Ivermectin and Silverleaf Nightshade

Ivermectin is approved in horses for the control of: large strongyles (adult) (Strongylus vulgaris, S. edentatus, S. equinus, Triodontophorus spp.), small strongyles, pinworms (adults and 4th stage larva), ascarids (adults), hairworms (adults), large-mouth stomach worms (adults), neck threadworms (microfilaria), bots (oral and gastric stages), lungworms (adults and 4th stage larva), intestinal threadworms (adults) and summer sores (cutaneous 3rd stage larva) secondary to Hebronema or Draschia Spp..

In horses, oral doses of 1.8 mg/kg (9X recommended dose) did not produce signs of toxicity, but doses of 2 mg/kg (10 X recommended dose) caused signs of visual impairment, depression and ataxia.

In the early 1990’s a syndrome affecting horses dosed with the proper dose of ivermectin and having exposure to silverleaf nightshade resulted in horses developing depression, ataxia, occasional evidence of liver involvement, sometimes hepatic encephalopathy, weight loss and at times comma and depression. By piecing together clinical signs of horses and extensive testing on the deceased horses a pattern of an exposure relationship between silverleaf nightshade and ivermectin evolved. The principle supplier of ivermectin when the syndrome was first noticed was contacted by a number of veterinarians and horse owners but the firm did not see the need for a warning label or a study.

In 1997 an investigation was undertaken using rabbits as models, because of a similar digestive physiology as horses. The findings in this investigation supported the theory of a relationship between ivermectin and the silverleaf nightshade This study was published in Garland, T, Bailey, EM, Reagor, JC, Binford, E; Probable Interactions between Solanum eleagnifolium and Ivermectin in Horses, In: Garland, T and Barr, C (eds.) Toxic Plants and Other Natural Toxicants, CAB International, Wallingford, Oxfordshire, England, 1998, p 423-427. The study was purchased by Merck for consideration of future studies in horses.

Horses consuming silverleaf nightshade, either the fresh standing plant, or baled into hay and are being regularly wormed with ivermectin based products have been shown to develop similar disease problems. Some of them may only develop liver disease, including hepatic encephalopathy and weight loss. They have mild neurological signs, such as weakness and ataxia. Horses having the plant removed from their diet, will often recover. Some horses may have less liver involvement and more neurological involvement. Those horses often progress to death or require euthanasia as they may get worse. In such case, brain is the organ of choice for assay and stomach contents are also helpful for identifying leaf fragments of silverleaf nightshade..

Silverleaf Nightshade (Solanum elaeagnifolium) appears to have the ability to alter the characteristics of the blood brain barrier and allow more ivermectin into the brain, where high levels can be fatal. It should be remembered that not all horses are affected and many can and do recover. Some do not, however.

For the past two years Dr. Lynn Post of the Food & Drug Administration, the Center for Veterinary Medicine has sought funding for a study in horses. Currently, a study has not been funded, but the anecdotal evidence continues to grow supporting the hypothesis that some constituent in silver-leaf nightshade promotes brain absorption of ivermectin.

The Texas Veterinary Medical Diagnostic Laboratory offers ivermectin testing on the brains of horses believed to be affected by the interaction of these components.

References:
Garland, T, Bailey, EM, Reagor, JC, Binford, E; Probable Interactions between Solanum eleagnifolium and Ivermectin in Horses, In: Garland, T and Barr, C (eds.) Toxic Plants and Other Natural Toxicants, CAB International, Wallingford, Oxfordshire, England, 1998, p 423-427
Ivermectin Oral Paste 1.87% (18.7 mg/gram) in 6.08 g syringes; Eqvalan® Paste 1.87% (MSD-AgVet), Zimectrin® Paste (Farnam); (OTC) Approved for use in horses (not intended for food purposes).

Ivermectin Liquid 1% (10 mg/ml) in 50 ml and 100 ml btls (for tube administration; not for injection);Eqvalan® Liquid for Horses (MSD-AgVet); (Rx) Approved for use in horses (not intended for food purposes).

Pictures of silverleaf nightshade are below.

Mature plant with berries

Plant with bloom

Close up of bloom