What causes ryegrass staggers

Overview

Ryegrass staggers, also known as ryegrass toxicity or paspalum staggers (though this often refers to a similar condition caused by a different fungus on paspalum grass), is a significant concern for livestock owners, particularly those with horses and cattle grazing on pastures containing perennial ryegrass.

This neurological condition is not caused by the ryegrass plant itself, but by a symbiotic fungus, an endophyte, that lives within the plant's tissues. This endophyte, scientifically known as Neotyphodium lolii, produces mycotoxins that are toxic to grazing animals when ingested. The most significant of these toxins in the context of ryegrass staggers is lolitrem B.

When animals consume ryegrass contaminated with the endophyte and its toxins, the lolitrem B interferes with nerve function, leading to a range of neurological symptoms that can be severe and, in some cases, fatal if the animal is not removed from the affected pasture.

What is Perennial Ryegrass and its Endophyte?

Perennial ryegrass (Lolium perenne) is a highly palatable and widely used pasture grass in temperate regions worldwide due to its good yield and nutritional value. However, many varieties are infected with the endophyte fungus Neotyphodium lolii. This symbiotic relationship benefits the grass by producing toxins that deter insects and herbivores, thus protecting the plant. In return, the fungus receives nutrients and shelter within the ryegrass plant.

The endophyte is present throughout the plant, including the leaves, stems, and seeds. The concentration of the endophyte and, consequently, the level of toxin production, can vary significantly depending on environmental factors such as temperature, moisture, and the specific ryegrass cultivar. Older, established pastures are more likely to contain endophyte-infected ryegrass.

The Role of Lolitrem B

Lolitrem B is a complex indole alkaloid that acts as a neurotoxin. Once ingested by an animal, it is absorbed into the bloodstream and travels to the brain. Lolitrem B is known to disrupt the function of the nervous system by interfering with potassium channels in nerve cells. This disruption impairs normal nerve signaling, leading to the characteristic symptoms of ryegrass staggers.

The amount of lolitrem B produced by the endophyte can fluctuate. Factors such as temperature, light intensity, and the plant's physiological state influence toxin levels. Generally, higher temperatures and good growing conditions can lead to increased lolitrem B production.

Symptoms of Ryegrass Staggers

The onset of symptoms can be rapid, often appearing within days of grazing infected ryegrass, especially if the pasture has high levels of endophyte infection and toxin. The severity of symptoms depends on the amount of toxin ingested, the duration of exposure, and the individual animal's susceptibility.

Common clinical signs include:

• Incoordination and Staggering: This is the hallmark symptom, where the animal appears unsteady on its feet, with a wide-based stance and difficulty maintaining balance.
• Tremors: Fine tremors, particularly of the head and neck, are common.
• Head Shaking: Animals may exhibit rapid head shaking or twitching.
• Loss of Appetite: Severe cases can lead to reduced feed intake.
• Hypersensitivity: Animals may become easily startled or agitated.
• Falls: In severe cases, animals may stumble and fall, having difficulty rising.
• Reduced Grazing Ability: The incoordination makes it difficult for animals to graze effectively, potentially leading to weight loss.

In extreme cases, animals may become recumbent (lie down) and unable to stand, which can lead to secondary complications like overheating or predation.

Factors Influencing Ryegrass Staggers

Several factors contribute to the risk and severity of ryegrass staggers:

• Endophyte Presence: The most crucial factor is the presence and viability of the Neotyphodium lolii endophyte in the ryegrass.
• Toxin Levels: The concentration of lolitrem B in the pasture is critical. This can vary seasonally and geographically.
• Pasture Management: Overgrazing can force animals to consume more of the plant, including the endophyte-infected portions. Grazing during periods of high toxin production (often warmer months) increases risk.
• Ryegrass Cultivar: Some perennial ryegrass cultivars are bred to be endophyte-free or have reduced endophyte levels, offering a lower risk.
• Animal Species and Susceptibility: Horses are generally more susceptible than cattle, although both can be affected. Individual animal physiology also plays a role.
• Diet: Animals grazing on a mixed diet with other non-infected forages may be at lower risk than those solely reliant on infected ryegrass.

Diagnosis and Management

Diagnosis is typically made by veterinarians based on the characteristic clinical signs observed in animals grazing ryegrass pastures, coupled with a history of exposure. Confirmation may involve pasture sampling and laboratory analysis to detect the presence of the endophyte and/or measure lolitrem B levels. Blood tests are generally not diagnostic for lolitrem B poisoning.

Management strategies focus on prevention and minimizing exposure:

• Pasture Management: Avoid grazing infected ryegrass during high-risk periods. Implement rotational grazing to prevent overgrazing and allow pastures to recover.
• Pasture Renovation: Replace infected ryegrass with endophyte-free cultivars or other safe forage species.
• Supplementation: Provide alternative feed sources if ryegrass is the primary forage.
• Monitoring: Regularly assess pastures for the presence of endophyte-infected ryegrass and monitor animals for early signs of staggers.
• Animal Removal: If staggers is suspected, immediately remove affected animals from the infected pasture and provide them with safe feed and water.

Recovery can occur once animals are removed from the source of the toxin, but severe cases may require supportive veterinary care. Long-term neurological damage is possible in some animals.