Introduction

Histoplasma capsulatum is a soil-borne dimorphic fungus that is found worldwide; however, most cases are reported around the Mississippi, Ohio, and Missouri Rivers.¹ Cats are more susceptible than dogs.¹ When dogs are affected, young sporting or working dogs are at greatest risk of infection, with a median age of 3 yr old at time of diagnosis.² Infection occurs via inhalation of microconidia from the mycelial phase, which develop into the yeast form within the lung. The yeast is then phagocytized by the mononuclear phagocytic system. Infection may be subclinical. Alternatively, infection may result in pulmonary granulomatous disease or become a disseminated infection.¹ The most common clinical signs of canine disseminated histoplasmosis are diarrhea, anorexia, weight loss, lethargy, and fever.² Although the radiographic findings of canine histoplasmosis are well documented, there are fewer reports in the veterinary literature that document abdominal ultrasound findings in canine disseminated histoplasmosis.^2,3 Biliary obstruction has been reported in human cases of disseminated histoplasmosis; however, it has not been reported in dogs.^4–6 This case report documents biliary dilation, consistent with biliary obstruction, in a young dog with disseminated histoplasmosis.

Case Report

A 4 mo old intact male Labrador retriever was referred to the authors’ institution for evaluation of a 3 wk history of weight loss (1.5 kg in 1 mo), ascites (diagnosed by the primary care veterinarian), and abdominal distension. The dog had no prior history of illness and no travel history and was up to date with vaccinations and preventive medications. The dog had a ravenous appetite and was fed a commercial puppy diet. On examination, the dog was bright, alert, and responsive, and his vital parameters were within normal limits. Thoracic auscultation revealed bilaterally increased bronchovesicular lung sounds. The dog weighed 14.0 kg, with a body condition score of 3/9. There was evidence of moderate generalized muscle atrophy as classified by the World Small Animal Veterinary Association Muscle Condition Scoring System. A neurological and ophthalmologic examination was within normal limits. The mandibular, superficial cervical, and popliteal lymph nodes were moderately enlarged. Cranial abdominal organomegaly was detected on abdominal palpation. No fluid wave was detected on abdominal palpation. A rectal examination revealed hematochezia.

Blood was drawn for a complete blood count (CBC) and serum chemistry. Laboratory values were compared with reference ranges for the age and breed of the puppy.⁷ The CBC revealed a moderate thrombocytopenia (128,000/μL; reference interval 379,000–478,000/μL) and a mild anemia for the patient’s age (packed cell volume [PCV]: 24.9 %; reference interval 30.7–33.4%). A serum biochemistry panel revealed a moderately elevated blood urea nitrogen (29 mg/dL; reference interval 14.9–18.9 mg/dL), moderate hypoalbuminemia (1.8 g/dL; reference range 2.4–2.6 mg/dL), and a normal total bilirubin concentration. Alkaline phosphatase (ALP) was mildly elevated (570 U/L; reference interval 418–465 U/L), which may have been associated with biliary disease or the patient’s age. ALP may be increased in serum in young animals owing to increased osteoblastic activity. Phosphorus was mildly elevated (7.9 mg/dL; reference range 2.5–5.0 mg/dL), likely associated with increased levels of growth hormone in growing patients, which leads to increased renal tubular reabsorption of phosphorous. Total calcium was within reference range. Ionized calcium concentration was not measured. A baseline cortisol and gastrointestinal panel^a were within normal limits.

Thoracic radiographs revealed a severe, diffuse unstructured interstitial and bronchial pulmonary pattern, in addition to a poorly defined ovoid soft tissue opaque mass dorsal to the second sternebra. Differential diagnoses for the pulmonary changes included infectious etiologies, sterile inflammation, noncardiogenic edema, hemorrhage, or, less likely, neoplasia. The soft tissue opacity dorsal to the second sternebra likely indicated sternal lymphadenopathy. Abdominal radiographs revealed hepatosplenomegaly and poor serosal detail. An abdominal ultrasound revealed many hepatobiliary abnormalities, including an enlarged hyperechoic liver, gallbladder sludge, biliary duct dilation (6.5 mm; reference interval ≤3 mm), and multiple tortuous and dilated intrahepatic bile ducts (Figure 1). The bile duct was traced back to the duodenal papilla, and no shadowing structures were noted. Dilation of the bile duct and intrahepatic ducts was consistent with extrahepatic biliary obstruction. Other abnormalities seen on the abdominal ultrasound included splenic nodules, scant free fluid, and pancreaticoduodenal and medial iliac lymphadenopathy. The pancreas was normal. Hepatomegaly in conjunction with muscle cachexia was considered the most likely cause of the abdominal distension.

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The patient was sedated with dexmedetomidine (2.5 µg/kg, IV) and butorphanol (0.2 mg/kg, IV) for ultrasound-guided fine-needle aspirates of the liver, enlarged lymph nodes, and splenic nodules. The risk of iatrogenic hemorrhage was discussed with the owner. Cytologic analysis of the spleen and liver aspirates revealed mixed inflammatory infiltrates with intracellular, round-to-oval, 2- to 3-μm organisms with pale basophilic cytoplasm, a small eccentrically located nucleus, and a thin clear halo (Figure 2). These organisms were consistent with H capsulatum. A urine Histoplasma antigen assay^b was submitted and was positive (11.8 ng/mL, no reference range; Table 1). A diagnosis of disseminated histoplasmosis was made. Antifungal therapy consisting of itraconazole^c (10 mg/kg per os [PO] q 12 hr) and terbinafine^d (35 mg/kg PO q 12 hr) was initiated, and the patient was monitored for 24 hr. Maropitant citrate (1.7 mg/kg PO q 24 hr) was prescribed to reduce inappetence associated with potential nausea. A dietary plan reflecting the patient’s caloric need was provided to the owner.

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TABLE 1 Correlation Between Ultrasonographic and Clinical Parameters

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Bloodwork and a focal ultrasound of the hepatobiliary system were repeated 7 days after initiating treatment. The owner reported that the dog was doing well and had an increased appetite. The CBC revealed an unchanged PCV and platelet count. The serum biochemistry panel revealed a lower ALP (365 U/L; reference range 418–465 U/L), an unchanged albumin level, correction of the previously elevated blood urea nitrogen, and a mild increase in total bilirubin (0.7 mg/dL; reference interval 0.2–0.6 mg/dL). The repeat focal ultrasound examination revealed an increase in the amount of hyperechoic material within the gallbladder. The examination also revealed focal progressive dilation of the bile duct, measuring 0.73 cm at its widest point. A choledocholith was present within this portion of the bile duct. The remaining bile duct had reduced in size, and the bile duct measured 0.47 cm at the duodenal papilla. The dog was discharged with instructions to continue itraconazole and terbinafine.

A second recheck examination was performed 21 days later. On day 21 the dog had an improved body condition score (4/9) and had gained 1.6 kg (weight: 15.6 kg). The CBC showed an increase in the PCV (33.0%), and the platelet count had returned to within reference range. The serum chemistry panel revealed a reduction in the ALP (235 U/L; reference range 306–353 U/L) and an increase in the albumin to 2.2 mg/dL. A repeat focal ultrasound revealed a reduced amount of hyperechoic material within the gallbladder. The liver had subjectively decreased in size, and the intrahepatic bile ducts were no longer tortuous but remained dilated, measuring 0.20 cm in diameter. The diameter of the bile duct was relatively unchanged and measured 0.75 cm at its widest point. The previously described choledocholith was no longer visible; it is unclear whether the choledocholith resolved or if it passed into the duodenum. The cystic duct had decreased in size and measured 0.36 cm in thickness (from 0.62 cm). All ultrasound examinations were performed by the same radiology resident under the supervision of two board-certified radiologists. A Histoplasma urine antigen test^b was repeated, and the antigen level had decreased (7.39 ng/mL; no reference interval). The dog was discharged with instructions to continue itraconazole and terbinafine.

A third recheck examination was performed approximately 2 mo later (day 78). A repeat CBC and serum chemistry were unremarkable. A repeat ultrasound revealed unchanged biliary duct dilation. No other abnormalities were detected within the limited examination of the hepatobiliary structures. A Histoplasma urine antigen test^b was repeated, and the antigen level had decreased (3.92 ng/mL; no reference range). The dog was discharged on continued itraconazole and terbinafine.

A fourth recheck examination was performed approximately 2 mo later (day 141). Bloodwork was unremarkable, and a focal ultrasound revealed a progressive decrease in the size of the bile duct. The urine Histoplasma antigen^b had decreased further (0.84 ng/mL). The dog was discharged on continued medical management (itraconazole and terbinafine).

A fifth recheck examination was performed approximately 2 mo later (day 202). Bloodwork was unremarkable apart from a mildly elevated cholesterol, 437 mg/dL (reference interval: 140–360 mg/dL). The cholesterol may have been elevated as a result of dietary factors or associated with biliary disease. The patient was, however, fasted prior to this sample. An ultrasound revealed a further decrease in size of the bile duct to 0.64 cm. A persistent choledocholith, measuring 0.54 cm, was noted. The significance of this choledocholith is unknown; however, it may have contributed to the biliary dilation. The urine Histoplasma antigen^b had decreased further (0.58 ng/mL; no reference range). The dog was discharged on continued medical management. At the time of submission, the dog was doing well at home and was consistently gaining weight (weight: 35.0 kg). The dog was receiving itraconazole (10 mg/kg PO q 12 hr) and terbinafine (35 mg/kg PO q 12 hr).

Discussion

The puppy in this report was diagnosed with disseminated histoplasmosis. Histoplasmosis is considered a diagnosis of young adult dogs, with previous studies documenting a median age at the time of diagnosis of 3 yr old.² Thus, the dog in this report was significantly younger than the typical age at diagnosis. This case report highlights the need to consider disseminated fungal disease as a differential diagnosis, even in young puppies. The authors hypothesize that the puppy’s clinical signs of weight loss, diarrhea, and hematochezia were likely due to infiltration of the small and large intestines by Histoplasma organisms; however, gastrointestinal biopsies were not obtained to confirm this suspicion.

The patient’s thoracic radiographs exhibited abnormalities consistent with intrathoracic histoplasmosis, including a diffuse unstructured interstitial and bronchial pulmonary pattern with sternal lymphadenopathy. Sternal lymphadenopathy is reported in approximately 12% of cases, although tracheobronchial lymphadenopathy in considered more common.² Although not seen in this case, previous case series have documented an alveolar pulmonary pattern and infrequently pleural effusion in cases of canine histoplasmosis.^4,8 Hepatosplenomegaly, as seen in this dog’s abdominal radiographs, is a common feature of histoplasmosis. The radiographic features of canine histoplasmosis are well documented in multiple case series; however, there are fewer reports that document, in detail, abdominal ultrasound findings with canine disseminated histoplasmosis.^2,4 Those reports that do exist document hepatomegaly, abdominal lymphadenopathy, splenomegaly, ascites, and intestinal wall thickening.^1,9 To the authors’ knowledge, this is the first case report of bile duct dilation consistent with a biliary obstruction associated with histoplasmosis in a dog. The initial ultrasonographic changes that occur with biliary obstruction include gallbladder and cystic duct enlargement within 24 hr, followed by dilation of the extrahepatic duct within 48 hr. This is later followed by distension of the intrahepatic bile ducts, which occurs at around day 5 to 7 after obstruction.^10,11 The dog in this case report had evidence of intrahepatic duct dilation, and as such, the extrahepatic biliary obstruction was likely at least 5–7 days old at the time of initial ultrasonographic examination at the authors’ institution. Use of additional imaging modalities such as contrast-enhanced ultrasound, computed tomography with biliary tract–specific contrast, or MRI may have helped to determine the exact cause of the biliary dilation in this case. Biliary obstruction can be caused by luminal obstruction of the cystic or bile duct, obstruction of the major duodenal papilla, or extraluminal compression of the cystic or bile duct. A potential cause of extraluminal biliary duct obstruction in this case would be compression of the biliary tree by the enlarged abdominal lymph nodes seen on ultrasound examination. Similar reports have been documented in human literature in cases of both histoplasmosis and diseases that cause hepatic granulomas such as sarcoidosis.^5–7,12 A congenital anomaly or stricture cannot be fully excluded, although these etiologies would fail to improve with therapy. A further potential explanation would be an intermittent intraluminal obstruction of the bile duct, as with a choledocholith. A choledocholith was noted in two of the six diagnostic ultrasound evaluations. The etiology of choledocholithiasis in dogs is not fully understood, but it is suspected that an imbalance between bile salts and cholesterol causes bile to become more viscous, predisposing to cholelith formation. To the authors’ knowledge, there are no known previous links between histoplasmosis and development of choledocholiths. The hyperechoic material within the gallbladder was likely associated with cholestasis.

Common bloodwork abnormalities seen in biliary obstruction include an elevated total bilirubin, ALP, and cholesterol level.¹³ There are a number of potential explanations for the lack of consistency between the biochemical data and ultrasonographic evidence of biliary obstruction. First, the ALP was greatest at the time of presentation and slowly decreased to within normal limits with treatment. This suggests that the improvement in biochemical data mirrored the ultrasonographic improvement in the biliary tract. Another potential cause for the decrease in ALP is patient aging. A second possible explanation for the lack of classic bloodwork abnormalities is a chronic intermittent obstruction with the choledocholith noted on day 7 and day 202. An intermittent obstruction could lead to biliary dilation without significant increases in biliary enzymes. A third potential explanation is that significant biliary disease can occur in the absence of hyperbilirubinemia, as documented in a recent case series of five dogs with gallbladder rupture and normal serum bilirubin concentrations.¹⁴ A fourth potential explanation is functional cholestasis, which is caused by disruption and dysregulation of bile acid excretory pathways. Endotoxins and inflammatory cytokines can affect sodium-dependent bile uptake and both bile salt–dependent and bile salt–independent bile flow. This disruption results in a functional cholestasis that is not always associated with increases in hepatobiliary enzymes.¹⁵ Thus, proinflammatory cytokines associated with histoplasmosis may cause cholestasis and bile duct enlargement in the absence of elevated hepatobiliary enzymes. A further interesting feature of this case report is that it documents concurrent ultrasonographic improvements in the hepatobiliary system with decreasing urine Histoplasma antigen levels. The H capsulatum antigen assay^b is an enzyme immunoassay that detects terminal (1,6)-α-D-galactofuranose, found in galactomannan, a component of fungal cell walls. It is highly sensitive (89.47%) and specific (100%) for the diagnosis of histoplasmosis in dogs.¹⁶ In this case, repeat Histoplasma antigen levels were performed approximately every 2 mo, based on extrapolation of data from a previous study that documented that antigen levels of Blastomyces decreased during treatment and increased with relapse.¹⁷

Treatment for histoplasmosis involves an azole antifungal drug such as ketoconazole, itraconazole, fluconazole, or posaconazole. Azole antifungal drugs inhibit sterol 14α-demethylase, which is involved in ergosterol synthesis, a key component of the fungal cell membrane. Ketoconazole is more likely to cause toxicity and is less efficacious than itraconazole.¹⁸ Potential adverse effects of itraconazole include vomiting, anorexia, hepatotoxicity, and ulcerative skin lesions. Itraconazole was given every 12 hr, rather than every 24 hr, as this dosing scheme is associated with decreased gastrointestinal signs in human patients.¹⁹ The dose of itraconazole used was greater than 10 mg/kg/day. Therefore, the patient in this study was at an increased risk of hepatotoxicity.²⁰ Frequent repeat serum chemistry panels were performed to monitor for this potential complication. Recent studies have documented no significant difference in survival, remission rate, or disease relapse rate when treated with fluconazole compared with itraconazole.² The dog in this report was also treated with terbinafine. Terbinafine is a synthetic allylamine, and its mechanism of action is to inhibit squalene epoxidase, a substance required for synthesis of fungal ergosterol and lanosterol. Terbinafine was used in this case, as it is inexpensive and blocks fungal ergosterol synthesis via a different mechanism than itraconazole. It has been reported that terbinafine may act synergistically with other antifungal drugs.²¹

Prognosis and length of treatment for canine disseminated histoplasmosis vary, and typically a minimum of 6 mo of antifungal treatment is required, with some animals needing several years of antifungal therapy. The dog in this case report is doing well at home 7 mo after initiation of therapy, with a 95.1% decrease in urine Histoplasma antigen levels. The dog will continue itraconazole and terbinafine until two negative urine antigen titer results occur, spaced 1–2 mo apart. The patient will have a serum chemistry, biliary tract ultrasound, and urine antigen test monitored every 6 mo to monitor for recurrence, following discontinuation of therapy.

Conclusion

Biliary obstruction is a rare complication associated with disseminated histoplasmosis in humans and typically results from extrahepatic compression of the bile duct by enlarged abdominal lymph nodes. To the authors’ knowledge, this case report is the first report of suspected biliary obstruction in a dog with histoplasmosis and may be due to extrahepatic compression of the bile duct with enlarged abdominal lymph nodes, intermittent obstruction from a choledocholith, or an inflammatory state leading to functional cholestasis. Disseminated histoplasmosis should be considered as a differential diagnosis for unexplained biliary dilation or obstruction. Dogs with histoplasmosis should be monitored for this potential complication.