Horses are at risk of getting struck by lightning when they are outside in the pasture during thunderstorms. Hundreds of livestock are killed annually by lightning worldwide, with many incidents never reported. The degree of risk is dependent on:

• The lightning flash or stroke density of the geographical area
• Topographical landscape
• Characteristics and distribution of trees (height, size, species, location relative to other trees, ground root system)
• Habits of animals during storms

Geographical Area
Horses living in regions with high flash or stroke densities are more likely to get struck by lightning then those that aren't.
In the United States, these areas predominately include the southeast (especially Florida), Gulf States, the Mississippi and Ohio River Valleys, the Front Range of the Rocky Mountains, and some other mountains in the Desert Southwest.

Layout of the Pasture
Lightning is caused by an imbalance of positive and negative charges in the atmosphere. During a storm, negative charges are generated along the bottoms of clouds. Positive charges are generated on buildings, trees, or any objects that protrude upwards from ground level. As both these charges increase, once they reach high enough levels, it causes a streamer of negative charges to move erratically in the direction of the earth. At the same time, a short leader of positive charges may move a short distance upwards into the air. Once the two charges meet, the negatively charged streamer from the clouds completes its grounding path as the positive charges move upwards along the same path, and into the clouds. Any trees, buildings, or other objects that are tall and isolated, tend to generate more positive charges. Therefore, pastures that are hilly, contain isolated tall trees, buildings or other objects, are more at risk of getting struck by lightning.

Trees
The taller and more isolated the tree, the more it's at risk of getting struck by lightning. Also, certain trees are more susceptible to lightning strikes than others. These trees should be identified and either protected or removed from pastures where horses are turned out into.

Susceptibility of non-protected temperate zone trees to lightning strikes

Tree genus	Common Name	Susceptibility to lightning strikes
Liriodendron	Tulip poplar	Very High
Robinia	Black locust	High
Fraxinus	Ash	High
Populus	Poplar	High
Tsuga	Hemlock	High
Ulmus	Elm	High
Quercus	Oak	High
Acer	Maple	High

Source: T Smiley, 2011

Animal Habits during Thunderstorms
The most common ways horses die from lightning strikes are not actually through a direct hit (direct strikes), but through indirect hits, which include:

• Ground current: Ground current strikes occur when lightning hits the ground, causing the current (often 20,000+ amperes worth) to spread out along soil surface, including any obstacles (such as horses) that may be in the vicinity.
  
  For a horse standing on the ground affected by the current, it can travel up a hind legs, through their entire body and down the front legs. Along the way, it can potentially stop the horse's heart or arrest it's breathing. The greater the space between where the current enters the horse's body and where it leaves, the greater the potential for serious damage.
• Side flash: Side flash occurs because lightning follows the path with the least resistance to electrical current to the ground. If a horse is standing underneath a tree that is struck by lightning, the lightning is more likely to jump from the tree to the horse. So one lightning strike to a tree or a strike to the ground can jump back and forth among all the animals nearby.
• Contact strike: Occurs when a horse is touching an object that is in the pathway of the lightning current, such as if it hits a water trough or electric fence.

Summary of Reported Horse Deaths Caused by Lightning Strikes

No	Location	How	Date	Time	Ref
3	MI	side flash; in pasture underneath tree	Aug-13	unknown	http://www.upnorthlive.com/news/story.aspx?id=931135#.U7wlurGa-yo
12	AL	Barn was stroke by lightning and caught fire	Jan-14	unknown	http://blog.al.com/breaking/2014/01/12_horses_die_in_barn_fire_at.html
2	WI	while riding - side flash or ground	Jul-13	unknown	http://www.wisn.com/news/south-east-wisconsin/waukesha/horses-hit-by-lightning-2-riders-injured-in-eagle/21093200#!baVYSC
2	CO	side flash; in pasture underneath tree	Sep-13	unknown	http://www.9news.com/news/article/354159/222/2-rescue-horses-struck--killed-by-lightning-
1	PA	side flash; in pasture underneath tree	Sep-13	unknown	http://wnep.com/2013/09/12/horse-struck-by-lightning-and-killed/
3	canada	side flash; in pasture underneath tree	Sep-13	unknown	http://www.cbc.ca/news/canada/british-columbia/lightning-strike-leaves-3-horses-dead-in-maple-ridge-1.1704715
2	MO	side flash; in pasture underneath tree	Jul-13	unknown	http://mtstandard.com/news/local/updated-lightning-strikes-kills-two-horses-south-of-butte/article_51cbe576-f8e0-11e2-a62b-001a4bcf887a.html
2	AL	Barn was stroke by lightning and caught fire	Sep-13	unknown	http://www.turnto10.com/story/23311955/lightening-strike-barn-kills-2-horses
3	canada	side flash; in pasture underneath tree	Sep-13	unknown	http://www.vancitybuzz.com/2013/09/lightning-strike-kills-3-horses-in-maple-ridge/
1	WY	side flash; in pasture underneath tree	Sep-13	unknown	http://archives.wcexaminer.com/index.php/2013/09/18/lightning-kills-horse-near-hop-bottom/
1	FL	Barn was stroke by lightning and caught fire	Apr-13	unknown	http://www.myfoxorlando.com/story/21893990/2013/04/05/lightning-strike-kills-horse-in-riverview
35	GA	Barn was stroke by lightning and caught fire	Jun-13	unknown	http://www.cbs46.com/story/22654992/several-dozen-horses
1	MS	girl riding	Jul-14	4:30pm	http://www.examiner.com/article/mississippi-woman-along-with-horse-she-was-riding-die-after-lightning-strike
4	MS	pasture underneath tree	Jul-15	unknown	http://www.wmcactionnews5.com/story/29479367/wlbt-exclusive-lightning-strikes-pasture-killing-several-horses
3	NC	pasture underneath tree	Jun-15	afternoon	http://www.wcnc.com/story/news/local/2015/06/30/therapeutic-horses-killed-by-lightning-strike-in-salisbury/29545515/
0	NC	hit barn and caused fire	Jun-15	6:00 PM	http://wncn.com/2015/06/25/trees-down-flooding-hail-as-storms-hit-durham-orange-person-counties/
2	MO	direct hit	Jul-15	12pm	http://www.webstercountycitizen.com/news/article_bea4f626-1f74-11e5-b4d1-4393e90fd35a.html
3	PA	standing underneath a tree while out in pasture	Jun-15	unknown	http://www.tiogapublishing.com/the_wellsboro_mansfield_gazette/news/lightning-kills-horses-damages-water-plant/article_4b7fb30a-14f4-11e5-afb5-73896965a3af.html
1	Southwest Florida	Standing underneath a large oak tree	Aug-16	early evening	http://www.news-press.com/story/life/2016/08/29/lessons-horse-sudden-death-rescue-lightning/89197328/
1	MI	Man and a horse were killed horseback riding outside a barn	Apr-16	unknown	http://www.clarionledger.com/story/news/2016/04/28/lightning-strike-kills-mississippi-man-horse/83640162/
2	OH	Woman was bringing in horses	Aug-16	unknown	http://www.news-journalonline.com/news/20160828/lightning-deaths-surge-upward-in-florida-nation
1	FL	Race horse Brillant Speed was found dead in his paddock after a thunderstorm hit	Jul-16	unknown	http://www.news-journalonline.com/news/20160828/lightning-deaths-surge-upward-in-florida-nation
1	AZ	Horseback riding, while heading home as storm was moving in; the horse was killed but than man survived.	May-16	unknown	http://www.abc15.com/news/region-west-valley/buckeye/rider-hospitalized-horse-killed-after-pair-hit-by-lightning-strike-in-buckeye
1	Mills River, NC	Lightning struck a large tree about 50 feet away from the horse trailer. The bolt traveled across the ground to the horses, killing one and injurying several others that were in a horse trailer hooked up to a truck.	Jul-16	2:00 PM	http://www.blueridgenow.com/news/20160719/lightning-strike-kills-horse-at-turkey-pen-trailhead
1	GA	Lightning hit a tree that horses' were standing nearby	Aug-16	unknown	http://www.wtvm.com/story/32601148/one-horse-recovering-after-two-struck-by-lightning
0	MD	Lightning struck a horse barn that caught fire	Jul-16	5:00 PM	http://baltimore.cbslocal.com/2016/07/24/lightning-strike-suspected-of-sparking-2-alarm-barn-fire/
2	UK	Horses were out in the pasture and found dead in the field	Jul-16	3:00 AM	http://www.horseandhound.co.uk/news/horse-struck-by-lightning-502717
2	Iowa	man and horse he was riding were killed by lightning strike	May-15	evening	http://qctimes.com/news/local/muscatine-man-horse-die-after-lightning-strike/article_dd6f399c-c2f0-5268-adf9-943c45ab4294.html