Low Antigen Titre Disseminated Cryptococcosis in Immunocompromised Hosts: Two Challenging Case Reports

Introduction

Cryptococcosis is a systemic fungal infection caused by the invasive fungus Cryptococcus. It primarily affects immunocompromised patients but can also occasionally occur in immunocompetent individuals. The infection often manifests with pulmonary involvement and central nervous system (CNS) infections, such as meningitis. Fungal spores are typically inhaled and usually cause few symptoms in healthy individuals. In immunocompromised patients, however, the infection can spread through the bloodstream, leading to disseminated infection characterized by a positive blood culture or cultures from at least 2 separate sites. Disseminated cryptococcosis is associated with significant morbidity and mortality. Bloodstream infections without CNS involvement are rare and underreported, making them a critical area of clinical focus [1,2].

This infection most commonly occurs in individuals with conditions such as HIV, diabetes, chronic renal disease, chronic liver disease, prolonged steroid use, and organ transplantation. In the United States, cryptococcal antigenemia is observed in 2.9% of HIV-infected patients with CD4 counts <100 cells/μL and in 4.3% of those with CD4 counts <50 cells/μL [1]. Additionally, 20% to 60% of cryptococcosis cases in HIV-negative patients occur in organ transplant recipients [3]. These statistics highlight the high prevalence of cryptococcal infections among immunocompromised populations, yet bloodstream infections without CNS involvement remain an atypical presentation. Such cases challenge traditional diagnostic and management strategies, as clinicians often prioritize CNS symptoms when suspecting cryptococcal infections.

This report presents 2 cases of cryptococcal bloodstream infection without CNS involvement, underscoring the diagnostic challenges and clinical implications of these atypical presentations. These cases highlight the importance of vigilance in recognizing disseminated cryptococcal infections and suggest the need for adjustments in antifungal therapy to effectively manage these unusual presentations.

Case Reports

CASE 1:

A 43-year-old man with a history of end-stage renal disease secondary to type 2 diabetes mellitus underwent an unrelated kidney transplant in 2022 and was maintained on tacrolimus monotherapy for immunosuppression. Two years after transplant, he presented with acute kidney injury and generalized fatigue. Because kidney rejection was suspected, he was empirically treated with intravenous (i.v.) methylprednisolone; however, biopsy findings ruled out rejection, prompting an infectious workup. Urine cultures were negative for cytomegalovirus (CMV) and BK polyomavirus virus, and a respiratory panel was also unremarkable. A chest computed tomography scan revealed multifocal bronchopneumonia with scattered lung nodules, leading to a bronchoalveolar lavage (BAL), which returned positive for Cryptococcus (titers: 1: 80). Cerebrospinal fluid (CSF) analysis showed no cryptococcal antigen or fungal growth. A diagnosis of pulmonary cryptococcosis was made, and the patient was started on oral fluconazole 200 mg daily, planned for a 12-month course.

Ten days after discharge, the patient returned with minimal symptoms of generalized fatigue; however, blood cultures grew Cryptococcus. Repeat investigations, including echocardiography, showed no evidence of fungal endocarditis, and lumbar puncture again demonstrated no cryptococcal growth. Based on positive blood cultures and BAL results, the patient was given a diagnosis of disseminated cryptococcosis. Treatment was escalated to i.v. amphotericin-B (4 mg/kg) and oral flucytosine (25 mg/kg), leading to negative repeat blood cultures and CSF. Amphotericin-B was subsequently discontinued, and fluconazole therapy was adjusted to 1200 mg daily for 10 days, followed by 800 mg daily for 8 weeks. Flucytosine (500 mg, 4 times daily) was continued for 9 days.

At the 1-month follow-up, the patient showed clinical improvement and was advised to continue fluconazole therapy for 1 year to prevent recurrence.

CASE 2:

A 44-year-old man with a known diagnosis of HIV and a history of non-adherence to antiretroviral therapy presented with worsening shortness of breath, high fever, and chills for 2 weeks. One week prior to admission, he had visited the Emergency Department with similar concerns and was discharged on levofloxacin. On current admission, the patient had hypoxia, prompting hospitalization.

On evaluation, laboratory investigations revealed a CD4 count of 9 cells/μL (reference range: 500–1500 cells/μL) and an HIV viral load in the millions. Chest X-ray showed diffuse alveolar and interstitial infiltrates, suggestive of Pneumocystis jirovecii pneumonia (PJP). He was started on high-dose trimethoprim-sulfamethoxazole (Bactrim) at 5 mg/kg every 8 h and prednisolone, with a planned 21-day course. Given the risk of opportunistic infections, prophylactic medications, including fluconazole, azithromycin, and cefepime, were initiated. Due to his resistance profile, his antiretroviral therapy was switched to Symtuza (darunavir, cobicistat, emtricitabine, tenofovir alafenamide). However, Symtuza was held for 1 week, due to concerns about immune reconstitution inflammatory syndrome, and restarted thereafter.

During his hospitalization, serum cryptococcal antigen returned positive, with a titer of 1: 80, prompting lumbar puncture to assess for CNS involvement. CSF analysis initially showed no growth, but a prozone effect was later ruled out through sample dilution. Blood cultures subsequently grew Cryptococcus, increasing suspicion for cryptococcal pneumonia in the setting of disseminated cryptococcosis. Simultaneously, beta-D-glucan >500 pg/mL (reference range: <60 pg/mL) and lactate dehydrogenase level of 422 U/L (reference range: 140–280 U/L) strongly supported PJP. Despite the absence of fungal growth on BAL, the BAL polymerase chain reaction was positive for Pneumocystis jirovecii.

The patient’s respiratory status deteriorated, necessitating intubation. A chest computed tomography scan demonstrated rapidly progressive multifocal pneumonitis and scattered lung nodules, complicated by pneumomediastinum. These findings, along with worsening hypoxia, raised concerns for superimposed cryptococcal pneumonia and possible immune reconstitution inflammatory syndrome. The patient was treated aggressively with amphotericin B, initially dosed at 5 mg/kg and increased to 10 mg/kg. However, amphotericin B was discontinued due to worsening renal function, and high-dose fluconazole (1200 mg daily) was continued. Concurrently, valganciclovir 450 mg daily was initiated for CMV viremia.

The hospital course was complicated by acute respiratory distress syndrome, requiring extracorporeal membrane oxygenation support. After a month of intensive multidisciplinary management, including treatment for PJP, cryptococcosis, and CMV, the patient’s condition stabilized. He was discharged on prednisone (tapered), trimethoprim-sulfamethoxazole (Bactrim DS) 1 tablet daily for PJP prophylaxis, fluconazole 400 mg daily for cryptococcal maintenance, valganciclovir 450 mg daily for CMV management, and Symtuza for HIV therapy.

At the 1-month follow-up, the patient showed significant clinical improvement, with stable respiratory status and improved overall condition. He was extensively counselled on strict medication adherence to prevent opportunistic infections and ensure effective HIV management.

Discussion

Here, we present 2 cases of disseminated cryptococcosis: one in a kidney transplant recipient on immunosuppressive therapy and the other in an HIV-positive patient who was non-adherent to treatment – both were at high risk for this infection. Despite being at high risk for opportunistic fungal infections, it is interesting to note that although both patients had a positive serum titer for cryptococcal antigen (1: 80), lumbar punctures excluded CNS involvement in both cases. Initially, both patients started treatment with fluconazole based on findings from BAL, clinical imaging, and high clinical suspicion of pulmonary cryptococcosis. Our first patient had a positive BAL and blood culture for Cryptococcus (Figure 1), while the second patient, who had HIV, had a negative BAL but a positive blood culture (Figure 2). As blood cultures later grew Cryptococcus, their diagnosis shifted to disseminated cryptococcosis, and their treatment changed from oral fluconazole to i.v. amphotericin-B.

According to a 2023 study published in Open Forum Infectious Diseases, disseminated cryptococcosis affects 5% to 8% of HIV-positive individuals who are not on effective antiretroviral therapy. Similar rates are observed among non-HIV immunocompromised patients, including organ transplant recipients and patients with cancer; however, these rates can vary depending on the specific illness and geographic location [4]. A prospective study of 235 cases of proven Cryptococcus species infection found that all cases were due to Cryptococcus neoformans (52 in solid organ transplant recipients, 107 in HIV patients, and 76 in neither). One of the study’s key findings was that solid organ transplant recipients receiving calcineurin inhibitors were less likely to have CNS involvement in cryptococcal infection (40.1% vs 66.7%), likely due to the interesting antifungal activity of calcineurin inhibitors in vitro. Tacrolimus, in particular, showed more promising antifungal activity than did cyclosporin [5–7]. In our patients, one had recovered from tacrolimus, while the second was HIV-positive and noncompliant with their medication. Despite these conditions, both patients were negative for CNS involvement.

In immunocompromised patients, meningoencephalitis is the most common manifestation, even though the lungs are the primary entry site for Cryptococcus. Interestingly, neither of our patients experienced symptoms of meningoencephalitis, despite Cryptococcus growth in their blood. Very few cases have been reported in the literature where blood cultures are positive for Cryptococcus without evidence of the fungus in CSF cultures [6–9]. A retrospective cohort study by Jean et al found that of 52 patients diagnosed with and treated for cryptococcemia, the major predisposing conditions were HIV (24/52, 46%), immunosuppressive therapy (12/52, 23%), and decompensated liver cirrhosis (11/52, 21%). CSF culture showed meningeal involvement in 32 (84%) of the 38 patients who had a lumbar puncture; 6 (14%) had negative CSF cultures, similar to our patients [8]. A case reported by Mishra et al also had a positive blood culture with negative serum cryptococcal antigen (CrAg) tests. The patient, a liver transplant recipient on immunosuppressive therapy being treated for Graft-versus-Host-Disease, developed a fever and grew C. neoformans on blood culture and Sabouraud dextrose agar. Due to thrombocytopenia, CSF analysis was not performed, and the patient’s CNS involvement was unclear, although magnetic resonance imaging of the brain was unremarkable. This patient was treated with amphotericin B and flucytosine for disseminated cryptococcosis, similar to our patient [9].

The CrAg lateral flow assay is highly sensitive and specific (approximately 95%). However, false-negative results can occur due to hemolysis or the postzone effect, which happens when antigen and antibody concentrations are not balanced. To minimize this, additional dilutions of the specimen are performed to achieve the zone of equivalency. Despite these measures, the CrAg test can still be negative in cases of low fungal burden or when the Cryptococcus species lacks the capsular antigen detected by the test [9]. We examined a diluted CSF sample and found no evidence of Cryptococcus in the CNS.

In cases in which patients have CrAg-positive results but lack symptoms of meningitis, the challenge lies in interpreting negative CNS findings despite positive blood cultures. World Health Organization guidelines recommend a preemptive course of fluconazole (800 mg/day for 2 weeks, then 400 mg/day for 8 weeks, and finally 200 mg/day pending immune reconstitution with antiretroviral therapy) for CrAg-positive patients. However, studies, including that of Wake et al, show that CrAg-positive patients remain at risk for cryptococcal meningitis and related mortality. Their research indicates that a CrAg titer of 1: 160 is associated with CNS involvement, suggesting that such patients should be treated as if they have meningitis [10]. In our cases, both patients had CrAg titers of 1: 80 in BAL and serum, respectively. Despite negative lumbar punctures for CNS infection, positive blood cultures for C. neoformans indicated disseminated infection, prompting a change in treatment strategy. This underscores the importance of obtaining blood cultures in such cases, to rule out dissemination, which can significantly affect treatment plans. It highlights the need for a comprehensive approach to CrAg-positive patients, emphasizing closer monitoring and potentially more intensive antifungal therapy, even when typical symptoms of meningitis are absent.

Our first patient was initially started on fluconazole after a negative lumbar puncture. However, based on the European Confederation for Medical Mycology/International Society for Human and Animal Mycology/American Society for Microbiology Global Guidelines for the Diagnosis and Management of Cryptococcosis [11], the treatment was escalated to standard induction therapy with amphotericin B and flucytosine. This decision was driven by the positive blood culture for C. neoformans, indicating disseminated infection despite the absence of CNS involvement. Treatment then progressed to consolidation therapy with high-dose fluconazole, followed by maintenance therapy with low-dose fluconazole.

In contrast, our second patient, who was HIV-positive, presented with severe respiratory symptoms. Although the BAL was suggestive of PJP and negative for Cryptococcus, the severity of his presentation warranted high-dose amphotericin B to cover possible cryptococcal pneumonia, alongside complicated pneumonia. A similar case reported by Subahi et al highlighted the potential for concurrent opportunistic infections, including PJP, non-tuberculous mycobacteria, and C. neoformans, in late-stage HIV infection [12]. This emphasizes the importance of maintaining a high index of suspicion for systemic mycoses and multiple opportunistic infections in immunocompromised patients, ensuring timely and appropriate management.

Conclusions

These cases highlight rare presentations of disseminated cryptococcal infection without CNS involvement, characterized by low serum CrAg titers (1: 80) and positive blood cultures. They emphasize the importance of considering disseminated disease even when serum antigen levels are low and CNS symptoms are absent. Positive blood cultures were key to diagnosis and guided the need for aggressive antifungal therapy. These findings support greater clinical vigilance and the role of blood cultures in managing atypical cryptococcosis presentations. Factors such as strain variations, low fungal burden, and unique immune responses can contribute to these unusual findings. Future research should explore these mechanisms, evaluate current CrAg titer thresholds, and refine diagnostic and treatment protocols, to improve outcomes and inform clinical guidelines.