What causes ojd

What is Ovine Johne's Disease (OJD)?

Ovine Johne's Disease (OJD), also known as Paratuberculosis or Johne's disease in sheep, is a serious and often fatal gastrointestinal disorder affecting sheep and other ruminant animals like cattle, goats, and deer. It is a chronic infectious disease characterized by progressive weight loss, diarrhea, and thickening of the intestinal wall. The disease is caused by a specific type of bacteria, *Mycobacterium avium* subspecies *paratuberculosis* (MAP). This bacterium invades the intestinal lining, specifically the ileum (the final section of the small intestine), and the associated lymph nodes. The infection triggers an inflammatory response that impairs the animal's ability to absorb essential nutrients from its feed, even if the animal is consuming adequate amounts of food. This malabsorption is the root cause of the progressive emaciation and other clinical signs associated with OJD.

What Causes OJD?

The sole cause of Ovine Johne's Disease is infection with the bacterium *Mycobacterium avium* subspecies *paratuberculosis* (MAP). This bacterium is shed in the feces of infected animals, making it highly contagious through fecal-oral transmission. Animals usually become infected by ingesting MAP bacteria from contaminated environments, such as pastures, water sources, or feed that has been exposed to feces from infected animals. Young animals, particularly lambs, are most susceptible to infection, typically acquiring it within the first few months of life. However, the incubation period for OJD is notoriously long, often spanning several years (typically 2-5 years, but sometimes longer). This means that animals can be infected as juveniles, harbor the bacteria without showing any outward signs of illness, and only develop clinical symptoms as they mature.

Transmission of MAP Bacteria

The primary mode of transmission for MAP is fecal-oral. Infected animals excrete MAP bacteria in their feces, even before they show clinical signs of the disease. This shedding can be intermittent or continuous, depending on the stage of infection. When susceptible animals, especially young ones, ingest these bacteria from contaminated environments, they become infected. Common sources of contamination include:

• Contaminated Pastures: Feces deposited on pastures can remain a source of infection for extended periods, especially in cool, moist conditions.
• Contaminated Water Sources: Runoff from infected pastures or direct contamination of water troughs can spread the bacteria.
• Contaminated Feed: Feedstuffs that have come into contact with feces, either during harvesting or storage, can transmit the infection.
• Vertical Transmission: While less common than horizontal transmission, there is some evidence suggesting MAP can be transmitted from an infected mother to her offspring during gestation or lactation, though this route is considered minor.
• Environmental Persistence: MAP is a hardy bacterium that can survive in the environment for a significant time, particularly in soil and manure.

Pathogenesis of OJD

Once ingested, MAP bacteria enter the intestinal lining, primarily in the ileum. They are taken up by macrophages, a type of immune cell, which normally engulf and destroy foreign invaders. However, MAP has evolved mechanisms to survive within these macrophages. The bacteria multiply within the macrophages and spread to the mesenteric lymph nodes, which drain the intestines. This leads to a chronic inflammatory response in the intestinal wall, characterized by granulomatous inflammation. The inflammation disrupts the normal structure and function of the intestinal villi, the finger-like projections that increase the surface area for nutrient absorption. As the disease progresses, the villi become thickened, flattened, and damaged, severely impairing the animal's ability to absorb fats, proteins, vitamins, and minerals from its diet. This malabsorption leads to the characteristic progressive wasting, despite the animal continuing to eat.

Clinical Signs and Diagnosis

The clinical signs of OJD typically appear in adult sheep, usually between 2 and 5 years of age, due to the long incubation period. The most prominent sign is chronic, progressive weight loss, often referred to as "emaciation." Infected sheep become increasingly thin and weak, even though they may appear to be eating normally or even have a good appetite. Another hallmark sign is persistent diarrhea, which is often watery and profuse, leading to "dags" or "scours" around the hindquarters. Affected animals may also experience a decrease in wool quality and quantity, and in severe cases, edema (swelling) of the jaw or brisket can occur due to low protein levels in the blood (hypoproteinemia).

Diagnosing OJD can be challenging due to the long incubation period and the fact that clinical signs can be subtle initially. Definitive diagnosis typically relies on laboratory testing, which can include:

• Fecal Culture: Detecting MAP bacteria in fecal samples. This test is specific but can be slow (up to 12 weeks) and may not detect low-level shedders.
• Stool PCR: A faster method to detect MAP DNA in feces.
• Blood Tests (ELISA): Detecting antibodies against MAP in the blood. This can indicate exposure but doesn't confirm active infection or shedding.
• Post-mortem Examination: Examining intestinal tissues (ileum and lymph nodes) for characteristic lesions and performing histopathology or bacterial culture.

Economic Impact

OJD poses a significant economic threat to the sheep industry worldwide. The disease leads to substantial financial losses through:

• Reduced Productivity: Infected animals produce less wool and have lower lambing rates.
• Mortality: Infected animals eventually die from the disease.
• Culling: Animals suspected of being infected may be culled prematurely.
• Trade Restrictions: Infected flocks or regions may face restrictions on animal movement and trade.
• Management Costs: Implementing control programs, testing, and biosecurity measures incur additional costs.

Effective management and control strategies are crucial for mitigating the impact of OJD on sheep farming operations.