It has been known for decades that American dog ticks (Dermacentor variabilis) can transmit Anaplasma marginale, the bacterium that causes bovine anaplasmosis. Laboratory studies established the tick’s role as a vector, but finding infected host-seeking ticks in the field has proven surprisingly difficult.
So when University of Missouri researchers finally detected A. marginale in a host-seeking American dog tick, the reaction wasn’t what many might expect.
“I wasn’t surprised that we found it,” says lead researcher Dr. Rosalie Ierardi, DVM. “I was surprised that we didn’t find more of it.”
That unexpected response reflects a question that has puzzled researchers for years: If American dog ticks transmit bovine anaplasmosis, why have infected ticks been so difficult to find?
The study published in the Journal of Medical Entomology may offer part of the answer.
Researchers Detect Anaplasma marginale in Host-Seeking American Dog Ticks
Between 2022 and 2024, Ierardi and her team sampled ticks from five Missouri beef cow-calf operations, collecting more than 29,000 ticks from pasture vegetation. The overwhelming majority were lone star ticks, but investigators also collected more than 1,500 American dog ticks.
After testing 692 adult male American dog ticks for A. marginale, only one pool of five ticks tested positive.
Although the prevalence was just 0.6%, the finding is significant. Previous field surveys in Oklahoma, Tennessee, the northwestern U.S. and southern Canada failed to detect A. marginale in host-seeking Dermacentor ticks, even in areas where bovine anaplasmosis is endemic.
This work represents what researchers believe is the first field detection of A. marginale in a host-seeking American dog tick, supporting decades of laboratory research showing that adult male ticks can acquire and carry the pathogen under natural conditions.
Why One Positive Tick Pool Could Change How Researchers View Bovine Anaplasmosis Transmission
At first glance, finding only one positive pool might appear reassuring. Ierardi cautions against that interpretation.
“I would consider even a low rate of detection to be significant, as it probably underestimates the true risk,” she says.
The explanation may lie not in how many infected ticks exist, but in how researchers search for them.
“The dragging method we used only captures ticks that are actively searching for a new host,” Ierardi explains. “Ticks that might move directly between cattle would not be captured.”
That distinction could help explain why infected ticks have remained so elusive.
The researchers propose that infected male American dog ticks may frequently move directly from one animal to another instead of dropping into vegetation to search for a new host. If that occurs, conventional drag sampling could miss many of the ticks most likely to transmit bovine anaplasmosis.
Previous experimental studies support that possibility. In one trial, marked male Dermacentor ticks transferred directly between calves housed together, while relatively few ticks were recovered from surrounding vegetation. Another study found tick movement increased when cattle huddled during cooler weather, creating more opportunities for ticks to change hosts.
Field Observations Support the Host-to-Host Transmission Theory
Although the study was not designed to observe tick movement directly, one field observation reinforced the hypothesis.
“On one farm we caught a lot of male D. variabilis ticks in a certain field,” Ierardi says. “The ticks were abundant in a patch of shade where the cattle had obviously been congregating, where the grass was trampled down to the dirt and it was a bit hollowed out. I was initially surprised because ticks, generally, don’t prefer bare soil. It made me wonder whether they had dropped off the cattle in that spot.”
While anecdotal, the observation illustrates how cattle behavior and tick behavior may interact in ways that traditional surveillance methods fail to capture.
What the Findings Mean for Bovine Anaplasmosis Research
The study does not change current recommendations for bovine anaplasmosis prevention. Tick control and reducing mechanical transmission through contaminated needles or blood-contaminated equipment remain important components of herd health programs.
Instead, the findings refine our understanding of how tick-borne transmission may occur under natural conditions.
Rather than suggesting infected ticks are rare, the study raises the possibility that researchers have simply been looking in the wrong place. If infected male ticks spend much of their time moving directly between cattle instead of questing on vegetation, conventional surveillance may underestimate their role in disease transmission.
That idea could also help explain why previous surveys in other endemic regions consistently failed to detect A. marginale in host-seeking ticks despite longstanding evidence that American dog ticks transmit bovine anaplasmosis.
What’s Next for Research on Tick Transmission of Anaplasma marginale?
The Missouri study raises several important questions.
We still do not know how commonly infected host-seeking American dog ticks occur in other endemic states or whether different surveillance methods could reveal additional infected ticks that traditional drag sampling misses.
For Ierardi, however, the findings reinforce rather than overturn existing knowledge.
“These results are consistent with previous work that shows adult male Dermacentor ticks transmit A. marginale,” she says.
Future studies will aim to determine whether the same pattern is found in other endemic regions. If so, we may need to rethink how they monitor tick populations and interpret surveillance data.
