INFORME BREVE

The feasibility of using antigens prepared with rough Brucella strains for diagnosis of canine brucellosis

G. I. Escobar¹, E. J. Boeri², S. M. Ayala¹, N. E. Lucero^1*        

¹Administración Nacional de Laboratorios e Institutos de Salud "Dr. Carlos G. Malbrán", Avda. Vélez Sarsfield 563 (1281) Ciudad Autónoma de Buenos Aires;
²Hospital "Luis Pasteur", Avda. Díaz Vélez 4821 (1405) Ciudad Autónoma de Buenos Aires, Argentina
*Correspondence. E-mail: nidia@elsitio.net

ABSTRACT

Clinical diagnosis of canine brucellosis is not sensitive enough and a negative blood culture cannot rule out the disease. Indirect methods of serological testing such as agar gel immunodiffusion (AGID), rapid slide agglutination test (RSAT) and indirect enzyme linked immunoassay (IELISA) are preferred for routine diagnosis. Since Brucella canis shares antigenic components with the Brucella ovis and Brucella abortus RB51 strain, it would seem that either strain could be used as antigen. We present data on AGID and IELISA tests using the B. ovis antigen, RSAT and IELISA using the B. canis antigen and IELISA using the B. abortus RB51 antigen. The cut-off values were adjusted by the ROC analysis; the IELISA-B. ovis cut-off value was 23 (%P) and the IELISA-B. abortus RB51, 24 (%P), with 100% sensitivity and 98.8% specificity. RSAT detected 100% of positive cases, while AGID was less sensitive. The sera from dogs treated with antibiotic showed that %P correlated well with the clinical course. Sera from dogs presumptively infected with B. suis were negative in all tests performed with the rough Brucella strains. RSAT is a very sensitive screening test and IELISA-B. canis, B. ovis and B. abortus RB51 could be used as confirmatory tests, since they show good specificity and sensitivity.

Key words: Brucellosis; B. canis brucellosis; Canine brucellosis; Diagnosis of canine brucellosis

RESUMEN

Utilidad de los antígenos preparados con cepas rugosas de Brucella en el diagnóstico de brucelosis canina. Las técnicas más usadas en el diagnóstico de brucelosis canina son la inmunodifusión en gel de agar (AGID), la microaglutinación en portaobjetos (RSAT) y el ELISA indirecto ya que el diagnóstico clínico es poco sensible y el bacteriológico no excluye la enfermedad. Como Brucella canis comparte componentes antigénicos con Brucella ovis y Brucella abortus RB51, estas cepas podrían ser usadas indistintamente como antígenos. En este trabajo presentamos datos sobre las pruebas de AGID e IELISA con antígeno B. ovis, RSAT e IELISA con antígeno B. canis e IELISA con antígeno B. abortus RB51. Los puntos de corte ajustados con la curva ROC fueron (%P) 23 para IELISA-B. ovis y (%P) 24 para IELISA-RB51 con 100% de sensibilidad y 98,8% de especificidad. RSAT detectó el 100% de los casos positivos, pero AGID fue menos sensible. Los sueros de los perros tratados con antibióticos tuvieron un %P que correlacionó con la evolución clínica. Los perros presumiblemente infectados con B. suis fueron negativos a todos los ensayos realizados con antígenos de cepas de Brucella rugosas. RSAT es muy sensible como prueba tamiz y los IELISA con antígenos de B. canis, B. ovis y B. abortus RB51 podrían ser usados como confirmatorios ya que han demostrado alta sensibilidad y especificidad.

Palabras clave: Brucelosis; Brucelosis canina; Diagnóstico de brucelosis canina; B. canis brucelosis

Canine brucellosis can be transmitted by infected dogs through natural mating or oronasal contact; male urine may be an important route in its natural spread (4, 13). Clinical diagnosis is not sensitive enough since signs range from asymptomatic to multiple conditions such as epididymitis, embryonic loss, abortion and testicular atrophy (3).
Bacteriological isolation is required for confirmation of the disease since asymptomatic dogs may harbour B. canis for prolonged periods and bacteremia can last for years. However, since the bacteremic period fluctuates, a negative blood culture cannot rule out the disease and the difficulties involved in bacteriological diagnosis have been stressed (4).
Indirect methods using serological testing are preferred for routine diagnosis and appear to be useful whenever brucellosis is suspected; rapid slide agglutination test (RSAT), agar gel immunodiffusion (AGID), indirect immunofluorescence assay (IFA), and enzyme linked immunoassay (ELISA) are alternative diagnostic methods (2, 5).
RSAT and indirect ELISA, using antigens prepared with the B. canis (M-) strain, have been proposed for diagnosing B. canis infection in dogs (6). More recently, a rough lipopolysaccharide of B. abortus RB51 as a common antigen to detect antibodies against B. ovis, B. canis and B. abortus RB51 by IELISA and fluorescence polarization assay (FPA) has been tested (10).
Close contact between humans and infected dogs increases the risk of transmission (7, 15), which augments the relevance of finding tests that are relatively easy to run and may have diverse applications.
We present data on dog sera using AGID and IELISA with the B. ovis antigen; RSAT and IELISA with the B. canis antigen and IELISA with the B. abortus RB51 antigen.
For detection of smooth Brucella antibodies, the buffered plate antigen test (BPAT) and the Rose Bengal test (RBT) were used as screening, while the standard tube agglutination test (STAT) and the 2-mercaptoethanol test (2MET) were used as confirmatory. The tests were run as previously described (1), with antigens prepared with the B. abortus 1119-3 strain at the INEI - ANLIS "Dr. Carlos G. Malbrán".
For detection of rough Brucella antibodies, RSAT was performed as screening test, as previously described (6); the antigen was prepared at ANLIS using the strain (M-) variant of B. canis.
For IELISA-B. canis the antigen was produced at ANLIS from the (M-) variant of B. canis as previously described (6) and used 1:2,000, after OD readings of various antigen dilutions using strong positive, weak positive and negative sera as controls. The antigen diluted in 0.06M sodium carbonate buffer (pH 9.6) was passively coated onto polystyrene plates (Nunc 2-69620, Denmark) 50 μl/well, incubated at room temperature (RT) for 18 h, then washed five times in 0.01 M phosphate buffered saline containing 0.05% Tween 20, pH 7.2 (PBS/T). Control and test sera were added 1:100 in PBS/T, 50 μl/well, at RT for 1 h. After five washes in PBS/T protein A/G horseradish peroxidase conjugated (from ImmunoPure, Pierce LAB) diluted 1:20,000 was added, 50 μl/well, and incubated 1 h at RT. After five washes in PBS/T, the final step was the addition of 100 μl of chromogenic substrate [4.0 mM hydrogen peroxide and 1.0 mM 2,2´-azino-bis (3-ethylbenz-thiazoline -6-sulfonic acid) diammonium salt in 0.05 M citrate buffer, pH 4.5] per well. The plate was shaken continuously in an orbital shaker and after 10 min the OD₄₁₄ was measured in a photometer (Labsystems Multiskan EX microplate reader) with 100 μl of chromogenic substrate in a plate used as a control for the microplate reader. The test is positive when OD₄₁₄ is equal to or above 0.281 or percentage of positivity (% P) > 29 (6).
The gel for AGID was prepared by dissolving 1.2 g Noble agar, 8.5 g NaCl and merthiolate 1:10,000 in 100 ml of TRIS buffer pH 8.2 in a boiling water bath. On a flat surface, a glass slide was covered with the gel (approximately 4 ml) and holes were cut using a gel puncher. The holes (3 mm in diameter and 3 mm apart) were arranged in an hexagonal pattern around a central hole (also 3 mm in diameter). Control and test sera were placed in outer wells with the B. ovis REO 198 strain antigen (SENASA, Argentina, Lot 01-05) in the central well. Results were read after 48 h incubation at RT in a humid chamber (11).
The antigen for IELISA-B. ovis, supplied by the CNEA (Comisión Nacional de Energía Atómica), was prepared from the B. ovis strain #11 as described by Vigliocco et al. (14), and used at a concentration of 1:2,500. The lyophilized protein A/G, horseradish peroxidase conjugated (ImmunoPure, Pierce LAB) was used at a concentration of 1:25,000 after testing various working dilution ranges with strong positive, weak positive and negative sera. Then, the same procedure as described for IELISA-B. canis was followed. The test is positive when color develops.
The antigen for the IELISA-B. abortus RB.51 was produced from the B. abortus strain RB 51 [origin Centro Panamericano de Zoonosis, OPS/OMS, Argentina] at ANLIS by the procedure previously described for B. canis (6) and was used at a concentration of 1:1,000. A lyophilized protein A/G, horseradish peroxidase conjugated was used at 1:15,000 after testing various working dilution ranges with strong positive, weak positive and negative sera. The same procedure described for IELISA-B. canis was used. The test is positive when colour develops.
A control standard serum, provided by SENASA (Servicio Nacional de Sanidad Animal), was used in AGID tests. Controls used for RSAT, IELISA-B. canis, IELISAB. ovis and IELISA-B. abortus RB.51 were strong positive [naturally infected dog, detected by serological and bacteriological methods (6)], weak positive [dog with weak serological titre (6)] and negative [dog from an uninfected kennel with no clinical, epidemiological or bacteriological evidence of brucellosis].
Animal blood samples were collected without anticoagulant by cephalic, saphenous or jugular venopuncture. Sera were separated and stored at -20 °C until testing for antibody detection.
Strains mostly isolated from blood cultures were cultivated in a basal medium (BBL, Becton Dickinson) with 3% equine serum (free from anti-B. canis antibodies) and incubated at 37 °C. Colonies were typed by standard procedure (1).
Eighty sera (18 males and 62 females) were from uninfected dogs with prolonged absence of clinical and epidemiological evidence of brucellosis, which tested negative for detection of smooth and rough Brucella antibodies and also to bacteriological studies. This group was used to determine the IELISA-B. ovis and IELISA-B. abortus RB 51 cut-off values and diagnostic specificity. Forty seven sera (16 males and 31 females) collected from naturally infected dogs with proven B. canis infection detected by bacteriological and serological methods were used to calculate the diagnostic sensitivity of IELISA-B. ovis and IELISA-B. abortus RB 51. Sera from 5 neutered dogs treated with antibiotics after diagnosis was confirmed (2 males and 3 females) were followed by serological and bacteriological tests. In addition, 3 dogs (1 male and 2 females) presumptively infected with B. suis were included.
Optical density (OD) values of IELISA-B. ovis and IELISA-B. abortus RB51 were compared to those of the control serum included in each 96 well plate and a relative percent positivity value (%P) was calculated: (%P) = [OD of the test sample / OD of the control serum] x 100.
Diagnostic specificity and sensitivity were initially determined with 95% confidence limits by plotting the data for negative and positive samples on a frequency histogram. Data were then analyzed by receiver-operator characteristics (ROC) (12).
The group of 80 sera from uninfected dogs found negative by bacterial culture, for detection of smooth Brucella antibodies and for RSAT and IELISA-B. canis tests had an IELISA-B. ovis mean (%P) value of 13.66 (SD 3.22), while the IELISA-B. abortus RB51 had a mean (%P) value of 14.63 (SD 4.18). A cut-off value of (%P) 20.1 (+2 SD) or (%P) 23.32 (+3 SD) was empirically determined by ELISA-B. ovis and (%P) 22.99 (+2 SD) and (%P) 27.17 (+3 SD) by IELISA-B. abortus RB51.
These cut-off values were adjusted by the ROC analysis using 80 negative and 47 positive sera, yielding an IELISA-B. ovis cut-off value of (%P) 23 and IELISA-B. abortus RB51 of (%P) 24, with 100% sensitivity and 98.8% specificity.
See Table 1 for a summary of tests of 47 infected dogs from which B. canis had been isolated in blood cultures. RSAT, IELISA-B. canis, IELISA-B. ovis and IELISA-B. abortus RB51 detected the 47 positive cases (100%); however AGID detected 12 cases (23.4%) and 3 presented non-specific precipitation lines.

Table 1. Tests to detect anti-B. canis antibodies in dogs from whose blood cultures B. canis was isolated

Table 2 shows the serological and bacteriological results of 5 neutered dogs treated with oral doxycycline 25 mg/kg twice daily for four weeks and IM streptomycin 25 mg/kg once a day after testing positive for canine brucellosis. B. canis was isolated on admission from the testicles of dog N° 1 and from blood cultures of the other animals. Dogs N° 2 and N° 4 presented a second isolation 10 and 20 months after the first, respectively, and another course of treatment was indicated in both cases.

Table 2. Bacteriological studies and tests to detect anti-B. canis antibodies in dogs with treatment

See Table 3 for serological tests ran on 5 sera from 3 dogs presumptively infected with B. suis.

Table 3. Tests to detect anti-B. canis antibodies in dogs presumptively infected with B. suis

Clinical signs of the 47 positive dogs (B. canis was isolated in all of them) ranged from asymptomatic (n = 12) to abortions (n = 15), fetal resorption (n = 1), discospondylitis (n = 3), epididymitis (n = 2), uveitis (n = 1), while we have no data for 13 of them (Table 1). On admission, the 5 dogs from Table 2 (2 males and 3 females), neutered and treated with antibiotic therapy, presented orchitis-epididymitis (n = 1), discospondylitis (n = 3) and acute pain in the spinal column (n = 1). Of the 3 dogs in Table 3, which belonged to the same owner, case N° 2 had aborted puppies and the others were asymptomatic. Our study agree with previous reports that clinical diagnosis is unspecific, since some infected dogs appeared to be clinically normal.
Although a bacteriological diagnosis is recommended as confirmatory test, a negative blood culture cannot rule out the disease (4). On admission, blood and testicle cultures were performed from dog N° 1 (Table 2), but B. canis was isolated only from the latter. The treated dogs (Table 2) had a positive isolation at admission, but cases N° 3 and N° 5 had another positive isolation 10 and 20 months after the first, respectively, indicating that the treatment had been only relatively successful (9). The 3 dogs in Table 3 had negative blood cultures.
Of the serological methods used to detect antibodies to B. canis, AGID has been shown to be a very simple and specific test, but, not sensitive enough to be used as screening test. When comparing RSAT to AGID in the group of 47 positive animals, the former detected 100%, while AGID detected only 25.5%; 3 sera presented unspecific precipitation lines, probably due to lypemic sera.
IELISA has been shown to be the most sensitive and specific test (6, 8). For the 80 sera from uninfected dogs, the IELISA-B. ovis and IELISA-B. abortus RB51 was 98.8% specific with a cut-off value of (%P) 23 and 24 respectively; sensitivity for the 47 sera from the infected dogs was 100% by both IELISA. When the cut-off value is adjusted upward, specificity may be increased to 100%, although some sensitivity is lost.
The study of 14 sera from 5 neutered and treated dogs (Table 2) revealed that all were positive by IELISA-B. canis, IELISA-B. ovis and IELISA-B. abortus RB51.
Dogs N° 2 and N° 4 relapsed 10 and 20 months after antibiotic treatment and presented persistent IELISA %P value while dog N° 3 followed up for 42 months showed decreasing serological titres that correlated well with the clinical course. These observations were similar to those observed during the follow-up of human brucellosis.
Three dogs (Table 3) were investigated for brucellosis because one of them (dog N° 2) had aborted puppies as a clinical sign. All resulted negative to tests that used rough strain antigens but 2 of them were positive to tests with B. abortus 1119-3 antigen. Dog N° 2 was euthanized, dog N° 3 was physically separated and dog N° 1 was treated with antibiotics. At that time the owner presented headache, fever and hepatosplenomegaly symptoms; brucellosis was confirmed by serology and B. suis biovar 1 was isolated from his blood culture. He admitted having handled raw swine viscera to feed his dogs.
In this study we presented data over 135 dogs addressing four major points. First, the B. canis (M-) strain variant has the advantage of making possible the development of a satisfactory antigen for agglutination tests. The RSAT has shown to be a useful and very sensitive screening test, simple to perform and easy to interpret, that would provide an adequate replacement for AGID.
Second, since IELISA-B. canis, IELISA-B. ovis and IELISA-B. abortus RB51 yielded the same specificity and sensitivity with minor differences, they could be used indistinctly as a confirmatory test. The data agree with studies published previously by Nielsen et al. (10). The standard control serum used on each plate makes it possible to convert the optical density reading to a percentage of positivity. Evaluation of the quality and standardization of both antigens and tests are important for consistent and replicable results.
The third point is that these tests could be used for monitoring the efficacy of treatment of canine brucellosis following a study of a larger number of animals.
The last is the fact that dogs presumptively infected with B. suis may not produce antibody responses that are readily detected by the tests that utilize rough Brucella antigens. Because dogs can be infected by B. suis, B. abortus, B. melitensis and B. canis, we suggest the use of BPAT and RSAT as screening tests for routine diagnosis of brucellosis.

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Recibido: 21/07/09
Aceptado: 24/11/09