8 Sept 2026
Hany Elsheikha BVSc, MSc, PhD, PgCHE, DipEVPC, FHEA considers how certain factors shape decision making in practice, and why we need to look beyond “good” and “bad” parasiticides.
Hany Elsheikha
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Image: Seventyfour / Adobe Stock
Parasiticides are a cornerstone of companion animal medicine, protecting animal health, reducing zoonotic risk and limiting the transmission of vector-borne pathogens (Giannelli et al, 2025).
Yet, public discussion increasingly frames their use in binary terms, with some compounds portrayed as inherently beneficial and others primarily as environmental hazards (Yoder et al, 2024). Such narratives risk obscuring a more fundamental reality: the consequences of parasiticide use do not arise solely from the intrinsic properties of active ingredients, but from the way these products are selected, applied and used within diverse clinical, ecological and social contexts.
Viewed through this lens, parasite control is better understood not as a single intervention but as a sequence of decisions, each made in the face of several challenges (Bagster and Elsheikha, 2022). Some decisions respond to known or suspected infection, where treatment is guided by clinical evidence; others are preventive, requiring clinicians and owners to act before infection occurs by estimating future exposure and weighing uncertain risks. It is within these preventive decisions that uncertainty becomes most apparent.
Questions of whether intervention is warranted, when protection should begin, how frequently treatment should be administered, how long protection should persist, and which formulation or combination of active ingredients is most appropriate rarely have universally accepted answers. Instead, they depend on an evolving assessment of parasite epidemiology, product efficacy and duration of activity, environmental persistence, host susceptibility, owner adherence and the potential for resistance to emerge. Even the choice of combination products illustrates these competing considerations: formulations containing multiple active ingredients may simplify treatment regimens and improve owner compliance, yet the same products may limit opportunities to tailor exposure to individual active substances according to the specific parasites and risks being targeted.
The outcomes of parasite control, therefore, emerge from the interaction between evidence, product characteristics, clinical judgement, animal owner behaviour and the wider epidemiological context, rather than from the properties of a particular active ingredient alone.
Clinical judgement is itself shaped not only by published evidence but also by professional experience, which inevitably influences how future risks are perceived; for example, veterinarians whose careers have unfolded during the widespread availability of highly effective isoxazolines may have little direct experience of managing the severe flea infestations that were once commonplace, leading them to view the consequences of reducing ectoparasite control differently from colleagues who practised before these products changed clinical practice.
These differences illustrate how experience can shape judgement through well-recognised perception, without diminishing the value of clinical expertise itself. Recognising this complexity does not imply that all interventions or perspectives are equally supported by evidence. Rather, it acknowledges that decisions must often be made before complete certainty is possible, requiring clinicians to integrate the best available evidence with individual patient needs, owner circumstances and the broader ecological consequences of intervention.
A central challenge in parasite control is that decisions are rarely made with complete knowledge of the true level of risk. Local parasite prevalence may fluctuate seasonally or geographically, while individual exposure depends on lifestyle, travel, housing and owner behaviour. At the same time, the environmental fate of parasiticides and the long-term dynamics of resistance development are often uncertain, or only partially observable at the point of decision making (Wells and Collins, 2022).
Consequently, parasiticide use operates within a probabilistic rather than deterministic framework. A treatment decision that is appropriate in one setting may be unnecessary or excessive in another, while a decision to withhold treatment may reduce environmental exposure but increase clinical or zoonotic risk.
The challenge is, therefore, not the identification of a universally “correct” intervention, but the navigation of uncertainty in a way that appropriately balances competing risks. Importantly, concern regarding environmental contamination is supported by emerging evidence and should not be dismissed (Perkins et al, 2024). However, the magnitude of environmental risk under different patterns of companion animal parasiticide use remains incompletely understood, making translation of this evidence into optimal clinical decision making inherently challenging. Under these circumstances, stewardship should focus not only on deciding whether treatment is appropriate, but also on ensuring that products are used according to the summary of product characteristics (SPC).
Measures such as appropriate disposal of faeces, avoiding swimming or bathing following topical treatment where recommended and adherence to product instructions can reduce environmental exposure while maintaining effective parasite control.
Parasiticide use can be conceptualised as an iterative decision process in which perception and outcome continuously interact. Clinical decisions are shaped by perceived risk, which leads to a chosen intervention, which in turn generates outcomes that feed back into future perceptions of risk.
This feedback loop helps explain why certain practices persist even when the underlying epidemiological context evolves; for example, routine preventive treatment may become normalised when short-term reductions in parasite burden reinforce the perception that continuous intervention is necessary. In many cases, these approaches represent pragmatic responses to uncertainty, the need for consistent preventive protection, and the practical realities of maintaining adherence within diverse client and pet populations. Conversely, increasing concern about environmental impacts may reduce treatment frequency even when epidemiological conditions still justify prevention.
Over time, such feedback processes can stabilise practice patterns that may not always fully reflect changes in underlying risk conditions. This pattern is not unique to parasiticide use and reflects broader features of preventive health care decision making under uncertainty.
Every decision about companion animal parasite control involves trade-offs. Effective prevention must protect animal health and welfare while reducing zoonotic risks to people – both within and beyond the household – minimising environmental impacts, remaining affordable and preserving the long-term effectiveness of antiparasitic treatments.
These objectives are not fully compatible, and improvements in one domain often require compromise in another; for example, increasing the frequency of preventive treatment may improve immediate protection against parasites but also increase overall environmental exposure and, in principle, contribute to selective pressures that could favour the emergence of resistance over time. Although confirmed resistance to companion animal parasiticides remains limited, preserving treatment efficacy remains an important stewardship objective. Conversely, reducing treatment may decrease environmental exposure but could, in certain contexts, increase the risk of infection or transmission. Practical considerations also influence these decisions.
The most appropriate parasite control strategy is not necessarily the one with the greatest theoretical efficacy, but the one that can be implemented consistently; for example, if an owner is unable to administer oral medication reliably, a topical formulation may represent a more appropriate choice, even if it involves different trade-offs. Likewise, long-term success depends on owner adherence to the prescribed treatment plan, making ease of administration and compliance important considerations when selecting an intervention.
The notion of a single, universally applicable “best” strategy is, therefore, difficult to achieve or sustain, as optimal approaches vary according to epidemiological context, animal characteristics, owner circumstances and clinical priorities. This framework should not be interpreted as an argument against preventive parasiticide use, which remains essential in many epidemiological contexts and continues to play a central role in protecting both animal and public health. Rather, it supports selecting the right intervention for the right patient at the right time, while ensuring that products are administered responsibly in ways that minimise unnecessary environmental exposure.
Even when robust scientific evidence is available, decisions about parasite control are shaped not only by evidence, but also by experience, values, and perceptions of risk (Bagster and Elsheikha, 2022). Under conditions of uncertainty, both clinicians and owners naturally draw on memorable events and previous experiences when weighing the potential benefits and harms of preventive treatment; for example, a veterinarian who has managed a fatal case of canine angiostrongylosis may perceive the consequences of failing to prevent Angiostrongylus vasorum infection as particularly severe and, therefore, be more likely to recommend routine prophylaxis than a colleague who has never encountered such a case. Both clinicians may be working from the same published evidence, yet their perception of risk is influenced by different clinical experiences.
Owners interpret risk in a similarly individual way. One owner may be comfortable adopting a risk-based approach, relying on regular inspection of their pet and seeking treatment only when parasites are detected. Another may have previously experienced a persistent household flea infestation that was difficult, costly and stressful to eliminate, leading them to favour continuous preventive treatment despite a similar underlying epidemiological risk. In these situations, the difference lies not necessarily in the objective level of risk, but in how that risk is perceived and valued. These differences in risk perception should not be regarded simply as barriers to evidence-based practice. Rather, they represent an inherent feature of decision making under uncertainty.
Recognising and openly discussing these differing perspectives allows veterinarians to build trust with pet owners, explore the reasons underlying treatment preferences, and develop parasite control plans that are both evidence informed and practical for the individual animal, owner and local epidemiological context.
Optimal parasiticide strategies are highly context dependent. Geographic variation in parasite prevalence, seasonal dynamics, climate change, indoor versus outdoor lifestyles, multi-pet households, travel behaviour and local resistance patterns all influence the risk–benefit balance of treatment decisions. Socio-economic constraints and access to veterinary care further shape what is feasible in practice.
Accordingly, parasiticide guidelines provide an essential foundation for clinical decision making, but cannot fully account for the diversity of context-specific factors that determine optimal practice. Parasite control should, therefore, be understood not as a fixed protocol, but as an adaptive process requiring ongoing reassessment and contextual interpretation. This does not diminish the importance of established guidelines, which remain essential for supporting consistent standards of care and reducing preventable parasitic disease risk in companion animals (www.esccap.org/guidelines/).
Despite advances in veterinary parasitology and preventive medicine, uncertainty remains an enduring feature of parasiticide decision making.
Local epidemiological data are often incomplete or not available, while resistance development may only become apparent after considerable delay. Environmental exposure pathways are also difficult to quantify precisely, and communication between stakeholders may be fragmented or inconsistent. At the same time, clinical decision making may be influenced by practical pressures to prioritise immediate prevention, client reassurance, consistency of care and feasible implementation in busy clinical settings, alongside longer-term considerations of optimisation and sustainability.
As a result, decisions are often made using heuristics and simplified rules rather than fully integrated evidence-based models, even in well-resourced settings.
Such approaches frequently reflect the practical need to make timely and consistent decisions in situations where complete information is not yet available. This reflects the practical realities of clinical medicine and should not be interpreted as a rejection of evidence-based practice, but rather as a consequence of making decisions under practical constraints and incomplete information.
Resistance represents an additional source of uncertainty. Although confirmed resistance to companion animal parasiticides remains unknown in the UK, continued surveillance is essential to detect emerging changes in efficacy before they become widespread.
Pharmacovigilance systems, together with systematic investigation of suspected treatment failures, therefore play an important role in distinguishing true resistance from more common causes of reduced effectiveness, such as incorrect product selection, inappropriate administration or poor owner adherence.
Given that parasiticide outcomes are shaped by decisions often made under uncertainty, improving outcomes requires attention to the quality of decision making rather than solely to product characteristics. This includes improving the accuracy of risk estimation, strengthening diagnostic support, and enhancing access to reliable local epidemiological data. It also requires clearer communication of uncertainty, ensuring that clinicians and owners understand both what is known and what remains uncertain.
Shared decision-making approaches with pet owners may help align treatment choices more closely with individual risk profiles, while adaptive management strategies can support the adjustment of interventions over time in response to changing conditions.
In this way, parasite control becomes a responsive and iterative process rather than a static protocol. Improving decision quality should, therefore, be understood as improving the transparency, contextual appropriateness and evidence basis of decisions, rather than imposing a single approach across all settings.
Improving decision quality also requires attention to how parasiticides are used once a treatment decision has been made. Stewardship extends beyond product selection to include adherence to the SPC and clear communication with owners regarding measures that minimise unnecessary environmental exposure. Advice on appropriate disposal of faeces, avoiding swimming or bathing after topical application where recommended, and correct administration of products should, therefore, be regarded as integral components of responsible parasite control rather than ancillary considerations.
In many situations, optimising product use may reduce environmental exposure more effectively than simply withholding preventive treatment.
Stewardship also requires ongoing monitoring of treatment effectiveness. Suspected treatment failures should not automatically be interpreted as evidence of resistance, as reduced efficacy may arise from incorrect administration, poor compliance, reinfection or failure to address environmental reservoirs. Appropriate pharmacovigilance and investigation of suspected loss of efficacy are, therefore, essential to identify genuine resistance while avoiding unnecessary changes in prescribing practice.
The framing of parasiticides as inherently “good” or “bad” is limited in its ability to capture the complexity of parasite control in companion animals. At the same time, ethical, environmental, clinical and public health concerns surrounding parasiticide use remain legitimate and important components of ongoing scientific and professional discussion.
Outcomes are not fixed properties of compounds, but emerge from decisions made under conditions of uncertainty, shaped by perception, context and behavioural bias.
Understanding parasiticide use as a dynamic decision problem highlights the importance of improving how decisions are made rather than focusing solely on what is used.
Sustainable and effective parasite control depends on the ability to navigate trade-offs, interpret incomplete information and adapt to changing epidemiological conditions. Ultimately, the challenge is not only how parasiticides are categorised, but how the quality of decisions governing their use – how, when and why they are applied – can be continuously improved.
Importantly, improving decision quality should support, rather than undermine, confidence in preventive veterinary medicine by promoting interventions that are both evidence informed and contextually appropriate.
Hany Elsheikha is a professor of interdisciplinary parasitology in the global health department in the School of Veterinary Medicine and Science, University of Nottingham (SVMS-UoN). He is also a European Veterinary Parasitology College diplomate, head of European Scientific Counsel Companion Animal Parasites UK and Ireland, and a fellow of the Higher Education Academy. Hany earned his PhD from Michigan State University and was awarded the prestigious American Society for Microbiology/National Center for Infectious Diseases post-doctoral fellowship. Since joining SVMS-UoN, he has been spearheading the development and delivery of parasitology teaching. Also, he has established a multidisciplinary research programme focused on decoding the interkingdom chemical communication between the host cells and neuropathogenic protozoan parasites, with a special interest in Toxoplasma gondii. Hany has published more than 270 peer-reviewed papers and many other articles in professional magazines and science communication journals. He has published eight books in veterinary, tropical and medical parasitology for students, residents and professionals.