History

A 22-month-old 568.2-kg (1,250-lb) Clydesdale filly was referred to the Purdue University Veterinary Teaching Hospital because of a 3-month history of an enlarged left mammary gland. The referring veterinarian originally attributed the enlargement to juvenile galactorrhea because of the filly's young age coupled with the onset of enlargement during springtime, when juvenile galactorrhea in foals and yearling mares is more common. The filly had never been bred and had no access to a stallion. To rule out excessive phytoestrogens in the filly's diet, alfalfa hay was removed from the diet and the mammary gland was closely monitored. Enlargement of the mammary gland persisted through the spring season.

On referral examination, findings were within reference limits, except for an abnormally enlarged left mammary gland that did not actively leak fluid and that on palpation elicited signs of pain (Figure 1). To allow for a thorough examination of the mammary glands, the filly was sedated with detomidine hydrochloride (0.01 mg/kg [0.0045 mg/lb], IV) and butorphanol tartrate (0.01 mg/kg, IV). Further palpation of the gland revealed a homogeneous feel with no obvious abnormal masses. Supramammary and superficial inguinal lymph nodes were not enlarged; however, there was mild ventral edema just cranial to the mammary gland.

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Picture of the udder of a 22-month-old 568.2-kg (1,250-lb) Clydesdale filly referred to the Purdue University Veterinary Teaching Hospital because of a 3-month history of an enlarged left mammary gland. The filly's head is toward the right.

Citation: Journal of the American Veterinary Medical Association 258, 5; 10.2460/javma.258.5.459

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Both the right and left mammary glands were evaluated ultrasonographically with a 5-MHz curved linear probe. No masses were observed, and the streak canals and septum were readily visible; however, abnormal accumulation of fluid was evident in the left mammary gland (Figure 2).

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Percutaneous lateromedial ultrasonographic image of the enlarged left mammary gland of the filly described in Figure 1 showing an accumulation of fluid (black arrow) in the mammary gland, homogeneously hyperechoic mammary tissue (double-headed arrow), and the medial lamina (white arrows) between the 2 halves of the udder. The image is at a level just dorsal to the mammary groove. The filly's head is toward the left, and the scale in the image is in centimeters.

Citation: Journal of the American Veterinary Medical Association 258, 5; 10.2460/javma.258.5.459

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Question

What are key differential diagnoses for enlargement of the mammary gland in a 22-month-old filly?

Answer

Differential diagnoses for enlargement of the mammary gland in a 22-month-old filly include infectious mastitis, galactorrhea, trauma, abscess formation in the mammary gland, cutaneous histoplasmosis, avocado (Persea americana) toxicosis, aberrant larval migrans, and premammary adipose tissue. Gland distention associated with weaning, periparturient udder edema, and neoplasia can also be added to the differential diagnosis list for mares.

Results

With light compression of the left mammary gland, copious white purulent material was expressed from the cranial and caudal streak canals. Routine aseptic technique was used to collect a sample of the purulent material from each streak canal for bacterial culture, susceptibility testing, and cytologic evaluation. Cytologic examination^a revealed a large number of neutrophils and cocci, and mastitis of the left mammary gland was diagnosed. The filly was empirically treated with trimethoprim-sulfamethoxazole (30 mg/kg [13.6 mg/lb], PO, q 12 h) and cephapirin sodium^b (5 mL/teat canal, intramammary infusion, q 24 h).

Results of bacterial culture and susceptibility testing isolated Staphylococcus spp, with susceptibility to the empirically chosen antimicrobial treatment; therefore, the prescribed oral and intramammary administration of antimicrobials continued for a total of 14 and 5 days, respectively. Hydrotherapy was also applied to the affected mammary gland daily to help reduce swelling.

Discussion

Infectious mastitis is an uncommon disease in horses. This is attributed to a relatively short period of lactation, the size and position of the mammary gland, limited udder milk capacity, and routine mare and foal management.^1,2,3 In addition, lysozyme activity is higher in mares, compared with cows, and thought to offer some protection against mastitis.¹

Mastitis most commonly occurs in nursing mares or mares that recently weaned foals. However, mastitis has also been reported in fillies^3,4 and older, non-pregnant and nonlactating mares.¹ A seasonal predilection, with most instances occurring during the summer, is associated with teat trauma from insects that allow the introduction of bacterial pathogens.^4,5

Clinical signs in affected mares vary and are patient dependent. The most common signs include unilateral swelling and firmness of the mammary gland, signs of pain elicited on palpation of the mammary gland, ventral edema, fever > 38°C (100.4°F; reference range, 37.2°C to 38.3°C [99°F to 101°F]), abnormally warm mammary gland skin, hind limb lameness, and purulent to serosanguineous exudate actively dripping from the affected gland. In addition, the cranial mammary vein on the affected side may be enlarged, and lactating mares may show signs of unwillingness to allow their foals to nurse. Rarely do affected mares show signs of systemic illness.

In horses with infectious causes of mastitis, entry of bacteria through the teat or streak canal is the most likely route of infection; however, hematogenous and percutaneous routes of infection have been described.¹ Single organism infections are most prevalent, and the most commonly cultured bacteria are Streptococcus equi subsp zooepidemicus (S zooepidemicus), Staphylococcus spp, Klebsiella spp, Actinobacillus spp, and Escherichia coli. Of these, S zooepidemicus was most commonly isolated (n = 7) from 17 mares with mastitis.⁵ Rarely, multiple different bacterial organisms or fungal agents (eg, Blastomyces, Coccidoides, or Aspergillus spp) may be involved.¹

Abscess formation in the mammary gland or adjacent lymph nodes is rare in horses with infectious mastitis^5,6 and is usually associated with Corynebacterium pseudotuberculosis, S zooepidemicus, Streptococcus equi subsp equi, or Staphylococcus spp. Horses with mastitis and associated abscess formation (vs no abscess formation) may have more severe signs (eg, hind limb lameness, ventral edema, and more severe signs of pain on palpation of the mammary gland), and abscess formation may occur regardless of lactation status. A diagnosis of abscess formation is based on findings from manual palpation and ultrasonographic evaluation of the mammary gland.⁶ A diagnosis of infectious mastitis is usually based on clinical signs and is confirmed by findings from cytologic examination and culture and susceptibility testing. Cytologic confirmation is important, as supported by findings from a study⁵ in which results were negative for bacterial culture performed on samples from 6 of 18 mares with clinical mastitis. For the filly of the present report, results of cytologic examination and bacterial culture performed on samples obtained from the affected mammary gland indicated infectious mastitis.

Ultrasonography of the mammary gland can be performed with a linear or curved linear array 5-MHz probe to help differentiate among reasons for mammary gland enlargement. Glandular tissue should have a homogeneous hyperechoic appearance. The interglandular septum can be identified as a hyperechoic line between the 2 halves of the udder, and teat sinuses are identifiable regardless of lactation status. When disease has progressed to the point of abscess formation in the mammary gland, ultrasonographic evaluation generally reveals subcutaneous edema and a well-circumscribed and delineated structure that has a hyperechoic wall and mottled or hypoechoic center, consistent with an abscess.⁶

Treatment of mastitis in horses is usually multimodal, including antimicrobial treatment that is systemic, intramammary, or both, and both approaches for antimicrobial treatment were used for the filly of the present report. Antimicrobial choice should be based on results from bacterial culture and susceptibility testing; however, broad-spectrum antimicrobials (eg, trimethoprim-sulfamethoxazole or penicillin and gentamicin in combination) may be chosen empirically until results are received.^1,5 In addition, frequent stripping of the affected gland is critical, and hydrotherapy, application of warm packs, and administration of NSAIDs (eg flunixin meglumine) may be needed. Extralabel use of commercially available intramammary products labeled for cattle with mastitis may also be effective^1,2; however, anatomic differences in the mammary structure of horses, compared with cattle, need to be considered and care has to be taken not to harm a horse when administering such products.

Noninfectious causes of mastitis should also be considered. For instance, trauma, frostbite, or avocado toxicosis could cause mastitis in horses and were ruled out in the filly of the present report. In addition, horses with mammary gland neoplasms (eg, adenocarcinoma or adenoma⁷) may also have mastitis, and secretions from ulcerated tumors are thought to be the inciting cause of mastitis in affected horses. Although mastitis secondary to neoplasia was placed low on our differential diagnosis list because of this filly's young age, results of palpation and ultrasonography of the mammary gland helped us rule out neoplasia.

Galactorrhea may initially be misdiagnosed as mastitis, and these conditions need to be differentiated through an examination of the udder combined with characterization of the mammary secretions by gross and cytologic examinations and with bacterial culture and susceptibility testing, as was performed on the filly of the present report. With galactorrhea and mastitis, the mammary gland is enlarged; however, heat and signs of pain are absent with galactorrhea.⁶ A common cause for galactorrhea in older mares is a rise in prolactin concentration secondary to equine pituitary pars intermedia dysfunction,⁷ whereas galactorrhea in young horses (neonates, weanlings, fillies, and, rarely, young adults) is a poorly defined phenomenon. Precocious mammary development is thought to be induced by high concentrations of lactogenic hormones.⁷ Juvenile galactorrhea has also been attributed to the ingestion of estrogenic components found in many types of spring grasses⁷ and was the referring veterinarian's primary differential diagnosis early in the management of the filly of the present report.

Outcome

The filly of the present report responded well to antimicrobial treatment, with the mammary gland returning to normal size over the following 2 months. During the latest follow-up conversation with the owner approximately 1 year later, the owners reported that the filly had been well and without relapse; however, the owners were concerned about potential permanent damage and loss of function in the previously affected mammary gland. The owners were informed that although long-term damage and loss of use may occur, such was uncommon^1,3 and that monitoring the filly's udder closely if they chose to have the animal bred would help to detect potential mammary-related issues early should they occur.