Introduction

Talaromyces (Penicillium) marneffei is an important thermally dimorphic fungus endemic across a narrow band of tropical South and Southeast Asia [1,2,3,4,5]. The fungus was first isolated from the swollen liver of a bamboo rat (Rhizomys sinensis) in Vietnam in 1956, which died spontaneously from the infection [6]. Professor Gabriel Segretain first described the mycology characteristic of T. marneffei, and its pathogenesis to a human when he accidentally pricked his finger with a needle filled with T. marneffei and developed a small nodule at the site of inoculation in 1959 [7, 8]. In 1959, Prof. Segretain formally described the mycology characteristic of T. marneffei and identified it as a novel species. To memory of Hubert Marneffe, director of Pasteur Institute in Indochinae, the fungus was named Penicillium marneffei. His important research was published in Mycopathologia in 1959. Taxonomically, it was classified into the Deuteromycotina, Hyphomycetes, subgenus Penicillium, and Subgen Biverticillium of Halomycetes according to its morphology characteristic [7]. Based on phylogenetic analysis and phenotypic analysis, T. marneffei was re-classified as a member of the family Trichocomaceae, order Eurotiales, class Eurotiomycetes, division Ascomycota in 2011 [9]. This microorganism is the only thermally dimorphic pathogen in over 200 species of Talaromyces. When cultured on SDA medium at 25 °C, the colony is velvety yellow–green or gray–green and producing diffusible red pigment in the culture medium. Microscopic morphology shows septate hyphae with branched or unbranched conidiophores with secondary branches. Phialides grouped in brush-like clusters (penicilli) at the end of conidiophores are arranged in whorls; flask-shaped phialides bearing unbranched chains of smooth or rough, and round to ovoid conidia (2–3 µm diameter) are also seen. When cultured at 37 °C or upon entering the human body, conidia convert to the pathogenic yeast phase and divide by fission [10] (Fig. 1).

Fig. 1
figure 1

Formal characterization of Penicillium marneffei, Segretain, Mycopathologia et Mycologia Applicata, 1959

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Epidemiology and Ecology

Endemic Regions

Talaromyces marneffei is an AIDS-defining illness in South and Southeast Asia, ranking just after tuberculosis and cryptococcosis [1, 11]. The endemic regions of the fungal disease include Northern Thailand, Southern China, Vietnam, Northern India, Hong Kong, and Taiwan [3,4,5, 12,13,14,15,16,17]. Travel-related talaromycosis is being increasingly recognized in non-endemic areas such as Australia, Belgium, France, Germany, Japan, the Netherlands, Oman, Sweden, Switzerland, Togo, the United Kingdom, and the USA [18,19,20,21,22,23,24]. In China, the highest incidence areas of T. marneffei are Guangxi, Guangdong [3, 25]. As the floating population number increases year by year, the disease has traveled far beyond the original epidemic area, and the other 21 provinces and cities have reported cases successively in China [26,27,28] (Fig. 2). Thus, the prevailing evidence strongly indicates an expansion of the known endemic region.

Fig. 2
figure 2

Geographical distribution of Talaromyces marneffei infection

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Host Susceptibility Risk Factors

HIV-infected individuals with CD4 cell count < 100 cells/μL are at particular risk and make up most infections in regions of endemicity [29]. Talaromycosis also has a higher mortality rate than most HIV-associated complications in Southern China [30]. In recent decades, improved treatment of HIV infection with highly active antiretroviral therapy and control of the HIV/AIDS epidemic with other measures has led to a decline in the incidence of T. marneffei infection among HIV-infected patients. An increasing number of T. marneffei infections have been reported among non-HIV-infected patients with impaired cell-mediated immunity [31]. Immunodeficiency due to anti-IFN-γ autoantibodies is an emerging adult-onset immunodeficiency syndrome associated with severe or persistent infections caused by T. marneffei in adult non-HIV-infected patients [32, 33]. The affected patients have high-titer serum neutralizing anti-IFN-γ autoantibodies that inhibit STAT1 phosphorylation and IL-12 production, leading to a severely compromised Th1 response [34]. Talaromycosis was also reported in other secondary immunodeficiency conditions, including autoimmune diseases (systemic lupus erythematosus, connective tissue diseases), cancers, hematological malignancies, solid organ or hematopoietic stem cell transplantation, and use of novel target therapy such as monoclonal antibodies against CD20 and kinase inhibitors [3, 29, 35,36,37,38,39,40,41,42,43] (Fig. 3).

Fig. 3
figure 3

Features of talaromycosis in patients with HIV. Photograph of skin lesions in a patient with talaromycosis (a). Fluorescence-stained touch skin smear showing sausage-like shapes yeast organisms. The arrowhead highlights the midline septum in a dividing yeast cell characteristic of Talaromyces marneffei (b). Morphology of T. marneffei colonies and T. marneffei cells grown at 25 °C and at 37 °C on Sabouraud agar medium (c)

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The proportion of T. marneffei infection is about 6–7.5% in reported cases of talaromycosis in which patient’s age ranged from 3 months to 16 years [25, 44]. Unlike the previous reports with most HIV-infected pediatric cases, recent studies showed most pediatric patients were HIV negative [45]. In recent years, improved immunological investigation and genetic studies identified various forms of immune-related underlying diseases and primary immunodeficiency (PIDs) associated with T. marneffei infection in children, including leukemia, hyper-IgM syndrome, hyper-IgE syndrome, mutations in CYBB, CD40L, or gain-of-function mutation in STAT1/STAT3 pathway, resulting in a functional defect of the IFN-gamma and IL-17 immune response [44, 46,47,48,49]. Hence, the epidemiology of T. marneffei infection transformed in the past three decades. T. marneffei infection is not limited to HIV-infected patients. This suggests that disseminated talaromycosis should be an indicator to look for underlying primary immunodeficiency after excluding secondary causes.

Ecology and Transmission

Subsequent studies showed that four species of bamboo rats (Rhizomys sinensis, R. pruinosis, R. sumatrensis, and Cannomys badius) were important enzootic reservoirs of T. marneffei [7, 8, 50,51,52,53]. 100% of the R. pruinosus rats captured in Guangxi and Guangdong Provinces in Southern China were positive for T. marneffei [54, 55]. Multi-locus genotypes show that T. marneffei isolates from humans are similar or identical to those infecting bamboo rats [55]. T. marneffei DNA has also been detected by PCR in nasal swabs of 13% of outdoor dogs in Thailand [56]. However, there is no evidence of direct animal-to-human transmission. T. marneffei has been isolated from bamboo rat feces and soil samples within or surrounding wild bamboo rat’s burrow, while no detection in the artificial farm of bamboo rats. Collectively, these epidemiological data suggest humans and bamboo rats are exposed to an as-yet-undiscovered common environment reservoir of infection, in which bamboo rats may be exploited by T. marneffei to expand its biomass and biogeography [54, 55].

History of exposure to or consumption of bamboo rats was not a risk factor for infection; instead, agricultural exposure to the soil during the rainy season [57, 58] serves as an important risk factor for T. marneffei infection and can be predicted by humidity levels [59]. So, infections in people probably occur through inhalation of T. marneffei conidia. According to the literature, the incubation periods of infection with T. marneffei are highly variable, which could be 1–3 weeks in acute disease or reactivation of a latent infection several years after exposure [21, 59].

Clinical Manifestations

Patients infected with T. marneffei would present various degrees of severity depending on the underlying immunocompromising condition and the timing of diagnosis. Generally, talaromycosis presented either as a local or disseminated infection. Local infections were rare, only involving single organs, such as corneas or lungs [21, 60]. Clinically, they only show local symptoms without systemic involvement. Disseminated infections were usually associated with HIV/AIDS patients or immunosuppressed HIV-negative patients with the pathogen isolated from more than one body site (noncontiguous) or from blood or bone marrow. The symptoms and signs of disseminated T. marneffei infection are atypical, including fever, weight loss, fatigue, hepatosplenomegaly, lymphadenopathy, respiratory, and gastrointestinal abnormalities. Central nervous system involvement is an emerging syndrome in T. marneffei infected HIV-positive patients, presented with an acute onset of altered mental status with confusion, agitation, or depressed consciousness [61, 62].

Skin lesions can be a single or first-episode symptom of T. marneffei infection [3, 31]. It often becomes the first sign to attract attention in disseminated cases and has a strong suggestive role in the diagnosis of this disease. Characteristic lesions are papules with central necrosis, predominantly on the head and upper chest, which presented in 70% of HIV-infected and 40% of non-HIV-infected patients and aid in the rapid diagnosis [3, 31]. In addition, papules, pustules, nodules, subcutaneous abscesses, cysts or ulcers can also occur. Reactive rashes, such as Sweet’s syndrome, can occur in non-HIV patients who suffered from adult-onset immunodeficiency syndrome due to anti-interferon-gamma autoantibodies. T. marneffei-associated immune reconstitution inflammatory syndrome (IRIS) has been reported as unmasking IRIS in patients starting ART. Skin lesions can be atypical, including erythematous nodules, verrucous lesions, or erythematous plaques [63].

In the literature, compared to HIV-infected patients, HIV-negative patients with T. marneffei infection have a longer diagnostic interval, a higher percentage of dyspnea and were significantly older, less likely to have fever, splenomegaly, and umbilicated skin lesions, and more likely to have Sweet’s syndrome and osteoarticular lesions. The non-HIV-infected patients also had higher leukocyte, CD4 lymphocyte and platelet counts, and lower alanine transaminase level and blood culture-positive rate [27, 29].

Diagnosis

Traditionally, the diagnosis of T. marneffei infection depends on identifying the fungus in clinical specimens by microscopy and culture. Microscopic examination of a direct smear of bone marrow aspirates, touch smear of skin biopsy, or lymph node biopsy specimens can lead to rapid presumptive diagnosis [1, 10, 64]. In patients with heavy fungemia, the organisms may be seen on the peripheral blood smear [65]. T. marneffei can be observed in histopathological sections [13, 66,67,68], preferably stained by Grocott methenamine silver or periodic acid-Schiff stain. T. marneffei yeast-like cells distributed within and outside macrophages and histiocytes, 2–3 µm in diameter with oval, round, elongated, sausage-like shapes, and divided by fission. The cross-wall formation can differentiate yeast cells of T. marneffei from those of Histoplasma capsulatum, which also appear as intracellular yeasts. The histopathological characteristic of T. marneffei infected tissues manifested as granulomatous, suppurative reaction, and non-reactive necrosis.

Mycological culture of all tissues or body fluids is the gold standard method for the diagnosis of T. marneffei infection. The high sensitivity samples are bone marrow (100%), skin biopsy (90%), and blood (76%), respectively [69]. Identification of T. marneffei is based upon the morphology of the colony, the organism’s microscopic morphology, and its mold-to-yeast conversion when moved from 25 °C (to) 37 °C [70]. Culture can be slow (3–14 days), resulting in diagnostic delay and raised mortality, particularly in patients without skin lesions [71].

The development of non-culture-based assays for rapid detection of T. marneffei infection has the potential to improve treatment outcomes. The commercial β-D-glucan assays or GM test (Aspergillus galactomannan antigen detection test) can be useful as a screening tool and adjunct to diagnosis [72, 73]. PCR-based assays such as nested PCR, TaqMan real-time PCR, targeting the ribosomal DNA (ITS1-5.8S-ITS2, 18S), or MP1 gene have been developed to detect T. marneffei in clinical samples including whole blood, plasma or paraffin-embedded tissue [74,75,76,77]. Evaluation of the tests revealed a high diagnostic specificity of the assay (100%), while diagnostic sensitivity ranged from 67 to 77%, respectively. These assays provide useful tools for the rapid diagnosis of T. marneffei infection. Recently, a monoclonal-based immunoassay has been used to detected T. marneffei Mp1p antigen in patient plasma with a sensitivity of 75% (15/20) and specificity of 99.4% (537/540). The assay detected Mp1p antigenemia in 9.4% of over 8000 HIV-infected patients in outpatient clinics, offering another tool for rapid diagnosis of T. marneffei infection [78, 79]. However, these methods require the physician to suspect the pathogen before examination, which might limit the application clinically. Recently, the next generation sequencing (NGS) based on metagenomics has been successfully applied in the diagnosis of disseminated T.marneffei infection, which does not depend upon prior assumptions from the physicians and provides a new technique for rapid etiological diagnosis [80]. NGS availability is limited especially in the resource-limited settings. Overall, it might be prudent to use a combination of several methods to achieve the highest diagnostic yield [81].

In Vitro Drug Susceptibility, Therapeutics Strategies, and Prevention

In Vitro Drug Susceptibility

At present, no existing guidelines were available for susceptibility testing of Talaromyces marneffei in vitro. However, studies using different methods showed that posaconazole, voriconazole, itraconazole, and other azole drugs had high activity against T. marneffei (except fluconazole), Amphotericin B showed intermediate antifungal activity, whereas echinocandins showed intermediate to resistance [82,83,84,85]. Besides, traditional herb, berberine had antifungal activity against T. marneffei, especially in combination with antifungal agents in vitro [86].

Therapeutic Strategies in Talaromycosis With or Without HIV-Infected

Amphotericin B deoxycholate (D-AmB) is still the first-line initial antifungal treatment for severe T. marneffei infection. International guidelines recommend for talaromycosis in HIV-infected patients is: D-AmB; 0.6–1.0 mg/kg per day for 2 weeks, followed by itraconazole 400 mg per day for 10 weeks, then with itraconazole 200 mg per day as secondary prophylaxis until CD4 counts are higher than 100 cells per µL for at least 6 months [87, 88]. Liposomal amphotericin B (L-AmB) at 3–5 mg/kg per day is effective and better tolerated than D-AmB. However, the drug is not available in a resource-limited situation [89]. Voriconazole is an effective therapeutic option for disseminated talaromycosis, given 6 mg/kg BID for one day, 4 mg/kg BID for 10–14 days, followed by oral voriconazole 200 mg BID for 12 weeks in HIV patients. It has been reported that voriconazole is effective in the treatment of patients who did not respond to amphotericin B initial therapy [90, 91]. Considering it can be administered by the oral route or by using intravenous-to-oral step-down therapy, voriconazole has the potential to be more convenient than the currently recommended treatment regimen.

In a multicenter open-label, non-inferiority trial comparing itraconazole (600 mg per day for 3 days, followed by 400 mg per day, for 11 days) with standard D-AmB initial therapy in HIV-infected talaromycosis patients reported mortality after 6 months is 11.3% and 21.0%, respectively. The study indicated D-AmB was superior to itraconazole as initial treatment for talaromycosis [92].

For prevention, HIV-infected people living or traveling in endemic areas, primary prophylactic treatment with itraconazole is recommended when the CD4 cell count is less than 100 mm3. Itraconazole can be taken orally for 200 mg once a day until CD4 cell count > 100 mm3 and more than 6 months [86].

At present, the standard recommendation regarding the appropriate duration of treatment and prophylaxis of T. marneffei among HIV-uninfected patients is unavailable. In the literature, the duration of treatment was significantly longer in HIV-uninfected patients compared to HIV-infected patients, and some cases may require lifelong treatment.

Special Population: Treatment of Non-HIV T. marneffei Infection in Children

Clinical experience in the treatment of non-HIV T. marneffei pediatric patients is limited. Studies showed when treated with voriconazole 7 mg/kg twice per day, for at least 12 days, followed by oral for at least 13 weeks, 70–80% children had a complete response to therapy at primary and long-term follow-up assessment [90, 93]. No adverse events were recorded during and after the treatment. D-AmB 1 mg/kg/d has been used for T. marneffei infected children, but the nephrotoxic side effects are significant. Regarding the dose-dependent nephrotoxicity, and electrolyte imbalance related to D-AmB treatment, pharmacokinetics and pharmacodynamics study has been used to evaluate the association between D-AmB exposure and response in the treatment of talaromycosis [94].

Despite antifungal therapy, case fatality rates among adult patients with or without HIV-infected are 20%, 29.4%, respectively. Whereas, mortality in the pediatric patients with talaromycosis could be up to 55% [42], higher than the adult patients, due to underlying immunodeficiency and delated treatment.

Bibliometrics of Talaromycosis and T. marneffei

We used a bibliometric analysis to highlight clinical and research trends and provide an impetus for future investigations [95]. Since the first description of experimental infection from France, the scientific literature on talaromycosis and T. marneffei has grown substantially with most contributions originating from investigators working in the endemic areas. With the search term “marneffei,” we found ~ 1000 articles in Google Scholar, 1070 articles in Scopus, and 942 articles in Web of Science (last accessed on November 19, 2019). We highlight well-cited articles on clinical disease, diagnosis and provide a summary of the highest-cited articles published in Mycopathologia since the original discovery by Prof. Segraetain (Tables 1 and 2). Late eighties–early nineties saw rapid advances in defining the clinical spectrum of talaromycosis (penicilliosis) especially in settings of HIV-positive patients [12, 13, 35, 58, 69, 96, 97]. The state-of-the-art serodiagnostic tests and clinical trials of amphotericin B and itraconazole were reported [11, 88, 98]. After Prof. Segretain’ initial description, Mycopathologia coverage described improved serological tests for the detection of P. marneffei antigenemia, antibodies, and exoantigens [7, 99, 100]. The test innovations also included antifungal susceptibility testing and PCR from paraffin-embedded tissues [76, 101]. Mycopathologia remained an important source of publications on the natural reservoir of the pathogen in wild rodents, and endemic and travel-related penicillosis [3, 4, 37, 52, 102,103,104]. Two notable contributions described an ex vivo host–pathogen interaction model and a novel gene that mediates oxidative stress in P. marneffei [105, 106].

Table 1 Well-cited clinical articles on Penicillium marneffei
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Table 2 Well-cited Mycopathologia articles on Penicillium marneffei
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Summary and Conclusions

Talaromycosis remains a significant infectious complication for patients with HIV/AIDS and other immune defects. The disease is being recognized with an increasing frequency well beyond the original endemic areas. Therefore, there is an urgent need for the development of rapid and affordable point-of-care diagnostic tests. More clinical studies are also needed to define the best therapeutic options for the management of talaromycosis patients.