Cats don’t have as many eye problems as dogs do, but when an eye disease occurs in a cat, it is usually chronic and sometimes is a lifetime problem for the cat. The most common cause of eye issues in cats is feline herpesvirus (FHV-1). Only cats get FHV-1-- it is not transmissible to people. Most kittens will become infected with FHV-1 and will typically develop sneezing (an upper respiratory tract infection) followed by conjunctivitis and sometimes keratitis. However, unlike other types of infections, FHV-1 infections last forever; the virus establishes latency (i.e. becomes dormant) and "sleeps" in nerve cells, then can flare up again at any time to cause repeat damage to eye tissues. Flare-ups are typically triggered by stress, because stress suppresses the immune system and allows re-activation of dormant virus particles.
Corneal erosion (stained with diagnostic fluorescein dye) glowing bright green under a blue light, in a cat with FHV-1 infection. Photo courtesy of Dr. Crystal Brazle
Acute conjunctivitis in a kitten.
Chronic conjunctivitis in a Persian cat.
Conjunctivitis is inflammation of the pink membrane part of the eye, which lines the white part (sclera) and the inner eyelid. The conjunctiva can become quite reddened and swollen in some cats, and often it is just in one eye and not in both. This causes intermittent or constant squinting for months to years. Conjunctivitis may occur without any other eye problems, or the eye may also have keratitis (corneal inflammation; the cornea is the clear windshield part of the eye), and/or uveitis (intraocular inflammation). Keratitis can manifest as a corneal ulcer (painful open sore on the cornea, where the surface cells of the cornea have been lost), or a cloudy cornea due to edema (swelling due to excess water), blood vessels, inflammatory cells, and/or scar tissue. Conjunctivitis, keratitis, and uveitis can manifest as separate entities or in combination, and are most often caused by feline Herpesvirus-1 (FHV-1). This article focuses primarily on ocular FHV-1 as the cause of squinting in cats.
Signs include squinting (which can be severe, especially if the cornea has an ulcer) and watery or mucoid discharge (that can be clear, gray, yellow, green, or a rusty dark red color that looks like dried blood but ISN’T). Cats that have this unusual dark red tear color are often Persians or Himalayans.
The conjunctiva is pink/red and sometimes swollen or thickened. It can be so swollen that the cornea is hidden from view. With keratitis, the cornea can be clear if there is an acute and very superficial ulcer (i.e. erosion), or can be cloudy. The iris (the colored part of the inside of the eye, that forms the pupil) can be a different color too (usually a “muddier”, duller color than normal) if uveitis is present. All of these eye signs may be present, or some of them, and it may be in both eyes or one eye. Sometimes the cat shows signs of an upper respiratory tract infection (URTI), and is sneezing. The URT often precedes ocular disease.
Another important clinical sign that is often present is pain. Some cats with ocular FHV-1 disease are very painful-- more than what you would expect from inflamed eye tissues. This is because the virus not only affects sensitive tissues of the eye, but also affects nerve tissue, and irritated or damaged nerves can cause severe pain (neuralgia). It can be very difficult to identify pain in cats: as a self-reliant, territorial species (both predator and prey), the domestic cat naturally hides signs of stress or pain. Also keep in mind that cats are genetically about 50% domesticated, compared with nearly 100% in dogs. Thus, cats try to instinctively hide their pain; they "shut down" and become lethargic. Depression, poor appetite, crabbiness, inappropriate elimination, hiding-- these can all be signs of pain in cats.
While there are several causes of conjunctivitis and keratitis in cats, the most common cause is FHV-1. However, trauma must be ruled out, especially in a multi-cat household and if your cat is allowed outside. Other causes (which are beyond the scope of this article) include eosinophilic keratitis, corneal sequestrum, lens luxation, uveitis, and glaucoma. While many different kinds of infectious organisms can cause conjunctivitis, there are only three that are common culprits: feline Herpesvirus-1 (FHV-1), feline Chlamydia (Chlamydophila felis), and feline Mycoplasma (multiple species). A case of conjunctivitis can have one, two, or all three organisms together causing the problem. Of these three organisms, only FHV-1 can also cause corneal ulcers.
This article will focus on FHV-1 ocular disease in cats, as this is by far the most common cause of squinting in cats, and is a very frustrating disease to treat. FHV-1 cannot be transmitted to people — only to cats. It is a species-specific virus.
FHV-1 is a very common virus in cats, and most of the cats on this planet have it and are exposed to it when they are small kittens-- typically infected by their mothers. The cat is infected when the virus either contacts the surface of the eye, or is inhaled into the upper respiratory tract (URT). The virus loves to attack epithelial tissue of the eyes and URT (epithelium is a layer of cells that lines tissues throughout the body, and there is epithelium lining the surface of the eye and the surface of the URT). After the initial (i.e. primary) infection subsides, FHV-1 will travel from the eye and URT epithelial cells into neighboring nerve cells, and establish lifelong latency (i.e. live in a dormant state for the cat's entire life) in those nerves-- but can flare up at any time. When the virus flares up (i.e. "recrudescence"), it activates and replicates (makes more copies of itself), and some of these new viral particles will travel back through nerve fibers to reach the epithelial cells and infect and damage them again. This activated virus can be "shed" from the body (via eye discharge and droplets from sneezing) and infect other cats.
As far as eye issues are concerned, after the primary infection in the conjunctiva and/or cornea, the virus can then travel to and live in a dormant state in the trigeminal ganglia, which are two clusters of nerve cell bodies located in both sides of the head. Often the virus infects only one trigeminal ganglion, so it is common in cats for flareups of ocular FHV-1 to always occur in one eye and not the other. A key factor in the severity of the disease and how recurrent it is, is STRESS. Anything that stresses the cat can suppress its immune system and allow FHV-1 to reactivate and cause problems. This is similar to coldsores in people, which are caused by a human Herpesvirus. Cold sores can worsen when the person is stressed. Cats are the same way!
Often, the medical history and the clinical signs present are sufficient to aid in diagnosis of the cause of squinting, but sometimes special tests are needed, and sometimes specialists are needed--- board-certified veterinary ophthalmologists (see www.acvo.org). Chylamidia and Mycoplasma infections will typically resolve on their own, hastened by topical antibiotic therapy. Diagnostic tests include (but are not limited to): collection of conjunctival and/or corneal cells for culture, or cytology (collecting conjunctival or corneal 'scrapings' to look at the cells under a microscope). Usually a diagnostic test is performed in which a green dye called fluorescein is applied to the eye. This dye sticks to the surface of the "raw" corneal surface wherever there is an ulcer on the cornea. The dye fluoresces bright green (making it easier to see the area of missing surface cells) when a special blue light is shone on the eye.
It is also important to know that clinically normal cats can test positive for Chylamidia, Mycoplasma, and/or FHV-1, so the results of diagnostic tests must be interpreted with caution. While there are special DNA tests to look for the presence of FHV-1, these are not recommended as they do not adequately diagnose the presence of active FHV-1 disease. This is because nearly 100% of all cats have been infected by FHV-1 when they are kittens; the virus will then establish latency in the trigeminal ganglion and the cats will intermittently shed the virus for the rest of their lives with or without signs of the disease, so a positive test is meaningless. Cats with clinical signs of ocular FHV-1 can also shed the virus at levels below what a test can detect. And no test detects the virus in all cats with disease, so a negative test is as meaningless as a positive test.
The diagnosis of ocular FHV-1 requires careful evaluation of the medical history and clinical signs, and evaluating the response to therapy.
Mycoplasmal infections usually respond well to topical antibiotics, and chlamydial infections often also respond but can recur. However, FHV-1 infections do not respond to topical antibiotic therapy, and are very frustrating to treat. Sometimes topical and/or oral antiviral medications are also used, but care must be taken in choosing these medications, due to cost and high frequency of dosing.
Because ocular FHV-1 is a very common and complicated disease, we will discuss this virus in more detail.
It is also important to know that FHV-1 conjunctivitis and/or keratitis cannot be cured, only controlled. FHV-1 viral particles live in the body forever. In between flare-ups, the cat’s eyes can be normal. Flare-ups can occur frequently, or there can be years in between each flare-up. Some cats will never have a flare-up.
There are many different possible treatments for conjunctivitis and keratitis caused by FHV-1, depending on the clinical signs present and tissues affected. If the cornea is involved, vision might be compromised if the virus is not controlled, but it is rare for a cat with ocular FHV-1 infection to become totally blinded by the disease. Rather, the disease is more likely to cause long-term pain and discomfort without proper treatment. Some cats with ocular FHV-1 disease are very painful; just as shingles in people (caused by a flare-up in adults of childhood chickenpox, which is a human Herpesvirus) can be quite painful, so can FHV-1 infection in cats if pain nerve fibers are affected. Additionally, a great challenge in treating cats with FHV-1 ocular disease is that the treatment itself can be stressful, and the stress can aggravate the disease.
In addition to specific antiviral therapy, alleviating stress is very important in cats affected with FHV-1. If possible, eliminate the stressors that are affecting your cat. Stress is triggered by any change in the daily routine. Your home is your cat's universe. Some common stressors are remodeling the home, boarding the cat, holidays, adding a new pet to the household, and the owner being gone for awhile. Please complete this Stress Audit for your cat, to help you determine your cat's stressors so that you can address them and help your cat.
Most cats with active ocular FHV-1 infections are painful. Remember that pain is also a stressor, and pain and stress can feed on each other, creating a vicious cycle. The pain can be due to corneal ulcers, but severe pain can also be caused by nerves infected and irritated by FHV-1-- similar to the neuralgia present in people with shingles. Treatment of pain MUST be addressed. Therapeutic options include (but are not limited to): oral pain medication (e.g. oral gabapentin for neuralgia; oral buprenorphine); identifying and reducing stressors; topical lubricants for dry eye (FHV-1 causes dry eye in most cats); topical antibiotic therapy if corneal ulcers are present; topical antiviral medication (e.g. cidofovir); oral antiviral medication (famciclovir); supplements to help boost the immune system and reduce stress; and pheromone therapy (e.g. valerian root) to help reduce stress. It is important that therapy does not cause MORE stress, and it is optimal that as many therapies as possible be given passively and not forced upon the cat. If possible, oral medications and supplements should be mixed with food. Each patient is unique in its stressors and reactions to stress, and therapy must be tailored to each cat. A word about famciclovir: this medication helps weaken the virus and reduce neuralgia and is critical for the well-being of many cats with severe ocular FHV-1, but it tastes terrible; pills should not be ground up and mixed in food because cats won't eat it. Cutting the pills in half is also not recommended, unless the cut edge can be coated so that the cat can't taste it. And this medication should not be tapered; always give the full dose (90 mg/kg twice daily). Because this medication is critical to give to some patients and because the pills are large (especially for obese/large cats), some owners elect to bring their cats to the veterinary clinic twice daily for veterinary staff to pill the cats-- at least for the first 3-5 days of therapy. It is often helpful to first give cats gabapentin for neuralgia and sedation, to then make it easier to administer famciclovir.
Cats may benefit from special oral nutritional antioxidant supplementation to strengthen immune function. Supplements that modulate the immune system ( Imuno-2865 ) and that help alleviate epithelial damage from viral infection ( SHaNa-Vet ) may help in supporting cats with FHV-1 infection, as long as administration is not stressful (such as mixing the contents of the capsules in food rather than pilling your cat). Imuno-2865 contains natural beta glucans derived from mushrooms and grasses, that support immune function. SHaNa-Vet contains the natural plant extracts docosanol and triacontanol, which belong to the family known as policosanols, and are predominantly found in naturally occurring phytowaxes such as rice bran and sugar cane waxes, as well as beeswax.
Docosanol inhibits fusion between lipid- enveloped viruses (such as Herpesviruses) and the plasma membrane of their host cells, thus blocking viral entry and replication, shortening both healing time and duration of symptoms. Triancontanol is present in plant cuticle waxes, tea leaves, and rice bran waxes, and exhibits potent anti-inflammatory action via lipid peroxidation. L-lysine is an amino acid that may help weaken viral replication.
Have your family veterinarian examine your pet. Sometimes your doctor may recommend referral to a veterinary ophthalmologist (which is often the case if FHV-1 is suspected to be present) if your cat’s eyes are not improving with medication or are worsening.
If traveling is stressful for your cat, please contact your family veterinarian to see if they can prescribe an oral sedative for you to give your cat prior to travel to the veterinary hospital. One medication that is often very helpful is gabapentin.