Who doesn’t remember Tricky Woo doing the “flop bott” in James Herriot’s stories about the veterinary surgery in Yorkshire, England? The vet would simply express the anal sacs of the overweight Pekinese, remind Tricky’s owner to cut down on the dog’s treats, then head off to his next call.

You may have never seen any of your own dogs do the flop bott, but you’ve likely witnessed other dogs as they drag their rear ends across carpet or grass. The behavior helps alleviate the discomfort that accompanies anal sac impaction.

Fortunately, veterinary medicine has progressed since Herriot practiced, and the possible causes of the condition are well-defined, according to Rebecca Ruch-Gallie, D.V.M., clinical coordinator of community practice at the Colorado State University Veterinary Teaching Hospital in Fort Collins.

This illustration shows the anatomy of a dog’s rear end. The anal sacs are located between 4 and 5 o’clock and between 7 and 8 o’clock on either side of the anus. Illustration reprinted with permission of the copyright owner, Hill’s Pet Nutrition Inc.; © Hill’s Pet Nutrition Inc.

The anal sacs contain a fluid, produced by the anal glands, that marks a dog’s territory when it has a bowel movement. That’s the only function of the sacs. It’s when the sacs aren’t fully emptied by the passing of fecal material that impaction occurs.

Ruch-Gallie says the two main reasons the sacs don’t function correctly are too much weight, as in Tricky Woo’s case, and allergies. Yes, allergies.

Before considering a possible allergy, a dog with the condition that arrives at the Colorado State veterinary teaching hospital has its weight assessed. When there is too much fat on and in a dog, the sacs can’t be properly – and naturally – emptied as the animal passes fecal material.

On a “nine-scale” body conditioning score, four to five indicates an ideal body weight. Nine is grossly obese, Ruch-Gallie says. If a dog comes in at six or seven and goes down to four or five, it may never have the problem again. “Or it may still have the problem,” she says.

Ruch-Gallie points out that although anal sac issues are often associated with small dogs, it’s not really about size. “It’s obese small dogs,” she says, that commonly have the problem. When a dog has a small frame and you add just two extra kibbles of food a day, it adds up, she says. “It’s easier to put weight on a smaller dog.” But larger dogs can also have anal sac problems when they build up too much fat around the rear end.

And if you’re thinking you can avoid this situation by steering clear of certain breeds, don’t even try. It has never been proven that any particular breed is more prone to anal sac problems.

After the weight comes off, “Most of the time, if a dog has [repeated] anal sac problems, it’s because of an underlying allergy,” she says. It can be an allergy to a variety of materials, but often it is food. “Generally dog allergies show up as skin problems, chronic ear infections, skin infections and in the anal sacs. They are lined with epithelium like our skin,” she says.

“Here at CSU, we go ahead and put them on a food trial” that includes a novel protein and novel carbohydrate – two ingredients that the dog hasn’t eaten up to that point. “We see if that helps with the issue,” she says.

The anal sac condition goes through several stages: as fluid builds up in the sacs, it may change color and thicken. Normally the liquid that passes through is essentially like light brown water. If it’s yellow, bloody or pus-like, it’s not moving through the sacs as it should. If you start smelling something different when your dog defecates, it may indicate impaction or infection. “There will be a kind of odor that wafts out,” Ruch-Gallie says. “It’s a very foul smell.” The fluid can have that smell if a dog is scared, “but if you smell that more regularly, probably there’s something going on,” she says.

Impaction just means that the fluid can’t pass from the sacs naturally.When you express the sacs, the fluid is forced to exit.

Many dog owners learn to express the sacs themselves, or they have their groomers do it. This does not, however, solve the problem, according to Ruch-Gallie. “The act of expressing them can, in itself, cause inflammation in the sac. If you rub on any body part for a long period of time, your skin will react to that.” If you, your assistant or groomer are expressing a dog’s anal sacs on a monthly basis, that’s too much, she says. “So we really want to increase the interval times that we have to actually express an anal sac. If not a lot is coming out, it doesn’t need to be done regularly.”

If a dog needs monthly anal sac expression, the dog’s veterinarian needs to know about it, she says. “They can check to make sure there’s not an underlying medical problem.” Dogs will sometimes do the flop bott when they have worms or other parasites. “When they’re passing, they feel a little odd,” Ruch-Gallie says. Certain cancers occur in that region as well. A veterinary workup is a must.

The next stage is infection, or sacculitis. Most dog owners won’t know when the condition has turned into an infection. A dog will lick, chew and flop bott, or scoot, at any stage.

Without treatment, however, an abscess may develop. By this point, the dog will have a fever and swelling, red at first then deep purple, usually on one side only, Ruch-Gallie explains. “The swelling of an abscess cannot be reduced by emptying the sac,” she says. “An abscess often ruptures through the adjacent skin, producing a draining tract. If the abscess has not ruptured spontaneously, it should be lanced by your veterinarian when it becomes soft and fluid-like.” At this time, the vet will flush the cavity “repeatedly,” she says, and prescribe oral antibiotics. “Once you can get that all cleaned out and get the infection under control, [the rupture] will heal. Then we’re back to ‘there’s some underlying disease process.’ If you don’t get that under control, it’s going to happen all over again.”

Ideally, however, you can get the issue resolved before it gets to the abscess-rupture stage.

“It’s really not normal for the sacs to be full,” Ruch-Gallie says.“Dogs should be able to express their anal sacs in the normal course of their living.”

That is, unless they have some anatomical abnormality. Most dogs’ anal sacs are at between 4 and 5 o’clock and between 7 and 8 o’clock when looking at the dog’s anus. But sometimes their rear ends aren’t built quite right. The opening of the sac may be rotated down, or it’s farther into the rectum, and the sac is longer than normal. In these cases, there simply is not enough pressure on the sacs to force the fluid out when the dog has a bowel movement. But that’s not typical, Ruch-Gallie says.

“Most of our dogs do really, really well as long as we can find out the underlying reason it’s occurring. Eighty to 85 percent, we can get under control.”

For that other 15 to 20 percent, the anal sacs can be removed. However, such surgery can result in fecal incontinence because the sacs are so close to the sphincter muscle. Ruch-Gallie says it’s important to have the surgery done by someone who is experienced at it.

Chances are that if one of your dogs is having an anal sac issue, your veterinarian can get to the bottom of it, and the flop bott will be a thing of the past in your home or kennel.