Executive Summary

At Health Canada's request, the Canadian Veterinary Medical Association (CVMA) established an Expert Panel to review the issues of the efficacy and safety of recombinant bovine somatotropin (rbST). The Panel was formed in March, 1998 and had expertise in epidemiology (Dr. Ian Dohoo - Chair), dairy health management (Dr. Luc DesCôteaux, Dr. Ken Leslie and Dr. Wayne Shewfelt), dairy nutrition (Dr. Alan Fredeen), livestock management and animal welfare (Dr. Allan Preston) and clinical pharmacology/large animal internal medicine (Dr. Patricia Dowling). The Panel operated completely independently from Health Canada and the CVMA.

The Panel reviewed material provided by Health Canada from Monsanto's submission to have rbST (sometribove) approved for use in Canada and carried out an extensive review of the published literature on the subject. While studies based on Monsanto's product and other companies' products were all considered, emphasis was placed on the former. The review process focused on studies which measured clinically relevant outcomes. The effects of rbST were assessed in the following main areas: milk yield, milk composition, nutritional implications, body condition, udder health, reproduction, lameness, other health concerns, culling and animal welfare. Within each area, key measures of effect (eg. 3.5% fat-corrected-milk for milk yield) were identified and all data from the literature review were extracted. These data were summarized in one or more meta-analyses to generate overall estimates of effect. Other related, but less commonly reported, measures of effect were also considered in a more subjective manner. If a detrimental effect was observed, the Panel discussed whether or not current dairy health management practices were adequate to control or eliminate the effect. Finally, the Panel discussed whether or not additional information was required in order to adequately assess the effects of rbST.

The Panel concluded that rbST does increase milk yield (3.5% FCM) by an average of 11.3% in primiparous cows and 15.6% in multiparous cows. There was considerable variation in the response between studies but all but one study reported a positive effect. There was evidence of a very small increase in the butterfat content (% fat) in the milk and in the protein content (% protein) in multiparous cows but the magnitude of the effects was too small to be of any consequence.

Treatment with rbST reduced the body condition of cows and although treated cows consistently increased their dry matter intake during the treatment period and on into the subsequent lactation, this did not appear adequate to offset the increased energy output associated with the higher yield. Consequently, treated cows started their next lactation in lower body condition than untreated cows.

Use of rbST increased the risk of clinical mastitis by approximately 25%. It appeared that there was also a slight increase in the prevalence of subclinical intramammary infections at the end of the treatment period. The Panel felt that while current dairy health management techniques could reduce this increased risk, they are not adequate to eliminate it. When the expected number of extra cases of mastitis was computed on a "per litre of milk shipped"basis, the increase was approximately 10%. Given this relatively small increase and the current programs for ensuring that antibiotic residues are not present in milk sold for human consumption, the Panel felt that the risk of increased antibiotic residues in dairy products was very small.

There were a number of effects on reproductive performance that were associated with the use of rbST. These included a substantial increase in the risk of non-pregnancy and a slight increase in days open in cows that do conceive. There was also inconclusive evidence of increased risks of cystic ovaries and twinning (multiple births). These adverse effects could be controlled by delaying use of the drug until cows were confirmed pregnant. There was some limited evidence of an increased risk of retained placenta and abortion/fetal loss in treated cows but there were insufficient data to draw a firm conclusion about these potential effects.

Treated cows experienced approximately a 50% increase in the risk of clinical lameness. Many of the lameness cases involved fore and hind limb joints. The Panel felt that current health management practices were not able to eliminate this increased risk.

Use of rbST reduced the risk of ketosis and some other metabolic diseases in the postpartum period in the lactation following one in which rbST had been used. This was probably due to a combination of the reduced body condition of cows at calving at the start of the next lactation and the higher levels of dry matter intake in the subsequent postpartum period.

Treated cows were at higher risk of being culled. This was particularly true in multiparous cows. Most of the data on culling did not include removal for reproductive reasons so the increased risk of non-pregnancy would exacerbate this problem in commercial dairy herds.

The Panel felt that there were a number of legitimate animal welfare concerns associated with the use of rbST. These included an increased risk of clinical mastitis and lameness, and a reduction in the lifespan of treated cows. Without better data on the frequency and severity of injection site reactions, the Panel could not determine if these represented a significant animal welfare concern.

In general, the Panel felt that there were sufficient data available to make a reasonably informed assessment of the effects of rbST. There were four specific conditions (risk of cystic ovaries, twinning, retained placenta, and abortion/fetal loss) for which there appeared to be an effect associated with the use of the drug, but for which there was insufficient evidence to draw firm conclusions. There was also insufficient information to determine how frequently injection site reactions occur. If the product is approved for sale, more information will be required about the nature of the increased risk of mastitis and lameness in order to manage those problems as effectively as possible.