Hemagglutination Test for Rapid Serodiagnosis of Human Pythiosis

Abstract

Human pythiosis is an emerging, life-threatening infectious disease, caused by the oomycete Pythium insidiosum. Thailand is an area where human pythiosis is endemic and the genetic blood disorder thalassemia is a predisposing factor. Patients with pythiosis present with arterial occlusions of the lower extremities, corneal ulcers, or chronic cutaneous infections. Diagnosis relies on time-consuming, relatively insensitive tests such as culture identification and immunodiffusion assay. Most patients undergo surgical removal of infected organs, and many die from the infection. Delayed diagnosis results in a poor prognosis. Here, we describe a hemagglutination (HA) test for rapid diagnosis of human pythiosis. Sheep red blood cells were coated with P. insidiosum protein extract and used in duplicated detection assays using serum samples from 33 patients with vascular (n = 27), cutaneous (n = 2), or ocular (n = 4) pythiosis and serum samples from 289 control patients with other infectious diseases (n = 77), with highly positive antinuclear antibody (n = 5), with thalassemia (n = 21), or with no known disorder (i.e., healthy blood donors) (n = 186). Based on receiver-operating characteristic analysis, a serum titer of 1:160 was selected as the cutoff point for the HA test. Serum samples that generated HA at the cutoff titer were read as positive, while samples that did not were read as negative. Positive results were obtained with the serum samples of all patients with vascular and cutaneous pythiosis and with two serum samples from the control group. Negative results were obtained with serum samples from all ocular pythiosis patients and the 287 remaining serum samples from the control group. Sensitivity and specificity of the HA were 88% and 99%, respectively. In conclusion, the HA test for detection of anti-Pythium antibodies is a simple, rapid, and reliable test for serodiagnosis of vascular and cutaneous pythiosis.

Pythiosis is can be a fatal infectious disease of humans and animals living in tropical and subtropical countries (2, 9, 15, 16, 18, 27, 30). The causative agent is the fungus-like organism Pythium insidiosum, which is a member of the family Pythiaceae, order Pythiales, class Oomycetes, and the phylum Pseudofungi in the kingdom Chromista (Stramenopila) (5, 9, 18, 30). Naturally, P. insidiosum inhabits swampy areas, where it is present in the form of mycelium or biflagellate zoospores (5, 19). The zoospore is an infective stage where it can swim, attach to, and penetrate host tissue, possibly leading to pathology (18, 19).

Although pythiosis in animals has been increasingly reported worldwide, most human pythiosis cases have been reported in Thailand, where it is considered to be endemic (8, 14, 16, 17, 26, 28, 30, 33). Thalassemia and agriculture-related careers are predisposing factors for human pythiosis (16, 17, 28). Clinical features of human pythiosis can be categorized into four forms as follows. (i) Vascular pythiosis (59% of reported cases) is an infection of the arteries leading to arterial occlusion and aneurysm. In advanced cases, many patients die, and since the main treatment is limb amputation, many patients become handicapped. (ii) Ocular pythiosis (33%) is an infection of the eyes, in which patients usually present with corneal ulcers or keratitis. Most of these patients undergo enucleation therapy to control the infection. (iii) Patients with cutaneous pythiosis (5%) present with granulomatous and ulcerative lesions confined to cutaneous and subcutaneous tissues. (iv) Disseminated pythiosis (3%) is an infection of other internal organs, such as the brain, sinuses, or gastrointestinal tract. The use of conventional antifungal drugs is ineffective in treatment of pythiosis because Pythium is only distantly related phylogenetically to fungi, and radical surgery is the main treatment option (16, 17, 29).

Delayed diagnosis leads to delayed treatment and a poorer prognosis in patients with pythiosis. Diagnosis by culture identification of P. insidiosum is time-consuming and laborious (3, 23). Serodiagnosis of pythiosis commonly relies on an immunodiffusion (ID) test. Although the ID test is highly specific, it has very poor sensitivity (11, 12, 21, 25). Subsequently, other diagnostic methods, such as an in-house enzyme-linked immunosorbent assay (ELISA), an immunochromatographic test (ICT), a Western blot assay, and a PCR assay, were developed and have good specificity and sensitivity (11-13, 20, 22, 32). However, the lack of diagnostic materials and special equipment needed for these tests limits their use, especially in rural areas where the disease is prevalent. Here, we describe a hemagglutination (HA) test to assist a rapid diagnosis of human pythiosis. The test is easy to perform, requires only routine laboratory equipment and could easily be adapted to a simple kit format.

MATERIALS AND METHODS

Serum samples.

A total of 33 serum samples from patients with pythiosis (27 vascular, four ocular, and two cutaneous) were recruited for the assay evaluation. Clinical information was recorded for each pythiosis patient and included clinical features, duration of symptoms before the first medical visit, underlying diseases, and method of diagnosis (Table 1). All pythiosis patients were diagnosed based on at least one of following criteria: (i) P. insidiosum isolated from infected tissue and confirmed by induction and identification of zoospores, or (ii) the presence of anti-P. insidiosum antibodies in blood samples; antibody detection was by at least one of the following well-established serodiagnostic tests: ID test, ELISA, Western blot analysis, or ICT (3, 11-13, 15-18, 20-23, 25, 32). Additional serum samples (n = 289) were collected as control samples that included (i) 186 randomly collected serum samples from healthy blood donors at the Blood Bank Division of Ramathibodi Hospital, (ii) 21 serum samples from healthy thalassemic patients without clinical evidence of pythiosis, (iii) five serum samples from patients with highly positive antinuclear antibody, and (iv) 77 serum samples from patients positive for other infectious diseases. The last group included 19 serum samples obtained from patients with proven cryptococcosis (n = 11), penicillosis (n = 7), or candidiasis (n = 1), as determined by criteria for invasive fungal diseases of the European Organization for Research and Treatment of Cancer/Invasive Fungal Infections Cooperative Group and the National Institute of Allergy and Infectious Diseases Mycoses Study Group (EORTC/MSG) (6). Of the remaining 58 serum samples, 20 were obtained from patients with aspergillosis (n = 4) or mucormycosis (n = 4) confirmed by culture identification, from patients that were fungal galactomannan antigen positive (n = 9), and from patients that were anti-Histoplasma capsulatum antibody positive (n = 3). However, information on host factors and clinical features and other mycological evidence for revalidation by EORTC/MSG criteria were missing for these 20 samples. The remaining 38 out of 77 serum samples comprised samples from cases with proven nonfungal infections according to established criteria (4, 34). These included samples that were positive for anti-human immunodeficiency virus antibody (n = 10), syphilis (n = 9), malaria (n = 5), mycoplasmosis (n = 4), toxoplasmosis (n = 2), leptospirosis (n = 2), melioidosis (n = 2), anti-Entamoeba histolytica antibody (n = 1), hepatitis A virus (n = 1), hepatitis B virus (n = 1), and hepatitis C virus (n = 1). One positive control serum sample was obtained from a rabbit immunized with P. insidiosum antigen. All serum samples were stored at −20°C until used.

TABLE 1.

Clinical information of pythiosis patients from whom serum samples were obtained for the HA test evaluation^g

Patient	Form of infection	Underlying diseasea	Clinical presentation	Duration of symptomb	Diagnosis			HA titer (test result)f
					Histologyc	Cultured	Serologye
S1	Vascular	Thal	Claudication, ulcer	5 mo	(+)	N/Ag	1 (+), 4 (+)	5,120 (+)
S2	Vascular	Thal	Claudication, ulcer	2 mo	(+)	(+)	1 (−), 4 (+)	1,280 (+)
S3	Vascular	PNH	Gangrenous ulcer	3 wk	N/A	(+)	1 (+), 4 (+)	1,280 (+)
S4	Vascular	Thal	Claudication, ulcer	1 mo	(+)	(+)	1 (+), 2 (+), 4 (+)	2,560 (+)
S5	Vascular	Thal	Claudication, ulcer	7 wk	(+)	(+)	1 (+), 2 (+), 4 (+)	1,280 (+)
S6	Vascular	Thal	Claudication, ulcer	5 mo	N/A	(+)	1 (−), 4 (+)	5,120 (+)
S7	Vascular	Thal	Claudication, ulcer	1 mo	(+)	N/A	1 (+), 4 (+)	20,480 (+)
S8	Vascular	Thal	Claudication, ulcer	3 mo	(+)	N/A	1 (+), 4 (+)	1,280 (+)
S9	Vascular	Thal	Claudication, ulcer	1 mo	(+)	(+)	1 (−), 4 (+)	320 (+)
S10	Vascular	Thal	Claudication, ulcer	10 days	(+)	(+)	1 (+), 4 (+)	5,120 (+)
S11	Vascular	Thal	Claudication, ulcer	3 mo	(+)	(+)	1 (+), 4 (+)	2,560 (+)
S12	Vascular	Thal	Gangrenous ulcer	12 mo	N/A	N/A	1 (+), 3 (+), 4 (+)	20,480 (+)
S13	Vascular	Thal	Claudication, ulcer	N/A	(+)	N/A	1 (+), 4 (+)	1,280 (+)
S14	Vascular	Thal	Claudication, ulcer	N/A	(+)	N/A	1 (−), 4 (+)	1,280 (+)
S15	Vascular	Thal	Claudication, ulcer	N/A	(+)	N/A	1 (−), 4 (+)	1,280 (+)
S16	Vascular	Thal	Claudication, ulcer	1 mo	(+)	N/A	1 (−), 4 (+)	160 (+)
S17	Vascular	N/A	Claudication, ulcer	N/A	N/A	N/A	1 (+), 4 (+)	1,280 (+)
S18	Vascular	N/A	Claudication, ulcer	N/A	N/A	N/A	1 (+), 4 (+)	1,280 (+)
S19	Vascular	Thal	Claudication, ulcer	N/A	(+)	(+)	1 (−), 4 (+)	320 (+)
S20	Vascular	Thal	Claudication, ulcer	3 mo	(+)	N/A	1 (+), 4 (+)	40,960 (+)
S21	Vascular	Thal	Claudication, ulcer	5 mo	(+)	N/A	1 (+), 4 (+)	10,240 (+)
S22	Vascular	Thal	Claudication, ulcer	N/A	(+)	N/A	1 (+), 4 (+)	640 (+)
S23	Vascular	N/A	Claudication, ulcer	N/A	N/A	N/A	1 (+), 3 (+), 4 (+)	10,240 (+)
S24	Vascular	N/A	Claudication, ulcer	N/A	N/A	N/A	1 (+), 3 (+), 4 (+)	1,280 (+)
S25	Vascular	N/A	Claudication, ulcer	N/A	N/A	N/A	1 (+), 3 (+), 4 (+)	10,240 (+)
S26	Vascular	N/A	Claudication, ulcer	N/A	N/A	N/A	1 (+), 3 (+), 4 (+)	2,560 (+)
S27	Vascular	N/A	Claudication, ulcer	N/A	N/A	N/A	1 (−), 3 (+), 4 (+)	160 (+)
S28	Cutaneous	Thal	Necrotizing cellulitis	6 days	N/A	(+)	1 (+), 4 (+)	320 (+)
S29	Cutaneous	HIV, ITP	Chronic ulcer	3 mo	N/A	(+)	1 (−), 3 (+), 4 (+)	640 (+)
S30	Ocular	N/A	Keratitis	10 days	(+)	(+)	1 (−), 4 (−)	80 (−)
S31	Ocular	N/A	Corneal ulcer	N/A	N/A	(+)	1 (−), 3 (−), 4 (−)	40 (−)
S32	Ocular	N/A	Corneal ulcer	N/A	N/A	(+)	1 (−), 3 (−), 4 (−)	40 (−)
S33	Ocular	N/A	Corneal ulcer	N/A	N/A	(+)	1 (−), 3 (−), 4 (−)	80 (−)

^aThal, thalassemia; PNH, paroxysmal nocturnal hemoglobinuria; HIV, human immunodeficiency virus; ITP, idiopathic thrombocytopenic purpura.

^bDuration of symptom starting from when it was first recognized by a patient before seeking medical attention.

^c(+), positive Grocott's methenamine silver staining for fungal elements compatible to P. insidiosum in infected tissue.

^d(+), successful isolation of P. insidiosum by culture identification method.

^e1, ID test; 2, ELISA; 3, Western blotting; 4, ICT; (+), positive result; (−), negative result.

^fHA titer represents reciprocal titer. (+), positive result; (−), negative result.

^gN/A, data not available.

Antigen preparation.

The P. insidiosum strain CBS119452 isolated from a Thai patient with vascular pythiosis was used to prepare the antigen. The microorganism was subcultured on Sabouraud dextrose agar and incubated at 37°C for 3 days. Several small blocks of mycelium were transferred to Sabouraud dextrose broth and shaken (150 rpm) at 37°C for 9 days. Merthiolate was added to the culture at a final concentration of 0.02% (wt/vol). The culture was filtered through a Durapore membrane filter (0.22-μm pore size). Phenylmethylsulfonyl fluoride (0.1 mg/ml) and EDTA (0.3 mg/ml) were added to the culture filtrate broth before concentration to ∼80-fold using an Amicron 8400 apparatus and an Amicon Ultra-15 centrifugal filter (10,000 nominal molecular weight limit; Millipore, Bedford, MA). The concentrated broth was referred to as culture filtrate antigen (CFA), and was measured for protein concentration by a spectrophotometer. CFA was stored at −20°C.

HA test. (i) Stabilization of sheep red blood cells.

Preparation of sheep red cells (SRC) for the HA test was modified from the methods of Hirata and Brandriss (7) and Petchclai et al. (24). The SRC were packed by washing with normal saline and centrifuging at 5,000 rpm for 3 min three times. Then, 1.25 ml of 0.15 M phosphate-buffered saline (PBS) (pH 7.2) and 0.25 ml of 2.5% glutaraldehyde in distilled water were added to 0.1 ml of the packed SRC, and then they were gently mixed by rotator at room temperature for 2 h. The SRC were washed with normal saline and centrifuged three times as described above. PBS with 0.1% sodium azide was added to the SRC to make a 10% glutaraldehyde-stabilized SRC suspension.

(ii) Preparation of P. insidiosum antigen-coated SRC.

To coat SRC with P. insidiosum antigens, 0.1 ml of 2-mg/ml CFA and 1 ml of 0.1 M acetate buffer (pH 4) were sequentially added to 0.1 ml of 10% glutaraldehyde-stabilized SRC suspension, before mixing and incubating at 37°C for 30 min. The CFA-coated SRC were washed with normal saline and centrifuged three times (as described above) and resuspended in PBS with 0.1% bovine serum albumin and 0.1% sodium azide to make a 0.5% CFA-coated SRC suspension.

(iii) HA assay.

To perform the HA test, 25 μl of 1:10 diluted serum was added to the first well of a 96-U-shaped-well microtiter plate. Then, the serum was diluted twofold from 1:10 to 1:20,480 by using a diluent (0.5% bovine serum albumin, 1% normal rabbit serum, and 0.1% sodium azide in PBS). Twenty-five microliters of the 0.5% CFA-coated SRC suspension was added to each well and mixed gently. The positive control well contained the CFA-coated SRC suspension mixed with a positive serum. The negative control wells contained CFA-coated SRC suspension and the diluent or a negative serum. Each sample was tested in duplicate. The plate was incubated for 1 h at room temperature. The presence of HA was read as a positive test result for P. insidiosum, whereas the absence of HA was read as a negative test result.

ID test.

The ID test was modified from the original method of Pracharktam et al. (25). Briefly, agar gel diffusion was carried out in a 5-cm-diameter petri dish with 2% agar in Veronal buffer (0.9% [wt/vol] C₈H₁₁N₂NaO₃, 0.05% [wt/vol] NaN₃ [pH 8.6]). The CFA and a serum sample were each added to 4-mm-diameter wells that were set 4 mm apart from each other. The plates were incubated in a moist chamber at room temperature for 24 h. The presence of an identity precipitation line with the positive control serum was considered a positive test result for P. insidiosum.

Statistical analysis.

Sensitivities, specificities, and accuracies were calculated from each cutoff titer of the HA test and displayed in a receiver-operating characteristic (ROC) curve using the Stata program, version 10.0 (StataCorp, TX).

RESULTS

The area under the ROC curve for the HA test results was 0.99 (Fig. 1). The best cutoff titer chosen for the HA test was 1:160 because it provided the highest accuracy (98.1%) (Table 2). At this cutoff titer, all 29 serum samples from vascular and cutaneous pythiosis patients, serum from one healthy blood donor, and serum from a patient with mycoplasmosis (i.e., two samples from the negative control group) gave positive test results, while all four serum samples from the ocular pythiosis patients and the remaining 287 serum samples from the negative control patients gave negative test results. Therefore, sensitivity and specificity of the HA test were 87.9% and 99.3%, respectively. Incubation time for the HA test was 1 h.

FIG. 1.

ROC curve for the HA test. The area under the ROC curve is 0.9931.

TABLE 2.

Sensitivity, specificity, and accuracy of the HA test at various cutoff serum titers^d

Cutoff point (reciprocal titer)	Sensitivity (%)a	Specificity (%)b	Accuracy (%)c
Undiluted	100.0	0.0	10.3
10	100.0	58.1	62.4
20	100.0	78.9	81.1
40	100.0	89.3	90.4
80	93.9	95.5	95.3
160	87.9	99.3	98.1
320	81.8	99.7	97.8
640	72.7	99.7	96.9
1,280	66.7	100.0	96.6
2,560	36.4	100.0	93.5
5,120	27.3	100.0	92.6
10,240	18.2	100.0	91.6
20,480	9.1	100.0	90.7
40,960	3.0	100.0	90.1

^aSensitivity = true positive/(false negative + true positive) × 100.

^bSpecificity = true negative/(false positive + true negative) × 100.

^cAccuracy = (true positive + true negative)/(true positive + true negative + false positive + false negative) × 100.

^dThe titer of 1:160 was selected as the best cutoff point because it provided the highest test accuracy (98.1%). This cutoff titer is indicated in boldface type.

To calculate positive and negative predictive values of the HA test, prevalence of pythiosis, test sensitivity (88%), and test specificity (99%) were used as in the statistic equation described elsewhere (1). At Ramathibodi Hospital during the year 2008, the prevalences of pythiosis in all patients (n = 190,415) and in patients with thalassemia (a prominent predisposing factor for pythiosis) (n = 372) were 0.002% and 0.8%, respectively. The positive predictive value (PPV) and negative predictive value (NPV) of the HA test using the prevalence in all patients were 0.2% and 100.0%, respectively, while the PPV and NPV using the prevalence in thalassemic patients were 41.7% and 99.9%, respectively.

To test the precision of the HA test, one each of the positive control and negative control serum samples was repeatedly assayed 20 times in both within-run and between-run analyses. The highest HA titer obtained from each precision analysis was 1:640 for the positive control serum, and test were negative at all titers for the negative control serum. The CFA-coated SRC were stable for at least 6 months, since reactions of the positive control serum remained unchanged at the highest HA titer (1:640) over the interval (i.e., at 2 weeks, 1 month, 3 months, 5 months, and 6 months after the preparation of coated SRC).

For the ID test, only 20 serum samples from patients with vascular (n = 19) and cutaneous (n = 1) pythiosis gave positive results. All of the control serum samples and 13 serum samples from patients with vascular (n = 8), ocular (n = 4), and cutaneous (n = 1) pythiosis gave negative test results. Therefore, sensitivity, specificity, and accuracy of the ID test were 60.6%, 100.0%, and 96.0%, respectively. Incubation time for the ID test was ∼24 h.

DISCUSSION

An HA test was developed to facilitate rapid serodiagnosis of human pythiosis. The large area under the ROC curve for the HA test (Fig. 1) indicated a very good discriminative power for identifying patients with human pythiosis. The serum dilution of 1:160 was selected as the cutoff titer for reading agglutination reactions, because it gave minimal nonspecific HA reactions (i.e., false-positive results) and high sensitivity and specificity (Table 2). The explanation for the two negative control serum samples that gave positive HA test results (i.e., from a healthy blood donor and a patient with mycoplasmosis) could be (i) the presence of anti-P. insidiosum antibodies in patients with subclinical pythiosis or with a previous P. insidiosum infection or (ii) nonspecific binding of antibodies to CFA or SRC.

As expected for the test, the calculated PPV was very low, and the NPV was very high, because pythiosis is a disease of very low prevalence. Thus, the results suggested that the diagnostic value of the HA test would be to rule out pythiosis. Using the HA test for patients with thalassemia gave a markedly higher PPV than that for general patients (i.e., 41.7% versus 0.2%, respectively). Based on the within-run and between-run analyses (see Results), the HA test showed good precision. The coated SRC had a long shelf life (≥6 months) when kept refrigerated.

Among the patients with well-developed pythiosis that gave positive HA results (Table 1), there were two cases (patients S28 and S29) with interesting and unusual clinical features. Patient S28 presented a history of 6 days of acute necrotizing cellulites in both legs. Initial diagnosis was in doubt because of a relatively short history of illness and presentation of unlikely symptoms for pythiosis. Another immunocompromised patient (S29) had a high human immunodeficiency virus load (418,000 copies/ml serum) and a low CD4 count (52 cells/μl). The HA test successfully detected anti-P. insidiosum antibodies in these cases, indicating good diagnostic efficiency.

A pitfall of the test evaluation was that it lacked serum samples from patients with early-stage pythiosis, during which typical clinical features may be absent or minimal. Such early-stage patients might have levels of anti-P. insidiosum antibody too low to be detected by the HA test, and this might lead to false-negative test results. In this regard, a prospective field trial of the HA test would be necessary. Similarly, use of the HA test in a seroprevalence study of thalassemic individuals predisposed to pythiosis would provide useful information on its possible application in screening people at risk and on the nature and epidemiology of the disease.

All serum samples from ocular pythiosis patients gave negative results by both the HA and ID tests, suggesting that patients with ocular pythiosis are seronegative. This phenomenon may due to the fact that the eye is classified as an immune privileged site that lacks normal immune functions such as T- and B-cell proliferation, generation and activation of NK cells, development of cytotoxic T cells, and immunoglobulin production in order to minimize immunopathology from local infections (10). Thus, due to very low antibody production, antibody detection is not generally used for diagnosis of ocular infections such as mycoses (31), and our results agree, showing that serological diagnosis of ocular pythiosis is not feasible due to the high likelihood of false-negative results.

When the HA test was compared to the ID test now commonly used for serodiagnosis of pythiosis, the specificity and the accuracy of the HA test (99.3% and 98.1%, respectively) and those of the ID test (100% and 96%, respectively) were similar. However, the sensitivity of the HA test (88%) was markedly higher than that of the ID test (61%). The ID test gave false-negative test results for nine proven cases of vascular and cutaneous pythiosis, while the HA test correctly detected these cases. Our work confirmed the results of previous studies showing that the ID test has poor sensitivity (11, 12). The assay turnaround time for the HA test (1 h) was significantly shorter than that for the ID test (24 h).

Animal pythiosis has been increasingly reported worldwide (9, 18, 27). Although Thailand is an area where human pythiosis is endemic (16), there have been no reports of animal pythiosis from this country. It may be that this is due to lack of a sensitive and specific test to facilitate its diagnosis rather than lack of its occurrence. Our HA test for efficient detection of anti-Pythium antibody in human patient sera does not require a host species-specific conjugate antibody as do other highly sensitive tests, such as ELISA, ICT, and Western blot analysis (11-13, 20, 22). Thus, we have shown that the HA test detected anti-Pythium antibody in the serum of the rabbit immunized with P. insidiosum antigen, and this suggests that it could be used to conveniently detect anti-Pythium antibody in the sera of other animals as well. As such, the HA test could be regarded as a sensitive and specific test for rapid serodiagnosis of pythiosis in both humans and animals.

Since the HA test had good precision, it was used for antibody tests during the follow-up period after surgical treatment for pythiosis in two patients with vascular pythiosis. Gradual decreases of the antibody titer occurred in both patients (dropping from 1:5,120 to 1:80 in one case, and from 1:640 to 1:80 in the other case) in a manner that correlated well with their clinical improvement after surgery (data not shown). The antibody titer started to decrease within 1 month after leg amputation. These preliminary results indicate that the HA test may be a useful tool for monitoring treatment success in human pythiosis. A study of a larger group of pythiosis patients would be required before this could be confirmed.

In conclusion, a reliable HA test for serodiagnosis of vascular and cutaneous pythiosis was developed, but it was not suitable for diagnosis of ocular pythiosis due to a high likelihood of false-negative results. Higher test sensitivity and shorter turnaround time were advantages of the HA test over the commonly used ID test. The HA test is easy to perform, and it is suitable for diagnosis of pythiosis in hospitals in rural areas where the disease is prevalent.