3. Does rbST use increase BSE risks?
A third concern we have examined is whether rbST use could increase the risk of bovine spongiform encephalopathy (BSE) in dairy cows. Evidence to support this concern is very limited at this point, but several lines of evidence suggest that it is a plausible question to pose. Given the potential economic implications for the dairy industry, and public health implications if it is eventually proved that BSE is causally connected to a new strain of the fatal human Creutzfeldt-Jakob Disease (CJD), it seems important to pursue this line of reasoning and to clarify priority issues for research.
There are two mechanisms whereby rbST use could potentially lead to an increase in BSE in dairy cows. First, increased circulating IGF-I levels might increase a cow’s susceptibility to BSE, should the animal be exposed to the infectious agent. And second, the rbST-treated cow’s increased protein need could magnify the odds of exposure to a BSE-infective agent.
Does IGF-I increase prion protein (PrP) production?
BSE belongs to a group of progressive neurodegenerative diseases of humans and animals called the transmissible spongiform encephalopathies (TSEs), which also includes scrapie in sheep, chronic wasting disease (CWD) in mule deer and elk, transmissible mink encephalopathy (TME), and Creutzfeldt-Jakob Disease (CJD) and several other rare diseases in humans. These diseases have been shown to be transmissible by eating parts of infected animals. They are also characterized by long incubation periods, invariably fatal outcome, and infective agents that are unusually resistant to most forms of sterilization (formaldehyde, 70% alcohol, heat, radiation).
The nature of the infective agent(s) is still imperfectly understood. A widely held theory is that the infectious agent is a protein. Prusiner (1982) coined the term “prion” to stand for “proteinaceous infectious” particles that he believed constituted the infectious agent. Further work showed that the prion protein is normally found in all animals and is encoded by a prion gene, PrP. As research has unraveled the causal process, TSE infectivity has been clearly associated with a protease resistant protein (called PrP-res, or PrP-sc), which is a posttranslationally modified form of the proteinase K-sensitive host-encoded prion protein (PrP or PrPc, c for cellular) (Prusiner, 1991). PrPc is a membrane-bound protein found on the surface of all nerve cells, some lymphocytes and some other tissues. Both isoforms of PrP (e.g. PrP-res and PrPc) have the same amino acid sequence, but the molecules differ in their three-dimensional structure.
According to the most widely held theory of how prion proteins behave, the abnormal form (PrP-res), in combination with some other factor, causes normal PrP to convert to the abnormal Prp-res form, which in turn causes more PrPc to convert to PrP-res in a kind of domino effect or crystallization process (Gajdusek, 1993). PrP molecules on the surface of cells do not stay there for the life of the cell; they are removed from the cell’s surface, transported through the cell membrane into the cell and digested, while new PrPc is made within the cell and transported to the cell surface. PrP-res cannot be digested by the cell, and builds up in the cell, eventually causing cell death. PrP-res molecules can form large oligomers that may eventually congeal (or precipitate) into large structures called prion rods or scrapie associated fibrils, which, if abundant enough, result in the appearance of large plaques. The role that PrP normally plays in nervous and lymph systems is not fully known.
One way that rbST use might increase BSE risks is through effects of IGF-I on prion gene expression. Lasmézas et al. (1993) demonstrated that IGF-I dramatically increased production of PrP mRNA in a laboratory system. That is, IGF-I leads to significantly increased synthesis of prion protein. Increased IGF-I levels in cows might therefore speed up the action of PrP-res in a BSE-infected animal, shortening the incubation period for BSE.
Work with transgenic mice showed that increasing the amount of PrPc in the brain (via increased PrP gene expression, as indicated increased PrP mRNA levels) increased the speed of progression of scrapie (Prusiner, 1991). In these experiments, mice were engineered with varying numbers of copies of the hamster prion gene (HaPrP). The mice were then exposed to a given dose of scrapie-infected hamster brains. Results showed that “The length of the incubation period after inoculation with Ha prions was inversely proportional to the level of HaPrPc in the brains of transgenic mice” (Prusiner, 1991: 1519). The transgenic mouse strain with the smallest amount of brain HaPrPc (Tg69) had an incubation period of about 275 days, while the strain with the highest amount of brain HaPrPc (Tg7), approximately six times the amount found in brains of Tg69 mice, had an incubation period of about 50 days, about one-sixth the incubation period of the Tg69 mice (Prusiner, 1991).
Lasmezas et al. (1993) undertook their study to see whether either human growth hormone (hGH) or human IGF-I affected the PrP gene. If such hormonal signals “switch on” the PrP gene, it would be good evidence that IGF-I plays a role in progression of the human form of the disease, CJD. They were interested in this question because 25 French children, who had contracted CJD after being treated with hGH extracted from apparently CJD-contaminated cadavers, had exhibited a particularly short incubation period. The investigators wondered if hGH or IGF-I may have hastened progression of the disease in these children. They used, as an experimental system, a line of rat cells (PC12 cells) that other studies had shown to be a good in vitro model for studies of TSE agents (Rubenstein et al., 1990). They found that while hGH had little or no effect on PrP gene expression, IGF-I induced dosage dependent increases in PrP mRNA levels. PrP mRNA levels increased 40% with IGF-I levels of 10 ng/ml and 100% at IGF-I levels of 100 ng/ml. The results are significant because, as the authors point out, “IGF-I is found in human serum at concentrations of up to 1 ug/ml, i.e. ten times more than the highest tested dose. The effect of IGF-I on PrP gene expression in our experiments therefore occurs within the ‘physiological’ concentration range of this factor” (Lasmézas et al., 1993: 1167-1168). They conclude that “an increase of the levels of PrP RNA messengers, as a result of gene activation or of transgenesis, can be deleterious for an [TSE] infected organism” (Lasmézas et al., 1993: 1167). The deleterious effects could include a decrease in the incubation period as well as a more rapid progression of the disease.
How is this work relevant to rbST use in cows? First, administration of rbST clearly elevates the level of IGF-I in the cow’s blood. Injecting cows with rbST increases bovine serum levels of IGF-I by at least 5- to 7-fold (Juskevich and Guyer, 1990). Indeed, the elevated IGF-I is believed to mediate the increase in lactation. However, from the work of Lasmézas et al. and Prusiner et al., one would also predict that the increased IGF-I would lead to increased prion protein (PrPc) production and possibly more rapid progression of BSE in animals exposed to the infective agent.
Could the increase in IGF-I levels in milk from rbST-treated cows, which we documented in section 1, also speed the progression of human CJD? This seems highly unlikely, for a number of reasons. First, milk levels of IGF-I are at least two orders of magnitude lower than serum levels. Second, the increased IGF-I in milk from rbST-treated cows would have to pass from the digestive system, into the lymph and circulatory systems (or lymphoreticular system), and then into the central nervous system and perhaps through a blood-brain barrier and into the brain, before it could affect prion protein (PrPc) production in the brain.
However, one cannot totally rule out a possible effect on human suscepti-bility. Based on the work of a number of scientists, the most likely route of the TSE infective agent in the human body would be the same as in animals: from stomach to intestines, through the peyer’s patch (in the intestine) into the lymphoreticular system into peripheral nerves and then into the central nervous system (Pattison and Millson, 1962, Blättler et al., 1997, Groschup et al., 1996, Brandner et al., 1996).
The intestines are one of the earliest sites of TSE infection. They are known to be among the infective parts of an animal, and since studies have shown that normal prion protein is required for spread of the infective agent (Brandner et al., 1996), intestinal cells probably express PrP. They could conceivably produce more PrP in response to IGF-I. Milk levels of IGF-I are high enough to potentially be of physiological relevance. Using the studies cited previously by JECFA, milk IGF-I levels in non-treated cows ranged from 3-4 ng/ml to as high as 28 ng/ml, and from 3-4 ng/ml to 35 ng/ml in rbST-treated cows (FAO, 1993). An IGF-I level of 10 ng/ml caused a 30% increase in PrP mRNA in the Lasmézas et al. study. Increased levels of IGF-I in milk that reached the intestine (as we have suggested, based on our review of current evidence, they are likely to do) could conceivably stimulate PrP synthesis in intestinal cells. Thus, one could hypothesize that stimulating PrP production in intestinal cells could increase the rate or ease with which the infective agent could then move into the lymph system. As noted, these concerns are highly speculative, and further research is needed.
Does rbST use increase use of protein rich diets?
There is a second way that rbST use could contribute to BSE risk: Changes in the diet of dairy cows associated with rbST use could increase the risk that the animals might be exposed to the BSE infective agent. Cows receiving rbST require more energy-dense food than control cows, as higher milk output increases the amount of protein needed. One major source of energy-dense foods is protein and energy supplements made from rendered animal remains. For example, in the U.S., an official of the Center For Veterinary Medicine stated in a 1991 memo, “There is a growing trend in the use of meat and bone meal for calf rations . . . Most is used as a protein source for high production dairy cattle and for feed lot cattle” (Osborne, 1991: 4).
The European Union has prohibited use of mammal protein in feed for dairy cows (poultry and fish protein is permissible) because of concern that rendered protein might contain TSE-infective agents. Cows in England are believed to have become infected by eating the rendered remains of scrapie-infected sheep. However, the U.S. allows swine and horse protein in dairy feed, as well as poultry and fish protein, despite some evidence suggesting presence of a BSE-type disease in swine in the U.S. (Consumers Union, 1997). Most other nations have no current restrictions on the types of protein in dairy feed. Although the Codex Committee on Food Hygiene is now considering “Good Animal Feeding Practices,” the Codex process typically takes years to produce consensus international guidelines.
Use of rbST will definitely increase the use of energy dense feeds and protein supplements. Although this increased need for protein could be met with feeds from plant sources, such as soybeans, farmers will undoubtedly buy protein supplements based on their relative cost. Indeed, as some countries control use of rendered animal feeds, the relative costs of such materials seems likely to decline in international markets, making rendered animal protein more attractive to farmers in unregulated markets.
Animal protein feeds banned in Europe and the U.S. might also be exported to developing countries. When the U.K. banned use of specified bovine offals (SBOs) in animal feed in 1989, because SBOs carried the infectious agent for BSE, rather than destroy the SBOs, the U.K. exported such materials to other EU countries that had no regulations and to developing countries, such as Thailand. There is now a global ban on such material from UK cows, but exports from other countries are unrestricted. The U.S., for example, recently banned feeding ruminant protein to ruminants, as a precautionary measure, but the regulation does not restrict feed exports.
There is no way to be sure rendered protein produced outside the United Kingdom is safe. We currently have no assurance that BSE is confined to the UK. Monitoring elsewhere is poor to non-existent (for example, the U.S. tests less than a tenth of a percent of slaughtered cattle for BSE; many countries do no testing at all). When European countries started more careful monitoring, they began finding cows with BSE (MacKenzie, 1997). A recently published statistical analysis of the cows exported from the U.K. during the 1980s predicts that a certain low percentage of cows would have been infected, yet the number of BSE cases reported by the various importing countries has been far smaller than the model predicts (Wilesmith et al., 1997). This suggests that there is chronic underreporting of BSE incidence.
On theoretical grounds, it is reasonable to expect a low natural background level of mutant PrPs in virtually all mammals. Dr. Clarence Gibbs of the U.S. National Institutes of Health states: “As to the possibility that BSE may become endemic, I have proposed the following hypothesis. Since we accept that sporadic CJD is the result of a configurational change in a normal protein that occurs at the rate of 1-2 cases per million population per year, and since normal prion protein has been detected in all mammalian species thus far tested, as well as in salmon fish and Drosophila, then the rare occurrence of spongiform encephalopathy may certainly take place but remain undetected due to its rare occurrence in nature” (Gibbs, 1997: 9). And there is both direct and indirect evidence to suggest that TSEs occur in numerous species. In the U.S., TSEs have been documented in sheep, mule deer and elk (chronic wasting disease), and mink (transmissible mink encephalopathy). A recent report of a cluster of 11 cases of CJD in rural western Kentucky, where all the cases ate squirrel brains, suggests that a TSE is present in squirrels (Berger et al., 1997; Blakeslee, 1997).
Swine may also be infected with a TSE in the U.S. Some 106 hogs in a 1979-80 USDA study at a packing plant in upstate New York showed many of the same behaviors found in BSE animals (Doi et al., 1979; Doi pers. com. 1997). The brain of one of the hogs also showed spongiform degeneration and some other signs of a TSE (Langheinrich, 1979). Dr. William Hadlow, one of the foremost TSE pathologists in the world, reviewed slides from the suspect pig brain. Dr. Hadlow felt the slides were suggestive of a TSE, although not definitive (Hadlow, 1997).
Case-controlled epidemiological studies of CJD patients in both the U.S. and UK point to dietary consumption of a number of animal products being associated with CJD. Two small case-controlled epidemiological studies of U.S. CJD patients suggest the presence of a TSE in swine. A 1985 study, involving 26 CJD patients, found that dietary consumption of six different pork products was associated with an increased risk of CJD compared to a control group: “An increased consumption among [CJD] patients was found for roast pork, ham, hot dogs (p < .05), roast lamb, pork chops, smoked pork, and scrapple (p < .10) . . . The present study indicated that consumption of pork as well as its processed products (e.g., ham, scrapple) may be considered as risk factors in the development of Creutzfeldt-Jakob disease. While scrapie has not been reported in pigs, a subclinical form of the disease or a pig reservoir for the scrapie agent might conceivably exist" italics added (Davanipour et al., 1985: 443, 448). The other study, from 1973, found that one third of the 38 CJD patients studied ate brains, much higher that the U.S. population overall, and that the patients had a preference for hog brains compared to controls (Bobowick et al., 1973). Both studies are small, but the results are highly suggestive. The U.S. Food and Drug Administration's August, 1997 regulation permits known TSE-positive material to be used in pet food, pig, chicken and fish feed. FDA requires only that it be labeled "Do not feed to cattle and other ruminants" when marketed in the United States. Finally, BSE-infected cattle typically do not exhibit symptoms of BSE until the end stages of the disease, following a long incubation period. There is evidence that rbST use reduces the useful lifespan of a dairy cow. Cows intensively treated with rbST from their first lactation cycle could be removed from herds after just two or three years rather than the now customary four to six years. Given that the incubation period for BSE is at least three to five years and perhaps longer, rbST-treated cows could harbor "hidden" BSE. That is, they might be infected but still asymptomatic when sent to slaughter. In summary, there is as yet no direct evidence that rbST use has, to date, contributed to or in any way caused the BSE epidemic. Studies comparing BSE incidence in dairy herds treated with rbST against that of untreated cattle would be required to answer that question, and no such studies have been done. Indeed, rbST has not been in use in the countries where BSE has been officially reported. But we believe there is a very sound theoretical basis for the hypothesis that rbST use could aggravate the risk of BSE. Cows treated with rbST require more energy-dense feed, which increases the likelihood they may be fed rendered animal protein. Use of such feeds has been causally linked with the spread of BSE, and current national and international controls cannot guarantee that these materials are free of TSE infective agents. Synthesis of PrP proteins has been shown to respond in a dose-dependent manner to IGF-I levels within the physiological range, and rbST use markedly increases serum levels of IGF-I in cows. It appears that rbST use could therefore decrease the incubation time for BSE in an infected animal. These hypotheses can neither be proven nor disproven, because of the amount of evidence currently available. But given the potential severity of the BSE problem, both for the economic health of the cattle industry and for public health, the possibility that rbST use could aggravate the BSE risk appears to demand intensive investigation.