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- MATERIALS AND METHODS
Purring is defined as a soft buzzing sound created by vibrations within the upper respiratory tract which varies rhythmically with breathing (Remmers & Gautier 1972, Frazer-Sissom et al. 1991). The precise behavioural role of purring has not been established although it is generally believed to be a form of communication. Several species of felids vocalise in this way (Stogdale & Delack 1985, Turner & Bateson 2000, Peters 2002). Purring is often cited as a sign of contentment in the domestic cat; on the other hand it has been observed that injured, sick or frightened cats may also purr, so the precise function of this phenomenon remains an enigma. Cats sometimes purr whilst undergoing physical examination at a veterinary practice, but no data have yet been published describing the prevalence of this behaviour. The sound of purring is a distraction when performing thoracic auscultation in this species, and important clinical abnormalities such as heart murmurs and cardiac gallop sounds (both of which are brief, low frequency sounds, often of quite low intensity) can be masked by purring. Purring may also hamper the measurement of heart rate and recognition of dysrhythmias during auscultation. Similarly, purring may prevent the effective auscultation of the respiratory system.
The purpose of this study was to establish the frequency with which cats purr during routine thoracic auscultation in veterinary clinical practice, and to look for possible associations with purring. A further aim was to identify an effective and safe intervention that could be used to halt purring of cats during a clinical examination. Such an intervention may be of use to clinicians in the course of their clinical work.
- Top of page
- MATERIALS AND METHODS
With all the interventions attempted the cat was held in light restraint (i.e. held gently); there was no systematic difference in the way the cats were held or carried associated with the different interventions although there was inevitably some undocumented variation.
Thousands of cats must be examined every day by veterinarians all over the world. If a cat is purring during an examination it becomes a challenge for the clinician to obtain even basic information such as heart rate, or to identify heart murmurs, dysrhythmias, or the presence of more subtle signs such as adventitious respiratory sounds or cardiac gallop sounds. It is surprising that no studies have been published describing the frequency with which cats purr whilst they are being examined. Nor, to the authors’ knowledge, have studies been performed to establish tactics that can be used to interrupt or stop a cat from purring. This study was performed to bridge that gap.
The findings demonstrated that in a single veterinary clinic, 18% of 341 cats presented for clinical examination were purring. Purring in the consulting room occurred in a substantial proportion of cats apparently irrespective of their health, age or sex. A larger study, or analysis of other variables, might have been able to identify some association but a tendency to purr may well just reflect the personality or demeanour of any particular cat.
The study design employed was relatively complex but reflects what might reasonably be expected to occur in a practice setting (i.e. if the first tactic fails to halt purring then try another tactic). This design complicates the statistical analysis as a simple chi-squared or Fisher's exact test cannot be used when tests are repeated on the same cat although these tests could be used to analyse the first attempts. Randomisation should ensure that any cats that spontaneously stopped purring, or cats that had an increased likelihood of stopping, were equally distributed among the three intervention groups. In an ideal world this trial would have used a negative control in order to establish the rate of spontaneous cessation of purring, but this was not one of the hypotheses of this study. It was not feasible to blind this trial but it is reasonable to assume that the investigator had no particular reason to favour a particular intervention. The fact that this was performed by a single veterinary surgeon in a single practice may reduce the external validity of the results, although it also ensures that the influence of operator or environmental variation on the outcomes was less than in a multi-centred study. The study was designed to minimise the bias that might occur due to a particular intervention being used more as a second or third attempt to stop purring. Figure 2 shows that each of the tactics was used first on an equal number of occasions. However, due to the different efficacies of the tactics there is some variation in the number of times each tactic was used as a second or third intervention, and inevitably a more successful intervention will end up being tested more frequently than a less effective one. It is also possible that there were other factors associated with the use of the interventions, that were not recorded, that could have accounted for the differences in their efficacy. For example, if the amount of handling or the type of handling was consistently different for each intervention this may be the explanation for the difference between interventions. Similarly the choice of the aerosol used may be important. The hypothesis was based on the possible aversion to the use of any ethanol-based aerosol but it cannot be discounted that the active ingredient (a pheromone in this case) may have been responsible for the effect observed.
The overall success rate in stopping purring in cats using one or more of the tactics under test was 93% (28 of 30). The most successful tactic was to hold the cat fairly close to a running tap which was successful in 17 of 21 of cats (81%) compared to use of the aerosol in 9 of 18 (50%) or blowing in 2 of 14 (14%). The statistical analysis of the first interventions attempted showed that the differences between the success rates were significant but did not show that the use of the tap was statistically significantly different from the use of the aerosol. There is no clear evidence that the use of other tactics previous to a successful intervention had any effect on the outcome (Fig 3) as the success rates broken down by attempt number appear similar for each tactic for each of the attempt sequence numbers and in a binary logistic model there was no statistically significant effect associated with attempt number. This analysis also enabled an investigation for the statistical significance of the individual interventions and showed that proximity to a running tap was significantly different to blowing at the ear (P = 0 · 001) but not significantly different to use of an aerosol (P = 0 · 057, a possibility of this result occurring by chance of 1:18). The inclusion of more cats in the trial would have resolved this uncertainty. It is unclear why a running tap works, and it has also not been established whether this has any negative consequences. It is possible that cats find the sound and sense of a running tap distracting, perhaps because of their acute sense of hearing or possibly because of a dislike of running water observed in most breeds and individuals. There does not appear to be a good explanation as to why the sound of a running tap should be more effective than blowing at the ear, or proximity to an ethanol-based aerosol spray. If it is just the sound of running water then a sound recording played to the cat might be equally effective as the real thing. This supposition would need to be tested in a further study, but if it were effective this could perhaps be incorporated into a “smart phone” application for veterinarians.
Heart disease is common in cats and it can be a diagnosis that is difficult to make with certainty (Stepien 1998). In contrast to dogs, many healthy cats have heart murmurs on auscultation (Cote et al. 2004, Wagner et al. 2010). Furthermore, cats with heart disease or even heart failure may have no discernible heart murmurs (Ferasin et al. 2009, Paige 2009, Smith & Dukes-McEwan 2012). In the face of this some clinicians might be tempted to ignore auscultation when a cat is presented for examination if it is purring. This would, however, be unwise as auscultation should enable the examiner to assess and establish clinical parameters such as heart rate and to identify amongst others the presence or absence of dysrhythmias, gallop sounds and the respiratory sounds. The study presented here shows that running a tap nearby, or failing that, spraying an ethanol-based aerosol onto the examination table near to the animal will induce most cats to stop purring for a sufficient time period to enable the clinician to perform a satisfactory auscultation on that animal and that this should be successful in 9 of 10 cats.
Conflict of interest
None of the authors of this article has a financial or personal relationship with other people or organisations that could inappropriately influence or bias the content of the paper.