English
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.
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]