Lawsonia intracellularis is an obligate intracellular pathogen. This means the bacterium can only multiply and survive inside the cells of a host organism, rather than growing freely in the environment or on standard laboratory media. It is a gram-negative, curved rod-shaped bacterium. Lawsonia intracellularis is found globally, contributing significantly to intestinal disease in a range of animal species.
Proliferative Enteropathy in Animals
Lawsonia intracellularis is the causative agent of Proliferative Enteropathy (PE), a widespread intestinal disease affecting various animal species. The bacteria invade and replicate within the enterocytes, which are the epithelial cells lining the intestine, particularly in the ileum, jejunum, cecum, and proximal colon.
The infection triggers uncontrolled proliferation of these intestinal cells, leading to a noticeable thickening of the intestinal mucosa. This overgrowth disrupts the normal function of the intestinal lining, impairing nutrient absorption and water balance. The thickened, often corrugated, intestinal wall is a hallmark lesion of PE, sometimes described as a “garden-hose gut” in severe chronic cases.
Clinical signs of PE vary depending on the affected animal species and the severity of the infection. In pigs, a common and economically significant host, chronic forms often manifest in growing pigs between 6 and 20 weeks of age, presenting as sporadic, moderate diarrhea, reduced appetite, weight loss, and uneven growth rates. While mortality in these chronic cases is generally low, there is a significant reduction in average daily weight gain.
An acute, more severe form, known as proliferative hemorrhagic enteropathy, typically affects older pigs. This acute form can lead to sudden onset of bloody diarrhea, often appearing black and tarry, along with weakness and pallor. Mortality rates can be high in these acute cases, sometimes affecting up to half of clinically ill animals.
Horses, particularly weanling foals, also experience PE, referred to as Equine Proliferative Enteropathy (EPE). Affected foals may display nonspecific signs like depression, lethargy, reduced appetite, weight loss, fever, and colic. Peripheral edema is also a common finding, often secondary to low protein levels in the blood.
Hamsters are another species susceptible to Lawsonia intracellularis, where the disease is commonly known as “wet-tail.” Similar to pigs, hamsters experience diarrhea and weight loss, accompanied by significant hyperplasia, or overgrowth, of the ileal segment of the small intestine. The impact of PE on animal health and productivity, particularly in swine and equine industries, can be substantial due to reduced growth, increased treatment costs, and potential mortalities.
Transmission and Detection of Lawsonia
Lawsonia intracellularis primarily spreads through the fecal-oral route, as animals become infected by ingesting contaminated feces. The bacteria are shed in the feces of infected animals, even those not showing overt clinical signs, making disease detection and prevention challenging.
The bacterium can survive for approximately one to two weeks in the environment at temperatures between 5°C and 15°C. This environmental persistence contributes to the widespread nature of the infection, as contaminated pens, equipment, and even wildlife or rodents can act as sources of transmission.
Detecting Lawsonia intracellularis infections involves several diagnostic methods. Polymerase Chain Reaction (PCR) is a widely used molecular method that detects the bacterium’s DNA in fecal samples or tissue specimens. Real-time quantitative PCR (qPCR) assays, targeting specific genes like 16S ribosomal DNA or aspA, are highly sensitive and specific for detecting Lawsonia intracellularis nucleic acid.
Fecal shedding of Lawsonia intracellularis DNA can be detected by PCR in foals around 12-18 days post-infection and may persist for 7-21 days. In pigs, shedding can begin about seven days after infection and continue intermittently for up to 12 weeks.
Serology is another diagnostic approach used to detect antibodies against Lawsonia intracellularis. Enzyme-linked immunosorbent assay (ELISA) tests are commonly employed for monitoring the prevalence of infection in animal populations. While serology indicates exposure to the bacterium, it does not necessarily confirm an active infection.
Histopathology, the microscopic examination of tissue samples, is often performed in post-mortem cases. This method allows for the visualization of the characteristic proliferative lesions in the intestinal lining and the presence of the curved, intracellular bacteria within the affected enterocytes. This direct observation in tissue provides a definitive diagnosis of proliferative enteropathy.
Treating and Preventing Lawsonia Infections
Treating Lawsonia intracellularis infections typically involves the administration of specific antibiotics. Common choices for antimicrobial treatment include tetracyclines, such as oxytetracycline, doxycycline, and minocycline, or chloramphenicol. The selection of the antibiotic considers the animal’s age and potential risks of gastrointestinal or renal toxicity.
Early intervention with antibiotics is important for successful outcomes, though prompt isolation and susceptibility testing can be challenging due to the bacterium’s intracellular nature. While macrolides are effective against Lawsonia intracellularis, their use in older foals and adult horses is associated with an increased risk of severe colitis.
Preventative strategies are important in controlling Lawsonia intracellularis infections. Vaccination is a primary preventative measure, with both live attenuated and inactivated bacterin-based vaccines commercially available for pigs. These vaccines aim to improve growth performance and reduce bacterial shedding in feces.
Biosecurity measures are also implemented to limit the spread of the bacterium. This includes strict control over animal movement, careful sanitation practices, and quarantine procedures for new animals. Prompt removal of feces and thorough cleaning and disinfection of animal housing after each group of animals can reduce environmental contamination.
Limiting exposure to wildlife and rodents, which can act as reservoirs for the bacteria. Maintaining strict “all-in-all-out” routines in animal housing further helps to break the chain of infection.