A Review of development of colonies of Anaplasma marginale in the gut of Dermacentor andersoni

Abstract

Anaplasmosis is a major disease of cattle in the United States and is one of several tick-borne maladies that severly hamper cattle production world-wide. The causative agent, Anaplasma marginale, is currently classified in the Family Anaplasmataceae, Order Rickettsiales according to the 8th Edition of Bergey’s Manual, but little is known about the developmental cycle of the organism in either the invertebrate (tick) or vertebrate (cattle) hosts. Previous to studies in our laboratory, the only form of A. marginale that had been identified and described was the erythrocytic inclusion body found in cattle with anaplasmosis. Although A. marginale had been reported by earlier workers to be transmitted by ticks, it was not clearly established whether the organisms observed in previous studies were those ingested as part of the blood meal or were stages that developed within tick cells.

The studies from our laboratory reviewed herein were undertaken to demonstrate A. marginale in Dermacentor andersoni and to describe its development from infection of nymphal ticks through transmission of the organism to cattle by subsequently molted adults. In these studies, A. marginale has been found to occur within membrane-bound inclusions that we have termed colonies. The colonies were first described in midgut epithelial cells of adult D. andersoni that were infected as nymphs. Since publication of that finding, colonies have also been found in replete nymphal ticks. The colonies at various stages of tick development were found, by light microscopy, to have different morphologic characteristics and to contain several types of organisms—representing part of a developmental sequence of the organism in the tick. This review of the development of A. marginale in ticks will be presented chronologically beginning from infection of nymphs (while feeding on a calf with anaplasmosis) and continuing through the time ticks molt to the adult stage.