The life cycle of Babesia bigemina (Smith and Kilborne, 1893) in the tick vector Boophilus microplus (Canestrini)

Abstract

The protozoan Babesia bigemina, a cause of tick fever in cattle, is transmitted in Australia by Boophilus microplus. Its development in the lumen of the gut of the tick during the first 24 hr after ingestion remains uncertain. It is believed, however, that most of the forms in the bovine erythrocytes are destroyed, and that only certain oval or spherical bodies survive and develop. These could be gametocytes. Invasion of the epithelial cells of the gut occurs about 24 hr after repletion, and subsequent multiplication of the parasite in these cells is by means of multiple fission. This leads, by about 72 hr, to the production of mature vermicules, which are released into the haemolymph. They measure about 11 µ by 2.5 µ, with a range of 9–13 µ by 2.0–2.9 µ. By about the fourth day, some of the vermicules invade the cells of the Malpighian tubules and of the haemolymph, and a secondary cycle of multiple fission gives rise to vermicules indistinguishable from those that had been produced in the epithelial cells of the gut. Others invade the ova of the tick, and subsequently undergo a similar cycle in the gut cells of the developing larva. The final cycle takes place in the salivary glands of the nymph, and the forms infective to the vertebrate host appear 8-10 days after larval attachment. Multiplication in this cycle is also by multiple fission, but it differs in detail from the preceding cycles. The resultant forms measure 2.2–2.7 µ by 1.0–1.5 µ, and are very similar to the pyriform bodies observed in bovine erythrocytes. Not all ticks develop an infection after engorging on a reacting animal. Infection of the invertebrate host depends, to some degree at least, on the parasite density in the blood of the bovine host. Also, cattle reacting to a tick-transmitted infection are more infective for the tick than those infected by blood inoculation. The vermicule is able to withstand low temperatures, but the earlier developmental stages in the female tick are inhibited by environmental temperature below 20°C. The distinctive characteristics of the vermicule could make this form of value in studies on the epidemiology of bovine babesiosis.