Many people associate the word tick with Lyme disease. Dr. Michael Yabsley, professor of Wildlife Disease Ecology at the University of Georgia, explained that in addition to the black-legged tick that transmits Lyme, other ticks are important in both veterinary and human medicine.
“Worldwide, ticks are second only to mosquitoes in transmission of pathogens,” said Yabsley. “In the United States, ticks are responsible for more pathogens than mosquitoes. In agricultural production, tick-borne diseases are important.”
Geographical location dictates the importance of pathogens harbored by ticks. “Ticks are bags of pathogens,” said Yabsley. “They can be associated with more than three pathogens as well as their normal microflora. We need to think of ticks as having suites of pathogens.”
Tick infestations on livestock can cause problems, even without the pathogens they potentially transmit. “With significant numbers, there are issues with blood loss,” said Yabsley. “Tick feeding can cause skin damage, which can lead to secondary infections and hide damage. All of that is very irritating, leads to reduced weight gain, reduced milk production and toxicoses and allergies.”
Yabsley said there’s nothing new about tick borne diseases – they’ve been around forever. “In the early 1900s, we recognized pathogens associated with ticks and people,” he said. “Over time, the number of pathogens and diseases we’ve identified has increased dramatically, especially in the past couple of decades.”
Ticks use slicing mouthparts to attach to their hosts and are essentially “glued in” to the skin, making them difficult to remove. Yabsley explained that as ticks feed, they release anticoagulants to keep the blood flowing, vasodilators to bring more blood to the tick and anti-inflammatories so the host doesn’t feel the tick.
Ticks fall into two general categories: soft and hard. There’s significant diversity between the two categories but all have the capability to transmit pathogens. While bacterial pathogens can usually be treated with antibiotics, viruses and parasites transmitted by ticks don’t respond to antibiotic treatment.
Hard ticks are the predominant type in the U.S. Only female ticks take blood, which they need for nutrition for egg production. The larvae of most hard ticks feed on three different hosts, which can be the same species or different species, depending on the tick. “The larvae feed on a host then drop off into the environment,” said Yabsley. “It lives for weeks or months, then molts to a nymph, which feeds on a second host. It engorges with blood, falls off and molts to adult and feeds again.” At each stage of feeding, there’s potential for the tick to pick up pathogens from the host animal.
Ticks feed for only a few days and spend the rest of their time living in the environment. Each tick species has a preferred habitat, although many species share habitats. Temperature and humidity dictate when ticks come out to seek a host and how well they survive the elements. Ticks may molt faster or lay more eggs in warmer weather.
A female lone star tick (Amblyomma americanum). Photo courtesy of Entomology Today (entomologytoday.org)
Yabsley described the most common ticks in the Northeast. The lone star tick (LST) was originally found primarily in the South/Southeast and is named for the white spot on the female’s back. The LST is now spreading to the Midwest and up into the coastal Northeast. “It’s marching north and northwest every year,” he said.
Many pathogens are associated with the LST, including several Erlichia species, which are bacteria that live in white cells. “White-tailed deer are the primary host,” said Yabsley. “They maintain these pathogens in nature, then the lone star tick transmits it from deer to other animals or people.”
The LST is responsible for spreading Heartland virus, a phlebovirus that has been responsible for several fatal human cases. “Antibodies to Heartland virus have been found in a number of wild animals in 13 states,” said Yabsley, “including all the way to Maine. It was thought the LST was the only vector, but data suggest there could be other vectors.”
The American dog tick is widespread in the East and some areas in the West. It’s associated with several pathogens that affect both animals and people. One is Rocky Mountain spotted fever (RMSF), which causes disease in people and dogs. The majority of RMSF cases are in the Southeast and Midwest, but the disease can show up just about anywhere. Yabsley said it’s been experimentally shown that the ALT (Asian longhorn tick) can transmit the organism that causes RMSF but hasn’t been found in the wild yet.
The Gulf Coast tick was historically restricted to the Gulf Coast region but it’s on the move northward. “There are a number of pathogens associated with it, for livestock in particular,” said Yabsley. “It causes gotch ear, which is swelling and damage to the ears. This tick is not as impacted by low humidity as other tick species.”
The black-legged tick is associated with numerous pathogens and is best known for transmitting Lyme disease. “It’s a rodent-tick cycle,” said Yabsley. “Deer are important because they maintain ticks in the environment, but they don’t maintain the pathogen – it still needs rodents.”
A serious meat allergy is associated with a tick bite, most often the LST but possibly others including the black-legged tick and ALT. The condition is known as alpha-gal syndrome. When ticks feed on humans, they inject a sugar called alpha 1,3 galactose (alpha-gal). The body recognizes alpha-gal as a foreign substance and mounts an allergic response. When the person eats foods containing alpha-gal, the allergy is triggered. Red meat, dairy products and items derived from meat and dairy such as gelatin or glycerin capsules cannot be consumed. People with alpha-gal allergy can safely eat fish and poultry because those animals lack the protein.
“About two to six hours after eating meat or dairy, people have symptoms that can vary,” said Yabsley. “Some people get a stomachache; others have more severe GI symptoms and/or hives. Sometimes people develop anaphylactic shock, which can be life-threatening.” He said alpha-gal is one of the major reasons for anaphylactic shock seen in emergency clinics.
“In the U.S., since 2004, we’ve seen tripling of vector-borne pathogen cases,” said Yabsley. “Much of that was associated with ticks. Lyme disease is a good example – there’s a 300% increase in Lyme disease cases in the northeast U.S. and a 250% increase in north-central states.”
Yabsley said tick ecology and tick-borne disease epidemiology is complex. “One thing we preach is how parasites are dynamic and ever-changing,” he said. “Ticks are expanding their range and new pathogens are being spread.” Factors for ticks’ range expansion include changes in vector distribution and abundance and introduction of exotic species.
Changes in vertebrate host communities also influence ticks’ range, as well as the changing demographics of people and animals. Yabsley said better diagnostics to detect novel pathogens also accounts for more diagnoses of tick-borne illnesses.
by Sally Colby
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