Although chewing disease, also known as equine nigropallidal encephalomalacis is not common in horses, it is deadly because of the brain lesions created by invasive plants causing the disease.
Knapweed = Irreversible brain damage in horses
Russian knapweed and yellow starthistle contain substances that cause lesions in the brains of horses resulting in irreversible brain damage causing horses to eventually die from starvation or dehydration.
Russian Knapweed and Yellow Starthistle are not similar in appearance, but both plants cause the same deadly disease in horses, and both are commonly found throughout the Northwest and California.
These plants contain substances that cause lesions to develop in the brains of horses that graze on them. The result is irreversible brain damage, and horses that have it will eventually die from starvation or dehydration.
“There is no treatment for chewing disease,” according to Dr. Patricia Talcott, a Washington State University associate professor who provides a diagnostic toxicology service for the Washington Animal Disease Diagnostic Laboratory in Pullman. “Once the lesion is there, it is permanent and euthanasia becomes the most humane option for affected horses.”
Only horses are susceptible to the disease, and to get it, they must eat 50 to 150 percent of their body weight of either plant over a period of one to three months. Thankfully, many horses and other livestock avoid grazing on Russian knapweed and yellow starthistle because of their bitter taste, as well as the physical discomfort that comes from eating the spines on yellow starthistle.
Horses will eat these plants, however, if a pasture or hay is heavily contaminated with them and other forage is minimal. “Exposure to chewing disease is cumulative and chronic, and initial signs can be mild and subtle, such as a horse that begins to have trouble swallowing or loses weight,” Dr. Talcott said.
“Significant changes seem to come on abruptly, however, and that is when most owners realize their horse is sick.” Chewing disease is well recognized, and most veterinarians are able to diagnose horses that have it. Signs that owners may notice include an acute onset of neurologic problems and assymetry to a horse’s face, Dr. Talcott said. Other signs include an inability to swallow food or water or use their lips normally.
Because of this, horses may spend a long time with their muzzle deep in water trying to drink. They may also look depressed, hang their head low, and make involuntary chewing movements. “There is not a large number of cases of chewing disease throughout the Northwest, but it is not rare either,” Dr. Talcott said. “It is totally preventable, so owners should really be aware of what they are feeding their horses.”
Yellow starthistle (Centaurea solstitialis) is a winter annual weed that is widespread throughout Washington, Oregon, and Idaho, frequently appearing on rangeland, pasture, and along crops. According to the Washington State Noxious Weed Control Board, it is widely distributed throughout eastern Washington and has spread as far north as Stevens County. It seems to grow best in deep silt loam on south-facing slopes.
Invasive plant - Deadly to horses
Yellow starthistle is actually a species of knapweed which begins its growth cycle in late fall then emerges in early spring, and bolts up to three feet in height.
Though it is called a thistle, yellow starthistle is actually a species of knapweed. It begins its growth cycle in late fall from seeds that germinate when moisture begins to collect. The plant emerges in early spring, and bolts up to three feet in height during May and June. Its flowers appear mid-summer, and by August, the plant dries and becomes an easily identifiable skeleton that is silvery gray with white cotton-like flowerheads.
The base of the flower, called a bract, contains sharp, needle-like spines that can grow up to two inches long. Each plant may produce as many as 150,000 seeds, which can remain viable in the soil for several years.
In contrast, Russian knapweed (Acroptilon repens) is a long-lived, creeping perennial that spreads mostly underground from vegetative root buds, as well as from seed, with growth characteristics similar to Canada thistle. Russian knapweed emerges in early spring, grows up to three feet tall during May and June, and flowers through the summer and fall. It regenerates by seed or by pushing up new plants from its extensive root system, which can survive for up to 75 years.
As a result, it is a difficult plant to eradicate. The Washington State Noxious Weed Control Board reports that Russian knapweed is found throughout the state, although the weed prefers the heavier, saline soils of bottomlands, and subirrigated slopes and flats.
In eastern Washington, it frequently appears on sites where basin wildrye grows, and competes well in pastures, hayfields, grain fields, and along roads and irrigation ditches. Russian knapweed is only one of several knapweeds that commonly grows throughout the Northwest, including diffuse and spotted knapweed.
Pasture managers may wonder how to distinguish Russian knapweed from other knapweeds. Russian knapweed has dark brown to black roots that extend downward and to the side, compared to diffuse and spotted knapweeds that are short-lived perennials with offwhite taproots similar to a dandelion. Russian knapweed flowers are pink to purple, similar to other knapweeds, but the bract underneath the flower is different.
Russian knapweeds have green or straw-colored bracts with paper-like tips, and produce ivory-colored seeds with feather-like plumes. Spotted knapweed bracts have a vein-like appearance, usually with black spots on the tips, and produce black to brown oval seeds. Diffuse knapweeds have sharp, spiny bracts that can puncture skin if touched, and also have a vein-like appearance. Both diffuse and spotted knapweeds are prolific seed producers, and do not spread underground like Russian knapweed.
“The area these plants are found growing in is spreading,” Dr. Talcott said. “So people should take time to find out what is growing in their pastures and recognize strange looking weeds in their hay.”
For help identifying yellow starthistle and Russian knapweed, horse owners and land managers can contact their local extension service or a weed identification specialist. Extension services usually have a specialist that can help a person identify weeds in a field or pasture, and can examine hay for undesirable plants. They can also recommend practices to control or minimize problem weeds. Controlling or eradicating yellow starthistle and Russian knapweed can be difficult, and may require an integrated approach.
More information about yellow starthistle or Russian knapweed can be found online at the Washington State Noxious Weed Control Board Web site at www. nwcb.wa.gov, or the USDA Natural Resources Conservation Service Plant Database Web site at plants.usda.gov.