Bovine babesiosis: Cattle protected in the field with a frozen vaccine containing Babesia bovis and Babesia bigemina cultured in vitro with a serum-free medium

Highlights

• Vaccine with Babesia bovis and B. bigemina cultured in vitro in a serum-free medium.

• Excellent protection for babesiosis with vaccine derived from in vitro culture in bovine serum-free medium.

• 100% protection against babesiosis in animals vaccinated with a novel vaccine.

Abstract

An attenuated live vaccine containing Babesia bovis and B. bigemina cultured in vitro with a serum-free medium was assessed for its clinical protection conferred of naïve cattle, under natural tick-challenge in a high endemicity zone to Babesia spp. Three groups of six animals were treated as follows: group I (GI) received a vaccine derived from parasites cultured with a free-serum medium; group II (GII) were immunized with the standard vaccine, with parasites cultured in a medium supplemented with 40% (v/v) bovine serum; and a control group (GIII) inoculated with non-infected bovine erythrocytes. Inocula were administered by IM route. Experimental animals were kept during 23 days after vaccination in a cattle farm free of ticks and Babesia spp. Thereafter, cattle were moved to a high endemicity farm for natural exposure to Babesia spp. transmitted by Rhipicephalus microplus ticks. Protection against clinical babesiosis was observed in bovines belonging to GI (100%) and GII (83.33%), while the control animals (GIII) were not protected, and showed severe clinical signs, closely related to babesiosis, were observed for at least three consecutive days during the challenge. These were fever, anemia, which were measured simultaneously, and circulating parasites were detected by optic light microscopy. All cattle showed B. bovis and B. bigemina in stained blood films during the challenge; B. bovis antibody titers were higher than those to B. bigemina in GI and GII, and lower titers were determined in GIII. The protective capacity of the vaccine derived from B. bovis and B. bigemina cultured in vitro in a serum-free medium was demonstrated.

Introduction

Bovine babesiosis is a disease caused by apicomplexan protozoa of the genus Babesia, it is considered one of the most important among diseases transmitted by arthropods to cattle, and it is characterized by fever, hemolytic anemia, hemoglobinuria, anorexia and often death [1], [2]. In Mexico, the main vector is the tick Rhipicephalus microplus distributed in tropical and subtropical regions, which transmits both Babesia bovis and B. bigemina [3]. Economic losses in cattle by ticks and transmitted diseases such as babesiosis and anaplasmosis, could be in the order of US$10 billion per year in worldwide [4]. The use of live attenuated vaccines is the best intervention strategy for the control of bovine babesiosis, because these can induce a protective immune response in naïve cattle [5]. The use of live vaccines with low virulence strains has been reported in countries such as Australia, Argentina, Brazil, Colombia, Mexico, Uruguay, Israel, Malawi and South Africa [6]. Some of these vaccine strains were attenuated either through serial passages in splenectomized cattle [7], [8], [9] or by using the culture in vitro of B. bovis and B. bigemina strains. In Mexico, there is an attenuated live vaccine against both species of Babesia that protects at least 80% of the vaccinated cattle exposed with infected ticks in endemic areas [3], [10], [11]. Thus, there is a need for continuous improvement of live attenuated vaccines to maintain their effectiveness and quality to protect cattle, despite possible antigenic variation and the potential for reversion to virulence of parasite strains [12], [13], [14], [15]. Recently the standard in vitro process for growing Babesia species was modified by Rojas et al. [16], who reported the elimination of bovine serum from the in vitro culture of B. bovis (Bbovis-SF) and B. bigemina (Bbig-SF), and supplemented medium with vital elements such as insulin, transferrin, selenite and putrescine [17], [18]. In this process, to optimize the proliferation of these parasites a perfusion bioreactor was used. However, it remains unknown whether these changes could somehow alter the antigenic make up of parasites grown under such culture conditions. Live vaccine production by using this new process could affect its immunoprotective capacity in susceptible animals during a natural challenge. Therefore, the objective in the present study was to compare the protection between a vaccine derived from B. bovis and B. bigemina cultured in a serum-free medium, supplemented with vital elements as compared to that conferred by and a standard vaccine obtained with culture medium with 40% (v/v) bovine serum as supplement. This experiment was performed by vaccinating susceptible cattle, and naturally challenging these cattle in the babesiosis endemic area in the Gulf coast of Mexico.