FELINE CALICIVIRUS
Feline calicivirus (FCV) is a highly contagious contributor to feline upper respiratory tract disease (FURTD) syndrome. Subclinical, latent or carrier states of infection are common, and affected cats usually have acute ocular and nasal disease, together with oral ulcers.
History of high-density housing is frequent.
Etiology and pathogenesis
FCV is most commonly transmitted through direct cat-to-cat contact and fomites.60 Virus is shed in oral, nasal and ocular secretions, and sometimes urine and feces. FCV remains infectious for up to 4 weeks in the environment and on fomites (e.g. cages, examination tables, food and water dishes, and human clothing). Uncommonly, aerosol transmission due to sneezing and coughing may occur.
FCV has an affinity for oropharyngeal and upper respiratory epithelium and pulmonary alveolar macrophages.
Signs develop after an incubation period of 2–5 days.61
Geography
Worldwide, although the prevalence varies geographically.
Incidence and risk factors
FCV is very common and believed to account for up to 50% of feline upper respiratory tract infections.
The prevalence of subclinical carriers is 5–10% in the healthy pet cat population, 20% in show cats and 25–75% in cattery and shelter cats.60, 61 The incidence of disease is poorly defined, but is highest in young kittens and unvaccinated cats.
Risk factors include history of stress, high-density group housing and close cat-to-cat contact in shelters, catteries and multi-cat households, especially where unvaccinated kittens are exposed to subclinical carrier adult cats.62, 63 Recovered cats can remain subclinical carriers and shed infectious virus continuously for weeks, months or even years.
Clinical signs
However, FCV typically causes oral ulceration(tongue, palate, gingiva) (Figs. 1.19, 1.20), mild rhinitis (sneezing, nasal discharge) and conjunctivitis (ocular discharge), accompanied by an abrupt onset of inappetence, lethargy and fever.
Clinical disease is usually self-limiting within 1–2 weeks. A subclinical or carrier state is common, and these cats often appear normal.
Viral pneumonia may occur in young kittens, which manifests as cough, labored breathing and secondary bacterial pneumonia. Transient synovitis, fever and joint pain (limping, reluctance to move), with or without concurrent respiratory signs, occur in some cats.
Highly virulent strains of FCV have emerged over the last decade in North America and Europe in sporadic outbreaks of severe virulent systemic disease with vasculitis, mostly in shelter cats.
Widespread vasculitis can cause epistaxis, GI bleeding, icterus, facial and limb edema, skin and footpad ulceration, abdominal pain, abdominal effusion and labored breathing (pulmonary edema, pleural effusion). The mortality rate is up to 50%, and even healthy, well-vaccinated adult cats can succumb rapidly. Persistent FCV infection is associated with chronic ulcerative–proliferative gingivostomatitis with signs of oral pain, dysphagia, halitosis, hypersalivation and bleeding.
Diagnosis
For individual infected cats, a presumptive diagnosis of “acute viral respiratory disease” or FURTD based on typical clinical signs and likelihood of exposure is usually adequate for patient management.
Mucosal specimens evaluated by virus culture, PCR or direct immunofluorescence can establish a definitive diagnosis of FCV.51 These tests are not routinely needed
in most individual cases because infection is typically self-limiting in 2 weeks or less. However, confirmatory testing is useful for evaluating disease outbreaks in catteries and shelters, and for diagnosis in individual cats with severe or atypical clinical signs. 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.
Diagnostic testing
Hematology and urinalysis
Cats with the virulent systemic form of FCV can have anemia, neutrophilic leukocytosis, thrombocytopenia, hyperbilirubinema, hypoalbuminemia, increased serum liver enzymes, increased serum creatine kinase (CK) and coagulopathies (e.g. disseminated intravascular coagulation, DIC).
Radiology and imaging
Thoracic radiography is usually normal, except in severe cases with pneumonia. Uncomplicated viral pneumonia causes an unstructured bronchointerstitial infiltrate, whereas secondary bacterial pneumonia causes an alveolar infiltration pattern and areas of lung consolidation.
Cytology
Non-specific mucopurulent or mixed inflammation may be evident on samples from affected areas of the upper or lower respiratory tract. Identifiable viral inclusions are not observed in FCV infection. Oral lesions consistent with lymphoplasmacytic stomatitis should be confirmed by biopsy.
Virus culture
Virus isolation is considered the “gold standard” for confirming FCV, but is rarely performed.
PCR
PCR testing can be performed on oropharyngeal, nasal and conjunctival mucosal swabs or scrapings, as well as airway lavage and lung specimens. PCR identification of FCV does not confirm causality of disease because non-clinical carriers can also be PCR positive. Therefore, results must be interpreted in conjunction with clinical signs and circumstances.
Direct immunofluorescence
FCV can sometimes be identified in swabs or scrapings of oropharyngeal, nasal or conjunctival mucosa submitted to a specialized lab. However, this is considered less reliable than virus isolation or PCR.
Treatment
In most cats, acute FURTD is self-limiting and the main treatment is supportive nursing and comfort care, such as gentle removal of ocular and nasal discharges. Dehydration should be prevented to minimize drying and thickening of respiratory secretions, which can occlude airways. Safe and effective antiviral drugs are not available for treating FCV. Antimicrobials may be considered in cats with mucopurulent ocular or nasal discharge, fever and lethargy, when secondary bacterial infection or co-infection is suspected. Typically doxycycline is chosen. 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). Life-threatening
pneumonia or virulent systemic FCV may require intensive care at a facility with an isolation unit and oxygen support.
Treatment of FCV-associated chronic gingivostomatitis
This refractory condition can be treated by extraction of teeth near the lesions, followed by systemic antibiotics and chlorhexidine mouthwashes. Oromucosal administration of recombinant feline interferonomega (0.1 MU q24h for 90 days) has also been shown to improve the condition.69
Prognosis/complications
With most FURTD cases, the prognosis is excellent and clinical signs resolve within 1–2 weeks.
Recovered cats remain subclinical carriers and persistently shed virus for months to years. Bacterial pneumonia is a rare but serious complication in young kittens, and the prognosis is more guarded.
Outbreaks of virulent systemic FCV with vasculitis are rare, but in these cases the disease is much more severe and a mortality rate of up to 50% is expected, even in previously healthy, vaccinated adult cats.
Prevention
Vaccination against FCV is recommended as a “core vaccine” for all cats. Vaccination is effective for preventing or minimizing clinical illness caused by FCV, 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.
FCV and other feline respiratory pathogens are highly contagious, so FCV-infected cats should always be isolated from other cats to prevent the spread of infection. In addition, routine infection control measures combined with reducing stress and overcrowding help prevent the spread of respiratory disease in catteries and shelters.