Zoonotic Implications of Changing Tick Populations

Between 1940 and 2004, the majority of emerging human infectious diseases worldwide were zoonotic. Of these, nearly one-quarter were arthropod vector-borne diseases, with ticks being the most common vector. In the United States, tickborne diseases account for about 95% of vector-borne diseases.

A recent review article in Veterinary Sciences examined many factors of tick biology, including the changing geographic distribution of tick populations and the impact of this change on associated tickborne diseases.

Ixodes Ticks and Associated Pathogens

Ixodid ticks exist worldwide. Warmer temperatures and changing humidity have allowed for northern expansion in North America, Europe, and Russia. Many significant zoonotic pathogens are carried by these ticks:

• Borrelia burgdorferi, the causative agent of Lyme disease, is now seen throughout the United States, Canada, and Europe.
• New Borrelia species identified worldwide have been implicated as additional causative agents of Lyme disease (Borrelia mayonii) and a relapsing fever (Borrelia miyamotoi).
• Babesiosis, caused by over 100 different Babesia species, is especially significant for cattle and humans. Human babesiosis cases are expected to be seen in Canada due to the increased number of Ixodes scapularis ticks, and new Babesia species are now seen in regions not previously known to have babesiosis.
• Anaplasma phagocytophilum is the causative agent of human granulocytic anaplasmosis (HGA), equine anaplasmosis, and febrile diseases in ruminants, cats, and dogs. Reports of HGA in the United States increased by a factor of 12 between 2001 and 2011.
• Co-infections are common in individuals exhibiting disease from an Ixodes tick vector. Ten percent of individuals infected with Anaplasma also had antibodies to B burgdorferi or Babesia microti.

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Ambylomma Ticks and Ehrlichia

In the United States, Amblyomma americanum ticks have expanded both north and west as white-tailed deer populations have increased in these regions. All life stages of this tick species can feed on humans and deer, increasing the potential for transmission of Ehrlichia chaffeensis and Ehrlichia ewingii, the most common causes of human monocytic ehrlichiosis.

In the rest of the world, other Amblyomma ticks serve as vectors for multiple species of Ehrlichia, including new genetic variants classified as Candidatus Neoehrlichia species in Europe and Asia. For veterinarians, heartwater disease, caused by Ehrlichia ruminantium, is an increasingly important reportable disease of ruminants in Africa and the Caribbean.

Viral Vector-Borne Diseases

Vector-borne viruses are another emerging global zoonotic threat. Many tick species carry viruses of increasing public health importance:

• Rhipicephalus microplus and Haemaphysalis longicornis ticks in China and Amblyomma americanum in the United States are known vectors of closely related viruses causing severe fever and thrombocytopenia. In the United States, this virus is known as heartland virus.
• Bourbon virus was recently discovered in the United States.
• Powassan virus is reemerging in North America.
• Tickborne encephalitis viruses are broadening in range throughout Europe as reforestation and movement of dogs allows the range of their vector, Dermacentor reticulatus, to expand into Germany, the Netherlands, and Poland.
• Crimean-Congo hemorrhagic fever virus is spreading to multiple countries in the Mediterranean, likely due to the transportation of its tick vector, Hyalomma marginatum, by birds from Africa, Asia, and Eastern Europe to Central Europe.

Take-Home Message

Practitioners in both veterinary and human medicine must remain aware of the changing geography of ticks and associated vector-borne diseases. The discovery of the Asian tick H longicornis in New Jersey and Virginia should be an important reminder of the fact that “ticks and tickborne pathogens do not recognize international boundaries.” Thus, “a robust international disease monitoring network” is needed to protect both human and animal health from both known and emerging tick-borne diseases.

Dr. Boatright, a 2013 graduate of the University of Pennsylvania, is an associate veterinarian in western Pennsylvania. She is actively involved in her state and local veterinary medical associations and is a former national officer of the Veterinary Business Management Association.