Prevalence of dental diseases is high in aging horses, including disorders of the surrounding tissue like gingivitis,1 tooth loss,2 malocclusion,3 and canine calculus.1,4 To maintain the physical condition of elderly horses, regular dental care is recommended.5 Management of keeping and feeding can impact aging processes of the oral cavity, such as dental wear, which depends on feeding behavior and the type of pasture.6 The influence of feeding (eg, rigidity of roughage, concentrated food) and keeping management (eg, continuity of mastication, posture during feeding) on dental health has yet to be the focus of many studies. Dental abnormalities have been shown to be less frequent in free-grazing Thoroughbred horses than in stabled ones.7 The effect on the aging processes of the rostral oral cavity has not yet been the subject of studies.
Genetic predisposition might also affect the progression of oral disorders in aging horses. Breed-dependent genetic influences can be excluded by focusing only on 1 breed. In Europe, the Icelandic horse seems to be prone to dental disorders like Equine Odontoclastic Tooth Resorption and Hypercementosis (EOTRH).8 Icelandic horses have been purebred in Iceland for over 1,000 years9 and are considered to be well adapted to the Icelandic living conditions in all respects. Approximately two-thirds of Icelandic horses are at present kept abroad.9 As a native breed placed in natural surroundings, the population in Iceland appears to be a reasonable study population to study the effects of living conditions on dental status. As far as the authors are aware, the status of the rostral oral cavity in Icelandic horses in their natural environment has not been the subject of any study yet.
This study aimed to put the results of dental health in Icelandic horses into context with feeding and living conditions as well as preventive dental care. Looking at only 1 breed will minimize genetic influences.
Methods
Malvaelastofnun (MAST), Selfoss, Iceland approved the study design (research No. 2021-08-05) by animal welfare policy.
Study location
The study was conducted in August and September 2021 in Iceland. About half of the examined horses were located in Northwest Iceland (region Norđurland vestra). Further examinations were performed in Búđardalur (region Vesturland) in West Iceland and the South of Iceland (region Suđurland) to include the dominant horse areas in Iceland. Horses on 21 farms were involved in the study.
Study population
The study population included 170 horses aged 15 years and older. The sample size was calculated based on the 13.1% prevalence of EOTRH found in Icelandic horses in Belgium8 and a probability (width) of ≥ 80% (achieved probability [width] = 91.9%) for an estimated CI half-width of 6%. These specifications required a sample size of at least 150 animals. Farms were selected based on their location in Northwest, West, and South Iceland. All horses in good overall health on any 1 farm were included in the study.
Data collection and clinical examination
The owner of each horse answered a questionnaire originally compiled in Icelandic about the horse and its living and keeping conditions (see Supplementary Material S1 for the English version).
The identity of each horse was verified by a microchip number. In Iceland, the microchip number of each horse is registered in the online database WorldFengur,10 which functions as the studbook of origin of Icelandic horses. The age of the horses was verified by birth date records in WorldFengur. The horses were checked in a general clinical examination, specifically looking at maintenance condition, posture, and behavior to prove medical suitability for sedation, which was needed in part II of the current study (Hain et al, DVM, DEVDC, University of Veterinary Medicine Hannover, Hannover Germany, unpublished data, 2023). Weight was defined according to Hennecke et al11 (overweight: body condition score [BCS] ≥ 7 and underweight BCS ≤ 3 in a 9-point BCS). The score from Tretow et al12 was used to assess the rostral oral cavity. This scoring system was used to show the relevance of several examined parameters in relation to EOTRH. These parameters, including fistula, gingiva retraction, dental calculus, gingivitis, tooth mobility, and bite angle, were considered. Documentation was supported by photographs. A scoring system was created for incisor and canine calculus (Table 1), and the Triadan system13 was used for numbering teeth. The incisal plane was evaluated by photographs. The physiological plane was present if the occlusal plane was level or slightly ventrally curved when viewed from the front. Any deviation was classified as “abnormal.” Deviation of the level plane was classified, observed in a front view of the incisors, and described from the position of the observer from left to right into diagonal bite (steady and even ascent or descent of the incisal plane), smile mouth (rounded incisal plane; maxillary incisor teeth are shorter laterally than mesially), wave mouth (uneven incisal plane, ascending and descending irregularly), step mouth (tiered ascending or descending incisal plane), and frown mouth (mesial maxillary incisors come below the normal incisal plane).
Classification of dental calculus (extent of calculus), gingival recession, and gingivitis according to Tretow et al.12
| Severity | Canine and incisor calculus | Gingival recession | Gingivitis |
|---|---|---|---|
| Mild | < 1/3 of the dental crown | ≤ 1/3 of the root | Focal |
| Moderate | At least ≥ 1/3 and < 2/3 of 1 tooth affected | ≤ 2/3 of the root | Areal |
| Severe | At least ≥ 2/3 of 1 tooth affected | Entire root exposed | Areal to bluish |
| Additionally, for canine teeth, calculus extended sideways > 1/2 of the tooth length |
Statistical analysis
The effects of feeding and keeping parameters were evaluated by using cross-tabulation with the chi-square test and Monte Carlo exact test. The appearance of incisor and canine calculus was considered in relation to each other. Feeding of minerals and concentrated food was put into context with dental calculus, gingivitis, and gingival recession. The duration of hay feeding and grazing time was allocated to 4 groups of 3 months each (0 to 3, 4 to 6, 7 to 9, and 10 to 12 months) and put into context with dental calculus, gingival recession, and gingivitis. The effects of localization on dental calculus, gingival recession, and gingivitis were considered individually and within the context of the summation of the scores. The impact of sex was estimated within the context of incisor calculus. Age dependence of canine calculus and gingival recession (dichotomous variable) was assessed using logistic regression. Significance was set at P < 05. Statistical software (SAS, version 7.1; SAS Institute Inc) was used to perform all analyses.
Results
Study population
The study group consisted of 170 horses (76 mares, 92 geldings, and 2 stallions). The average age was 19.0 years (15 to 30 years). A total of 61.7% of the horses were between 15 and 19 years, 31.7% were between 20 and 24, and 6.6% of the horses were 25 years and older.
A total of 83.5% (142/170) of the animals were riding horses, which mainly consisted of horses used for long-riding tours and sheep round-ups as well as horses used for rental purposes. A total of 10.6% (18/170) of the owners categorized their horses as being used for other purposes. These were primarily breeding mares. A total of 5.9% (10/170) of the horses were used for competition.
Clinical examination
All the horses were in good general condition. The average BCS was 5.3. A total of 91.2% of the horses had normal weight. The distribution of BCS was as follows: BCS 3, 1.2%; BCS 4, 8.2%; BCS 5, 60.6%; BCS 6, 22.4%; BCS 7, 7.1%; and BCS 8, 0.6%. A total of 1.2% of the horses were underweight (n = 2), and 7.6% of the horses were overweight (13).
Keeping, feeding, and dental care
The study group spent an average of 11 months a year on pasture and 1 month stabled. Two/170 horses (1.2%) spent 9 to 12 months stabled. Both were competition horses. A total of 135 (77.6%) spent 10 to 12 months of the year outside. Almost half of them (47.5%) additionally received wrapped hay for 5 to 6 months. Concentrated food was fed to 5.3% of the horses at certain intervals when stabled. A total of 71.2% of the horses usually had free access to salt stones for at least half of the year. A total of 8.2% received both minerals and had free access to salt stones, while 2.4% only received mineral supplements and no additional salt. The feeding practice was strongly dependent on the farm rather than on the usage of the horse. The feeding and keeping management were similar in Northwest, West, and South Iceland.
A total of 30.6% of the horses (52/170) had received regular dental treatment in the past. A total of 92.3% (48/52) of them were checked once a year. In 2 cases, “regular” meant 1 treatment every second year. Two owners did not make any specifications. A total of 28.2% (48/170) of the horses did not receive any dental treatment, including 1 of the competition horses. Six of the 10 competition horses received regular dental treatment. Altogether, breeding mares had received no treatment or only 1 treatment in the past. Forty-three of the 151 horses (28.5%) used for riding had not received any dental treatment. Again, both the frequency of the dental treatment and the qualification of the person administering the treatment tended to depend on the owner than on the use of the horse since horses of 1 owner usually received the same dental treatment regardless of their use.
Dental status
A total of 110/170 of the examined horses showed no abnormality in the incisors and canines (64.7%). Two horses had 1 absent incisor each. Two horses showed an interdental space between 301 and 401, 1 of them including a wave mouth. One horse (aged 30 years) had severe attrition of the maxillary incisors. A total of 70.6% (120/170) had an even incisal plane. A total of 18.2% (n = 31) of the horses had a smile mouth, 5.9% (10) a wave mouth, 3.5% (6) a step mouth, 1.2% (2) a diagonal bite, and 0.6% (1) a frown mouth. The bite angle of incisors seemed to decrease slightly as a result of aging but had not been measured reproducibly. No bulbous distension nor tooth mobility was found. The results were independent of the localization of the farm in Iceland (P > .05).
Gingiva
A total of 38.1% of the horses had no gingival recession, 50.0% showed mild, and 11.3% had moderate gingival recession. Only 1 horse (0.6%) had severe gingival recession. The degree of severity was related to the feeding of hay (P = .047) but did not increase with age (P > .05). A total of 4.7% of the horses suffered from mild gingivitis, and 0.6% showed moderate gingivitis. No relation with feeding practice was found, and fistulae were not present.
Dental calculus
Both incisor and canine calculus occurred at a mild and moderate severity level (Figure 1). Severe amounts were only seen in canines. Male horses tended to be more frequently affected, but the finding was insignificant (P > .05). A total of 128/170 horses could be identified as having canine teeth, including even 50.7% (38/75) of the mares. A total of 23.7% (9/38) of these had mandibular and maxillary canines. Of the 128 horses with canine teeth, 74.2% (96/128) were affected by calculus on at least 1 canine. A total of 52.3% (67/128) of them showed calculus only in mandibular canines. Calculus in maxillary canines was always accompanied by mandibular calculus. A total of 14.8% (19/128) of the horses showed neither calculus in incisor teeth nor in canine teeth. The likelihood of horses developing incisor calculus increased significantly with age (P = .002), whereas there was no age dependence in canine calculus (P > .05). Canine calculus was not correlated with feeding concentrated food and minerals (P > .05).
Percentage prevalence of dental calculus in incisors (n = 170) and canines (128).
Citation: Journal of the American Veterinary Medical Association 261, S2; 10.2460/javma.23.06.0303
Discussion
This study is the first examination of the status of the rostral oral cavity in elderly horses of a native breed living in their original environment in the context of keeping, feeding, and dental care.
Relatively few horses (30.6%) received regular dental treatment compared to Swedish observations (60%).14 Dental care is considered an essential tool for maintaining the health of geriatric horses in general5 and is recommended by Icelandic veterinarians as well.15 Health deficiency might be expected in the study population. However, the Icelandic horses in fact showed an excellent dental status concerning incisors, canines, and surrounding soft tissues. The quality of dental treatment was not in any way considered in the questionnaire. Both technique (manual or electrically driven rasping) and experience of persons performing treatment play a decisive role in the outcome as does the decision to perform incisor treatment (leveling).
Most of the horses were in reasonable body health condition, although the horses were elderly and body condition is expected to decrease with age.2 Even though the health condition was a limiting selection criterion, only 1 presented horse was excluded for age reasons at the request of the owner. Horses with dental disease are prone to lose weight due to chewing difficulties. Underweight animals or poor conditions caused by dental disorders might have urged owners to euthanize some of the aged horses in the past, so these horses were not even presented for examination. This could have caused an inadvertent selection of the study population and could have caused a relevant bias in the results.
Another unintentional selection might have biased the results. Since horse owners in Iceland often own a great number of horses, most of them keep their horses abroad in herds and only fence them or stable them if required.16 Especially during the summer, even saddle horses are normally kept in the field when currently not being ridden. Most of the examined horses originated from these herds and were only fenced or stabled for examination purposes. Due to those circumstances, an unintentional selection toward free-grazing horses was made. Since feeding and keeping conditions are very likely to affect dental health, this certainly had an impact on the results. The study might not reflect the horse population in Iceland in its entirety, but it indeed gives an overview of the majority of them. Intensively managed horses like competition horses might show differences in dental health. All in all, horses in Iceland showed very good dental health and that of surrounding tissue, which might be a result of natural grazing behavior.
Large areas of Icelandic grassland are not available for farming due to the rough terrain and are therefore used for extensive grazing of livestock.17 Cultivated land is rare and mostly used for haymaking and grazing of dairy cattle. Extensive grazing means high seasonal variations in flora. Flowering and lignification have an impact on the ingredients and texture of the plants.18 Horses in Iceland select forage plants due to availability during seasonal progression.19 Diversity is found to be high in Icelandic pastures, including plants of the Cyperaceae, Asteraceae, and Equisetaceae families. In contrast, cultivated grass strains such as Lolium perenne (Poaceae) are usually cultured regarding energy content, digestibility (ie, low fiber content), and growing qualities20 and are often grown as a result of monoculture farming. Research about the ingredients in Icelandic grass strains is lacking, but those extensively grown plants are expected to contain a great deal of crude fiber and less carbohydrates along with increased rigidity.
Salivation plays a significant role in dental health.21 Naturally, free-grazing horses spend about 60% of the time grazing, slowly moving along with their head down.22 Since the salivary flow in horses strictly depends on mastication itself,23 saliva might constantly flow in free-grazing horses. Steady salivary flow is determinative for dental health in facilitating the steadiness of the chemical milieu (buffering effect of bicarbonate) in the horse's mouth and inducing a constant rinsing effect and secretion of lysozyme and lactoferrin with direct antibacterial function.21 In addition, microflora changes depend on the chemical milieu.21 It is known that even IgA and IgG underlie considerable intraindividual variation24 and could also depend on feeding practice. According to that, a lack of saliva is known to promote several diseases like gingivitis, caries, and calculus in humans, affecting the microflora.25 A constant flow of saliva with rinsing, buffering, and anti-inflammatory effects might be crucial for the health of the surrounding tissue of the tooth. Free-grazing Thoroughbreds aged 5 to 15 are known to show fewer dental abnormalities than stabled ones,7 which can be confirmed regarding the health of surrounding tissue. Conducting investigations of saliva flow and the microbiological milieu of the oral cavity in free-grazing and stabled horses would be an interesting field of further research. The current study revealed a prevalence of gingivitis in the rostral mouth of 5.3% of the horses, which is consistent with earlier findings (6%).1 Memedi1 reported an increased prevalence of 20% in horses aged 21 and older. Prevalence did not increase with age in the current study. Three out of the 9 horses with gingivitis lived on the same farm in South Iceland. Certainly, the number of these horses is too small to provide sufficient evidence, but the pasture on this farm was seeded and manured and nevertheless was used for grazing horses.
Memedi1 as well as Brigham and Duncanson26 found low to moderate prevalence of canine calculus (53% and 28%, respectively) in male horses. Memedi1 described differences in disposition with a higher prevalence in ponies (55%) than in warmbloods (33%), Thoroughbreds (30%), and trotters (12%). Pony breeds seem to score high on canine calculus, analogous to the current study where it was predominantly present (74.2%), and canine calculus has been previously found, especially in mandibular teeth.4
In the current study, remarkably, many mares were identified with canines (50%) compared to other observations (8.8%1 and 22%27). Therefore, a genetic predisposition in native breeds might be present.
The prevalence of incisor calculus in horses seems relatively high as previously stated; 29%22 in the United Kingdom as well as 33.0%27 and 44.8%28 in horses in Chile. Those studies22,27,28 neither narrowed down the age of the horses nor did they measure the amount of calculus. Even if almost 50% of horses in Iceland were affected in the current study, most showed remarkably little calculus compared to routine clinical findings in Germany (authors' observation). Incisor calculus did not correlate with the feeding of concentrated food. This observation is not definite because of the lack of horses fed with concentrates.
This study revealed a significant difference between the degree of calculus in canine and incisor teeth. The incisor and canine prevalence did not correlate (P > .05). Dental calculus is composed of food residuals and calcium,4 which is an ingredient of a horse's saliva23 and might, as a result, promote calculus formation. Mandibular canines are more frequently affected by dental calculus and located close to the ducts of sublingual and mandibular glands.29 Following gravity, saliva mixed with food and bacteria passes the mandibular canines more than the maxillary ones. As seen in free-grazing horses, increased saliva flow and a lowered head position might predispose the animals to canine calculus. Tooth contact and attrition while chewing clean teeth,4 which is impossible in canine teeth, which have no mutual contact. The occlusal rinsing effect is believed to be more effective the more tooth occlusion occurs. Thus, natural grazing behavior with continual food intake and constant mastication promotes this rinsing effect. The harshness of forage influences chewing intensity, saliva flow,30 and abrasion31 of dental substances and might even free the incisors from appositional calculus. Most likely, other ingredients in Icelandic pasture plants, like phytoliths,31 play a determining role in this process. Indeed, it is questionable if flora in Iceland can be mimicked elsewhere.
The bite angle between maxillary and mandibular incisors usually becomes more acute with age.5 If angulation does develop unevenly in maxillary and mandibular incisors, normal prehension can be inhibited.5 In the present study, the bite angle of incisors seemed to change little with aging. This was also seen in Przewalski's horses kept in a zoo.32 There is evidence of breed-dependent dental wear.33 Przewalski's horses showed slower dental wear compared to warmblood horses where breed-dependent resistance of the incisors to dental wear and adaptation to living conditions in Mongolia were assumed.32 It might be that native breeds in general show genetically higher wear resistance than other breeds, but this has not been the focus of any study yet. The time of tooth occlusion, which increases with larger food intake and prolonged chewing time, is assumed to be a relevant factor for dental wear in humans.34 Attrition in natural living conditions in balance with well-adapted tooth resistance dental wear might have affected the bite angle of the horses in Iceland.
An abnormal plane of the incisors is often caused by cheek teeth diseases,35 which might influence the range of motion and balance of the jaw as well as the proportion of dental attrition. A total of 24% of the horses (12/50) with irregular incisal planes received regular dental care. A total of 70.3% of the horses (90/128) did not receive regular dental care and did not show irregularities of the incisal plane either. Chewing intensity with a higher range of jaw motion and high abrasion might also affect the cheek teeth and play a role in the evenness of incisors. Cheek teeth were not examined in this study. The horses that showed abnormal incisal planes would certainly benefit from enhanced dental care. Examination of cheek teeth would lead to more comprehensive information about the dental health of free-grazing horses in Iceland.
Natural living conditions including continuity, intensity of chewing, and nativeness of forage might support natural attrition and a balanced milieu in the oral cavity, promoting the health of incisors and tissue surrounding the tooth even in elderly horses. Dental findings are influenced individually. Despite natural living conditions, recommendations for routine dental care still depend on individual factors.
Supplementary Materials
Supplementary materials are posted online at the journal website: avmajournals.avma.org
Acknowledgments
We thank Dýralaeknaþjónusta Suðurlands, Dýralæknar Sandhólaferju-Rauðalæk, Dr. Hróbjartur Darri Karlsson, Ingunn Reynisdóttir, Gisli S. Halldórsson, Dr. Susanne Braun, Dr. Johanna Becker, Franziska Mahler, and Hólar University, Iceland, for providing materials and local cooperation.
Disclosures
The authors have nothing to disclose. No AI-assisted technologies were used in the generation of this manuscript.
Funding
The authors have nothing to disclose.
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