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Fillies And Tying Up
It’s often said that fillies are most prone to tying-up. A recent study by Dr. Cathy McGowan of the Royal Veterinary College in the United Kingdom suggests this is true. A survey of 1,276 horses in training identified risk factors as female, nervousness and being a two-year-old. A similar pattern was found in Dr. Jennifer MacLeay’s survey of 948 Thoroughbreds on Midwestern tracks. But they don’t answer the question why.

Simply being nervous may contribute, since the stress hormones epinephrine and cortisol can interfere with the muscle being able to use glucose as a fuel. It’s unlikely, however, that it’s enough. It’s more likely to have something to do with the filly’s sex hormones.

No studies have been done in horses, but in human female athletes the sex hormones can have significant effects on muscle metabolism. One striking study showed that intense training every few days during the phase of the cycle when estrogen is at its highest, with training only once a week in the progesterone phase, led to significant improvements in strength and muscle size compared to training every three days throughout the entire cycle.

For a mare/filly with a regular 20-day cycle who is in estrus/heat for seven days, the high estrogen phase would be the entire time she is in season plus the three days immediately before that. For a filly/mare with a 20-day cycle, if the day she goes out of season is “Day 0,” your ideal time for heaviest work would be days 10 through 20. This doesn’t mean you have to do an abrupt on and off, but cluster heaviest work during that period of time, tapering down as she comes out of season, avoiding hard work especially around days 4 to 7, picking it up again after that.

Studies that have looked at muscle’s response to estrogen versus progesterone would support these findings. Estrogen’s effects include:

• More efficient aerobic energy generation, including reliance on fats.

• Better endurance.

• Improved blood flow through the horse’s muscle.

• Enhanced release of tissue growth factors that favor muscle repair and remodeling.

• Improved uptake of glucose and synthesis of glycogen, as well as improved utilization of glycogen.

• Strong anti-inflammatory effect.

The effects of progesterone are basically the opposite.

Erratic estrus cycles and low estrogen, or imbalance of progesterone and estrogen, may play a role. High levels of soy/soy-fortified feeds may also be involved, since soy contains high levels of phytoestrogens, which can bind to the estrogen receptors and block the effects of natural estrogen. (For more information on tying up, see October 2000.)

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New Test For West Nile In The Works
Horses with severe West Nile Virus infections may test negative on current tests early in the course of their disease if the antibody titers aren’t yet detectable. It takes seven to eight days after infection for the titers to start to rise.

Because of the problems last year with people becoming infected with WNV from blood transfusions, the development of an ultra-sensitive viral DNA test for checking of blood bank samples has been on the fast track and will likely be available early this summer.

ELISA testing would remain as the first test of choice for active WNV infections, but early cases negative on ELISA could be confirmed using the new test.

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Racing And The Immune System
Several studies show that flat racing has negative effects on the lungs’ ability to withstand infections, contributing to the high incidence of respiratory problems on racetracks. However, a recent South-African study shows that endurance exercise also has a negative effect on the horse’s immune system, as it does in human runners.

Blood cortisol, white-cell counts and white-cell function were studied in eight horses after completing an 80 km race. Shifts in the relative percentages of white-cell types immediately after the race were consistent with a stress response, and the ability of both monocytes and neutrophils to respond to a challenge was depressed and remained depressed even after three days.

Careful attention to providing high-quality protein and generous levels of essential fatty acids and key antioxidant vitamins and minerals — vitamin E, selenium, copper and zinc — during training and racing put the horse in the best position to bounce back quickly from the severe stresses of endurance work. This study also emphasizes the importance of adequate rest and avoiding stress and sick horses after a race.

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Sugar Levels In Horse Feeds
Anne Rodiek of the Department of Animal Sciences, California State University, published the results of a study looking at the glycemic index of common horse feed ingredients.

The glycemic index is a measure of the blood sugar rise produced when the food is eaten. Horses that are overweight, insulin-resistant, have EPSSM or trouble digesting starches should avoid high glycemic index ingredients. Based on these findings, corn is the worst thing you could feed these horses, while beet pulp is best.

The findings, listed in order of highest glycemic index through the lowest, were:

Corn 117
Oats and molasses 105
Barley 101
Oats 100
Oats and oil 86
Alfalfa and molasses 85
Wheat 71
Vetch blend hay 53
Carrots 51
Wheat bran 37
Timothy hay 32
Alfalfa cubes 30
Alfalfa hay 26
Bermuda grass hay 23
Rice bran 22
Beet pulp 1

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Rhino Vaccine Effectiveness
A Gluck Equine Research Center study sheds light on why rhinopneumonitis (EHV) vaccines often don’t provide much protection from disease.

Weanlings vaccinated with typical intramuscular rhino vaccine, or given an experimental modified live intranasal EHV, failed to produce detectable levels of EHV-specific antibody at the level of the nasal mucosa, the horse’s first line of defense. A group given a dose of actual live virus developed high levels of mucosal antibodies, which persisted throughout the 13-week testing period.

This means the horse’s best protection against the disease appears to develop through a mild case of the disease rather than a vaccine.