Figure 1. Characteristic prolonged hyeprflexion of the hind limb as seen in pasture-induced stringhalt.
Stringhalt is a condition well known to Australian and New Zealand horse owners. However, its prevalence belies how little is known about this debilitating condition. Dr Charlie El-Hage from the University of Melbourne recently produced a review article on this enigmatic condition and his paper was published in Equine Veterinary Education, the official journal of the American Association of Equine Practitioners.
Equestology- Sport Horse Science proudly presents this summary of his work.
Clinical Signs:
Pasture associated stringhalt is most often reported in horses grazing poor quality, drought-affected pastures and, although occasionally asymmetrical in presentation, the condition is invariably bilateral.
Initially described in Australia and New Zealand, this condition has also been reported in Europe, North and South America, and Asia.
Horses with pasture-associated stringhalt have characteristic, though very variable, hyperflexion of both hindlimbs when attempting to move (Figure1.). In mild cases, the gait abnormality might only be apparent when the horse is excited or nervous, when it is turned sharply, or when it is forced to walk backward. The severity of hyperflexion might decrease after the initial few steps, but the gait typically remains abnormal. Clinical signs can often be exacerbated while walking the horse down a slope, after a sudden stop or following hard exercise.
Horses with more severe pasture-associated stringhalt can exhibit a hopping action in which hyperflexion becomes progressively more exaggerated and of increased duration. In some horses, flexion is so extreme that the front of the fetlock contacts the underside of the abdomen and the duration of hyperflexion becomes so prolonged that both hind feet leave the ground, almost at the same time, even while walking. In extreme cases, horses are unable to rise without assistance.
The gait abnormality is often worse in cold weather and appears to be worse in horses that appear agitated.
Diagnosis:
Diagnosis of stringhalt is based on the clinical signs and absence of other neurologic and orthopaedic abnormalities.
A similar condition but one which affects only one hind limb is the trauma induced stringhalt known as ‘unipedal stringhalt.’ The distinction between pasture-associated stringhalt (which invariably affects both hind limbs) and classical unipedal stringhalt is usually obvious on observation and is accompanied by evidence or history of trauma to the hock region.
The gait abnormality in pasture-associated stringhalt can vary considerably between individual horses and, as mentioned, some horses can display changes in their gait that are not typical of pasture-associated stringhalt. Thus, in some cases, pasture-associated stringhalt might be difficult to distinguish from a number of other neuromuscular or musculoskeletal conditions, particularly when the patient is only mildly affected.
The differential diagnoses for pasture-associated stringhalt in less than obvious cases may include:
Shivers,
Acquired equine polyneuropathy (Europe),
Lathyrism (South Asia),
Fibrotic myopathy,
Upward fixation of the patella, and
Conditions affecting the brain or spinal cord.
Hind limb pain or irritation- horses with pain, or even irritating stimuli (such as application of a leg wrap) relating to the distal limb or hoof may produce a transient stringhalt-like gait.
History, careful examination and the presence or absence of known exposure to the below described epidemiological factors will differentiate between these conditions and pasture-associated stringhalt.
Epidemiology:
The presence of Hypochoeris radicata (Figure 2.), which is commonly known as Flatweed, False Dandelion or Catsear, in pastures grazed by horses that develop pasture-associated stringhalt has been consistently reported.
An Australian study reported that 50 of 52 horses diagnosed with pasture-associated stringhalt had grazed on pastures containing a large amount of H. radicata and a more recent report from France identified exposure to H. radicata in 69 of 70 affected horses.
Other weeds, including Taraxacum officinale (Common Dandelion) and Malva parviflora (Marshmallow, Mallow Weed) have also very occasionally been associated with cases of pasture-associated stringhalt.
H. radicata ia a very hardy plant with a long tap root which affords the plant some resistance to drought conditions. This plant is often the only source of green feed in paddocks with otherwise sparse dry forage. However, some horses have also been observed to selectively graze H. radicata.
Multiple horses in the same paddock are usually affected and, occasionally, all exposed animals will develop clinical signs. However, it is not uncommon for only a proportion of horses from the same paddock to develop signs, suggesting that exposure alone is insufficient to cause disease.
Most cases of pasture-associated stringhalt occur in mature full-sized horses, with taller animals appearing to be at greater risk, although the condition has been described occasionally in ponies, younger horses and donkeys.
It is also important to note that the onset of clinical signs can be delayed and the first signs of the disease may not be evident for 1 to 3 weeks after removal from affected paddocks.
Interestingly, cattle and sheep grazing pastures incriminated in pasture-associated stringhalt are not affected. This presumably is because toxins are broken down by rumen microflora.
What Causes Stringhalt?
The neurologic lesions in horses with pasture-associated stringhalt have been entirely restricted to the peripheral nerves to date. Brain tissue and the spinal cord does not appear to be affected.
Figure 2. Hypochoeris radicata commonly known as flatweed or false dandelion. The leaves are serrated, shiny and hairy with a blunt tip. The plant has characteristic yellow daisy like flowers which grow on multiple upright, divided stems.
Although there is a consistent epidemiological association between the presence of H. radicata and the development of pasture-associated stringhalt, attempts to experimentally induce stringhalt have been limited and largely unsuccessful. Injection of a “concentrated extract of H. radicata into laboratory animals” did not induce clinical signs consistent with pasture-associated stringhalt.
Mycotoxins are a well recognised causes of neurological diseases in grazing animals and have frequently been suggested as a cause of pasture-associated stringhalt. However, pathological lesions consistent with known mycotoxicoses have not been detected in cases of stringhalt and so are not implicated in this disease. Similarly, fungal elements have not been found in samples of H. radicata or pasture litter from pastures grazed by affected horses.
There is however evidence to suggest that H. radicata plants produce toxic metabolites following chemical or climate induced stress. This of course is consistent with the climatic conditions that often proceed “outbreaks.”
Treatment and Management
Numerous therapies have been suggested for the treatment of pasture-associated stringhalt and while some of these have been described in the peer-reviewed literature, none have been assessed in large-scale blinded or controlled studies.
Results of routine blood tests from affected horses are typically unremarkable. Low plasma Vitamin E concentrations and mild increases in muscle and liver enzyme activity have been noted in occasional cases of pasture-associated stringhalt but abnormalities are inconsistent, usually mild and unlikely to be of clinical significance.
The reported success of any treatment for pasture-associated stringhalt must, of course, be considered in the context that the vast majority of affected horses will recover spontaneously following removal from implicated pastures without treatment.
Most horses recover over a period of 6–18 months, but mildly affected horses will often recover more quickly. It should however be noted that recovery can be extremely prolonged (requiring over 2 years for complete resolution) in some severe cases.
Severely affected horses may need to be euthanised on humane grounds if they become recumbent for prolonged periods. There are also a very small number of horses that never appear to recover completely, which may be due to severe muscle wastage or fibrosis.
How to Treat Stringhalt:
Treatments have included:
Surgery- myotenectomy of the lateral digital extensor is contentious given the spontaneous recovery of many horses, but one study reported success in 11/13 horses with pasture-associated stringhalt.
Botulinum toxin A infiltration- injected multiple times over a three-week period was observed to have a prolonged clinical improvement in one report that looked at two horses suggesting that further investigation is warranted.
Muscle relaxants- Oral administration of phenytoin at 15mg/kg bodyweight every 12-24hours has been shown to result in a degree of clinical improvement while the drug is administered. The thought is that the mild tranquillisation that often accompanies phenytoin might be responsible for the amelioration of clinical signs. It is important to note that there has been some variation in the pharmacokinetics following both oral and IV administration so plasma concentration monitoring may help improve efficacy.
Miscellaneous:
- Thiamine (Vitamin B1) and Taurine may help with behavioural changes
-Antioxidants: Vitamin E, Vitamin C and DMSO as well as L-tryptophan, magnesium potassium bromide and mycotoxin binders have all been promoted for both the treatment and prevention of pasture-associated stringhalt. However, despite the anecdotal evidence there is little theoretical basis for many of these supplements and no scientific evidence to support the use of any of them.
Future Research:
The definitive identification of a causative toxin and an understanding of the exposure required to cause disease would be a major advance in the management of grazing horses in high-risk pastures. The invitro and laboratory animal studies evaluating extracts from (stressed) H. radicata appear promising and might provide sensible targets for a preventative strategy.
Prevention Is Better Than Cure:
Until a definitive lesion and/or toxic agent are identified, recommendations for the prevention and management of pasture-associated stringhalt must remain empirical.
Pasture-associated stringhalt is well known to occur in certain geographical regions and might even be restricted to specific paddocks. Reducing known risk factors by improving poorer pastures with fertilisers and/or introduced grasses such that H. radicata and other weeds are out-competed and other weed management strategies would seem logical.
Supplementary feeding to reduce reliance on grazing might be a useful strategy especially when there is minimal pasture available and horses cannot be moved to another paddock. [The use of slow grazing hay nets will also occupy a horses grazing instincts and prevent foraging for roots].
[Monitor horses closely for gait abnormalities and changes in demeanour and inspect paddocks regularly for weed growth].
Reference:
El-Hage C. M., Huntington P. J., Mayhew I. G., Slocombe R. F. and Tennent-Brown B. S. (2019) Pasture-associated stringhalt: contemporary appraisal of an enigmatic syndrome. Equine Vet. Educ. 31 154-162