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Date Posted: 22:17:02 05/03/09 Sun
Author: Sheena
Subject: Demodex...again
In reply to: DKH 's message, "You gotta be kidding me Tashi..." on 14:47:49 05/02/09 Sat

This is not a post made lightly, as I have zero desire to engage in an online feud. However, the issue of demodex continues to surface in these posts, so additional information on the topic is warranted.

“You used one of my culls at stud, a demo (GENERALIZED) dog…”
This issue of generalized vs. localized in regards to this particular dog is an interesting one. Let’s do a brief recap. Norman, the dog’s owner at the time he exhibited demodex, (the very experienced dog-man that Harris thought highly enough of to give the puppy to), stated that the dog had LOCALIZED demodex (just putting it in caps so the information is not missed). Norman’s vet (the vet that examined the puppy, the vet of the experienced dog-man) stated the puppy had localized demodex. Harris, the dog’s breeder (the breeder that was multiple states away from the dog at the time of the demodex incident) states the dog had generalized demodex (and also states that every puppy in the litter had generalized demodex).

Now, for those that may be reading wondering about the relevance of localized vs. generalized (although I think everyone on this board has likely seen this topic beat to death enough to understand the differences), localized is not considered anything to be concerned about, it is called ‘puppy demodex’ and it is reasonably common in growing puppies. Generalized demodex is a result of a genetic susceptibility to demodex infection, and current information on the topic states that it is inherited in an autosomal recessive manner.

Autosomal recessive. What does that mean? That means for the trait to be manifest, the affected dog must have two copies of that particular gene, which means that one copy is inherited from each parent. This means that both parents of a dog with an autosomal recessive trait are carriers of the gene. The parents could also be affected themselves (have two copies of the gene), or just carriers (one copy of the gene).

What is the relevance in this situation? As mentioned, data indicate that susceptibility to generalized demodex is inherited in an autosomal recessive manner, which means that both parents of puppies that exhibit generalized demodex are (at least) carriers.

For anyone that may be interested in a little more detail, we can break this down into some numbers. Let’s say that the dam of the litter was a carrier (heterozygous, only one copy of the gene). Now, as mentioned, in accordance with current knowledge on the topic, for any puppies in the litter to exhibit generalized demodex , then the sire was a carrier as well. You cannot have offspring exhibit a recessive trait without both parents possessing at least one copy of the gene.

Those of you familiar with inheritance and probabilities are probably already figuring out the numbers in your head. For a puppy to be born with a recessive trait, and if both parents of a given puppy were heterozygous, then the probability of a given puppy being homozygous (having the recessive trait) is 25%. The odds change a little when you start to ask “what is the probability that all puppies in a litter from carrier parents are affected?” Continue to read if you want an example of this probability (but this is going to get a little dry…).

So, for this example, we’ll say that there are 8 puppies in a litter (8 seems to be a reasonable number for a litter, perhaps a bit low for this breed, but a workable number). As stated, the probability that any given puppy is affected with a trait for which both of its parents are carriers is 25%, or ¼. To determine the probability that every puppy in a litter of 8 would be affected if both parents were carriers, you multiply the probability of a given puppy being affected, by the probability that any given other puppy is affected, by the probability of any given other puppy being affected… etc. for the number of puppies. So, your equation would look like this: (1/4) x (1/4) x (1/4) x (1/4) x (1/4) x (1/4) x (1/4) x (1/4). This gives you the number 0.000015, or 0.0015% (another way to state it is 15 out of every 1,000,000 (one million) such sample litters). That’s a pretty low probability!

The chances increase a bit if one parent was affected (homozygous for the trait), and not just a carrier. Both parents still need to be carry the gene in order for affected puppies to be born, but the odds (of all puppies being affected) are higher in this situation. The probability that any given puppy would be affected if born to a carrier/affected mating is ½, or 50%. Using the same calculations as above, the probability that every puppy is affected (of an 8 puppy litter) is 0.00390625, or 0.39% (or 39 out of 10,000). Still pretty low, but obviously higher than the previous example.

The final example is a puppy born in a litter where both parents are affected (homozygous) and not just carriers. The odds of any given puppy in a litter being affected is 1/1, or 100%. Subsequently, the odds that every puppy in the litter is affected is also 100%.

You can draw from this information the conclusions you feel are warranted. Either every puppy in the litter originally spoken of was not affected (which disagrees with what the breeder states), or every puppy in the litter was affected (which is what the breeder states). If every puppy was affected, then you can decide for yourself which scenario listed above and based on current knowledge is most likely to be the case in this situation. Were both parents carriers (chances of all puppies affected = 0.0015%)? Was one parent a carrier and one parent affected (chances of all puppies affected = 0.39%)? Or were both parents affected (chances of all puppies affected = 100%)?

Any way you look at it, there really is not a lot of room for stone-throwing in this situation. Either the breeder is incorrect in stating that every puppy in the litter was affected with this recessive trait (which agrees with what the dog owner and his vet stated), or the breeder is correct in stating every puppy was affected, which means both parents at least carried the gene (and choose your own situation based on the probabilities).

“You appear to be either naive, ignorant, or dis-honest, I can’t decide.”
A reflective statement, given the probabilities of this situation, and the fact that Gordo’s sire continued to father litters after the birth of the ‘demodex’ litter. Harris, if you truly believed the entire litter was affected with generalized demodex, and feel as strongly about it as you evidently do (considering you’ve stated the dam of the litter was culled, and the reaming you continue to heap upon Norman and Tashi), then would it not have made sense to cull the sire as well?

In my dealings with Tashi, she has not exhibited ignorance, and certainly not dishonesty (whether or not you like her is a different situation…). I have found her to be more forthright and forthcoming with information than anyone else I have spoken with in this breed.

Believe what you like, but do it with your eyes open.

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