Conjunctivitis (Pink Eye) in Horses: Causes, Treatment & Prevention

Conjunctivitis, also known as “pink eye”, refers to inflammation of the conjunctiva, the mucous membrane that covers the front of the eye and lines the inside of the eyelids. This condition is common in many species of animals, including horses. ^[1]

Various factors can trigger conjunctivitis in horses, including bacterial or viral infections, allergies, irritants, tumors, and injuries. In some cases pink eye develops in response to a direct trigger and in others it arises secondary to another underlying condition. ^[2]

By identifying pink eye early and taking action promptly, horse owners and care takers can limit discomfort and potential complications in affected horses.

Early diagnosis and treatment are key to easing symptoms, curbing the spread of infection and ensuring a complete recovery, safeguarding the horse’s overall eye health.

Conjunctivitis in Horses

There are multiple forms of pink eye in horses, categorized according to their underlying cause. Infectious conjunctivitis is caused by pathogens such as bacteria, viruses, or fungi. This type leads to inflammation characterized by redness, discharge, and swelling, and can spread between horses or to the other eye. ^[1][2]

In contrast, non-infectious conjunctivitis results from factors such as allergies, irritants, or injuries. While this form also causes inflammation, it does not involve an infection and there is no risk of spread between horses. Management focuses on addressing the underlying cause, such as controlling allergens or removing irritants.

Conjunctivitis can affect either one eye (unilateral) or both eyes (bilateral). Unilateral conjunctivitis is often caused by localized issues such as trauma, a foreign body, or a specific infection. It may also result from localized irritants or chemical exposure.

Bilateral conjunctivitis can be associated with systemic (whole body) causes or widespread exposure. ^[1] Systemic causes include viral infections, such as equine herpesvirus (EHV), and environmental allergens or irritants.

Symptoms

Recognizing the signs of conjunctivitis early is key to ensure timely intervention and treatment. Horses may exhibit signs of conjunctivitis in one or both eyes.

Common clinical signs include: ^[1][2][3][4]

• Red or pink eyes
• Chemosis (swelling of the conjunctiva)
• Watery or thick discharge from the eyes (clear, yellow, green)
• Tearing and squinting
• Rubbing and pawing at the eyes
• Lacrimation (shedding of tears)

If any of these symptoms are observed, it’s essential to seek veterinary attention promptly. Early diagnosis and proper treatment are key to alleviating discomfort, preventing complications, and ensuring the horse’s overall eye health.

Causes

The severity of conjunctivitis and appropriate treatment varies depending on the underlying cause. Pink eye in horses can be broadly categorized as resulting from infectious or non-infectious causes.

Infectious Causes

Infectious causes are related to some form of pathogenic infection. In horses, infections that cause pink eye may be associated with:

• Bacteria
• Viruses
• Fungi

Bacterial Conjunctivitis

Like the body’s other orifices, the conjunctiva of the eye is naturally colonized by various bacteria. Under normal conditions, these bacteria inhabit the eye without causing harm. However, when this balance is disturbed by factors such as surgery, trauma, or other underlying conditions, healthy flora can become opportunistic pathogens. ^[5][6]

Primary bacterial conjunctivitis in horses is often caused by the bacterium Moraxella equi. Systemic infections, such as those caused by Streptococcus equi, the bacterium responsible for strangles, can also lead to secondary conjunctivitis. ^[2][3][6][7]

Horses with bacterial conjunctivitis typically exhibit a thick, yellow-green discharge, which may appear pus-like and can crust around the eyes, leading to further irritation. This condition typically develops in both eyes and has the potential to spread between horses, particularly in environments with poor hygiene or close confinement. ^[7]

Effective management requires prompt veterinary attention to initiate appropriate treatment and prevent the spread of infection.

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Viral Conjunctivitis

Viral conjunctivitis in horses is commonly caused by viruses such as equine herpesvirus (EHV), leading to inflammation of the conjunctiva accompanied by mild eye redness. ^[8]

Unlike bacterial conjunctivitis, which often presents with thick, pus-like discharge and significant swelling, viral conjunctivitis typically results in clearer, more watery discharge and may be associated with respiratory symptoms. ^[2][8]

While bacterial conjunctivitis can be severe, viral conjunctivitis tends to have a more gradual onset and generally resolves with supportive care as the viral infection runs its course. ^[8]

In cases where standard treatment is insufficient, topical antiviral ophthalmic preparations such as trifluridine and interferon are used to manage chronic or non-responsive cases of EHV conjunctivitis. ^[8][9]

Other Types of Infectious Conjunctivitis

In addition to EHV and strangles, less common infectious causes of pink eye in horses include: ^{[1][2][10][11][12][13]}

• Chlamydial Conjunctivitis: This type of conjunctivitis in horses is caused by the bacterium Chlamydia psittaci, which can lead to inflammation and swelling of the conjunctiva. It is relatively uncommon in horses but may occur, especially in environments or among animals where Chlamydia is common, such as sheep.
• Mycotic Conjunctivitis: This form of pink eye is caused by fungal infections, typically following traumatic eye injuries. Fungi such as Aspergillus or Candida infect the conjunctiva, leading to chronic inflammation, redness, and a discharge that may have a distinctive odor. This condition is relatively rare in horses.
• Parasitic Conjunctivitis: This type of conjunctivitis is caused by various parasites including Habronema megastoma, H. muscae, Draschia megastoma, Onchocerca cervicalis, Thelazia lacrimalis, and Trypanosoma spp. These parasites infect the conjunctiva and surrounding tissues, resulting in inflammation and clinical signs of conjunctivitis in horses.

Non-Infectious Causes

Any cause of conjunctivitis in horses that is unrelated to pathogens is non-infectious. These forms do not transmit between horses. Non-infectious causes in horses include:

• Allergies
• Injuries
• Tumors/Neoplasia

Allergic/Irritant-Induced Conjunctivitis

Allergic conjunctivitis in horses occurs when the conjunctiva becomes inflamed due to an allergic reaction. This type of conjunctivitis can result from direct topical application of various substances, such as medications, as well as exposure to environmental allergens like pollen and aerosol sprays. ^[2][14]

Allergic reactions are often bilateral and usually accompanied by systemic symptoms, such as itching (pruritis) and coughing.

In contrast, irritant-induced conjunctivitis is related to direct exposure to irritants such as dust, environmental pollutants, or applied chemical substances. While this type of conjunctivitis is also typically bilateral, it usually lacks the broader systemic signs associated with allergic responses, such as coughing and sneezing. ^[14]

Traumatic Conjunctivitis

Eye trauma can lead to conjunctivitis in horses when direct injury or foreign objects cause inflammation of the conjunctiva. Common causes of eye trauma include being struck by branches, rough handling, or accidental injury from other animals or equipment. ^[1][2]

The affected eye may show signs such as scratches or abrasions on the cornea, and the discharge can range from watery to pus-like, depending on whether a secondary infection develops.

Conjunctival Neoplasia

Conjunctival neoplasia refers to inflammation of the conjunctiva resulting from the presence of tumors or abnormal growths on the eye’s mucous membrane. This condition can be triggered by both benign and malignant tumours. ^[4][15]

Among the various types of tumors, squamous cell carcinoma (SCC) is a common and notable malignant tumor in horses that often affects the conjunctiva. SCC is characterized by the uncontrolled growth of squamous epithelial cells and can lead to severe ocular issues if not treated promptly. ^{[2][16][17][18]}

The presence of such tumors can cause chronic inflammation of the conjunctiva, which appears as persistent redness, swelling, and discharge. In advanced cases, SCC can lead to more severe complications, including ulceration and vision loss.

Diagnosis

Investigation of conjunctivitis includes both an ocular and physical examination. During the ocular exam, the veterinarian inspects the conjunctiva for redness, swelling, and discharge, noting its color and consistency, which can indicate the type of conjunctivitis. ^[1]

Illustration: Dr. Ana Mesa, PhD

They also assess the cornea for ulcers or abrasions using fluorescein dye and check the eyelids for swelling or foreign bodies. The physical examination includes a general health assessment to identify any systemic symptoms or underlying conditions, and evaluates the horse’s environment for potential irritants or allergens. ^[2][3]

Additional Diagnostic Testing

Additional diagnostic testing for conjunctivitis in horses may be necessary to identify the underlying cause. Further diagnostics may distinguish between bacterial, fungal, viral, allergic, irritant, neoplastic or trauma-related origins.

By pinpointing the exact cause, veterinarians can determine the most effective treatment, such as selecting the appropriate antibiotics or antifungal medications based on culture and sensitivity results.

Additional tests include: ^{[1][3][18][19][20]}

• Conjunctival Swabs: Used to collect samples from the surface of the conjunctiva for laboratory analysis. These swabs are essential for culturing bacteria or fungi to identify infectious agents and perform sensitivity testing to determine the most effective antimicrobial treatment.
• Conjunctival Scrapings: Involves taking a sample of conjunctival tissue for microscopic examination. This procedure provides detailed cytological information that can reveal the presence of bacteria, fungi, parasites, or abnormal cells. Scrapings are particularly useful for diagnosing chronic or unusual cases where other tests might be inconclusive.
• Schirmer Tear Test (STT): Measures tear production by placing a special strip of paper under the lower eyelid. This test helps assess whether the horse is producing adequate tears, which may indicate inadequate tear production is contributing to conjunctivitis.
• Biopsy: Involves removing a small sample of conjunctival tissue for histopathological examination. This test is used to rule out or confirm neoplastic conditions, such as tumors, and to evaluate the extent of any pathological changes. Biopsy results are vital for diagnosing tumors and determining the appropriate treatment strategy.

Treatment

Treating conjunctivitis in horses is challenging due to the various factors that can cause the condition, all of which present with similar symptoms like redness, swelling, and discharge. This overlap makes it difficult to pinpoint the exact cause based on symptoms alone, so discussing treatment options with a veterinarian is necessary.

Prior to administering medication, the veterinarian cleanses the eye by irrigating the conjunctival sacs with pharmaceutical grade saline to remove any debris. If there is a related issue like dacryocystitis (blocked tear duct), the treating practitioner may also flush the duct system with saline or antiseptic wash. ^[1]

For bacterial or fungal infections, broad-spectrum antimicrobials are typically administered immediately upon diagnosis. Treatment may be adjusted based on culture and sensitivity results if there is no improvement within 7 – 10 days. ^[1][2]

Viral conjunctivitis is managed primarily with supportive care. For persistent or severe cases, such as those caused by equine herpesvirus (EHV), topical antiviral treatments like trifluridine or interferon may be used to control the virus and alleviate symptoms. ^[1][2][9]

In cases of allergic conjunctivitis, antihistamines and corticosteroids can help relieve symptoms. If irritants are the cause, removing the irritant and using soothing treatments like artificial tears or anti-inflammatory medications can reduce inflammation and discomfort. ^[1][14]

Neoplastic Conjunctivitis

Treatment for neoplastic conjunctivitis in horses depends on the type and extent of the neoplasia. Surgical removal is often the primary treatment for localized tumors like squamous cell carcinoma (SCC), aiming to excise the tumor completely while preserving healthy tissue. ^[4][18]

Other potential treatments for neoplastic conjunctivitis include: ^[4][21][22]

• Cryotherapy: utilizes liquid nitrogen to freeze and destroy abnormal tumor cells, particularly effective for superficial or smaller tumors
• Laser Therapy: employs a laser to precisely target and remove or reduce the size of the tumor, useful for delicate or hard-to-reach areas
• Radiation: applied when tumors are inoperable, have metastasized (spread), or when surgical options are limited. This method targets cancerous cells with focused radiation

Each treatment option is selected based on the tumor’s characteristics and the horse’s overall health. Early intervention provides the best chance of a positive outcome.

Prognosis

The prognosis for conjunctivitis in horses largely depends on its underlying cause. Bacterial and fungal conjunctivitis usually resolves with appropriate treatment, often within 5 – 7 days.

Persistence or recurrence can occur if there is an underlying issue or if the infection is resistant to the type of antibiotic prescribed. ^[1]

Viral conjunctivitis often resolves on its own with supportive care and, in some cases, antiviral medications. However, it can recur, especially with equine herpesvirus (EHV), which may lead to a guarded prognosis. ^[8]

Allergic and irritant conjunctivitis usually improve once the allergen or irritant is identified and removed, but it’s important to note that allergies are a life-long condition. ^[14] Any horse with known allergies is expected to have recurring symptoms whenever they are re-exposed to their specific allergen, which may be unavoidable in some cases.

For neoplastic conjunctivitis, outcomes vary based on the type and extent of the tumour, affecting both treatment and prognosis. In cases where conjunctivitis is secondary to systemic or other ocular diseases, prognosis depends on the severity and management of the underlying condition. ^[1]

Overall, most cases of conjunctivitis have a favorable prognosis if the underlying cause is effectively addressed.

Prevention

Preventing conjunctivitis in horses isn’t always possible, but reducing its occurrence involves a combination of good management practices, environmental control, and regular veterinary care. ^[1]

By implementing strategies to maintain eye health and minimize the risk of both infectious and non-infectious causes, horse owners can effectively lower the likelihood of conjunctivitis outbreaks.

Prevention strategies include: ^[7][14][23]

• Maintain Effective Hygiene: Regularly clean and disinfect feeding equipment, water troughs, and stable areas to minimize exposure to bacteria and irritants. This helps reduce the risk of infections that can lead to conjunctivitis. Use clean, dust-free bedding and keep it dry to prevent mold and other contaminants that could irritate the eyes.
• Manage Environmental Allergens: Reduce dust in the environment by regularly sweeping or dampening surfaces with water. Whenever possible, limit the horse’s exposure to known allergens and irritants. This can be accomplished by managing pasture conditions and controlling the presence of weeds and other potential allergens.
• Adhere to Robust Biosecurity Practices: Implement quarantine measures for newly introduced horses or those showing signs of illness to prevent the spread of infectious conjunctivitis and other diseases, such as Strangles. Use personal protective equipment, such as gloves and face masks, when handling horses with eye infections to reduce the risk of spreading disease.
• Avoid Unnecessary Medications and Irritants: Avoid applying unnecessary topical medications or treatments that could irritate the eyes. Always follow veterinary advice when using any eye treatments to prevent adverse reactions.

Summary

Conjunctivitis, or “pink eye,” refers to inflammation of the conjunctiva and is the most frequently acquired eye disease in animals.

• Conjunctivitis in horses can result from infectious or non-infectious causes, including bacteria, viruses, fungi, allergies, irritants, trauma, and tumors
• Clinical signs include redness, swelling, discharge, tearing, squinting, and eye rubbing
• This condition is diagnosed in horses through physical and ocular exams, swabs, scrapings, tear tests, and biopsies
• Treatment depends on the cause and may include antibiotics, antifungals, antiviral medications, anti-inflammatory drugs, surgical removal for tumors, and supportive care
• The prognosis for horses with pink eye is generally favourable, but varies based on the specific cause and severity of the condition