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Viral Upper Respiratory Infections in Cats


I could hear it the moment he came through the door. A 9 year-old neutered male domestic shorthair, was in his carrier, sneezing his head off. “He’s been doing this for the last two days,” said his owner. “He must have sneezed about fifty times yesterday.” A few minutes later, in the exam room, it only took a few moments to figure out his problem. The sneezing, the watery eyes, the snotty nose… this was your classic upper respiratory infection.

Upper respiratory infections (URIs) are common in pet cats. Several infectious organisms are responsible for causing signs of upper respiratory infection, however, viral URIs are the most prevalent. In the cat, the majority of upper respiratory infections (80% to 90%) in cats are attributable to either feline herpes virus, feline calici (pronounced “kuh-LEE-see”) virus, or both. Other respiratory pathogens seen in the remaining 10% to 20% include Chlamydophila (formerly called Chlamydia), Mycoplasma, and Bordatella.

Clinical Signs

Sneezing and nasal discharge are the main clinical signs of acute viral URIs. The viruses irritate the nasal membranes and trigger episodes of sneezing. Episodes of sneezing tend to be intermittent initially, increasing in frequency and severity over a 3 to 5 day period. In the early stages of infection, the nasal discharge tends to be serous (watery and clear), and is often not even noticed by the cat owner. As the disease progresses, nasal discharges may become mucoid (thick and yellowish), indicating that a secondary bacterial infection has developed on top of the initial viral infection. Fever is a common finding.

Other signs associated with URIs in cats may include epistaxis (bloody nose) due to violent sneezing, difficulty eating or chewing food (due to oral ulcers caused by calicivirus), and a discharge from the eyes. If the eye discharge dries out and becomes pasty, the eyelids may seal shut. This is more common in kittens. If the nostrils become occluded with discharge, it may be unable to breathe through the nose and the cat may show open-mouth breathing. Because open-mouth breathing is also associated with severe lung or heart disease, a cat displaying open-mouth breathing should be treated as an emergency until a diagnosis of upper respiratory infection can be verified. The herpes virus causes more sneezing compared to the calici virus, however, the calici virus may cause oral ulceration, which can lead to increased salivation, dehydration, difficulty eating or chewing, poor appetite, and weight loss. Kittens are more susceptible to viral URIs, and the disease in kittens may be severe.

A diagnosis of acute viral URI is based on the history and clinical signs. Although both feline herpes virus and calici virus can be isolated from tissue cultures, it is rarely necessary to isolate the virus for a definitive diagnosis. Furthermore, it is not usually important to distinguish between herpes and calici since the treatment and management strategies are similar for each.

Routine laboratory tests, such as a complete blood count, serum chemistry evaluation, and urinalysis are usually of little diagnostic value in cat with acute upper respiratory disease. Cats, however, should be tested for the presence of concurrent feline leukemia virus (FeLV) or feline immunodeficiency virus (FIV) infection, since either of these viruses can suppress the immune system and may be the underlying cause for the cat’s upper respiratory disease.

Although some veterinarians submit samples of the nasal discharge for bacterial culture, this is not necessary and rarely gives useful information, since the organisms isolated tend to be the normal flora that are typically found in the nasal cavity.


Treatment for viral URI consists of antibiotics, making sure the cat stays well-hydrated, and ensuring the cat meets its daily nutritional needs. An oral antibiotic such as amoxicillin (Amoxidrops®), or amoxicillin combined with clavulanic acid (Clavamox®), is an excellent first choice for cats with URI. Antibiotics, it should be noted, do not treat the viral infection. Antibiotics treat the secondary bacterial infection – the mucoid ocular and nasal discharge – while the cat’s immune system defeats the virus on its own.

Lysine can help the cat’s immune system keep the herpes virus in check. Lysine is an amino acid that has been shown to reduce the severity of the clinical signs of in humans, and this has been shown to be the case in cats as well. Lysine is available in vitamin shops and health food stores, however, the pills are large and hard to administer to cats. Recently, a palatable lysine paste (Enisyl-F ™) has become available for veterinarians to prescribe to cats with URIs due to herpes virus. A study in the American Journal of Veterinary Research, published in 2002, confirmed the effectiveness of lysine in reducing the clinical signs of herpes virus infection in cats, compared with placebo.

Cats with fevers often will refuse to eat. Likewise, cats with nasal discharge may have trouble smelling their food, causing a decreased interest in eating. Meeting the nutritional needs of these sick cats may require force feeding, either by the owner at home, or by trained veterinary staff during a brief hospital stay. Liquid diets specifically formulated for cats and kittens, such as Clinicare®, are recommended, as are nutritional recovery diets, such as Hill’s a/d or Eukanuba Maximum-Calorie. These diets are of a consistency that is ideal for feeding through a syringe or a feeding tube, and they contain additional calories, which is appropriate for sick and debilitated animals, as illness increases a cat’s caloric requirements. Most cats with URIs will continue to drink water, however, cats with fevers have increased fluid requirements and often don’t meet their daily fluid needs. These cats may need to be given subcutaneous (under the skin) fluids either at home, by the owner, or by a veterinarian in a hospital setting.


The prognosis for recovery from viral upper respiratory infections is excellent, with the majority of adult cats making a full recovery. Kittens usually have a good prognosis if they receive prompt, thorough medical care. Occasionally, kittens will succumb to severe infections. In general, morbidity and mortality rates increase as the age of the exposed cat decreases.

Compared to the herpes virus, the calici virus causes milder clinical signs. However, an outbreak involving a highly virulent vaccine-resistant strain of calici virus occurred in northern California in 1998, in which 33% to 50% of affected cats died. Since the report of that outbreak, at least 4 similar outbreaks have been identified in Pennsylvania, Massachusetts, Tennessee, and Nevada. Although infection with this virulent calici virus remains rare, rapid recognition and treatment is imperative.


Cats and kittens are routinely vaccinated against herpes and calici virus, however, feline viral URI is still the most prevalent infectious disease of cats today. One reason for this has to do with the persistent nature of herpes viruses. Once a cat is infected with herpes, a chronic carrier state is established, and transmission from an infected carrier cat to a susceptible cat or kitten is easily accomplished either by direct contact or by aerosol transmission. It is suspected that about 80% of cats that recover from acute viral URIs will become chronic carriers, capable of infecting other susceptible cats and kittens.

Most cats that are vaccinated against the upper respiratory viruses have already been exposed to the virus and are already harboring it in their body. Vaccination induces immunity, but does not prevent infection. In other words, cats who are properly vaccinated will usually only show mild clinical signs of URI if they’re exposed to and infected with herpes or calici, or if the dormant herpes or calici virus becomes reactivated in cats previously infected with these viruses. Cats that are inadequately vaccinated are at increased risk of significant illness from these viruses.

The typical vaccination protocol involves administering the vaccine to kittens starting at 6 – 8 weeks of age, and again at 10 – 12 weeks, and once again at 14 – 16 weeks of age. Cats then receive a booster one year later, and then every three years afterward, as it has been shown that the vaccine confers immunity for at least 3 years, and possibly longer in some cats. In lieu of revaccination, cat owners may elect to have their cats’ antibody level checked. Cats with protective levels of antibodies against herpes and calici do not need to be revaccinated, however, they should have their antibody level checked yearly, and be revaccinated if and when their antibody level is deemed too low to be protective. Forum Index -> Cat Diseases Library Section
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