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Abscesses and Cysts

by Debbie “The Rat Lady” Ducommun

Updated 6/22/17

Tumors are common in rats, but so are abscesses and cysts. If you suspect a lump on your rat’s body is a tumor, it’s worth it to wait 2-4 weeks to see if it is an abscess or cyst instead. Most abscesses are fairly harmless and either clear up on their own or with minor treatment. Generally cysts are also harmless. Abscesses can “pop up” overnight and they tend to grow more quickly than cysts or tumors. With most tumors on the body the skin moves slightly independently of the lump, but abscesses or cysts are usually inside the skin so moving the skin moves the lump.

Abscesses are usually small infections, often caused by bite or scratch wounds, which the skin encapsulates to isolate. The wound usually heals before the abscess appears. However, sterile abscesses can be caused by inflammation due to an injury or a foreign body, such as sutures. After a rat has been neutered, a sterile abscess in the scrotum is common and can be treated as a simple abscess.

It is fairly common for male rats to develop abscesses in the groin area. These often are noticed only after the abscess opens and drains and therefore appear as a hole in the skin. In most cases, groin abscesses heal quickly once they open and drain.

An abscess on the face, in a tooth socket, or near lymph glands, can be more serious, often indicating a cancer. An appropriate oral antibiotic should be given for a serious abscess; start with amoxicillin.

An abscess that won’t heal after a week or two of treatment may be a cyst, a sterile abscess caused by a foreign body, or it may indicate a cancer, such as squamous cell carcinoma, especially on the face. You can try to treat a cancer with prednisone at 1 mg/lb twice a day, which in some cases can shrink or slow the growth of a tumor, and try to control the infection with amoxicillin, but there is no cure.

A Simple Abscess

In rats, an abscess on the body (including the scrotum) can usually be allowed to progress on its own. The body has a natural healing process to open the abscess so it can drain. This process may take several weeks, but as long as the rat seems well and isn’t bothered by the abscess you don’t have to do anything. Abscesses on the underside of the body generally progress more quickly because gravity helps the opening process. Abscesses on the back can take much longer to open.

The first sign that the abscess is starting to open is often a dark area on the top of the lump. Most commonly, the skin on the top of the lump will die and turn dark, hard and “crunchy.” Eventually the disc of dead skin will fall off leaving a hole, often quite a large hole, leading to a pocket. This is all okay.

In most cases, the pus will drain out and the rat will lick the pocket clean, so you don’t have to do anything. The hole will usually heal up in a few days. Sometimes, the dead skin will not come off completely and will need to be cut away.

This small abscess on my hairless rat’s ear took quite while to open. I took the first photo on 9/25/10, and you can see the yellow pus through the thin layer of skin over the top of the lump. The next photo was 12/23/10, and I finally got tired of waiting and squeezed it out. You can see the blob of pus that came out. The black dot on Munchkin’s shoulder is a tiny sebaceous cyst.

Here is a sterile abscess that developed after a neuter due to a reaction to the sutures. See the dark spot where the hole will form. (Note to veterinarians: because such abscesses are common, I recommend the incision for a neuter be done at the distal end of the scrotum and not so close to the penis.)

Here is a series of photos I took of my rat Comet who developed a sterile abscess in his scrotum nearly 5 months after he was neutered! The first two photos were taken on 5-12-11, one before and one after I shaved his scrotum. You can see the dark area of dead skin that will come off.

I took the next picture on 5-16-11, and you can see at the bottom edge of the dark area, some greenish pus starting to seep out. I took the last picture on 5-21-11, after the dead skin had fallen off, and you can see the wound is already healing well.

Abscesses that develop under or next to the ear are likely to be caused by a cancer.

Helping an Abscess

If you want to speed up the process of a developing abscess, here are some things you can do. First, carefully trim the hair away from the lump; then apply a hot compress twice a day to help bring the infection to a head so it will open and drain. To make a hot compress, wet a cloth with water as hot as you can stand, or fill a latex glove with hot water. To reheat the compress, microwave it for a few seconds. Always test the temperature on your forearm first; then hold it against the lump for as long as your rat will let you. You can also apply an acne treatment containing benzoyl peroxide, or Swedish bitters (an herbal mixture), to the top of the abscess daily to help it open.

Treating an Abscess

In some cases when the abscess is open, the pus will be too thick for the rat to clean out so you need to clean the abscess by gently wiping out all the pus with a cotton swab. This usually does not cause the rat pain. If it does, try applying some Bactine.

If you can’t get all the debris out of the abscess you can flush the wound with Betadine diluted with 5 parts water or hydrogen peroxide diluted with 10 parts water, using a syringe without a needle. Then you can spray on some Bactine. If the abscess doesn’t heal up quickly, you can clean it out twice a day.

Repeat this treatment twice daily if an abscess is open, but doesn’t heal right away. An abscess must heal from the inside out. If the skin heals over the opening too quickly, the infection will recur.

On rare occasions, an abscess will come back. Should this happen, treat the rat with oral amoxicillin. If this doesn’t clear the abscess up you should suspect the underlying cause is cancer. Otherwise you may need to have your veterinarian do a culture and sensitivity test to determine the best antibiotic to use for the infection.

Tooth Abscesses

Tooth abscesses are rare in rats, and I’ve only seen them affect incisors, not molars. The first sign of a tooth abscess is often black staining on the tooth, caused by the infection in the root. There is usually no swelling visible on the face because the infection is inside the jawbone. A foolproof sign of a tooth abscess is that the entire tooth will come out easily when pulled. While the tooth must be removed, it can still be difficult to cure the infection remaining in the tooth socket and jaw. A powerful antibiotic good for anaerobic bacteria, such as metronidizole or clindamycin, is necessary. You might need to have your veterinarian do a culture and sensitivity test to find out what antibiotics are best.

Cysts

A cyst is a sac filled with a liquid or semi-solid substance. The most common type of cyst found in rats is the sebaceous cyst. These cysts develop from sebaceous oil glands in the skin. They are most often found in male rats whose sebaceous oil glands are more active, and usually located on the back or side. These cysts look much like a small abscess, although they fill with waxy or oily sebaceous secretions, not pus.

Sebaceous cysts tend to grow more slowly than abscesses or tumors and usually don’t get as large. Most sebaceous cysts eventually develop a small hole in the top that can appear dark because of dirt on the secretions inside the cyst.

Sebaceous cysts typically don’t cause the rat any pain or problem, but if a cyst continues to grow, it should be emptied to keep it from getting too big. (I once saw a sebaceous cyst on the back of a female rat that the owner thought was a tumor because it was about 1½ inches long!) A cyst will recur until its internal sac is destroyed. Keep squeezing the cyst and it will eventually go away.

Here are 3 photos of tiny new sebaceous cysts. Of course, they are much easier to see on hairless rats.

Here’s a little sebaceous cyst that I squeezed out, showing all the stuff that came out, and what it looked like afterwards.

Here is an older sebaceous cyst that is open, showing the accumulation of dirt on the top that makes it look like a scab.

Photo of Marty by Sarah Hunt.

Here is a much older sebaceous cyst that has been open for a while, allowing the sebaceous secretions inside to dry out and turn yellow. This cyst has even developed a second opening on its side. This cyst was only about ¼ inch across.

Two other less common kinds of cysts that can occur in the skin of rats are hair cysts and dermoid cysts. These are strange cysts that can contain pieces of hair, bone, teeth, etc. Treat them the same way you treat sebaceous cysts. They are not anything to worry about.

Warts

Rats can grow warts, moles, and funny skin tags which tend to look similar to those humans can get. If something looks like a wart, it probably is. Strangely, in 2 cases on my hairless rats, I have seen a sebaceous cyst turn into a wart after repeated squeezing.

This large wart is about ¾ inch across and was under the rat’s arm. The dark areas are probably where the rat has scratched it and caused scabs.

Here’s a wart that grew on one rat’s tail. It was about 5/8 inch long.

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