By Kim Campbell Thornton

Parasites are among the most persistent organisms that plague dogs and cats. Fleas, ticks, and mosquitoes spread bloodborne, intestinal, and zoonotic diseases, to name just a few. The bite of mites and fleas can cause pets to have unbearably itchy skin. Plus, many parasites are more widespread than ever, thanks to expanding ranges of the animals that carry them—a result not only of climate change in regions that were once less hospitable to them, but also of wider and more frequent travels by humans and their domestic animals.

In a 2021 forecast statement¹ from the Companion Animal Parasite Council (CAPC), Christopher Carpenter, DVM, now CAPC's executive director, said, "Over the years, we've seen the risk for parasitic diseases continue to increase and expand into areas that have had historically lower prevalence."

That's because people are living in greater proximity to nature, according to CAPC board member I. Craig Prior, BVSC. "We're seeing expanding ranges of animals with changing migration patterns due to climate change, which in turn enhances the ability of insects and arachnids to change or improve their range as well. As a result, parasites are dynamic and ever-changing."

A 2022 global study² found climate change is projected to continue contributing to the spread of Lyme disease, particularly in Canada, where vector surveillance has documented³ a geographic range expansion of the black-legged tick, Ixodes scapularis.

"This expansion is associated with elevated temperatures, the emergence of tick populations, increases in their range and recent geographic spread, as well as with a rapid increase in human Lyme disease cases," the researchers reported.

In addition, the season for Lyme disease in the U.S. is projected to expand in the event of a 2 C warming scenario. This would result in a 20-percent increase in cases over the coming decades, leading to an earlier onset and a longer duration of the annual Lyme disease season.⁴

Cases of another potentially fatal tick-borne disease, Rocky Mountain spotted fever (RMSF), have had an overall dramatic increase since 2000, according to the U.S. Centers for Disease Control and Prevention (CDC). Warmer, wetter climates have contributed to the range expansion of the ticks that spread the disease.

While it is not transmitted directly from dogs to humans, RMSF is one of the vector-borne diseases in which dogs play a crucial role as sentinels for human disease risk. Epidemiological tracking of canine RMSF is difficult, and data generally are not available, according to a 2024 paper published in the Journal of the American Veterinary Medical Association (JAVMA).⁵

However, the researchers write: "…seroprevalence studies in dogs can highlight regions where R. rickettsia is circulating and may represent risk to people. Even if a dog is not seropositive against a rickettsial organism and ticks are not seen, we can infer brown dog tick exposure and therefore possible risk of RMSF based on canine serology surveys for other brown dog tick-transmitted pathogens."

Not surprisingly, then, parasites remain a serious threat to patients and, often, their people. A 2020 study⁶ found 20 percent of 3,006 dogs at 288 dog parks across the U.S. had intestinal parasites. The most common gastrointestinal parasites were Giardia, Ancylostoma caninum, and Trichuris vulpis.

Preventive developments

While parasite preventives have improved immensely over the past 40 years—gone for the most part are the days of dips, powders, and sprays—parasites have evolved just as rapidly, leading to resistant populations. Hookworms are a prime example.

Chemical classes used to fight parasites include isoxazolines, ivermectin, milbemycin, and lufenuron, among others. The most recent, isoxazolines, first came out in 2013. They target fleas and several tick and mite species.⁷

Isoxazolines have revolutionized the approach to ridding pets of fleas and ticks, says Amelia White, DVM, MS, DACVD, clinical professor of dermatology at Auburn University College of Veterinary Medicine. She appreciates their safety, efficacy, and oral and topical formulation options for dogs and cats.

"Extra-label use of these drugs is reported in the literature for treatment of ectoparasites in other species as well," Dr. White says. "As a dermatologist, I have especially appreciated the broad spectrum of activity of this class against other ectoparasites, including demodex, sarcoptes, and otodectes."

The broader protection and longer duration of action that these products offer is important for filling in compliance gaps, Dr. Prior says. Extended-duration products provide up to 12 months of protection. Depending on the active ingredient and country in which the product is sold, registered target species include fleas, American dog ticks (Dermacentor variabilis), lone star ticks (Amblyomma americanum), black-legged ticks (Ixodes scapularis), paralysis ticks (Ixodes holocyclus), brown dog ticks (Rhipicephalus sanguineus), bush ticks (Haemaphysalis longicornis), demodectic mange mites (Demodex spp.), sarcoptic mange mites (Sarcoptes scabiei), and ear mites (Otodectes cynotis).

"These products have had label extensions, and increased global research studies show on- and off-label efficacies against different parasites," Prior says. "This includes around the different routes of administration. The same molecule may provide different blood levels when given by injection versus transdermal topically. It's important for the veterinarian to understand these differences and nuances of routes of administration and doses between different molecules, as well as on- and off-label claims."

For clients, the benefits lie in greater convenience. The advances mean they can give fewer products with the same or better protections because molecules from multiple products are being combined into one product, Prior says.

Some clients resist giving preventives. White says a helpful strategy is to educate them about the consequences of ectoparasite presence—infectious diseases, skin infections, pain and discomfort, pruritis, allergy, anemia, and increased risk for secondary bacterial and fungal infections—as well as the risk to themselves and their families of contracting these sometimes deadly and debilitating infectious diseases.

"Remind them these ectoparasites are difficult to see and most of the lifecycle does not occur on the pet, making it more challenging to notice when they are present," she says.

Identifying pathogens fast

Diagnostics have improved, as well. Prior sees more options for patient-side testing, as well as expanded offerings for send-out testing for internal parasites using rapid PCR, antigen, and AI machine learning.

Vector-borne disease testing using molecular or serological techniques can rapidly detect pathogens spread by fleas, mosquitoes, and ticks, such as Lyme, ehrlichia, anaplasma, and heartworms, allowing them to be identified and treated before infections become severe.

For fast-moving and deadly protozoal infections, such as Cytauxzoon felis, some panels include a marker associated with atovaquone resistance. Treatment with a combination of atovaquone and azithromycin results in a 60 percent survival rate, according to a 2020 study published in the Journal of Veterinary Internal Medicine,⁸ but some strains may be resistant to standard therapy. Early identification of resistant species can help to guide treatment.

Finally, integration of lab results into all the various practice management systems is in the works, Prior says.

Still fighting fleas

With so many new and improved methods of parasite control, it might seem as if fleas are no longer a concern, but Prior says that's not the case, particularly in cats. Improved diagnostics have shown the true prevalence of the flea tapeworm,⁹ and it's not a pretty picture.

"The statistics make us realize that our flea control in cats is lacking," he says. "It's a huge area of opportunity to protect cats and protect more people, as we know fleas also carry zoonotic diseases." Those include plague, cat scratch disease, and tapeworms.

Tips and resources

Parasites and parasite control are topics always of interest to researchers. Prior recommends the open-access journal Parasites and Vectors (https://link.springer.com/journal/13071) for keeping up with the latest news.

Knowing what's going on in your client's backyard is where CAPC comes in. It offers prevalence maps (https://capcvet.org/maps#/), helpful for educating pet owners about the parasites in their area, as well as a 30-day forecast for individual counties and disease alerts (www.petdiseasealerts.org). The parasite forecast for 2026 is in progress and will be released in April.

"These sites help with education and engagement and help drive compliance, but it's also important to understand your client," Prior says. "Are they able to administer medications to their pets? Are they forgetful? Do they just like convenience? Finding the pockets of resistance will help to circumvent these to recommend products that best suit their lifestyle."

White advises clients to set an alarm on their phone or smart speaker to remind them on the same date every month to give preventatives. They may also want to consider long-acting injectable options if that better suits their lifestyle.

"It's more expensive to treat the consequences of ectoparasites than it is to prevent them," she says.

Kim Campbell Thornton has been writing about dogs, cats, wildlife, and marine life since 1985, and is a recipient of multiple awards for her articles and books from the Cat Writers Association, Dog Writers Association of America, and American Society of Journalists and Authors.

References

1. https://capcvet.org/about-capc/news-events/companion-animal-parasite-council-releases-annual-2021-pet-parasite-forecast/
2. Semenza, J.C., Rocklöv, J. & Ebi, K.L. Climate Change and Cascading Risks from Infectious Disease. Infect Dis Ther 11, 1371–1390 (2022). https://doi.org/10.1007/s40121-022-00647-3
3. Clow KM, Leighton PA, Ogden NH, et al. Northward range expansion of Ixodes scapularis evident over a short timescale in Ontario, Canada. PLoS ONE. 2017;12(12): e0189393.
4. Monaghan AJ, Moore SM, Sampson KM, Beard CB, Eisen RJ. Climate change influences on the annual onset of Lyme disease in the United States. Ticks Tick-Borne Dis. 2015;6(5):615–22.
5. Foley J, Álvarez-Hernández G, Backus LH, Kjemtrup A, Lopéz-Pérez AM, Paddock CD, Rubino F, Zazueta OE. The emergence of Rocky Mountain spotted fever in the southwestern United States and northern Mexico requires a binational One Health approach. J Am Vet Med Assoc. 2024 Feb 27;262(5):698-704. doi: 10.2460/javma.23.07.0377. PMID: 38417252.
6. Stafford, K., Kollasch, T.M., Duncan, K.T. et al. Detection of gastrointestinal parasitism at recreational canine sites in the USA: the DOGPARCS study. Parasites Vectors 13, 275 (2020). https://doi.org/10.1186/s13071-020-04147-6
7. https://www.merckvetmanual.com/pharmacology/systemic-pharmacotherapeutics-of-the-integumentary-system/antiparasitic-drugs-for-integumentary-disease-in-animals
8. Hartley AN, Marr HS, Birkenheuer AJ. Cytauxzoon felis cytochrome b gene mutation associated with atovaquone and azithromycin treatment. J Vet Intern Med. 2020 Nov;34(6):2432-2437. doi: 10.1111/jvim.15935. Epub 2020 Nov 4. PMID: 33146909; PMCID: PMC7694809.
9. https://capcvet.org/maps#/2025/all-year/flea-tapeworm/cat/united-states