Presented by Professor T Gruffydd-Jones, University of Bristol, at the FAB Breeders Conference in March 2003

What do we now know about the coronaviruses and their role in the pathogenesis of feline infectious peritonitis (FIP)?

The relationship of the different coronaviruses, particularly between feline enteric coronavirus and feline infectious peritonitis virus, has been the subject of considerable confusion for some time. However recent studies which have involved studying different isolates has helped to clarify the relationships of coronaviruses. This information is not of just academic interest but is of considerable practical relevance to the development of diagnostic

tests, understanding the pathogenesis of FIP, understanding the origin of disease and how it may be controlled.

Feline coronaviruses

Feline coronaviruses can be identified as varying according to

• serotype
• biotype

The feline enteric coronavirus form is very common and lives in the intestine of many cats that appear healthy. It mutates very readily and just occasionally this mutation enables the virus to escape from the gut, spreading throughout the body and causing FIP.

A number of factors are believed to influence the rate of mutation of feline coronavirus in an individual cat and the likelihood of FIP developing -

• number of cats in the household
• stress
• age
• compromised immune function
• genetic factors
• immunity to coronavirus

Comparison of isolates of feline coronavirus isolated from cats which have developed FIP in the same household, around the same time, show significant genetic differences which suggest they result from independent mutation from the same ‘parent' coronavirus.

The key point is to think in terms of feline coronavirus infection and that differences in isolates, biotypes, etc, are not of much practical relevance.

Diagnosis of FIP

The diagnosis of FIP remains one of the most difficult challenges. No definitive test to identify infection with a particular FIPv biotype exists. Diagnosis therefore often depends on assessment of a number of tests which influence the index of suspicion of FIP.

The initial suspicion of FIP is usually based on the findings emerging from the history and clinical examination derived at consultation which prompts further diagnostic investigation.

Around 75 per cent of cats with FIP show effusions and analysis of the effusive fluid is one of the most useful diagnostic aids. The fluid is classically straw coloured and proteinacious which gives a tacky feel and frequently leads to clot formation. However FIP fluids can be relatively colourless and may appear relatively thin and watery grossly. Total protein and globulin analysis of the fluid are the most useful laboratory investigations, particularly for their negative predictive value. The total protein of ascitic fluid in FIP is almost invariably greater than 35 g/l with at least 50 per cent globulin. The cellular content is very variable in effusions in FIP but the cell count is generally low. There is usually a mixture of inflammatory cells often including lymphocytes, neutrophils and macrophages.

The main confounding disease for ascites (with or without jaundice) is lymphocytic cholangitis which can result in ascitic fluid with very similar characteristics to that seen with FIP together with very similar routine blood laboratory changes. Thoracic radiography can be a very useful aid to differentiating these two diseases. Thoracic fluid does not accumulate in lymphocytic cholangitis but is seen in around 25 per cent of cases of FIP with concurrent ascites. Accumulation of pericardial fluid is uncommon in cases of FIP but gives a strong diagnostic indication. Fluid accumulation is restricted to the thoracic cavity in around 10 per cent of cases of effusive FIP.

The clinical features of the non-effusive forms of FIP are much more variable and can represent a major diagnostic challenge. The most common sites of involvement in such cases seen in our clinic are the eye and the CNS. The kidney is an uncommon site in contrast to popular belief.

The distinction between effusive and non-effusive forms of FIP is not clear cut and there is considerable overlap. Up to 50 per cent of cases of FIP with predominantly non-effusive features will show some fluid accumulation even if subclinical. Imaging to detect such fluid and laboratory analysis of the fluid can prove useful diagnostic aids.

It is important to be realistic about the value of coronavirus serology in diagnosis. Coronavirus serology is no more or less valuable than most other diagnostic tests. A titre simply indicates exposure to a coronavirus which is common in cats, particularly pedigrees. However very high titres increase the index of suspicion.

Various other tests have been offered which are claimed to reliably differentiate infection with FIP virus (biotype) from a variety of approaches. However,none of these tests work.

There is no consistent difference between the relatively minimally pathogenic biotypes and those that induce FIP.

Histopathology is generally regarded as the only definitive test for FIP and this is not feasible pre-mortem in most cases.

Vaccination against FIP

No vaccine is available in the UK but ‘Primucell', produced by Pfizer, has been available in the USA and some European countries for a number of years.

There are some inherent problems in producing an effective vaccine against FIP. The phenomenon of antibody dependent enhancement (ADE) occurs in FIP. Antibodies are not protective and can be an important contributory factor in the pathogenesis, particularly of effusive cases. Mutation to allow infection of macrophages is believed to be a crucial factor in allowing dissemination of virus and development of disease. This may afford the virus some protection in evading the immune system of the cat.

Primucell is a temperature sensitive mutant coronavirus which is administered intranasally. This approach is aimed at inducing local mucosal immune response and cell mediated immunity. There is considerable controversy regarding this vaccine. Different studies have given markedly varying results for efficacy of protection against disease in both experimental challenges and field conditions.

The manufacturers of the vaccine reported a preventable fraction of around 65 per cent in an experimental study using either homologous or heterologous challenge. Another experimental study showed a preventable fraction of 80 per cent (Hoskins et al, 1994) but preventable fractions as low as 33 per cent (Scott et al, 1992) or no observable protective effect (McCardle et al, 1995, Scott et al, 1992) have been reported.

The results of the efficacy of protection in field studies have been equally contradictory. A preventable fraction of 75 per cent was found when using the vaccine in a very large cat shelter in the USA (Postorino Reeves, 1995) Studies in Europe reported by Fehr and colleagues (Fehr et al, 1995) showed a preventable fraction of 70 per cent when the vaccine was used in non-breeding, multicat households but no protective effect when used in pedigree Persian breeding colonies.

Concerns have also been raised about the safety of the vaccine and, in a disease for which ADE is recognised, the possibility of vaccination potentiating disease. The results of Scott's studies suggested that this may arise with the use of Primucell although the results of other studies have contradicted this conclusion.

The more readily cultured Type II serotypes of feline coronavirus are more common in the USA whereas in Europe the Type I biotype predominates. Therefore efficacy of a vaccine against USA field strains does not necessarily predict similar efficacy in the UK.

Primucell is licensed for use from 16 weeks of age in the USA. This poses an important practical limitation since most infections with feline coronavirus at least in pedigree breeding colonies are probably derived from the dam and occur at a young age, before 16 weeks.

Control of FIP

The issue of paramount importance to breeders is the control of FIP following occurrence of a case of the disease in their cattery. This is not straightforward and there is no generally agreed practical and effective scheme for achieving control - as there is for other important infectious agents of cats such as respiratory viruses and FeLV.

Coronaviruses are highly infectious. Their stability in the environment is poor but they can survive for weeks in ideal conditions. Infection is most often derived from contact with an infected cat involving faecal-oral transmission. All the cats in a household are likely to be infected with feline coronavirus by the time a case of FIP has arisen unless they have been strictly segregated. Serological surveys of coronavirus titres have shown a prevalence of around 90 per cent in multicat households. Even in the general cat population the prevalence of positive coronavirus titres is up to 40 per cent.

This underlines not only how common infection with coronavirus is in cats, but also how this background prevalence of titres complicates their interpretation in individual cases. Although infection with feline coronaviruses is clearly commonplace, the prevalence of FIP disease is low and probably less than 1 per cent in the general cat population. In catteries with apparent endemic problems with FIP, the prevalence is still usually less than 5 per cent although occasionally it may be greater than 10 per cent and in exceptional circumstances may exceed 30 per cent.

FIP is generally a sporadic problem but occasionally clusters of cases arise, sometimes with most or all kittens from the same litter developing FIP within a short period of time. It was presumed that lateral, horizontal spread of a strain (biotype) of coronavirus which had become pathogenic through mutation in an individual cat accounted for these apparent clusters of cases. However recent evidence comparing the sequence of coronavirus isolates from different individual cats within a cluster of FIP cases, sometimes from the same litter, has shown that these differ and do not show the same pathogenic mutation, although they may derive from the same original non-pathogenic ‘parent' enteric biotype. This suggests that is it rather the factors such as genetic make-up overcrowding, etc, which increase mutation pressure that are the common element in explaining clustering of cases.

Several different strategies for control have been suggested

• establishing a coronavirus-free colony
• early weaning
• reducing risk factors

Establishing a coronavirus free colony

The only way of ensuring that cats do not develop FIP is to guarantee that they remain free of all coronavirus infections. The serious disadvantage of this approach is the difficulty and sacrifice in achieving coronavirus free status and then in maintaining it.

An approach which is sometimes suggested to attain coronavirus free status is based on the assumption that if a stable group of cats is maintained, coronavirus infection will eventually be extinguished and this can be monitored by progressive decline in titres. There are, however, some flaws in this approach. It is generally agreed that there is no prospect of infection being eliminated if there are more than four to six cats in the household. There is also considerable experience to suggest that changes in coronavirus titre are a very poor predictor of coronavirus status and the likelihood of developing FIP disease. Little is known about coronavirus carrier status and most of this has been derived from study of type II isolates which may not reflect the epdidemiology of the more common type I isolates.

Early weaning

The mother is probably the most common source of infection to kittens. If the queen has coronavirus antibodies, these will confer maternally derived immunity five to six weeks of age. If the kittens are weaned before then and isolated from any potentially positive coronavirus cats, they should remain coronavirus free. The success of this approach depends on strict isolation and avoidance of cross contamination. It has been used by some breeders to establish a group of coronavirus free cats within their household for future breeding - although again the success of this system is entirely dependent on future protection from exposure. This approach of maintaining kittens which have been early weaned in isolation can also be used to allow breeders to sell on kittens directly from isolation in the confidence that they are coronavirus-free and cannot act as a source for introducing infection into their new households. Kittens to be retained within the colony will have to be reintroduced to the main group but delaying this until they are older, their immune system has had more opportunity to mature and some of the potential stresses of early life, such as vaccination, have passed may reduce the chance of them subsequently developing FIP.

This method has the advantage of also contributing to control of other infectious agents, most notably respiratory virus infection. The main concern is potential disadvantages in socialisation and an increased chance of behavioural problems developing.

Reducing risk factors

This is the approach adopted by most breeders following a case of FIP.

It is generally recommended that breeding is stopped for six to nine months following a case of FIP.

Consideration should be given to reducing the risk factors for FIP -

• Restrict the number and frequency of litters of kittens
• Segregate the cats in small, stable groups
• Select against individuals/lines which produce cases of FIP
• Avoid exposure to multiple concurrent stresses
• rehoming
• weaning
• vaccination
• neutering
• Ensure maintenance of good health - address any

concurrent health problems

• Maintain high standards of hygiene - particularly attention to

litter trays/feeding bowls

• Consider early weaning/isolation

Serology is of questionable value in such cases. There is no convincing evidence that it will aid in predicting the likelihood of development of FIP or excretion of coronavirus. Other cats from a household which has had a recent case of FIP will almost certainly be seropositive.