Research Article: Iron and iron/manganese ratio in forage from Icelandic sheep farms: relation to scrapie

Date Published: August 31, 2006

Publisher: BioMed Central

Author(s): KB Gudmundsdóttir, S Sigurdarson, J Kristinsson, T Eiríksson, T Jóhannesson.


This study was undertaken in order to examine whether any connection existed between the amounts of iron in forage and the sporadic occurrence of scrapie observed in certain parts of Iceland. As iron and manganese are considered antagonistic in plants, calculation of the Fe/Mn ratios was also included by using results from Mn determination earlier performed in the same samples. Forage samples (n = 170) from the summer harvests of 2001–2003, were collected from 47 farms for iron and manganese analysis. The farms were divided into four categories: 1. Scrapie-free farms in scrapie-free areas (n = 9); 2. Scrapie-free farms in scrapie-afflicted areas (n = 17); 3. Scrapie-prone farms (earlier scrapie-afflicted, restocked farms) (n = 12); 4. Scrapie-afflicted farms (n = 9). Farms in categories 1 and 2 are collectively referred to as scrapie-free farms. The mean iron concentration in forage samples from scrapie-afflicted farms was significantly higher than in forage samples from farms in the other scrapie categories (P = 0.001). The mean Fe/Mn ratio in forage from scrapie-afflicted farms was significantly higher than in forage from scrapie-free and scrapie-prone farms (P < 0.001). The results indicated relative dominance of iron over manganese in forage from scrapie-afflicted farms as compared to farms in the other categories. Thus thorough knowledge of iron, along with manganese, in soil and vegetation on sheep farms could be a pivot in studies on sporadic scrapie.

Partial Text

The prion protein (PrP) occurs naturally in most organs. It is believed to have a role in copper metabolism and possibly also in oxidative defense and functions of the central nervous system. The protein is present in both a free and a glycosylated form, bound to cell membranes. In prion diseases (also called transmissible spongiform encephalopathies (TSEs)), the prion protein takes on a pathological, misfolded form (often called PrPsc), leading to depositions of extracellular aggregates and spongiform degeneration (vacuolation) in the brain. TSEs are always lethal. As the name implies, a distinguishing feature of TSEs is their transmissibility between individuals of the same species, or even between individuals of different species [1-4].

The iron concentration in the forage samples ranged from 57 mg kg-1 to 1379 mg kg-1. In 14 of the samples (ca. 8%) the iron concentration was less than 100 mg kg-1 and in three of the samples (ca. 1.7%) it was above 1000 mg kg-1. It was considered that the three samples with iron concentration above 1000 mg kg-1 were contaminated by soil, and these samples were thus excluded from further processing. The mean iron concentration in forage from scrapie-free farms (categories 1 and 2 combined) was 223 mg kg-1 (91 samples). On scrapie-prone farms the mean iron concentration was 221 mg kg-1 (40 samples), and on scrapie-afflicted farms the mean iron concentration was 343 mg kg-1 (36 samples). The mean iron concentration was thus significantly higher in forage from the scrapie-afflicted farms than in forage from farms of the other scrapie categories (P = 0.001) (Fig. 1). The mean iron concentration in forage from scrapie-free farms in scrapie-free areas was 193 mg kg-1 (27 samples), compared to the mean iron concentration of 236 mg kg-1 in the forage from scrapie-free farms in scrapie-afflicted areas (64 samples). The difference between these two categories was however not statistically significant (P > 0.05).

The amounts of soluble iron in Icelandic soil are known to be high [10]. The iron concentration in grass (and forage made from grass) in the country is accordingly high. The high concentrations of soluble iron in soil in Iceland can probably influence the absorption of other trace elements in plants, such as manganese, copper, zinc and cobalt, which, like iron, are predominantly absorbed as divalent cations. Icelandic forage is also known to contain somewhat low amounts of zinc and cobalt, although no difference was seen between the amounts of these trace elements in forage from farms of different scrapie categories [11]. This might nevertheless be of importance for the general health status of the livestock. There is, however, at the present time no indication of manganese or copper deficiency in grass or sheep in Iceland [6,12]. This might be ascribed to the high levels of soluble manganese and copper in Icelandic soil [[10], Th. Gudmundsson, personal communication 2006]. In this context it is of interest that the high iron levels found in forage from scrapie-afflicted farms border on the toxic levels of the metal for plants [13].




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