Ticks responsible for most U.S. vector-borne diseases
R. Scott Nolen
January 15, 2020
Standing in a New England forest on a summer day, you wouldn’t suspect you’re surrounded by dozens—possibly hundreds—of ticks wanting to feed on you.
“Most people don’t see ticks. If they did—if they saw how many are waiting on twigs and leaves and in the brush—they would never go outside again,” observed entomologist Thomas Mather, PhD, director of the Center for Vector-Borne Diseases at the University of Rhode Island.
Seven of the 17 vector-borne diseases reportable to the Centers for Disease Control and Prevention are tick-borne. Nine species of tick found throughout North American are known vectors for these pathogens. Of these, Ixodes scapularis, the blacklegged or deer tick, is the most medically important tick species. Not only is the blacklegged tick a vector for five of the six pathogens, but it is also the primary carrier of the bacterium Borrelia burgdorferi responsible for causing Lyme disease in humans as well as dogs, horses, and cats.
Dr. Mather was studying ticks before 1981, when Lyme disease was first identified as a tick-borne pathogen. Since then, he has seen the discovery of several new blacklegged tick diseases, including anaplasmosis, Powassan virus disease, and ehrlichiosis. And in that time, he has seen the tick’s habitat in the Northeastern, mid-Atlantic, and upper Midwestern regions of the United States expand as a result of human activities.
“Ticks are in more places, and that’s true for blacklegged ticks in particular,” said Dr. Mather, who oversees TickSpotters.org, an online, crowd-sourced tick survey.
Among the estimated 50,000 human cases of vector-borne disease reportedly acquired in the U.S. each year, approximately 80% are associated with ticks, according to the CDC. Lyme disease is by far the most common of the tick-borne diseases and is among the fastest-growing infectious diseases in the United States with 33,666 cases reported in 2018, although the real number of cases is thought to be far higher.
Veterinarians are seeing more cases of Lyme disease, as well. According to the Companion Animal Parasite Council, the number of canine Lyme disease cases increased from 245,971 in 2015 to 336,200 in 2019. Increases in the numbers of other tick-borne diseases were also reported, including canine anaplasmosis, up from 117,203 in 2015 to 207,825 in 2019, and canine ehrlichiosis, up from 107,985 in 2015 to 186,075 in 2019.
“That’s been the pattern for the last few decades, and we’re seeing that trend in dogs as well as in people,” confirmed Dr. Susan Little, co-director of the National Center for Veterinary Parasitology at Oklahoma State University. “We’ve seen a dramatic increase in the number of cases, not just in Lyme disease but in human anaplasmosis, human ehrlichiosis, and Rocky Mountain spotted fever.
“It’s almost an exponential increase in identified cases of infection and associated disease.”
In its 2018 Lyme Consensus Statement, the American College of Veterinary Internal Medicine states that most dogs exposed to Borrelia burgdorferi will develop subclinical infection. The small number of dogs that develop acute Lyme borreliosis may present with fever, shifting leg lameness, swollen joints, enlarged lymph nodes, lethargy, depression, and anorexia. Affected dogs typically recover following treatment with doxycycline.
While pet owners have the option of vaccinating dogs against Lyme disease, the six-member panel of ACVIM and European College of Veterinary Internal Medicine diplomates that authored the consensus statement were split on the issue. Duration of immunity, cost, and lack of controlled studies are some of the objections raised by three panelists.
If you have a dog maintained on tick control, the ticks the dog encounters are killed, and they’re not in the home or in the environment and able to transmit infection. It’s a perfect example of one health.
Dr. Susan Little, co-director, National Center for Veterinary Parasitology, Oklahoma State University
The ACVIM found no evidence linking B burgdorferi with illness in cats.
With horses, it’s more complicated. In a separate 2018 statement, the ACVIM emphasizes that a positive blood test result only indicates past or current exposure to the bacteria and not whether an animal will develop clinical signs of the disease. The tissues most commonly affected by B burgdorferi in horses include the nervous system, ocular tissues, and skin. Because many other diseases can cause clinical signs similar to those seen with Lyme disease, the ACVIM strongly recommends ruling out other diseases that could be responsible for a horse’s clinical signs rather than relying on response to treatment to conclude that a horse has Lyme disease.
Horses are not to be treated for Lyme infection solely on the basis of a positive blood test result, the ACVIM advises, but if treatment is pursued, tetracycline and beta-lactam antimicrobials should be considered.
Prior to 1970, the blacklegged tick was not considered an important vector of human pathogens.
Blacklegged ticks were restricted to the Northeast and upper Midwest following decades of deforestation to accommodate agriculture. Plus, excessive hunting and habitat loss nearly wiped out white-tailed deer, the primary host for the blacklegged tick.
Beginning in the second half of the 20th century, suburbia increasingly crept into forested portions of the Northeast. Moreover, farmland was reforested, and deer populations recovered. These factors, in conjunction with milder winters, earlier springs, and longer, warmer summers because of climate change mean blacklegged ticks are succeeding in new geographic areas.
“These are the factors we think are driving the expansion of the blacklegged tick as well as the expansion of Lyme disease and the other diseases that are carried by this tick,” said Ben Beard, PhD, deputy director of the CDC’s Division of Vector-Borne Diseases. Dr. Beard is the CDC representative on the Tick-Borne Disease Working Group, an advisory committee that makes recommendations on ways the federal government can address this public health threat.
Dr. Beard said the CDC estimates the actual number of Lyme disease infections in people to be eight to 12 times the number reported each year. Lyme disease is notoriously difficult to diagnose. Apart from the bull’s-eye rash, which may or may not be present and lasts for around a month if it is, clinical signs of Lyme disease are subtle and often misdiagnosed. Current tests for the disease are based on antibody response. Consequently, false negative findings are common in early infection.
Because of this, the CDC recommends that physicians in Lyme endemic areas don’t wait for the test results to treat suspected cases of Lyme disease. “We encourage physicians not to wait for test results to start treating patients suspected of Lyme disease if the patient lives in or has traveled to an area where Lyme is common,” Dr. Beard said.
Last October, the advocacy group Center for Lyme Action launched to lobby Congress for additional federal funding to accelerate the development of new drugs and diagnostic tests for Lyme disease. That same month, the Senate Health Committee passed legislation introduced by U.S. Sens. Susan Collins of Maine and Tina Smith of Minnesota to improve research, prevention, diagnostic testing, and treatment for tick-borne diseases. The bill—the Kay Hagan Tick Act (S 1657)—is named in honor of the former U.S. senator from North Carolina, who died Oct. 28, 2019, of complications from Powassan virus disease.
Although no vaccine for Lyme disease is currently available for humans, Dr. Beard said one is expected to enter into phase two clinical trials in the near future.
One health in action
Dr. Mather believes everyone should be better educated about ticks and able to identify the small number of tick species in the United States that threaten human health. “Different ticks carry different diseases in different parts of the country,” he explained.
Concerning the blacklegged tick, adult males and females are active October through May so long as the daytime temperature remains above freezing. Nymphs are active May through August and are most commonly found in moist leaf litter in wooded areas or at the edge of wooded areas. Both the nymph- and adult-stage blacklegged ticks can transmit infectious diseases.
Veterinarians play a part in preventing Lyme disease infections, according to Dr. Little.
“I always encourage veterinary students to think about controlling ticks as a public health service,” she explained. “If you have a dog maintained on tick control, the ticks the dog encounters are killed, and they’re not in the home or in the environment and able to transmit infection. It’s a perfect example of one health.”
The Center for Disease Control and Prevention’s reference manual on ticks and tick-borne illnesses of the United States is available at the CDC website (PDF).