Friday, August 6, 2010

Do Vaccines Weaken the Health of a Breed?

Do vaccines affect the genetic diversity or strength of a breed?? Does vacccination and the protection provided by herd immunity produce a weaker population? Would it be better to allow "survival of the fittest"; avoiding vaccination completely and breeding only those who can survive infectious disease, with their presumed superior immune systems? Some proponents of natural rearing believe so. Here is another point of view from a canine genetics email list, reposted here by permission of the author. SB 

Is dying from a disease organism, adapted by its nature to try to overcome an animal’s immune system, a sign of “weakness” or “inferiority” of an organism? Is an animal immune-compromised if it dies of rabies? Since rabies is virtually 100% lethal in all mammals except bats (where it also apparently causes mortality, but not 100%), are we to assume that all mammals except bats have weak immune systems?

What about distemper or parvovirus? If dogs have become “weak” because vaccination has allowed the weaker individuals to live, then you might expect wild carnivores to be more resistant to these diseases. In reality, both distemper and parvo often cause high mortality among wild canids. Distemper affects many carnivore species other than canids and has been catastrophic for some of them. (I didn’t have much luck in a quick search for scientific articles accessible to the public without a university connection, but the Wikipedia article on distemper in dogs gives a few examples of wild species severely affected by dog distemper or strains of distemper that apparently evolved from dog distemper. I also found this interesting article in ScienceDaily )

Distemper seems to be a recently evolved disease, maybe a few hundred years old. Parvovirus may be very recent, only a few decades old. Like many infectious diseases, I think these are both diseases that were aided in their evolution by the increasing population density of the affected organism (dogs, in this case, but also people because the distemper virus apparently mutated from the human measles virus). As population density increases, a disease organism has an easier time finding new individuals to infect. It has more individuals to provide a home to incubate in and undergo mutations that might help it be more successful, which to a disease organism means being better able to spread and overcome the victim’s defenses.

It was no accident that Europeans were the victims of so many more diseases than Native Americans prior to Columbus. Europeans had higher population densities that allowed the successive evolution of all sorts of infectious diseases (smallpox, measles, etc.) The Europeans didn’t have to face all these diseases all at once. They developed their resistance over many centuries of separate epidemics. The Native Americans got hit all at once with all these diseases that had forced Europeans to adapt (via massive high-mortality epidemics). By some estimates, 90% of Native Americans were wiped out by infectious diseases carried by Europeans in the decades following Columbus’s trips. Would you consider the pre-Columbus Native Americans genetically inferior or “weak” because they had been isolated from those diseases prior to Columbus? Would you consider Black-footed Ferrets (a species where dog distemper causes huge mortality) to be genetically “weak” compared to dogs that have had hundreds of years of exposure to distemper?

The point is, there will always be new, evolving diseases. All organisms are in an arms race with pathogens. Pathogens are constantly evolving new ways to overcome the immune defenses of organisms, which forces organisms to come up with new defenses against the pathogen. In some cases, the species cannot adapt fast enough and goes extinct.

All organisms face multiple challenges on all fronts: competition with other species, the need to survive the abiotic environment (heat, cold, storms, etc.), the need to find enough food, the need to find sufficient nutrients that may be difficult to get through their diet, and the need to defend against predators, parasites, and pathogens. What is most adaptive depends on the relative selective intensity of all these challenges. An arctic animal may face few challenges from infectious diseases but faces brutal environmental challenges, hence an arctic species may be more vulnerable to certain diseases it is rarely exposed to. A similar species adapted to a milder climate is probably better at disease resistance but could not survive the rigors of the arctic winter. So, a species ability to respond to challenges depends on how important those challenges are to them. No species can be good at all things. There is always some cost to an adaptation, even if it is only the cost in lost diversity of the animals that could not adapt.

So, the question for dog breeders is, how much value do you want to place on the ability of dogs to survive parvo or distemper without vaccination? Do you want to make that the first cut (very literally!) in your breeding selection by letting the puppies be exposed to see who lives or dies? If so, do you want to make sure they get exposed at their most vulnerable age to make it the best test possible? What about rabies? Surely, among the millions of living dogs, there must be a few that could survive rabies. After all, bats somehow evolved the ability to survive the disease. Shall we expose all unvaccinated dogs to rabies and breed from the 5 or 6 or so that survive? If none survive, well, then the weak dog species deserved to go extinct, right?

Also, a question for those that don’t vaccinate for parvo or distemper, but treat aggressively at the first sign of illness. If you want to select for stronger dogs, why do you treat at all?

Personally, I choose vaccination. To me, a weak immune system is NOT one that cannot fight every single new disease that evolves. It is one that cannot defend against non-pathogenic organisms that are always present or that is incapable of mounting a defense against most of the “normal” pathogens.

If you were to only select dogs resistant to distemper and parvo, then you would most likely be reducing diversity of the population, which would make it MORE vulnerable to the next completely new disease. The ability of a species (not an individual) to defend against a novel threat (one a species has never faced before) depends on the population having enough diversity that it increases the chances of some individuals having the adaptations they need to survive.

Kelly Cassidy, PhD
Curator, Vertebrate Museum
Washington State University

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