INTRODUCTION
Protozoa are microorganisms that cause public health problems in humans. Among the most important protozoa, are Trypanosoma cruzi, which causes American Trypanosomiasis or Chagas disease 1 and Leishmania sp. which causes leishmaniasis. These agents are transmitted by a bed bug (Triatoma dimidiata) and a blood-sucking mosquito (Lutzomyia and Phlebotomus) respectively. However, recent studies have indicated that agents such as ticks of the genus Rhipicephalus sanguineus may be playing a role in the transmission of canine visceral leishmaniasis 2.
American trypanosomiasis, also known as Chagas disease, is a parasitic zoonosis caused by a protozoan, and characterized by using two guests, vertebrate (dog) and other invertebrate (triatoma) to complete its life cycle 3; Leishmaniasis is a disease caused by a complex group of protozoa. The life cycle takes place in two evolutionary forms, one transmitted by the bite of a sandfly Diptera (vector) and one in the host (dogs) that often acts as a reservoir of the parasite 4. According to the epidemiological importance of dogs in the transmission of Chagas disease as reservoirs of the parasite in endemic areas, it is essential to know the prevalence of infection, as a measure of active surveillance to identify the circulation of the parasite before it infects the human population susceptible 5. Some studies carried out in Yucatan on Trypanosomiasis in dogs reported seroprevalence of 10.74% (Merida, Molas and Xcalacoop) and 21.34% in the state of Quintana Roo (Playa del Carmen, Akumal Xcalac) 6. Leishmania sp. is very common in almost all the world, especially in tropical and subtropical areas.
The prevalence in the Yucatan Peninsula reported for L. braziliensis (7.52%), L. infantum (6.07%) and L. mexicana (20.63%) 7, while it was reported 41.4% for the peninsula in general 8 and 30.2% 9 for Merida. The most common worldwide used techniques for detection of these agents are diagnostic techniques Enzyme-linked Immunosorbent Assay (ELISA), indirect immunofluorescence (IFI) and Polymerase Chain Reaction (PCR). ELISA test Fe-SOD has high sensitivity for the diagnosis of Leishmaniasis, since it has a specificity between 99 and 99.7%; and a sensitivity between 96.2 and 100% 7,10,11. The use of Western Blot is justified by high sensitivity compared to ELISA, however their values are consistent with those obtained with ELISA Fe-SOD, but has a higher cost. 7.
Since there is not accurate information regarding the frequency and scope of these diseases in the state of Tabasco, this work aimed to estimate the frequency of Trypanosoma cruzi circulating antibodies, Leishmania mexicana and Leishmania braziliensis in a population of dogs using Fe-SOD ELISA and Western blot diagnostic techniques.
MATERIALS AND METHODS
Field of study. An epidemiological cross-sectional for convenience study was carried out in the canine population of the Chontalpa subregion of the state of Tabasco, Mexico (latitude 17°15’- 18°39’, longitude 19°00’- 94°17’). The climate is tropical, with temperatures of 15-44°C and relative humidity above 90%.
Sample size. A sample size of 119 individuals, was calculated for an infinite dog population, with 95% confidence level, an error of 5% and an expected prevalence of 10% 6 using the statistical software WinEpiscope 2.0.
Blood sampling. Blood samples were collected during the months of July-August 2013, taking into account, the ethical, technical, scientific and administrative standards of the Autonomous University of Tabasco (UJAT), as well as the consent of the dogs´ owners through an informed consent. Before samples were taken, a general physical exam was carried out on each dog along with the identification of skin lesions and general health status. 5 ml of blood were taken of the cephalic vein by puncture using Vacutainer system. The samples were collected in tubes without anticoagulant and refrigerated at 4°C. Then, the samples were centrifuged at 1500 rpm per 10 minutes to obtain the serum. Once the serum was obtained, it was stored at -20°C and subsequently processed at the Faculty of Sciences of the University of Granada, Spain.
Statistical analysis. For the analysis of the results descriptive statistics were used and the Kappa value was calculated as well as the sensitivity and specificity for the ELISA test by means of a 2x2 table.
Laboratory analysis. For the samples analysis, a parasite culture was carried along with the extraction and purification of the Fe-SOD excreted; subsequently the serological ELISA test and Western blot were performed. Each or these procedures are described below.
Parasite culture, extraction and purification of the Fe-SOD excreted. Parasites were cultured in vitro in sterile medium liquid MTL (Trypanosomes Liquid Medium) supplemented with 10% (v/v) inactivated fetal bovine serum at 56° C/30 minutes. The parasite inoculum at the beginning of the culture was 5 × 104 cells/ml in 5 ml of media in Falcon plastic jars of 25 cm2 and they were kept in an oven at 28°C. The cultures were routinely performed until the needed cell mass for further studies was obtained. The extraction and purification of Fe-SOD was obtained through cultures in exponential growth phase.
Parasites were washed twice through a procedure known as free of serum MTL. The parasites were counted using a hemocytometer and then distributed into aliquots of 5x109 parasites/ml. Subsequently a second parasite culture through medium MTL was performed without serum for 24 hours, a supernatant was obtained by centrifugation at 1500 xg for 10 minutes, filter after centrifugation was obtained, using a filter of 0.45 microns. The filtrate was washed twice with ammonium sulfate at concentrations of 35% and 85% respectively, then centrifuged at 9000 xg for 20 minutes at 4°C, obtaining a precipitate which was re suspended in 2.5 ml of potassium phosphate 20 mM pH 7.8, 1 mM EDTA as buffer solution. Additionally the precipitate was dialyzed through a Sephadex G-25 (Pharmacia, PD 10), previously equilibrated with buffer solution to a total volume of 2.5 ml of Fe-SOD. Protein content was determined using the Bio-Rad assay, according to the Bradford method (Inmunochemicals Sigma, St. Louis), using bovine serum albumin as standard as described by Marin 12. The Fe-SOD obtained was used as antigen in the ELISA and Western Blot for the diagnosis of T. cruzi, L. mexicana and L. braziliensis 8.
ELISA serological test. For the ELISA test, polyethylene microtiter plates (Nunc, Denmark) were sensitized with antigen Fe-SOD fraction at a concentration of 1.5 g in carbonate buffer (pH 8.2) for 2 hours at 37°C. Plates were washed 3 times with phosphate buffered saline (PBS), 0.05% tween® 20 (wash buffer) removing unbound antigens. The free adsorption sites were blocked for 2 hours at 37°C using a blocking solution (PBS-Tween 20® at 0.2% bovine serum albumin at 1%). The plate was then incubated for 45 minutes in the presence of dog serum at a dilution of 1:200. After this further washing was performed and it was incubated for 30 minutes at 37°C with inmunoconjugate (Anti IgG with peroxidase anti-dog, Sigma Immunochemicals) at a dilution of 1:1000. The enzymatic reaction with the chromogenic substrate was OPD (hydrogen peroxide O-phenylenediamine-Sigma®, Madrid, Spain) and 10 ul of 30% H2O2 per 25 ml for 20 minutes in darkness. The reaction was blocked by adding 50 ul of HCl3N. Absorbance was read at 492 nm in a microplate reader (SunriseTM, TECAN). All samples were tested in triplicate in microtiter plates. The median and standard deviation (SD) of optical density of the negative control sera (belonging to the faculty of the University of Granada, Spain) were used to calculate the cutoff value (mean +3xSD) according to the technique suggested by ongoni 8.
Western blot analysis. The fraction of Fe-SOD antigen (concentration of 1.5 µg of protein) of T. cruzi, L. mexicana and L. braziliensis, was merged with IEF 3-9 gels and subsequently transferred to nitrocellulose for 20 minutes, as described in the Phast system manual. The membrane was blocked for two hours at room temperature using 0.4% gelatin and 0.2% Tween 20 in PBS, followed by three washes in 0.1% Tween 20 in PBS (PBS-T), subsequently they were incubated for two hours at room temperature with dog sera in a dilution of 1:100. Before the washing, the membrane was incubated for two hours mass room temperature with the second antibody, anti-dog immunoglobulin G conjugated with peroxidase (Sigma Immunochemicals; dilution 1/1000. The latter was washed with diaminobenzidine substrate (0.5 mg/ml in Tris buffer/HCl 0.1M pH 7.4 containing H2O2 1/5000 [10 v/v]) and was added to the reaction, which was stopped with a number of washings with distilled water according to the technique suggested by Marin et al 13.
RESULTS
The frequency of seropositive dogs by ELISA (Fe-SOD) was 6.7, 15.1 and 14.2 for T. cruzi, L. mexicana and L. braziliensis respectively; while the frequency of seropositive dogs by Western blot was 3.4, 9.2 and 10.1 for T. cruzi, L. mexicana and L. braziliensis respectively (Table 1).
Table 1 Sera of dogs positive to T. cruzi, L. mexicana and L. braziliensis using the ELISA and Western blot-sod tests, carried out in the Chontalpa subregion of Tabasco, Mexico.
Antigen | Elisa - SOD | Frequency | Western Blot | Frequency |
---|---|---|---|---|
Trypanosoma cruzi | 8 | 6.7% | 4 | 3.4 % |
L. mexicana | 18 | 15 % | 11 | 9.2 % |
L. braziliensis | 17 | 14.2% | 12 | 10.1% |
Total serum | 119 |
Table 2 shows the results of reliability analysis of the ELISA test (Fe-SOD) for the detection of T. cruzi antibodies, L. mexicana and L. braziliensis present presenting Kappa values of 0.65, 0.71 and 0.68 respectively .
Table 2 Reliability indices of the diagnostic test (ELISA) using Fe-SOD fraction of T. cruzi, L. mexicana, L. braziliensis in sera from dogs of the Chontalpa subregion of Tabasco, Mexico.
T. Cruzi | L. mexicana | L. braziliensis | |
---|---|---|---|
Sensitivity | 100% | 100% | 100% |
Specificity | 96.5% | 89.1% | 88.2% |
Negative predictive value | 100% | 100% | 100% |
Kappa Value | 0.65 | 0.71 | 0.68 |
DISCUSSION
In the present study the frequency of antibodies to Trypanosoma cruzi, Leishmania mexicana and L. brasiliensis was lower than previously reported in the Yucatan Peninsula 9 (Table 1), and the State of Morelos 14; as well as studies in Latin American countries like Paraguay and Colombia 15. The prevalence found in this study may be related to factors such as: a) changes in the environment, b) presence of jungle-peridomestic wildlife, c) Actions of prevention - control of the vector, d) the diversity and abundance of vectors related with the disease (influenced by environmental conditions of the geographical area), e) the socioeconomic conditions of the study area (urban vs. rural; in rural areas high prevalences have been reported 29.9%, due to close contact with areas of forest, favoring the interaction of animals like dogs and farm animals, with triatomine at certain seasons of the year); f) Age of the animals (As age of dogs increases the prevalence increases); and g) The handling of pets in urban vs. rural areas, in rural areas the dogs are released, providing that come into contact with vectors. The condition of freedom for dogs, allow dogs to remain in surrounding areas to the House-hold, in those areas higher collections and diversity of triatomine collections have been reported, giving as a result higher serological frequency in dogs 16-19.
Monitoring of diseases such as Trypanosomiasis and Leishmaniasis in pets, is of great importance 16, as animals like dogs, act as reservoirs of the parasite in endemic areas, and it is necessary to identify the movement of agents in order to prevent possible infection in humans 4. Pets like dogs, cats and rodents, presented high prevalence of T. cruzi; according to some studies, the presence of dogs in homes increases the risk of infection for the human population of 3 to 5 times 20,21. In states such as the State of Mexico, Puebla and Morelos has reported a correlation of seropositivity among humans (7, 4 and 1.2%) and dogs (21, 10 and 24.2%) respectively; according to these results, the presence of dogs in homes represents a risk factor for acquiring Chagas disease 20. Studies in the State of Quintana Roo, Mexico indicate that frequent diagnosis of canine leishmaniasis, may be related to increases in the presentation of the disease in infected pet owners 8,17. Most pets living with the human population, shows a connection between urban - rural and peridomestic, bringing along several repercussions on public health 22. Therefore, the use of specific tests for the monitoring and control of diseases such as Trypanosomiasis and Leishmaniasis, is very important; in this study, Western blot test showed values of 100% sensitivity and specificity for T. cruzi, L. mexicana and L. brasiliensis, compared to ELISA (Table 2), however, considering the low cost and easy reproducibility of the ELISA test, should be considered as a viable option for the development of monitoring and control programs in endemic areas Tryopanosomiasis and Leishmaniasis tool. In conclusion, there are circulating antibodies to T. cruzi, L. mexicana and L. braziliensis in dogs from Chontalpa region of the State of Tabasco, Mexico. The frequency of antibodies to T. cruzi, L. mexicana and L. braziliensis in dogs Chontalpa region of the State of Tabasco, is lower than reported by other authors for the Mexican southeast and Latin America. It is suggested to conduct epidemiological studies on population dynamics and diversity of vectors in the study area, in order to design effective control plans for T. cruzi, L. mexicana and L. braziliensis in dogs.