adult worms may sit under the eyelids, around the third eyelid, or within the nasolacrimal system, where they irritate delicate eye tissues.

The irritation can range from mild tearing to more painful inflammation. Signs reported across affected animals include watery eyes, discharge, conjunctivitis, keratitis, and in more severe cases corneal clouding or ulcers. The worms themselves may sometimes be seen as thin, whitish threads moving on the eye surface during an exam.

Eyeworms are much better described in cattle and horses than in pigs, so in pigs they are considered an uncommon or poorly documented problem rather than a routine diagnosis. Still, the same type of parasite-related eye irritation is possible, especially in outdoor settings with heavy fly exposure. Because many other eye problems can look similar, your vet needs to confirm whether parasites are truly present before treatment decisions are made.

Eyeworm infestations are caused by parasitic roundworms, most often Thelazia species in large-animal medicine. These worms are not spread directly from one pig touching another pig. Instead, they rely on certain flies as intermediate hosts.

The life cycle starts when adult female worms release larvae into tears and eye secretions. Flies that feed on those secretions ingest the larvae. Inside the fly, the larvae develop into an infective stage over the next few weeks. When that fly feeds on another animal's tears, it can deposit larvae into the new host's eye, where the worms mature.

Risk tends to be higher in warm seasons when fly activity increases. Outdoor housing, manure buildup, moist shaded areas, and limited fly control can all raise exposure. Because pigs can also get tearing and conjunctivitis from dust, trauma, foreign material, bacterial infection, or other parasites, your vet will need to sort through several possible causes.

Diagnosis starts with a full eye exam. Your vet will look for conjunctivitis, corneal injury, discharge, and any visible worms under the eyelids or around the third eyelid. Topical anesthetic may be used so the eye can be examined more comfortably and thoroughly.

In large animals, gross inspection can sometimes reveal adult worms, especially when they are in more superficial locations. If worms are suspected but not obvious, your vet may gently manipulate the eyelids, flush the eye, or collect tear fluid for closer evaluation. In some cases, the recovered parasite is submitted for identification.

Just as important, your vet will rule out other causes of eye irritation. Corneal scratches, ulcers, foreign bodies, trauma, entropion, bacterial conjunctivitis, and irritants in the environment can all mimic parasite-related eye disease. That is why home treatment without an exam can delay the right care and increase the risk of permanent eye damage.

Prevention is mostly about reducing fly exposure and catching eye irritation early. Because eyeworm larvae are carried by flies that feed on tears, good fly control matters. That can include manure management, keeping bedding and housing drier, reducing standing moisture, and using farm-appropriate fly-control strategies recommended by your vet or herd veterinarian.

Outdoor pigs may have more exposure during warm months when flies are active. If possible, pay extra attention to pigs with watery eyes during spring, summer, and early fall. Early treatment of eye irritation can help prevent rubbing, secondary infection, and more serious corneal injury.

Routine observation is important. Check for tearing, redness, discharge, squinting, or face rubbing, especially in pigs housed outdoors or in areas with heavy fly pressure. If one pig develops suspicious eye signs, ask your vet whether herd-level monitoring or environmental changes are needed.

There is no one-size-fits-all prevention plan for every pig setup. Small backyard groups, sanctuaries, and production settings all have different risks. Your vet can help you choose a practical prevention plan that fits your pig's environment, handling style, and overall health needs.