14 Dec 2015
Kit Sturgess
Job Title
Figure 1. Factors affecting vaccination given to an individual patient.
“What vaccination protocol should we use for dogs and cats in our practice?” is not an uncommon question, often combined with “How do we keep our vaccination protocol simple so pricing, owners and staff are clear and mistakes are unlikely?” and “When did we last review our practice policy and is it up to date?”
Despite its routine nature it is important vaccination is not only seen as population protection, but as individual patient care rather than a one-size-fits-all mechanical procedure.
How individualised to make patient vaccination is the synthesis of a wide variety of different inputs (Figure 1).
In an attempt to clarify the situation, groups have identified some vaccines, as “core” (they should be given to all individuals) and others as “non-core” vaccines, with use more closely aligned to individual assessment.
In the UK 12 diseases have a vaccine licensed for use in dogs (Table 1a) and seven for cats (Table 1b).
Particularly for dogs, there is pressure to complete primary vaccination quickly to allow socialisation of puppies as early as possible. Early vaccination, however, increases the potential risk of a poor response associated with high maternally derived immunity.
Practice policy may need to be developed that, in part, is related to local disease prevalence on the benefits of a third vaccination at 16 to 20 weeks of age for some puppies.
Table 1a gives a potentially bewildering array of protocols. It is important to realise in many cases variations between products reflect the date they were licensed and the specific investigations undertaken to obtain the licence, rather than genuine difference between vaccines.
For a particular infectious disease, there can be a relatively limited number of vaccines, with several manufacturers using the same vaccine under a different trade name.
However, genuine differences do exist – not only in the formulation, but also in adjuvants and the strain used in the vaccine – that may affect the quality, breadth and duration of immunity.
In most cases no information is available on whether one vaccination is clearly more efficacious than another. For parvovirus, for example, when deciding on a practice policy – particularly where clinical cases are relatively common – ensuring the vaccine used provides immunity to the local parvovirus strain or strains (2a, 2b or 2c) responsible for disease is important. In the past, there has been more extensive work on the relative efficacy of FeLV vaccines.
Response to vaccination and duration of immunity studies cannot be expected to measure the variability associated with the wide genetic diversity within the cat and dog population, nor the general health status of individuals when presented for vaccination.
Some dog breeds are reported to be more susceptible to parvovirus infection (Rottweilers, Dobermanns, Labrador retrievers, American Staffordshire terriers, German shepherds and Alaskan sled dogs; Greene, 2012) and there is older anecdotal evidence their response to vaccination may be less robust, leading to some veterinarians recommending a third parvovirus vaccination at 16 to 20 weeks of age.
Subsequent studies with more immunogenic vaccines (Hoskins, 1997) have not supported a failure to seroconvert and the value of a 16 to 20-week vaccination should be based on individual cases associated with their likely maternally derived immunity, local disease prevalence and virulence and patient-specific factors.
Anecdotal reports suggest pedigree cats (Siamese and Persian) may respond less favourably to vaccination and have a higher incidence of vaccine issues than domestic shorthaired cats.
These reports are unsubstantiated and are mostly from the late 1970s and 1980s.
Injection site sarcoma (ISS) is reported to occur at a rate of 0.021 to 0.61 sarcomas/10,000 doses of FeLV vaccine in the UK. ISS is more rarely reported in dogs. The general consensus is ISSs are more commonly associated with FeLV and rabies vaccinations.
Due to the difficulty in managing ISS in the interscapular area, recommendations have been made that different vaccines are given at different sites on the distal limbs so if ISS occurs amputation of the limb is possible. Vaccination at these sites is technically more difficult and less well tolerated by most cats compared to injection in the interscapular area and there is little evidence this is a widely adopted regime in the UK due to the low prevalence of ISS. It should also be remembered ISS is reported with other injectable agents.
In the author’s opinion, the first booster at around 15 months of age is a critical part of the vaccination process as it will catch patients that have failed (usually through high maternally derived immunity) to respond to their primary vaccination course.
Relatively recently, Leptospira vaccines containing four different serovars (genomospecies) have been introduced on to the market (Table 1a) as there was evidence in serologic surveys the most common reactivity was other than to canicola and icterohaemorrhagiae.
However, at least 10 serogroups are thought to be important in dogs, with cross-reactivity between serogroups being variable. No published data exists in the UK on the prevalence of naturally occurring antibodies to the various genomospecies, nor is there a clear understanding of the prevalence of leptospirosis, particularly chronic infection leading to glomerulonephritis or uveitis, for example, within the canine population in the UK.
It is reasonable to assume inclusion of more genomospecies provides wider protection against leptospirosis, with newer vaccines also potentially preventing renal colonisation. Studies have shown a decline in immune protection over time and not all individuals are protected against challenge at one year (preventable fraction ranging from 50% to 100% at 56 weeks) making annual vaccination essential to provide most individuals with reasonable protection (Greene, 2012).
It is important to realise any vaccination protection is not absolute and the risk of succumbing to infection is a combination of immune status, infectious dose, infectious strain, route of infection, presence of intercurrent disease and general health, for example, nutritional status.
Ideally, booster vaccination frequency should be related to duration of immunity for the individual, but this is not practical. There is good evidence immunity to canine distemper, adenovirus and parvovirus is long-lived (studies on some vaccines conducted at 56 months postvaccination showing protection) and for distemper immunity may be lifelong following an initial course and first booster, making a three-yearly cycle a good compromise between risks and benefits.
Similarly, evidence in cats indicates duration of immunity to feline parvovirus is long-lived. Evidence of duration of immunity to feline herpes and calicivirus is conflicting, with older studies suggesting poorer duration of immunity.
There is significant difference in the recommended frequency of boosters for cat vaccinations between manufacturers (usually yearly) and a number of other authorities where the suggestion is respiratory virus should be boosted every three years after the initial course and first vaccination after one year and parvovirus no more frequently than three years.
To some extent this reflects the date at which products were licensed and the data on file (if field tests only looked at one year immunity then this is the frequency given on the data sheets and relicensing would be required to extend this to three years or beyond, which is a costly undertaking).
When developing a policy, it is important a risk benefit analysis for the individual patient and the practice disease demographics is considered and there is informed client consent, especially if vaccination frequency is without the manufacturer’s recommendations.
Concerns, often driven by owners, about the risks of over-vaccination have led to the use of antibody measurements to determine duration of immunity to infection following vaccination in an individual patient.
Interpretation of antibody titres has a number of potential issues, not least because it does not take into account cellular immunity that is often the critical arm of the immune system in providing protection against viral disease.
Antibody titres will provide insight into the degree of protection against self-limiting, systemically spread infection, such as distemper, parvovirus (cats and dogs) and adenovirus, but much less information about mucosal infections, for example, calicivirus and herpes virus, bordetellosis and so on.
The absolute values of antibody titres indicating protection vary depending on the laboratory and method used, so laboratory-specific indices should be used associated with evidence of how the quoted protective titre was derived.
A few reported cases of vaccine-induced infection have occurred following the use of modified live vaccines in immunosuppressed patients, for example, feline parvovirus in FIV-positive cats (Buonavoglia et al, 1993) as well as abnormal response to vaccination in FIV-positive cats (Dawson et al, 1991).
In patients that are immunosuppressed due to drug treatment, such as ciclosporin, the recommendations are to avoid modified live vaccines (Nuttall et al, 2014) and to realise response to vaccination may be curtailed (Roberts et al, 2015). For patients on chronic low-dose glucocorticoids no data are available, but the risks are likely to be low.
A greater risk is the quality of the immune response to vaccination may be subnormal (Robinson et al, 2004), leading to a false sense of security a patient is protected against a specific disease when this is not the case.
There is no evidence vaccination will affect chronic diseases such as kidney or liver disease, osteoarthritis, heart failure, inflammatory bowel disease or bronchitis. Overall, the benefits of vaccination are likely to significantly outweigh risks as wild-type infection will place additional burden on major organ function, and if infection has its major effect on the primary organ affected by the chronic disease, such as bordetellosis in a chronic bronchitis, the effects could be extremely severe.
Some retrospective studies have suggested a link between immune-mediated disease and vaccination because there is a higher proportion of cases of immune-mediated diseases in the month(s) following vaccination than over the rest of the year (Duval and Giger, 1996); however, such an association has not been a consistent finding (Rose et al, 2014; Huang et al, 2012). Current opinion is vaccination is unlikely to be a cause of immune-mediated disease.
Vaccination of patients with a known previous history of immune-mediated disease is controversial; no studies have looked at this in cats or dogs. Concern centres on whether activation of the immune system by vaccination could trigger recurrence of clinical disease.
For many owners, the severity of previous immune-mediated disease is such they are reluctant to take even a theoretically small risk and such cases need careful individual risk assessment and informed owner consent before any decisions are made.
Whether to continue vaccinations requires careful consideration in elderly dogs and cats as some have little or no contact with other dogs and cats.
The consequences of low contact are:
This low risk needs to be balanced against:
On balance, for patients from a stable household that do not have contact with other pets and receive routine health checks the risks/costs outweigh benefits, but such patients are particularly vulnerable if a new pet, especially a puppy or kitten, is brought into the household. For all other geriatric patients, benefits of vaccination are likely to outweigh risks.
An additional consideration is the effect lack of vaccination may have on “herd” immunity. However, if the patient has no contact with members of the “herd” it will have no effect on population immunity.
