Penn Vet | Fact Sheet Detail




By: Wildlife Futures Team Date: Nov 1, 2020


Anaplasmosis is a tick-borne disease caused by the bacteria Anaplasma phagocytophilum. Anaplasmosis was originally believed to infect only ruminants such as cattle, sheep, goats, deer, elk, bison, antelopes etc. Following recent reclassification, some former members of the Ehrlichia group that infect humans, dogs and horses, are now considered to be part of the group Anaplasma.


With few exceptions, Anaplasmosis is not considered a disease that affects wildlife (see Clinical Signs). In humans, known as human granulocytic anaplasmosis (HGA), it can cause serious illness though it is rarely fatal. People can acquire the disease from the bite of a tick carrying the bacteria, from blood transfusions, or organ transplants. Tick bite prevention is key to protecting humans and domestic animals from this disease. In some areas, ticks can be infected with multiple bacteria at the same time, like A. phagocytophilumBorrelia burgdorferi (Lyme) and Ehrlichia.

Species Affected

Rodents, like the white footed mouse, deer and birds are considered the main reservoirs of A. phagocytophilum. However, the bacterium can be found in many wild ruminants including white-tailed deer, black-tailed deer, mule deer, elk, bison, pronghorn antelope, moose, bighorn sheep and several exotic species without apparent illness. Non-ruminant wildlife species that may be infected include, but are not limited to, rodents, coyotes, ­fishers and mountain lions. Humans, dogs and horses can also be infected.


Different species of Anaplasma are found worldwide. In the United States, human cases of anaplasmosis are most common in the upper Midwest and Northeast regions of the country with occasional cases on the West Coast. These areas correspond to known geographical distributions of the Ixodes scapularis and Ixodes pacificus ticks, the primary vectors of A. phagocytophilum. In 2017, the states with the highest reported cases of Anaplasmosis included Connecticut, Maine, Massachusetts, New Hampshire, New York, Rhode Island, Vermont, New Jersey, Minnesota and Wisconsin.


The most common mode of transmission for anaplasmosis is through the bite of a tick carrying the bacteria. A. phagocytophilum can survive and reproduce in the tick. In the United States, the bacteria is transmitted by the deer tick, I. scapularis, in the Northeast and Midwest and by the western black-legged tick, I. pacificus, along the West Coast. Biting flies can also transmit the bacteria but are less efficient vectors because the organism does not survive very long on the mouthparts of flies. Rarely, anaplasmosis can also be transmitted by way of blood-contaminated needles or surgical equipment.

Most cases of anaplasmosis are reported during the summer months, typically June and July which corresponds with the active nymphal tick stage. A second smaller peak occurs in October and November which corresponds to adult tick activity periods.

Clinical Signs

Clinical signs are rarely observed in wildlife. Deer and other wild ruminants are commonly infected with Anaplasma but do not usually become noticeably ill. Clinical signs observed in domestic ruminants, dogs and humans include anaemia, fever, lethargy, loss of coordination, muscle pains, shortness of breath, rapid pulse and yellow membranes. The urine may appear brown in color and the spleen is often enlarged. Clinical manifestations can range from mild to life-threatening depending on the age of the individual affected and general health.

There have been two reports of giraffes with fatal anaplasmosis infections. Bighorn sheep also seem to be more susceptible to clinical illness than other wild ruminants.


Several laboratory tests are used to diagnose anaplasmosis. Using a special stain, blood smears may be examined microscopically to look for the characteristic morulae life-stage of A. phagocytophilium.


There is no vaccine for anaplasmosis. Antibiotics, specifically Doxycycline, are used to treat anaplasmosis in humans, domestic animals and captive wildlife species, but this treatment is not feasible for free-ranging wildlife. Tick and fly control can be used as a preventative measure in captive species. In wildlife, topical tick controls applied at feed or feeding stations have been tried and been effective at controlling tick populations in an urban environment.  However, this is not a practical method of tick control in the wild.


Elimination of anaplasmosis from wildlife is likely not feasible. The best method to prevent infection of domestic animals and humans is vigilant tick control.

For humans, protective clothing such as long pants and long sleeve shirts and repellents containing DEET help to minimize tick exposure. Permethrin-based repellents can also be sprayed on boots and clothing. Hunters and meat processors should wear gloves when preparing carcases to prevent tick bites. Ticks should be promptly removed if found on people or pets. Domestic animals can introduce ticks to their owners and can themselves become infected with anaplasmosis.

Suggested Reading

  • Anaplasmosis. 2015. The Merck Veterinary Manual.
  • Centers for Disease Control and Prevention (CDC). 2011. Anaplasmosis.
  • Davidson, W. R., and W. L. Go . 2001. Anaplasmosis. Pages 455-466 inE. S. Williams and I. K. Barker, editors. Infectious diseases of wild mammals. Iowa State University Press, Ames, Iowa, USA.
  • Dugan, et al. 2006. Evaluation of white-tailed deer (Odocoileus virginianus) as natural sentinels for Anaplasma phagocytophilum. Vector-Borne and Zoonotic Disease 6: 192-207.
  • Jessup, D. A., W. L. Go , D. Stiller, M. N. Oliver, V. C. Bleich, and W. M. Boyce. 1993 A retrospective serologic survey for Anaplasma spp. infection in three bighorn sheep (Ovis canadensis) populations in California. Journal of Wildlife Diseases 29: 547-
  • Foley, J., N. C. Nieto, J. Madigan, and J. Sykes. 2008. Possible differential host tropism in Anaplasma phagocytophilum strains in the western United States. Animal Biodiversity and Emerging Diseases: Annals of the New York Academy of Sciences 1149: 94-97
  • Kirby C. Stafford, III, Scott C. Williams. 2017. Deer-Targeted Methods: A Review of the Use of Topical Acaricides for the Control of Ticks on White-Tailed Deer, Journal of Integrated Pest Management, Volume 8.