The White Paper

Texas Department of Public Health

Zoonosis Control

Options for Rabies Vaccination

of Dogs and Cats in Texas

I. Purpose of the Report

The purpose of this report is to explore the options for rabies vaccination protocols in dogs and cats in Texas which would adequately and appropriately protect the public’s health.  Six options have been proposed that comply with current statutory law:

1.  annual vaccination using any approved vaccine

2.  annual vaccination using a vaccine approved for three years (triennial vaccine)

3.  vaccination every two years with a triennial vaccine

4. vaccination every three years with a triennial vaccine

5. vaccination according to manufacturer’s recommendations
initial vaccination with titer check

It is evident that many dogs and cats in Texas are not receiving any rabies vaccinations at all.  Pet owners’ complete failure to vaccinate their animals presents a serious public health danger; however, total noncompliance cannot be remedied through vaccination protocols but must be addressed through education and enforcement of existing laws.  The purpose of this document is to explore options for rabies vaccination protocols; therefore, discussion will focus on vaccination intervals rather than the complete failure to vaccinate pets.

II. Overview of Rabies & Rabies Vaccine

Rabies is a universally fatal disease of mammals, including humans.  Aggressive rabies control measures in animals and postexposure treatment in humans have resulted in a low incidence rate of the human rabies in the United States.  Between 1990 and 2000, there were 32 human deaths due to rabies in this country.  Of those, 24 cases were the result of variants of the rabies virus commonly found in bats; six were exposed to rabies outside of the United States and died of rabies variants not found in this country.  The remaining two cases lived in Texas and contracted the variant of rabies that infects domestic dogs and coyotes.  Exactly how these last two people were exposed to rabies could not be determined.

Life-saving treatment is available for people and is effective if given soon after exposure to the rabies virus.  Once clinical signs develop, treatment is no longer effective and the outcome is death.  The cost for rabies biologicals is significant and ranges from $900 to $1,400 per person depending on body weight, exclusive of physician charges and associated costs such as transportation and time away from work.[1] 

Nationwide in 1998 at least 795 persons required post-exposure prophylaxis as a result of exposure to 267 rabid dogs and cats.

Between 1998 and 2000, an average of 573 people annually received rabies biologicals from the Texas Department of Health due to exposure to potentially rabid dogs and cats.  The quantity of rabies biologicals dispensed through private sources, such as hospitals, is unknown.

There are many types of rabies viruses, each of which is called a variant.  Each individual variant of rabies will perpetuate itself in nature through repeated transmissions only in the mammalian species for which it is adapted.  Although “spillover” of the particular variant might occur in an alternate species (such as the gray fox rabies variant in a cow), spillover rabies usually presents as a single rabies case and does not continue to spread.

Texas is the only state in the U.S. in which rabies outbreaks due to the domestic dog/coyote variant of the virus have occurred in recent years.  Throughout history, the canine variants of the rabies virus have caused millions of deaths worldwide.  The canine variants are regarded as especially dangerous because they are readily transmitted amongst and between wild and domestic canines (such as coyotes and dogs) and, subsequently, to people due to their close association with pet dogs.  Texas’ Oral Rabies Vaccination Program has curbed the outbreak of the domestic dog/coyote variant in Texas with only an occasional case occurring near the US-Mexico border.

Rabid domestic animals are far more likely to expose humans to rabies than are rabid wild animals.  For every 100 dogs and cats that are tested for rabies in Texas and found to either have positive or inconclusive results, 170 people are potentially exposed while only 16 people are potentially exposed for every 100 terrestrial wild animals that tests positive or inconclusive. 

[1] Unpublished data – Texas Department of Health, Zoonosis Control Division

The number of laboratory-confirmed rabid dogs and cats in Texas is contained in Figure 1.  According to figures from the American Veterinary Medical Association, over 60% of Texas households own companion animals, with an estimated 5.9 million dogs and 6.6 million cats residing in the state.

                                    Figure 1.  Laboratory-confirmed cases of rabies in Texas
 

During the 1950s, two concurrent events contributed to a dramatic reduction in the number of rabid dogs in the United States.  First, a safe, effective rabies vaccine for dogs was developed.  Second, cities began enacting more stringent animal control laws, including the removal of stray dogs.  The number of confirmed cases of rabies in dogs in the United States was reduced from 6,648 in 1941 to 160 in 1989, despite an increase in the number of confirmed cases of rabies in wildlife.  

Two general types of rabies vaccines for dogs and cats are currently available in the United States: vaccines that are approved for use on an annual basis and vaccines that are approved for use on a triennial basis.  Advances in technology are
occurring which may result in new vaccines being introduced. 

The United States Department of Agriculture (USDA), based upon Title 9 CFR §113.209, governs preparation and testing of rabies vaccines.  The USDA requires that rabies vaccines be tested using challenge trials that include a minimum of 25 vaccinated animals and 10 unvaccinated controls.  The animals in the vaccine group receive one dose of rabies vaccine.  During the first year, all test animals receive five blood tests for neutralizing antibody titers to the rabies virus.  For duration of immunity trials of more than one year, the test animals are monitored serologically every six months for the remaining time of the trial.  At the end of the trial period, the vaccinates and the controls are both challenged by injection with virulent rabies virus into the masseter (jaw) muscle using USDA-provided virus of the New York City dog strain.  The challenged animals are then observed for 90 days.  The brains of test animals that die following challenge are examined for evidence of rabies infection using fluorescent antibody testing.  A successful challenge trial requires that 80% of the controls die of rabies and 87% of the vaccinates survive for 90 days.

III. Current Recommendations & Laws Regarding Rabies Vaccination

The Compendium of Animal Rabies Prevention and Control sets forth the recommendations of the National Association of State Public Health Veterinarians for rabies control in the United States.  The Compendium recommends that a triennial vaccine be administered to dogs and cats in accordance with the manufacturer’s specifications.  The product labels for the triennial rabies vaccines currently on the market call for the first vaccination to be administered at three months of age with a booster given one year later and then every three years thereafter.  The product labels for most of the annual rabies vaccines states that the animal should be vaccinated at three months of age and annually thereafter.  One manufacturer’s annual rabies vaccine for cats is approved by the USDA for use in animals as young as eight weeks of age.

Rabies vaccination laws for dogs and cats vary widely among the states with some states following the Compendium’s recommendations and others opting for different protocols due to the unique characteristics of their states.  Current state rabies laws are as follows (Figure 2):

Ø      33 states permit vaccinating according to the Rabies Compendium;

Ø      10 states allow local jurisdiction for rabies vaccination intervals;

Ø      2 states require a rabies vaccination every two years;

Ø      5 states require rabies vaccination each year.

Texas state law allows only veterinarians to administer rabies vaccinations.[1]  Using this law as its basis, the Texas Department of Health has administrative rules which require that dogs and cats be vaccinated by four months of age and annually thereafter. [2]   These administrative rules require that a vaccine with a three-year duration of immunity be used in dogs.  This requirement for dogs was instituted in 1996 after then Governor Ann Richards declared rabies to be a public health emergency in Texas.  The vaccine to be used in cats is not specified in the administrative rules and, therefore, can be either the one-year or the three-year product.

IV. Vaccination Efficacy

One common concern about vaccine efficacy is the actual duration of immunity conferred by triennial vaccines.  In other words, manufacturers of triennial vaccines have satisfied USDA requirements to demonstrate that the vaccine confers immunity for at least three years, but how much longer than three years does the protection last?  Vaccine manufacturers consider duration of immunity studies to be proprietary information.  While manufacturers are required to provide test results to the USDA, neither they nor the USDA are required to reveal those results to the public, and they opt not to do so.  

Very few studies on the duration of immunity of rabies vaccines can be found in the literature, with none that demonstrate duration of immunity for vaccines currently used in the United States.  A vaccine manufacturer, Merial Inc, performed the longest challenge trial by testing cats at 44 months.  The vaccine was 100% effective in these cats.  In another study, Dr. Ronald Schultz of the Madison College of Veterinary Medicine at the University of Wisconsin found antibodies to rabies in dogs at seven years post-vaccination, but he did not perform rabies challenge studies.[1]  The scientific consensus is that data are not available to validate protection beyond three years.