Professor, The Feline Centre, Department of Clinical Veterinary Science, University of Bristol, UK
Vaccination is the most important health measure that veterinary surgeons undertake. Practitioners in many countries report that they no longer ever see cases of feline infectious enteritis (FIE) since vaccination became widely used. There is a strong view that the prevalence of feline leukaemia virus (FeLV) has dropped considerably since the introduction of effective vaccines.
Although the benefits of vaccination are widely recognised, vaccination protocols have become a controversial topic in recent years and traditional protocols have been questioned. The main discussion centres around what should be included in routine vaccinations and how often boosters should be used. A number of expert bodies have considered these questions and formulated recommendations for vaccination protocols, although there are significant differences between these.
The key to decisions is based on consideration of the benefits of vaccination compared to the potential drawbacks. The crucial benefits depend on the prevalence of the infection, the severity of associated disease and the efficacy of protection induced by vaccination. The most problematic of these factors to evaluate is the prevalence and there is surprisingly little reliable data available from the field of the prevalence of even the major infectious diseases. Most data are based on the proportion of positive test results derived from diagnostic laboratories which represents a highly biased population and may provide a poor guide to the overall prevalence. Geographical variations in the prevalence, both between countries and within different part of individual countries are also important but largely unknown.
One of the major issues that have led to reconsideration of vaccination is potential side effects. Some practitioners report that mild vaccine reactions shortly after vaccination consisting of transient lethargy and associated pyrexia or mild swelling at the site of vaccination, are common. However, in a large retrospective analysis of the health records of around half a million cats vaccinated in practise in the USA, the prevalence of recorded vaccine reactions was found to be 0.5%.
The major concern in recent years has been over the potentially more serious issue of sarcomas associated with vaccination sites. This problem was first highlighted in the USA. Very different estimates of the incidence of vaccination site sarcomas have been made and there have been limitations in some of the epidemiological studies. More recent figures based on large studies (Gobar & Kass 2002; Kass et al, 2003) have estimated an incidence of around 0.2-1/10,000 vaccinations.
The pathogenesis of injection site sarcomas is not clear. Early reports suggested a particular link with rabies and FeLV vaccines with speculation of the role of the adjuvant aluminium hydroxide, but more recent studies have shown that sarcomas may develop.
Another concern that has been raised about potential side effects associated with vaccination is that it may be a contributory factor in the subsequent development of renal failure later in life. This is based on experimental studies which demonstrated inflammatory infiltrates in the kidneys of cats which had been immunised with lysates of cell cultures of the type used for propagation of feline vaccine viruses (Lappin et al, 2006). However, there is no evidence to substantiate any involvement of vaccines in renal disease in the field.
What Should be Included in Routine Vaccination?
Some Vaccine guidelines incorporate the concept of "core" vaccines which should be given to every cat. There is general agreement that this should include FIE, feline herpesvirus (FHV) and feline calicivirus (FCV). More controversial is whether or not FeLV should be included as part of routine vaccination. Risk analysis to tailor decisions on which components should be given to each individual cat is suggested, and has the potential advantage of avoiding unnecessary vaccination for infectious agents to which a cat will not be exposed. However, ensuring that risk analysis can be undertaken effectively, particularly as the "vaccination interview" at the time of the first consultation for a new kitten, raises concerns. Local knowledge of the prevalence of the different infectious agents is a key factor to be taken into account and in many countries FeLV is still a common infection and major cause of mortality.
Decisions about infectious agents which are regarded as less serious, based largely on their prevalence, severity and whether or not they are treatable, is more controversial--e.g., for Chlamydophila felis. However such infections cause distress to infected cats and this has to be balanced against any significant adverse effects.
How Often Should Boosters be Given?
The AAFP has also played a prominent role in suggesting changes to the recommended intervals between booster vaccinations. Traditionally this has been dictated by data sheet recommendations and has been based on annual boosters. It has been suspected for some time that immunity following vaccinations lasts considerably longer than one year for some infectious agents and there have been some reports which corroborate this. It was the study by Scott and colleagues at Cornell which prompted the AAFP to make new recommendations suggesting tri-annual boosters for the "core" vaccines (Scott and Geissinger, 1999). In this study cats were maintained in isolation for extended periods following vaccination and their protection assessed by periodic monitoring of antibody levels followed finally by challenge. The value of serological titres in predicting protection has been argued but in some studies the presence of any discernible titre has been shown to indicate protection (Lappin et al, 2002). Some cats may have residual protection despite undetectable antibody tires and may show an amnaestic serological response following re-exposure. So it is likely that the presence of antibody titres underestimates the prevalence of protection. Field studies of pet cats have also been undertaken to determine the prevalence of antibody titres (Lappin et al, 2002) and also to assess whether an amnaestic serological response occurs following administration of a booster which is interpreted as indicating pre-existing protection (Mouzin et al, 2004). These studies have shown that approaching 100% of cats have either serological titres against FPV and FCV or show an amnaestic response following booster vaccination, and this is corroborated by experimental studies which provide compelling evidence that immunity lasts at least three years and probably considerably longer in many cats. Whilst immunity against FHV lasts longer than a year in the majority of cats, there is a significant proportion of cats vaccinated against FHV in which this is not true and serological studies have shown that around 30% of the cat population in the USA has undetectable titre. Vaccination protocols should be designed to provide the minimum levels of protection that will include cats that respond less effectively and therefore it is recommended that boosters for FHV should be given annually.
The question of boosters for FeLV is more problematic, particularly since many vaccinated cats do not show antibodies following vaccination, even though they are protected, and will develop an amnaestic serological response if exposed to FeLV. Titres do not, therefore, provide a satisfactory marker of protection. There is circumstantial evidence that immunity to FeLV may not be long lasting and if there is continuing risk of exposure, careful thought should be given to boosters. It is a common misconception that adult or older cats cannot become infected but maintenance of protection of cats at continued risk is important.
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