Genomic study and immunological characterization of cattle resistant to bovine babesiosis and analysis of the genetic diversity of Babesia bovis and Babesia bigemina

Abstract

Cattle tick fever (CTF) is one of the main parasitic diseases afflicting cattle. In Brazil the most recent estimates are that this ailment causes yearly losses to breeders amounting to more than 500 million dollars, due to mortality, morbidity, reduced milk and meat production, stillbirths, reduced fertility and expenses for vaccines and treatment of sick animals. CTF is a complex formed by two ailments: babesiosis and anaplasmosis caused by different etiological agents, but with similar clinical signs and epidemiological patterns. Bovine babesiosis is due by the protozoa Babesia bovis and Babesia bigemina which parasitizes erythrocytes and induces hemolytic anemia. The occurrence of babesiosis coincides with the dispersion of the cattle tick Rhipicephalus (Boophilus) microplus considered the only biological vector of these parasites. The control of babesiosis in herds requires maintaining a minimum level of infestation by these ticks. The greater resistance of Bos taurus indicus (zebu) animals to this tick and agents of CTF has prompted Brazilian stockbreeders mainly in regions with hot climates to prefer animals of this breed in detriment to B. taurus taurus (taurine), even though the latter are considered more productive (quantitatively and qualitatively). Studies have investigated the genetic aspects of resistance to ticks by cattle of these two subspecies and crosses between them. The results show it is possible selectively breed animals for increased tick resistance but since the phenotypic evaluation is difficult, genetic improvement programs rarely involve this trait. Therefore, little has been done regarding breeding cattle for resistance to babesiosis. Taurine animals are considered less resistant in the sense of presenting more evident clinical symptoms and higher mortality. The emergence of molecular techniques based on PCR has allowed identifying the regions where babesiosis is endemic. The findings have shown that even among Zebu animals, virtually 100% are carriers of B. bovis and B. bigemina. The aim of this study is to evaluate the possible causes of variation in resistance of cattle to babesiosis, in two breeds resulting from crossing taurines and zebus. Immune response of calves of the Canchim breed will be studied in the first year. In the first three months, when the animals have innate immunity and the immunity conferred by the colostrum we will study the frequency of transplacental transmission and neonatal infection by the Babesia ssp. The next phase, until 180 days of age is more critical for the acquisition of adaptive immunity, and by 12 months of age it is believed that adaptive immunity typically reaches a point of stability. In animals of the Braford breed, the investigation will focus on the quantitative genetic components of the variation in the number of copies of B. bovis and B. bigemina in the blood of animals between 12 and 24 months old, when immunity is stabilized. This project is divided into four studies. In the first one, biological materials will be collected from 50 calves, to allow evaluation of the immunological process in the periods before and after weaning. Four measures indicating the immune response will be ascertained: rate of babesiosis infection, humoral and cellular immune responses and expression of genes associated with the immune process. In the second study, the samples collected will be used for molecular characterization of the strains of babesiosis agents from the genetic and virulence standpoints. These differences can influence the immune response and be the cause of variation, to controlled in assessing the host's response. The third study will investigate aspects of transplacental transmission, especially its role in the development of adaptive immunity. The fourth study will focus on quantification of the level of babesiosis infection in about 1,800 animals to estimate the genetic parameters and shed light on genomic selection and genomic association in general. (AU)