When dealing with these arthropod disease vectors, timing is paramount
Ticks are nothing new: They have been found encased in 99-million-year-old amber.1 A mummified Egyptian dog dating back to approximately 4000 bc was unearthed with 61 ticks attached,2 dead proof that these arachnids have long liked canine blood.
If ticks aren’t new, then why are they news? Perhaps because human and pet exposure to ticks is increasing, said Caroline Sobotyk, DVM, PhD, MSc, an assistant professor of clinical parasitology at the University of Pennsylvania School of Veterinary Medicine in Philadelphia.
Humans—along with our dogs and cats—are treading further into tick territory by hiking, camping and even taking up residence in nature. “There is a lot of overlap between us and wildlife that makes us more exposed to ticks and the diseases they carry,” Sobotyk explained.
Human population growth has marred habitat for both tick predators and their food sources. Declining are nature’s tick exterminators, such as spiders, beetles, frogs, lizards, ground birds, rodents, and other foragers, particularly possums (which can individually consume some 5000 ticks each season).3-6 When ticks’ food sources, such as deer, decrease in regions saturated by humans, ticks are sometimes forced to make other dietary choices. “Balance is everything,” Sobotyk said. “If we have a good amount of wildlife around, the ticks will be on them and not us.”
Evolving weather patterns may be morphing in ticks’ favor as temperatures and humidity climb.7,8 This offers ticks a year-round presence in places where they once were active in summer only. “We have high numbers of ticks in certain seasons, but that doesn’t mean we aren’t going to also have ticks in the winter,” Sobotyk said.
The greatly accelerated transport by animal welfare communities of dogs, plus their ticks, is one of the factors in the introduction of novel species into naive regions of North America.9-12 Results of a recent study revealed sprawling distributions of certain species, such as the Asian longhorned tick (Haemaphysalis longicornis) and the Gulf Coast tick (Amblyomma maculatum).13 The lone star tick (Amblyomma americanum), once just a denizen of southeastern states, has pushed northward.14 The brown dog tick (Rhipicephalus sanguineus), which made its original Canada border crossing awhile back but never really took hold,15 is now a familiar local throughout the US.
In a large-scale study at Oklahoma State University (OSU) in Stillwater,13 10,000 tick submissions poured in from across the US over a 13-month period. Despite the shifting forces impacting tick habitat and distribution, investigators still found plenty of “same-old, same-old,” with the greatest numbers arriving during July and from the South and Midwest.
Now that we better understand the when and the where, let’s look at the who.
Like their spider cousins, ticks have 8 legs. These are attached to a bulb- shaped or rounded body, which is fronted not by a true head but by a harpoonlike feeding apparatus.15 Ticks have 4 life stages: egg, larva, nymph, and adult. Larvae, or seed ticks, are built like their adult selves, but are the size of a pin head; nymphs are slightly bigger.15
Ticks exist in 2 families.15 Argasidae, or soft ticks, are rapid feeders that quickly drop off their host after a blood meal. More of a concern for farmers and ranchers, they prefer cattle, horses, and fowl. Ixodidae, on the other hand, readily infest dogs and cats. These hard ticks—named for their tough, protective coat—dine slowly, sometimes for days to weeks.
The hard ticks that prey on our pets are generally 3-host species.16-18 The larva siphons blood from its first host, usually a small, ground-dwelling animal. Once full, it drops off and molts into a nymph, which awaits a second host to feed on. After eating, the nymph detaches and matures to an adult, which parasitizes a larger mammalian host. The engorged adult falls to the ground and, if female, lays eggs.
Ticks are ambush predators; they don’t pursue prey. When triggered by vibrations and carbon dioxide16-18 signaling animal activity, they position themselves in a spot where they can passively hitch onto passersby, such as atop tall grasses, among bushy vegetation, and within leaf piles.
Most of the tick’s life is spent in its off-host environment,17 where it can survive without blood for months to years. Every species and life stage has its individual host preferences, but as opportunistic feeders, ticks will cling to anything they can draw blood from.
A tick bite can have consequences such as attachment site rash, allergic reactions, anemia (with severe infestations), toxicosis, and pathogen transmission. Tick-borne illnesses that are significant in animals and humans include11,18-28:
Lyme disease, caused by Ixodes-borne B burgdorferi, is often subclinical. It occurs throughout the US but most canine cases are reported in the northeastern and mid-Atlantic states.28 Affected dogs, and possibly cats, may exhibit joint swelling, lameness, fever, and appetite loss. In severe canine cases, glomerulonephropathy, heart disease, and nervous system disorders can occur.
Anaplasmosis, is caused by A phagocytophilum, which can infect the white blood cells of dogs and, rarely, cats. Signs are subclinical to vague, and include fever, lethargy, and appetite loss. It is transmitted by Ixodes spp, and coinfections with B burgdorferi can occur.
Ehrlichiosis occurs through transmission of Ehrlichia spp by the brown dog tick or lone star tick. Caseloads peak in the southwest and Gulf coast regions of the country. These bacteria dismantle white blood cells and platelets, which can lead to inflammatory conditions and coagulopathies. Depression, appetite loss, chemosis, ecchymosis, epistaxis, and lameness may occur.
Rocky Mountain spotted fever can follow the bite of an American dog tick; its cousin, the Rocky Mountain wood tick (D andersoni); the brown dog tick; or the lone star tick infected with the coccobacillus R rickettsia. Endemic to the Rocky Mountain states, this microbe has expanded throughout the US and Canada. Affected dogs can experience acute onset of fever, lethargy, arthropathies, and neurologic deficits. RMSF comes on strong, lasts a few weeks, and can lead to death.
Babesiosis is tied to the Babesia spp of protozoan, which is transmitted by brown dog ticks. Affecting dogs throughout the country, babesiosis causes red blood cell destruction, anemia, weakness, vomiting, hyporexia, and weight loss.
Tick paralysis is not an infection, but rather an acute reaction to saliva injected via a tick bite. It’s unclear whether this ascending paralysis, which begins hours to days into tick attachment, is the manifestation of an allergic reaction to salivary proteins or the fallout from toxins in the tick’s saliva. Although dogs in North America can get tick paralysis from many tick types, Dermacentor spp are the main culprits.20-22 Potentially fatal and fortunately rare, this condition recedes as soon as the feeding tick is removed.
A tick bite is hardly synonymous with disease. OSU researchers found less than 1% of the ticks they studied were infected with (and capable of transmitting) a pathogen.23 In their survey of local ticks,23 for instance, fewer than 1 in 1000 harbored RMSF. Similarly, investigators in New Hampshire evaluated almost 3000 native I scapularis ticks and found just 37% to be infected with B burgdorferi, and 6% with A phagocytophilum.24 Transmission modifying factors–such as pathogen, vector, and host species, plus host immunocompetence–also determine whether a specific host will contract disease from an infected vector.
The site of infection within the tick dictates how long the tick must be affixed to transfer its germs. Transmission is easier for pathogens that can migrate rapidly from the tick’s midgut to the salivary glands without a prior blood meal, such as Anaplasma spp, Ehrlichia spp, and R rickettsii. B burgdorferi, on the other hand, cannot budge from its midgut home to the salivary glands until it undergoes a change in its outer surface proteins. For this, hours of dining are required, thereby slowing transmission.25
Anaplasma and Ehrlichia spp and R rickettsii can be transferred to the host within 24 hours, sometimes within just 4 hours, whereas B burgdorferi requires 48 to 72 hours.25,26 This provides time to remove the tick before disease spreads. Following infection, seropositivity occurs 2 to 3 weeks post bite for Anaplasma spp, 3 to 4 weeks for Ehrlichia spp, and 4 to 6 weeks for B burgdorferi.27
When we think about ticks, we often think about dogs. Ticks bite cats, too, and can inject them with their pathogens.
Cats accounted for just 20% of the tick submissions in the OSU study.13 However, investigators concluded that they were likely underrepresented as tick hosts, partly because the data were generated by veterinary visits,13 which are fewer for cats than dogs.28 The lone star tick, American dog tick, and black-legged (I. scapularis) tick accounted for 80% of feline samples.13
Cats can acquire anaplasmosis, ehrlichiosis, cytauxzoonosis, and tularemia from ticks. The latter 2, although uncommon in domestic cats, can cause severe illness. B burgdorferi antibodies have long been found in feline serum, but whether the spirochete causes clinical Lyme disease is controversial; such dissonance is also true of Rickettsia spp in cats.29-33 Feline tick paralysis, although common in Australia, has rarely been reported in North America.35,36
It matters less what type of tick, what stage, what season, what part of the country if a pet is being administered tick preventives. The newest generation, isoxazolines, inhibit the GABA- and glutamate-gated chloride channels, leading to hyperexcitation and death of the tick. These compounds, which include afoxolaner (NexGard; Boehringer Ingelheim), fluralaner (Bravecto; MSD Animal Health/Merck & Co), sarolaner (Simparica; Zoetis), and lotilaner (Credelio: Elanco), start killing ticks within hours of their attaching. This is well within the grace period for pathogen transmission.
Even treated dogs and cats should be given a once-over before coming in from the outdoors, because ticks crawling on their fur can end up on humans inside, Scimeca cautioned. Homeowners can head off tick activity by keeping the grass around their dwellings mowed. These common-sense precautions shouldn’t give way to tick mania, Scimeca warned. “We don’t need to be completely paranoid about ticks. But we do need to be aware of the potential problems a tick bite can cause,” she said.
Joan Capuzzi, VMD, is a small animal veterinarian and journalist based in the Philadelphia, Pennsylvania area.
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