those four-and-a-half-years. the incubation period was seven months and up. only 30% of the sheep receiving a subcutaneous inoculation of the suspension developed scrapie in that time, and the incubation period in this group was 15 months and up. the researchers concluded that the infective agent was probably a filtrable virus." contaminated mycoplasma agalactiae vaccine. in 1997 and 1998 there was an outbreak of scrapie in sheep and goats in italy, and the cause was attributed to the vaccine. "an accidental infection from a vaccine was suggested as the explanation for the sudden increase in outbreaks of scrapie in italy in 1997 and 1998. this paper describes a recent outbreak of scrapie in sheep and goats which were exposed to the same vaccine. no ewes or goats had been imported into the herd since 1992, but a vaccine against mycoplasma agalactiae had been administered twice, in 1995 and 1997. high rates of crude mortality and scrapie incidence were experienced by both species, all birth cohorts were involved and a large proportion of aged animals was affected. a pattern of brain lesions was observed, with slight differences between the sheep and goats, which was very similar to the pattern observed in animals previously exposed to the same vaccine but clearly different from that observed in the brains of sheep with scrapie in a flock not exposed to the vaccine. regardless of their exposure status, genotype analysis of the sheep showed the presence of polymorphism only at codon 171. the patterns of both incidence and brain lesions provide evidence that the epidemic of scrapie was due to the use of the vaccine." "an accidental intra- and interspecies transmission of scrapie occurred in italy in 1997 and 1998 following exposure to a vaccine against mycoplasma agalactiae. prpsc in affected sheep and goats, collected from a single flock exposed to vaccination 2 years earlier, was molecularly typed. in five animals with iatrogenic scrapie, a prpsc type with a 20 kda core fragment was found in all areas of the brain investigated. in three sheep and one goat, this isoform co-occurred with a fully glycosylated isoform that had a protease-resistant backbone of 17 kda, whereas in two sheep and four goats, the two prpsc types were detected in different regions of the brain. in sheep with natural field scrapie, a prpsc type with physico-chemical properties indistinguishable from the 20 kda isoform was found. the present results suggest the co-presence of two prion strains in mammary gland and brain homogenates used for vaccination." exposure through contaminated soil ingestion of soil by grazing sheep has been measured in two soil types, at two stocking rates, and over two grazing seasons. animals ingested up to 400 g soil per kg of body weight between may and november. rainfall and stocking rate emerged as factors influencing ingestion. the effect of soil type and vegetation type was less evident. the average weight of an adult sheep is around 250 pounds. if an adult sheep ate 400g/kg of soil as predicted by d. mcgrath et al., then the average sheep would ingest about 45,000 g over six months, or 251 g per day. assuming the soil was contaminated with prions (prpsc) from feces or birth fluids, then potentially the sheep would become infected. the concentration of the prions in the soil is uncertain, and concentration is not directly proportional to infectivity. factors affecting prion infectivity in the soil have been shown to include the length of time in the soil and the binding abilities of the soil. for a detailed risk assessment of scrapie-contaminated soil, it was of major importance to analyze whether the detectable prpsc in the soil extracts still exhibited oral infectivity after incubation times up to 29 months. a bioassay with syrian hamsters was performed by feeding the animals with contaminated soil or aqueous soil extracts that had been collected after soil incubation for 26 and 29 months, respectively. hamsters fed with contaminated soil exhibited their first scrapie-associated symptoms at two weeks to six months (95% ci) after the first feeding. the hamsters reached the terminal stage of scrapie at five to 21 months (95% ci) after the first feeding. this indicated substantial amounts of persistent infectivity in soil that had been incubated for 26 and 29 months. in iceland in 1978, a program was implemented to eradicate scrapie, and affected flocks were culled, premises were disinfected, and sheep houses were burnt; after two to three years, the premises were restocked with lambs from scrapie-free areas. between 1978 and 2004, scrapie recurred on 33 farms. nine recurrences occurred 14–21 years after culling as a result of persistent environmental contamination with prpsc. the binding abilities of different soil types have been shown to enhance disease penetrance into a population. soil containing the common clay mineral montmorillonite (mte) and kaolinite (kte) binds more effectively with the prions than soil containing quartz. enhanced transmissibility of soil-bound prions may explain the environmental spread of scrapie despite low levels shed into the environment. the mechanism by which mte or other soil components enhances the transmissibility of particle bound prions remains to be clarified. prion binding to mte or other soil components may partially protect prpsc from denaturation or proteolysis in the digestive tract, allowing more disease agent to be taken up from the gut. adsorption of prpsc soil may alter the aggregation state of the protein, shifting the size distribution toward more infectious prion protein particles, thereby increasing the infectious units. for prion disease to be transmitted via ingestion of prion contaminated soil, prions must also remain infectious by the oral route of exposure. researchers at the university of wisconsin investigated the oral infectivity of mte-bound and soil-bound prions. the effects of prion source (via infected brain homogenate and purified prpsc) and dose on penetrance (proportion of animals eventually exhibiting clinical signs of scrapie) and incubation period (time to onset of clinical symptoms) was evaluated. about 38% of animals receiving orally 200 ng of unbound, clarified prpsc derived from soil exhibited clinical symptoms, with an incubation period for infected animals of 203 to 633 days. all animals orally dosed with an equivalent amount of mte-bound prpsc manifested disease symptoms in 195 to 637 days. by contrast, animals orally receiving mte soil alone or one-tenth as much unbound clarified prpsc (20 ng) remained asymptomatic throughout the course of the experiment. these data established that mte-bound prions remain infectious via the oral route of exposure, and that the binding agent mte increases disease penetrance, enhancing the efficiency of oral transmission. exposure through contaminated hay mites "with scrapie, the archetypical tse, which is a natural disease in sheep and goats, the disease can appear suddenly in a flock in the absence of any known exposure to infected flocks (palsson, 1979). finally, fields in iceland, that were left empty for up to 3 years after the destruction of scrapie-infected flocks, were repopulated with known scrapie-free sheep, and some of the sheep in this latter group subsequently developed scrapie (palsson, 1979). this last 'experiment in nature' has yielded similar results a number of times in iceland and in the united kingdom. in one icelandic farm, flocks have been eradicated three times; each time, the farm was left without sheep for 2 years, and after restocking with sheep from scrapie-free areas, the disease reappeared. several years ago, a suggestion was made (s sigurdarson, personal communication) that hay mites would be a good candidate as a vector for scrapie; this led to infection of mice with mite samples prepared from hay obtained from five icelandic farms. ten of these 71 mice became sick after injection with mite preparations from three of the five farms (wisniewski et al, 1996; rubenstein et al, 1998). the incubation periods ranged from 340 days to 626 days, and these mice had the protease-resistant form prpsc, of a host-coded glycoprotein, prpc . the protease-resistant form is a marker of tse disease (prusiner, 1991; parchi et al, 1996). for some of these clinically positive mice, the banding pattern on wb analysis was unique (wisniewski et al, 1996; rubenstein et al, 1998)." transmission summary prions (prpsc) are shed from sheep and goats in birth fluids, feces and other excrement. the concentration of the prions is uncertain, but is not directly proportional to infectivity. sheep ingest a considerable amount of soil, so soil represents a plausible environmental reservoir of scrapie prions, which can persist in the environment for years. longevity of the prions and the attachment of soil particles likely influences the persistence and infectivity of prions in the environment. effective methods of inactivating prions in the soil are currently lacking, and the effects of natural degradation mechanisms on prion infectivity are largely unknown. an improved understanding of the processes affecting the mobility, persistence and bioavailability of prions in soil is needed for the management of prion-contaminated environments. a system for estimating the prion-binding capacity of soil on farms using simple soil analysis may allow an estimate of the prion risk in the environment, and whether altering prion binding by the use of soil amendments may help to mitigate the infectious prions. lichens, specifically, parmelia sulcata, cladonia rangiferina and lobaria pulmonaria, may have potential for reducing the number of prions because some lichen species contain proteases that show promise in breaking down the prion. further work to clone and characterize the proteases, assess their effects on prion infectivity, and determine which component organism or organisms present in lichens produce or influence the protease activity is warranted and is currently under investigation. genetics the prion gene that codes for the prion protein is highly conserved in most mammals, meaning the gene is similar and present in most species of mammals. three locations on the prion protein gene have been identified as highly polymorphic and may have an effect on scrapie: codons 136, 154, and 171. codon 154 has not shown any evidence of having a high effect on scrapie susceptibility, but is most likely having an effect on incubation time of the disease. codons 136 and 171 are thought to control the incubation time as well as the susceptibility of the disease, and are the ones the usda uses in its breeding standard. codon 171 has been determined to be the major genetic factor in scrapie susceptibility.