An abscess is defined as: “A collection of pus in tissue, organs, or a confined space, which is usually caused by a bacterial infection.”
A sterile abscess can be defined as a localized swelling filled with fluid where no organism growth is obtained on culture.
- An abnormal swelling of tissue filled with pus.
- A swelling or lump that develops quickly from one to a few days. It may be located anywhere on the body and may cause tenderness.
- The center of the lump may appear white as the skin starts to thin signifying rupture, or may show a scab at the site of the lump where injury was initiated.
- If a deep abscess is present, or if there is systemic involvement, signs may include: anorexia, weight loss, possible limited mobility, and possible abnormal organ function.
*Note: for additional information on recognizing various signs of pain or discomfort refer to: Signs of Pain In Rats.
An abscess is caused by disruption of a normal tissue barrier through penetrating trauma, bites, established infections, or by migration of normal flora to other sterile areas of the body. It is an infection and inflammatory process; where-by leukocytes (white blood cells which protect the body from invading organisms) form a wall around infectious agents in tissue, organs, or confined spaces, to localize inflammatory exudate (pus) in an attempt to prevent further spread of infection. Exudate (pus) is made up of dead leukocytes, dead organisms, and fluid from blood.
Exudate from an abscess may:
- autolyze and be reabsorbed resolving the inflammation and abscess.
- further develop under the skin surface and appear visually as a pus filled lump. This pus filled lump may then “come to a head” by itself, rupture and drain, or through local application of heat, break through the skin and allow the pus to drain thereby resolving the abscess.
- rupture and drain through a sinus or tract to another organ or tissue resulting in further inflammation to those areas.
When an abscess continues to deepen, it will often rupture, allowing infectious agents to enter the blood stream. This is called bacteremia (bacteria in the blood). In the event the pathogenic agents (germs) multiply, which is common, toxins are released into the blood stream causing septicemia (commonly known as “blood poisoning”).
The causative organisms of abscess formation are varied and may reflect the area where the abscess is located. Some examples include:
- Cutaneous or skin and soft tissue abscesses commonly associated with Staphylococcus aureus. Staphylococcal strains may show rapid necrosis, and early suppuration, with a large amount of pus. It liberates lethal cellular toxins, but is more able to be quickly walled off by surrounding tissue before it can spread.
- Oral and perineal abscesses may be produced by anaerobic bacteria and contain a brown foul smelling pus.
- Cervical abscesses may be produced by Klebsiella pneumoniae, Pasturella pneumotropica, and Streptococcus spp.
- Organism most often seen for facial, orbital and tail abscesses is Staphylococcus aureus.
- Abscesses of the abdomen may be a mixture of both anaerobic or aerobic Gram-negative bacilli such as Klebsiella and E. coli, and anarobes such as Bacteriodes fragiles.
- Organisms affecting the preputial gland (seen as either a unilateral or bilateral swelling around the penis) are Pasteurella pneumotropica and Staphylococcus aureus. *Note: a unilateral swelling may be indicative of a neoplasm.
- Those abscesses produced by streptococcus show little to no necrosis, and contains serous exudate (clear fluid with cellular debris). Because the organism does not cause intense tissue destruction walling off takes much longer, thus allowing greater tendency to spread throughout the body.
Factors which may contribute to the formation of abscesses are an impaired immune system, excessive fluid accumulation in tissue, hematomas, or where normal drainage of specific organs may be obstructed.
Case Histories and Photos of Abscesses
- (Skin, limbs, tail)
- Fig. 1a: Abscess of cat bite in hairless rat
- Fig. 1b: Abdominal skin abscess on hairless rat (Odie)
- Fig. 1c: Abdominal skin abscess in male rat
- (Including skin, eyes, ears, mouth, sinuses, and brain)
- Fig. 2a: Facial abscess, scab removal in female rat (Dameon)
- Fig. 2b: Facial abscess in male rat (Bramble)
- Fig. 2c: Retrobulbar abscess in female rat (Alexandria)
- Fig. 2d: Retrobulbar abscess with enucleation in 1-year-old rat (Leon)
- Fig. 2e: Oral Abscess
- Fig. 2f: Tooth Abscess
- Fig. 2g: Left eye enucleation with postop abscess in female rat (Yasmine)
- Fig. 2h: Mandible abscess stemming from probable tooth root infection in female rat (Opossum)
- Figures pending
- Fig. 4a: Multifocal abscesses in young rat (Benny Simpson). Case history and photos by Dr. Mimi Ehrlich.
- Fig. 4b/Fig. 3: Urolithiasis in 8-month-old neutered male rat. Multifocal abdominal abscesses identified in post mortem histopathology (Gambit). *Warning photos graphic!* (Additional example of internal abdominal abscessing shown from article on Urolithiasis)
- Fig. 5a: Small groin abscess
- Fig. 5b: Scrotal abscess in 9-month-old male rat (Osmolarity)
- Fig. 5c: Inguinal canal abscess in 22-month-old rat (Bumpus)
- Fig. 5d: Preputial abscess in adult male rat (Omni)
- Fig. 5e: Post surgical abscess related to neutering in 2-month-old male rat(Gambit)
- Fig. 5f: Post neuter abscess. A possible reaction to absorbable suture material used in male rat (Beni)
Many abscesses can be diagnosed by fine needle aspiration of pus, or by spontaneous rupture of the abscess and the discharging of pus.
Gram stains of purulent exudate.
Lab work for culture and sensitivity may be obtained to determine the antibiotic of choice.
If the abscess is relatively small and seems to have a point of entry, moist heat at 105 to 110 degrees F. or 40.6 to 43.3 degrees C. may be applied. This causes peripheral vasodilation, which speeds leukocytes to the site, accelerating pus formation. Doing this may open the tract and allow the pus to drain. Refrain from leaving a warm compress on for extended periods as this will cause vasoconstriction to occur.
The aspiration of pus from an abscess that appears ready to rupture can also be done. Once the pus is drained or aspirated, the small abscess can be cleaned twice daily or as directed by veterinarian. The solution of choice for abscess care would be normal saline .9%. (If using sterile contact lens solution make sure it is preservative/additive free.), an over-the-counter wound care solution such as Blairex, or in the absence of any of those, warm water and a mild antibacterial soap may be used.
Other solutions that may be used to flush wound are: 1% chlorhexidine solution diluted with water to the color lighter than a robin’s egg blue, or Betadine solution diluted 1:5 (1 part Betadine to 5 parts water) once a day or twice daily as directed by veterinarian.
In abscesses that are large or deep, where an incision is required to be done over the suppurative area, irrigate the wound with normal saline twice a day or as directed by your veterinarian, and keep the incision open to allow drainage and promote healing. Irrigating a surgical wound with solutions other than normal saline should be done only at the direction of your veterinarian. Using an antibiotic ointment in a large or deep abscess may cause the abscess to heal from the outside trapping bacteria in. Abscesses heal more efficiently from the “inside out”.
Where a rat continues to pull out sutures exposing wound or where wound can not or should not be closed (requiring healing by second intention) as a result of infection, purulent (pus-filled), contaminated state the use of gentle packing of the wound, twice a day, with granulated sugar or medicinal honey into the wound, following gentle irrigation of wound with warm (room temperature) normal saline and patting wound dry, aids in inhibiting and preventing adherence of bacterial growth and promotes granulation of the tissue for healing. In those wounds where tissue is dry the use of hydrogels (e.g., Intrasite) may be judiciously applied to rehydrate tissue.
Home made sterile saline solution
16 ounces water (use distilled if your tap water is hard)
1 teaspoon table salt
You can use warm water or you can boil the solution then cool to room temperature before use.
*Note: boiling is usually not necessary when flushing a non sterile wound such as an abscess.
*Note: Broad spectrum antibiotics may be prescribed for large, deep, or persistent abscesses. Keep in mind that the abscess itself does not have a blood supply, and that the antibiotics are used to prevent recurrence and possible spread of infection.
- Amoxicillin, or Clavamox (generic name: amoxicillin trihydrate/clavulanic acid). If it is known that the rat has an allergy to amoxicillin and alternative antibiotic to use is Erythromycin.
In the event where an abscess is chronic, and culture has shown positive for a species of methicillin resistant staphylococcus, the antibiotic clindamycin (ClindaDrops) may be selected by the veterinarian for use. *Note: clindamycin belongs to the class of drugs called Lincosamides. While Lincosamides are contraindicated in rabbits, hamsters, guinea pigs, horses, chinchillas and ruminating animals due to the potential for adverse gastrointestinal effects, they are tolerated well in rats. Because this class of antibiotic has the potential to affect normal gut flora it is recommended to give a probiotic daily, while administering clindamycin, and to continue for 3 weeks following completion of the antibiotic.
In the event surgery intervention is required the following post-op analgesia may be given:
- *Note: for pain not controlled by the use of an NSAID (e.g., Banamine, meloxicam, or carprofen), alone, consider alternating or co-administering with a narcotic (e.g., buprenorphine or butorphanol) or narcotic-like (e.g., tramadol) medication.
Refer to the Rat Medication Guide for information on medications.
The goal in keeping an abscess open to drain is to prevent a reaccumulating of pus, and allow the wound to heal from inside to outside.
- Avoid using chlorhexidine at a concentration of more than 1% or even more than once a day, since it will delay wound healing in granulating wound beds of abscess pockets. Diluted solution should appear lighter than a robin’s egg blue.
- Continue treatment or follow-up care until wound heals.
- If antibiotics are given remember to include Bene-Bac, or yogurt with live active cultures, to prevent normal gut flora from being destroyed by the antibiotics.
For those abscesses that require surgical intervention:
- Provide hospital cage in the event of surgery and during recuperation if there are concerns that their cage mates may groom wound site.
- Provide clean bedding daily such as felt, soft t-shirt type material or ink-free paper towels. Avoid using material such as terry cloth type towels that can ravel. Also avoid litter-type bedding, post-op, until healed to prevent the chance of wound contamination or infection.
- Provide additional warmth to maintain body temperature within normal limits. It is essential that the rat does not become overheated or dehydrated. The rat should also be able to move away from the heat source if it becomes uncomfortable. If the rat is unconscious or immobile extreme care must be taken to keep the heat low and stable.
- You can use an isothermic product that is heated in the microwave such as SnuggleSafe®. Make sure to follow the product directions carefully and wrap in a towel before placing in the cage. SnuggleSafe® will provide heat for 12 hours before needing to be reheated. Other similar types of product may vary in re-heat time. Check directions for individual product.
- If using a heating pad (good for long term use) use only the low heat setting, put a thick towel in between the pad and the cage bottom, and place beneath a corner of the cage.
- If none of these options are available you can use a plastic bottle filled with hot water, and wrapped in a towel, in the corner of the cage.
- Provide pain medication if needed post-op, based on pain severity. Check with veterinarian for most appropriate pain medication to give.
- Provide high calorie foods or food supplements such as Nutri-Cal Paste, canned Ensure, soy or soy formula, during recuperation.
- Encourage fluid intake while recuperating, such as water, Jell-O water , or electrolyte replacement drinks such as Pedialyte or Gatorade (which can be found in local grocery stores).
- Please note that Pedialyte is only good refrigerated for 24 hours after opened, but can be frozen as ice cubes and thawed as needed.
- *Note: juicy types of fruit also provide an additional fluid source in the diet.
- Observe and contact veterinarian if there are: further signs of infection, if the wound does not appear to be healing, or if the wound heals but elevation or lump returns.
Although abscesses can be persistent in returning, requiring additional treatment, many do resolve.
Factors that may affect the outcome
- Nutritional status
- Status of the immune system, and its ability to initiate and resolve inflammation
- Abscess resolved
- No advanced systemic infection
- Practice good sanitation of cage environment.
- Cover wire floors, or rough surfaces with a sturdy covering to prevent abrasions. Examples of coverings can include vinyl floor covering, plastic needlepoint canvas, carpeting, towels, self stick tiles, plastic place-mats, fiberboard, and Plexiglas.
- Treat all wounds promptly and seek attention at the first sign of infection.
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