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Vaccine Boosters and Titres

Dr. Susan Wilkinson, DVM

Do we over-vaccinate our pets? Maybe.

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However, the trick is protecting our friends (and ourselves) from infectious diseases, while at the same time not over-exposing them to the risks associated with vaccines, or any medication. This is an area of some controversy among the veterinary profession. Most veterinarians are struggling to sift through all the data, both from the scientific and lay community, about the efficacy and duration of vaccines. One thing we all can agree on is that we don’t want to see a resurgence of those diseases that are thankfully rare today thanks to our established vaccine protocols. Personally, I’ve never seen a clinical case of canine distemper in a dog, and I’m not anxious to break that record (the raccoons I have seen with distemper are pathetic indeed – it’s not a nice way for an animal to die).

Most vaccines marketed today have a one year label claim to efficacy. Rabies vaccines however, commonly carry a three year label claim. Exceeding the manufacturer’s recommendation constitutes an “off-label” use of the vaccine -- which legally could be a problem for veterinarians if a disease outbreak where to occur. However, off-label use in the face of peer-reviewed and published studies substantiating that use is justifiable legally.

Unfortunately, the peer-reviewed studies have been slow in appearing. There was a small study conducted several years ago among a small population of laboratory cats that suggested that the common feline combination vaccines did offer longer that one year of protection. Protection against what, though? Those cats were challenged by laboratory strains of disease, not the wild variants found in the real world. Many veterinarians doubt the validity of such a study and want to see more work done before risking their patients’ health through possibly under-vaccinating. Many accept the conclusions, and are anxious to not risk their patients’ health through over-vaccinating. Neither group is wrong. The American Animal Hospital Association (AAHA) has recently published new vaccine protocols that generally recommend all vaccines in adult animals previously vaccinated (for two consecutive years), be boostered every three years. Bearing in mind that legal requirements for boosters may alter this (i.e. rabies vaccine, some municipalities still require annual vaccination).

The decision to vaccinate or not to vaccinate needs to be carefully evaluated, weighing the risks vs. the benefits for each individual or groups of individuals. Geographic location also plays a major role in the decision process, as not all diseases are prevalent in all areas. For the vast majority of my patients (rural area, lots of wildlife, lots of unvaccinated feral dogs and cats, lots of wilderness areas to walk your dog in), I still recommend annual vaccine boosters – the risk of meeting a sick or unvaccinated animal is fairly high. My own dogs cornered a sick raccoon in my backyard a couple of years ago. The poor thing likely had distemper, and I caught and euthanised it. Also, many animals are not seen on an annual basis anyway; they get vaccinated when they’re seen!

Puppy vaccines are another matter. The AAHA guidelines still support the the accepted protocol of several boosters given three to four weeks apart from 6-8 to 16-20 weeks, then an additional booster in one year. The multiple boosters are due in large part to the uncertain levels of maternal antibodies acquired by the puppy from it’s mother’s milk (the colostrum). While these maternal antibodies provide the puppy with vital protection, they also wane at an uncertain period, not providing a protective amount of antibodies but at the same time interfering with the vaccine’s action of stimulating an immune response within the puppy. Thus, it is necessary to administer more than one vaccine in order to ensure the puppy generates an adequate immunity.

So how do I know if my dog is protected and when do you booster their vaccines? What about vaccine titres you ask? Surely that will tell me if my dog is protected. Well, yes and no. A vaccine titre is calculated by the serial dilution of blood serum to determine the highest dilution that still contains detectable amounts of antibody. Better yet is an “end-point” titre that measures antibody activity by the dilution of antibody in a constant quantity of antiserum. The dilution in which flucculation occurs most rapidly is the end point.

A single titre is just a number and really doesn't tell us too much – what may be a low and unprotective level in one animal would be a perfectly normal protective level in another. No one knows if such and such a number is protective. So we have to look at the trend in the numbers. If a titre increases or decreases 2 fold, then that's considered to be a "normal" fluctuation". A titre that increases 4 fold over a previous titre means that the animal has been exposed to the disease and the immune system has responded appropriately, there is no need for a booster vaccine. If the titre has decreased 4 fold, then that means the immune system is losing "memory", it likely will not be able to respond appropriately to a challenge, and a booster vaccine is advised. In order for this logic to work, end-point titres are necessary. Ideally two separate blood draws at least three months apart, and submitted to the same laboratory.

All well and good, let’s titre our dogs. OK, but there are a couple of snags. Unfortunately, vaccine titres are a relatively new thing and as such aren’t widely available and are very, very expensive at the present time. Also, not all diseases provide us with a reliable useable titre. In dogs, only rabies, canine distemper, and canine parvovirus are useful titres. Diseases such as leptospirosis and lyme disease are not worthwhile measuring due to the many different strains that exist.

As you can see there are many questions, and no easy answers, nor even right vs. wrong answers. I struggle with the issues on a daily basis – for my patients, as well as for my own pets.

Dr. Susan Wilkinson, DVM

August, 2003