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Volume 51, Issue 4
A flagship study details tick infestations in pet dogs and cats in the United States.
Pets and people share environments, which means they also share parasites and their associated zoonotic diseases. Perhaps the scariest of the blood-sucking hangers-on are ticks. But our understanding of tick parasitism has been sparse thus far.
An 2007 review of more than two million veterinary records in 40 states found ticks that ticks were present on 1.3% of dogs.1 Yet until now, little was known about the respective prevalence of individual tick species and their specific life stages on dogs and cats. In a large-scale study conducted by investigators at Oklahoma State University, ticks collected from pets were identified by species, gender and life stage; characterized by attachment site; and delineated by month and geographic location found.
From February 2018 to January 2019, tick submissions were solicited from 190 veterinary practices throughout the United States: A total of 10,978 ticks were harvested from 1,494 dogs and 336 cats.
Veterinary practices were provided with kits that facilitated submission and sample quality control. Sample data included tick removal date; geographic location and attachment site on pet; age, gender, weight and spay/neuter status of pet; and estimated percentage of time the pet spends outdoors.
Morphologic examination was conducted on each tick to determine its genus/species, gender and life stage. Molecular identification was used when morphologic identification was precluded by specimen damage or ambiguity.
Ticks were submitted every month of the year, with the highest numbers of ticks recovered from both dogs and cats in July, from the South and Midwest.
Canine tick submissions originated at 263 veterinary hospitals in 49 states, and data were collected for most of the dogs. They ranged in age from 40 days to 19 years, with an average weight of 20 kg. Gender and neuter status did not differ significantly from the general population, although fewer dogs were spayed than in the general population (63.1% vs. 67.4%).2 Half of the dogs spent more than 30% of their time outdoors.
Although one dog was infested with over 4,700 ticks, only 5.5% of the 1,494 dogs in the study bore more than nine; most had just a single tick.
A total of 14 tick species were identified among the canine samples. The greatest portion of dogs—35.6%—was infested with the American dog tick, Dermacentor variabilis. The black-legged (“deer”) tick, Ixodes scapularis, was found on27.4% of dogs, followed by the Lone Star tick, Amblyomma americanum (23.1%) and the brown dog tick, Rhipicephalus sanguineus (11.4%). A smaller number of dogs were infested with Amblyomma maculatum, Otobius megnini, Haemaphysalis longicornis, and other Ixodes and Dermacentor species. Co-infestations with more than one tick species were documented for 93 dogs (6.2%).
When investigators tabulated tick species with respect to percentage of total ticks, the brown dog tick was by far the most prevalent in the canine samples, at 62%; the presence of this large number of ticks—6,252—on a mere 174 dogs suggests large groupings of larvae/nymphs, which can number a few thousand in a single hatch. The Lone Star tick composed 19.1% of submissions; the American dog tick, 10.2%; and the black-legged tick, 5.7%.
Attachment site data from single-species infestations of adult ticks (available for over half of the canine submissions) showed species distinctions in attachment site preference. The American dog tick, the black-legged tick and the brown dog tick were more commonly attached dorsally, specifically to the head, ears and neck; brown dog ticks were also attached, albeit less frequently, on the abdomen, axillae, groin, legs and paws. The Lone Star tick was more commonly attached ventrally, to the abdomen and axillary/inguinal regions.
Feline submissions came from 109 veterinary clinics in 39 states. Cats ranged in age from 18 days to 18 years, and averaged 4.4 kg. Fifty-nine percent were male compared with 49.6% for the general pet cat population; a disproportionately small percentage of the cats were sterilized.2 Over 56% of the cats were reported to spend more than 70% of their time outdoors.
Infestation intensities ranged from one to 38 ticks, with an average of 2.6 ticks collected from each cat. Twelve tick species were identified. The black-legged tick was present on 46.4% of the cats, the Lone Star tick on 29.5% and the American dog tick on 17.9%. A smaller number of cats hosted O. megnini, R. sanguineus, A. maculatum, H. longicornis, and other Ixodes and Dermacentor species. Co-infestations were documented in 14 cats (4%).
Most tick submissions consisted of the Lone Star tick (38.5%), the black-legged tick (32.2%) and the American dog tick (13.7%).
Attachment site data from single-species infestations were provided for 33 cats (10%). As in the dog sample, the American dog tick and the black-legged tick were more commonly attached dorsally, and the Lone Star tick ventrally (tail and perianal region in cats).
Several new findings were generated by this study. First, the primary ticks parasitizing the dogs were the brown dog tick, the Lone Star tick and the American dog tick, composing 95% of submissions. For the cats, the Lone Star tick, the American dog tick and the black-legged tick accounted for 80% of samples.
Although some 40% of the ticks removed from dogs and cats in this study were adults—primarily females—nearly half were larvae and 16% were nymphs. Larvae and nymphs constituted the majority of submissions of Lone Star ticks from dogs and cats; in dogs, larvae and nymphs constituted the majority of brown dog tick submissions. This finding confirms that immature stages of some common tick species are significant players in tick parasitism of pets. It also suggests the importance of tick control, as these small stages can be overlooked easily.
The study also revealed an expanding geographic distribution of certain tick species, such as the Gulf Coast tick (A. maculatum) and the Asian longhorned tick (H. longicornis),and a year-round presence of feeding ticks in this country.
While most pets in this study had outdoor access, a small portion were reported to rarely or never go outside, suggesting that ticks hitch rides indoors on clothing and other pets.
Investigators also concluded that cats may be underrepresented as hosts for ticks: While they outnumber dogs as pets in the U.S.,3 cats accounted for only 20% of tick submissions in this study. However, the data in this study were generated by veterinary visits, which are fewer overall for cats than dogs.4
Dr. Joan Capuzzi is a small animal veterinarian and journalist based in the Philadelphia area.