Notes on Animal Health, December 2023: Pets vs. Pests

For many of us, our pets truly are part of our family. They share our homes, our beds, go to work and on vacation with us. They entertain us and protect us. They will be by our sides and on our laps as we celebrate this holiday season with family and friends. We keep our pets close with little concern that they might share fleas, ticks, and intestinal parasites with us. That, however, has not always been the case.

For those of us who came of age before the 1990’s, infestations of fleas and ticks often relegated our pets to the yard. We had to bathe or “dip” them regularly in noxious, and often toxic, chemicals to keep the pests at bay. Flea infestations were common and often caused severe and life-threatening anemia in dogs and cats, particularly puppies and kittens. And it was not just our pets that suffered. Fleas also bite humans and can carry zoonotic diseases, including cat scratch disease (Bartonella henselae), flea-borne spotted fever (Rickettsia felis), murine typhus (Rickettsia typhi), and bubonic plague (Yersina pestis). As if these ectoparasites weren’t bad enough, our pets can also carry endoparasites, including hookworms, roundworms and tapeworms, which also present zoonotic disease risks.

The development of effective and safe products to treat and/or prevent external and internal parasite infestations made it safer and easier for pets to become integral parts of our family and share our homes. The first "modern" anthelmintics with relatively wide safety margins and activity against a broad range of intestinal parasites were the benzimidazoles (BZDs), developed and commercialized in the 1960s and 70s.  Advances in these agents, as well as the development of other structurally unrelated compounds, such as pyrantel, were safer still and more active than their predecessors.  Many are still in use today.

An even more significant advancement came with the discovery of ivermectin in 1975 and the subsequent development of other macrocyclic lactones, including moxidectin and milbemycin oxime. Macrocyclic lactones are extremely safe and effective in the control of many intestinal parasites, including hookworms and roundworms in dogs and cats. They also have excellent activity against the infective larval stages of filarial worms including Difilaria immitis, which causes heartworm disease in dogs and Onchocera volvulus, which is the cause of onchoceriasis in humans. Contemporaneously, other agents, including pyrantel and praziquantel, demonstrated excellent efficacy against tapeworms. In short, in the span of 20-some-odd years the treatment and prevention of intestinal parasites in dogs and cats had been revolutionized.

A breakthrough in the control of flea infestations, however, did not arrive until the 1990’s with the approval of lufenuron. Prior to the launch of that product, fleas were controlled primarily by treating both the environment and the pet on a regular and frequent basis using sprays, dusts and collars. The active ingredients in these products included pyrethrins, synthetic  pyrethroids, chlorinated hydrocarbons, organophosphates, and carbamates. While these products were effective against adult fleas, some of the actives were toxic, while others were safe but degraded rapidly. Obviously, cuddling up to a pet wearing a collar that slowly released a pungent organophosphate, like chlorpyrifos, was not particularly appealing. Lufenuron was a total game changer, because it was extremely safe for pets and humans, administered orally once a month, and could break the life cycle of the flea by preventing flea eggs and larvae from developing into adults. Although somewhat underappreciated today, at the time lufenuron was a phenomenal advancement in flea control. Nevertheless, because pets can pick up fleas in a variety of environments, lufenuron is most effective when used in combination with products active against adult fleas. Serendipitously, the first modern adulticide, imidacloprid, which is both safe and effective, was also launched in the early 1990’s. Many other products containing actives, such as fipronil and nitenpyram, followed soon after, and by the early 2000’s there were numerous products available to control fleas on our pets.

Once we could protect cats and dogs from fleas and internal parasites, we opened our homes to our pets and now we take the effectiveness of these products for granted. However, the battle is not over, because over time parasites inevitably develop resistance to the agents we use to kill them. For example, the first report of drug resistance in the dog hookworm, Ancylostoma caninum, was reported in Australian racing Greyhounds in 1987. In the U.S. reports of multidrug resistant hookworm isolates are now common. In addition to intestinal parasites, resistance to macrocyclic lactones has also been demonstrated in the U.S. heartworm population. Finally, our current parasiticides are used not only in our pets, but also in livestock and humans, thus increasing the chances and the consequences of the development of further resistance.

Insects, such as fleas, can also develop resistance to the agents we use to control them. Resistance to carbamates, organophosphates, and pyrethroids has been shown to be widespread, though resistance to imidacloprid has yet to be documented. A new class of insecticides, isoxazolines, have a novel mechanism of action, and this drug class is proving extremely effective against fleas, and in some cases, ticks, and their effects can last from weeks to months. Generally considered safe, isoxazolines have been associated with an increased risk of seizures in some dogs, although the reported incidence is low. Combinations of these agents with other parasiticides have yielded once-a-month products that can treat or control intestinal parasites, fleas, and ticks, while also preventing heartworm disease. While extremely popular, there is growing concern that these cocktail approaches may increase the risk of the development of resistance within the parasite population.

No one wants to go back to the days of keeping our family pets at arm’s length for fear of flea and tick bites or intestinal parasites. This means we need new, active ingredients and novel formulations to simply maintain the status quo of external and internal parasite control. We also need products affordable to pet owners with low incomes, otherwise, their communities will be at greater risk of zoonotic diseases. The animal health industry has made amazing progress in protecting us and our pets against external and internal parasites, but we must remain vigilant against the development of resistance. Judicious use of our current arsenal, as well as the development of new actives, will be necessary to protect our own and our pets’ health and welfare moving forward.

– Cindy Cole, DVM, PhD, DACVCP



First Five

1/ New uses for AI: Goose beak recognition
Researchers from The Konrad Lorenz Research Center for Behavior and Cognition have developed a of facial recognition AI program that can ID a goose, by looking at specific features of its beak with a 97% accuracy rate. Geese, however, seem to have no use for such a program as they appear to recognize photos of their partners and friends. The research suggests that geese can also recognize and remember human faces. Just another reason to always be kind to our feathered friends.

2/ Gender equality in the benefits of dog hugs
The results of numerous studies have shown that dog therapy programs can improve a person's social and emotional wellbeing, however, the majority of participants in many of those studies were female. A recent study conducted at the University of British Columbia sought to determine if individuals identifying as female benefited more than others from interactions with therapy dogs. A total of 163 students—49% women, 33% men, and 17% non-binary and other genders—participated in 20-minute sessions with the dogs in groups of three to four before completing a survey. The results showed that there was a significant increase in wellbeing and a decrease in homesickness, stress, and loneliness across all gender identities. Confirming that is little dog love is good for all.

3/ Pet cats are amazing hunters
A recent comprehensive global assessment identified 2,084 species eaten by cats, of which 347 (16.65%) were of conservation concern. While the effects of the domestic cat on wild bird populations has been speculated, the actual number of different vertebrate, and non-vertebrate, species potentially affected by are pet cats was surprising. Birds, reptiles, and mammals constituted ~90% of species consumed, with insects and amphibians being less frequent. Unfortunately, no easy solution has been found. Confinement and collar-mounted devices may reduce the numbers of animals killed, but some owners do not wish to inhibit what they see as natural behavior,  or they perceive safety risks associated with collars. One recent study found that high protein meat based diets, more play time with their owners, and devices such as puzzle feeders can decrease the rates of predation.

4/ Go fetch, kitty?
Domesticated animals are famous for the ease with which they can accommodate to diverse human environments and roles, but less well-studied is the ease with which domestic animals can manipulate their human caregivers to their own ends. A recent study surveyed 924 cat owners who report fetching behavior in 1154 cats. The overwhelming majority (94.4%) of the owners reported that fetching emerged in the absence of explicit training. Fetching was primarily first noticed when the cats were less than one year old (n = 701) or 1–7 years old (n = 415). Cats initiated and terminated fetching bouts more often than did their owners. Thus, cats who fetch largely determine when they engage in fetching sessions and actively influence the play behavior of their owners.

5/ Effects of neutering on lifespan in Rottweilers
Although surgical sterilization or neutering of dogs is common in many countries, concerns have been raised regarding possible side effects of neutering, including increased risk of certain neoplastic, musculoskeletal and endocrinological conditions. A recent study compared the lifespans of neutered and sexually intact male and female Rottweilers. Male and female Rottweilers neutered before 1 year of age (n = 207) had expected lifespans 1.5 years and 1 year shorter, respectively, than their intact counterparts (n = 3085; p < 0.05). Broadening this analysis to include animals neutered before the age of 4.5 years (n = 357) produced similar results. Further research to establish reasons for this finding is warranted. In addition, similar studies on different breeds of dogs may clarify whether this effect is unique to Rottweilers.