Ultrastructure of Babesia bovis sexual stages as observed in Boophilus microplus cell cultures

How relevant are in vitro culture models for study of tick-pathogen interactions?

Although tick-borne infectious diseases threaten human and animal health worldwide, with constantly increasing incidence, little knowledge is available regarding vector-pathogen interactions and pathogen transmission. In vivo laboratory study of these subjects using live, intact ticks is expensive, labor-intensive, and challenging from the points of view of biosafety and ethics. Several in vitro models have been developed, including over 70 continuous cell lines derived from multiple tick species and a variety of tick organ culture systems, facilitating many research activities. However, some limitations have to be considered in the translation of the results from the in vitro environment to the in vivo situation of live, intact ticks, and vertebrate hosts. In this review, we describe the available in vitro models and selected results from their application to the study of tick-borne viruses, bacteria, and protozoa, where possible comparing these results to studies in live, intact ticks. Finally, we highlight the strengths and weaknesses of in vitro tick culture models and their essential role in tick-borne pathogen research.

Three-Dimensional Culture of Rhipicephalus (Boophilus) microplus BmVIII-SCC Cells on Multiple Synthetic Scaffold Systems and in Rotating Bioreactors

Tick cell culture facilitates research on the biology of ticks and their role as vectors of pathogens that affect humans, domestic animals, and wildlife. Because two-dimensional cell culture doesn't promote the development of multicellular tissue-like composites, we hypothesized that culturing tick cells in a three-dimensional (3-D) configuration would form spheroids or tissue-like organoids. In this study, the cell line BmVIII-SCC obtained from the cattle fever tick, Rhipicephalus (Boophilus) microplus (Canestrini, 1888), was cultured in different synthetic scaffold systems. Growth of the tick cells on macrogelatinous beads in rotating continuous culture system bioreactors enabled cellular attachment, organization, and development into spheroid-like aggregates, with evidence of tight cellular junctions between adjacent cells and secretion of an extracellular matrix. At least three cell morphologies were identified within the aggregates: fibroblast-like cells, small endothelial-like cells, and larger cells exhibiting multiple cytoplasmic endosomes and granular vesicles. These observations suggest that BmVIII-SCC cells adapted to 3-D culture retain pluripotency. Additional studies involving genomic analyses are needed to determine if BmVIII-SCC cells in 3-D culture mimic tick organs. Applications of 3-D culture to cattle fever tick research are discussed.

Babesiosis and heartwater: threats without boundaries

Suppose one of your clients from southern Florida starts talking about cattle egrets while you are vaccinating her cat. It seems she found a nearly dead egret near the cattle pen a few days ago, picked it up, and noticed a number of what looked like small ticks on the legs. Or, suppose you are called out to a small dairy in central Texas to look at some cows that are feverish and anemic. The first animal you examine has a few brown ticks attached just under the tail. Finally, perhaps you are looking at a lame tortoise for a reptile fancier, a new client, and find a large, colorful tick on a hind leg, well up under the shell. Ring any bells? Egrets are great hosts for the immature stages of Amblyomma ticks and have been captured and marked in the eastern Caribbean, then recaptured in the Florida Keys. Those cattle ticks in Texas might be acaricide-resistant Boophilus ticks that originated in Mexico. The Amblyomma tick on the tortoise could well have "hitch-hiked" all the way from South Africa. By now you remember that both Amblyomma and Boophilus ticks are efficient vectors of two tickborne diseases in this hemisphere, heartwater (in the case of Amblyomma) and babesiosis (transmitted by Boophilus ticks). Both of these diseases are exotic to the United States, and because our livestock are considered to be totally susceptible, an introduced infection could result in high initial death losses (approximately 70%); thus, both the ticks and the diseases pose immediate threats to the health and economic security of United States animal industries. Most importantly, you, whether as a small animal or large animal practitioner, are the first line of defense against such exotic diseases and their vectors.

Development of tick gut forms of Babesia bigemina in vitro

Development of a laboratory cultured tick-transmissible strain of Babesia bigemina was followed in vitro after addition of gut material from engorged female Boophilus microplus ticks and incubation at 37 degrees C. Sequential development of stages, from intraerythrocytic strahlenkörper through multiplication to the fusion of what is assumed to be two gametes, is described. A change in physical environment (temperature, gas composition) experienced during passage of Babesia stages into the in vitro culture tubes possibly mimics the changes experienced in passage from host blood to the midgut of the tick vector. The effect in vitro was to induce the erythrocytic parasites to remain inactive at a trophozoite-like stage. Addition of factor(s) within midgut initiated further development of strahlenkörper. Two populations of strahlenkörper were recognized; an elongated form which did not appear to develop further, and a polymorphic population which underwent further multiplication initiated while the parasites were still within the erythrocyte, and continuing after they had emerged. These strahlenkörper increased in size as multiple division of nuclei occurred. with cell division being completed more slowly. Large aggregations of multinucleated strahlenkorper formed, but once division was complete, single-nucleated strahlenkörper emerged from the aggregates. Two individuals of post-aggregation strahlenkörper, assumed to be gametes, fused together. The morphology and ultrastructure of all stages of development are described and compared with forms already described from the tick midgut.

Ultrastructure of Babesia bigemina gametes obtained in "in vitro" erythrocyte cultures

Propagation of Babesia bigemina in "in vitro" erythrocyte cultures resulted in the appearance of the sexual stage of the parasite, characterized by cytoplasmic projections and microtubules. These morphologically distinct parasites were found to adhere to each other, with later fusion of their membranes. No nuclear fusion was observed. The ultrastructure of the sexual stages was described.