I have no idea if those deaths have any connection to the electrical phenomena, and it's a mere speculation, but thought of putting it here in case others have additional ideas or professional opinions (Herr Eisenheim?).
I heard about horses being highly sensitive to electrical currents before, but I cant say I know much more then you about the subject. It is perhaps worth noting that its not just horses who are highly susceptible, so are the cows and any other large four legged animals.
Here is a good summary:How shock injuries occur
Several factors determine the nature and extent of electrical injury in humans and horses. The type of current being used is the first important piece of information. High-voltage direct current (DC) injuries are not common in horses because there are few sources of this type of electricity in their environment.
Batteries and generators are the principal types of DC that a horse might encounter. This type of electrical contact results in a strong and extremely rapid muscle spasm, often throwing the victim forcefully away from the current and resulting in blunt-force injury. Occasionally, short-circuited batteries in trailers or some electric fences can produce this type of injury, but the shock causes the horse rapidly and forcefully to back away from the source, and the injury usually is not severe.
Alternating current (AC) is the standard type of electricity found in power cords, electrical plugs, and most standard wiring in houses and barns. AC is three times more dangerous than DC of the same strength or voltage because muscle contracture occurs when a person or animal contacts AC even at low levels. This contracture then preserves the contact between the victim and the current, which leads to a longer duration of contact and greater electrical injury.
Since horses commonly chew on wiring or step on electrical cords, this contracture makes them unable to release a wire from their teeth or to move off a "live" power cord.
The amount of electricity needed to produce muscle contracture in humans is low. An electrical source producing one to two mA (milliamperes) of electricity will cause a tingling sensation, but a 200-pound human physically cannot let go of a wire carrying nine mA. Since this mA threshold depends on the size of the victim, one would think that horses could tolerate more voltage, but other factors contribute to making them even more sensitive.
Resistance is the tendency of a material to resist the flow of electrical current. Higher resistance tends to slow the passage of electricity through the body and increases the chance that the electrical energy will be changed to thermal energy, which causes the tissue to heat up and become damaged. Tissues in the body that are good conductors of electricity and consequently lower in resistance--nerves and blood vessels--tend to be less affected by low-level electrical contact. Dry skin and muscles are moderately resistant; tendons, ligaments, and bones are the most resistant.
Horses proportionally have more of the types of tissues that resist electricity. Their heavy muscles, thick tendons and ligaments, and large, bony skeleton make them especially sensitive even to low levels of current. Another factor that determines the severity of an electrical injury is the duration of the shock or contact with the current. Because horses are composed of a majority of tissues that slow down or resist the passage of electricity, they tend to be vulnerable to injuries caused by longer duration of contact, as well. Higher resistance and longer duration of exposure combine to be severely detrimental to the horse.
Mary Ann Cooper, M.D., of the University of Illinois at Chicago-based Lightning Injury Research Program, described why four-legged animals such as horses and cows are much more sensitive to electricity than are two-legged animals.
"The most disastrous individual electrical injury occurs when the person or animal becomes part of the electrical arch, since the temperature along the arch in a lightning strike can reach 2,500¡," she wrote.
Because a horse's front feet are a relatively long distance from its back feet in terms of the travel of electrical current, the horse's body usually becomes part of the arch of the current's path during a shock or lightning strike. Horses walking in an area with stray electrical ground voltage or horses exposed to "step voltage" (current spreading rapidly through the ground) are especially sensitive. Electricity enters one part of the body and travels through the body to exit at another.
The longer the distance between parts of the body, the greater the arch and the higher the temperature in the tissues. This high temperature causes massive tissue destruction throughout the body. Most deaths caused by electricity in horses occur before this tissue destruction comes into play, however, since most deaths are caused by short-circuiting of the body's electrical systems such as the heart, the respiratory center, or the nervous system. Cardiac arrest, respiratory arrest, and nerve instability are the rapid consequences of substantial electrical injury in the horse.
Another reason horses are more likely than humans to be injured fatally from electrical shock is what happens following the incident. Humans are much more likely to receive quick medical attention, whereas horses often are out at pasture during a storm without many people observing them. Injuries in barns, such as chewing through cords or wires, often are unobserved.
Rapid attention and cardiopulmonary resuscitation (CPR) often is successful in humans following electrical injury. This factor may be the most important reason why most horses struck by lightning die, while 70% of humans survive. CPR in the horse is difficult at best, and because many electrical injuries are unobserved, there is often no chance even to try.