We’re deep into tick season in the Eastern US. As dog parents, we give our pup monthly flea and tick medication and check her for ticks if she’s been in fields or wooded areas. We thought we were in the clear, but when we went to the vet one time, her routine bloodwork came back positive for anaplasmosis, a tick-borne illness.
I had no idea what this disease was (maybe this is embarrassing to admit as a microbiologist) and was surprised since she did not seem sick. I had to dig deeper into this lesser known tick-borne illness.
What is anaplasmosis and how do you get it?
Anaplasmosis is transmitted through bites from ticks infected with the bacterium Anaplasma phagocytophilum (or less commonly, Anaplasma platys). Humans, dogs, cattle, goats, and horses can get anaplasmosis. The stats vary based on location and time of year, but many regions are showing an uptick in anaplasmosis cases. For example, the Pennsylvania Department of Health has reported an increase of anaplasmosis cases from 2008 to 2017.
Since ticks can also carry the bacteria behind Lyme disease, it’s possible to get anaplasmosis and Lyme disease at the same time. However, the data that exists on this vary. One source reports ~45% of dogs are positive for both anaplasmosis and Lyme disease, but I couldn’t verify this claim in peer reviewed literature. Another study found that co-infection, based on antibody positivity for both diseases, occurred in ~4% of dogs based on routine wellness screening. The study was from 2014 so that number is likely different now and will be different depending on geographic location.
What happens during Anaplasma phagocytophilum infection?
A. phagocytophilum can only survive and reproduce within a type of immune cell called neutrophil. Once transmitted by a tick, A. phagocytophilum injects molecules into the neutrophil, attaches to its surface, and enters the cell. A. phagocytophilum interferes with normal neutrophil functions, preventing it from killing the bacterium. It uses the host cell’s nutrients to replicate and form an intracellular cluster called a morula. Lastly, the newly multiplied A. phagocytophilum exits the cell to continue infecting other cells.
My dog has anaplasmosis, but no symptoms
The symptoms for anaplasmosis include fever, joint pain, decreased appetite, and limping or the inability to walk properly. Symptoms usually begin within one to two weeks of infection. With co-infection, it’s more likely that dogs will have symptoms of disease.
I got curious about asymptomatic A. phagocytophilum infections since my dog did not have symptoms but tested positive. It turns out that a large fraction of dogs are positive when tested for anaplasmosis but don’t have symptoms. A lot of this may be explained by the type of test the dog got.
While I don’t know exactly which test my dog got, below are a few different tests used to diagnose anaplasmosis:
- Polymerase chain reaction: detects A. phagocytophilum DNA, low-level chronic infections might not detectable levels of DNA
- ELISA: detect antibodies made against A. phagocytophilum, positive tests can indicate prior infection (maybe this is the kind of test my dog got?)
- Blood smears: to visualize the morula from infected immune cells, positive test can indicate current infection, but morula may be missed in low-level chronic infection
More information on anaplasmosis diagnostic results and symptoms can be found here.
For those of you wondering about human anaplasmosis: you can’t get this from your dog. But, if your dog has anaplasmosis, it’s likely that you may have been exposed to ticks as well.
Further reading
CDC: Clinical Testing and Diagnosis for Anaplasmosis. 2024
CDC: Tickborne Diseases of the United States. 2022
This Podcast Will Kill You: Episode 35 Lyme Disease: I’d like to check you for ticks. 2019
Featured image: Anaplasma phagocytophilum infection in sheep. Source: Alan R Walker.
