Chinchilla Malocclusion - A Polygenic Trait
"Some traits show a wide range of phenotypes because these traits are controlled by many genes. The genes act together as a group to produce a single trait...Traits are often influenced by the organism's environment." -Biological Sciences
Malocclusion ( mal, meaning 'bad,' and occlude, meaning 'to fit' or 'to align' ) is the misfit of the teeth. It can occur in the front or the back teeth of a chinchilla. Malocclusion of the molars has caused the most trouble in the chinchilla. It is usually a fatal condition. Most chinchillas that are going to malocclude will do so by the age of three, but it can show up as late as five years of age or more. When maloccluding, one or both eyes may look wet, the chinchilla may lose weight, or the chinchilla might drool or have a wet lower jaw called 'slobbers.' There is usually little use in trying to correct the maloccluding back teeth. They are difficult to trim, regrow quickly, and the fatal spurs are left intact. Malocclusion of the front teeth (as long as it is not due to malocclusion of the molars) can be much less serious, and survivable if tended to correctly. An injury, usually resulting from a fall, an abscess, or food or debris wedged between the teeth can cause the teeth to malocclude. This kind of malocclusion is easier to treat. The front teeth can be trimmed back or corrected so that the animals can eat and survive.
In general, recessive mutations like ebony, sapphire, and violet, along with hybrids, are more likely to malocclude than standards and dominant colors, although any chinchilla can potentially malocclude. Also, chinchillas that grow rapidly, and chinchillas with the extremely short brevicaudata head shape are more likely to malocclude, as are inbred chinchillas with the above traits. It is also suspected that an inability to absorb calcium and other nutrients plays a role in setting a chinchilla up to malocclude. Environmental factors also play a role. Diet, including the intake of excessive simple sugars, can contribute to malocclusion. Additionally, illness, hay wedged between the teeth, foreign particles stuck between the teeth or under the gums, teeth stuck in breeding collars or wire, and abscesses under the gums can contribute to malocclusion.
An easy way to check for early signs of malocclusion is to run your fingers along the bottom of the jaw bone, until you get to the large soft jaw muscle. You are looking for "tooth bumps", which are palpable roots of the maloccluding teeth. They feel like small hard round bumps, about 3-5mm in diameter, and are usually located just prior to the jaw muscle. Don't confuse these bumps with the jaw muscle itself. You can also feel along the bottom of the eye sockets for roots of maloccluding upper teeth. It is best if someone experienced with checking for malocclusion can guide you through the process the first few times.
In 1994, Laurie Schmelzle built a hobbyist chinchilla herd from breeding stock acquired at PSK chinchilla in California. She decided early on to only buy foundation breeding stock that was 3 years old or older, with no signs of malocclusion, and only from parents that were still alive and in breeding, that also had no signs of malocclusion. Since she had access to such a large herd, she was able to look through the breeders and see the parents and in some cases the grandparents of the chinchillas that would eventually be selected as breeding stock. After going to such an extremely selective process, she thought that she would certainly have an extremely low percentage of malocclusion in her herd. After breeding 7 generations of chinchillas, however, she quickly learned that eradicating malocclusion was not going to be accomplished so easily.
In 2004, Chinchillas.com funded a biotech research project conducted by a private company in New York. One of the things they were hoping to develop was a DNA based blood test that would allow breeders to know if their chinchilla was a 'carrier' for malocclusion. However, since the genes that predispose a chinchilla to malocclude are not a simple set of alleles at a single loci on the chromosomes, it is possible that the ability to detect potential maloccluders from a blood test may be out of reach for us in this lifetime. If it were as simple as finding the recessive violet gene, for example, it would be a test that we could hope to develop possibly in the next few years. It was discovered during the feasibility study stage that large pharmaceutical companies have spent millions of dollars trying to find a way to test for malocclusion in rats and mice. They have thus far been unsuccessful for the same reasons that plagued us in our research.
Unfortunately, malocclusion is not a simple recessive gene that can be culled or tested for in a simple routine manner. In December of 2007, Laurie Schmelzle of Chinchillas.com was asked to give a lecture on chinchilla breeding and genetics to junior and senior year biology and pre-med students at the Benedictine College in Atchison, Kansas. One of the things discussed during the lecture was polygenic traits. Malocclusion in chinchillas, rabbits, mice, and rats is, to the best of our well researched knowledge, a polygenic trait. That means that multiple inherent factors, like head shape, rate of growth, nutrient absorptive ability, and other inherited traits, can predispose a chinchilla to malocclude. Malocclusion does tend to run in family lines, but not in a predictable or consistent manner. This is why we are careful to use the word "predispose" in association with polygenic traits, as there is not one simple "cause." Some chinchilla families will produce only a single maloccluder, and others will produce many. In families that consistently produce high percentages of maloccluders, breeders should cull the suspected producers.
Chinchilla owners must understand that owning chinchillas (and this applies more as you acquire more chinchillas) means that you will eventually encounter malocclusion ( as well as fur chewing, skin fungus / ringworm, and a number of injuries and infectious diseases ). Since chinchillas have teeth that constantly grow, there is always a risk that at some point the teeth will not align properly, and either the roots will overgrow into the eye sockets or lower jaw bone, or the teeth themselves will become so misaligned that they can no longer grind food.
If your chinchilla is maloccluding, and he is growing spurs in his eye sockets and is unable to eat, it is probably best to euthanize the animal. It is usually not practical to try to continually grind his teeth down and feed him soft foods, and expect him to live a long happy life. The teeth will grow back quickly, the spurs remain, and you will most likely be left with a large vet bill and a chinchilla that will die anyway.
Breeders need to be responsible, and cull lines that are consistently producing maloccluders, but even the most responsible breeders cannot rid their herds of malocclusion completely.
Chinchillas.com makes all of the chinchillas they sell available for pre-purchase vet exams if the buyer elects to have them done. The buyer may have radiographs, ultrasounds, or any lab tests they deem necessary prior to purchase. However, even if a chinchilla is completely clinically normal one year, that does not guarantee that it will be immune to malocclusion for the rest of its life. The same hold true for breeding stock purchased from any breeder. No breeder, no matter how responsible, can honestly guarantee that a chinchilla will never malocclude.
It is not necessarily a bad reflection on a responsible breeder if an animal which they produced maloccludes. Granted, there are some terribly irresponsible breeders who do not cull properly, or who do not outcross sufficiently, or who have poor husbandry practices, and they produce a lot more maloccluders than average. However, one has to look at a breeder's practices as a whole when forming on opinion on a breeding herd. Do not base a positive or negative conclusion on a single incident.