Basically, megacolon is a condition that occurs in many species, including humans, though in humans it is called Hirschsprung's Disease. Affected animals are missing nerve cells in the wall of their colon and/or their rectum that are necessary in order to move stool along and out. In rats, this means that the stool will back up, and they will bloat. This is most likely painful and eventually results in the death of the animal. Rats don't usually have problems with it until they are weaned, though you might see symptoms of the condition in an affected animal before weaning such as poor muscle tone (especially in the hind legs) and a general failure to thrive. Though this condition can occur in any rat, it is very often seen in rats with what I call "high-white" markings. We'll go into which markings these are throughout the article, but the gene or genes that cause these markings are directly related to the condition.

First, I want to say that I have not ever seen megacolon in a rat that was not a high-white. Next, I want to say that not all high-whites will have megacolon. Before we go any further, yes, I have had one rat that I bred that had a case of megacolon that was so mild that he was able to lead a semi-normal life. One. He had to have a special diet including papaya (and other fruits with high moisture content) and stool softeners every day. He had to be helped to potty twice a day every day for his entire life. Mind you, his case was very, very mild. A case like his is very rare. I know of two like him in six years of breeding. The other was a rat in Florida. During the time I have been breeding, I know of literally hundreds of rats who have died of this condition, some in my rattery. Those are the ones I know of. There were probably thousands more with hobby, feeder and commercial breeders. Don't think it is something you can treat easily. It isn't. Pups who get it will die. Rats who have it should not be bred, will most likely not even live long enough to be bred. Go ahead and argue that it is self-limiting. It is. Yes, it is. I do agree with you. I also agree, that you can reduce the risks, but you cannot breed it out of high-white lines. It is a part of the gene or genes that make the markings. I do not believe someone who is new to breeding should be working with high-white, and it should not be taken on by the faint of heart. I won't tell you not to breed it at all because it is not my place. Others, however, will. The purpose of this article is to inform, not to preach.

Those three darling babies in the picture in the ramekin at the top of the page were from a litter of five that I bred. They were born in November of 2000 to a blazed father and a self mother. At that point, I was still working out how to work with blazes as safely as possible. I was still under the misconception that the tendency toward megacolon could somehow be bred out by selective breeding. I had learned that his marking was the dominant blazing, the high-risk type, and that the safest way to breed him would be to a self, so that is what I did. The black boy on the left died first. Another black blazed in the litter died next. The blue girl in the middle (and in the picture, left), SRR Borrowed Angel, died at 46 days old. It was awful. I loved her, not for her markings or her color, but for her personality. She wasn't breeding quality, even without the megacolon, but she was a special pet for the time that I had her. The blue wedge blaze boy on the right (and in the picture, left) lived a normal life as a pet. He was unaffected. I kept none as I had already decided to discontinue the sire's line at my rattery, but the other doe from the litter, a blue self, was placed in a breeding home (with full disclosure, of course), but she died giving birth at barely four months of age. The point of this is the markings on all three of those rats is a giveaway that they are at-risk. If you saw them as adults, you would know. You would (or should after reading this article) know to think long and hard before breeding them.

There are many different genes that make markings (white) on rats. There are the genes on the "hooding" locus which is also called the white spotting locus or the H locus. That is where you will find HH (self), Hh (berkshire), hh (hooded), and all the modifiers to those genes that make Irish, bareback (pictured right), capped, masked, etc. These include dalmatian and variegated and the modifiers to those markings. The gene or genes, "high-white" as I am referring to them in this article, that bring with them the increased risk for megacolon are not believed to be here. Do not confuse "high-white" with dalmatian, variegated, roan, Essex, downunder or any other of the marking/pattern genes. For my purposes here, I am referring to the gene or genes that bring with them the increased risk of megacolon. As mentioned before, there is always a risk, but the genes on the "hooding" locus do not appear to bring with them any increased risks.

A very common marking seen in high-white rats is the blaze. Not all blazed rats are high-whites, but most seen in the fancy in the US seem to be as it is a dominant marking (some blazing is recessive, but this is much less common). The rats below have dominant blazes (high-white), though they are not always "wedge" blazes; some are "lightning" blazes or other asymetrical blazes:

Dominant blazes are not the only markings that should raise red flags. High-white means white markings in places you do not normally see them. For instance, when you see or think of a berkshire rat, it has color on its back and sides (face, legs most of the way down, etc.), and if you flip it onto its back, it has white on its tummy. This marking is not coming from a high-white gene; it is coming from the marking on the "hooding" locus (Hh). When a rat has white coming farther up the sides than it should, that should be a sign to you that something is not right. The rats pictured below are what is often called "banded," and this marking usually comes from the high-white gene(s) changing a berkshire (may or may not have a blaze):

High-white can put markings in other places that should raise suspicion. The markings on these rats are all the result of dominant high-white:

There is one other dead-giveaway to a high-risk of megacolon; odd-eye. If a rat has odd-eyes, you can almost bet it came from high-white (especially in the US) as it is one of the effects sometimes seen in high-white (not a separate gene). These rats are examples:

Pictures are not always reliable, and what you see when you look at a rat is not all there is to know. They are a start, though. For instance, the spot over the eye on the picture of the rat above, the next row above the odd-eyes, SRR Confection, is a sign that she is a high-white, and she is. A spot near the eye and some near the ears of another rat such as the one pictured at the left is probably a sign that he/she is not a high-white, but it is not definite (if I did not know, from just the picture, I would guess dalmatian but not be sure). Then, there is the one at the right with just a spot near her ear. That's not suspicious. Why, you ask? Well, the placement of the spots and the type/quality of the spots. High-whites tend to have fairly clear and defined markings, and the spots tend to be solid-colored. In other words, they do not have any silver-colored hairs in the spots. The rat at the right has silver-colored hairs in the spot which is typical of spots caused by dalmatian/variegated genes. You would see that this is more visible if you saw the rat in person, but the picture is terrible quality. Spots caused by a dalmatian/variegated gene tend to be at the base of the ear first. If there are more spots, they may be on the back and/or face and they may spread to the sides such as in the rat at the left. By the way, the rat at the left was not a high-white. She was from dalmatian lines. The photos below illustrate "silvering" in dalmatian/variegated spots. This "silvering" is not actual silvering caused by the silvering gene; it's a dilution caused by the genes that create the marking. As I mentioned, it is typical of the spotting on dalmatian and variegated rats.

We have covered some pictures, and now we can look into the pedigrees that back up the rats in the pictures. Most of these specific rats can be looked up in the registry (NARR), but we can look at the markings you will find that should raise red flags for you. Here is a pedigree for a rat that is fairly "safe." I highlighted in red some potential issues that stand out, though.

Wow! Look at all those blazes. Yes, the pedigree is pretty safe, nonetheless. NWSSR Sniper's parents were a blue agouti and a chinchilla blazed (oversimplified, a chinchilla is a high-white agouti selected for no brown or gold tones so that it has the bands of colors on each hair but in shades of black/gray and white, basically). They went back to mostly satin selfs, but there are a couple of Irishes and a variegated mentioned in Sniper's pedigree. The only real issue in there that I see is the odd-eyes and chinchillas, but they are a couple of generations back, and the rats in the pedigree my rats come from are dalmatians. They are not odd-eyes, and they are not high-whites. The blazes in the pedigree are coming from dalmatians and variegated rats. They are not dominant blazes; they are recessive ones like the one on the rat pictured at the right. RRR Hara was from the same litter as RRR Bianca, so you can see her pedigree there. Recessive blazes are not as easy to make as the dominant ones, but I never concentrated on them myself in breeding my dalmatian line. I expect with selective breeding, one could do quite well with them in a line from dalmatians, and as long as high-white was kept out of the line, they would be free from the issues that come along with high-white, specifically megacolon. There are many other patterns that will come from dalmatian and variegated on the way to the standardized types depending on the mate that is chosen. The rats in the pictures below are three of RRR Hara's pups (the sire of the litter was RRR Miles' brother, RRR Pongo; no high-white in the litter and no recessive blazes):

Not all blazes come from high-whites or recessive blazing. There is another type of marking that always has a blaze, the desired type of which is a "wedge blaze," and it is megacolon-free, but it is not commonly available in the United States (first imported November of 2004). It isn't really a marking, but more of a pattern. It is called roan, and it originated in the United Kingdom. Roan is recessive and comes in two patterns: roan, which is basically a berkshire-type pattern with a blaze, and striped roan, which US breeders would compare to "banded" with a blaze (see photo at left). As they age, their color fades until, in old age, they look white. There are a few US breeders working on a line of rats that originated in the US that seem to be just like the roans in the UK, but there has not been an opportunity, yet, to find out if they are compatible with the UK rats. The US line is also not high-white.

Here is a pedigree for a rat that is not "safe." Since the rat has already been bred, I can confirm what we knew would happen. MAR Sampson and SRR Samson both produced megacolon in litters though both produced surprisingly low numbers of cases. Regardless, we knew they would, and they did.

(This is an old pedigree, and it is not as current as the one in the registry. We are using it as an example here. We will not go into the things in the pedigree that could not be correct knowing what we know about these rats just from looking at the pedigree. The breeders determined what these rats were before they knew what they would produce; they did not have the information we have now.)

SRR Samson is the odd-eye in the photo on the left up a ways on this page. The terms "odd-eye," "split-cap," "split cap/stripe," "collared," etc. are all very clear indications that high-white is at work. A "split cap" is a blaze that went farther up the head than it should (according to standards) and divided the cap in half. In a "split-cap/stripe," the rat basically started with a hooded-type marking before high-white was added. The blaze (from high-white) split the hood and took it completely off the shoulders, but the high-white also put more white at the top of the spine line. A nickname for a variation of this is called "collared" where high-white has put white in between the spine line and the hood so there is basically a white collar. The rat may or may not have a blaze, but the general markings are the same. The effects are similar, and that is what I am trying to point out in this article. Terminology that should raise flags include but are not limited to: "blazed," "bi-color," "chinchilla," "patched," "spotted," "banded," "blaze-banded," "black-eyed white" (unless it comes from dalmatian/variegated lines with no high-white for a few generations), "masked" (depending on the lines the rat comes from), "husky," "odd-eye," "split-cap," "split cap/stripe," "cap/stripe," "collared," "Aurora," "Dutch" and "badger."

Terms for markings that are generally "safe" include: "self," "berkshire," "Irish," "English-Irish," "hooded," "bareback," "capped," "masked" (there is a modifier for masked on the "H" locus that is not high-white, and they do pop up in dalmatian lines), "dalmatian," "variegated," "variberk," "possum" or "possum-faced" (if the rat came from dalmatian or variegated lines), "mottled," "roan" (from UK lines and from some US lines), "Essex" (from UK lines), "Roberts" (exists only in the UK) and "downunder" (when they have not been mixed with high-white).

There is a marking pattern referred to as downunder that was imported to the United States from Australia in December of 2002. It is also available in Europe as it was sent there around the same time. It does not appear to be a white marking gene but rather a dominant gene that puts more color on the rat (not less) For instance, if the rat is hooded, addition of the downunder gene will also put marking similar to the dorsal stripe on a hooded rat on the rat's belly. They have been bred with selfs, berkshires, hooded, dalmatian and variegated rats with no health issues. Downunders do not have a tendency toward megacolon, and when they were brought to the US, it was with the understanding that US breeders were not to breed them to high-whites. A few have bred them to high-whites anyway, and they got the expected cases of megacolon. Kept out of high-white lines, they have remained megacolon-free. There have been no health issues related to the gene, and they can be bred together safely. The rat pictured at the left is a downunder from a downunder variegated father and a self mother.

There is another marking pattern called Essex (formerly known as Roberts) which has a headspot, and the color gradually fades. Markings vary from what I have been able to find, but they range from berkshire to variegated usually with the headspot. Though the gene is believed to be lethal when homozygous, it is not a type of high-white. It is available in the UK and possibly in other parts of the world. In the United States, it is only in southern California at the time of this writing (in 2005 before the 2007 edit but author has no new information in regard) as it was imported April of 2004.

It has been known for a long time that breeding high-white to high-white was a recipe for trouble, but there has also been the hope that the issues with megacolon could be "bred out" of the rats while keeping the markings by breeding to "safe" markings such as self or berkshire. At this time, I do not believe that to be true. I believe the risks can be reduced and that there are lines that have fewer incidences than others, but I do not believe that it can be bred out altogether while retaining the desired markings. The affect of the gene(s) on color, the depigmentation, is a warning that there was a problem in the development of the rat before it was born. It is a sign that the nerves in the colon might not have developed properly (or at all). It is not merely a marking gene. It is a warning that there may be a problem. Judging by what I have seen in my rattery, it is a warning to be heeded. I do not have the figures to give out of how many of the high-white marked rats got megacolon versus how many did not in my breedings, but an estimate would be close to 40% with megacolon. I have been lucky, actually, as I am not one of the breeders who has lost entire litters to it, but that doesn't make it any less heartbreaking.

This article is meant only to help breeders recognize high-risk markings and to be able to tell them from low-risk ones, in order to make more educated breeding decisions. I would encourage all to continue researching the genetics of markings and disease whether or not high-whites might be included in their breeding programs.

Can you pick out the pups from the litter below that have "suspicious" markings just from looking at the picture?

"High-White and High-Risk" article and photos © 2000-2007, E. Brooks of Spoiled Ratten Rattery, and photo © 2004, E. Sandford of The Rats of Alpha Centauri. All rights reserved.

A huge thank you goes out to Sarah Easter of Lone Star Rats of Central Texas for her help as proofreader and editor on this article!