Two submissions to TVMDL contained the following histories:
Case 1 – A seven-year-old, female, spayed Chihuahua mixed breed dog had suddenly become blind about one month prior to presenting at the submitting veterinarian’s clinic. The dog had enlarged lymph nodes and a large abscess on the left side of the head. It was open-mouth breathing, and the owner reported coughing, but no nasal discharge was noted on examination.
Case 2 – A six-year-old, female Dachshund presented with a three-week history of a one-inch, non-healing fistulous mass expressing a purulent discharge on the left thorax, along with enlargement of the right prescapular lymph node and mediastinal lymph nodes. The submitting veterinarian suspected that there was also a mass near the base of the heart and tracheal bifurcation. The dog had a persistent fever, ranging from 102.8 F to 105 F, and was being treated with Clavamox and Zeniquin.
Both submissions requested bacterial culture and sensitivity; Case 1 requested fungal serology, while Case 2 requested fungal culture and dermatohistopathology. While both dogs presented with very different clinical signs, the astute submitting veterinarians wisely included fungal etiologies in their lists of differentials. In Case 1, the fungal serology panel was positive for coccidioidomycosis, and Coccidioides posadasii was isolated on the bacterial culture plates. In Case 2, C. posadasii was isolated on both the bacterial and fungal cultures, and the fungal organisms were visualized on the biopsy samples taken from the thorax.
Coccidioidomycosis is the most common systemic fungal infection diagnosed by culture at TVMDL. The disease is caused by Coccidioides posadasii (formerly known as C. immitis; the latter species name is now reserved for fungal organisms isolated in California). The organism is endemic in areas of the southwestern U.S., including Texas, as it favors an arid environment. The primary route of infection is by inhalation of the arthrospores, though rare cases of infection by direct injection of the spores have been reported.
In a review of TVMDL diagnostic submissions, 90% of coccidioidomycosis cases were diagnosed in canines, 5% were cases from camelids (llamas/alpacas), and the remaining 5% of cases were diagnosed in other species, including felines, equines, bovines, a snake, and a wallaby. This mirrors reports in the medical literature that while dogs and camelids are over-represented, a wide variety of other species can become infected, including marine mammals and reptiles.
These two submissions serve to highlight several aspects of the diagnostic challenges presented by this disease. Although it most often begins as a primary pulmonary infection, it is commonly diagnosed after infection has disseminated to other tissues. In the dog, the most common site of dissemination appears to be bone, and frequently the soft tissues over infected bone fistulate and present as chronically draining lesions that are unresponsive to antibiotics. This is the most common scenario described in the clinical histories of Coccidioides culture-positive cases at TVMDL. While fungal serology is considered useful in supporting a diagnosis, it should always be used in conjunction with methods that definitively confirm infection by identification of the fungal organisms on cytology, histopathology, and/or culture. It is recommended that whenever possible, submissions should include biopsies or aspirates that are submitted for both culture and histopathology or cytology, to maximize the opportunities for a definitive diagnosis. Because this organism readily grows on blood agar plates, and the arthrospores are easily aerosolized and highly infectious, the culture of draining tracts should always be performed by qualified microbiologists using bio-containment and personal protective equipment.
For more information about TVMDL’s test catalog, visit tvmdl.tamu.edu.
