Feline calicivirus (FCV), a small, single-stranded nonenveloped RNA virus, is one of the major etiologic agents of upper respiratory tract disease in cats. Viruses, principally FCV and the feline herpesvirus-1 (FHV-1), are responsible for approximately 80% of feline upper respiratory tract infections. Although vaccination against FCV and FHV-1 is widely employed as a core vaccination recommended for all cats, many kittens are exposed and infected prior to receiving a first vaccination. A chronic carrier state involving continuous shedding of FCV often ensues. Other, less common, syndromes associated with FCV infection in cats include polyarthritis, interstitial pneumonia, and nasal planum ulceration. In the last two decades a highly contagious, virulent strain of FCV has been identified (virulent systemic feline calicivirus; VS-FCV) which can cause severe clinical signs in infected patients, including edema of the face and paws, high fever, icterus, and hemorrhage, and involves mortality rates up to 50%.
In the case of FHV-1, infection is lifelong and the carrier state involves stress-triggered viral shedding. An antiviral agent, famciclovir, has proven to be useful in ameliorating clinical signs of FHV-1 associated ocular and/or upper respiratory infections. However, no direct-acting antivirals have been approved for treatment of FCV infections. In this study, 15 different compounds representing a variety of four different antiviral classes were tested in vitro for efficacy against FCV using protein and cell culture assays. The compounds were evaluated for their ability to inhibit FCV protease or polymerase, both of which are essential for viral replication. Famciclovir was among the 15 compounds tested for FCV antiviral activity; it demonstrated minimal anti-FCV activity even at high concentrations, and therefore is not a good option for treatment of FCV infections.
This study identified two in vitro assays that will allow the testing of agents that inhibit FCV protease or polymerase, and promote the development of FCV-targeted antivirals for clinical use. Some of the compounds that demonstrated inhibition of either FCV protease or polymerase, however, did not inhibit replication of this virus in cell culture, but two of the drugs did. These were nitazoxanide, a broad-spectrum antimicrobial licensed for treatment of gastroenteritis caused by Cryptosporidium parvum and Giardia intestinalis in humans, and a nucleoside analogue, 2′-C-methylcytidine (2CMC). They both emerged as potent inhibitors of FCV replication in cell culture, and have a synergistic effect when combined. Nitazoxanide has also been shown to eliminate shedding of Tritrichomonas foetus in cats, and has also been used in treating giardiasis and cryptosporidiosis in dogs.
However, 2CMC is known to have dose-related gastrointestinal side effects in humans, and this may limit its future clinical use in treating caliciviral infections. Nitazoxanide also is associated with concerns about gastrointestinal side effects (diarrhea and vomiting) in cats, and has a narrow in vitro therapeutic index. Given their synergistic inhibitory effect on FCV, the combination of these two agents might allow reduction in the effective concentration of nitazoxanide, thereby limiting its cytotoxic effeects, while simultaneously improving drug efficacy. In the future, other potentially clinically useful anti-FCV antivirals may be identified using the in vitro assays developed in this research. [PJS]