A monoclonal antibody-based urine Histoplasma antigen enzyme immunoassay (IMMY®) for the diagnosis of histoplasmosis in cats

Clinical utility of fungal culture and antifungal susceptibility in cats and dogs with histoplasmosis

BACKGROUND

Culture can be used for diagnosis and antifungal susceptibility testing in animals with fungal infections. Limited information is available regarding the diagnostic performance of culture and the susceptibility patterns of Histoplasma spp. isolates.

HYPOTHESIS/OBJECTIVES

Describe the clinical utility of culture and the susceptibility patterns of Histoplasma spp. isolates causing histoplasmosis in cats and dogs.

ANIMALS

Seventy-one client-owned animals, including 33 cats and 19 dogs with proven or probable histoplasmosis.

METHODS

Culture was attempted from tissue or fluid samples. Diagnostic performance of culture, cytopathology, and antigen detection were compared with final diagnosis. Susceptibility to antifungal agents was determined for a subset (11 from dogs, 9 from cats) of culture isolates.

RESULTS

Culture had a diagnostic sensitivity of 17/33 (52%; 95% confidence interval [CI], 34%-69%) and 15/19 (79%; 95% CI, 61%-97%) and specificity of 6/6 (100%; 95% CI, 54%-100%) and 10/10 (100%; 95% CI, 69%-100%) in cats and dogs, respectively. Culture was not positive in any animal in which cytopathology and antigen testing were negative. Target drug exposure (area under the concentration curve [AUC]/minimum inhibitory concentration [MIC] >25) should be easily achieved for all isolates for itraconazole, voriconazole, or posaconazole. Five of 20 (25%) isolates had fluconazole MIC ≥32 μg/mL and achieving target drug exposure is unlikely.

CONCLUSIONS AND CLINICAL IMPORTANCE

Fungal culture did not improve diagnostic sensitivity when used with cytopathology and antigen detection. Susceptibility testing might help identify isolates for which fluconazole is less likely to be effective.

Invasive fungal rhinitis with adnexal involvement caused by Histoplasma capsulatum in a cat from a non-enzootic location

Case summary

This report describes a cat with a rare form of histoplasmosis: invasive rhinitis with adnexal involvement, mimicking disease more commonly caused by cryptococcosis or aspergillosis. This case is especially noteworthy as it was from an area where histoplasmosis is not enzootic.

Relevance and novel information

Invasive fungal rhinitis causes significant morbidity in cats. Diagnostic investigation of more common pathogens includes detection of fungal antigen (Cryptococcus) or antifungal antibodies (Aspergillus). This case demonstrates that histoplasmosis can present as chronic nasal disease in cats. Histoplasma antigen testing provides a non-invasive diagnostic option. Moreover, this case serves as a reminder that histoplasmosis can affect cats anywhere, even in non-enzootic areas.

Diagnosis of histoplasmosis: current status and perspectives

Histoplasmosis is a worldwide-distributed systemic mycosis caused by the dimorphic fungus Histoplasma capsulatum. Its clinical manifestations range from subclinical or mild respiratory illness to progressive disseminated histoplasmosis (PDH), a life-threatening disease, whose accurate diagnosis is still challenging and limited in many countries, where this disease is highly endemic. In this regard, Histoplasma antigen testing is now included in the WHO Essential Diagnostics List. The final diagnosis of histoplasmosis is established by culture and/or visualization of the yeast cells by cytology or histopathology using specific stains. However, both procedures have limited sensitivity to detect the disease and cultures are time-consuming. Antibody detection assays are effective for the subacute and chronic clinical forms of histoplasmosis. However, their sensitivity is low in the immunocompromised host. Several molecular "in-house" tests were also developed and showed promising results, but none of these tests are commercially available and their standardization and validation are still pending. Antigen detection assays have high sensitivity in PDH cases and are of great value for the follow-up of patients with histoplasmosis; however, cross-reactivity with other related fungi are common. In addition, this assay is expensive and only performed in few laboratories. Novel protein antigen candidates have been recently identified and produced by DNA-recombinant techniques in order to obtain standardized and specific reagents for the diagnosis of histoplasmosis, as opposed to the unspecific antigens or crude extracts currently used. This review describes the currently available assays, highlighting their strengths and limitations and reports the latest approaches to achieve reliable and rapid diagnostic tests for histoplasmosis. KEY POINTS: • PDH causes thousands of deaths per year globally. • Rapid accurate diagnosis of PDH is unfeasible in many regions. • Fast, accurate, and low-cost diagnostic alternatives are currently under development.

Evaluation of a novel monoclonal antibody-based enzyme immunoassay for detection of Histoplasma antigen in urine of dogs

Background

Commercially available, noninvasive testing options for histoplasmosis are limited outside of the United States.

Objectives

To describe the diagnostic performance of a novel Histoplasma antigen enzyme immunoassay (IM EIA) for the diagnosis of histoplasmosis in dogs.

Animals

Twenty dogs with histoplasmosis, 79 dogs without histoplasmosis, and 11 unclassified dogs providing 202 urine samples.

Methods

This a prospective study using stored urine samples. Samples were analyzed with the IM EIA and with the commercially available Histoplasma antigen EIA (MV EIA). Dogs were classified based on final proven diagnosis and performance of the IM EIA was described and compared with the MiraVista enzyme immunoassay (MV EIA).

Results

The diagnostic sensitivity (DSe), specificity (DSp), and accuracy (DAc) of the IM EIA were 70% (51%‐89%), 99% (97%‐100%), and 93% (81%‐100%), respectively. The DSe, DSp, and DAc for the MV EIA were 95% (85%‐100%), 99% (97%‐100%), and 98% (95%‐100%), respectively. The area under the receiver operator characteristic curve was significantly smaller for IM EIA (0.87) as compared with MV EIA (0.97, P = .03). This was primarily due to 6 false negative IM EIA results, 4 from dogs with disease localized to the gastrointestinal tract. The MV EIA was positive in 5/6 of these dogs.

Conclusions and Clinical Importance

The IM EIA might be useful for the diagnosis of disseminated histoplasmosis in dogs, but clinical usefulness will be limited in dogs with histoplasmosis localized to the GI tract.

Timely Diagnosis of Histoplasmosis in Non-endemic Countries: A Laboratory Challenge

Human histoplasmosis is a fungal infection caused by the inhalation of microconidia of the thermally dimorphic fungi Histoplasma capsulatum. Autochthonous cases of histoplasmosis have been diagnosed in almost every country, but it is considered an endemic infection in specific areas of the world. Many of them are popular travel destinations or the source of migratory movements. Thus, the vast majority of the registered cases in non-endemic countries are imported. They correspond to people having been exposed to the fungus in endemic locations as immigrants, expatriates, transient workers or tourists, with reported cases also associated to organ donation. Misdiagnosis and delays in initiation of treatment are not uncommon in cases of imported histoplasmosis. They are associated to high fatality-rates specially in patients with compromised cellular immunity in which progressive disseminated forms develop. The diagnosis of this infection in non-endemic countries is hampered by the lack of clinical suspicion and a dearth of available diagnostic tools adequate to offer rapid and accurate results. Non-culture-based assays such as nucleic-acid amplification tests present as a suitable alternative in this situation, offering improved sensitivity and specificity, shortened turnaround time, and increased biosafety by avoiding culture manipulation. In non-endemic regions, molecular techniques are being used mainly in laboratories from countries that have registered an increase in the incidence of imported cases. However, the number of published techniques is limited and lack consensus. Efforts are currently under way to standardize nucleic acid amplification-based techniques for its implementation in areas registering a rising number of imported cases.

A monoclonal antibody-based urine Histoplasma antigen enzyme immunoassay (IMMY®) for the diagnosis of histoplasmosis in cats

BACKGROUND

An in-house Histoplasma urine antigen test for cats might be desirable in certain situations.

OBJECTIVE

To validate and compare the diagnostic performance of a monoclonal antibody-based IMMY urine Histoplasma antigen enzyme immunoassay (IMMY EIA) to the commercially available urine Histoplasma antigen enzyme immunoassay (MiraVista Diagnostics, MV EIA).

ANIMALS

One hundred ninety-three urine samples from 105 client-owned and purpose-bred research cats.

METHODS

Cats were classified as Histoplasma positive or negative based on diagnostic investigation. The IMMY EIA and MV EIA were performed on all urine samples. Correlation and agreement between the assays were determined. Diagnostic performance was determined and compared between assays.

RESULTS

The IMMY EIA, with a 0.25 ng/mL diagnostic cutoff, provided a diagnostic sensitivity (DSe), diagnostic specificity (DSp), and diagnostic accuracy (DAc) of 89% (95% confidence interval [CI]; 73%-97%), 80% (67%-89%), and 83% (74%-90%), respectively. The IMMY EIA, with a 1.1 ng/mL diagnostic cutoff, provided a DSe, DSp, and DAc of 77% (95% CI 60%-90%), 97% (88%-100%), and 89% (81%-95%), respectively. The MV EIA provided a DSe, DSp, and DAc of 94% (95% CI 81%-99%), 97% (89%-100%), and 96% (90%-99%), respectively. Moderate overall agreement was found between MV EIA and IMMY EIA using the 0.25 ng/mL cut-off (к = 0.44; 95% CI 0.31-0.57) and the 1.1 ng/mL cut-off (к = 0.43, 95% CI, 0.31-0.56).

CONCLUSIONS AND CLINICAL IMPORTANCE

The IMMY EIA might be useful as a diagnostic test for histoplasmosis in cats. Further modifications of the IMMY EIA are required to achieve the diagnostic performance of the MV EIA.