Osteopathology and selenium deficiency co-occurring in a population of endangered Patagonian huemul (Hippocamelus bisulcus)

Osteopathology in huemul both in Argentina and in Chile

The descriptions of pathological lesions encountered in cranial bones of huemul from
the RNLC region coincide with those observed in specimens from Argentina 2]. The degree of the lesions from RNLC described here is considered severe and appears
to surpass the cases from Argentina, like the present case 3 regarding maxillary lesions,
or case 2 regarding mandibular lesions including a fractured main body. The prevalence
in an Argentine population was at least 57%, whereas the data from the RNLC does not
allow its estimation. Nonetheless, the encounter of the four described cases during
a short study indicates that such osteopathology is not a rare event.

Regarding the potential etiological factors postulated by Flueck and Smith-Flueck
in 2008 2], fluorosis could be discarded for huemul in Argentina 10], which likely applies to the RNLC region as no reports of fluorosis in livestock
nor people for that region were found. In contrast, the occurrence of iodine deficiency
in southern Chile has been documented in people 28] and livestock 36], 37]. Primary iodine deficiency thus may also play a role in huemul, but in addition is
aggravated by Se deficiency due to its key role in iodine metabolism. The occurrence
of Se deficiency in huemul from RNLC has now been confirmed 22], 23] and coincides with other reports about Se deficient plants and livestock in southern
Chile (reviewed in 43]). Soil samples from Argentina in areas used by extant huemul corroborate the deficient
levels in forage, livestock and huemul by also revealing highly deficient levels 35].

Are huemul from RNLC (Chile) deficient in selenium?

The alternative interpretation 23] of the first-ever study on Se status in huemul from RNLC related to the conclusion
by Chihuailaf et al. 22] that “Se values were within the interval reported for white-tailed deer”, and that
Se concentrations from huemul were “within reference intervals calculated for other
deer”, referring to the study by 24]. However, the review by 24] was not based on data from deer “in one area” as asserted by Chihuailaf and Corti
44], but provided results from 8 different populations with means differing by up to
542%. These results certainly cannot serve as reference intervals, but mainly because
24] mentioned deer and livestock in their study areas as having been diagnosed with white
muscle disease, and 50% of the deer were severely deficient and 75% had less than
the critical concentration by livestock standards.

More importantly, we questioned the removal of 36% of data points from the analysis
by 22], based on the claim that these were outliers. In response, Chihuailaf and Corti 44] only mention that Friedrichs et al. 45] suggest the removal of outliers to minimize error when establishing reference intervals.
However, according to Friedrichs et al. definition of outliers we must ask: were these
eliminated data points considered not to belong to the underlying distribution of
the overall data set? or were they the result of preanalytical, analytical, and postanalytical
errors? or did they result from erroneous inclusion of results from individuals that
did not satisfy selection criteria? Instead, the samples eliminated from analysis
by 22] were removed only because values were below the detection limit (bDL) of their equipment.
Deleting these bDL data points resulted in a type I left-censored data set, an erroneous analysis and conclusions that Se levels were
within reference intervals of other deer. However, these bDL data cannot be ignored
nor deleted from subsequent analyses. Rather, commonly used substitution methods are
replacement of bDL data by zero, or by half of DL, or the DL itself 46], 47], with DL/2 performing well in small samples 48]. By using the conservative value of DL for these previously discarded data, the huemul samples indicated 64% severely
deficient, 9% deficient, and 18% marginal values 23].

Chihuailaf and Corti 44] further suggested that “North America, where acid rain produces soils low in selenium”,
would be problematic for their wildlife, as opposed to southern South America which
“lacks chronic atmospheric pollution”. This assertion does not hold up, first because
only limited parts of North America with low soil Se are impacted by acid rain. Then
to contradict this suggestion, 22] stated: “huemul currently inhabit mountainous ecosystems… those environments are
formed on volcanic soils with pH tending to acidity”. In fact, data on soil pH of
South America indicate that most all soils of the current huemul distribution are
acidic (Fig. 1). Moreover, the fact that soils in areas with huemul are deficient in Se was highlighted
by 23], averaging only 0.19 mg/kg of total Se 35]. These low soil Se levels and general acidic soil conditions best explain the well
documented occurrence of Se deficiency in plants, livestock and huemul in southern
Chile.

The nexus between osteopathology in huemul and nutritional factors

The comprehensive literature on Se and iodine deficiency and their effects on physiology,
including osteopathology, among many mammalian species, allows predictions on their
effects on huemul. Moreover, basic mammalian biochemical functions demand a clear
lower limit for Se provision to be able to provide adequate physiological responses.
Due to remarkable interspecies similarities of the action of Se, there are few exceptions:
direct extrapolations of Se requirements between species have a high degree of correlation
49]. This commonality has its roots in the fact that Se functions at very basic biochemical
levels. First, the genetic code contains a codon which results in a Se-containing
amino acid. Second, the rather recent discovery of the function of this codon revealed
this new Se amino acid which was recognized as the 21st naturally occurring amino
acid (reviewed in 25]). Thus, although the Se enzyme GSH-Px was hypothesized to exhibit higher activity
per unit of blood Se in wild bighorn sheep (Ovis canadensis) than in domestic cattle, this however, was not the case 50]. In general, compared to non-ruminants, the dietary Se requirement is higher in ruminants,
with more severity in small ruminants, due to lower diet Se absorption (ruminants:
11–35%, nonruminants: 77–85%: 51]). Specific circumstances may result in higher dietary requirements, such as the increased
exposure to interfering elements like Cd, Hg or increased oxidative stress (reviewed
in 25]). Moreover, subclinical effects may be the more frequent situation, yet these are
best demonstrated mainly with production trials 52].

The argument that huemul have survived since the Pleistocene in habitats dominated
by volcanic soils lacking in Se 44] needs qualification. First, during glaciation events, including the last glacial
maximum, the area of the remaining extant huemul groups was under an ice cap reaching
1,800 m in thickness. Therefore, during these events, huemul in Patagonia existed
only in areas east of the continental divide 3] and hence, far from soils dominated by volcanic ash. Moreover, animal species commonly
occupy the landscape with strong presence in good habitat (source areas), but also
areas that are marginal, and even areas incapable of sustaining populations (sink
areas). The fact that most of the remaining 100 huemul groups fail to recover, or
are diminishing, or have recently disappeared indicates that areas used by these extant
huemul are not good enough as source areas. Lastly, the argument omits the crucial
fact that the area with extant huemul contains a highly heterogeneous landscape due
to the topography, yet huemul now are only using a very restricted portion which tends
to be marginal or deficient regarding micronutrients. However, huemul formerly also
utilized more fertile portions of this landscape particularly during winter, namely
the places which became occupied with humans, their agriculture and livestock, or
by towns and cities.

Several mineral nutrients like Se are not distributed homogeneously in the landscape,
being consistently more concentrated in lower elevations in valley bottoms, flood
plains, and in drier sites 30], 53]. Frequently, geological processes result in concentrations which become mineral licks
used by wild ruminants, and these occur most often, even exclusively, in low elevations
and on winter ranges 54], 55]. For instance, bighorn sheep made bimonthly short trips during the summer, to visit
mineral licks at up to 2,000 m lower in elevation on traditional winter ranges, which
replenished an otherwise Se-deficient summer diet 56]. Similarly, domestic ruminants have been shown to be Se deficient at high, but not
at low elevation in the Columbian Andes, with Se enzyme activity differing by 41%
57]. Thus, if southern Chile has documented deficiencies of iodine and Se in livestock
and humans living in the supposedly more fertile portion of the landscape, it follows
that huemul in their restricted areas might be more compromised, as indicated by low
survival of offspring 22], low longevity, and high prevalence of osteopathology 2]. Many of the 100 populations are thus declining, very few are increasing, and the
species is numbering only about 1,500 individuals partitioned in over 100 groups.