Disturbed soils, waste areas, pastures and lawns in most States
native to Europe and now present in many areas of North America. It has been reported as problem to horses in California, Washington State and North Carolina
Unknown. There are likely multiple factors involved. Feedng the plant experimentally to horses has reproduced the disease. Horses apparently must graze the plants for extended periods before signs develop. Signs generally appear in late summer.
Other plants have been associated with similar 'stringhalt' lameness including dandelions and Malva parviflora (common mallow)
Flatweed is commonly mistaken for the common dandelion as it has mutiple basally clustered leaves that are irregular in shape with multiple lobes. The leaves can be up to 12 inches in length and are hairy. Unlike the dandelion, Flatweed has multiple branching flowers on stems up to 18-24 inches in height, each with a single, yellow, dandelion-like flower. Seeds are tipped with a circle of bristles that aid in wind distribution.
Horses grazing on flatweed have been reported to develop hindleg lameness that resembles stringhalt. Hypermetria and hyperflexion resembling stringhalt result in a high stepping gait of the hind legs. In Australia it has been referred to as 'Australian stringhalt'. The affected horses develop unilateral or bilateral excessive flexion of the stifle and hock joints that causes a characteristic 'stringhalt' gait and eventual recumbency. Less severely affected animals may drag the hind hooves and show ataxia. Chronic cases show muscle atrophy.
Removing the affected horse from the flatweed generally results in the animal's recovery over a period of months. Treatment with phenytoin and thiamine have been reported to have some beneficial results.
Segental demyelination of nerves of the hindlegs causing a periheral neuropathy and neurogenic muscle degeneration and atrophy