This chapter describes the principal infectious diseases of cats, but septic infections are dealt with separately in Chapter 19 while dermatomycoses are discussed in Chapter 15. Although some feline infectious diseases have been recognised clinically for many years knowledge of their aetiology is far from complete. This is particularly true of feline influenza and feline enteritis, for it is only recently that viruses associated with these conditions have been isolated in vitro. On the other hand, it seems probable that more cases of feline infectious anaemia occur in Great Britain than are actually diagnosed, and toxoplasmosis has not yet been recorded in British cats. It is apparent that much remains to be done by both clinicians and research workers before the diagnosis, treatment and prophylaxis of feline infections can be considered to be satisfactory.
Feline respiratory virus infections
These are essentially diseases of catteries in the broadest sense, that is to say any premises where a number of cats come together for any reason.
Cases which are sporadic can usually be traced to a recent sojourn in boarding kennels, visit to a cat show, or contact with a cat recently returned from either. Local epidemics tend to arise from individuals recently released from cat kennels and are commoner at holiday periods, viz. during the summer, Easter, or Christmas. Due to the fact that outbreaks in kennels can often be associated with the recent admission of a particular cat which itself remains healthy, a state of affairs which may occur annually, the conviction has arisen that a carrier state exists.
In Great Britain there has been a tendency to call all feline respiratory infections Feline Influenza; however, it now seems certain that several different viruses play a part in what is probably a complex of diseases.
One respiratory disease of cats, feline pneumonitis, has been clearly defined, but although this infection has occasionally been detected in Britain, its occurrence does not appear to have been widely recognised.
For present purposes feline pneumonitis has been described separately below, while other respiratory diseases have been described under the term ‘feline influenza complex’. Many viruses and strains of viruses that produce a cytopathic effect on cells grown in tissue culture have recently been isolated from cats with the feline influenza complex, and it is to be hoped that before long the aetiology of this condition will be more fully understood.
This disease is caused by a member of the psittacosis group of large viruses. It differs somewhat from feline influenza in that it runs a protracted course and is rarely fatal. More information is required about its occurrence and symptomatology in Britain, but since one attack confers appreciable immunity, it might be inferred that feline pneumonitis, unlike feline influenza, is a disease of kittens and cats not previously exposed to infection.
Clinical symptoms. The following description is taken from that of Baker (1944). The incubation period ranges from 6-10 days. Apart from a slight transient rise in temperature the disease is almost afebrile.
There is increased lacrimation, which is followed after a day by conjunctivitis and a muco-purulent discharge from the eyes and nose, with photophobia. Some loss of weight occurs shortly after the onset of illness. Handling tends to provoke prolonged attacks of sneezing. A cough usually develops, and this is indicative of the involvement of the lower respiratory tract, frequently with pneumonia, but apart from coughing, symptoms of lung involvement are usually minimal. Signs persist for about 2 weeks, and affected cats return to normal within 1 month.
Pathology. Pathological changes may be confined to inflammatory changes in the upper respiratory tract. In those cases developing pneumonia dense pinkish grey consolidations occur particularly in the anterior lobes of the lung. Other organs are not involved. Histological sections of the lung show bronchial exudates containing polynuclear and mononuclear cells and a serous cellular exudate in the alveoli with monocytes as the predominating cells. Elementary bodies can be seen in the cytoplasm of the mononuclear cells.
Etiology. Feline pneumonitis virus infects cats, mice and other laboratory animals, and can be cultivated in embryonated eggs. It has therefore been studied rather extensively in the laboratory and this has resulted in the production in the United States of prophylactic vaccines.
The virus is sensitive under laboratory conditions to penicillin, chloramphenicol and the tetracyclines, but in experimental infections in cats McKercher (1953) found that although penicillin and chloramphenicol produced clinical improvement, treatment with the tetracycline group had a more pronounced effect on virus suppression. Virus has been detected in the tissues of some cats 1-2 months after recovery. There would obviously be danger of infection if such cases were brought into close contact with other susceptible cats.
Diagnosis and Differential Diagnosis
The differentiation of this disease from other respiratory infections may present difficulties. Typically, feline pneumonitis is afebrile and runs a course of about a month. The occurrence of pneumonia is significant, but this may not be apparent from the symptoms. A favourable response to antibiotics also suggests that the antibiotic-susceptible agent of feline pneumonitis is being treated.
Prognosis. Most cases recover, despite the extended course of the disease.
Treatment. McKercher (1953) noted clinical improvement after four days treatment with tetracyclines, penicillin or chloramphenicol, but virus was still present in the tissues of some kittens at this time. Relapses in one group of kittens treated with Chlortetracycline responded to a second course of treatment with the same drug.
Prophylaxis. Inactivated and attenuated live virus vaccines prepared in the United States have been claimed to confer satisfactory levels of immunity.