Most cats develop an upper respiratory infection at least once in their lifetime, with most infections occurring during kittenhood or early adulthood. The pathogens that cause these infections are highly contagious among cats, so most cats are eventually exposed, and they often become life-long carriers after their acute infection has resolved. Several of the most common upper respiratory infections in cats can also affect their eyes and cause acute conjunctivitis or more chronic inflammatory problems. These infections are extremely common, so the Envision More Veterinary Ophthalmology team wants to explain how they can affect your cat’s long-term eye health. 

How respiratory infections affect cats

Respiratory infections are extremely common in cats, and especially in kittens, young adults, and those housed in a large multi-cat environment where disease easily spreads. Most of these infections are caused by a handful of organisms—feline herpesvirus type 1 (FHV-1), calicivirus, and chlamydia or mycoplasma bacteria—that cause not only respiratory signs, but also conjunctivitis in cats. Typical infection signs may include:

  • Sneezing
  • Runny nose or thick nasal discharge
  • Lethargy
  • Decreased appetite
  • Red, swollen conjunctiva
  • Squinting
  • Eye discharge

Although vaccines can help to reduce upper respiratory viruses’ severity and spread, they don’t prevent infection entirely, so most cats acquire FHV-1 and/or calicivirus at some point. The initial infection can be diagnosed using a PCR test that looks for viral or bacterial genetic material, but is often treated presumptively with antibiotic and antiviral medications, since testing is not always reliable. Bacteria are usually cleared completely after initial infection, but viruses can remain in the body and re-activate during times of stress or other illnesses.

Herpesvirus in cats

FHV-1 affects nearly all cats, similar to the way the human cold sore virus—also in the herpes family—infects the majority of children. Most cats become sick with typical respiratory signs during the initial infection, and then the virus becomes dormant inside nerve cells. Similar to a cold sore showing up when you are most stressed, FHV-1 can reactivate throughout a cat’s lifetime. Some cats may have only one or two flares, or none at all, while others experience frequent problems. 

FHV-1 is the leading cause of conjunctivitis in cats, and often also leads to spontaneous corneal ulcers. Ulcers are painful open areas on the eye’s surface that are prone to secondary bacterial infection, and can lead to eye rupture and blindness in severe cases. In some cats, FHV-1 eye involvement becomes chronic and causes keratitis (i.e., an inflamed cornea that may become cloudy or discolored and obscure vision). Severely affected kittens can also suffer with secondary problems, including obstructed tear ducts or eye tissues that stick together (i.e., symblepharon).

FHV-1 reactivation often affects only the eye, but may also cause sneezing or a runny nose. Eye treatment includes antiviral eye drops, antibiotic eye drops if ulcers are present, and possible systemic medications in severe cases. L-lysine is a supplement with conflicting data surrounding efficacy, but may reduce recurrence in some cats. New research has shown probiotics may also effectively reduce recurrence. Cats who experience frequent FHV-1 flare ups will require a veterinary ophthalmologist’s long-term care.

Calicivirus in cats

Calicivirus is another common respiratory pathogen that may sometimes cause conjunctivitis. Cats with calicivirus may show no signs, while others can be severely affected and develop systemic illness in their lungs and joints. Most cats also develop painful oral ulcers. Many cats with calicivirus get sick only once, but then become chronic carriers and may pass the virus to others. Antivirals don’t work well for calicivirus, so treatment includes encouraging eating, treating congestion, and preventing secondary infections with oral and topical eye antibiotics.

Bacterial infections in cats

Chlamydia and mycoplasma bacteria may become secondary invaders in a cat with viral conjunctivitis, or may be the primary problem. Culture or other testing can sometimes determine whether bacteria are present in conjunctivitis, but most often your veterinarian will decide to treat an assumed bacterial infection based on the eye’s appearance. Antibiotic eye drops and ointments are effective in eliminating bacterial conjunctivitis. 

Nearly all cats are exposed to the viruses and bacteria that cause conjunctivitis, but most do not have long-term problems after initial treatment. Some cats will develop secondary problems or chronic, recurrent viral flares, which are best managed by a qualified veterinary ophthalmologist. Contact us at Envision More Veterinary Ophthalmology if your cat shows chronic conjunctivitis signs, or you have any other concerns about your cat’s eye health.