Feline influenza is a highly contagious infection principally involving the upper respiratory tract. The disease affects cats of all ages, and recovered animals remain resistant to reinfection for short periods only, so that one cat may experience several attacks. Recently a large number of viruses has been isolated from the upper respiratory tracts of cats, and it seems probable that the feline influenza complex, like human respiratory infections may be caused by a variety of viruses (Piercy & Prydie 1963). The morbidity in feline influenza is reported to be high, but mortality is fairly low. The disease resembles feline pneumonitis, but tends to be of shorter duration.
Clinical symptoms. Under experimental conditions the incubation period of influenza-like diseases has ranged from 1-7 days. In this condition, it seems probable that the initial virus infection is frequently complicated by a superimposed secondary infection, which may be bacterial. The initial viral infection may be mild and not observed by the cat owner; in other instances it may be noted that the cat has serous ocular and nasal discharges, and that these later become purulent, presumably as secondary infection occurs.
Sneezing is the classical early symptom. A sneeze is the one sound dreaded by all cattery personnel. Sneezing becomes more frequent and is soon followed by a watery discharge from eyes and nose. At this stage appetite is often maintained and the cat remains bright and active. Mild cases may not progress beyond sneezing and aqueous oculo-nasal discharge but in most cases by the third or fourth day discharges become mucoid or muco-purulent. Conjunctivitis with a typical oedema reaction may be present. As the discharges become more tenacious and block the eyes and nose, appetite is lost, and increasing depression is evident. The course varies from a few days to several weeks and a chronic infection may supervene.
Signs found on clinical examination. These vary with the stage at which the cat is presented. In the very early stages the owner may seek advice on account of sneezing and other signs may be absent. In the first two days an excess of watery secretion from the nose and a moist appearance of the eyes may be associated with a normal or only slightly raised temperature. Even in the most severe cases the temperature rarely exceeds 40-3° C. (104-5° F.). When muco-purulent discharges cake the eyes and nostrils, the cat presents a sorry sight, and is usually severely depressed. The sternal posture is usually adopted, with the head drooping. Respirations are often noisy due to mechanical interference with air flow at the nostrils, and mouth breathing may occur in spasms or permanently.
Aetiology. A virus was isolated by Grandell and Maurer (1958) from the throats of cats affected with a disease of the upper respiratory tract.
This virus was later named feline rhinotracheitis virus, and strains were isolated from different localities in the United States. The virus grew well in cat kidney tissue cultures where it produced multinucleated giant cells and intranuclear inclusion bodies. A second virus isolated shortly afterwards from a cat with respiratory disease in California did not produce inclusion bodies and appears to be unrelated to the first isolate (Crandell & Madin 1960). Further virus isolates have been made in America and in this country, and some of these have been shown to produce clinical feline influenza when inoculated into susceptible cats.
Although the role of the various isolates in producing defined clinical disease is at present confused, it is to be hoped that the aetiology of the feline influenza complex will become more clear in the near future.
Diagnosis. Based on clinical signs and epidemiology.
Differential Diagnosis. 1. In the early stages this disease may be indistinguishable from an allergic rhinitis and conjunctivitis which, while not common, does occur. The subsequent course is the only guide.
- Chronic nasal catarrh and/or sinusitis. In these cases discharge is confined to the nose and is less profuse and more purulent. A chronic infection of this type is found in young kittens or cats previously subject to an attack of a feline respiratory virus infection.
Prognosis. Prognosis is difficult. The disease is usually mild even when pneumonia is present, but severity varies with the strain of virus, and this cannot be easily established for prognostic purposes. Fatal cases are relatively few and usually result from inanition due to a protracted course. Recovery may be prolonged with a slow convalescent period.
A small proportion of patients are left with chronic nasal catarrh or sinusitis which runs a long course without spontaneous recovery.
Treatment. None specific. The use of antibacterial therapy is justified to avert secondary bacterial infection or to treat this in those cases in which secondary infection has already occurred. Other treatment is mainly directed at keeping up the patient’s ‘morale’ by assiduous nursing, maintenance of nutrition and restoration of appetite. Owners should be instructed quite clearly that a case of moderate to severe feline respiratory infection cannot be properly nursed unless frequent (2-3 hourly) attention is possible; these patients cannot be looked after by someone who is out at work all day and it is the veterinary surgeon’s duty to say so. Hospitalisation in the average veterinary establishment is utterly impossible and its wisdom even in isolation wards is debatable because of the tendency for bacterial superinfection to complicate the infection and produce a far more serious syndrome.
Nursing must include maintenance of an even temperature with good ventilation, the frequent cleansing away of ocular and nasal discharges, and the tempting of appetite. The latter is best achieved by offering foods with attractive (catwise!) pungent odours, e.g. kipper, mackerel, hare, since stimulation of appetite in the cat is very dependent upon the sense of smell, which is lost for long periods during this illness. Forced feeding with nutritious fluids such as chicken or beef essence, beef tea or hydrolysed protein may be necessary if the course is protracted and condition is being lost.
The use of inhalations to keep airways clear is often helpful either by vaporising Compound Tincture of Benzoin (Friars’ Balsam) or oil of eucalyptus on hot water—after initial resistance many cats seem to appreciate this attention—or the judicious application around the nostrils of proprietary vapour rubs diluted wth 1 or 2 parts yellow or white soft Paraffin (Vaseline).
In cases where conjunctival oedema is a serious factor the use of eye drops rather than ointments is indicated. The author has found it useful to dispense eye drops of Collosol Argentum (Crookes) together with adrenalin 1 : 500,000. This reduces the oedema more quickly than any other dressing tried and it is well tolerated.
Vitamin therapy is beneficial when convalescence is protracted.
Vitamin C in high dosage is helpful in rendering tenacious catarrhal discharges more fluid and thus more easily eliminated.
So far as possible systemic drugs should be administered by injection, i.e., antibiotics such as penicillin, streptomycin and chloramphenicol in parenteral form. Certain vitamin preparations intended for intramuscular injection may safely be given subcutaneously to cats if diluted in 1-2 volumes of saline or glucose-saline.
Prophylaxis. Since it seems that many viruses are involved in the feline influenza complex and that immunity persists for only a few weeks the difficulties involved in obtaining a serviceable level of immunity with a reasonable duration are considerable. However, Evans (1963) has reported briefly that an autogenous vaccine (prepared from cats in a local outbreak of respiratory disease) administered to cats prior to admission to a boarding and quarantine cattery gave promising results.
Wider application of vaccination in the present state of knowledge is unlikely to prove useful.