Inaccurate dosing and repeated administration of equine anthelmintic from the same drug class are indicated as factors associated with equine helminth resistance. Evidence also suggests that resistance is specifically related to the under-dosing of anthelmintic products (Batson, 2012; Austin, 2017). The challenges associated with equine helminth resistance have led to the development of new worming protocols and current recommendations suggest that horses are only administered an anthelmintic product when it is specifically required, rather than at regular intervals (Kaplan, 2002; Kaplan and Nielsen, 2010; Matthews, 2014; Peregrine et al, 2014; Tzelos and Matthews, 2016). An inappropriate dose of anthelmintic is one that is not suitable or ideal for use, either an under-dose or an overdose of anthelmintic (Shalaby, 2013). In order to avoid the inappropriate dosing of a pharmaceutical product, it is recommended that body-weight should always be determined as accurately as possible prior to administration (Kaplan and Nielsen, 2010). With significant differences between the estimated body-weight and the accurate bodyweight of equine species, the only way to establish an accurate bodyweight is by using an equine weighbridge, however research indicates that that many owners may still rely on visual estimation to determine a horse's bodyweight (Ellis and Hollands, 1998; Ellis and Hollands, 2002).
Rationale
An online survey conducted by Allison et al (2011) was carried out to establish owners' beliefs, attitudes, and practices with reference to the use of anthelmintic drugs. The survey identified that horse owners were concerned about the issue of anthelmintic resistance and highlighted that less than 60% of all respondents were comfortable with their existing anthelmintic programme, and 25% wanted to use less anthelmintics in their horses. Despite these concerns, and even though it is recommended that all anthelmintic products should be considered in accordance with professional guidance and the relevant pharmaceutical data sheets to ensure correct use and administration, less than half of the respondents included ‘veterinary surgeon’ among their sources of advice when it came to worming (Vercruysse and Claerebout, n.d.; Allison et al, 2011).
Many anthelmintic products have a wide safety margin, and moderate overdosing, although not recommended, is unlikely to cause many significant or detrimental problems. Macrocyclic lactones, which are commonly used in horses, have proven to cause no adverse effects when given as a single administration at ten times the recommended dose, and only mild clinical signs have been observed when ivermectin has been given at nine times the recommended dose (Vercruysse and Claerebout, n.d.). In comparison, however, praziquantel has shown adverse effects at just three times the recommended dose (Vercruysse and Claerebout, n.d.). In contrast, under-dosing takes place when parasites are exposed to an anthelmintic at an insufficient dosage to kill them resulting in them passing on resistant genetic information to subsequent generations (Brady and Nichols, 2009; Matthews, 2010). Matthews (personal communication, 2018) suggested that the higher the under-dose the more likely the chance of selecting for anthelmintic resistance; however this is dependent on a number of factors including the stage and species of the parasite, and the individual host. In support of this, Warnock (personal communication, 2018) would never recommend any amount of anthelmintic under-dose, and Naylor (personal communication, 2018) stated that any anthelmintic under-dose is a risk. To ensure safety and to avoid inaccurate dosing, it is vital that anthelmintic products are administered at the appropriate dose, and in order to achieve this, bodyweight should always be determined as accurately as possible (Batson, 2012; Austin, 2017).
With reference to the ability of horse owners to accurately estimated the bodyweight of their horses, Ellis and Hollands (1998) highlighted the significant differences between the estimated and the accurate weights of equine species. Research also highlights that horse owners may still rely on visual estimation to determine a horse's bodyweight, even though of the methods analysed by Ellis and Hollands (1998) this was the one method that was indicated as definitely not recommended for use due to being significantly inaccurate (p<0.001) result (mean (± SD) 88.3% ± 20.1%). It is stated that using a weightape or a weight estimation formulae can provide more accurate information than a standalone visual estimation of bodyweight, however it is also documented that a weight estimation formula can give results which are significantly different from the actual weights, and weightapes need to be used with consideration as the mean difference between actual weight and weightape weight can be in excess of 60 kg (Ellis and Hollands, 1998; Hintz, 2002; Wagner and Tyler, 2011). Carroll and Huntington (1988) suggested that if the approximate height and body condition score (BCS) of a horse is known, then the weight of the horse could be estimated, but this is potentially difficult to achieve; Jensen et al (2016) highlighted that the ability of owners to accurately determine the BCS of their horse was poor, and Martinson et al (2014) felt compelled to encourage the equine industry to provide training to owners on proper use and accurate assessment of BCS. The results of such research indicate that there is no single most accurate method that can be used for estimating bodyweight, and that the only method that can be used to obtain an accurate bodyweight is a weighbridge (Ellis and Hollands, 1998).
Aims and objectives
This pilot study aimed to identify whether horse owners administer inappropriate doses of anthelmintic based on an inaccurate perception of their horse's bodyweight.
The objectives of the pilot study were:
Methods
Data were collected using a mixed method allowing for both quantitative and qualitative data to be obtained and analysed.
Participants
Following a convenience sampling strategy, a number of participants with varying experience and from a variety of equine disciplines were invited to take part in this pilot study via an announcement on social media. 16 participants volunteered to be involved in the pilot study and travelled with their horses to The Equine Therapy Centre, Hartpury University, Gloucestershire. A variety of horse breeds and types were presented (Table 1). All of the owners were female and above 20 years in age, 88% (n=14) were 30 years old and above, and had varying amounts of equine experience; categorised into 15 years or less of experience (owners with less experience n=8) and more than 15 years of experience (owners with more experience n=8).
| Number | Breed | Sex | Age (years) | Height (hh) | Discipline/activity |
|---|---|---|---|---|---|
| 1 | Section A | Mare | 18 | 11.2 | Pony Club |
| 2 | Section A | Gelding | 22 | 11.1 | Pony Club |
| 3 | Danish WB | Mare | 15 | 17.1 | Dressage |
| 4 | Irish | Gelding | 18 | 15.2 | Eventing |
| 5 | ID x TB x WB | Gelding | 5 | 17.1 | Hacking |
| 6 | ISH | Gelding | 7 | 16.1 | Pony Club |
| 7 | Dartmoor | Mare | 9 | 12.2 | Pony Club, show jumping |
| 8 | ISH | Gelding | 8 | 17 | Dressage |
| 9 | Cob x TB | Gelding | 15 | 13.3 | Pony Club |
| 10 | Shetland | Gelding | 3 | 33-34inch | Companion |
| 11 | TB | Gelding | 24 | 16.2 | Hacking, dressage |
| 12 | Section D | Mare | 19 | 14.2 | Pony Club |
| 13 | TB x | Gelding | 13 | 15.2 | Eventing |
| 14 | ISH x New Forest | Mare | 10 | 14.2 | Pony Club, show jumping |
| 15 | ISH | Mare | 10 | 14.3 | Hacking, dressage |
| 16 | Appaloosa x | Gelding | 10 | 13.3 | Pony Club |
Measures
Each owner answered a series of questions in order to capture individual owner information and experience, horse details, and factors that were taken into consideration when estimating a horse's bodyweight (kg). Each owner was asked to provide a perceived bodyweight by estimating the bodyweight of their horse, to provide the dose of anthelmintic (kg) that they had most recently administered to their horse, and to provide a BCS for their horse using the World Horse Welfare (2018) BCS 0–5 scale guide provided.
Each horse was then weighed on a calibrated equine weighbridge (Figure 1) in order to obtain an accurate body-weight (kg) (Ellis and Hollands, 1998). The estimated body-weight was compared with that of the accurate bodyweight, and the dose of anthelmintic was compared with that of both the estimated bodyweight and the accurate bodyweight. The BCS provided by each owner was compared with the BCS provided by the researcher.
Ethical considerations
Prior to the conduction of this pilot study, a project proposal form was submitted to Hartpury University and ethical approval was obtained. Owners were required to complete a participation consent form prior to participating in the research (Gov.uk, 2018). The data collected were stored according The Data Protection Act 2018, the UK's implementation of the General Data Protection Regulation (GDPR), and handled with confidentiality at all times.
There were no obvious welfare concerns regarding human or animal participants. The methods used were non-invasive, and all of the horses were given as much time as was required to acclimatise to the weighbridge prior to data collection. The health and safety implications of working with horses were considered, and owners were asked to handle their own horses and take the necessary precautions by wearing suitable personal protective equipment to include a riding hat, and sensible clothing and footwear (Middlecote and Chandler, 2011).
Data analysis
All statistical analysis was carried out using IBM Statistics Package for Social Scientists (SPSS) version 24.0. Data were tested for normal distribution using the Shapiro-Wilks test and the data considered to be normally distributed are presented as mean (± SD). The differences between estimated bodyweight and owner experience were analysed using a paired-samples t-test. The differences between estimated bodyweight and accurate body-weight, estimated bodyweight and the administered dose of anthelmintic, and accurate bodyweight and the administered dose of anthelmintic, were analysed using an independent-samples t-test.
Results
Estimation of bodyweight
100% (n=16) of owners inaccurately estimated the body-weight of their horse (mean (± SD) 51.9 kg ± 29.6 kg); 44% (n=7) of owners under-estimated the bodyweight of their horse (mean (± SD) 51.6 kg ± 26.4 kg), and 56% (n=9) of owners over-estimated the bodyweight of their horse (mean (± SD) 52.2 kg ± 33.4 kg). The data were analysed using a paired-samples t-test; there was no statistically significant difference between the estimated bodyweight and the accurate bodyweight (p=0.738). Owners with less experience more accurately estimated the bodyweight of their horse (mean (± SD) 39.2 kg ± 19.8 kg) when compared with owners with more experience (mean (± SD) 64.6 kg ± 33.3 kg). The data were analysed using an independent-samples t-test; there was no statistically significant difference in accuracy between the two groups (p =0.085).
Factors taken into consideration when estimating bodyweight
None of the owners involved in the study had used a weight estimation formula to inform their estimation of body-weight. None of the owners with more experience had used a weightape, but 63% (n=5) of owners with less experience had used a weightape to inform their estimation of body-weight. 69% (n=11) of owners had relied on veterinary advice, horse height, and comparison to other horses to inform their estimation of bodyweight.
Estimation of bodyweight and dose of anthelmintic
When comparing the estimated bodyweight to the most recent dose of anthelmintic provided, 38% (n=6) of owners had administered a dose of anthelmintic in line with the perceived bodyweight. 56% (n=9) of owners had administered an overdose of anthelmintic (mean (± SD) 48.9 kg ± 38.2 kg), and 6% (n=1) of owners had administered an under-dose of anthelmintic (50 kg) (see Figures 2 and 3). Data were analysed using a paired-samples t-test; there was a statistically significant difference between the estimated bodyweight and the administered dose of anthelmintic (p=0.034).
Owner experience
Of the 38% (n=6) of owners that had administered a dose of anthelmintic in line with the perceived bodyweight, 63% (n=5) were owners with less experience and 13% (n=1) were owners with more experience. 38% (n=3) of owners with less experience had administered an overdose of anthelmintic (mean (± SD) 15 kg ± 10 kg), and 75% (n=6) of owners with more experience had done the same but with a greater overdose (mean (± SD) 65.8 kg ± 35.6 kg) in comparison to the owners with less experience. The 6% (n=1) of owners that had administered an under-dose of anthelmintic (50 kg) were owners with more experience.
Accurate bodyweight and dose of anthelmintic
When comparing the accurate bodyweight to the most recent dose of anthelmintic provided, 75% (n=12) of owners had administered an overdose of anthelmintic (mean (± SD) 55.5 kg ± 45.5 kg), including 19% (n=3) of owners who had underestimated bodyweight but had then subsequently overdosed their horse with anthelmintic (mean (± SD) 18 kg ± 19.7 kg). 25% (n=4) of owners had administered an under-dose of anthelmintic (mean (± SD) 48.8 kg ±18.5 kg) (see Figures 4 and 5). Data were analysed using a paired-samples t-test; there was no statistically significant difference between the accurate bodyweight and the administered dose of anthelmintic (p =0.074).
Owner experience
63% (n=5) of owners with less experience had administered an overdose of anthelmintic (mean (± SD) 25.9 kg ± 23.7 kg), and 88% (n=7) of owners with more experience had done the same, but with a greater overdose in comparison to the owners with less experience (mean (± SD) 76.7 kg ± 46.5 kg). 19% (n=3) of owners with less experience had administered an under-dose of anthelmintic (mean (± SD) 57.7 kg ± 5.9 kg), and 25% (n=1) of owners with more experience had also done the same but to a lesser extent than the owners with less experience (22 kg).
BCS
100% (n=16) of owners provided a BCS that was accurate/the same as the assessment made and documented by the researcher.
Discussion
The results of this pilot study support the work of Ellis and Hollands (1998; 2002) indicating that many horse owners still rely on visual estimation to determine a horse's bodyweight. Although a statistically significant difference was not found between the estimated bodyweight and the accurate bodyweight (p=0.738), or between the ability of owners with less experience and owners with more experience to accurately estimate the bodyweight of their horse (p=0.085), it is apparent that visual estimation results in an inaccurate estimate of bodyweight (mean (± SD) 51.9 kg ± 29.6 kg). The results further substantiate evidence that suggests that the only method by which an accurate equine bodyweight can be obtained is by using a weighbridge (Ellis and Hollands, 1998).
None of the owners mentioned using a weight estimation formula to inform their estimation of bodyweight, and owners with more experience appeared to rely on using a visual estimation based on experience, veterinary advice, horse height, and comparison to other horses, to inform their estimation of bodyweight. Owners with less experience were more likely to use a weightape to inform their estimation of bodyweight, and although Wagner and Tyler (2011) and Hintz (2002) identified that weightape weight can be in excess of 60 kg inaccurate, it is apparent that using a weightape provides more accurate information than a visual estimate of bodyweight (Ellis and Hollands, 1998). Owners were not questioned as to the reason/s why they chose the method/s that they did to inform their estimation of bodyweight, and it would be appropriate to more fully investigate this.
There was no statistically significant difference between owner experience and an accurate estimation of bodyweight (p=0.085), however of the owners with less experience, 63% (n=5) had used a weightape and had more accurately estimated the bodyweight of their horse (mean (± SD) 39.2 kg ± 19.8 kg) when compared with owners with more experience (mean (± SD) 64.6 kg ± 33.3 kg).
There was a statistically significant difference between the estimated bodyweight and the dose of anthelmintic administered (p=0.034). The results suggest that owners administer inappropriate doses of anthelmintic based on an inaccurate perception of their horse's bodyweight. Although 63% (n=5) of owners with less experience were more likely to administer a dose of anthelmintic based on the estimated bodyweight when compared with 13% (n=1) of owners with more experience, it is more likely that an owner will administer an overdose (75% (n=12)), rather than an under-dose (25% (n=4)) of anthelmintic. With 75% (n=6) of owners with more experience having administered what they felt to be, based on estimated bodyweight, an overdose of anthelmintic (mean (± SD) 65.8 kg ± 35.6 kg) and one owner with more experience having administered an under-dose of anthelmintic (50 kg), it would be particularly relevant to further consider and assess the reasoning for this.
Contradictory to the research carried out by Jensen et al (2016), 100% (n=16) of owners were able to provide a BCS that was accurate/the same as the assessment made and documented by the researcher. This could be considered with reference to the work of Carroll and Huntington (1988) who indicated that the weight of the horse could be estimated if the approximate height and BCS of a horse is known.
Limitations
This pilot study had both a limited sample size and a minimal owner demographic. In order to gain a better representation the population, a sample size of at least 100 participants is recommended for this type of study (Ellis and Hollands, 1998). It should also be noted that all of the owners were female and above 20 years in age, with the majority being 30 years and above in age. A more diverse demographic would be ideal to allow for stronger representation of the population and more detailed analysis of whether gender and age have any influence on the outcomes of the study. It is worth considering, however, that there may potentially be some difficulty with being able to recruit equal numbers of male participants when according to The British Equestrian Trade Association's (BETA) National Equestrian Survey (2015) there remains a strong gender bias in equine sports, with females representing 74% of the total population.
The owners had varying amounts of equine experience. The number of years of experience is a fairly subjective measure, particularly within this participant sample, and without further questioning and data collection it was difficult to interpret exactly how experienced participants were based solely on the number of years that they had owned horses, or been involved in the equine industry.
Conclusion
The administration of inappropriate doses of anthelmintic can have a detrimental effect on equine health. The results of this pilot study suggest that horse owners administer inappropriate doses of equine anthelmintic based on an inaccurate perception of their horse's body-weight, with a statistically significant difference between estimated bodyweight and the dose of anthelmintic administered, however the horse's estimated bodyweight does not appear to be the only influencing factor when deciding on the dose of anthelmintic to be administered.
Further research with an increased sample size, comprehensive owner focused discussion, and additional appropriate statistical analysis is required to more fully investigate this area of enquiry.