To boost or not to boost...

Recently an interesting article appeared in The Veterinary Record, the vet’s professional magazine, about the duration of immunity in dogs after vaccination or naturally acquired infection. In other words, it discussed what is known about the protection against infectious diseases after having been in contact with them, or vaccinated against them, and how often we should be giving booster vaccinations. Vet Peter van Dongen summarises.

Vaccination is an important component of a complete health programme. Vets are generally recommended to follow the manufacturer’s directions in administering vaccines, which often include once yearly re-vaccination. However, recently questions have been raised about the duration of immunity and about whether adult dogs really need yearly ‘boosters.’

History
Widespread vaccination became possible in the 1940s with the development of modified live vaccines for Distemper and Adenovirus (This virus causes viral hepatitis.) Early work showed that the duration of protection against Distemper was thought to be less than one year and it was suggested to recommend annual vaccination. Since the 60s yearly vaccination of dogs has become established as a standard of practice and this has reduced the incidence of disease considerably. For example, the incidence of Distemper decreased from 10% between 1919 and 1964 to 0.1% between 1980 and 1991! However, some people have called into question the safety of vaccines and reports of for instance auto-immune diseases have resulted in widespread discussions about the appropriate use of vaccines.

Measurement of immunity
The immune response of dogs to infection or vaccination has often been evaluated by measuring titres of antibodies in the blood. However, although these titres correlate with protection against Distemper, Parvo, Hepatitis and Leptospira (Weil’s Disease), they do not necessarily correlate with protection against Bordetella, Para-Influenza virus (the latter two are involved in ‘Kennel Cough’) and Corona virus. Also, dogs respond to antigenic challenge (disease or vaccination) in various ways, only one of which is the production of antibodies.

Duration of immunity after vaccination
Unfortunately there is not a lot of information about how long immunity lasts after vaccination. This is mainly because this information has historically not been needed for the authorisation or licensing of vaccines. This has changed with Rabies vaccines though: the vaccine has to be proven to give protection for at least one year, to be licensed. For most other vaccines the duration of immunity has been estimated on the basis of the persistence of antibodies in the blood, although this may not always be adequate, as outlined above.

The following information is known about the major components of modern vaccines:-

Rabies
After natural infection with Rabies most dogs will die and this makes it difficult to estimate the duration of immunity. The evidence of long term immunity after vaccination is based on challenge and the measurement of antibody levels. As part of the Pet Travel Scheme, dogs are required to have an antibody level of at least 0.5 IU/ml, but dogs with less than this have been known to survive challenge. Most dogs will be protected for between one and three years.

Distemper
If dogs are infected with Distemper and survive, they will normally have a long lived immunity. After vaccination, the level of immunity is measured by determining antibody levels. Information about the protection in humans against Measles after vaccination is also used, as Distemper and Measles are related. In some dogs protection can last very long, but in high risk patients, such as certain young dogs and old dogs, the determination of antibody titres can indicate whether re-vaccination is required. A recent study suggested that yearly re-vaccination should be maintained.

Parvo
Protection against Parvo can last quite long after natural infection, if the patient survives. The persistence of immunity against Parvo after vaccination probably lasts at least one year, but long term studies have not been conducted. It is probably shorter than in Distemper or Rabies.

Adenovirus
There are two types of this virus, one causes viral hepatitis and the other respiratory disease. Immunity against one protects against the other as well. Not enough is known about the duration of immunity after natural infection, but after vaccination protection probably lasts at least one year.

Leptospira
Nobody knows how long immunity lasts after natural infection! We do know that the bacteria can persist for a very long time in dogs which have been exposed to the disease, even if they have high levels of antibodies. Immunity after vaccination is thought to be short lived and long term studies have not been performed.

Bordetella & Para-Influenza virus
After natural infection with either of these causative agents of ‘Kennel Cough’, protection is thought to be short lived, probably in the region of seven months. Long term protection after vaccination has not been evaluated properly yet.

Duration of immunity standards
There are several scientific, technical and economic reasons which have made it difficult to carry out studies which give good information about the duration of immunity. These include the necessity to include unvaccinated control animals, kept under specialised conditions, but also the fact that individual vaccines can differ in their characteristics and formulations. Therefore, each and every vaccine would have to be evaluated separately. In the absence of valid data, an annual re-vaccination ensures a minimal level of protection under the most challenging circumstances!  

Vaccines as an important component of a complete pet health programme
There are two purposes of vaccination: firstly to protect individuals from disease, but also to maintain a sufficiently large immune population so that disease is not readily transmitted. For each individual disease there is a certain proportion of the population which has to be protected to prevent outbreaks. If this proportion is lower than the threshold, because of waning of vaccine immunity or lack of vaccination, outbreaks are more likely to occur. In humans this has happened with measles, polio and diphtheria in the last few decades. In dogs the threshold levels are not known for most diseases. However, where most dogs are vaccinated, diseases such as Distemper, Parvo, Hepatitis and Rabies are rare. But outbreaks can occur if the population immunity decreases, just like in humans. This occurred in Finland, where 5,000 dogs were affected by Distemper recently, after not having had the disease for 16 years! Just like in human medicine, it is important to remember that to continue protection against canine diseases it is imperative to maintain an adequate population immunity. Therefore more veterinary research is necessary to develop appropriate vaccination protocols.

Vaccination recommendations
Every dog is unique in its breeding, lifestyle, life stage, geographical location and risk of exposure to disease. It is, therefore, difficult  to make blanket recommendations on the use of vaccines. All the information about a certain individual dog must be evaluated to tailor its vaccination to its specific needs. The problem is that the persistence of immunity against all the diseases of concern is not easily determined in each individual dog. Even blood tests are not always helpful, particularly in cases where we know that immunity does not correlate to antibody levels.

Reference: Coyne, M. J. et al. (2001) Duration of immunity in dogs after vaccination or naturally acquired infection.
The Veterinary Record 149, 509-515

About the author...
Peter van Dongen qualified as a vet at the Utrecht Veterinary school, The Netherlands, in March 1990. He worked in a mixed practice in Louth, Lincolnshire for three years, before moving to Borough Green, Kent. At the same time he limited himself to small animals only. Since December 1996, he has run a branch practice in Allington, Maidstone, Kent.

In May 1995 Peter started agility (after years of just thinking about it!) with his Jack Russell Cross 'Basil' (a bitch!), then five years old. Since then they have qualified for many finals, including Crufts and Olympia. Basil, Peter's first and still only agility dog, is now an 'Advanced' dog - the highest level in the UK - and still going strong at the age of 11 years! Basil has won the coveted Crufts 2001 title in the individual Mini agility.

Peter passed the Agility Club Instructors' exam in October 1999 and has since done the British Agility Club Judging Workshop.

Peter and his wife Carry still live in Borough Green with their two dogs and two cats.

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