Infectious Disease
1910 — South Africa, Arnold Theiler's identification of Anaplasma marginale in cattle; human disease first described 1990, United States
Anaplasmosis, a tick-borne disease caused by the bacterium Anaplasma phagocytophilum, was entirely unrecognized as a distinct entity until the final decades of the twentieth century. For most of medical history, its existence was simply unknown, and patients who suffered from it were either diagnosed with other febrile illnesses or went unexplained. The scientific story of anaplasmosis is therefore a modern one, rooted in the development of veterinary microbiology before it migrated into the realm of human medicine.
Historical Narrative
Unlike many classical diseases with roots stretching back to antiquity, anaplasmosis has an almost entirely modern medical history, making it a rare example of a condition whose entire arc of discovery took place within living memory of the late twentieth century. The organism responsible, along with the broader family of bacteria to which it belongs, was first encountered not in human patients but in livestock.
The genus Anaplasma was first described in the context of veterinary disease. Sir Arnold Theiler, a South African veterinary researcher, identified Anaplasma marginale in 1910 as the causative agent of anaplasmosis in cattle, a disease that devastated herds across southern Africa and caused enormous economic losses. Theiler's work established the foundational understanding that certain small parasitic organisms could infect red blood cells in animals and be transmitted by ticks. His discovery, while entirely focused on bovine disease, created the conceptual and taxonomic groundwork that would later prove essential when the human variant attracted scientific attention.
For decades, the possibility that closely related organisms might infect human beings was not seriously considered in mainstream medicine. Tick-borne diseases were increasingly studied in the early twentieth century — Rocky Mountain spotted fever, identified and investigated by Howard Taylor Ricketts and others in the early 1900s, demonstrated that ticks could serve as vectors for serious human pathogens. This work opened broader scientific curiosity about tick-associated febrile illnesses, but the particular organism responsible for what would eventually be called human granulocytic anaplasmosis remained invisible to researchers.
The decisive turn came in 1990, when a critically ill patient in Wisconsin who had been bitten by ticks was found to have unusual inclusions within his white blood cells — specifically within granulocytes. The patient died, and post-mortem examination and subsequent study by a team led by Johan Bakken and Jochen Dumler revealed an organism resembling Ehrlichia species that had previously been known only from veterinary contexts. This case prompted retrospective investigation and, eventually, formal description of a novel human pathogen.
Bakken, Dumler, and colleagues published their seminal findings through the early and mid-1990s, establishing that a distinct tick-borne bacterial infection was circulating in the upper Midwest of the United States. The organism was initially placed within the genus Ehrlichia and designated Ehrlichia phagocytophila, reflecting its visible tropism for granulocytes. The taxonomic situation was complicated and debated among microbiologists for years, as molecular techniques revealed the organism's closer relationship to members of the Anaplasma genus. In 2001, formal reclassification placed it within Anaplasma, renaming it Anaplasma phagocytophilum.
The identification of the Ixodes scapularis tick — the same black-legged tick known as the primary vector for Lyme disease — as the principal vector in the northeastern and upper midwestern United States added anaplasmosis to a growing list of tick-borne threats that public health researchers began mapping with increasing urgency in the late twentieth century. The realization that a single tick bite could potentially transmit multiple pathogens simultaneously opened new research questions about co-infection that occupied researchers through the 1990s and into the 2000s. Anaplasmosis thus emerged as a wholly twentieth-century discovery, its history compressed into a remarkably short span of scientific inquiry.
Key Historical Figures
Historical narrative only — this page describes how Anaplasmosis was understood historically. It is not medical advice and does not describe current diagnosis or treatment. Sourced from verified medical history references (NIH, Encyclopaedia Britannica, and standard medical history texts). See our medical disclaimer.