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3 CE Credits

Allison Werner, DVMBrykerwood Veterinary Clinic Austin, Texas

Frank Norton, DVM, MS, DACVIMVCA Berwyn Berwyn, Illinois

Abstract: For more than 100 years, blastomycosis has been recognized as causing significant morbidity and mortality in people and dogs. The disease is rare in cats. Isolation of the organism is difficult, and novel methods to culture environmental samples are forthcoming. The most significant clinical dilemma is the inability to make a timely diagnosis when multiple cytologic samples are unrewarding. This article reviews the literature on advances in epidemiology and serology, clinical presentations, new antifungal drugs, and progress in formulating a vaccine.

Blastomycosis

Blastomyces dermatitidis is a well-known, infectious, fungal organism. Blastomycosis has been characterized for more than 100 years and affects almost all mammalian species,

most commonly dogs and humans. Although reported world-wide, the organism is endemic in North America. Research interests in the veterinary and human medical fields include environmental detection, serologic diagnosis, new treatments, and vaccine formulation.1–3

EpidemiologyVeterinarians located in the Mississippi, Missouri, and Ohio river valleys as well as those located in southeastern Canada should have a high index of suspicion for this infection. The disease is endemic in most of the mid-Atlantic and southern states; states with the highest incidences include Kentucky, Louisiana, Mississippi, South Carolina, Virginia, and Washington.4 Northern Illinois and southern Wisconsin also have high incidence rates. In such hyperendemic areas, >1% of the total population of dogs is affected.2 Sporadic cases have also been reported in Colorado and New York State, suggesting a wider range of endemic areas than previously thought.5,6

There are very few reports of the organism being isolated from nature, all using animal inoculation techniques, with thousands of cultures unsuccessfully attempted.2 Investigating sources of outbreaks is thus difficult. A soil isolation method using poly-merase chain reaction–based detection of B. dermatitidis DNA was developed in 2006.4 This test yielded positive results in three out of eight soil samples near a dog kennel in Lexington, Kentucky, where 35 of 100 dogs had reportedly contracted the disease the year before. This is the first reported detection of B. dermatitidis DNA from a natural source, possibly providing a means of rapid

isolation for epidemiologists at sites of outbreaks as well as possibly replacing animal inoculation techniques.2

Areas from which the organism has been successfully isolated generally have acidic or sandy soil, decaying wood, and animal feces.7 Humidity, fog, rain, and heavy dew are thought to facilitate fungal sporulation and loosening of conidia from conidiophores.4,7 Proximity to waterways has also been implicated as a risk factor for infection.7

Signalment plays a role in the occurrence of the disease in dogs, with risk factors including young age, breed (Doberman pinschers, golden retrievers, and Labrador retrievers), and possibly male sex.7–9 In humans, diagnosis of blastomycosis is four to 15 times more common in males than in females.1

Pathogenesis and Clinical PresentationThe organism is primarily transmitted by inhalation of conidia that are in the mycelial phase.7 Infection occurs when these conidia are deposited into the alveoli, phagocytized by pulmonary macrophages, and transported to the pulmonary interstitium,7 where normal body temperatures promote transformation to the yeast phase.3 The disease may stay localized in the lungs or may spread hematoge-nously or lymphatically to other body systems.7 Dermal or lingual inoculation of the organism has also been reported, causing a localized granulomatous disease; however, this form of transmission is uncommon.8 The infection is not transmissible between humans, between animals, or from animals to humans and, therefore, is not considered contagious or zoonotic.1,4

A typical case presentation is a young, male, large-breed dog presenting with coughing, tachypnea, or dyspnea. Skin lesions that appear papular or plaque-like are commonly present, occasionally with draining cutaneous tracts7,9 (FIGURE 1). Other common

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Blastomycosis

physical examination find-ings are lymphadenopathy, ocular abnormalities such as uveitis, and pyrexia.7,8 The disease is usually pulmo-nary in origin and self-lim-iting.9 However, dissemi-nation is possible, producing a systemic, pyogranuloma-tous infection that can affect the lymphatics, eyes, bones, skin, central nervous and urogenital systems, and many organs, including the heart.4,10 Dogs may present with respiratory signs, a heart murmur, and a history of syncope suggestive of heart failure.10 Heart lesions caused by blastomycosis, although rare, can range

from myocarditis to endocarditis due to pyogranulomatous and fibrous masses in and around the heart.10

Blastomycosis is very rare in cats. To date, three review articles account for all of the reported cases in cats. The presentation is similar to that in dogs and does not appear to be related to immunosuppression caused by retroviruses.11,12 A 2006 study11 of eight cases of blastomycosis in cats revealed respiratory prob-lems as the most common chief complaint, which is consistent with previous reports. Respiratory signs were primarily dyspnea, tachypnea, and coughing. Lymphadenopathy is generally not appreciated in cats; however, fever and skin lesions are prominent physical examination findings.11 Skin lesions reported in cats are mostly dermal masses on the head and trunk.11 Ocular abnor-malities such as retinal granulomas and retinal detachment are reported to occur in 18% to 32% of cases.11–13 Central nervous system disease is relatively common in cats with systemic blasto-mycosis and carries a poor prognosis.11,12

DiagnosisHematologyRoutine hematologic screens are nonspecific for the diagnosis of blastomycosis. However, high band neutrophil counts have been correlated with decreased survivability in canine patients with pulmonary blastomycosis.14

CytologyCytologic identification of blastomycosis organisms is the gold standard and most common method of diagnosis. However, this method has variable sensitivity, and multiple cytologic or histologic samples may be needed for a diagnosis to be confirmed (FIGURE 2). A 2008 study indicated that transthoracic fine-needle aspiration is more sensitive (81%) than transtracheal washing (69%) for obtaining a diagnostic sample in dogs with pulmonary blastomycosis.14

RadiologyA diffuse miliary or nodular interstitial pattern has been reported as the most common radiographic manifestation of pulmonary blastomycosis7,8 (FIGURE 3). In a 2008 study of 125 dogs with pul-monary blastomycosis, most dogs (59%) had localized infiltrates rather than diffuse, nodular, interstitial disease.15 These nondiffuse patterns could mimic bacterial pneumonia or neoplasia, high-lighting the need for a confirmatory diagnosis. Tracheobronchial lymphadenopathy is seen in 25% to 29% of cases and is not correlated with extent of lung disease or survival times.7,15 In the study of 125 dogs, the number of affected lung lobes was the only radiographic finding that was found to be significantly correlated with decreased survivability.15 Of the 125 dogs in the study, 88% of dogs with fewer than four lobes affected survived compared with 52% of dogs with four or more lobes affected.

Figure 2. Prostate fine-needle aspirate from an intact male dog with prostatitis due to B. dermatitidis (arrows). Courtesy of Frank Norton, DVM.

Figure 3. Lateral thoracic radiograph of a dog with pulmonary blastomycosis. Courtesy of Frank Norton, DVM.

Figure 1. Draining tract on the lateral hock of a German shepherd. B. dermatitidis organisms were confirmed on cytology.

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Blastomycosis

SerologyAntibody DetectionSerologic tests that detect antibodies evaluate the humoral immune response, which is not always indicative of active infection and varies among patients. The most widely available serologic tests to detect antibodies in veterinary medicine is agar gel immunodiffusion (AGID), which has a reported sensitivity ranging from 17.4% to 90%.4,9,16,17 A study testing the antibody responses of dogs with confirmed blastomycosis used a novel radioimmunoassay (RIA) and compared the results with those from the traditional AGID test.16 The reported difference in sensitivity was striking: 92% and 41% for the RIA and AGID methods, respectively. The specificity of both tests was 100% when tested against negative controls. The RIA method detected antibodies up to 1545 days after initial diagnosis, whereas the AGID method detected <10% of positive infections after day 10.16 Although this study revealed that the RIA method is superior to the AGID method of antibody detection, the RIA is unavailable for commercial use.

Another, more recent study17 compared the sera antibody de-tection rates of an enzyme immunoassay (EIA; MVista Blastomyces dermatitidis antibody EIA, MiraVista Diagnostics) and the AGID test (Meridian Bioscience) in dogs with confirmed blastomycosis. The sensitivity of the EIA and AGID test was 76.1% and 17.4%, respectively.

Antigen DetectionBecause up to 30% of dogs lack detectable antibodies at the time of presentation, studies are now looking at antigen assays as a method of detecting B. dermatitidis infection.18 MiraVista Diagnostics (Indianapolis, IN) has developed an EIA for detecting antigen in serum and urine specimens (MVista Blastomyces dermatitidis

quantitative antigen EIA).17,19 Urine specimens from people with blastomycosis and other fungal infections were used in studying this test.19 In the study, the test was highly sensitive, detecting almost 93% of blastomycosis cases. Furthermore, none of the urine samples from healthy people yielded positive results. However, the test was not specific for B. dermatitidis, having significant cross-reactivity with Histoplasma capsulatum, Paracoccidioides brasiliensis, and Penicillium marneffei. Although this test is imperfect, the benefit of diagnosing a fungal infection and initiating treatment early may outweigh the ability to differentiate organisms.

A competitive binding inhibition ELISA was 100% successful in detecting B. dermatitidis antigen in urine samples from dogs with blastomycosis in one study3 and has been used in university settings. Cross-reactivity with H. capsulatum and a positive result in a negative control were issues with this test. Re-search is still in progress to increase the specificity of this test and to define its optimal parameters.

The study that compared the AGID and EIA methods of detecting antibodies17 also evaluated the EIA method of antigen detection in dogs. The study used paired canine urine and serum specimens of 46 confirmed blastomy-cosis cases and 44 control cases negative for fungal infection. Urine specimens

Drug and DosageTotal Number

of Dogs Number Cured Mortality Number

Relapsed Study

Ketoconazole, 10 mg/kg/d for 60 days 9 3 (33%) 3 (33%) 3 (33%) Legendre 1984b

Amphotericin B, 1 mg/kg/dose for 8–9 mg/kg 35 20 (57%) 8 (23%) 5 (20%) Legendre 1984b

Amphotericin B, 1 mg/kg/d for four doses, then ketoconazole, 10 mg/kg/d for 60 days

18 11 (61%) 4 (22%) 3 (17%) Legendre 1984b

Amphotericin B and ketoconazole, 10 mg/kg/d for 60 days

19 12 (63%) 7 (37%) Not stated Arceneaux 19987

Itraconazole, 10 mg/kg/d for 60 days 56 30 (54%) 14 (25%) 12 (21%) Legendre 199621

Itraconazole, 5 mg/kg/d with food for 60 days 35 19 (54%) 9 (26%) 7 (20%) Legendre 199621

Itraconazole, 10 mg/kg/d for 60–90 days 31 16 (52%) 10 (32%) 5 (16%) Arceneaux 19987

Overall with amphotericin B, amphotericin B + ketoconazole, or itraconazole

195 108 (55%) 52 (27%) 35 (18%) Arceneaux 1998,7 Legendre 1996,21 Legendre 1984b

aWheat J, Legendre A. Blastomycosis in dogs. Accessed March 2011 at http://www.miravistalabs.com/Files/pdf/BlastomycosisinDogs2007.pdf. Used with permission.bLegendre AM, Selcer BA, Edwards DF, Stevens R. Treatment of canine blastomycosis with amphotericin B and ketoconazole. JAVMA 1984;184:1249-1254.

Table 1. Summary of Treatment Studiesa

Key Facts

The epidemiologic study of •Blastomyces dermatitidis is precluded by the difficulty in culturing the organism from nature.

The “classic” radiographic pattern of •pulmonary blastomycosis is being recharacterized.

Recent advances in the antigen •detection of B. dermatitidis are improving the reliability of serologic diagnosis.

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Blastomycosis

yielded a sensitivity of 93%, whereas serum specimens yielded a sensitivity of 87%. The specificity was excellent (100%) for both specimen types using the negative controls. Cross-reactivity with other fungal organisms was not investigated in this study. The findings of this study are promising for the use of EIA antigen testing in the early diagnosis of blastomycosis in dogs.

TreatmentThe treatment for blastomycosis depends on the severity of disease. If significant pulmonary compromise is present, amphotericin B is recommended along with itraconazole for the first 4 to 7 days, followed by itraconazole for 4 to 6 months or 1 month past radio-graphic resolution.14,20 In less severe cases, itraconazole alone is recommended for at least 60 days or 1 month past resolution of clinical or radiographic signs of disease.21 Ketoconazole is less effective than itraconazole; however, if cost of treatment is a concern, using amphotericin B for the first 4 days along with ketoconazole results in similar response rates to use of itraconazole alone.20 Fluconazole is another azole antifungal that is minimally protein bound and highly water soluble, penetrating the blood–brain and prostatic barriers in higher concentrations than other azoles. This drug may play a role in treating central nervous system or prostatic blastomycosis; however, itraconazole is more effective in most cases. Corticosteroids are reserved for dogs with dyspnea and should only be used after treatment with antifungals has been initiated.4 In general, 25% of cases fail to respond to treatment, and of the cases that respond to treatment, 25% relapse when medication is discontinued. The overall treatment success rate is about 70% to 80%.4,8,21 A summary of treatment studies is outlined in TABLE 1.

Alternative treatments include the newer triazoles voriconazole and posaconazole.22 Both show promising results in humans and mice.23–26 Voriconazole, a derivative of fluconazole, has shown to lengthen the life of mice infected with pulmonary blastomycosis in a dose-dependent fashion.24 With its wide tissue distribution, this drug may be effective in treating central nervous system blastomycosis; it has been successful in treating a human case of cerebral blastomycosis.27 Posaconazole is an analogue of itracon-azole. This drug improved survivability in mice infected with B. dermatitis compared with itraconazole, fluconazole, and am-photericin B.23 Also, samples of lung tissue that were obtained and cultured after treatment with posaconazole were negative for B. dermatitis, in contrast to the other three drugs.23

VaccineStudies are ongoing in formulating an effective and safe vaccine against blastomycosis. Early studies used a recombinant BAD1 (an immunodominant surface antigen) vaccine to initiate an immune response.28 The increase in survival time of vaccinated mice compared with unvaccinated control mice was modest. Monoclonal antibodies against BAD1 were then investigated for their protective value.29 None of the mice was protected against experimental pulmonary infection, indicating that inducing an antibody response is not protective.29

A recombinant, live attenuated vaccine has also been formulated. Mice injected subcutaneously with live yeast of this mutant strain of B. dermatitis had significantly improved survival when challenged with nonmutant B. dermatitidis.30 This genetically engineered strain of B. dermatitidis shows promise in the future of vaccines against the organism, both therapeutically and preventively. This vaccine is being evaluated for trials in human volunteers.1

References1. Pappas P. Blastomycosis. Semin Respir Crit Care Med 2004;25(2):113-121.2. Burgess J, Schwan W, Volk T. PCR based detection of DNA from the human pathogen Blastomyces dermatitidis from natural soil samples. Med Mycol 2006;44:741-748.3. Shurley J, Legendre A, Scalarone G. Blastomyces dermatitidis antigen detection in urine specimens from dogs with blastomycosis using a competitive binding inhibition ELISA. Mycopathologia 2005;160(2):137-142.4. Legendre A. Blastomycoses. In: Greene CE, ed. Infectious Diseases of the Dog and Cat. St. Louis: Saunders; 2006:569-576.5. De Groote MA, Bjerke R, Smith H, Rhodes LV III. Expanding epidemiology of blas-tomycosis: clinical features and investigation of 2 cases in Colorado. Clin Infect Dis 2000;30:582-584.6. Côté E, Barr SC, Allen C, Eaglefeather E. Blastomycosis in six dogs in New York State. JAVMA 1997;210(4):502-504.7. Arceneaux KA, Taboada J, Hosgood G. Blastomycosis in dogs: 115 cases (1980-1995). JAVMA 1998;213(5):658-664.8. Kerl ME. Update on canine and feline fungal diseases. Vet Clin North Am Small Anim Pract 2003;33:721-747.9. Rudmann DG, Coolman BR, Perez CM, Glickman LT. Evaluation of risk factors for blastomycosis in dogs: 857 cases (1980-1990). JAVMA 1992;201(11):1754-1759.10. Schmiedt C, Kellum H, Legendre AM, et al. Cardiovascular involvement in 8 dogs with Blastomyces dermatitidis infection. J Vet Intern Med 2006;20:1351-1354.11. Gilor C, Graves TK, Barger AM, O’Dell-Anderson K. Clinical aspects of natural infec-tion Blastomyces dermatitidis in cats: 8 cases (1991-2005). JAVMA 2006;229(1):96-99.12. Miller PE, Miller LM, Schoster JV. Feline blastomycosis: A report of 3 cases and a literature review (1961-1988). JAAHA 1990;26(4):417-424.13. Davies C, Troy GC. Deep mycotic infections in cats. JAAHA 1996;32:380-391.14. Crews LJ, Feeney DA, Jessen CR, et al. Utility of diagnostic tests for and med-ical treatment of pulmonary blastomycosis in dogs: 125 cases (1989-2006). JAVMA 2008;232(2): 222-227.15. Crews LJ, Feeney DA, Jessen CR, Newman AB. Radiographic findings in dogs with pulmonary blastomycosis: 125 cases (1989-2006). JAVMA 2008;232(2):215-221.16. Klein BS, Squires RA, Lloyd JK, et al. Canine antibody response to Blastomyces dermatitidis WI-1 antigen. Am J Vet Res 2000;61(5):554-558.17. Spector D, Legendre AM, Wheat J, et al. Antigen and antibody testing for the diagno-sis of blastomycosis in dogs. J Vet Intern Med 2008;22(4):839-843. 18. Spector D, Wheat J, Bemis D, et al. Antigen testing for the diagnosis of blastomyco-sis. Proc 24th Annu ACVIM Forum 2006:731.19. Durkin M, Witt J, LeMonte A, et al. Antigen assay with the potential to aid in diagno-sis of blastomycosis. J Clin Microbiol 2004;42(10):4873-4875.20. Wheat J, Legendre A. Blastomycosis in dogs. http://www.miravistalabs.com/Files/pdf/BlastomycosisinDogs2007.pdf. Accessed March 2011.21. Legendre AM, Rohrbach BW, Toal RL, et al. Treatment of blastomycosis with itracon-azole in 112 dogs. J Vet Intern Med 1996;10:365-371.22. Whelan N. Voriconazole, posaconazole, ravuconazole: an update on emerging azole antifungal agents. Proc 20th Annu ACVIM Forum 2002:468-470.23. Sugar AM, Liu XP. In vitro and in vivo activities of SCH 56592 against Blastomyces dermatitidis. Antimicrob Agents Chemother 1996;40(5):1314-1316.24. Sugar AM, Liu XP. Efficacy of voriconazole in treatment of murine pulmonary blas-tomycosis. Antimicrob Agents Chemother 2001;45(2):601-604.25. Jeu L, Piacenti FJ, Lyakhovetskiy AG, Fung HB. Voriconazole. Clin Ther 2003;25(5): 1321-1381.26. Nagappan V, Deresinski S. Reviews of anti-infective agents: posaconazole: a broad-spectrum triazole antifungal agent. Clin Infect Dis 2007;45(12):1610-1617.27. Bakleh M, Aksamit AJ, Tleyjeh IM, Marshall WF. Successful treatment of cerebral

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blastomycosis with voriconazole. Clin Infect Dis 2005;40:e69-e71.28. Wüthrich M, Chang WL, Klein BS. Immunogenicity and protective efficacy of the WI-1 adhesin of Blastomyces dermatitidis. Infect Immun 1998;66(10):5443-5449.29. Wüthrich M, Klein BS. Investigation of anti-WI-1 adhesin antibody-mediated protec-

tion in experimental pulmonary blastomycosis. J Infect Dis 2000;181:1720-1728.30. Wüthrich M, Filutowicz HI, Klein BS. Mutation of the WI-1 gene yields an at-tenuated Blastomyces dermatitidis strain that induces host resistance. J Clin Invest 2000;106:1381-1389.

1. What currently available test method is the most sensitive for diagnosing B. dermatitidis infection?

a. AGID antibody test

b. EIA antigen test

c. cytology of lung aspirate

d. chest radiography

2. Patients with blastomycosis may present with

a. uveitis.

b. neurologic signs.

c. heart murmur.

d. all of the above

3. Blastomycosis is transmitted primarily by

a. dermal inoculation

b. contact with other infected animals

c. inhalation of fungal conidia

d. ingestion of fungal conidia

4. Which diagnostic test is most commonly used in clinical practice to diagnose blastomycosis?

a. serology

b. histopathology

c. fungal culture

d. cytology

5. Which method is most likely to obtain a diagnostic sample for cytology?

a. transtracheal wash

b. transthoracic fine-needle aspiration

c. impression smear of nasal discharge

d. pharyngeal swab

6. What is the overall treatment success rate for blastomycosis?

a. 25% to 35%

b. 45% to 55%

c. 70% to 80%

d. 90% to 100%

7. In cats, the most common physical examination finding related to blastomycosis is

a. lymphadenopathy.

b. neurologic impairment.

c. ocular abnormalities.

d. respiratory problems.

8. Which is the drug of choice for treatment of uncomplicated blastomycosis?

a. itraconazole

b. amphotericin B

c. fluconazole

d. voriconazole

9. A negative prognostic indicator for dogs with pulmonary blastomycosis is

a. an alveolar pattern.

b. tracheobronchial lymphadenopathy.

c. involvement of four or more lung lobes.

d. an interstitial pattern.

10. The most common radiographic finding(s) in pulmonary blastomycosis is/are

a. a diffuse interstitial pattern.

b. a localized infiltrative pattern.

c. tracheobronchial lymphadenopathy.

d. a and b

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