Individualized assessment offers insight to patient's risk exposure to disease

In the March/April issue of the Journal of the American Animal Hospital Association (AAHA), an Executive Summary of the 2003 Canine Vaccine Guidelines was published. A full text of the guidelines, recommendations, and supporting literature is available to AAHA members on the organization's Web site (www.aahanet.org). The 2003 Canine Vaccine Guidelines represent the most comprehensive, current review of canine vaccines published in more than 14 years . . . . it's very much worth a look.

Table 1: CORE canine vaccines, suggested booster interval and estimated duration of immunity.

The Task Force that authored the 2003 Canine Vaccine Guidelines have made a comprehensive effort to review each type (not individual vaccine products) of licensed canine vaccine in the United States. Taking into consideration such factors as age, consequences of infection, and duration of immunity of individual vaccine types, an extended table summarizes recommendations for selecting and administering canine vaccines. What the task force did not do was to make specific recommendations on how to implement the canine vaccine guidelines. That task, quite clearly, has been left to the individual practitioner.

Implementing the AAHA guidelines, it would appear, is a somewhat more arduous task made somewhat more difficult by the fact the task force has recommended triennial booster vaccination for some (not all) vaccines. Here's the problem: For years, veterinarians have endorsed and recommended annual vaccinations which, when appropriately linked to annual health care examinations, provide a standard of care the profession strives to maintain. Under the recommendations put forth by the AAHA Task Force, how is it possible to implement triennial vaccination in practice when annualized health care is the objective?

What follows is a practical primer on health risk profiles and how such assessments can be used to implement your own guidelines using the 2003 Canine Vaccine Guidelines as a starting point.

Understanding terminology

For those who have actually reviewed both the 2003 Canine Vaccine Guidelines, and the 2000 Feline Vaccination Guidelines, it will be apparent that both sets of recommendations have been based around core vaccines and non-core vaccines. Applying this terminology will help practicing veterinarians make clear distinctions for their staff and help explain recommendations to clients.

Table 2: NON-CORE canine vaccines, booster interval, and estimated duration of immunity.

Core vaccines, by definition, are those that pose a serious health risk to every dog and every cat presented to the practice. These are the vaccines that every dog and every cat will receive. And everyone in the practice knows that. Vaccines included in this category are those that protect against highly contagious, life-threatening infections and/or zoonotic infections. For the sake of consistency, everyone in the practice should know which vaccines are included in this category (Table 1, p. 36).

Non-core vaccines are perhaps best defined as those vaccines that, in the opinion of the clinician, should be made available because infection poses a realistic, but variable, risk to individual dogs (Table 2). The decision to administer, or not to administer, any of the vaccines in this category should be based on an assessment of the individual animal's health risk profile and an effort to determine whether the patient is at low risk, moderate risk, or high risk for infection.

An additional category, not generally recommended, has been included in the AAHA guidelines to address those vaccines that, based on available data, do not meet reasonable efficacy or safety standards (Table 3, p. 38).

The message here is quite clear: all patients do not share equal risk for exposure and infection. Obviously, it is impractical to think that a single vaccination protocol can be designed to fit the needs of all patients. For the individual practice, categorizing the list of canine and feline vaccines into these categories is an important and valuable exercise . . . implementing guidelines, however, requires additional effort. Ideally, all clinicians in the practice will reach agreement as to which vaccines are considered core. On the other hand, the decision over which non-core vaccines to administer entails selecting, from a growing list of options, vaccine(s) on the basis of the individual patient's risk for exposure to the infectious agent. Vaccination does, in fact, require a medical decision. What's more, health risk profiling is fundamental in deciding which vaccines to use and when.

The health risk profile

In the context of this discussion, "risk" is a subjective term used by the clinician to estimate the consequence of infection in the individual patient following exposure to an infectious agent. Risk, therefore, is ultimately determined by a combination of three factors:

1. Host Factors

2. Environmental Factors

3. Agent (or pathogen) Factors

All that considered, vaccination becomes a critical tool used to mitigate risk following exposure to an infectious agent.

Host Factors: The consequences of exposure to an infectious disease are expected to be worse among animals that are malnourished, have concurrent infection or illness, or are receiving regular doses of immune suppressive drugs. Additional intrinsic factors considered to influence the outcome of infection include heritable resistance (and possible susceptibility) factors and stress. Age at the time of exposure is an important, independent variable in assessing an individual's risk to an infectious agent. Although no age group can be considered entirely free of risk, dogs and cats less than 6 months of age are generally more susceptible to infection than adult animals. Puppies and kittens, therefore, represent the principal target population to benefit from vaccination. In the canine and feline guidelines, it should be noted that for both puppy and kitten vaccination protocols, vaccine intervals and frequency are unchanged.

The presence of maternal antibody is an intrinsic host factor known to protect a puppy or kitten following exposure to an infectious agent. However, interference of vaccine antigen by maternal antibody is the single most common cause of vaccination failure. Administering vaccine to healthy puppies and kittens through the first two to four months of life, at the time when maternal is declining become especially critical in effectively lowering risk.

Table 3: Canine vaccines not generally recommended.

Environmental Factors. Population density, cleaning techniques, air exchange, construction/floor plan, and opportunities for exposure to other animals are perhaps among the most critical issues affecting the risk of exposure to an infectious organism. Consideration of such factors when contemplating administration of non-core vaccines becomes fundamental in deciding whether to administer vaccines against infections caused by Bordetella bronchiseptica, feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV). Cats and kittens clustered within a single household, for example, are at substantial risk of exposure to herpesvirus and calicivirus. Viral infection with either virus virtually ensures development of a chronic carrier state and persistent (for years) shedding of virulent virus. Geographic distribution of various infectious agents, e.g. Lyme borreliosis, may represent a significantly different risk profile to animals living in different locations within the United States and should be considered when determining which non-core vaccines would be appropriate. Sustained high ambient temperatures and humidity, in addition to housing environments with fewer than 12 air exchanges per hour, significantly increase the risk of animals' exposure to respiratory pathogens. In kennels, a single adult dog with infectious tracheobronchitis could put a large percentage of the canine population at risk for weeks threatening facility depopulation.

Agent Factors. Independent agent-associated variables, such as virulence, dose, and mutation (often related to population density and rate of virus replication) do influence the outcome of infection but are difficult to objectively assess in the shelter setting. However, in the domain of risk assessment, it is the interaction between the agent, the host, and the environment that dictates the outcome following exposure and infection. The severity of an infection, particularly a viral infection, is highly variable within a population of animals with similar exposure to the same agent. Clinical illness in susceptible animals exposed to the exact same pathogen can range from inapparent or mild to severe, acute illness to chronic or latent infection. Predicting the health impact of numerous variables on dogs and cats known to be at high risk of exposure to infectious diseases can be among the most difficult decisions facing veterinarians in clinical practice.

As a profession, vaccination is among the most frequent and important clinical services we provide. Today, with the recent introduction of several new vaccines, the decision over which vaccines to give, and when to give them, has become complex. Furthermore, the promise of new vaccines and new manufacturing technology assures that the challenge of administering vaccines in the future won't be reduced to a single, simple protocol.

The publication of vaccination guidelines for cats, as well as dogs, has been an important step in providing much-needed information to veterinarians committed to implementing rational vaccination protocols. These are important publications that should at least be reviewed by anyone engaged in companion animal practice. And despite the concern…and the controversy over somewhat dramatic changes, it must be remembered that the 2003 Canine Vaccine Guidelines are just that…guidelines….they are not standards against which practicing veterinarians are to be held accountable. Veterinarians can use this information to develop their own protocols to meet the individual needs of their clients and patients.