Staggers in Horses: Ryegrass Contamination Poisoning – Toxicology, Treatment & Prevention

Ryegrass is a popular forage choice worldwide, as it is a nutritious source of protein and energy for horses and other grazing animals. There are two types of ryegrass: perennial and annual.

Although both species of ryegrass are good sources of nutrition and are not toxic in and of themselves, both are susceptible to bacterial and fungal infestation. Ingestion of these pathogens can cause poisoning in horses, referred to as ryegrass staggers.

Which form of staggers horses are at risk of depends on which species of ryegrass is available regionally. Cases of annual ryegrass staggers are mostly reported in Australia, with some occurrences in South Africa, while perennial ryegrass staggers affects horses worldwide.

As there is no specific diagnostic test available, veterinarians diagnose staggers based on history of exposure, clinical signs and feed analysis. Once diagnosed, affected horses can only be treated symptomatically, as no antidote is currently available.

Read on to learn everything you need to know about ryegrass staggers so you can ensure your herd’s pasture is in tip-top shape all year round.

Ryegrass for Horses

Lolium spp., commonly known as ryegrass, is a genus of tufted grass and includes a number of different species. Two of these species are associated with staggers in horses: ^[1][2]

• Perennial Ryegrass (Lolium perenne)
• Annual Ryegrass (Lolium rigidum)

Both annual and perennial ryegrasses are frequently found in grazing pastures. Ryegrass is a popular forage choice for multiple reasons, including: ^[3]

• High protein, fiber and carbohydrate content
• High digestibility and palatability
• Low Cost
• Supports soil erosion control

Perennial Ryegrass Distribution

Perennial Ryegrass (Lolium perenne) is a temperate grass native to Europe, the Middle East, and Central Asia. Today, perennial ryegrass is found on every continent, and is commonly cultivated for grazing and fodder.

As well as being nutritious and favored by horses, L. perenne is a useful crop for pasture improvement and soil stabilization. It is the most common ryegrass in pastures throughout South Africa, the United Kingdom, Australia, New Zealand, and North and South America. ^[4]

Ryegrass is a popular forage as both a pasture grass and in hay form. This underscores the importance of limiting the risk of staggers on farms where this species is a substantial part of the horse’s diet.

Annual Ryegrass Distribution

Annual Ryegrass (Lolium rigidum) is a temperate tufted grass, growing up to 3 feet (1 m) in height. This species is less common than L. perenne. It grows in small clusters of South America, western North America, and southern Africa. ^[5]

The majority of L. rigidum populations are in Oceania and south western Europe, which correlates with higher rates of occurrence of annual staggers being reported in Australia compared with the rest of the world. ^[5]

Ryegrass Nutritional Composition

Ryegrass hays and pastures have a nutrient content similar to other cool-season grasses. Like other cool season grasses, ryegrass pasture and hay cut at mid-maturity are good sources of energy, protein, and fiber for horses.

While the table below gives a good picture of the average nutrient content of Ryegrass forages, it’s important to note that many variables such as season, weather, and time of cutting will impact the nutrient content of specific hays and pastures.

The best way to determine the nutrient content of your hay or pasture is to conduct a forage analysis.

Table 1: Nutritional composition (% DM) of ryegrass pasture and hay ^[6]

Physical Characteristics

Both annual and perennial ryegrass are tufted grasses, but they have slight differences in physical presentation.

Table 3: Comparison of Annual and Perennial Ryegrass Phenotypes

Perennial Ryegrass Toxicity

Perennial ryegrass toxicosis is caused by the toxin Lolitrem B, a neurotoxic mycotoxin produced by the Epichloë festucae fungus (previously known as Neotyphodium lolii). This fungus is an endophyte, meaning it has a symbiotic relationship with the plants it uses as a growth substrate. ^[7][8]

E. festucae can infect all parts of the ryegrass plant. ^[7][8] These microorganisms play a crucial role in the ecological survival of this vegetation, providing benefits to the “infected” grasses including: ^[8]

• Increased drought tolerance
• Increased flood tolerance
• Protection against herbivores
• Increased nutrient uptake

E. festucae is transmitted to other plants through the seeds of infected grasses and does not transmit from one mature plant to another. ^[4]

While E. festucae provides biologic benefits to ryegrass, its production of Lolitrem B poses a significant threat to grazing animals, including horses. Lolitrem B acts on the central nervous system (CNS) by: ^[10]

• Interfering with the calcium-activated potassium channels (BK channels)
• Blocking the action of gamma-aminobutyric acid (GABA) receptors. GABA receptors are fundamental neurotransmitters; when blocked by toxic effects of Lolitrem B, neuron firing throughout the body increases, causing an increase in neurological symptoms

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Annual Ryegrass Toxicity

Similar to perennial staggers, annual ryegrass toxicosis is related to toxins secreted by another organism the ryegrass plant is hosting, but in this case the pathogen is bacterial instead of fungal.

The annual ryegrass form of staggers is caused by ingestion of bacterial corynetoxins produced by the Rathayibacter toxicus bacterium, which is found in the seedheads of annual ryegrass. R. toxicus is a gram-positive species that requires a nematode vector to infect plants. ^[1][11]

The most common nematode utilized by the R. toxicus bacteria is Anguina spp. This nematode picks up the bacteria present in the soil and transports it into the developing ryegrass plant. The presence of Anguina nematodes carrying the bacteria causes the formation of abnormal swelling of the plant tissues (galls) around the seed head, replacing the seeds with galls. ^[1][11]

Once in the seed head – and in the presence of favorable environmental conditions – the R. toxicus bacteria proliferate. As the bacteria population grows, it produces corynetoxins: the toxic compound that causes annual ryegrass staggers. ^[1][11]

Once ingested, corynetoxins inhibit an enzyme called glutamate pyruvate transaminase (GPT). Inhibiting this enzyme disrupts a number of cell activities that lead to tissue damage, giving corynetoxins a cumulative effect.

This means horses grazing on small quantities of infected ryegrass over a long period are at similar risk of staggers as those who ingest a single large dose of the toxin. ^[1][11]

Symptoms

Both types of toxicosis present with similar symptoms in horses, including: ^[10][12]

• Muscle tremors and twitching
• Stumbling
• Stiff or spastic gait
• Ataxia (incoordination)
• Opisthotonos (neck spasms)
• Nystagmus (involuntary eye movement)
• Extensor rigidity
• Recumbency
• Collapse
• Heat stress
• Dehydration

Symptoms of both annual and perennial ryegrass staggers tend to worsen after external stimuli such as loud noises, exercise, and other stressors. ^[10][12]

While the two conditions present the same symptoms, the timing of onset varies. Annual ryegrass staggers symptoms occur approximately two to six days after ingestion of the toxin. The onset of perennial ryegrass staggers usually takes place within two to three days from consumption of infested grasses. ^[1][2]

Diagnosis

Veterinary diagnosis of both annual and perennial ryegrass staggers in horses is based on:

• Known history of exposure
• Presence of clinical signs
• Feed analysis

Currently, there is no definitive diagnostic test for staggers. However, veterinary evaluation, combined with the horse’s history and ruling out other possibilities, usually leads to an accurate diagnosis. ^[10]

Feed analysis tests include: ^[10]

• ELISA test for the detection detection of conotoxin (annual ryegrass)
• Microscopic analysis of ryegrass for either the detection of fungus (perennial ryegrass) or galls (annual ryegrass)

Differential Diagnosis

Several other toxins and conditions can cause symptoms similar to those of ryegrass staggers. These include: ^[10]

• Locoism (Astragalus spp.) toxicosis
• White Snakeroot (Eupatorium rugosum) toxicosis
• Encephalopathy
• Paspalum Staggers
• Fescue poisoning
• Ergotism

Post mortem examination

Unfortunately, the mortality rate of annual ryegrass staggers is high, and affected horses often die before diagnosis. In these cases, horses may undergo a post-mortem examination (necropsy) to confirm the cause of death.

Findings on necropsy in cases of staggers may include: ^[1][2]

• Enlarged liver
• Lung edema
• Lung congestion
• Hemorrhages in multiple tissues

Treatment

Treatment options are limited, as no antidote for either form of ryegrass poisoning is currently available. Treatment is focused on minimizing further exposure, general supportive care, and minimizing external stressors.

General care provided to horses diagnosed with either form of staggers may include: ^[1][2]

• High-quality nutrition
• Plentiful water to prevent dehydration
• Rest
• Limited exposure to sunlight and heat
• Anticonvulsants

Prognosis

The prognosis of ryegrass staggers varies depending on the type of toxin involved, the relative size of the dose of toxin ingested, health status of the horse at the time of exposure, and how promptly the horse is removed from exposure and receives veterinary intervention.

Perennial Ryegrass Staggers

Perennial ryegrass staggers carries a good to guarded prognosis. With prompt and appropriate treatment most mildly affected horses recover completely within days once they are removed from infected pasture. ^[1][2]

Horses that develop severe symptoms may require longer to recover. Horses presenting seizures and other severe neurological symptoms are at a higher risk of severely injuring themselves, leading to further health complications. ^[1][2]

Annual Ryegrass Staggers

Annual ryegrass staggers carries a poor prognosis, with most horses dying within 24 hours following the onset of clinical signs.

Horses with the most severe symptoms are more likely to succumb. Horses with milder symptoms can recover eventually with aggressive treatment, but may have long-term or permanent neurological deficits. ^{[1][2][10][12]}

Prevention

Due to the absence of an antidote and limited treatment options available, prevention is the best way of ensuring horses’ well-being and safety.

Since both annual and perennial ryegrasses are good sources of nutrition, robust pasture management and early identification of pasture infections provide the best conditions to limit the risk without compromising pasture quality. ^{[1][2][10][12]}

Prevention of Perennial Ryegrass Staggers

There are a number of strategies to prevent perennial ryegrass staggers, including: ^[1][2]

• Reduce overgrazing: introducing other crops and rotational grazing can limit the amount of infected grass consumed, and minimize toxin concentrations in the infected grass.
• Select endophyte-free seeds: when planting new pastures, purchasing endophyte-free ryegrass seed (seeds stored for at least 18 to 24 months are less likely to contain viable endophytes) is a reliable strategy to reduce the risk of the presence of toxins in pastures.
• Select novel endophyte ryegrass strains: a number of perennial grasses are infected with specific strains of endophytes that are unable to produce Loritem B. These strains are not considered 100% effective, but do increase the safety of ryegrass-containing pastures.

Prevention of Annual Ryegrass Staggers

The risk of annual ryegrass staggers can be reduced by recognizing infected annual ryegrass in pastures early. Identifying infected grass is straightforward and does not require special training or equipment.

Normal seed heads have rounded topped seeds almost as wide as the husks, while infected seed heads have galls half as wide as the husk and a pointed top. ^[1][2]

Summary

Annual and perennial ryegrass are widely used in pastures for their nutritional value, but can pose risks due to fungal or bacterial contamination. Consumption of contaminated grasses can lead to ryegrass staggers in horses.

• Perennial ryegrass (Lolium perenne) is found worldwide, while annual ryegrass (Lolium rigidum) is predominantly found in Australian pastures.
• Perennial ryegrass staggers result from Lolitrem B toxin produced by Epichloë fungus, while the Rathayibacter toxicus bacteria cause annual ryegrass staggers.
• Both conditions share symptoms, including uncoordinated stiff or spastic gait, recumbency, and muscle tremors; annual ryegrass staggers has a higher mortality rate.
• No specific diagnostic test is available. Diagnosing involves discussing the horse’s potential history of exposure, clinical signs, and feed analysis.
• Treatment focuses on supportive care.
• Horses diagnosed with annual ryegrass staggers have a poor prognosis, while those with perennial ryegrass staggers have a guarded outlook.
• Key prevention measures include pasture management, using endophyte-free seeds, and early identification of infected pastures to minimize exposure risks