Masters of ride-sharing well before Uber and Lyft came along, ticks hitch rides with white-tailed deer, migrating birds, and other animals, making their way to locales where they didn’t exist in the past. Now, one or more tick species are found in every state, including Alaska and Hawaii. And milder winters mean that even if ticks aren’t out 365 days a year, they are seen 12 months out of the year. Dogs, cats, and humans are at risk for various tick-borne infections, making them a public health issue.
In fact, dogs are sentinels for Lyme disease in humans, according to the Centers for Disease Control and Prevention. While dogs don’t transmit the disease themselves, they can bring ticks into contact with humans. A CDC research paper shows a direct correlation between a 5 percent or higher incidence of Lyme disease in dogs in an area and development of Lyme disease in humans in that area, according to Craig Prior, BVSC, president of the Companion Animal Parasite Council.
“Ticks that infest dogs can also feed on people, and some of the pathogens they carry can also infect people,” said Meredith Miller, DVM, DACVIM, at Cornell University College of Veterinary Medicine in Ithaca, N.Y. “The pathogens they transmit can vary from very treatable to life threatening or even cause death.”
Affecting tick numbers
Because disease-spreading ticks have a two- to three-year lifecycle, it’s difficult to predict just how numerous and widespread ticks and tick-borne diseases will be in 2018, but the blood-sucking arachnids typically are most active in spring, summer, and early fall. Factors affecting their numbers include temperature, rainfall, humidity, and the availability and numbers of host animals such as mice, deer, dogs, and other animals. In any given year, the CDC reports, the number of ticks in an area varies from region to region, state to state, and county to county.
“The seasonality of ticks varies geographically,” said Dr. Miller. “Some ticks can infest homes or kennels and survive cold temperatures.”
They do this by seeking shelter beneath logs or leaf litter, which provide thermal insulation, and reducing their activity level to conserve water. Ticks are vulnerable primarily to dehydration. Short of massive drought or an extremely cold winter with no snow, tick populations are unlikely to decrease. The minimum temperature from which various tick species can recover without mortality ranges from 3.2 degrees Fahrenheit to 17.6 degrees Fahrenheit, said Michael Dryden, DVM, in a lecture at VMX in February.
To aid veterinarians in understanding the risk in their area, CAPC’s website (capcvet.org) provides maps for Lyme disease, ehrlichiosis, and anaplasmosis. The prevalence maps show the percentage of pets tested who are positive for a given infection. Factors influencing results include the number of pets tested, the history of the pets prior to testing, the reason the pets were tested, and the assays used.
“We’ll be able to add soon a local forecast for one month in advance,” Dr. Prior said. “You’ll be able to see what’s going on in your local county on a month-by-month basis. We’re getting really good information out there so people can understand what their risks are.”
In Wisconsin, for instance, CAPC’s figures show a total of 172,888 dogs were tested in 2017, with 14,813 testing positive for Lyme disease. That’s approximately 1 out of 12 dogs, for an 8.6 percent infection rate.
The University of Georgia also offers a forecasting map detailing which areas in the 48 contiguous states are most likely to see Lyme disease infections in dogs, information that could also predict the infection in humans.
Tick-borne diseases in dogs
It’s a common misconception that dogs don’t get Lyme disease, transmitted by blacklegged ticks—Ixodes scapularis—and, on the West Coast, Western blacklegged ticks—Ixodes pacificus. Although 95 percent of dogs exposed to Lyme do not show signs, those who do may experience fever, anorexia, depression, shifting lameness, swollen joints, and enlarged lymph nodes.
Dogs with chronic Lyme disease may exhibit persistent polyarthritis, acute progressive renal failure, and in rare instances, neurological signs such as sudden unexplained aggression. In areas with high numbers of ticks, consider running a tick panel on dogs presenting with unexplained signs of aggression, depression, fear, or a combination.
According to CAPC, prevalence of Lyme infection in dogs ranges from as high as 16.37 percent at ground zero in Connecticut, where the disease first emerged, to as low as 0.19 percent in Louisiana. Some surveys by CAPC found that more than half of questing nymphs (immature ticks seeking hosts) and adult ticks harbored Borrelia burgdorferi, the spirochete that causes Lyme disease, putting dogs living in tick-infested environments at high risk of infection. Adult ticks transmit most canine Lyme infections, while ticks in the nymph stage transmit most human Lyme infections.
A Lyme disease vaccination may benefit dogs at high risk such as hunting dogs in Lyme-endemic areas. According to the 2017 American Animal Hospital Association vaccination guidelines, dogs in areas where risk of exposure is sustained should be vaccinated annually with a single dose after the initial two-dose series. In areas where risk is not high, the vaccine remains noncore.
Two tick-borne disease threats, both deadly to dogs, are Rocky Mountain spotted fever (transmitted by the brown dog tick, Rhipicephalus sanguineus) and an emerging disease called American canine hepatozoonosis, which is transmitted indirectly when dogs ingest infected Gulf Coast ticks (Amblyomma maculatum) or intermediate hosts such as rabbits.
Name notwithstanding, RMSF isn’t limited to the Rocky Mountain region. It occurs east of the Mississippi, including in New York. Again, dogs do not transmit the disease to humans, but if dogs are exposed to ticks, humans likely are as well.
Signs of the rickettsial infection include high fever, anorexia, muscle and abdominal pain, stiff gait, and vomiting and diarrhea. Unless effectively treated, RMSF has high morbidity and mortality in dogs and humans.
“There’s been a case where two dogs in a family and a person have all three died of Rocky Mountain spotted fever,” Dr. Bowman said.
Dogs with hepatozoonosis develop fever, depression, weight loss, muscle atrophy and pain, hypertrophic osteopathy, and weakness. Treatment with antibiotics brings temporary relief, but without continued support dogs typically do not live more than one or two years after diagnosis. Infection is not known to be zoonotic.
Ticks may also infect dogs with anaplasmosis, ehrlichiosis and, less commonly, babesiosis. Anaplasma phagocytophilum is transmitted primarily by the blacklegged tick and is seen most commonly in the northeast and in California. Signs of infection include lameness, diarrhea, vomiting, and fever. The term ehrlichiosis refers to a group of bacterial diseases usually transmitted by the lone star tick and the brown dog tick. Babesiosis, a protozoal infection transmitted by blacklegged ticks, invades red blood cells, triggering fever, anemia and weight loss. It is seen in the south and in California.
Cats at risk, too
While cats are typically more resistant than dogs to tick-borne diseases, outdoor cats are increasingly at risk of a serious and often fatal disease called cytauxzoonosis, transmitted by the lone star tick (Amblyomma americanum) and to a lesser extent the American dog tick (Dermacentor variabilis). Those ticks are most commonly seen in southeastern and south-central states such as Arkansas, Florida, Oklahoma, and Texas, but their range continues east to the Atlantic coast and north to North Dakota.
The protozoal infection, caused by Cytauxzoon felis, usually occurs between spring and fall, when ticks are most active. Disease can develop five to 20 days after an infected tick bites a cat. Signs include high fever (up to 106 degrees Fahrenheit), lethargy, anorexia, and dehydration.
Cats succumb quickly to cytauxzoonosis (popularly known as bobcat fever after the wildcats who appear to be the primary reservoir for the disease). Even with treatment—a combination of antiparasitic and antibiotic drugs, plus IV fluid therapy, heparin, a feeding tube, and oxygen therapy and blood transfusions as needed—the mortality rate can be greater than 50 percent. Cats recover best with minimal stress and handling.
“Cytauxzoon is deadly,” said Dwight D. Bowman, Ph.D., a parasitologist at Cornell University College of Veterinary Medicine. “It’s a reason why cats should not even be allowed outside in certain parts of the country.”
More rarely, ticks can transmit Ehrlichia spp., Mycoplasma spp., and Anaplasma spp. to cats. Signs of Ehrlichia or Anaplasma infection in cats include fever, lethargy, anorexia, weight loss, joint pain, and vomiting and diarrhea. A 1997 report in JAVMA of a suspected ehrlichial infection in five cats from the same household notes that it may be associated with anemia, leukopenia, thrombocytopenia, or dysproteinemia. The cats responded to a 21-day course of doxycycline.
Mycoplasma infection in cats with feline leukemia virus or feline immunodeficiency virus as well as some healthy cats may cause severe and in some instances fatal hemolytic anemia.
Cats do not appear to be susceptible to Lyme disease, but that is not definitive.
“At this point in time, there is no documented evidence that cats get Lyme disease,” Dr. Dryden said.
There are, however, anecdotal reports of cats with Lyme-like signs responding to Lyme disease antibiotic therapy, so the possibility shouldn’t be ruled out. While there are no feline-specific tests for exposure to tick-borne diseases, canine SNAP-4DxPlus tests can be used to screen for exposure to tick-borne pathogens such as ehrlichiosis, anaplasmosis, and Lyme disease in cats.
Disease transmission: What we know
Less is known than you might think about the time frame in which pathogens are transmitted from a tick bite. At his VMX lecture, Dryden reviewed several studies from the past century that contained no actual statistics or records to support the generally accepted time frame of 24 to 48 hours for RMSF pathogens to be transmitted from tick to host. The same was true for other diseases.
It’s understood, though, that the longer a tick remains on the host, the more likely the host is to become infected. It’s rare for B. burgdorferi to transmit infection within 24 hours, but transmission rates increase markedly after 42 hours. Studies do support that Babesia transmission takes 48 to 72 hours. Data is limited for other infections.
What is known is that ticks transmit disease through saliva, but when they begin to feed, pathogens are in the mid-gut, not the saliva. Pathogens must be reactivated to migrate from mid-gut to salivary glands so they can be transmitted. Repelling, detaching, or killing the tick during the time following attachment and before deposit of pathogens helps to prevent disease transmission,Dryden said. This phenomenon is documented for RMSF, Lyme disease, babesiosis, and Anaplasma phagocytophilum.
Most important, pets—especially those who go outdoors—should have tick protection year-round throughout life. Which preventive is best depends on the pet’s lifestyle and the prevalence of ticks in the area. Currently, there’s no evidence of tick resistance to acaricides, with the exception of the brown dog tick. Counsel owners to perform tick checks regularly and remove ticks as soon as they find them, especially during warmer months.