Feline HerpesVirus 1 (Feline Viral Rhinotracheitis)
Feline herpesvirus 1 (FHV-1) is a common and highly contagious contributor to feline upper respiratory tract disease (FURTD), most commonly manifesting as acute rhinotracheitis and keratoconjunctivitis.
Etiology and pathogenesis
FHV-1 is an enveloped DNA virus that is endemic in the domestic cat population. It is most commonly transmitted through direct cat-to-cat contact and via fomites.Virus is shed in oral, nasal and ocular secretions of acutely infected cats and those with recrudescence of latent infection. It remains infectious for up to 18 hours on fomites (e.g. cages, examination tables, food and water bowls, and human clothing). Less commonly, aerosol transmission occurs via sneezing and coughing.
FHV-1 has an affinity for conjunctival, corneal, nasal and laryngotracheal epithelium, where multifocal epithelial necrosis results in acute rhinitis, tracheitis, laryngitis and keratoconjunctivitis after an incubation period of 2–6 days. In some cats, FHV-1 also causes severe damage (necrosis and osteolysis) to the nasal turbinates. Rarely, FHV-1 has been associated with chronic ulcerative dermatitis and chronic corneal disease in cats.
Nearly all cats that recover from FHV-1 infection become latent subclinical carriers for life. These latent carriers can shed FHV-1 intermittently, with or without episodic clinical signs, when reactivation is triggered by factors such as stress, parturition, lactation or glucocorticoid therapy.
Geography
FHV-1 is prevalent in cats worldwide.
Incidence and risk factors
FHV-1 is common, and accounts for up to 40% of FURTD infections. The incidence of FHV-1 infection is highest in young kittens and unvaccinated cats. Risk factors include stress, high-density group housing and close cat-to-cat contact, as in shelters, catteries and multi-cat households, especially where unvaccinated kittens are exposed to subclinical carrier adult cats. Shedding of FHV-1 is often triggered by parturition and lactation in latent carrier queens, which is an important source of infection in newborn kittens. The prevalence of subclinical carriers of FHV-1 in the healthy pet cat population ranges from 2% to 10%, but is up to 40% in shelter and cattery cats.
Clinical signs
Diagnosis
For individual infected cats, a presumptive diagnosis of “acute infectious respiratory disease” or FURTD based on typical clinical signs and likelihood of exposure is usually adequate for patient management.
Mucosal specimens evaluated by PCR, direct immunofluorescence or virus culture can establish a definitive diagnosis.51, 75 These tests are not routinely needed in most individual cases because infection is typically self-limiting in 2 weeks or less. Confirmatory testing can be useful for evaluating disease outbreaks in catteries and shelters, and for diagnosis in individual cats with severe or atypical clinical signs.
Diagnostic testing
Hematology and urinalysis
Routine tests are normal in most infected cats. Cats with secondary bacterial pneumonia may develop a neutrophilic leukocytosis. Cats with severe or prolonged signs of FURTD should be tested for feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV) as potential causes of immunosuppression.
Radiology and imaging
Thoracic radiography is normal in most cats with FHV-1 infection, except in severe cases with pneumonia.
Nasal CT and rhinoscopy are useful for evaluating cats with chronic nasal sequelae, such as frontal sinusitis, severe turbinate damage or nasopharyngeal stenosis, and to differentiate these from other causes of chronic nasal disease.
Cytology
Cytology from affected areas of the upper or lower respiratory tract shows non-specific mucopurulent or mixed inflammation. Intranuclear viral inclusions can occasionally be identified early in the disease in conjunctival scrapings or mucosal biopsies.
PCR
PCR testing can be performed on oropharyngeal, nasal, conjunctival and corneal swabs or scrapings, or airway lavage or lung specimens.PCR identification of FHV-1 does not confirm causality of disease because non-clinical carriers of FHV-1 infection canalso be PCR positive. Results must be interpreted in conjunction with clinical signs and circumstances.
Direct immunofluorescence
FHV-1 can be identified in swabs or scrapings of nasal mucosa or conjunctiva submitted to a specialized lab, but this is less sensitive than PCR.
Virus culture
FHV-1 can be isolated in cell culture from swabs of the oropharynx, nasal cavity or conjunctiva submitted to a specialized lab, but this is less sensitive than PCR.
Serology
Measurement of FHV-1 serum antibodies is of limited use for diagnosis because most cats have antibody titers induced by prior vaccination or subclinical exposure.
Treatment
In most cats, acute FURTD is self-limiting and the main treatment is supportive nursing and comfort care, sometimes combined with antibiotics or antiviral therapy, depending on the severity.
FHV-1 disrupts the ocular tear film, so topical tear replacement eyedrops may be beneficial.76 Dehydration should be prevented to minimize drying and thickening of respiratory secretions, which can occlude airways. Infectious respiratory disease should be treated on an outpatient basis whenever possible to prevent contamination of the veterinary facility and spread of infection to hospitalized cats. Rarely, severe or complicated infections (mostly in young kittens) or prolonged anorexia may require additional care (e.g. parenteral fluid therapy, oxygen therapy, tube feeding or appetite stimulants).
Antibiotics have no effect on FHV-1, and are not routinely needed in typical self-limiting viral respiratory infections, but antibiotics such as doxycycline may be considered in cats with mucopurulent naso-ocular discharge, fever and lethargy, if secondary bacterial infection or co-infection is suspected.
Oral l-lysine has been used to inhibit FHV-1 replication in herpes conjunctivitis and to reduce shedding of FHV-1. However, recent studies have concluded that lysine is not effective for prevention or treatment of FHV-1.77–79
Antiviral drugs are used when FHV-1 signs are severe, persistent or recurrent. Famciclovir (90 mg/kg PO q8–12h) is the most effective and well tolerated in kittens and adult cats for inhibiting FHV-1 replication, decreasing viral shedding, shortening recovery time, and treating both acute and chronic disease caused by FHV-1.80 Other oral antiviral drugs used in human herpesvirus infection (e.g. acyclovir, valacyclovir) are less active against FHV-1 and are toxic for cats. Interferon has not been effective for treating FHV-1.
Topical ophthalmic antivirals are beneficial in cats with severe or refractory FHV-1 eye disease (e.g. keratitis, corneal ulcer, chronic keratopathies).
These include cidofovir 0.5% (1 drop OU, q12h), idoxuridine 0.1% (1 drop OU, q4–6h) or vidarabine 3% (1 drop OU, q4–6h).81 Eye irritation should be monitored as a potential side effect of ophthalmic antivirals. Topical corticosteroids should be avoided.
Prognosis/complications
The prognosis is usually excellent and clinical signs resolve within 1–2 weeks. Recovered cats remain latent subclinical carriers for life. Bacterial pneumonia is a rare but serious complication in young kittens, and the prognosis is more guarded.
Uncommon chronic complications and sequelae of FHV-1 include chronic keratoconjunctivitis, chronic rhinosinusitis, nasal turbinate damage resulting in fibrosis and stenosis of nasal passages, and chronic ulcerative dermatitis.
Severe keratoconjunctivitis can result in adhesions of the conjunctiva to the cornea (symblepharon). In neonatal kittens, panophthalmitis (ophthalmia neonatorum) can cause irreversible blindness.
Prevention
Vaccination against FHV-1 is recommended as a “core vaccine” for all cats.Vaccination is effective for preventing or minimizing clinical illness caused by FHV-1, but it does not completely prevent infection, eliminate the chronic carrier state or prevent virus shedding.
Modified live virus (MLV) injectable, inactivated (killed virus) injectable and MLV intranasal (IN) vaccines are available, and all are reasonably effective.
IN vaccines induce faster and possibly better protection while avoiding adjuvant-related side effects, but mild sneezing and oculonasal discharge are common after IN vaccination. Commercial trivalent core vaccine products combine FHV-1 with feline calicivirus (FCV) and feline panleukopenia virus (FPV).
FHV-1 and other feline respiratory pathogens are highly contagious, so FHV-infected cats should always be isolated from other cats. In addition, routine infection control measures combined with reducing stress and overcrowding help prevent the spread of respiratory disease in catteries and shelters.