Trends in tick-borne diseases
by Jenny Alonge
In the past two decades, the annual number of tickborne disease cases has more than doubled in the U.S. Potential reasons include growing tick populations, rising heat and humidity that provide a good tick environment, wildlife population changes, increased wildlife interactions in newly developed and reclaimed areas, pet rehoming, and improved disease tracking. The Companion Animal Parasite Council (CAPC) and other organizations track U.S. tick-borne disease trends to determine the risk for people and animals.
Lyme disease trends
Lyme disease is the most common tick-borne disease. The Centers for Disease Control (CDC) records about 36,000 human cases every year, but many cases are underreported, and the agency estimates that as many as half a million Americans are diagnosed and treated annually for Lyme disease.
The disease vector, the blacklegged tick (Ixodes scapularis), continues to expand its geographic range and is considered an important One Health pathogen. CAPC findings include:
- Lyme disease is expanding southward and westward outside the historically high-risk Northeast and Upper Midwest regions.
- Minor increases are expected in northeastern Tennessee, western Michigan, and Ohio. High-risk areas are predicted in northern and western lower Michigan, and southern and northeastern Ohio.
- A higher-than-normal risk is expected in North Dakota, northeastern South Dakota, Iowa, Illinois, and eastern Kentucky.
- Increasing risk is expected in North Carolina, with prevalence rates in some eastern North Carolina counties as high as 6% to 9%.
Ehrlichia spp. trends
Ehrlichiosis is transmitted through the bite of an infected lone star tick (Amblyomma americanum), brown dog tick (Rhipicephalus sanguineus), or blacklegged tick. The organism Ehrlichia canis most commonly causes illness in dogs, while E. chaffeensis causes most human cases. Ehrlichiosis high prevalence areas are more widespread and less well-defined than for other vector-borne pathogens. CAPC findings include:
- Ehrlichiosis prevalence is expected to increase across most of the U.S.
- Ehrlichiosis risk is high for dogs throughout the southeast, southwest, southcentral, and coastal Atlantic states.
- Increasing numbers of seropositive dogs are expected in central California, northern Idaho, western Washington, Vermont, New Hampshire, and Connecticut. Many parts of Colorado and Wyoming are also at risk.
- E. muris eauclarensis is a new Ehrlichia species that is emerging in parts of Wisconsin and Minnesota.
Anaplasma spp. trends
Anaplasmosis phagocytophilum, which is transmitted primarily by the blacklegged tick and the western blacklegged tick (Ixodes pacificus), is most common in the upper Midwest and Northeastern states. The organism was previously known as Ehrlichia phagocytophilum, but a taxonomic change in 2001 identified that the organism belonged to the genus Anaplasma, and the name was changed to A. phagocytophilum. The organism A. platys can also cause disease in dogs and is transmitted by the brown dog tick, which is most commonly found in the U.S. southern and western areas. Anaplasmosis cases reported to the CDC have increased steadily since the disease became reportable in 2000. CAPC findings include:
- Anaplasmosis seroprevalence in dogs follows the expanding blacklegged tick range.
- Dogs in the Northeast and upper Midwest are at increased risk.
- Seroprevalence is expected to increase considerably throughout the Northeastern and Mid-Atlantic regions, in the upper Michigan regions, and western North Carolina.
- A. platys prevalence is predicted to increase in California and Texas.
Babesiosis spp. trends
Babesiosis is transmitted by blacklegged ticks, brown dog ticks, and the American dog tick (Dermacentor variabilis). Several disease-causing species are important for dog health, including Babesia canis (subspecies vogeli, canis, rossi), B. gibsoni, B. vulpes, and B. conradae, while B. microti causes most human cases.
In March, the CDC reported an increase in babesiosis incidence throughout the U.S.
Three states (i.e., Maine, New Hampshire and Vermont), where babesiosis had not been considered endemic, had significantly increasing incidence and reported case counts similar to or higher than those in the seven states (i.e., Connecticut, Massachusetts, Minnesota, New Jersey, New York, Rhode Island, and Wisconsin) with known endemic transmission.
This information highlights the need for year-round parasite protection and annual testing. Year-round use of parasite prevention products provides the best comprehensive internal and external parasite control, and annual testing can help monitor compliance and preventive efficacy.
For more information about the tick-borne diseases prevalent in your area, access the CAPC website’s parasite prevalence maps.
The council now offers a flea activity forecast at www.petdiseasealerts.org that displays a national view of flea activity based on environmental conditions that is updated daily. This is another reminder about the importance of year-round parasite control to help limit infestations and prevent fleas from establishing populations in and around homes.
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