Differential diagnoses for draining tract lesions in dogs and cats (Proceedings)


Draining tracts or fistulous tracts are tract lesions that connect with a central area or focus of inflammation to the skin's surface. Draining tracts are most commonly associated with nodules which are large (greater than 0.5 cm), firm, raised usually circular lesions that extend down into the dermis.

Draining tracts or fistulous tracts are tract lesions that connect with a central area or focus of inflammation to the skin's surface. Draining tracts are most commonly associated with nodules which are large (greater than 0.5 cm), firm, raised usually circular lesions that extend down into the dermis. These nodular lesions occur because of neoplasia or inflammation. This makes the differential diagnoses list extensive. As a result, draining tract lesions can be frustrating and a diagnostic challenge for many veterinarians. Therefore, it is important to be familiar with the differential diagnoses, diagnostic tests available and necessary to treat draining tracts and treatment options for these cases. The prognosis will vary depending on the reason that the dog is developing these lesions. An accurate diagnosis is therefore imperative for the pet's health.

Clinical signs

No specific age, sex or breed predispositions exist for dog developing draining tract lesions. Draining tract lesions can be solitary lesion or can be associated with multiple lesions. These lesions can be anywhere on the body but the most common lesion locations are on the feet and trunk. As stated previously, the draining tract lesions are most commonly associated with nodules. The draining tract lesion occurs when the nodule ruptures. The draining tract has scant to copious amounts of exudates coming out of this skin lesion. This exudates can be serous, serosanguinous, or purulent. Granules may also be present. Dogs and cats may lick the draining tract area the area from discomfort. This excessive licking results in partial to complete alopecia being present around the draining tract lesion. If the draining tract lesion is located on the foot or feet then lameness may occur. The presence of depression or inappetence may be present when systemic disease is present.

Differential diagnoses

As previously stated, draining tract lesions can be due to a wide variety of underlying etiologies. The four most common causes are infectious, parasites, neoplasia and noninfectious/non-neoplastic causes. Bacteria and fungus are cause infection in the skin and draining tract lesions. These lesions can be due to any bacteria but some of the more common bacteria that cause draining tract lesions are Actinomycosis, Nocardiosis, Mycobacteriosis, Botrymomycosis, Actinobacillosis and L-form infections. Examples of some of the more common fungal infections that can cause draining tract lesions include: Blastomycosis, Histoplasmosis, Crytpococcosis, Coccidiomycosis, Aspergillosis (usually nasal area), Pythiosis, Sporotrichosis, Phaehyphomyocosis, Kerion (dermatophytosis). Parasites of the skin that can cause draining tract lesions are demodicosis, cutaneous, Dirofilariasis and Leishmaniasis. Some of the more common neoplasms that can cause draining tract lesions are mast cell tumor, melanomas, SCC, cutaneous histiocytosis, systemic histiocytosis, malignant histocytosis, and cutaneous lymphoma. Examples of non-infectious/non-neoplastic causes for draining tract lesions include: foreign bodies, eosinophilic granulomas, sterile nodular panniculitis, sterile granulomas, nodular sebaceous adenitis, calcinosis cutis, and juvenile cellulitis.

Diagnostic tests

Several diagnostic tests are required in order to get a more definitive diagnosis for the cause of the dog or cat developing draining tract lesions. Diagnostic tests that are beneficial include cytologies, skin biopsies, culture and sensitivity, imaging and advanced special procedures. Specific information for each of these tests will be discussed individually.

A cytology is the quickest, most inexpensive way to try to get an idea of whether an infection or cancer is present. The Romanowsky-type staining (Dif-Quick) can be used initially. The cytology sample is obtained from the exudates, aspirates from the nodules and aspirates from any enlarged lymph nodes that are present. If granules are present on cytology then higher bacteria such as actinomycosis, botryomycosis, actinobacillosis, eumycoctic mycetoma and nocardiosis are most likely. Impression smears can be made directly from the skin lesion or from the excised tissue that is biopsied. Special stains such as acid-fast stain to aid in visualizing higher bacteria or modified periodic acid-Schiff (PAS) to see fungal elements may also be necessary

One of the most valuable diagnostic tests when dealing with draining tract lesions is the skin biopsy with histopathology. Since many of the organisms that cause draining tract lesions are difficult to find, special stains should be requested. A minimum of three skin biopsies should be taken in effort to increase the chances of getting a definitive diagnosis. The exception to this rule is if a solitary lesion is present then the whole lesion should be removed if possible and submitted for histopathology. The biopsy site should not be cleaned since surface material may be important to the diagnosis. A wedge shaped or elliptical biopsies should be taken. If a biopsy punch is used then multiple samples and deeper tissue samples (i.e. subQ) should be obtained. The biopsy sample should be divided into two portions (one part for histopathology and the other part for culture).

Bacterial (aerobic and anaerobic), mycobacterial and fungal cultures should be performed on draining tract lesions. After biopsy collection, the tissue for culture can be sent in a sterile leak proof container and submitted to the laboratory for analysis so that the sample arrives within 12 to 24 hours. Please make sure that the sterile container does not contain any anticoagulants or preservatives that might affect the organism's ability to grow. Since refrigeration can delay proliferation of some fungi such as aspergillosis, pythiosis and zygomycetes, these samples should be stored at room temperature. Failure to grow an organism can be a false negative result due to sampling and transport mistakes, previous antibiotic therapy and infectious causes due to fastidious organisms such a as nocardia, actinomyces, L-form bacteria, mycobacterium, obligate anaerobes, spirochete and rickettsias.

Diagnostic imaging may be necessary in draining tract cases. Survey radiographs may be useful if a foreign body is suspected. A fistulogram (infusion of iodinated contrast into the draining tract and taking radiographs) is useful for identifying potential locations of the foreign body and identifying the internal features of the draining tract lesions. Radiographs may also be useful in cases that have concurrent systemic disease such as osteomyelitis and pulmonary involvement or if metastasis is present if neoplasia is present. Depending on the results of radiographs, additional imaging tests such as ultrasound, CT or MRI may be necessary.

Other advanced special procedure may also be necessary in some cases of draining tract lesions. Examples of additional special procedure include electron microscopy, immunostaining with polyclonal anti-Mycobacterium bovis (BCG), enzyme histochemistry, immunocytochemistry and immunofluorescence staining. An addition special procedure that may be needed to be considered in some draining tract lesions is serology. Antibodies to a specific organism can be useful but this finding indicates previous exposure and does not automatically prove that an active infection exists. Therefore, two samples collected 1 to 3 weeks apart are recommended. If the clinical signs of disease are present along with a 4-fold rise in titer then the diagnosis can be officially made. It is important to know that false negative results can occur early in infection, with chronic infection or primary cutaneous disease. Titers may persist for months to yeasts after the clinical disease has resolved.

Serology can be useful for diagnosing certain medical conditions. Serology is useful for disseminated cases of cryptococcosis. A commercial kit that uses latex agglutination to the capsular antigen is available with 90% to 100% sensitivity and 97% to 100% specificity in cats. An agar-gel immunodiffusion test (AGID) is currently the most used serologic test available for balstmycosis and has a sensitivity and specificity in the dogs of greater than 90%. A western blot serology for Pythium insidiosum and other oomycotic pathogens, enzyme-linked immunosorbent assay (ELISA) serology, and immunohistochemical staining for Pythium insidiosum are available. The Western blot is used for the initial serologic diagnosis and then the ELSIA to follow anti-Pythium titers after treatment. Follow-up titers every 2 to 3 months starting 2 months after surgical resection and continuing for a year is recommended. Dogs have had successful surgical resections usually drop their titers significantly within 2 to 3 months.

If a higher bacteria is suspected or an organism is grown on culture but cannot be easily identified then a polymerase chain reaction (PCR) for some of the higher bacteria (i.e. Nocardia, Atypical mycobacteria) might be useful. Macerated tissue cultures are especially useful for obtaining a better representative sample of the bacteria that is present.


The treatment for draining tract lesions is based primarily on identifying the underlying etiology. Infectious causes for draining tracts need to be treated with systemic antibiotics and/or anti-fungal medications. Bacterial infections usually need to be treated for a minimum of 6 weeks and in cases of higher bacteria may need to be treated for 1 year. Antifungal agents are administered for a minimum of 6 weeks or 2 weeks beyond when the skin looks normal.

Parasitic causes for draining tract lesions should be treated with appropriate anti-parasitic treatments. The most common parasite causing draining tract lesions are demodex. Ivermectin or milbemycin are the most common systemic drugs used. Mitaban is the most common topical medication for treating demodex.

Sterile nodular draining tract lesions are treated with immunosuppressive drugs. The majority of the time oral steroids control the problem. If dogs or cats do not tolerate the steroids due to side effects then steroid sparing drugs such as azathioprine or leukran may need to be added into the treatment protocol.

The treatment for cancer will depend on the type of cancer which is present and the extent that the cancer has already metastasized to the body. Surgery, radiation, and chemotherapy are the types of therapies most typically used. A consult with a veterinary oncologist is advisable for these cases.

Surgery is required for foreign bodies. In cases of severe pododermatitis which chronic furunculosis and scar tissue then a fusion podoplasty (surgical removal of the webs of the feet) may be necessary.


Many different causes for draining tract lesions in dogs and cats exist. A though history, physical examination are a vital start to have some idea as to which differential diagnoses is most likely. However, in order to provide definitive diagnoses diagnostic tests need to be performed. Only after a though work-up has been done will the clinician be able to develop an effective treatment plan or management protocol. The pet owner needs to have patience because most draining tract lesions take a months to get under control.

Selected readings

1. Beale KM. Nodules and draining tracts in feline dermatology. Vet Clin North Am Small Anim Pract 5:887-899,1995.

2. Daigle JC, Kerwin S, Foil CS, et. al. Draining tracts and nodules in dogs and cats. Clin Tech Sm Anim Pract 116(4):214-218, 2001.

3.Nelson RW, Couto LG. Polysystemic mycotic infection, in Nelson RW, Couto LG (eds). Small Animal Internal Medicine. St. Luis, MO, Mosby, 1999, pp. 1302-1312.

4. Gross TL, Ihrke PJ, Walder E (eds): Veterinary Dermatopathology. St.Louis, MO, Mosby, 1992.

5. Jang S, Biberstein E: Laboratory diagnosis of fungal and algal infections. In Greene Infectious Disease of the Dog and Cat. Philadelphia, PA, Sanders, 1998, pp 349-357.

6. Jones R: Laboratory diagnosis of bacterial infections. In Greene Infectious Disease of the Dog and Cat. Philadelphia, PA, Saunders, 1998, pp. 179-185.

7. Bonenberger TE, Ihrke PJ, Naydan DK, et al: Rapid identifications of tissue micro-organisms in skin biopsy specimens from domestic animals using polyclonal BCG antibody. Vet Derm 12:41-47, 2001.

8. Lamb, CR, White RN, McEvoy FY: Sinography in the investigation of draining tracts in small animals: Retrospective review of 25 cases. Vet Surg 23:129-134, 1994.

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