Eyeworms in Sheep: Thelaziasis Symptoms, Treatment & Prevention

Eyeworm disease, also called thelaziasis, is a parasitic eye condition caused by Thelazia nematodes. These thin, whitish worms live on the surface of the eye and nearby tissues, including under the eyelids and around the tear ducts. Sheep are not the most commonly discussed species, but they can be affected along with goats, cattle, horses, and other large animals.

The worms irritate delicate eye tissues as they move and feed in tear-rich areas. That irritation can start as mild tearing and redness, then progress to conjunctivitis, keratitis, or even corneal damage if the infestation is not addressed. In some flocks, only a few animals show obvious signs, while others may carry worms with milder irritation.

For pet parents and flock managers, the practical takeaway is that this is a treatable but uncomfortable condition. Early veterinary care matters because ongoing irritation can make a simple parasite problem turn into a more serious eye problem.

Eyeworms are caused by infection with Thelazia species. In large animals, these parasites are spread by non-biting flies that feed on tears and eye secretions. When a fly feeds on an infected animal, it can pick up larval stages of the worm. Those larvae develop inside the fly and are then deposited onto another animal's eye the next time the fly feeds.

This means the disease is not usually spread by direct sheep-to-sheep contact alone. Instead, fly activity is the key driver. Risk tends to rise in warmer months, in areas with higher fly pressure, and in management settings where animals are grouped closely or where fly control is limited.

Once on the eye, the worms mature in tissues around the conjunctiva and eyelids. Their presence causes mechanical irritation and inflammation. Secondary bacterial infection can follow, which is one reason some sheep develop more severe discharge, corneal haze, or ulcers than others.

Diagnosis usually starts with a careful eye exam. Your vet may gently part the eyelids and inspect under the third eyelid and within the conjunctival sac, where adult worms often hide. In some sheep, the worms are directly visible as thin, white, moving parasites on the eye surface or under the lids.

Your vet will also look for the degree of inflammation and whether there is damage beyond irritation alone. That may include checking for conjunctivitis, keratitis, corneal opacity, and ulcers. If the cornea looks abnormal, your vet may use fluorescein stain to look for an ulcer and help guide treatment choices.

Because several eye problems in sheep can look similar, diagnosis is not only about finding worms. Your vet may also consider pinkeye, trauma, foreign material, entropion, or other causes of ocular discharge and pain. That full exam matters because treatment options and urgency change if the cornea is injured.

Prevention focuses mostly on reducing fly exposure and catching eye irritation early. Because Thelazia worms are spread by flies that feed on tears, practical fly control can lower risk. Depending on your flock setup, that may include manure management, reducing wet organic buildup, strategic insecticide use approved for sheep, and seasonal fly-control planning with your vet.

Routine observation also matters. Check sheep during high-fly months for tearing, squinting, or discharge so affected animals can be examined before corneal damage develops. Separating animals with active eye disease may help with monitoring and reduce crowding around shared fly-heavy areas, even though flies rather than direct contact are the main route of spread.

If your flock has repeated summer eye problems, ask your vet about a herd-level prevention plan. That may include reviewing pasture conditions, handling areas, stocking density, and whether broader parasite or fly-control measures fit your operation. The best prevention plan is the one that is realistic for your farm and matched to local parasite pressure.