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Veterinary Forum May 2007 (Vol 24, No 5)

Forget Everything You Know About Ticks

by Marie Rosenthal

    The epidemiology of tick-borne disease is changing: ticks are moving into new areas and are being linked to new diseases. VETERINARY FORUM talks with leading experts to update you on the problem.

    The tick population in the United States is exploding. Driven by socioeconomic factors, changing climate and an increase in the number and distribution of wildlife hosts, ticks have successfully spread across the country, changing the landscape of zoonotic diseases (see box).

    Two ticks are particularly troublesome in the United States: Amblyomma americanum (the Lone Star tick) and Ixodes scapularis (the black-legged or deer tick). They have spread across nearly half of the country, overlapping in many states, and have been implicated in transmitting nearly a dozen infectious disease pathogens to both humans and animals.

    "Based on clinical evidence and the literature, there appears that at least two major tick species have expanded their ranges and even their density within those ranges over the past 15 to 20 years," explains Michael W. Dryden, DVM, PhD, who is professor of veterinary parasitology at the Kansas State University College of Veterinary Medicine. "Those tick species are Ixodes scapularis in the eastern half of the United States, which is our major Lyme disease and human anaplasmosis vector, and Amblyomma americanum, which is a big ehrlichiosis vector."

    I. scapularis has been reported from Minnesota to Miami and from Bangor, Maine, to Corpus Christi, Texas, and A. americanum has been found from Corpus Christi to southern Michigan and from southern Florida to Maine, according to Dryden.

    Katherine M. Kocan, PhD, agrees, adding that the rise in the role of A. americanum has been one of the most fascinating aspects of her field of study. "Historically, the Lone Star tick, or A. americanum, was considered insignificant as a vector of disease, but now it is considered the number one vector. It used to be if someone got a Lone Star tick on them, I'd say don't worry, you don't have any risk of disease," says Kocan, who is regents professor and Walter R. Sitlington endowed chair of food animal research in the department of pathobiology at the Center for Veterinary Health Sciences at Oklahoma State University. "But in the past 15 years or so, the Lone Star tick has become our number one tick vector in the United States. I think it is one of the most interesting and dramatic things going on in the study of tick-borne diseases in the United States."

    And then there were?

    Ticks are the most common transmitters of vector-borne diseases in the United States. They wait in vegetation for a host and then attach themselves using a hypostome to pierce the skin. During a blood meal, they transmit a variety of pathogens, including bacteria, spirochetes and rickettsiae.

    Most of the pathogens linked to A. americanum cause emerging diseases, Kocan explains, and the number associated with the tick has risen from zero to seven: Ehrlichia chaffeensis, which causes human monocytic ehrlichiosis; E. ewingii, which causes canine and human granulocytic ehrlichiosis; Francisella tularensis, the cause of tularemia; Rickettsia amblyommii, the cause of a rickettsial spotted fever that has not yet been named; nonpathogenic Theileria cervi in deer; and Borrelia lonestari, which has been associated with southern tick-associated rash illness (STARI), a newly described disease in humans. Another newly described disease that is associated with the Lone Star tick but does not yet have a name has been found in a goat in the United States and is caused by Ehrlichia ruminantium, which appears to be closely related to the pathogen that causes the African cattle disease known as heartwater.

    "In Africa, Ehrlichia ruminantium causes the cattle disease heartwater. It was reported in the Caribbean islands and Puerto Rico some years ago, and there is a big push to eradicate the tick from the Caribbean and Puerto Rico to reduce the risk of introducing heartwater into the United States. Heartwater is a major disease in cattle in Africa," Kocan explains. "Just recently, a closely related Erhlichia organism was described in a goat in the Northeast."

    I. scapularis transmits Anaplasma phagocytophilum, which causes human granulocytic anaplasmosis; Babesia microti, the cause of rodent babesiosis as well as human infection; B. odocoilei, which appears to be host-specific for deer (cervid babesiosis) and Borrelia burgdorferi, the cause of Lyme disease. These diseases occur in a variety of animals and humans (see chart).

    There are two other major tick vectors in the United States that are especially important for animal diseases: the brown dog tick and the American dog tick. Rhipicephalus sanguineus (the brown dog tick) transmits pathogens that cause the following canine diseases: Anaplasma platus, which causes a milder form of ehrlichiosis; Babesia canis, which causes babesiosis; and Ehrlichia canis, which causes canine monocytic ehrlichiosis. Outside the United States, the tick has been implicated in the transmission of Hepatozoon canis, which causes canine hepatozoonosis, and B. gibsoni, which causes babesiosis. The brown dog tick has also been implicated in the spread of Rickettsia rickettsii, the cause of Rocky Mountain spotted fever (RMSF) in both humans and dogs.

    Another vector of RMSF is D. variabilis or the American dog tick, which also transmits Anaplasma marginale, the cause of anaplasmosis in cattle; Cytauxzoon felis, the cause of cytauxzoonosis in domestic and wild cats; Ehrlichia canis, the cause of canine ehrlichiosis; and E. chaffeensis, which causes ehrlichiosis in humans and dogs.

    Although it may seem as if ticks are being infected with more organisms, which is why they are transmitting more diseases, both Dryden and Kocan say that probably isn't the case. Instead, researchers and epidemiologists have the technical means to find pathogens they couldn't find before. "Ticks are not changing their ability to transmit disease," explains Dryden, "but we are recognizing more diseases because we are looking for them."

    Adds Kocan: "Molecular technologies are fabulous. We can do things that we could never do before. For instance, you can collect ticks and, using polymerase chain reaction [PCR], you can test for half a dozen parasites at one time. So ticks may have carried these pathogens all along, but we just haven't recognized them."

    Because of tick spread, increased tick density and recognition of more diseases from tick vectors, veterinarians might find it useful to include vector-borne diseases that are not common in their area as part of the diagnostic differentials if the animal's clinical signs and presentation match the disease definition. Dryden adds: "What I tell practitioners when I get in front of an audience is very simple: "Whatever you knew about ticks 5 years ago, it is different today and whatever you know about ticks today will be different 5 years from now."

    "And that is because these ticks continue to move and expand and increase in importance. Just because you did not diagnose this disease in your practice yesterday doesn't mean you won't diagnose it tomorrow. The situation is not static. This is a dramatic, dynamic situation we have today in the eastern half of the United States."

    Hard ticks begin their life cycle as larvae, which hatch from eggs and immediately begin seeking hosts, often rodents. After the larvae successfully feed, they fall off the host and live in the soil and decaying vegetation where they then molt into nymphs, which seek their blood meal from a small vertebrate. If a nymph fails to find a blood meal, it dies. If it succeeds, it falls off the host and lives in the soil to molt into an adult. The adult seeks a larger host for its blood meal, and the preferred host for I. scapularis and A. americanum is the white-tailed deer, where the adult feeds and mates over the winter. The adult female tick is capable of laying several thousand eggs before she dies, according to Kocan. (See figure for example of hard tick life cycle.)

    "The thing that drives the tick populations is deer. If we didn't have deer, we wouldn't have half the tick populations that we have," Kocan says.

    When the United States was young, white-tailed deer flourished in this country, but they were over-hunted, explains Dryden. Through the 1800s, they were a major source of food in this country; even Army units supplied themselves by hunting deer. "The estimates indicate there were only between 200,000 and 300,000 white-tailed deer left in the early 1900s. Then it became illegal to hunt deer in many states," Dryden says, as many state gaming departments tried to repopulate the species.

    It was one of the most successful conservation efforts in history, and today, there are more than 30 million deer in this country. Humans also have eliminated the natural predators of deer, so they have flourished.

    "By anyone's estimates, there are almost 100 times the deer today than there was 100 years ago," Dryden says. "I don't care where you live, whether it is Kansas or New Jersey, when you were a kid, to see a deer was a phenomenon. Now they are everywhere, and the white-tailed deer is the primary host for adult I. scapularis and a major host for all stages of the Lone Star tick. Neither of those ticks can be maintained in high numbers in an ecosystem without white-tailed deer."

    Other driving forces

    The other major driving forces behind the emergence of tick-borne diseases are the suburbanization of America; climate change; an increased incidence in outdoor recreational activities and migrating birds, according to the Centers for Disease Control and Prevention (CDC).

    "You have changing trends in tick transmission and diseases because the environment and people are changing," Kocan says. As people move from urban areas to suburbs and rural neighborhoods, they drive animals and their attached ticks to outlying areas moving them across the map.

    In addition, the trend today is not to cut forests to build tract houses but to integrate the homes with the natural surroundings. Although this is aesthetically pleasing, it brings people, wildlife and ticks together. "Significant reforestation has occurred in the eastern United States over the past 100 years from massive clear cutting that occurred in the 1700s and 1800s," Dryden says. "We just are not doing that to the extent that we once did."

    In addition, people are spending more recreational time outdoors, often with their dogs, providing another opportunity for ticks to attach. "Dogs are the ones that go out and run around and bring ticks back into the human environment," Kocan says. "I don't think people understand that dogs can be the big connector between people and ticks."

    Migratory birds are also being recognized as both a host for various zoonotic pathogens and a carrier of ticks, which can drop off and lay eggs in new territory. According to a recent article in Emerging Infectious Diseases, North American birds host vector-borne pathogens, such as Anaplasma spp and B. burgdorferi, that infect Ixodes larvae. Although rodent species, such as the white-footed mouse, are the most common hosts of immature Ixodes ticks, alternative hosts, such as migratory birds, play a larger role than previously thought and may be hosts of epidemiologically important vector ticks.

    Finally, climate change is also influencing tick populations. "We are not experiencing as hard and as cold winters that we used to historically for whatever reason," explains Dryden. "We need to protect ourselves and our dogs and cats. With the winters not being as hard as they used to be in many areas, we've gone to year-round tick control for our pets."

    Although the winters didn't necessarily kill ticks, they stopped attaching during the really cold months. Now, as soon as there is a sunny day, they are out again.

    "We have ticks all year-round," explains Kocan from Oklahoma, "and each tick has its own biology and its own seasons. So, this time of year, we have the Lone Star tick, and by the end of the summer, it is I. scapularis, and then later, the Dermacentor albipictus comes out.

    "People think they only need tick control in the summer, but we have something out there all year around. Oklahoma has really become a hotspot for tick-borne diseases."

    Although tick-borne diseases and ticks are a major problem in the Northeast, South and Midwest, it doesn't mean the West is without a tick problem, according to Curtis Fritz, DVM, MPVM, PhD, DACVPM (epidemiology) and Anne Kjemtrup, DVM, MPVM, PhD, both of the vector-borne disease section of the California Department of Health Services.

    "There are changes in tick densities and numbers yearly, but we don't see an overall increase or change in location as they are seeing in other parts of the country," Kjemtrup says. People moving into rural or coastal areas encounter more wildlife and ticks, but most transmission occurs through recreational activities.

    "We developed differently out here," she says. "People live in high-density areas, where they don't end up with large backyards. Tick-borne diseases in California for the most part are a result of recreational activities, where people and animals go out into wooded areas."

    There is a seasonality to tick infestations in the West, she adds, and different ticks spread different diseases. For instance, Ixodes pacificus, instead of I. scapularis, is the tick that transmits Lyme disease in California. That tick primarily feeds on lizards, instead of deer, and lizards have a natural substance in their bloodstreamthat kills Borrelia organisms. "As a result, our adult tick populations are infected at a much lower prevalence with Borrelia than is seen in the eastern United States," Kjemtrup says.

    Still there are some novel pathogens being seen, Fritz adds. For instance, Arizona recently reported an outbreak of RMSF, which doesn't normally occur in that state, and it probably resulted from a different tick vector, the brown dog tick.

    Kjemtrup adds that canine babesiosis is a problem in California. Three different species of canine Babesia have been documented in Californian dogs: Babesia canis, B. gibsoni, and the recently described B. conradae. Epidemiologists are reporting B. gibsoni in dogs used in dog fighting, and it is related not only to tick transmission, but also to the exchange of body fluids during the fights. "This is something that has been reported in the literature recently," Kjemtrup says, adding that "scary dog fighting websites have started posting treatment recommendations" because the owners are concerned about this disease.

    There are also more reports of Lyme disease in canines, she added. In fact, IDEXX Laboratories recently reported that Lyme disease has been found in canines in 48 states. However, Fritz says this doesn't mean that dogs have a current infection, but that some dogs were exposed to B. burgdorferi at some point and developed antibodies. Nevertheless, these reports suggest that exposure of dogs to B. burgdorferi may be more widespread than previously supposed.

    "The tick populations have increased dramatically and moved into new areas. If there is one tick out there, it's not a big deal. If there are 1,000 ticks out there, it is a big deal," Kocan explains. "The average tick burden of a moose in Canada is about 37,000 ticks."

    Most readers think about ticks in reference to dogs and cats. The average pet owner might find one or two ticks and pick them off, but ticks are a larger problem in the cattle industry, where a steer can harbor more than 3,000 ticks at once. As the ticks feed, they can lead to weight reduction and secondary infections, she says. Kocan and her team are working on a tick vaccine for A. americanum, she adds. There is a tick vaccine for the cattle tick, Boophilus, that is being used in Cuba, which Jose de la Fuente developed. He is now at Oklahoma State University working with Kocan. "We are doing a lot of work to find tick-protective antigens and develop tick vaccines," she says.

    Ticks have few natural predators. Fire ants destroy them but the cure is worse than the problem, and Guinea hens eat them but they are not a preferred food for most other birds. So, the best defense is a good offense, all the experts say. Recommend year-round tick protection for animals and humans, and advise owners to do tick checks before returning home if they walk or play in wooded areas. "If you don't keep up with that, an engorged female can drop off in your yard and lay 5,000 eggs. You'll have a big problem in a short period. Tick control is a major consideration," Kocan says.

    In addition, tell owners to make sure their home is not attractive to wildlife and ticks. "There are a lot of things you can do around your house to make sure ticks don't become established," Kocan says. "You can cut down bushes and mow your grass and put a gravel area around your yard, because they won't cross that. Use an integrated pest management approach."

    Dryden adds: "Try things that reduce animal harborage. You want to reduce rodent populations. Don't keep a wood pile next to your house where the rodents might move in. You want to keep your house from being attractive to a tick host, be that a deer or a rodent. One of the worst things that a person can do is put a deer feeding station in their backyard, but people do that all the time. There are enough deer out there, so we don't need to be doing that.

    "I'm not advocating getting rid of the deer, but we need to recognize that there has been a consequence to moving so close to nature.

    "Owners understand why this has happened, then they will be far more willing to accept the consequences, which is year-round, lifelong tick control for their pets."

    For more information:

    1. Comstedt P, Bergstrom S, Olsen B, et al: Migratory passerine birds as reservoirs of Lyme borreliosis in Europe. Emerg Infect Dis [serial on the Internet]. 2007 Jul [date cited]. Available from http://www.cdc.gov/ncidod/EID/ vol12no07/06-0127.htm.

    2. Kocan AA, Kocan KM: Tick-transmitted protozoan diseases of wildlife in North America. Bull Soc Vector Ecol 1991;16(1):94-108.

    3. Raghavan M, Glickman N, Moore G, et al: Prevalence of and risk factors for canine tick infestation in the United States, 2002-2004. Vector-borne Zoonotic Dis 2007;7:65-75.

    4. Dryden MW, Payne PA: Biology and control of ticks infesting dogs and cats in North America. Vet Ther 2004;5:139-152.

    5. Kjemtrup AM, Wainwright K, Miller M, et al. Babesia conradae, sp. nov., a small canine Babesia identified in California. Vet Parasitol 2006;138: 103-111.

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    Did you know... Environmental changes due to human encroachment, climate changes, and other anthropogenic effects will likely modify tick-transmitted diseases and increase their incidence. Read More

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