ABSTRACT
Anaplasma marginale (A. marginale), es una bacteria del orden de las Rickettsias que ocasiona la anaplasmosis bovina en regiones tropicales y subtropicales del mundo. Esta enfermedad, trasmitida principalmente por tábanos y garrapatas, se desarrolla típicamente en una etapa inicial aguda con manifestaciones clínicas caracterizadas principalmente por anemia y fiebre. Después de un par de meses, los animales recuperan su condición física y se hacen asintomáticos, siendo incapaces de eliminar completamente la bacteria, convirtiéndose en animales persistentemente infectados. Esto se debe a la capacidad de A. marginale para evadir el sistema inmune. En este sentido, se ha demostrado la existencia de un mecanismo de variación antigénica en las proteínas MSP1, MSP2 y MSP3 de la bacteria. Al evaluar la familia multigénica que codifica para la MSP2, se determinó que está conformada por dos regiones conservadas que flanquean una región central hipervariable. De esta manera, al expresarse cada una de las 52 variables de la MSP2, se expresa un epítope diferente. Cuando se describió el genoma completo de este hemotrópico, se encontró también la presencia de 16 pseudogenes msp2, los cuales pueden ser recombinados dentro del sitio de expresión del operón de la MSP2, constituyendo un segundo mecanismo de variación. Además de ello, los fragmentos hipervaribles y los pseudogenes se pueden combinar entre sí, en un proceso denominado conversión génica, creando nuevos epítopes recombinantes, confiriendo una capacidad de variabilidad antigénica casi infinita al A. marginale (tercer mecanismo). Un cuarto mecanismo de variación antigénica, lo constituye la dimerización de la MSP2 sobre la superficie del A. marginale, debido a que la expresión simultánea de variantes conforman epítopes únicos. En conclusión, la recombinación génica de la MSP2 y su dimerización en la membrana, constituye un mecanismo muy eficiente de variación antigénica para eludir el sistema inmunológico del hospedador.
Anaplasma marginale (A. marginale) is a bacterium of the Rickettsiales order that causes bovine anaplasmosis in tropical and subtropical regions worldwide. This disease, mainly transmitted by ticks and horseflies, typically develops in an initial acute stage, with clinical signs characterized by anemia and fever. After two months, animals recover their original physical condition and become asymptomatic, being unable to completely eliminate the bacterium, turning into persistently infected animals. This is due to the ability of A.marginale to evade the immune system. In this regard, the existence of a mechanism for antigenic variation in proteins of the bacterium, such as MSP1, MSP2, and MSP3, has been demonstrated. When assessing the multigenic family which encodes for MSP2, it was determined that it consists of two conserved regions flanking a central hypervariable region. Thus, when expressing each of the 52 MSP2 variables, a different epitope is also expressed. When the entire genome of this parasite was decoded, the presence of 16 pseudogenes for MSP2 was also discovered. These pseudogenes can be recombined within the operon expression site of MSP2, providing a second mechanism of variation. Moreover, both the hypervariable fragments and pseudogenes can combine among them, in a process called gene conversion, creating new recombinant epitopes, conferring the A. marginale with an almost infinite capacity for antigenic variability (third mechanism). A fourth mechanism of antigentic variation consists of the dimerization of MSP2 on the surface of A. marginale, because the simultaneous expression of variants creates unique epitopes. In conclusion, gene recombination of MSP2 along with the dimerization of MSP2 on the membrane provides a very efficient mechanism for antigenic variation for evading the hosts immune system.
ABSTRACT
Animal trypanosomiasis is a disease caused by parasites of the genus Trypanosome. This malady is widely distributed in many countries, located in tropical and subtropical areas of the world where blood-sucking flies are present. Water buffaloes are important domestic animals used for meat and milk production, and draught power. Buffalo herds are raised in areas where trypanosomiasis is endemic. In Venezuela, the buffalo industry is becoming a very important and common livestock. However, animals imported from non-endemic areas may suffer severe infections. The development of methods which ensure an efficient epidemiological surveillance against this disease is of great relevance. The immunological tests are of great importance for this purpose, because of the low sensitivity of the current parasitological methods, due to the low parasite burden that occur in subclinical and chronic infections caused by trypanosomes. To estimate the serological prevalence of trypanosome in water buffaloes, an enzyme-linked immunosorbent assay (ELISA) was used in buffalo samples of healthy animals from the municipalities of Rómulo Gallegos, Ricaurte and Girardot, in the State of Cojedes, Venezuela. Additionally, samples were also assessed with the indirect fluorescence antibody test (IFAT) and the microhematocrit test (MHCT). A total of 180 blood samples, none of which had an active parasitemia by TMC, were assessed. The prevalence determined by ELISA was 45.56%, which was significantly (P<0.05) higher than that obtained by IFAT (28.89%). The results of the experiments showed a moderate Kappa index of concordance of 0.45 (95% CI: 0.31-0.58); whereas the concordance value for both tests was 73.33%. Both the sensitivity and specificity of ELISA, compared to the IFAT, was 82.69% and 69.53%, respectively. The predictive positive and negative values were 52.44% and 90.82%, respectively. The findings suggest an endemic condition, with moderate infection values caused by Trypanosoma spp. in buffaloes from these regions of Venezuela and show, for the first time, the usefulness of ELISA for epidemiological studies of trypanosomiasis.