The White Paper page 2

Options for Rabies Vaccination
of Dogs and Cats in Texas

V. Vaccination Compliance Rates

Human behavior is the most difficult part of evaluating vaccination policies.  Public health officials in most states do not know the vaccination rates for dogs and cats in their states.  There are a number of obstacles to obtaining that data.  When conducting surveys, it is difficult to validate the owner’s ability to recall if and when the pet was vaccinated and to ensure honest responses in these surveys.  As an example of survey accuracy, the American Society for Microbiology conducted a national telephone survey of 1,021 adults asking if they washed their hands after using public restrooms.  They also placed observers in public restrooms in five major metropolitan areas who observed the behavior of 7,836 men and women.  Ninety-five percent of those interviewed said they washed their hands; however, only 67% were actually observed washing their hands.

 and 60% of dogs are vaccinated against rabies in Alabama each year.[1]  It is unknown whether the difference between Alabama and the New England states is because of lack of recall and/or dishonest responses on the part of the owner or if a difference in compliance truly exists between the populations.

The Texas Department of Health reviewed case investigations involving 198 cats and 1,233 dogs in Texas which had been exposed to potentially rabid animals during the years 1998 through 2000.  The analysis revealed that 19% of the animals had received a rabies vaccination in the past but were delinquent at the time of exposure (Figure 3).  For those dogs and cats which were delinquent in receiving their rabies vaccinations, the average length of time between the animal’s last vaccination and its exposure to a rabid animal was19 months (i.e., it was 7 months overdue for its annual booster).

Figure 3.  Rabies vaccination status of dogs and cats potentially exposed to a rabid animal, 1998-2000

One of the arguments for yearly vaccination of dogs and cats is that owner compliance will be higher than with triennial vaccination because annual events present an easily recognized and remembered routine.  Also, most veterinarians mail reminders to clients to prompt them to obtain their pets’ rabies vaccinations.  One of the concerns about vaccinations given every three years is that, in today’s mobile society, clients will move and not receive mailed reminders, the result being that the rabies vaccination will not be obtained or will be delinquent.  If the client obtains preventive care other than rabies (such as other vaccinations, heartworm testing and prevention, or dental prophylaxis), these reminders are sent on a more frequent basis.  Therefore, the concern about owners failing to receive rabies reminders focuses primarily on the owners who obtain only rabies vaccinations for the pet without other preventive care.

[1] unpublished data provided by Bill Johnston, DVM, DACVPM, of the Alabama Department of Public Health

In an attempt to see if a difference in delinquency rates exists between states requiring rabies vaccination each year (one-year states) and states requiring rabies vaccinations every three years (three-year states), the Texas Department of Health obtained data from Banfield Corp., which owns VetSmart.  Banfield Corp. provided data from seven states where they have clinics (Texas and Florida which are one-year states and Arizona, California, Illinois, North Carolina, and South Carolina which are three-year states).  The data contained the vaccination status of 1,600,000 dogs and 582,000 cats based on a form completed by the veterinarian indicating if the vaccine was administered early, late, or on time.  When looking at the data for the people who elected to have their pets receive rabies vaccination only and no other preventive care, comparison of the one-year states and the three-year states demonstrates no difference in the delinquency rates.

 VI. Potential Adverse Effects of the Rabies Vaccination

In the early 1990s, veterinary oncologists and pathologists noted an increase in the occurrence of tumors in cats known as sarcomas.  While the exact cause of this increase in sarcomas has not been determined, evidence indicates a causal relationship between vaccination against rabies and/or feline leukemia virus and the development of these sarcomas at the site of vaccination.  These retrospective studies have also demonstrated that vaccination-site sarcomas tend to occur in younger cats and are less responsive to therapy than sarcomas not associated with vaccination sites.  Currently, insufficient data exist to assess the relative risk of administering a particular vaccine or antigen to an individual cat, but ongoing research is attempting to define the scope and incidence of the problem and determine causal and prognostic factors relating to the disorder.  The true incidence of vaccine-associated sarcomas in cats is unknown but attempts to quantify its occurrence range from 1 case/10,000 cats to 10 cases/10,000 cats. 

Although most of the discussion about adverse consequences of vaccine administration has been focused on cats, some evidence associates detrimental effects with vaccine administration in dogs.  In a case-control study at the University of Pennsylvania, researchers reached the conclusion that a marked difference existed in the frequency of immune-mediated hemolytic anemia in dogs which had been vaccinated versus the control group.

The veterinary profession has begun to question some previously held beliefs regarding vaccination practices.  Increasing numbers of veterinarians and professors at colleges of veterinary medicine in the United States are advocating that vaccination is a medical procedure that should be undertaken with the same thoughtful consideration as any other medical procedure in veterinary practice, to be performed after careful assessment of the needs of the patient with consideration given to the medical importance and zoonotic potential of the infectious agent and the patient’s risk of exposure.

VII. Rabies Titer

One alternative to requiring regularly occurring rabies boosters in dogs and cats would be an initial series of rabies vaccinations with the option, in lieu of boosters, to periodically perform a blood test to determine if the animal has a titer against the rabies virus.  If the animal had an adequate titer, no booster vaccination would be necessary; if the animal had no titer or a low titer, a booster would be required.  Rabies prevention in humans presents a model for this concept in that it is recommended that people who are at high risk for exposure to rabies (such as veterinarians, animal control officers, and spelunkers) receive pre-exposure rabies vaccinations.  Rather than receiving boosters at regular intervals, the person should have his or her titer tested and receive a booster only if the titer has dropped below acceptable levels. 

At least two reasons exist for why blood testing is not an acceptable substitute for rabies vaccination in animals.  First, most veterinary immunologists agree that rabies titers provide a sketch of the animal’s past exposure to the rabies virus but do not indicate a level of protection.  A titer of 0.5 IU/ml is considered acceptable, but it is not known whether it is protective.  Studies have demonstrated that dogs with low titers and even no titers were protected when injected with rabies virus.  This may be due to the fact that a titer measures only one of the two types of immunity.  In addition to the humoral, or antibody, response measured by titer tests, rabies vaccine produces cell-mediated immunity.  This cell-mediated response is important in protecting against rabies, but no commercial assay is currently available that offers an economical, accurate, and rapid assessment. 

The second reason that human recommendations cannot be extrapolated to animals lies in the chain of events that occurs after a human, dog, or cat is exposed to rabies.  Typically when a human is bitten by an animal, it is determined whether or not the animal has rabies.  If the animal is rabid, the person undergoes a series of post-exposure treatments to prevent rabies from developing.  Post-exposure treatment is recommended regardless of whether the individual had pre-exposure vaccinations or an acceptable titer.  In contrast to humans, when a dog or cat is bitten, the owner is often not aware that a bite occurred or that the biting animal was rabid, and no post-exposure treatment is administered. 

VIII. Summary

For reasons cited in this document, determining a rabies titer in lieu of a vaccination is not an acceptable means of protecting the public’s health.

Historical use of the annual and triennial rabies vaccines has shown them both to be effective in preventing rabies in dogs and cats.  A paucity of scientific data exists to demonstrate a clear public health benefit of a one-year vaccination protocol versus a three-year vaccination protocol.

The issue of vaccination frequency is highly polarized with various groups advocating a preferred interval.  The Texas Department of Health will seek stakeholder input in the form of comments made both in writing and at public meetings.  Upon the conclusion of the comment period, the Texas Department of Health will review the scientific data in combination with the input given by stakeholders to determine if the current Rules for Rabies Control and Eradication adequately and appropriately safeguard the public’s health.

The fact that approximately 33% of the dogs and cats70 in Texas have never received a rabies vaccination and 19% are overdue for their rabies vaccination represents a serious public health danger.  Failure to comply with rabies vaccination regulations must be addressed through education and enforcement of existing laws.

References

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Centers for Disease Control and Prevention: Compendium of Animal Rabies Prevention and Control, 2001: National Association of State Public Health Veterinarians.  MMWR 2001;50(No. RR-8):1-8

Clark KA, Wilson PJ: Postexposure rabies prophylaxis and preexposure rabies vaccination failure in domestic animals.  JAVMA 1996;208, No. 11:1827-1830.

Cohen Michele, Wright James C, Brawner William R, Smith Annette N, Henderson Ralph, Behrend Ellen N: Use of surgery and electron beam irradiation, with or without chemotherapy, for treatment of vaccine-associated sarcomas in cats: 78 cases (1996-2000)

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Hendrick MJ, Goldschmidt MH, Shofer FS, Wang Y-Y, Somlyo AP: Postvaccinal Sarcomas in the Cat: Epidemiology and Electron Probe Microanalytical Identification of Aluminum.  Cancer Research 1992;52:5391-5394.

Hendrick MJ, Shofer FS, Goldschmidt MH, et al: Comparison of fibrosarcomas that developed at vaccination sites and at nonvaccination sites in cats: 239 cases (1991-1992).  JAVMA 1994;205, No. 10:1425-1429.

Hyde B: America’s Dirty Little Secret – Our Hands.  http://www.washup.org/page03.htm  American Society for Microbiology, September 18, 2001

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