Spring Fever

Spring means vaccines for many mammals and discussions with veterinarians

April 1, 2013 (published)


Ah spring! Marshmallow clouds, chocolate bunnies hatching hard-boiled eggs, lawns on steroids, and best of all...


Spring means shots for horses and many mammals, and, for veterinarians, long discussions about shots. The first vaccine was developed in 1796 against smallpox by a clever guy named Edward Jenner. I wonder how Jenner would feel knowing that over 200 years later, a few folks still look at vaccination as some sort of radical government-takeover plot.

The basic principle behind vaccination is simple. Bodies – human, animal, squid, or Tribble –come equipped with an anti-intruder system. This feature, otherwise known as the immune system, fights off and kills foreign invaders such as bacteria, viruses, parasites, and IRS agents. When the immune system sees proteins that it identifies as “other,” it develops smart bombs (antibodies) designed to destroy anything attached to that protein.

Vaccines are a way of putting up “WANTED. SHOOT ON SIGHT” posters in the hallways of the immune system. A vaccine contains a very small dose of an infectious agent, usually virus or bacteria, or some recognizable piece of that agent, such as a sequence of DNA. When a whole virus or bacterium (a single member of the bacterial hordes) is used, it is incorporated into the vaccine in a form that will not cause disease in the animal. Like computers, vaccine technology is progressing toward the smaller and more efficient. With the huge leaps in understanding of genetics over the last couple of decades, manufacturers have been able to develop vaccines that use only a small piece of the viral or bacterial DNA to stimulate the immune system – think of it as putting a nametag on the pathogen rather than having to drag everyone down to the police station to identify the germ from a lineup.

The veterinarian injects a vaccine into the animal. The immune system sees the proteins in that vaccine, also known as the antigen, and says, “Hey. That guy looks suspicious. Let’s check it out.” The immune system doesn’t have antibodies prepped and ready to combat agents that it’s never seen before – this is why one dose of a vaccine doesn’t usually protect against disease. (There are some exceptions out there, but they are rare.)

However, the immune system takes a “fool me twice, shame on me” approach. Once it has spotted an intruder, it decides not to get caught with its pants down again. (Nothing worse than facing an invasion force and not being able to find your pants.) Once it has seen an antigen, the immune system develops antibodies to target that antigen the next time it crosses the threshold.

If an animal’s first exposure to an antigen comes in the form of legions of viruses causing full blown disease, the immune system might as well just raise the white flag and roll over. On the other hand, if the enemy is first spotted in a vaccine, the immune system can marshal its forces. Think of it this way; if aliens were landing on earth in thousands of big shiny ships with scary lasers, wouldn’t it be better if we got a warning like a meteor rock with a message reading “THESE GUYS ARE COMING, BUT YOU CAN MELT THEM IF YOU MIX COFFEE, CHOCOLATE, AND WHISKEY IN THESE PROPORTIONS.”

This need to develop an early warning system is the reason most vaccines require a two to three dose initial series to prime immunity and why it’s a bad idea to skip any of those doses. Have you ever been introduced to someone at a party after a drink or four and then run into them at the grocery store? You know that the person looks vaguely familiar but you can’t remember if you liked her or if she babbled about her nephew’s tax problems.

There are two tricky concerns left to address, and they are the ones that (right after “How much does this cost?”) are most likely to prevent folks from vaccinating their child, horse, dog, or Tribble.

“I’ve heard that vaccines are dangerous. I don’t want to do anything that will hurt my animal.”

“How often does my animal REALLY need booster shots?”

Let’s take the easy question first (easy meaning: sort of like to sticking an arm in the alligator enclosure). Are vaccines dangerous? Of course. So is oxygen. So is water. Life is 100% fatal.

When I was in vet school a professor made a statement that stuck with me: “Nothing we do is benign.” For veterinarians, who, like our physician counterparts, have the motto “First, do no harm” tattooed on our brains, this statement isn’t trivial and it always lurks in some corner of our brain. An animal could panic during an exam and injure itself, us, an innocent bystander, or a small village. A bandage could tighten and cause pressure necrosis. A medication that we’ve prescribed thousands of times could wind up being incompatible with this one patient’s physiology. And a vaccine could cause an adverse reaction.

In order to stick as close as possible to the “Do no harm” line, veterinarians constantly weigh risk vs. benefit. This is why your vet may occasionally take the seemingly insane path of losing money by talking you out of a medication or procedure for your pet. If everything has an associated risk, we want the benefits of that choice to be heavier on the balance than the risks.

Just like the meteor warning of the pending alien invasion might crash through the roof of a crowded mall, a vaccine can have the potential to cause an adverse reaction. However, the consequences that seem to most concern the public and the real risks of vaccination are sometimes divergent.

Things vaccines WON’T do:

  1. Vaccinating your horse won’t make her sterile.
  2. Vaccinating your dog won’t make him grow an extra head.
  3. Vaccinating your cat won’t cause brain damage.

Some weird accusations about vaccines have been thrown around the Internet and occasionally lobbed into the popular press. While many of these theories seem potentially logical or have high emotional impact, they often rely on the false logic that correlation equals causation and simply don’t stand up to credible research.

One example of this phenomenon is a rumor that circulated widely in the horse world when vaccines against West Nile Virus were first released. West Nile Virus is spread by mosquitoes. Mosquitoes emerge in the spring, as do foals. Thus there is a seasonal overlap between needles being poked into horses and foals poking out of horses. In any given year, a certain percentage of mares will “lose” their foals, generally meaning that an early pregnancy fails to implant or sometimes that a mare miscarries part way through gestation.

It is human nature to want to find a reason when things go wrong, preferably a more specific reason than “excrement occurs.” When faced with something like the loss of a foal, the first question horse owners ask is, “What have I done this season that is different from previous years?” If something “different” is identified, some part of the human brain says, “Aha! I did X and Y happened, therefore X causes Y.” Tidy logic, but seriously flawed. For example, if I decide to cook eels for dinner for the first time and my stove blows up that night, it cannot be reasonably concluded that cooking eels causes stoves to blow up.

Is it possible that some factor related to the eel caused the cooking range cataclysm? Sure. Maybe some sort of weird eel-oil vapors or something. However, it’s also possible that rodents chewed into the wiring, the stove was old, or the neighbors really don’t like me. The only way to have a better idea of the risks of cooking eels would be to do a scientific study comparing stove explosions in households that prepare eel with those in eel-free domiciles.

Believe it or not, these sorts of studies ARE run on veterinary vaccines (minus the eels). Before a vaccine can be licensed by the USDA it must clear a veritable steeplechase of hurdles proving that it meets the standards for purity, potency, safety, and efficacy.

In the case of the West Nile vaccine kerfuffle, not a single abortion in a single mare was ever definitively linked to West Nile vaccination. For that matter, the vaccines have been tested in hundreds of mares without any statistical difference in foaling rates between vaccinated and unvaccinated mares. Infection with West Nile Virus, however, does pose a real risk to both mare and foal. In this case, the vaccine risk vs. benefit math lands us squarely in the ‘benefit’ column.

What are some of the REAL risks of vaccination?

  1. Anaphylaxis
  2. Delayed onset systemic reaction
  3. Injection site reaction
  4. Injection site abscess
  5. Injection site tumors (mostly cats).

Anaphylaxis, systemic reactions, and injection site reactions are all the products of an overly-diligent immune system that freaks out not just at invasion but at random noises in the middle of the night. The system that is programmed to target and destroy the foreign invaders sometimes goes a bit haywire and starts attacking everything. Allergies are the most common manifestation of a super-enthusiastic immune system. All that sneezing, watering eye stuff in the spring? That’s a sign that your immune system is taking its job way too seriously and really needs a vacation on a beach with a cold beer.

Of the events on the list, anaphylaxis is the heavy-breathing, scary-mask-wearing baddie. Anaphylaxis is an immune hypersensitivity on PCP. Though life-threatening, the good news is that anaphylactic reactions to vaccination are fairly rare, happen almost immediately after vaccination, and are usually treatable with the proper medications - a good reason to have your veterinarian vaccinate your animal.

General systemic ickiness characterized by mild fever, loss of appetite, a desire to watch reality TV and drink cheap beer, and localized reactions (stiffness, swelling, pain, and heat at the site of an injection) are much more common adverse consequences of vaccination. These reactions, while unpleasant for owner and patient, are still not common and are much less dangerous than most of the diseases for which we vaccinate. Once again, risk vs. benefit lands us with benefit and minimal risk.

Do I ever recommend NOT vaccinating an animal? Sure. Risk vs. benefit. When making vaccine recommendations, I look at several things:

  1. What is this animal’s risk of contracting the disease? For example, in Central California, we don’t see Venezuelan equine encephalomyelitis (VEE) in horses. Our mosquitoes just don’t happen to come VEE-flavored. So unless I have a patient traveling to a high-risk area, I discourage owners from vaccines containing VEE antigen. It’s not necessary, so why risk a reaction?
  2. How protective is the vaccine? No vaccine is 100% effective because every animal is different. That said, some vaccines are better than others efficacy wise. If a patient has a low risk of disease exposure and the vaccine for the disease is not highly effective, I may question the value of vaccinating that particular animal with that particular vaccine.
  3. How serious are the consequences of the disease? Say an animal has a low-ish exposure to two different diseases. Disease A is an influenza virus – causes a fever, some lethargy, disgusting snot, but not generally too horrific. Disease B – let’s call it rabies – is 100% fatal and has the possibility of being transmitted to humans. I may say, “Yeah, you can probably skip vaccinating against Disease A,” but you can damn-skippy bet that I’ll arm wrestle you to get that vaccine against Disease B into your animal.

Let’s get to the tricky question. “How often does my critter need those darn booster shots?” The answer to this one changes.

Booster recommendations may change based on:

  • Changes in our understanding of duration of immunity: Most dog and cat vaccines are no longer recommended at yearly intervals since it was found that immunity may persist for two or three years.
  • The type of vaccine: Some vaccines are formulated to produce a longer lasting immunity than others.
  • The degree of exposure: An injury such as a puncture wound may make it advisable to booster a tetanus shot sooner than expected.
  • The best medical guess: For some vaccines in some animals, we don’t yet know the duration of immunity, so the current recommendation tends to err on the side of “play it safe.”

Bottom line: Talk to your veterinarian. Ask questions. Express your concerns (hint: you’ll get a more informative response if you avoid the phrases, “I was reading on the Internet,” “My friend the breeder/the kid at the feed store says,” “government conspiracy,” and “tinfoil hats”). Remember that as a rule, invasion warning beats the actual invasion. Just look at human medicine. How many people do you know who have been afflicted with polio or died of smallpox? Those diseases didn’t just go off to find other planets to conquer. The massive reduction in polio cases and elimination of smallpox were direct results of vaccination.


Christy Corp-Minamiji 
April 2, 2013

Hi, TAM.  You bring up a good point.  Titres (measures of antibody levels) are used in some cases to attempt to evaluate an animal's immunity to a particular disease.  However, as far as recommendations go; I'm not comfortable enough with the current correlations in most species between titres and protective immunity to suggest checking antibody titres in lieu of vaccination in most cases.  I think that for some animals, this may be a valuable tool, but I'm not super comfortable with it as an across the board recommendation yet.  I think I'd default back to the "talk with your veterinarian."  Each animal is an individual, and the risks of disease exposure vary a lot by region and even by the type of housing for the animal. 

April 2, 2013

How about adding a discussion of titres vs. just re-vaccinating?


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