Understanding Virulent Systemic Feline Calicivirus The emerging hypervirulent strains of this virus can be particularly devastating to cats around the world. Through careful research, doctors will have a better chance of successfully managing future outbreaks.
Patricia A. Pesavento, DVM, PhD
Kate F. Hurley, DVM, MPVM
Through our involvement in the Shelter Animal Medicine Program at the University of California-Davis School of Veterinary Medicine, we have developed a deep appreciation for the pathaogenic potential of emerging RNA viruses. RNA viruses that infect dogs and cats include canine influenza virus, feline and canine coronaviruses, feline immunodeficiency virus, and feline calicivirus (FCV). The newly emerging hypervirulent strains of FCV are potentially devastating. We are working to understand these strains so we can protect the feline population.
FELINE CALICIVIRUS BASICS
FCV infection is common and can be detected in up to 50% of cats in multiple-cat households, shelters, and catteries. In single-cat homes, the prevalence is lower (5% to 20%). FCV infection can be clinically inapparent or associated with various acute and chronic disease syndromes. Most commonly, field strains are associated with upper respiratory tract disease accompanied by glossopharyngeal ulceration. The typical FCV disease course is mild, which belies the virus’s potential. In the past 10 years, a spectrum of potentially severe clinical diseases has been attributed to FCV infection. Collectively, the causative viruses of these severe cases are termed hypervirulent FCV’s.
WHAT IS HYPERVIRULENT FCV?
Compared with typical field strains, hypervirulent FCV strains are marked by aberrant clinicopathologic features or an intensive disease course. Within the last 10 years, multiple independent outbreaks of hypervirulent FCV strains have been described worldwide. Acute hypervirulent FCV infections have been associated with pyrexia, depression, dyspnea, pneumonia, shifting lameness, and abortion.
One hypervirulent form of FCV, virulent systemic FCV (VS-FCV), is particularly devastating. More than eight outbreaks have been rcognized in the United States and Europe since it was first described in 1998. Initially, these strains were termed hemorrhagic FCV’s, but the disseminated intravascular coagulation and hemorrhage seen early on have not been a consistent feature of subsequent outbreaks.
VIRULENT SYSTEMIC CALICIVIRUS
VS-FCV has a mortality rate of up to 50%. Clinical signs include high fever, facial and limb edema, and ulceration with crusting of the pinnae, muzzle, and paw pads. Glossopharyngeal ulceration has been present in some VS-FCV cases, and it is also common in field-strain caliciviral infections. Death may occur in some cats with minimal clinical signs. Variable associated findings include pancreatitis, hepatitis, and ulcerative rhinitis and sinusitis. Findings on blood chemistry panels are nonspecific; changes can reflect either the repercussions of fever or variability in organ involvement (e.g., hyperbilirubinemia, thrombocytopenia, neutrophilia, lymphopenia, and increased creatine kinase activity).
Several researchers, ourselves included, have collected and analyzed VS-FCV-type viruses from a number of outbreaks. While infection from mild field strains appears to be limited to epithelial cells of the oral cavity and respiratory tract, VS-FCV strans are found consistently in both the epithelium and endothelium. Clinical signs reflect vascular damage, presumably as a direc result of the expanded tissue tropism (e.g., edema, ulcers, multiple organ failure).
WHAT CATS ARE AT RISK
In most outbreaks, the index case animal originates from a multiple-cat environment. Fatal cases occur in both vaccinated and unvaccinated kittens and cats, providing strong evidence that current vaccines do not protect against these strains, although only a limited number challenge studies have been attempted.
Generally, the incubation period is one to five days. A few cases have developed up to 12 days after the last known exposure. The systemic virus may be shed in feces; sloughed skin and hair; and nasal, ocular, and oral secretions. Asympotomatic and mildly affected cats may transmit fatal disease to other cats; therefore, all exposed cats should be considered a potential infectious risk to naïve cats. VS-FCV has been carried between clinics and homes via contaminated fomites (e.g., clothing or hands), infecting cats that were never directly exposed to a sick cat. In other words, the disease can be transmitted throough minimal exposure. For example, a healthy cat visiting a contaminated clinic for a simple weight check can develop fulminant disease a few days later. Some cats have continued shedding the virus intermittently for at least four months after clinical signs resolve. Although there are no documented cases where a fully recovered cat has transmitted VS-FCV, the potential significance of chronic shedders and the factors that lead to long-term persistence of infection are unknown.
DIAGNOSING VS-FCV IS NOT EASY
Common tests used to detect all known FCV strains, including VS-FCV, include viral culture and polymerase chain reaction. The virus is most often isolated from oropharyngeal swabs obtained from an acutely affected cat. The caveat: Depending on the environment, any strain of FCV can be present incidentally in some cats. Moreover, the mutation rates for FCV are so high that, by approximation, a virus isolated from any one cat will be genetically different (in at least one nucleotide) from a virus isolated from any other cat. This leaves us without a means of genetically distinguishing hypervirulent strains from incidental or mild infection.
So how does a doctor know when a strain is hypervirulent? It depends on cumulative data from ruling out other pathogens or disease states, detecting virus in serum and on oropharyngeal swabs, isolating and sequencing an identical strain from more than one affected cat, and finding the virus in situ through biopsy or necropsy. Practitioners must also consider history, clinical signs, and examination findings.
PROTECTION FROM NEW STRAINS
Presently, we cannot anticipate new outbreaks of VS-FCV. Although this syndrome is relatively uncommon compaired with mild strains, occasional outbreaks and focal clusters of VS-FCV have been documented throughout the United States. In an attempt to defend cats, several veterinary groups at universities are focusing their efforts toward understanding VS-FCV pathogenesis. We hope future insights into microbial evolution will identify targets for new vaccines and thereby improve disease management. Until then, some precautions practitioners can take include:
Using a disinfectant effective against FCV, particularly when examining cats with signs of upper respiratory infection or those recently exposed to a high-risk environment.
Vaccinating against respiratory infection at the recommended intervals. Although vaccination will not protect against all FCV strains, it will protect cats from severe disease caused by some strains and help prevent false alarms regarding VS-FCV.
Tell all your family and friends about this deadly disease if they have cats. Get your cat vaccinated against VS-FCV. This vaccine can save his/her life. As of now, CaliciVax is the only vaccine licensed and proven effective against this killer. CaliciVax contains a VS-FCV strain and a traditional calicivirus strain, providing broader protection for your cat. It has been proven safe on more than 700 client-owned cats.
Vaccinate before it is to late. Your cat should receive an initial vaccination followed by a booster about 3 weeks later. The product label indicates your cat should be re-vaccinated on an annual basis. Always consult with your veterinarian to determine the best vaccination protocol for your pet.
Now, however, pet owners and veterinarians can take preventive action with a new vaccine that protects against VS-FCV. CaliciVax™, from Fort Dodge Animal Health, is the only vaccine licensed and proved effective against VS-FCV. It also protects against the traditional calicivirus, providing broader protection.
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