Can Cattle Eyeworms Infect Humans? Thelazia Symptoms, Treatment and Prevention

By Dr Duncan Houston

A moving white thread inside the eye is not ordinary conjunctivitis.

The cattle eyeworm, Thelazia gulosa, normally lives within the eyes and tear ducts of cattle. In very rare circumstances, an infected face fly can deposit immature larvae onto a person’s eye, where the worms can mature and cause persistent irritation.

The sight of a worm is understandably alarming, but the documented human cases have generally resolved after the worms were removed. The more immediate concern is protecting the cornea, finding every remaining worm and distinguishing the infection from other causes of eye pain, inflammation or vision loss. (PubMed Central (PMC))

Quick Answer

Yes, cattle eyeworms can infect humans, but this is extremely rare. Human T. gulosa infection occurs when an infected tear-feeding face fly deposits infective larvae onto the eye, not simply from touching cattle.

A visible worm, persistent foreign-body sensation, worsening redness, light sensitivity, eye pain or any change in vision warrants prompt examination by an ophthalmologist. In cattle, excessive tearing, conjunctivitis, corneal cloudiness or an ulcer requires veterinary examination because eyeworm disease can resemble pinkeye, trauma and other serious ocular conditions. (CDC)

Thelazia at a Glance

(CDC)

What Is Thelazia gulosa?

Thelazia gulosa is a spirurid nematode that primarily parasitises cattle. Adult worms may live beneath the eyelids, behind the third eyelid, within the conjunctival sac, in the nasolacrimal ducts or near the excretory ducts of the lacrimal glands.

Adults are thin, pale and thread-like. Human-infecting Thelazia species may reach approximately 2 centimetres in length, although the worms recovered in the second U.S. T. gulosa case were described as approximately half an inch long. (CDC)

Cattle can also be infected by other Thelazia species, including:

• Thelazia skrjabini

• Thelazia rhodesii

Worldwide, most human thelaziasis is caused by Thelazia callipaeda, while T. californiensis and T. gulosa are less common human pathogens. (CDC)

How Does the Cattle Eyeworm Life Cycle Work?

The life cycle requires both a mammalian host and a tear-feeding fly.

1. Adult female worms live in or around the eye.

2. Females release first-stage larvae into tears and ocular secretions.

3. A face fly feeds on those secretions and ingests the larvae.

4. The larvae develop inside the fly into infective third-stage larvae.

5. The infective larvae migrate to the fly’s mouthparts.

6. When the fly feeds on another animal’s tears, the larvae enter the new host’s conjunctival sac.

7. The larvae mature into adult worms within the ocular tissues.

Larval development inside the fly generally takes two to four weeks. Once deposited in cattle, the worms may become sexually mature within approximately one to four weeks, depending on the species. (CDC)

Is the Face Fly Just Carrying Worms on Its Feet?

No.

The face fly is a biological intermediate host, not merely a dirty insect mechanically carrying worms from one eye to another. The larvae must develop inside the fly before they become infective.

This also means ordinary contact with cattle, manure or cattle bedding is not considered the usual route of human infection. A suitable infected fly must feed on the person’s ocular secretions and deposit infective larvae. (CDC)

Is Musca autumnalis a Stable Fly?

No.

Musca autumnalis is the face fly. It is a non-biting muscid fly that feeds around the eyes, muzzle and other moist body surfaces.

Stable flies are blood-feeding flies with a different biology and are not the established North American vector of T. gulosa. The face fly’s preference for tears makes it particularly well suited to transmitting eyeworm larvae. (CDC)

Can Cattle Eyeworms Really Mature in Humans?

Yes.

Humans are accidental hosts, but the two landmark U.S. cases demonstrated that T. gulosa can mature within the human eye.

In the second case, the recovered adult female contained eggs with developed larvae. This showed that the parasite had reached reproductive maturity in its human host rather than simply surviving briefly as an immature larva. (OUP Academic)

That does not mean the infection spreads directly between people. Another face fly would have to ingest larvae from an infected eye, allow the larvae to develop and then deposit them into another host.

What Happened in the Two U.S. Human Cases?

The First T. gulosa Human Case

The first recognised human infection with T. gulosa involved a 26-year-old woman in Oregon in 2016. She had been working with horses in a cattle-farming region and developed irritation and a foreign-body sensation in her left eye.

Fourteen adult worms were removed over approximately 20 days. Her symptoms resolved after complete physical removal, and no lasting ocular effects were reported. The case was published in 2018. (PubMed Central (PMC))

The Second T. gulosa Human Case

The second case involved a 68-year-old Nebraska resident who spent winters in Carmel Valley, California. In February 2018, she recalled running into a swarm of small flies while trail running in cattle-ranching country.

Eye irritation began in March. She removed two worms through irrigation and manual extraction, an ophthalmologist removed a third, and a fourth appeared later. Mild bilateral conjunctivitis subsequently resolved after the worms were removed. The report was published online in October 2019 and appeared in the February 2020 issue of Clinical Infectious Diseases. (OUP Academic)

This corrects the common retelling that a new infection first occurred in 2020. The human exposure and illness occurred in 2018.

Does This Mean Human Cattle-Eyeworm Infection Is Increasing?

Not necessarily.

Two U.S. cases occurring within a short period prompted the authors to suggest that T. gulosa deserved closer surveillance. However, routine monitoring of cattle eyeworm infection is limited, so the available evidence cannot show whether cattle prevalence, human exposure or zoonotic spillover is genuinely increasing. (OUP Academic)

Warmer conditions may influence fly activity and transmission seasons, but the two T. gulosa case reports do not prove that climate change caused these infections or that human risk is now rising.

The evidence supports vigilance, not panic.

Why Does the Broader Thelazia Story Still Matter?

The North American Thelazia picture is changing, but not only because of the cattle eyeworm.

CDC-associated researchers have documented Thelazia callipaeda infections in U.S. dogs and cats and subsequently identified adult T. callipaeda worms in an American black bear in Pennsylvania. This suggests that a wildlife transmission cycle may be becoming established in parts of the United States. (CDC)

This is a different parasite with different vector ecology:

• T. gulosa is associated primarily with cattle and Musca autumnalis.

• T. callipaeda commonly affects carnivores and is transmitted by tear-feeding Phortica flies.

• T. californiensis has traditionally been associated with western North American wildlife and dogs and is transmitted by Fannia flies.

These species should not be grouped together as though every new animal detection means cattle eyeworm is suddenly spreading to people. Precise species identification matters. (CDC)

What Are the Signs of Thelazia Infection in Humans?

Human symptoms depend on the number and location of the worms and how much damage their movement causes.

Possible signs include:

• A sensation that something is moving in the eye

• Persistent foreign-body sensation

• Redness

• Excessive tearing

• Itching

• Ocular discharge

• Mild swelling

• Conjunctivitis

• Light sensitivity

• Eye pain

• A thin, pale worm visible beneath the eyelid or moving across the conjunctiva

Heavier or prolonged infections may cause:

• Keratitis

• Corneal abrasion or ulceration

• Reduced visual acuity

• Significant photophobia

• Persistent inflammation

• Rarely, permanent visual damage

The published U.S. T. gulosa cases resolved without long-term complications after the worms were removed, but delayed or incomplete removal can allow continued corneal irritation. (CDC)

Can Both Eyes Be Affected?

Yes.

The second U.S. case developed bilateral foreign-body sensation and papillary conjunctivitis, although recovered worms were associated mainly with the initially affected eye. Thelazia infection should not be excluded merely because symptoms become bilateral. (OUP Academic)

What Are the Signs in Cattle?

Many North American cattle infections appear to be subclinical or mild. The worms may be found incidentally during surgery or post-mortem examination.

Clinical disease can still occur, causing:

• Excessive tearing

• Wet staining beneath the eye

• Clear, mucous or purulent discharge

• Conjunctival redness

• Swollen eyelids

• Repeated blinking

• Squinting

• Head shaking or rubbing

• Sensitivity to light

• Keratitis

• Corneal cloudiness

• Corneal ulceration

• Subconjunctival cysts or inflammatory masses

• Reduced grazing when both eyes are painful

• Vision loss in severe cases

T. gulosa and T. skrjabini may occupy deeper lacrimal tissues and can be harder to see than T. rhodesii, which is more often visible within the conjunctival sac. (Merck Veterinary Manual)

Can Thelazia Be Mistaken for Pinkeye?

Yes.

Eyeworm disease may resemble infectious bovine keratoconjunctivitis, particularly when cattle have:

• Tearing

• Conjunctivitis

• Squinting

• Corneal opacity

• Ulceration

• Failure to improve after ordinary pinkeye treatment

An unusual report documented T. gulosa within inflammatory, granuloma-like tissues behind the eye after cattle had undergone several ineffective treatments for presumed infectious keratoconjunctivitis. (PubMed)

The mistake I would avoid is repeatedly treating presumed pinkeye without performing a proper ocular examination when the lesions, treatment response or season do not fit.

How Worried Should You Be?

Lower Risk

In a person

• A fly briefly contacted the face

• There is no continuing eye discomfort

• Vision is normal

• No worm or persistent redness is seen

Action: A gentle eye rinse may be reasonable after direct fly contact. Monitor for persistent irritation, redness or a moving object.

In cattle

• Mild tearing only

• No squinting

• Clear cornea

• Normal appetite and behaviour

Action: Examine the eye closely and monitor. Arrange veterinary assessment if tearing persists, worsens or affects several animals.

Moderate Concern

In a person

• Persistent foreign-body sensation

• Redness or tearing lasting several hours

• Mild conjunctivitis

• Recent exposure to dense face-fly activity around cattle

Action: Arrange prompt medical or optometric examination. Do not assume it is only allergy or dust.

In cattle

• Persistent tearing

• Repeated blinking or rubbing

• Conjunctival redness

• Visible white thread-like material beneath the eyelid

Action: Arrange veterinary examination within 24 hours. Earlier removal reduces ongoing ocular trauma.

High Concern

In a person

• A visible moving worm

• Increasing eye pain

• Photophobia

• Discharge

• Symptoms returning after one worm was removed

• Reduced or blurred vision

Action: Seek same-day ophthalmic care. More than one worm may be present.

In cattle

• Corneal cloudiness

• Marked squinting

• Ulceration

• Significant swelling

• Reduced feed intake

• Several cattle developing eye disease

• Failure to respond to previous treatment

Action: Seek same-day veterinary care and evaluate the herd’s fly-control programme.

Critical

In a person

• Sudden major vision loss

• Severe eye pain

• A white or blue corneal defect

• Penetrating eye injury

• Blood within the eye

• Inability to open the eye

• Severe swelling after attempted home extraction

Action: Attend an emergency department or emergency ophthalmology service immediately.

In cattle

• Deep or perforated corneal ulcer

• Apparent blindness

• Severe ocular trauma

• Extensive tissue damage

• Major secondary infection

• Inability to navigate, eat or drink normally

Action: Obtain urgent veterinary treatment. Salvage of the eye may depend on how quickly treatment begins.

What Else Can Look Like Human Thelaziasis?

A moving worm is strongly suggestive, but most irritated eyes are not caused by parasites.

Important alternatives include:

A history of fly exposure near cattle increases suspicion, but it does not replace a slit-lamp examination and proper identification of any recovered organism.

What Else Can Look Like Thelaziasis in Cattle?

Important cattle differentials include:

• Infectious bovine keratoconjunctivitis

• Grass awns or other foreign material

• Corneal trauma

• Ulcerative keratitis

• Infectious bovine rhinotracheitis

• Uveitis

• Eyelid disease

• Neoplasia

• Photosensitisation

• Chemical irritation

• Other ocular parasites

More than one problem may occur at the same time. Face flies can transmit eyeworm larvae while also irritating the eyes and contributing to pinkeye transmission.

When Is This an Emergency?

Human emergency signs

Obtain urgent medical care for:

• A visible worm

• New visual impairment

• Severe pain

• Increasing photophobia

• A cloudy cornea

• Significant discharge

• Bleeding

• Penetrating injury

• Symptoms that continue after one worm is removed

• Swelling or pain caused by attempted extraction

A worm seen on the ocular surface should be treated as an urgent eye-care problem, even when vision initially remains normal.

Cattle emergency signs

Seek prompt veterinary attention for:

• Corneal clouding

• A visible corneal ulcer

• Severe squinting

• Apparent blindness

• Deep ocular swelling

• A protruding or ruptured eye

• Reduced eating or drinking because of bilateral eye pain

• Several animals developing similar signs

• Failure of presumed pinkeye treatment

Corneal ulceration, perforation and permanent fibrosis are recognised complications of severe bovine thelaziasis. (Merck Veterinary Manual)

What Should a Person Do Right Now?

1. Do Not Use Forceps at Home

The conjunctiva and cornea are delicate.

Attempting to grasp a moving worm without magnification, topical anaesthesia and proper instrumentation can cause:

• Corneal abrasion

• Conjunctival tearing

• Bleeding

• Incomplete worm removal

• Retained worm fragments

• Secondary infection

Home extraction is particularly risky when the worm disappears beneath an eyelid or into the tear drainage system.

2. Do Not Rub the Eye

Rubbing can increase corneal trauma and drive a foreign body or worm against the ocular surface.

3. Arrange an Eye Examination

An ophthalmologist or appropriately equipped eye-care clinician should:

• Examine the conjunctival sacs

• Evert the eyelids

• Inspect beneath the upper lid

• Examine the cornea

• Assess the nasolacrimal region

• Look for additional worms

• Check for abrasion or ulceration

One visible worm does not prove it is the only worm present. Four worms were recovered in the second U.S. case and 14 in the first. (PubMed Central (PMC))

4. Do Not Discard a Recovered Worm

Place a spontaneously removed organism in a clean, sealed container and ask the clinician or diagnostic laboratory how it should be preserved.

Preservative choice matters. Formalin preserved the worms well enough for morphological identification in the second U.S. case but prevented useful DNA extraction for molecular confirmation. (OUP Academic)

5. Avoid Unprescribed Eye Medication

Do not use leftover:

• Steroid drops

• Antibiotic drops

• Veterinary eye medication

• Redness-relief drops

• Antiparasitic products

Treatment depends on whether the cornea is intact, whether infection is present and whether all worms have been removed.

What Should a Cattle Producer Do Right Now?

1. Restrain and Examine the Animal Safely

Look for:

• Worms beneath the eyelids

• Discharge

• Corneal opacity

• Ulceration

• Foreign material

• Swelling

• Signs affecting the opposite eye

Topical ocular anaesthesia may be needed for a complete veterinary examination and safe tissue manipulation. (Merck Veterinary Manual)

2. Do Not Scrape the Eye or Use Unclean Forceps

Mechanical removal can be effective, but it should be performed under appropriate restraint, lighting and local anaesthesia.

3. Review the Entire Herd

Check other cattle for:

• Tearing

• Squinting

• Facial fly burden

• Corneal lesions

• Reduced grazing

• Failure of pinkeye treatment

The visible animal may not be the only infected or exposed animal.

4. Review Recent Anthelmintic Use

Some cattle may already have received a macrocyclic lactone with activity against adult eyeworms. Others may not have received an effective product, dose or formulation.

Do not redose the herd blindly. Product approval, resistance considerations, cattle class and meat or milk withdrawal periods must be reviewed.

5. Intensify Face-Fly Control

Eyeworm control is unlikely to succeed long term when face flies remain abundant.

How Is Human Thelaziasis Diagnosed?

Diagnosis relies primarily on finding and identifying adult worms.

The eye-care examination may include:

• Direct observation

• Slit-lamp examination

• Eversion of the eyelids

• Corneal examination

• Fluorescein staining where abrasion or ulceration is suspected

• Removal of worms under topical anaesthesia

Species identification requires microscopic examination of features such as:

• The buccal cavity

• Cuticular striations

• Position of the vulval opening

• Tail morphology

• Anal opening

• Male spicules

Molecular testing may provide additional confirmation when a specimen has been preserved appropriately. (CDC)

How Is Thelaziasis Diagnosed in Cattle?

Veterinary diagnosis may involve:

• Direct ocular inspection

• Eversion of the eyelids

• Examination behind the third eyelid

• Topical anaesthesia

• Examination of the conjunctival sac and lacrimal tissues

• Microscopic examination of tears for eggs or larvae

• Fluorescein staining for corneal ulceration

• Removal and morphological identification of adult worms

Detection can be difficult because T. gulosa may live deeper in the lacrimal ducts and glands rather than remaining visibly exposed on the ocular surface. (Merck Veterinary Manual)

How Is Human Thelaziasis Treated?

Complete Mechanical Removal

The central treatment is removal of every accessible adult worm.

Both U.S. T. gulosa cases were managed through physical removal, with resolution of the associated conjunctivitis. Worms may appear over several days or weeks, so continued symptoms require repeat examination rather than assuming the initial procedure was complete. (OUP Academic)

Are Human Dewormers Needed?

There is no standardised human T. gulosa drug protocol established by the two U.S. case reports.

Systemic ivermectin was considered in the second case but ultimately deferred. The infection resolved after four worms were removed and the eyes were monitored. This does not prove ivermectin is ineffective; it shows that drug treatment was not necessary in that individual case. (OUP Academic)

Antiparasitic medication should be considered only by an infectious-disease or ophthalmology team familiar with the patient and the identified species.

Are Antibiotic Eye Drops Needed?

Not automatically.

The second patient received topical tobramycin to reduce the risk of secondary bacterial infection, but this was a case-specific decision. Antibiotics do not kill Thelazia and should not replace worm removal. (OUP Academic)

What About Steroids?

Steroid eye medication should only be used under ophthalmic direction after the cornea has been assessed and infectious causes considered.

How Is Thelazia Treated in Cattle?

Mechanical Removal

Accessible worms may be removed with fine forceps after topical local anaesthesia.

This is particularly useful when worms are visible in the conjunctival sac, although deeper T. gulosa infections may be harder to clear mechanically. (Merck Veterinary Manual)

Systemic Anthelmintic Treatment

Veterinary references report activity against bovine Thelazia from:

• Levamisole

• Ivermectin

• Doramectin

• Selected pour-on macrocyclic lactones

• Eprinomectin in appropriate dairy settings

Doramectin is approved in the United States for adult eyeworms in cattle. The veterinarian must still select the correct product, route and formulation and observe all food-animal label requirements and withdrawal periods. (Merck Veterinary Manual)

Treatment of Ocular Damage

Additional treatment may be required for:

• Corneal ulceration

• Keratitis

• Secondary bacterial infection

• Severe inflammation

• Pain

• Dehydration or reduced intake in bilateral disease

Anthelmintic treatment removes the parasite burden. It does not automatically repair an ulcerated cornea.

What Is the Prognosis?

Humans

The prognosis is generally good when the worms are recognised and completely removed before major corneal injury develops.

In the two U.S. T. gulosa cases, the patients recovered without documented long-term visual damage. Symptoms may persist or recur temporarily when additional worms remain hidden. (PubMed Central (PMC))

Cattle

The prognosis is good for uncomplicated conjunctival infection treated before severe corneal damage occurs.

The outlook becomes more guarded with:

• Deep tissue involvement

• Corneal ulceration

• Perforation

• Permanent fibrosis

• Secondary bacterial infection

• Delayed recognition

• Bilateral vision loss

(Merck Veterinary Manual)

How Can Human Infection Be Prevented?

Because infection is rare, no prevention method has been proven to eliminate human risk completely.

Practical precautions during heavy face-fly activity include:

• Wearing wraparound protective eyewear near cattle

• Avoiding dense fly swarms where practical

• Using hats or face nets during high-exposure work

• Keeping flies away from the eyes without applying repellent directly to the ocular surface

• Washing hands after handling cattle eyes or ocular secretions

• Seeking care when irritation persists after fly exposure

People working closely with cattle are not expected to undergo routine parasite screening. The key is recognising the unusual combination of persistent ocular irritation and visible moving material.

How Can Cattle Infection Be Reduced?

Control Face Flies

Face-fly control may include appropriately labelled:

• Insecticide ear tags

• Dust bags

• Face rubbers

• Sprays

• Pour-on products

• Other integrated fly-control measures

No single method provides perfect control because face flies spend much of their time away from cattle.

Treat Clinically Affected Cattle

Removing adult worms and using an effective veterinary anthelmintic reduces the source of first-stage larvae available to flies.

Examine Persistent Eye Cases

Cattle that repeatedly fail pinkeye treatment should be assessed for:

• Eyeworms

• Foreign bodies

• Deep corneal ulceration

• Other ocular disease

Monitor During Warm Fly Seasons

Clinical signs in cattle are commonly associated with warm-season fly activity, although adult worms may be detected at other times of year. (MSD Veterinary Manual)

Common Thelazia Mistakes

Calling the Worm an Insect

Thelazia is a nematode. The fly is the intermediate host.

Calling the Vector a Stable Fly

The main North American T. gulosa vector is the face fly Musca autumnalis.

Assuming Humans Catch It by Touching Cattle

The established life cycle requires development inside a tear-feeding fly.

Trying To Pull a Worm From a Human Eye at Home

Uncontrolled extraction can injure the cornea and may leave other worms behind.

Assuming One Removed Worm Means the Infection Is Over

Multiple worms were present in both landmark U.S. cases.

Treating Every Case With Antibiotics

Antibiotics do not kill eyeworms.

Repeatedly Treating Cattle for Pinkeye Without Looking for Parasites

Thelaziasis can mimic infectious keratoconjunctivitis and may occupy deeper ocular tissues.

Conflating T. gulosa With T. callipaeda

They have different principal hosts, vectors and epidemiological patterns.

Claiming Climate Change Has Already Caused a Human Outbreak

The available T. gulosa case reports do not establish that conclusion.

Frequently Asked Questions

Can cattle eyeworm spread directly from a cow to a person?

Not through ordinary contact. The larvae must develop inside a tear-feeding face fly before they become infective and are deposited onto another host’s eye. (CDC)

Can a worm live and reproduce in a human eye?

Yes. An adult female T. gulosa recovered from the second U.S. patient contained developed larvae, showing that reproductive maturation can occur in humans. (OUP Academic)

Will eye drops kill Thelazia worms?

Ordinary antibiotic or anti-inflammatory drops do not reliably eliminate adult worms. Complete mechanical removal remains the central human treatment.

How many worms can be present?

More than one. Four worms were recovered in the second U.S. case and 14 in the first. (PubMed Central (PMC))

Can cattle recover completely from eyeworm disease?

Yes. Most uncomplicated infections respond well to removal, an appropriate systemic anthelmintic and fly control. Corneal ulceration, deep tissue invasion and delayed treatment worsen the prognosis. (Merck Veterinary Manual)

Final Thoughts

Cattle eyeworm infection in humans is real, but it remains extremely unusual.

The most important points are:

1. Thelazia gulosa is a nematode, not an insect.

2. Cattle are its main host.

3. The face fly Musca autumnalis is its primary North American vector.

4. The fly is a biological intermediate host, not merely a mechanical carrier.

5. Humans do not ordinarily acquire infection by touching cattle.

6. Two landmark U.S. human cases involved multiple adult worms.

7. Both patients recovered after physical removal.

8. A visible worm or persistent foreign-body sensation needs prompt ophthalmic examination.

9. One removed worm may not be the only worm present.

10. Antibiotics do not replace worm removal.

11. Cattle infections may resemble pinkeye or ocular trauma.

12. Some bovine worms occupy deeper lacrimal tissues and are difficult to see.

13. Mechanical removal, veterinary anthelmintic treatment and face-fly control are the main cattle-management tools.

14. Current evidence supports surveillance but does not prove a climate-driven human cattle-eyeworm outbreak.

15. Separate Thelazia species now emerging in North American wildlife and pets should not be confused with T. gulosa.

The real concern is not that every face fly near cattle carries a human parasite. It is recognising the rare situation in which persistent eye irritation is accompanied by a visible moving worm and responding before the cornea is damaged.

ASK A VET™ can help cattle owners review eye photographs, herd patterns, previous treatments and fly-control measures while arranging examination by a livestock veterinarian. A person with a visible worm, eye pain, light sensitivity or any change in vision needs direct medical or ophthalmic care rather than veterinary telehealth.