The feral horse foot. Part B: radiographic, gross visual and

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The feral horse foot. Part B: radiographic, gross visual and
histopathological parameters of foot health in 100 Australian
feral horses
BA Hampson,* MA de Laat, PC Mills, DM Walsh and CC Pollitt
Background It has been proposed that the feral horse foot is a
benchmark model for foot health in horses. However, the foot
health of feral horses has not been formally investigated.
Objectives To investigate the foot health of Australian feral
horses and determine if foot health is affected by environmental
factors, such as substrate properties and distance travelled.
Methods Twenty adult feral horses from five populations
(n = 100) were investigated. Populations were selected on the basis
of substrate hardness and the amount of travel typical for the population. Feet were radiographed and photographed, and digital
images were surveyed by two experienced assessors blinded to
each other’s assessment and to the population origin. Lamellar
samples from 15 feet from three populations were investigated
histologically for evidence of laminitis.
Results There was a total of 377 gross foot abnormalities identified in 100 left forefeet. There were no abnormalities detected
in three of the feet surveyed. Each population had a comparable
prevalence of foot abnormalities, although the type and severity of
abnormality varied among populations. Of the three populations
surveyed by histopathology, the prevalence of chronic laminitis
ranged between 40% and 93%.
Conclusions Foot health appeared to be affected by the environment inhabited by the horses. The observed chronic laminitis may
be attributable to either nutritional or traumatic causes. Given the
overwhelming evidence of suboptimal foot health, it may not be
appropriate for the feral horse foot to be the benchmark model for
equine foot health.
poor foot conformation and a high prevalence of foot abnormalities in
the Kaimanawa feral horses of New Zealand.4 Additionally, in the first
extensive study of the morphology of feral horse feet, the same authors
reported aspects of foot morphology that were of concern and warranted further investigation.5 The foot type from some feral horse
populations displayed a very large dorsal hoof wall–distal phalanx
distance (HWDPD) and an unusually large coronary band–extensor
process distance, indicating possible pathology of the distal phalanx
suspensory apparatus. The investigation of the foot health of the same
subjects in the current study aimed to determine whether the unusual
morphology was a beneficial adaptive response to biomechanical
stress or a pathological consequence of excessive interaction with the
environment.
Although genetics may play a part in the differences observed between
populations of horses, a genetic survey revealed no significant difference in the breed origins of Australian feral horse populations that
were geographically separated by up to 2000 km.6 Likewise, a previous
survey of the nutrition of the feral horses in the current study failed
to find significant differences in nutritional parameters that were
likely to be responsible for the morphological differences in the feet
observed among the populations.7
The aims of the current study were to determine the prevalence and
range of feral horse foot abnormalities across a variety of habitats and
to investigate whether environment and lifestyle affected foot health.
Materials and methods
T
Subjects
Twenty mature age feral horses from each of five different environments were obtained at necropsy (n = 100) following standard, controlled feral horse culling operations unrelated to the research. The
study group had a mix of the sexes (58 males, 42 females) and all
subjects were at least 4 years of age, as assessed by dentition. The
results of the detailed morphometric analysis of the same 100 feet,
together with a detailed description of the horses and their habitats,
are described elsewhere.5 DNA analysis of horse hair samples from
four of the five populations in the current study confirmed that 93% of
the genetic material was common to each population and there was no
measurable difference in breed origin among the five populations.6
*Corresponding author.
Australian Brumby Research Unit, School of Veterinary Science, The University of
Queensland, St Lucia, Queensland 4072, Australia; [email protected]
Horse populations
The feral horse populations were selected on the basis of the substrate
type, landscape topography and availability of food sources in relation
to water sources in each habitat (Table 1). Substrate referred to the
type of ground surface (footing) over which the horses were travelling.
Keywords
brumbies; feet; laminitis; pathology
Abbreviations GPS, global positioning systems; HWDPD, hoof
wall–distal phalanx distance; PEL, primary epidermal lamellae;
SEL, secondary epidermal lamellae
Aust Vet J 2013;91:23–30
doi: 10.1111/avj.12017
he wild horse foot has been proposed as the ideal model
for equine foot health, despite a lack of detailed empirical
investigation.1–3 It has been suggested that the free-roaming
lifestyle of the wild horse promotes ideal foot health because of longdistance travel over variable terrain, a varied natural diet and the
absence of the perceived harmful effects of domestication, including
some traditional farriery practices. However, Hampson et al. reported
© 2013 The Authors
Australian Veterinary Journal © 2013 Australian Veterinary Association
Australian Veterinary Journal Volume 91, No 1–2, January/February 2013
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Table 1. Substrate type and travel pattern of feral horse populations
(n = 20) in five different locations in Australia
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Population
Babbiloora
Kings
Mussellbrook
Cliffdale
Palparara
Substratea
Mixed
Hard
Hard
Sandy
Sandy
Travelb
Medium
High
Medium
Medium
High
a
Substrate refers to the type of ground surface on which the horses
travelled. Sand was considered soft and hard substrate consisted
primarily of rock. Mixed substrate refers to a combination of soft- and
hard-substrate types.
b
By identifying food and available water sources in each habitat, and
combining this information with GPS data derived from feral horse
tracking,8 the general travel patterns of the horses in each habitat could
be predicted and was defined as low (0–5 km/day), medium (5–10 km/
day) or high (>10 km/day).
Sand was considered a soft substrate and a hard substrate consisted
primarily of rock. A mixed substrate was a combination of soft- and
hard-substrate types. The travel pattern of feral horses was determined
by the spatial relationship between food and water in the habitat.8,9 By
identifying available food and water sources in each habitat, and combining this information with global positioning systems (GPS) data
derived from feral horse tracking,9 we estimated the general travel
patterns of the horses in each habitat (Table 1). Travel distance
was defined as low (0–5 km/day), medium (5–10 km/day) or high
(>10 km/day).
Foot collection
The left forelimb of each horse was used for the radiographic and
photographic assessments. Limbs were disarticulated at the radiocarpal joint, immediately packed in ice and frozen within 8 h of collection. Limbs were thawed to room temperature on the day of the
laboratory study, within 2 weeks of collection.
Radiographic assessment
Feet were thoroughly cleaned in preparation for radiography.
A thumbtack was inserted at the frog tip–sole junction for landmark
identification. The hairline was clipped to expose the coronet.
To identify the coronary band on the radiograph, a 2-mm leadimpregnated rubber strip was placed circumferentially around the
distal hairline–proximal hoof wall junction and taped in place on
the lateral coronet. Another 2-mm strip was taped in place from the
most distal hairline on the coronet, down the midline dead centre
of the hoof wall and passing under the distal wall and sole, ending
at the frog tip–sole junction, to highlight the hoof wall and sole
profile.
Each limb was loaded in a custom-built hydraulic loading device
designed to apply a nominated downward vertical force of
1300 newtons (force exerted by 30% body weight in a 400-kg horse) to
simulate a standing radiograph. The starting position for loading was
with the third metacarpal bone positioned vertically and the loading
arm positioned over the centre of the sagittal centre of the foot. The
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Australian Veterinary Journal Volume 91, No 1–2, January/February 2013
load was applied through the proximal carpus via a cup attached to the
distal end of the hydraulic ram. To calibrate measurements, a 100-mm
long stainless steel rod was placed on the ground surface in alignment
with the sagittal axis of the foot. Lateromedial and dorsopalmar radiographs were obtained using a portable X-ray machine at a film-focal
distance of 700 mm. Radiographs were electronically scanned to allow
digital measurement and image enhancement using digital image analysing software (ImageJ v1.38, National Institutes of Health, Bethesda,
MD, USA).
Photographic assessment
Following the radiographic assessment, feet were photographed while
positioned on a custom-built photographic jig. A Nikon D100 Digital
camera fitted with a 55-mm Micro Nikkor lens and Nikon SB-800DX
Speedlight flash was screw-mounted to the proximal end of the jig,
700 mm from the foot. The camera was aligned to the centre of each
foot and a 100-mm ruler with a hoof label was placed in the measurement plane to provide image calibration and foot identification. The
feet were photographed from the dorsal, medial, lateral, solar and
caudal views. Feet were then sectioned in the sagittal plane with a band
saw and the medial portion was photographed.
Foot health assessment
The health of each foot was assessed by two operators experienced
in foot health assessment (D.W. and B.H.). Digitised radiographs
and digital photographs of the feet were analysed for the presence
of visible abnormalities. Radiographs and photographs were coded
so that the assessors were blinded to the population origin of each
foot. The assessors were also blinded to each other’s assessment. Feet
were assessed in random order by drawing the horse number from a
container and five feet were assessed twice to determine internal
validity (coefficient of variation <5%). A customised assessment
protocol containing 55 points of assessment was used. Only the
abnormalities detected by both assessors were recorded in the data
set. Each assessor was told to assess the feet for abnormalities. An
abnormality was defined as a variation from normal that, in the
assessor’s experience, may affect the function of the hoof. Following
completion of the foot health survey, the two assessors ranked the
list of abnormalities identified as more or less significant based on
the assessors’ experience of the effect of hoof abnormalities on hoof
function.
Histology
Lamellar samples were collected for histological analysis from the
Kings, Palparara and Babbiloora populations. Logistical problems
prevented the Mussellbrook and Cliffdale populations from being
assessed histologically. Fifteen horses were selected at random from
each population and the right forefoot was removed and processed
for histology within 60 min of death. A 30-mm full thickness sagittal
sample of the dorsal hoof wall that included lamellae and sublamellar
dermis was taken 3 mm proximal to the distal tip of the distal phalanx
and placed immediately in 10% neutral buffered formalin. Following fixation, a 5-mm3 tissue block was cut from the centre of the
30-mm sample and embedded in paraffin wax. Blocks were sectioned
transversely at 5 mm, mounted on Superfrost Plus slides (Menzel,
Braunschweig, Germany) and stained with haematoxylin and eosin.
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Ethical considerations
The project was approved by the University of Queensland Animal
Ethics Committee (AEC-PCA), monitoring compliance with the
Animal Welfare Act (2001) and The Code of Practice for the care and
use of animals for scientific purposes.
Results
Foot health assessment
There were 377 abnormalities identified in the left forefeet (n = 100).
Each of the five feral horse populations (n = 20) had a similar prevalence of abnormal feet in the overall survey, ranging from 16% to 22%.
The reported abnormalities were separated into those considered to
have a minimal effect and those with a more significant effect on foot
function (Table 2). A clear trend in different foot health of horses
emerged in the comparison between the hard (Mussellbrook) and soft
(Cliffdale) substrate habitats. Although the Mussellbrook horses had
the lowest prevalence of less significant abnormalities, they also had
the highest prevalence of more significant abnormalities. The situation was the reverse for the Cliffdale horses, with the highest prevalence of less significant abnormalities and the lowest prevalence of
more significant abnormalities. Three feet from horses from the Kings
population had no abnormalities detected.
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One author (C.C.P.), experienced in describing lamellar histopathology, used a light microscope (Olympus BX-50, Olympus Corporation
Lifesciences, Tokyo, Japan) to rank each section for laminitis (nil,
acute, mild chronic, moderate chronic, severe chronic).
Feet from soft substrate generally had some hoof wall flaring
(Figure 1A), but the internal anatomy appeared normal (Figure 1B).
Feet from hard-substrate environments appeared normal externally
(Figure 2A), but showed internal derangement, including excessive
HWDPD and increased coronary–distal phalanx distance (Figure 2B,
C) and occasional bony abnormalities (Figure 2C).
Histology
Histological examination showed that the prevalence of chronic laminitis was 67%, 40% and 93% for the Kings, Palparara and Babbiloora
populations, respectively (Table 3). No case of laminitis was described
as acute (acute basement membrane separation, leukocyte influx) and
Table 2. List of abnormalitiesa observed in a foot health survey comprising five photographic and two radiographic views of 100 left forefeet from five
feral horse populations in Australia
Less pathogenic abnormalities
Capsule deviation
Medio/lateral imbalance
Hoof wall cracks
Hoof wall rings
Medio/lateral wall flare
Frog and bars
Narrow heels
Underrun heels
Uneven heel length
White line defect
Long dorsal wall
Dorsal wall flare
Total
% contribution
More pathogenic abnormalities
DP/hoof wall alignment
DP bony abnormality
Excessive HWDPD
Ungual cartilage calcification
Total
% contribution
Mussellbrook
(n = 20)
Cliffdale
(n = 20)
Kings
(n = 20)
Palparara
(n = 20)
Babbiloora
(n = 20)
Total
(n = 100)
1
5
2
2
4
5
7
2
1
1
1
0
31
11
2
14
2
12
14
16
0
4
0
0
4
6
74
26
2
12
11
13
7
11
0
1
0
1
0
1
59
21
1
6
7
18
20
4
0
3
1
3
0
1
64
23
0
11
3
14
12
1
1
2
0
4
0
5
53
19
6
48
25
59
57
37
8
12
2
9
5
13
281
100
0
4
14
14
32
33
0
1
2
1
4
4
1
3
13
3
20
21
0
6
10
1
17
18
4
10
8
1
23
24
5
24
47
20
96
100
a
For the purpose of the study, an abnormality was defined as a variation from what the assessors considered as optimal and was considered to
negatively affect the structure and/or function of the foot. The prevalence of abnormalities is shown for each population (n = 20), as well as the total
population (n = 100). Each population shared a comparable prevalence of abnormality in the overall survey; however, the prevalence of less and
more significant abnormalities, in terms of pathogenicity, varied among the populations.
HWDPD, distance between the dorsal surface of the distal phalanx (DP) and the outer hoof wall.
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(A)
(B)
Figure 1. (A) Typical soft-substrate foot showing medial and lateral wall flaring with some distal hoof wall chipping or breaking. (B) The anatomy of the
distal phalanx in relation to the hoof wall appears normal. The hoof wall–distal phalanx distance (16 mm) and coronary–extensor process distance
(6 mm) are within normal limits. The hoof wall is the major weight-bearing structure of the hoof capsule and the sole is raised slightly above the
ground-bearing surface (X).
Table 3. Prevalence of laminitis and severity of the disease detected in
Australian feral horses from each of three populations (n = 45)
Laminitis rankinga
Absent
Mild chronic
Moderate chronic
Severe chronic
Laminitis prevalence
Kings
(n = 15)
Palparara
(n = 15)
Babbiloora
(n = 15)
5
5
5
–
67%
9
4
1
1
40%
1
14
–
–
93%
a
Laminitis grade was ranked as acute, mild chronic, moderate chronic or
severe chronic. No acute laminitis histopathology was detected.
only one case of severe, chronic laminitis was detected. All other hoof
sections were ranked as either mild or moderate chronic laminitis. Of
the horses diagnosed with chronic laminitis, the most prevalent histopathological features were attenuated secondary epidermal lamellae
(SEL), multibranched SEL, dystrophic tips (axial ends) of primary
epidermal lamellae (PEL) and the presence of cap horn (Figure 3).
It was the combination of these and other features that was used to
subjectively grade the severity of laminitis.
Discussion
This observational study investigated the foot health in five populations of Australian feral horses. Of the 100 left forefeet examined, only
three were considered to be free of abnormalities and these were from
the same hard-substrate, high-travel population (Kings). This same
population also contained a high degree of abnormality, including a
67% prevalence of histopathological chronic laminitis. Overall, the
majority of the feral horse feet were assigned a low to moderate abnormality score, although there was evidence of severe abnormality in
some feet.
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It has previously been assumed by some authors that the feral horse
foot is an ideal model on which to base equine foot care practices.1–3
The adoption of this model by some groups has shifted the focus of
hoof trimming away from the traditional farriery model, with a tendency towards excessive removal of the bearing border of the distal
hoof wall by natural foot advocates.A previous study of the foot health
of a population of feral horses in New Zealand reported a similar
high prevalence of abnormalities as in the current study, including
chronic laminitis.4 Thus the practice of using the ‘natural’ foot model
as the optimal morphometric model on which to base foot trimming
practices may need to be reconsidered.
In the present study, the external appearance of the typical hardsubstrate brumby foot, which is often used as the benchmark model,
was aesthetically pleasing, with little visible pathology. However, this
superficial impression was misleading, as a more thorough investigation of the internal foot structure using radiographic and lamellar
histological assessment revealed significant pathology. Previous
observers and proponents of the ‘natural’ foot model, apparently
unaware of this internal pathology, have, regardless, made assumptions and recommendations for domestic foot care, such as promotion
of solar loading and excessive bevelling of the distal hoof wall.
We selected the habitats for the current study on the basis that they
contained different substrate types, which determined different travel
patterns of the horses. Our previous study, which investigated the hoof
morphology of the same 100 horses, found that habitat influenced
37 of the 40 morphological parameters measured.5 An earlier genetic
study of four of the five feral horse populations in the current study
found no significant difference in the genetic origins of the horses6
and there were no significant differences in nutritional parameters
likely to be responsible for the differences in foot health.7 From our
separate considerations of the effect of environmental substrate and
travel patterns, we concluded that both factors influenced foot
morphology. Each of the five populations had a different hoof
type and each displayed foot abnormalities that were unique to their
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(A)
(C)
environment. Two of the most significant abnormalities detected were
secondary remodelling of the distal phalanx and increased HWDPD.
Previous reports suggested that these changes are consistent with
chronic laminitis10–15 and this was confirmed by histopathological
examination in three of the five populations. There is evidence to
support the possibility that feral horses from hard-substrate environments may suffer from foot pathology that is similar to traumatic
laminitis in domestic horses. Linford13 induced traumatic laminitis by
trimming the bearing border of the hoof wall to the level of the sole
and then housing the horses on a hard surface for 4 months. The
findings reported were disruption of the lamellar architecture, solar
chorium haemorrhages, solar margin fractures and distal phalanx
remodelling.
The necessity for some feral horses to travel long distances over hard
substrate may lead to overuse or concussive injuries to the distal
phalanx and lamellar suspensory apparatus of the distal phalanx.
The high prevalence of calcified ungual cartilages in feet from the
hard-substrate environments, particularly the Mussellbrook population (70%), supports a concussive injury aetiology. Although horses
in the Kings population were able to travel long distances to food
and water along soft tracks worn through the hard substrate, the
Mussellbrook horses were restricted to hard, rocky substrate, unaffected by stock erosion, for most of their travel. The difference
© 2013 The Authors
Australian Veterinary Journal © 2013 Australian Veterinary Association
(B)
Figure 2. (A) Typical hard-substrate foot showing mediolateral symmetry
with minimal hoof wall flaring. The gross external appearance of the hoof
capsule is aesthetically pleasing, with little sign of deformity. (B) Internal
anatomy shows derangement of the position of the distal phalanx with
respect to the hoof capsule, with excessive hoof wall–distal phalanx distance (27 mm) and the appearance of distal displacement of the distal
phalanx by an excessive coronary–extensor process distance (16 mm). In
this foot type, the peripheral sole is forced to bear considerable weight
because of the excessively worn distal hoof wall, exposing the sole
to weight-bearing (X). (C) Lateromedial radiograph of the same hardsubstrate foot shown in A and B shows the derangement of the radiographic anatomy of the distal phalanx in relationship to the hoof capsule.
There is remodelling of the dorsal surface of the middle phalanx (+) and
distal tip (*) and extensor process (#) of the distal phalanx and calcification of the cartilages of the foot (**).
between the two areas in the mechanical properties of the substrate
may account for the different prevalence of calcified ungual cartilages
in the two populations. Likewise, the difference in substrate hardness
may also account for the difference in the HWDPD and other signs
of traumatic laminitis among the five populations. Although the
hard-substrate foot type had significant internal pathology, horses
in these populations appeared sound and were able to travel large
distances over very challenging terrain.9 Feral horses living on
hard substrate had significantly greater mean epidermal sole depth
(12.6 ⫾ 0.77 mm) than either the feral horses from a soft-substrate
environment (9.6 ⫾ 0.59 mm) or domestic Thoroughbred horses
(10.6 ⫾ 0.52 mm).16 Horses living in a hard-substrate environment
also had a shorter toe length (Kings 29.1 ⫾ 0.6 mm) than either the
horses from a soft-substrate area (Cliffdale 34.6 ⫾ 1.4 mm) or the
domestic horses from previous studies (Kummer: 36 ⫾ 0.47 mm;17
Linford: 31.4 ⫾ 3.0 mm13). A reduction in toe length has been shown
to reduce the peak moments substantially at the onset of breakover.17,18 It may be that the robust foot structure and the unique foot
morphology of these horses is protective against the mechanical
trauma and pain often associated with chronic laminitis in domestic
horses.
The feet from the soft-substrate, moderate-travel horses (Cliffdale) did
not have abnormal radiographic features. In the present study, the
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Figure 3. Photomicrographs of mid-dorsal hoof wall lamellar sections (H&E) from Australian feral horses. (a) Hoof epidermal lamellae are uneven in
length, with blunt, rounded tips (black arrowhead) devoid of secondary epidermal lamellae (SEL). The keratinised axis of some primary epidermal
lamellae (PEL) is convoluted and compressed (white arrowhead). Lamellae adjacent to the hoof wall contain cap horn (white arrow). (b) Isolated islands
of lamellar epidermis at PEL tips (black arrowhead) are dyskeratotic and resemble tubular cap horn. SEL occasionally present (black arrow). (c) The
keratinised axis of the PEL is sometimes discontinuous in zones that are convoluted (black arrow). Instead of being organised into symmetrical rows
of SEL, epidermal-basal cells form a contiguous mass on either side of the PEL (black arrowhead). When present, SEL are often oriented towards the
hoof wall (white arrowhead), which is the reverse of the normal arrangement. (d) Some midlamellar SEL are long and narrow with pointed tips (black
arrowhead) instead of the usual rounded tips. PDL, primary dermal lamellae.
HWDPD of the Cliffdale population was significantly less (P < 0.05)
than that of the horses from the hard-substrate environments in our
previous study5 and was within the normal range for sound domestic
horses.13 There were no radiographic signs of laminitis in the Cliffdale
(soft substrate) population. Hoof wall flaring and splitting were
common in feet from soft-substrate environments, but were absent in
feet from hard-substrate areas. The hard substrate may have induced
significant internal abnormalities, but the high rate of hoof wear may
have protected against less significant foot abnormalities. The significance of this protective effect, however, is not fully understood.
A straight, short-walled hoof capsule is a feature of horses surviving
in a hard-substrate environment and appears to withstand the pathological consequences of this lifestyle.
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Substrate hardness may not be solely responsible for the more significant abnormalities observed in the Australian feral horses. The gross
observations of laminitis in the Palparara and Babbiloora populations
were confirmed by histopathology. These environments did not
contain substantial areas of hard substrate compared with the Mussellbrook and Kings areas. The presence of laminitis in these populations may be explained by nutritional factors. In Palparara, a
significant flood occurred 3 months prior to sample collection, generally considered to be a ‘once-in-10-years’ event,19 and caused a flush
of previously dormant plants, including grasses, forbes, legumes and
native sorghum (Echinochloa turnerana), which is reported by local
stockmen to cause severe lameness in horses allowed to graze it. The
feral horses had free access to this pasture for the duration of its
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The lifestyle of feral horse populations can be very challenging.
In some parts of Australia they walk extremely long daily distances
because of the long distances (>65 km) between food and water
sources.8,9 Hampson et al. reported, from data collected by GPS, that
the majority of feral horse travel is performed at a slow walking pace,9
yet significant foot abnormalities develop under this regimen. Thus,
repeated low-load, concussive conditions, such as unavoidable travel
over hard terrain, predispose horses to the foot abnormities described
here. The pathogenesis of the laminitis detected in some populations
may be attributable to free grazing of pastures and browse that are
high in non-structural carbohydrates, despite the horses being extensively free-ranging. It appears that the vulnerability of Equus caballus
to foot pathologies such as laminitis extends even to horses leading a
‘natural’ existence in ‘outback’ Australia.
Study limitations
A limitation of the current study is the qualitative and subjective
nature of the identification and classification of hoof abnormalities.
Whenever possible, we used recognised objective measures of pathology indicators, such as the HWDPD, to support a subjective assessment, but this was not possible with the majority of abnormalities
identified. This assessment was performed in the same manner that a
practising veterinarian or experienced farrier would approach a clinical case and the same assumptions were made concerning the effect
that the abnormalities may have on hoof function. Clearly, there is a
lack of objective evidence to validate the pseudoclinical assessment
performed in this study. However, the combination of photographic
and radiographic images with lamellar histological examination
enabled the assessors to obtain a more informed view of foot health
than is available to many clinicians. Because of the logistical challenges
© 2013 The Authors
Australian Veterinary Journal © 2013 Australian Veterinary Association
of obtaining detailed data in remote areas, the left and right forelimbs
were used in combination to describe foot health. The assumption was
then made that the feet presented symmetry. Although left/right
asymmetries are frequently encountered because of the athletic pursuits of domestic horses, we have found symmetry in feral horse feet,
as described previously.5
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survival and we hypothesise that the high prevalence of laminitis
(40%) in their feet was related to carbohydrate overload from opportunistic feeding, similar to laminitis caused by grain overload.18
The prevalence of laminitis was high in the Babbiloora horses (93%),
whose environment was unique among the feral horse habitats
because it is situated within prime cattle grazing country that is cultivated with exotic pastures, including buffel grass (Cenchrus ciliaris),
Rhodes grass (Chloris gayana), green panic (Panicum maximum var.
trichoglume) and silk sorghum (Sorghum halepense ¥ S. roxburghii ¥
S. arundinaceum) species, all of which are known to contain high
levels of non-structural carbohydrates and have been linked to laminitis in horses.20 A nutritional survey of feral horses in the current
study found higher values than would be expected in high-quality
pasture for water- and ethanol-soluble carbohydrates in all locations,
but these parameters were highest in the Cliffdale population,7 which
showed no radiographic signs of laminitis in the current study.
Carbohydrate levels were lowest in the Babbiloora population, which
had the highest prevalence of laminitis in the current study. However,
the nutritional survey was performed during a sustained period of
drought and thus does not relate to the sporadic events (i.e. flood)
proposed to have been responsible for the nutritionally-induced laminitis in the current study. We suggest that high dietary carbohydrate
levels may be tolerated by feral horses because of their high daily
activity levels,8,9 but sporadic environmental events, as described,
producing extremely high levels of dietary carbohydrate may not be
tolerated without consequences for foot health.
Our observations were limited to gross external, internal and radiographic assessments of foot health within five feral horse populations
and histopathological confirmation of chronic laminitis in three.
Logistical difficulties prevented collection of lamellar samples for histology in two of the populations. It was not known if subject horses
experienced clinically detectable lameness, stress fractures, ligament
or other soft tissue injuries. The population samples were potentially
biased because they were limited to horses that were still alive after
surviving the extremes of several Australian environments. Many
horses experiencing similar or more profound pathologies to those
observed in this study may have died as a consequence of failure to
cope and thus were not included in the sample. Finally, environmental
conditions were severe during the sample collection period, with some
populations experiencing severe drought stress and bush fires while
others had recently experienced unseasonal rain and flooding, with
the associated abundance of carbohydrate-rich food.
Conclusion
Horse owners face the challenge of creating a housing environment
that is most conducive to their horse’s health. This may include the
provision of a surface under foot that balances hoof growth and wear
and promotes musculoskeletal preparedness for the environment
in which the horse will be used. There is a choice of substrates for
housing horses and there are active feeding systems that can modify
the daily distance travelled by horses.21 The present study identified
the negative long-term implications of substrate and travel pattern on
foot health. Although feral horses living and travelling on hard substrate appear to have robust feet, modified by the environment and
able to withstand locomotion over hard substrates, the current study
suggests that there are some negative consequences associated with
this lifestyle.
The results of this analysis of the foot health of feral horses contain a
warning of the risks and consequences for domestic horses engaged in
extreme pursuits, such as endurance racing, and police patrol work on
hard concussive footing such as paved roads. Recommendations can
be drawn from this study regarding the design of equine housing,
particularly with respect to the hardness of footing on which horses
are kept and the access to pastures that are high in non-structural
carbohydrates.
These data could be used by the equine husbandry community to
make informed decisions on the value of using the feral horse foot to
guide domestic horse foot care practices. The study of foot health in
extreme environments may also provide data that enables a better
understanding of the risks to domestic horse foot health associated
with equine sporting and occupational pursuits, such as endurance
riding and sustained activity over hard footings. Furthermore, the
effect of the free consumption of native and exotic pasture and browse
Australian Veterinary Journal Volume 91, No 1–2, January/February 2013
29
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EQUINE
(trees and shrub) species on the foot health of the feral horses
described here may provide insight into dietary-related foot disease in
domestic horses.
This work highlights the importance of using empirical methodology,
a large sample size, thorough investigation and independent, blinded
observations to obtain data for use in the guidance of hoof care
practice. Further investigation of the feet of horses is warranted to
determine the existence of pathology before the consequences of the
environment in which they live and work can be fully established, and
to fully understand the effect of various footings on the health and
wellbeing of domestic horses in managed care. Currently, there is no
clear evidence to suggest that the feral horse foot is preferable to the
model of foot care currently accepted and used by mainstream veterinarians and farriers. However, it is possible that the hard-substrate
foot type, because of its robust nature and biomechanical function,
allows the feral horse to withstand significant foot pathology without
showing overt lameness, thus assisting the horse to survive in extreme
environments. The modern horse may have been physiologically
and structurally adapted for superior speed and agility. Human intervention in breeding programs in pursuit of specific sporting and performance goals may have pushed the species towards vulnerability in
some tissues, such as the lamellar suspensory apparatus of the distal
phalanx. However, the feral cousins of the modern domestic horse are
also vulnerable to foot pathology, despite being free from the confines
and influence of human intervention.
Acknowledgments
This research was funded by a grant from the Rural Industries
Research and Development Corporation, Australia, and from internal
funding of the Australian Brumby Research Unit, School of Veterinary
Science, The University of Queensland.
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(Accepted for publication 31 July 2012)
© 2013 The Authors
Australian Veterinary Journal © 2013 Australian Veterinary Association