The Basics of Feline Herpes Virus-1 with Dr. Shelby Reinstein, DACVO
In this VETgirl online veterinary continuing education blog, Dr. Shelby Reinstein, DVM, DACVO reviews ocular disease associated with Feline Herpes Virus-1 and what you need to know! Check out this 2-part blog to learn how to diagnose and treat your cats with chronic ocular discharge!
Feline herpes virus -1 (FHV-1) is the causative agent of feline viral rhinotracheitis (FVR) and is a very common cause of upper respiratory tract disease in cats. It is considered to be the most common cause of ocular disease in cats. The virus is transmitted through direct contact with salivary, ocular, and nasal secretions and is highly contagious. Some kittens may become infected in utero from maternal transmission, however it is more common to become infected between 8 – 12 weeks of age, when the maternal antibodies are waning. The disease will spread rapidly through shelters or catteries.
When exposed for the first time, FHV-1 infects and replicates within the epithelial cells of the upper respiratory tract, conjunctiva, and cornea. This is called the primary infection. The virus induces cytopathic effects with cell lysis, and quickly spreads to adjacent epithelial cells. After the primary infection, approximately 80% of cats develop dormant virus in the trigeminal ganglion. Clinical flare ups, referred to as recrudescent disease, is associated with stressful events such as comorbid conditions, travel, introduction of a new pet, pregnancy or lactation, or iatrogenic steroid treatment (topical or systemic). The disease manifests differently in kittens with primary infections as compared to the recurrent disease in adult cats.
OCULAR DISEASES FROM FHV-1
Primary Infection versus Recrudescent Disease
The initial infection with FHV-1 leads to severe upper respiratory and ocular disease. Kittens may develop a significant fever and malaise, sneezing, coughing, and purulent nasal and ocular discharge. This is in contrast to adult cats that develop disease secondary to reactivation of latent FHV-1. Often milder in severity, recrudescent disease may manifest as transient conjunctival hyperemia and mild blepharospasm and may be unilateral. These flare ups, however, may become severe and be associated with significant discomfort, chemosis, and purulent ocular discharge. Sneezing or other signs of upper respiratory tract disease often do not accompany a flare up.
Conjunctivitis is the most common ocular manifestation of FHV-1, and is characterized by hyperemia, chemosis, blepharospasm, and ocular discharge. The conjunctival inflammation may be quite severe, particularly in kittens, leading to the formation of fibrinous membranes and areas of conjunctival ulceration due to epithelial cell loss. The formation of significant conjunctival adhesions can occur quickly and must be broken down to prevent the formation of permanent adhesions, known as symblepharon. Neonatal ophthalmia, or neonatal conjunctivitis, is a condition seen in kittens in which FHV-1 infection leads to severe conjunctivitis with or without secondary bacterial infection shortly after birth. This often leads to dramatic distension of the eyelids due to severe chemosis and inflammatory debris. Many times the accumulation of purulent discharge and inflammatory membranes leads to adhesions between the conjunctivae (eyelids, third eyelid, bulbar) called symblepharon, or extensive corneal scarring.
Corneal epithelial cell infection with FHV-1 leads to corneal ulceration, which is the second most common ocular manifestation after conjunctivitis. Early corneal ulceration is confined to the epithelial cell layer and has a dendritic or branching pattern. However, this is rarely seen clinically as these areas develop quickly into larger areas of corneal ulceration. Secondary bacterial infection will lead to progressive ulceration with increasing ulcer depth associated with loss of the corneal stroma. Conjunctivitis is usually present in conjunction with corneal ulceration and increases the risk of symblepharon formation. Some cats will recover from FHV-1 ulceration without therapy, and others will become a chronic, non-healing ulcer, which often incites intense vascularization.
Stromal keratitis is an inflammatory condition of the cornea suspected to result from an immune-mediated reaction to FHV-1 viral particles present in the epithelium or stroma. Clinically, deep corneal vascularization develops in association with areas of corneal haze from inflammatory cell infiltrate. Corneal ulceration is not present in the majority of cases with stromal keratitis, and there is often less dramatic discomfort. Treatment with topical steroids has been proven to increase the risk of development of stromal keratitis.
Chronic corneal ulceration from FHV-1 may lead to the development of a corneal sequestrum. Clinically recognized as auburn to black-brown discoloration in the cornea, the lesion develops due to necrosis of the corneal stroma. Early studies proposed that melanin granules were the source of the color change, however more recent, sophisticated imaging techniques have demonstrated that neither melanin pigment nor porphyrins are present in corneal sequestra. Often associated with a dense vascular response, sequestra may remain superficial, or can extend very deep into the corneal stroma to the level of Descemet’s membrane. Superficial ulceration surrounds the sequestrum, leading to further discomfort and risk for secondary bacterial infection.
Also known as proliferative keratitis, eosinophilic keratoconjunctivitis is an inflammatory condition that has been suggested to be associated with FHV-1 infection. The disease is characterized by infiltration of eosinophils, plasma cells, lymphocytes, and neutrophils that often begins at the lateral limbus. Clinically, white to pink, raised inflammatory tissue develops on the surface of the conjunctiva and cornea, and is usually associated with vascularization. Superficial ulceration is often present surrounding the areas of infiltrate and contributes to corneal haze. The 3rd eyelid may also be affected and appear to be thickened and inflamed. Cats exhibit differing degrees of pain, often depending upon the degree of ulceration, and will have variable ocular discharge ranging from epiphora to mucopurulent.