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Photographic essay of chronic fluorosis in horses
Part I: first four adult horses
Larry H. Kelly, DVM
Lomita, California
USA
email: hossdox@sbcglobal.net
2
Copyright © 2022 Larry H. Kelly. All rights reserved. All data and images are the property of
Larry H. Kelly, DVM. For reprint permission, please email hossdox@sbcglobal.net.
Introduction
This photo-essay series documents an investigation of chronic fluorine toxicosis (fluorosis) in horses and
other livestock in the region of Twentynine Palms, a desert township in southern San Bernardino County,
California (USA).
The investigation began in 2012 with a report that three Quarter Horse foals on the same ranch, each
about 3 months of age, were eating copious quantities of sand, an abnormal behavior they had exhibited
since birth. Two of the foals died; on postmortem examination, their intestines were filled with sand,
creating a sausage-like appearance. The surviving foal was immediately confined to a stall that had a
concrete floor, thus preventing death from further sand ingestion.
Notably, the ranch had switched their principal forage source from alfalfa to bermudagrass hay
approximately two years earlier. The average calcium content of alfalfa hay is 15 g/kg dry matter (DM),
while the average calcium content of bermudagrass hay is 5 g/kg DM. Ordinarily, grass hays have
adequate calcium for maintenance in adult horses. But given the pathophysiology of fluorosis (see below),
this switch from a high-calcium forage to one with only a moderate calcium content likely was pivotal,
both for the pica displayed by these rapidly growing foals and for the dental and skeletal lesions
documented in this series.
This decade-long project germinated when Dr Kelly noticed a sign for a water defluorination plant (Fig
A) while driving through the town after the deaths of the two ranch colts. As shown in Fig B, Twentynine
Palms is in a region of the US that, geologically, is relatively high in fluorides (≥ 1.5 parts per million) in
the soil and ground water. According to data provided by the Twentynine Palms Water District, well
water in this region has a fluoride concentration, [F-], of 3–22 parts per million (ppm) or milligrams
per liter (mg/L).
For reference, the US Department of Health and Human Services currently recommends a maximum [F-]
of 0.7 mg/L (ppm) in drinking water, and the World Health Organization recommends an upper limit of 1.0
mg/L (ppm). Fluorosis is considered a risk above this threshold.
Well water from the subject ranch was found to have a [F-] of 12–15 ppm. Later hay analysis revealed
that the well water alone contributed approximately 90% of the total daily F- intake in the ranch horses.
The ranch owners generously allowed Dr Kelly to document the dental and skeletal lesions of fluorosis in
their cattle and horses. The ranch cattle are bred and raised for use in team roping and as a meat source.
A
B
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The horses are bred and trained for team roping competition. As the ranch is in a desert region, the
livestock are all kept in fenced dry lots (sandy topsoil) and fed a commercially grown hay as their
principal forage source; well water is available ad libitum.
In 2020, Dr Kelly and colleagues documented two cases of fluorosis in these ranch horses, publishing
their findings in the Journal of Veterinary Diagnostic Investigation. One gelding had lived on the ranch
for 15 years, having arrived as a yearling; the other had lived on the ranch for 10 years, since his arrival as
a 4-year-old. At around 10 years of age, each horse began showing a vague, shifting-limb lameness with
enlargement of both carpal joints. The condition slowly progressed to the point where the horses had
difficulty lying down and getting up. Owing to the chronic and progressive lameness, the horses were
euthanized at 16 and 14 years of age, respectively.
In the JVDI paper, the images illustrating the dental and skeletal lesions in these two horses were limited
to eight (four dental and four skeletal), including two photomicrographs, so this photo-essay series was
borne of the desire to document the range of clinical abnormalities that were found in these and several
other horses with chronic fluorosis. A similar photo-essay of comparable lesions in cattle has also been
prepared.
Lesion Types
The types of dental and skeletal abnormalities associated with chronic fluorosis in horses may best be
appreciated by understanding the basic biochemistry:
• hydroxyapatite, Ca10(PO4)6(OH)2, is the principal form of mineral deposit in teeth and bones
• F- binds to many different cations, including Ca++
• at low concentrations (≤ 10 ppm), F- displaces some of the OH- ions in hydroxyapatite, forming
fluorapatite, a stable but not insoluble molecule
• at high concentrations (≥ 100 ppm), F- binds to the Ca++ ions in hydroxyapatite, displacing them as it
forms calcium fluoride (CaF2); thus, dissolution of apatite predominates
As reported in more detail later, tissue [F-] in the sampled teeth and bones of these four index cases were
all > 100 ppm; they ranged from 290 ppm to 2800 ppm, and were > 700 ppm in all alveolar bone samples.
While we don’t know the molecular form(s) of F- incorporation in these tissues, both abnormal formation
and dissolution of apatite are evident in multiple tissues from all ranch horses and cattle examined to date.
In other species, fluorosis disturbs the normal balance of apatite formation and dissolution required for
normal tissue maintenance, repair, and response (remodeling). In the equine specimens presented in this
series, we see evidence that abnormal dissolution of apatite has structurally weakened the tooth or bone,
stimulating abnormal apatite deposition, particularly at sites of enthesis and other areas of biomechanical
stress, including the periodontal ligaments.
Also, important to appreciate is that horses have hypsodont teeth. The adult or permanent incisors and
‘cheek teeth’ (premolars and molars) continue to erupt throughout life as the tooth is worn away at the
occlusal surface. These teeth have the following characteristics:
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Copyright © 2022 Larry H. Kelly. All rights reserved. All data and images are the property of
Larry H. Kelly, DVM. For reprint permission, please email hossdox@sbcglobal.net.
• long crown, most of which is situated below the gum line in the young to middle-aged horse
→ the subgingival part is the ‘reserve’ crown
→ elongation of the erupted or exposed crown normally occurs with advanced age
→ premature or exaggerated elongation occurs with abnormal loss of alveolar bone and associated
gingival recession
• enamel extends below the gum line, on the reserve crown
→ at the occlusal surface of the newly erupted tooth, the ‘peripheral’ enamel invaginates into a well-
defined central infundibulum (except in the mandibular cheek teeth, which do not have
infundibula)
→ in the incisors, the gradual attrition of the infundibulum and its ‘central’ enamel with wear is
useful in estimating the horse’s age when it is unknown or in doubt
• cementum overlies the peripheral enamel for the entire length of the crown, both exposed and reserve
crown
→ thus, ‘peripheral’ cementum extends above and below the gum line
→ as with the enamel, cementum lines the infundibulum on the occlusal surface
• dentin is exposed at the occlusal surface as the peripheral folds of cementum–enamel are worn away
from the newly erupted tooth
→ thus, dentin fills the space between the peripheral cementum-enamel and the infundibulum
• this arrangement is simple and clear in the incisors but is less so in the cheek teeth, with their multiple
roots and pulp cavities, and various folds and invaginations
• [F-] in enamel does not appreciably increase after a tooth has erupted, but [F-] in dentin and cementum
continues to slowly increase with ongoing F- exposure
Table 1 on the next page summarizes the types of dental lesions that were found in horses following
chronic consumption of well water with a [F-] of 3 ppm or higher.
Of note, the dental abnormalities were bilaterally symmetrical and remarkably consistent among
horses in that age category (foal/deciduous teeth, adult/permanent teeth) who were chronically exposed to
well water with a high [F-].
In addition, there was a distinct pattern to the dental lesions in the rostral part of the mouth (incisor and
canine teeth): the lesions were more extensive distally than mesially.* For example, the canine teeth
(teeth ’04 in the equine Triadan system) and the corner incisors (teeth ’03) were more severely affected
than the central incisors (teeth ‘01). This pattern is the opposite of the order in which the incisor and
canine teeth erupt, which is mesial to distal.
* ‘Distal’ in reference to the tooth or teeth refers to the surface or aspect that, in situ, is furthest from the
midline of the rostral maxilla or mandible, the reference point which bisects the dental arcade into left and
right sides. In contrast, ‘mesial’ refers to the surface or aspect that is closest to the rostral midline.
5
Copyright © 2022 Larry H. Kelly. All rights reserved. All data and images are the property of
Larry H. Kelly, DVM. For reprint permission, please email hossdox@sbcglobal.net.
Table 1. Dental abnormalities associated with chronic fluorosis in horses
Tissue
Gross abnormalities
enamel
general: dark discoloration, denting, pitting, premature attrition
deciduous incisors of foals born to ranch mares: enamel hypoplasia, erosions, mottling, pitting, staining
permanent incisors of foals born to ranch mares: enamel hypoplasia manifested as a deep vertical fold or
dent on the labial surface of the tooth; the enamel appeared abnormally thin in those areas
- in several horses, a full-thickness enamel fold created a vertical fissure on the palatal/lingual surface of
the corner incisors (Triadan teeth ’03) which extended to the infundibulum*
(Note: adult horses introduced to the ranch after 5 years of age did not show enamel hypoplasia, denting,
or pitting.)
cheek teeth of ranch horses: excessive enamel wear and accelerated dental attrition
cementum
general: irregular surface deficits or defects, such as pitting and clefts, cracks, or fissures; in some cases,
the cementum had an abnormally pale, smooth, ‘porcelain-like’ appearance
deciduous incisors of foals born to ranch mares: cemental hypoplasia
permanent teeth of ranch horses: hypercementosis, obvious clinically on the exposed crown but often more
severe on the reserve crown and roots, in some instances resulting in ankylosis**
- a combination of hypercementosis with deep pitting was seen in some cases
dentin
discoloration (e.g., dark-brown staining) on the occlusal surface
gingiva
gingival recession: the gingival margins often appeared thickened and “ropey”; recession caused diastema
formation, with impaction of feed material**
alveolar bone
postmortem/radiographic findings: decrease in bone density, ranging from osteoporosis to frank bone loss
at the alveolar margins**
* This unusual lesion may be pathognomonic for chronic fluorosis in horses.
** Although a clear association has not been proven, the dental lesions of chronic fluorosis in the adult horses bore a striking
resemblance to those of equine odontoclastic tooth resorption and hypercementosis (EOTRH). The possible involvement of
chronic fluorosis in horses with EOTRH is worth investigating, given that both conditions generally manifest in older adult horses
(teens and twenties).
Table 2 on the next page summarizes the types of skeletal lesions that were found in horses following
chronic consumption of well water with a [F-] of 3 ppm or higher.
As with the dental lesions, the skeletal abnormalities were strikingly bilaterally symmetrical. Not only
were both limbs of the pair affected (e.g., left and right third metacarpal bones), but the osseous lesions
were symmetrical around the long axis of the affected bone. Chronic, ‘shifting’ lameness involving
multiple joints and multiple limbs was common.
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Copyright © 2022 Larry H. Kelly. All rights reserved. All data and images are the property of
Larry H. Kelly, DVM. For reprint permission, please email hossdox@sbcglobal.net.
Table 2. Skeletal abnormalities associated with chronic fluorosis in horses
Tissue
Gross abnormalities
bone
exostosis, particularly at sites of fibrous enthesis; the proximal and distal ends of the long bones were
most obviously affected, contributing to joint swelling, but entheses on the vertebrae and occiput were
also affected*
osteoporosis: grossly, the surface of the bone was visibly porous; radiographically, there was substantial
reduction in radiodensity*
joint
gross enlargement of affected joints; the abnormal tissue was firm or fibrous; affected joints were stiff
and painful on manipulation, with reduced range of motion
articular cartilage erosions, notably in areas with subchondral bone lesions*
connective tissue
ossification of soft tissues, * e.g., interosseous ligaments, origin of the suspensory ligament (interosseus
tendon), collateral ligament/joint capsule of the intertarsal joints
horn
abnormal overgrowth of the hoof wall, like that seen with chronic laminitis (“slipper foot”)*
similar overgrowth of other keratinized tissue, such as the ergot and chestnut*
laminitis, with severe rotation of the distal phalanx, leading in some cases to solar penetration
* Involvement of the axial skeleton helps distinguish the bony lesions of chronic fluorosis from those of hypertrophic osteopathy
(HO, Marie’s disease), in which periosteal new bone principally forms on the diaphyses and metaphyses of the long bones of the
limb in horses. Osteoporosis, articular cartilage erosions, ossification of soft tissues, abnormal keratin production, and the
characteristic dental lesions of fluorosis also distinguish these two conditions, as does the fact that, with fluorosis, multiple
animals and various species are affected on the same ranch. Chronic fluorosis may be expected to develop over time in all
animals consuming elevated levels of fluoride in food or water unless the diet is supplemented. (Aluminum, boron, calcium,
magnesium, and sodium may all reduce the absorption/availability, and thus toxicity, of ingested fluoride.)
Initial Case Series
This photo-essay summarizes the findings for the first four adult horses examined postmortem for
evidence of chronic fluorosis. The horses were sampled at a rendering plant, so complete postmortem
examination was not possible. The horse’s age was unknown in three cases, but all were mature (teens or
twenties). While all four horses had been living for years in the Twentynine Palms area, duration of
exposure to high [F-] well water was unknown.
Three standard tissue samples were collected from each horse for measurement of [F-]:
• tooth — mandibular right corner incisor (Triadan tooth 403)
• jaw — alveolar bone supporting the index tooth (403)
• tail — last two caudal (coccygeal) vertebrae
Before analysis, all tissue samples were defatted and dehydrated, so [F-] values are reported as parts per
million (ppm) of dry matter. In horses, a [F-] > 500 ppm in bone is considered abnormal.
All photos in the following pages were taken postmortem.
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Copyright © 2022 Larry H. Kelly. All rights reserved. All data and images are the property of
Larry H. Kelly, DVM. For reprint permission, please email hossdox@sbcglobal.net.
Case 1: Aged gelding (22 years old).
A: Vestibular/labial surfaces of the mandibular incisors and gingiva on the horse’s right side (Triadan
teeth 401–403), showing gingival recession (g), with food packed into the resulting diastema between
teeth 402 and 403. The cementum on the surface of tooth 403 is irregularly thickened, with a long,
ragged, inverted-L shaped cleft (black arrows) extending coronally from the gum line. The red bracket
shows the area (mid-crown) sampled for histopathology.
B: Photomicrograph of an approximately 100-µm thick cross-section of the tooth highlighted in A,
following decalcification and routine staining. The cementum is irregularly thickened, with microscopic
pitting and cracking (red bracket).
C: Magnified and polarized image of the tooth surface bracketed in B. There is a regionally extensive
area of irregular cementum deposition (red bracket) adjacent to, but not involving, a moderately sized
resorption cavity.
Fluoride concentrations in this horse’s tissues were as follows: tooth, 220 ppm; alveolar bone, 710
ppm; caudal vertebrae, 890 ppm. The [F-] in tooth 403 was 31% of that in the adjacent alveolar bone,
and 25% of that in the caudal vertebrae.
[Photomicrographs courtesy of Dr. Brett Webb.]
A
B
C
403
402
401
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Copyright © 2022 Larry H. Kelly. All rights reserved. All data and images are the property of
Larry H. Kelly, DVM. For reprint permission, please email hossdox@sbcglobal.net.
Case 2: Teenage gelding.
Upper Row: Vestibular/labial surfaces of the mandibular incisors and gingiva on the horse’s left side
(Triadan teeth 301–303; the right central mandibular incisor [401] is partially visible in A).
A: All four teeth have discolored, irregular cementum (c). In addition, tooth 302 has a large cemental
defect (^). On all four teeth shown, there is gingival recession, and each tooth is outlined by a distinct rim
of mineralized material (tartar or cemental hyperplasia, red arrows).
B: In this closer and more lateral view, the rim of tartar/cemental hyperplasia is more extensive at the
base of the exposed crown on tooth 303 (red v). The other cemental defects shown in A are more
apparent, and on close-up the peripheral cementum is thickened and porcelain-like. On tooth 303, the
cementum thins and then disappears coronally, exposing the underlying enamel near the occlusal surface.
Lower Row: Occlusal surfaces of the mandibular incisors (teeth 403–303) and canine teeth (404, 304).
The tongue has been retracted. Image D shows a close-up view of the central pair of incisors (401, 301).
The peripheral cementum (c) is porcelain-like, in contrast to the brown-discolored dentin (b) and adjacent
peripheral enamel (*).
Fluoride levels in this horse’s tissues were as follows: tooth, 690 ppm; alveolar bone, 1700 ppm; caudal
vertebrae, 510 ppm. The [F-] in tooth 403 was 41% of that in the adjacent alveolar bone, but 135% of
that in the caudal vertebrae.
A
B
C
^
D
*
c
b
401
301
302
303
302 303
404
403
402
401
301
302
303
304
401
301
^
c
b
*
c c c c
c c
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Case 3: Aged mare (20+ years old).
A: Vestibular/labial surfaces of the mandibular incisors and gingiva on the horse’s right side (Triadan
teeth 401–403; the left central mandibular incisor [301] is partially visible as well). Gingival recession has
allowed food to pack into the resulting diastemata (red ^). On much of the exposed crown, peripheral
cementum (pc) is absent, exposing the underlying peripheral enamel (e).
B, C, D: Occlusal surfaces of the mandibular incisors. The black bar is a reference point for all three
images, as D is an isolated and flipped (rotated 180°) view of the teeth shown in C. Gingival recession has
resulted in diastemata that are packed with feed material (red > <). There is no distinct peripheral
cementum layer at the occlusal surface. Exposed dentin (d) is yellow-brown.
Fluoride levels in this horse’s tissues were as follows: tooth, 290 ppm; alveolar bone, 810 ppm; caudal
vertebrae, 370 ppm. The [F-] in tooth 403 was 36% of that in the adjacent alveolar bone and 78% of that
in the caudal vertebrae.
A
B
C
D
d
pc
pc
e
e
403
402
401
301
402
403 402 401 301 302 303
e
pc
e
403
402
401
V
V
402
403
d
d
10
Copyright © 2022 Larry H. Kelly. All rights reserved. All data and images are the property of
Larry H. Kelly, DVM. For reprint permission, please email hossdox@sbcglobal.net.
Case 4: Aged gelding (20+ years old).
A: Vestibular/labial surfaces of the mandibular incisors and gingiva on the horse’s right side (Triadan
teeth 402 and 403). Gingival recession (g) is characterized by marginal thickening, giving the gingival
margin a “ropey” appearance. The peripheral cementum is thin, absent in places, and irregularly mottled
or pitted (red outline).
B, C: Oblique views of the lingual, lateral/distal, and occlusal surfaces of the teeth shown in A. The black
bar marks the mesial aspect of 402. The peripheral cementum (c) has a porcelain-like appearance.
Gingival recession (g) has resulted in a diastema that is packed with food material (red >). On the occlusal
surface, the exposed dentin is discolored deep brown (db).
D: Occlusal surface of a cheek tooth (premolar or molar) from the same horse. As with the incisors, the
exposed dentin is discolored deep brown (db).
Fluoride levels in this horse’s tissues were as follows: tooth, 1400 ppm; alveolar bone, 2700 ppm;
caudal vertebrae, 2800 ppm. The [F-] in tooth 403 was 52% of that in the adjacent alveolar bone and
50% of that in the caudal vertebrae.
A
B
db
c
db
db
C
D
403
402
402
403
402
403
g
g
c
g
c
c
db
g
db
db
11
Copyright © 2022 Larry H. Kelly. All rights reserved. All data and images are the property of
Larry H. Kelly, DVM. For reprint permission, please email hossdox@sbcglobal.net.
Summary
In summary:
• all four horses had at least one tissue sample that was abnormally high in F- (> 500 ppm)
• in all four horses, the [F-] in the alveolar bone supporting tooth 403 (the index tooth) was > 700 ppm
• in all four horses, the [F-] in the alveolar bone was 2- to 3-fold higher than the [F-] in the tooth itself
• all four horses had gingival recession and diastema formation between 402 and 403
Tooth -mandibular right corner incisor (Triadan tooth 403)
jaw — alveolar bone supporting index tooth 403
tail — last two caudal (coccygeal) vertebrae
1 2 3 4
tooth 220 690 290 1400
jaw 710 1700 810 2700
tail 890 510 370 2800
220
690
290
1400
710
1700
810
2700
890
510
370
2800
TISSUE FLUORIDE (PPM)
TISSUE FL UO RI DE (PPM )
tooth jaw tail
12
Copyright © 2022 Larry H. Kelly. All rights reserved. All data and images are the property of
Larry H. Kelly, DVM. For reprint permission, please email hossdox@sbcglobal.net.
Recommended Reading
The following references, listed in reverse chronological order, have informed the text of this photo-essay
series on chronic fluorosis in horses and cattle.
Kelly LH, Uzal FA, Poppenga RH, et al. Equine dental and skeletal fluorosis induced by well water
consumption. J Vet Diagn Invest 2020; 942–947.
Reddy DR. Metabolism of fluoride. In: Reddy DR, ed. Endemic Skeletal Fluorosis. Saarbrucken:
Lambert Academic Publishing, 2016; 13–30.
Choubisa S.L. Bovine calves as ideal bio-indicators for fluoridated drinking water and endemic
osteo-dentalfluorosis. Environ Monit Assess 2014; 186 (7):4493–4498.
Kowelka M. Clinical and radiographic findings in 60 horses diagnosed with equine odontoclastic
tooth resorption and hypercementosis, in Proceedings of AAEP Focus on Dentistry 2013; Aug 4-6;
Charlotte, North Carolina.
Baratt RM, Clinical management of equine odontoclastic tooth resorption and hypercementosis. in
Proceedings Focus on Dentistry 2011;112–118.
Buzalaf MAR, Whitford GM. Fluoride metabolism. In: Buzalaf MAR, ed. Fluoride and the Oral
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Copyright © 2022 Larry H. Kelly. All rights reserved. All data and images are the property of
Larry H. Kelly, DVM. For reprint permission, please email hossdox@sbcglobal.net.
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