Feline infectious diseases: part one

Transcription

1 Vet Times The website for the veterinary profession Feline infectious diseases: part one Author : Martin Atkinson Categories : Vets Date : June 1, 2009 Martin Atkinson debates the prevention versus vaccination issue in the first of a two-part article, and underlines his belief that reliance on rumour is to be avoided THE purpose of this article is to compare the benefits of vaccination with other methods of feline infectious disease prevention. Therefore, it will not dwell on symptoms of individual diseases or treatment options other than to review the epidemiology, where appropriate but it will evaluate some of the more recently available material. Part one of this article will deal with general principles, feline parvovirus (FPV), cat flu and Chlamydophila. The second part will cover Bordetella, feline leukaemia virus (FeLV) and feline immunodeficiency virus (FIV), coronovirus/ feline infectious peritoni peritonitis, and haemoplasma, as well as a number of less common infectious diseases. Introduction It is a simple fact that if infectious diseases could be avoided or prevented, then vaccination would be unnecessary. Theoretically, if a cat that came from a pathogen-free mother was taken into a catfree home before the loss of its maternally derived antibody (MDA), and was subsequently never exposed to other cats or fomites from an infected environment, it would have no need for vaccination. 1 / 12

2 However, the likelihood of this happening is remote in the extreme, given the high carrier status rate of many infectious diseases in breeding females, and the inevitability that even indoor cats will come into contact directly or indirectly with others (such as in a cattery) at some time in their lives. Consequently, for most cats, vaccination is the mainstay of protection against the common, serious and infectious diseases. Nonetheless, as we will see, vaccination is not foolproof, so subsidiary preventive strategies may also need to be adopted. Protection is the only way of protecting against some diseases where no vaccine is available. Prevention Some generalisations on prevention are highlighted in Table 1, to avoid cluttering the text, and can be referred back to if required. Vaccination The vaccination issue continues to provoke controversy. Few, if any, clinicians are short sighted enough to blindly trust in vaccination as foolproof or believe that everything manufacturers tell them is beyond dispute. We should challenge accepted practice, but it would be wrong to go to the other extreme and believe that vaccination is a major cause of other disease processes (as some action groups would have us think). Equally, the recommendations made by some expert committees to vaccinate only every three years do not take into consideration the possibility that an individual may have failed to have achieved adequate immunity (for one reason or another) at its last vaccination and would, therefore, be at risk if it had to wait another three years for the next instalment. The Vaccinations Guidelines Group (VGG) has recommended that three vaccinations should be given against core diseases to kittens at eight, 12 and 16 weeks of age, with a first booster after 12 months and subsequent boosters every three years or more. It also recommends that adult cats of unknown vaccine status should receive one vaccine, followed by a second 12 months later. The European Advisory Board on Cat Disease broadly agreed with these findings, but believed that feline herpesvirus (FHV) vaccination should be more frequent. In addition, core vaccines vary according to differing groups criteria. As available vaccines are multivalent, this presents a problem and vaccine data sheets do not yet reflect this advice. Of course, there is a danger that vaccine companies commercial interests may override best practice and be reflected in their recommendations. Nonetheless, more frequent vaccinations 2 / 12

3 should probably be administered to cats entering high-risk situations, and most catteries still expect annual boosters. However, there is evidence that antibodies against feline calicivirus (FCV), for example, may be present at four years and even up to seven years post-vaccination. Even in previously vaccinated cats with negligible antibody detected, high immunity can be induced with a single booster vaccination, suggesting the presence of cellmediated immunity that can rapidly respond under challenge. Decisions on vaccination frequency should be made on an at-risk basis, in association with information from manufacturers data sheets, and will vary in individual animals and populations. The measles, mumps and rubella (MMR) debacle in human medicine where there were unfounded fears the vaccine was linked to autism shows how rumour can led to a dangerously low uptake of vaccines. This resulted in the first increase in measles and child deaths from the disease in the UK in a generation. Vaccines are far from perfect, but millions of animals lives have been spared and untold suffering prevented by routine vaccination. With a more feline-specific reference, FCV vaccines with the F9 strain have been shown to have only 40 per cent effectiveness in some instances, although there is evidence to show that response has greatly improved with the incorporation of the newer FCV serotypes. Vaccine site sarcomas have been connected to FeLV vaccines, but no link has been proven and the prediction of an epidemic appears unfounded. However, although the mild transient effects of vaccination are quite common, the incidence of suspect adverse reactions is very low, and serious reactions are incredibly rare. Vaccination will remain at the heart of any strategy for disease prevention, but its limitations must be recognised. Therefore, established or innovative preventive methods still have a vital part to play. A number of specific feline infectious diseases will now be discussed in the context of prevention versus vaccination, where applicable. FPV, feline infectious enteritis and panleukopaenia Epidemiological studies have shown that when canine parvovirus (CPV) emerged in the late 1970s, it was a mutation of feline panleukopaenia, which is now more frequently referred to as FPV. Although FPV would appear to have remained stable and the feline enteritis vaccine is still effective (resulting in very few clinical cases in vaccinated populations), CPV has mutated in the dog population and new variants have emerged. Research has also shown that a full circle has turned, with CPV now the more likely cause of 3 / 12

4 parvovirus-related disease in cats in some populations. The virulence and ease of FPV spread in an immunologically naïve cat population and the need to take aggressive steps to prevent its transmission (see Table 1) is well understood. However, for this particular challenge, vaccination of cats in contact with dogs should, perhaps, be added to the list begging the question of whether current FPV vaccine strains will be effective against this new challenge. Cat flu (overview) Cat flu is a broad term covering a group of infectious agents that may be involved in the process of this disease (these are listed in Table 2). Clinical cases may include infection with a combination of a number of these agents. Vaccination is primarily aimed only at FHV and FCV, with Chlamydophila less frequently included in vaccination programmes, and Bordetella only rarely. Therefore, it is clear that a strategy for cat flu prevention must include measures other than vaccination. Complicating factors include the fact that vaccination does not produce a sterilising immunity, so it cannot protect against the disease itself, only against clinical signs of disease. There is, at present, no vaccine against the other contributory agents. Influenza viruses (FHV and FCV) FHV vaccination is effective, and newer FCV vaccine strains have shown promise. However, some older FCV vaccines may be ineffective. Coupled with the high carrier state incidence in queens (meaning that many kittens are infected before vaccination can commence) and the emergence of feline calicivirus virulent dystemic disease (FCV-VSD), where vaccination in some outbreaks had no benefit, it is clear that preventive tactics need to be a major part of cat flu control. Early vaccination before the decline in MDA has been shown to have some benefit. Live cat flu vaccines should be administered with care, as creation of an aerosol or spillage on to the coat (which is subsequently licked off) may induce symptoms of disease and virus shedding. Shed virus may then cause disease in in-contact, immunologically naïve cats. A high proportion of cats that have been infected with cat flu viruses will become carriers (the majority with FHV and up to 40 per cent with FCV, many of them asymptomatic). Therefore, it may not be apparent, when introducing an apparently healthy cat into andisease in in-contact, immunologically naïve cats. A high proportion of cats that have been infected with cat flu viruses will become carriers (the majority with FHV and up to 40 per cent with FCV, many of them asymptomatic). Therefore, it may 4 / 12

5 not be apparent, when introducing an apparently healthy cat into an unvaccinated population, that the cat is shedding virus. Viral studies by isolation or PCR may identify these carriers, but it is probably best to assume that all cats are potential carriers and follow a quarantine procedure (Table 1), and vaccinate all at-risk cats prior to introduction. The author has had some success treating acute cases of FHV infection in kittens and immunologically naïve adults with recombinant interferon of feline origin. It is likely that this treatment will reduce the number of viral organisms in the environment, but, as yet, there is little information about whether it will prevent the progression to a carrier state. FCV has been shown to evolve, and infections within cat colonies can be very persistent. Cats tend to fall into three groups according to their virus shedding patterns: always negative, intermittent shedders and continual shedders. This may be as a result of infection persistence with one FCV strain, re-infection with a new strain or a mutated variant of the original strain. As mentioned previously, in some instances a FCV variant has become highly virulent. Unfortunately, it is not practical, at present, to identify virulent strains by the methods available. Any FCV-VSD outbreak must be dealt with by implementing very conscientious barrier nursing, and the isolation of cats into small groups according to their known disease status. In outbreaks in the USA, personnel have been shown to have transported infection between premises. Complete clothing changes and attention to personal hygiene are, therefore, essential for preventing spread. Chlamydophila Chlamydophila was recognised as a contributory factor in cat flu before FHV and FCV, but interest in it as a causative agent has decreased since the discovery of the respiratory viruses. Surveys demonstrate that the incidence of C felis infection runs at five to 10 per cent of the cat population, but this may be higher. In the author s experience of dealing with relatively large numbers of stray and rehomed cats through a feline charity, the incidence of Chlamydophila appears to be at least as high as the flu viruses in cases of feline upper respiratory tract (URT) infection. It is surprising, therefore, that vaccination against Chlamydophila is not universally included in vaccination protocols. The symptoms of Chlamydophila are characteristic in the earliest stages of the disease (unilateral conjunctivitis and/ or chemosis, with purulent discharge and little URT involvement), but as it may become bilateral and associated with other causative agents before presentation, it may be overlooked as a contributory agent. 5 / 12

6 Following the acute stage, symptoms become milder and gradually asymptomatic, but they may remain subclinical. The method of Chlamydophila transmission between cats is not fully understood, but probably involves close contact and transmission via ocular and possibly nasal discharge. Kittens are typically infected from their mothers as MDA declines. Vaccination of breeding queens is useful to produce good levels of MDA in kittens, and there is some evidence that immunity may be derived from early vaccination in the face of MDA. However, in multi-cat environments with endemic infections, treating all in-contact cats with doxycycline for at least four and, preferably, six weeks may be effective in eliminating the organism and be a useful adjunct to vaccination. The role of carriers in the transmission of C felis is unknown, but affected cats can shed organisms for more than 200 days. In other species, it has been demonstrated that Chlamydia can remain subclinical. Therefore, although a true carrier status may not exist in cats, this subclinical state may contribute to the persistence of organisms in the environment. Experience certainly seems to show that some cats that have previously suffered from C felis infection will exhibit continual symptoms, and others will suffer a periodic recurrence of clinical symptoms. C felis has also been demonstrated in the intestinal and reproductive tracts, which may also contribute to the pathogenesis. Diagnosis, or detection of the presence of subclinical infections to initiate a protocol for treatment or prevention, can be confirmed by demonstration of C felis on conjunctival swabs by isolation or PCR. Due to the low viability of the organism, even in viral transport media, PCR testing may be more reliable. In the author s experience, Chlamydophila is one of the few diseases in which the recurrence of clinical symptoms in a population apparently decreased following vaccination in this case, with a monovalent inactivated feline Chlamydia psittaci vaccine available at the time. In one small clinical trial in a multi-cat household, which had previously only vaccinated against FPV, FHV, FCV and FeLV, three cats were asymptomatic, three periodic sufferers and four continual sufferers. These were confirmed by the absence or isolation of C felis in conjunctival swabs. Within this group, the continual sufferers improved following vaccination to become periodic sufferers, and the original periodic sufferers improved to become asymptomatic. Unfortunately, due to cost restraints, it was not possible to perform repeat testing to see if Chlamydia was still present in the conjunctiva of the newly asymptomatic cats following vaccination. In addition, no serology was performed, but this new disease status was maintained with periodic vaccination until it was lost to follow up. References are available on request to the editor. 6 / 12

7 Below: chlamydiosis typically starts with a unilateral chemosis. The author is surprised that vaccination is not universally adopted. 7 / 12

8 Above: facial oedema and ulceration are common symptoms of virulent systemic disease 8 / 12

9 Classical FCV lingual ulceration. There is evidence that antibodies against FCV, for example, may be present at four years and even up to seven years post-vaccination. 9 / 12

10 10 / 12

11 TABLE 1. Prevention of disease spread 11 / 12

12 TABLE 2. Infectious agents involved in cat flu 12 / 12 Powered by TCPDF (