The conjunctiva is the thin vascular membrane that arises at the edge of the cornea and extends over the sclera as the white part around the eye and is folded back forming the conjunctival sac before covering the third eyelid and then lining the inner surface of the upper and lower eyelids. The conjunctiva is important in preventing infections reaching the deeper ocular structures - it has a dense network f blood vessels and abundant lymphoid tissue - involved in the immune response to infectious organisms such as viruses, bacteria and fungi.
The most common condition seen affecting the equine conjunctiva is conjunctivitis. This can be allergic just as in humans or related to various viral, bacterial and parasitic diseases. For instance foals with various systemic bacterial infections often present with conjunctivitis (and sometimes corneal ulceration).
Horses affected with the nematode parasite Habronema
may develop conjunctivitis as well as skin lesions around the eyes or elsewhere on the body. The nematode parasite Onchocerca
causes conjunctivitis which often present with a loss of pigment and swelling on the lateral side of the eye with infiltration into the corneal tissue and occasionally uveitis. Scraping and biopsies of the tissues are needed to diagnose either of these 2 diseases.
Allergic conjunctivitis may be seen with focal accumulations of eosinophils in the conjunctiva or even the peripheral cornea. This is diagnosed based on appearance and cytology of the lesions. Corticosteroids are usually effective in treating this condition.
In some horses the nasolacrimal ducts can become obstructed as part of the inflammatory process and may require irrigation or antibiotic infusion to clear.
Signs of conjunctivitis include ocular discharge, which can be watery or thicker and colored yellow or green (especially in bacterial infections), redness (engorgement of the conjunctival blood vessels) and swelling of the conjunctiva due to fluid (edema) developing in and beneath the conjunctiva (chemosis).
Diagnosing the cause of conjunctivitis involves on tests of tear production, cultures for bacterial infection, and scraping of the conjunctival tissue for cytology. Various tests for other systemic diseases may be needed.
Treatment of conjunctivitis depends on the specific cause. Treatment of specific underlying disease is often needed. Reduction of inflammation can be achieved with non-steroidal anti-inflammatory drugs and corticosteroids but these need to be used with care in case they potentiate any infection (particularly fungal infections of the cornea).
Tumors of the conjunctiva are quite common in the horse. The most common tumor (neoplasm) seen is squamous cell carcinoma
. Other tumors seen include more benign papillomas, hemangiomas and hemangiosarcomas and axially melanomas. The conjunctiva can be involved in cases of systemic lymphoma and present with bilateral swelling of the conjunctiva along with lymph node enlargement throughout the body. Depending on tumor type, excision and treatment with crytherapy, radiation or local chemotherapy are often effective in treating these diseases.