Ovine and Caprine Paratuberculosis (Johne's Disease)

Ovine and caprine paratuberculosis, commonly known as Johne's disease, is a chronic, contagious, granulomatous enteritis of ruminants caused by Mycobacterium avium subspecies paratuberculosis (MAP). The disease is characterized by a prolonged subclinical phase followed by progressive weight loss, diarrhea (less common in sheep and goats than in cattle), and eventual debilitation and death. In sheep and goats, the disease is often insidious, with clinical signs appearing in adult animals (typically 2-4 years of age) after a long incubation period of 1 to 3 years. The infection primarily affects the distal small intestine (ileum) and associated lymph nodes, leading to malabsorption and protein-losing enteropathy. The disease has significant economic impact due to premature culling, reduced milk production, decreased fertility, and increased susceptibility to other diseases. In dairy goat herds, MAP infection can also pose a potential zoonotic concern, as MAP has been controversially linked to Crohn's disease in humans, although a definitive causal relationship has not been established. The disease is endemic in many sheep and goat populations worldwide, and control is challenging due to the lack of effective treatment and the limitations of diagnostic tests in subclinically infected animals.

The causative agent is Mycobacterium avium subspecies paratuberculosis (MAP), a slow-growing, acid-fast, Gram-positive bacterium. MAP is an obligate intracellular pathogen that primarily infects macrophages in the intestinal wall. The organism is highly resistant to environmental degradation and can survive for months in feces, water, and soil, especially in cool, damp conditions. MAP is shed in the feces of infected animals, and transmission occurs primarily via the fecal-oral route, through ingestion of contaminated feed, water, or pasture. In utero transmission can occur, and the organism can also be shed in colostrum and milk, leading to infection of neonates. The bacterium has a long generation time (approximately 22 hours) and requires specific mycobactin for in vitro growth, which complicates culture. Strains of MAP are often classified into 'C' (cattle) and 'S' (sheep) types, with sheep and goats being susceptible to both, but the S type is more common in sheep. The S type is more fastidious and difficult to culture. MAP infection leads to a granulomatous inflammatory response in the intestinal mucosa, with infiltration of macrophages, lymphocytes, and giant cells, resulting in thickening of the intestinal wall and impaired nutrient absorption.

Paratuberculosis affects sheep and goats worldwide, with prevalence varying by region and management system. In sheep, the disease is often seen in intensive or semi-intensive flocks, while in goats, dairy operations are particularly affected. The disease is more common in adult animals, with clinical signs typically appearing between 2 and 4 years of age, although infection occurs in young animals, usually within the first few months of life. The incubation period is long, and many infected animals remain subclinical for years, serving as sources of infection. Morbidity in affected flocks can range from 5% to 20%, with mortality approaching 100% in clinically affected animals if not culled. The disease is more prevalent in flocks with poor biosecurity, high stocking density, and inadequate hygiene. Breed susceptibility may exist, with some breeds showing higher resistance or tolerance, but this is not well-defined. In goats, dairy breeds such as Saanen and Alpine are commonly affected. The disease is often introduced into a flock through the purchase of apparently healthy but infected animals. Once introduced, MAP can persist in the environment for extended periods, making eradication difficult. Economic losses arise from premature culling, reduced milk production, decreased weight gain, and increased veterinary costs. In endemic flocks, the disease can cause significant production losses, and control measures are essential to reduce prevalence.

The pathogenesis of paratuberculosis begins with the ingestion of MAP by young animals, typically within the first few months of life. The bacteria cross the intestinal epithelium via M cells in Peyer's patches and are taken up by macrophages in the underlying lymphoid tissue. In the early stages, the bacteria survive and replicate within macrophages, evading the host immune response. The infection remains subclinical for a prolonged period, during which the cell-mediated immune response (Th1) predominates, containing the infection. As the disease progresses, a shift to a humoral (Th2) immune response occurs, leading to the development of clinical signs. The bacteria disseminate within the intestinal mucosa, causing diffuse granulomatous inflammation, particularly in the ileum, jejunum, and associated lymph nodes. The intestinal wall becomes thickened, with infiltration of macrophages, epithelioid cells, and multinucleated giant cells. This inflammation disrupts the normal absorptive surface, leading to malabsorption of nutrients, protein-losing enteropathy, and impaired intestinal barrier function. The result is progressive weight loss, diarrhea (though less common in sheep and goats than cattle), and hypoproteinemia. The disease also causes a chronic inflammatory state, leading to cachexia and immune dysfunction. In advanced cases, there is extensive fibrosis and mineralization of the intestinal wall. The exact mechanisms of the immune shift are not fully understood, but genetic and environmental factors may play a role. The disease is ultimately fatal, as the intestinal damage is irreversible.

Several factors predispose sheep and goats to paratuberculosis. The most critical is age at exposure; young animals (less than 6 months of age) are highly susceptible, while adults are relatively resistant. Management practices that increase fecal-oral transmission, such as overcrowding, poor sanitation, and contaminated feed and water, significantly increase the risk. The use of pooled colostrum or milk from infected dams can transmit the infection to neonates. In dairy goat operations, the intensive nature and high stocking density facilitate spread. Nutritional stress, such as inadequate nutrition or concurrent parasitic infections, may exacerbate the disease. Genetic susceptibility may play a role, with certain breeds or individuals being more resistant or susceptible. Stress factors, including parturition, lactation, and transport, can trigger clinical disease in subclinically infected animals. The presence of other immunosuppressive diseases, such as caseous lymphadenitis or caprine arthritis encephalitis, may also increase susceptibility. Environmental factors, such as cool, damp climates, favor the survival of MAP in the environment, increasing the risk of transmission. Finally, the introduction of new animals into a flock without adequate quarantine and testing is a major risk factor for introducing the disease.

Clinical signs of paratuberculosis in sheep and goats are often subtle and progressive. The hallmark is chronic, progressive weight loss despite a normal or even increased appetite. Diarrhea is less common in sheep and goats than in cattle, but may occur intermittently, with soft, pasty feces. Affected animals may have a rough, dull hair coat, and reduced milk production in dairy animals. As the disease progresses, animals become emaciated, with muscle wasting, particularly over the back and hindquarters. Submandibular edema (bottle jaw) may develop due to hypoproteinemia, but is less common than in cattle. The animals remain alert and afebrile until the terminal stages. In sheep, the disease may present as a wasting syndrome without diarrhea, and in goats, diarrhea may be more frequent. Clinical signs often appear after a stressful event, such as lambing or kidding, and may be exacerbated by concurrent parasitic infections. In advanced cases, animals become weak, recumbent, and eventually die. The clinical course can range from several weeks to months. Flock-level signs include an increased incidence of culling due to poor condition, reduced fertility, and increased mortality in adult animals. It is important to note that many infected animals are subclinical and show no signs, but can shed bacteria and transmit the disease.

Differential diagnoses for paratuberculosis in sheep and goats include: 1) Parasitic gastroenteritis (e.g., haemonchosis, trichostrongylosis) - causes weight loss, diarrhea, and anemia; fecal egg counts and response to anthelmintics help differentiate. 2) Caseous lymphadenitis (CLA) - causes abscesses in lymph nodes and weight loss; culture of Corynebacterium pseudotuberculosis and ultrasound findings. 3) Caprine arthritis encephalitis (CAE) - causes chronic weight loss, arthritis, and indurative mastitis in goats; serology for CAE virus. 4) Ovine progressive pneumonia (OPP) - causes progressive weight loss and respiratory signs; serology for maedi-visna virus. 5) Malnutrition or starvation - due to poor feed quality or competition; dietary history and body condition scoring. 6) Chronic fascioliasis - causes weight loss, anemia, and submandibular edema; fecal fluke egg detection and liver enzymes. 7) Johne's disease in cattle - similar presentation, but species-specific. 8) Intestinal neoplasia or other chronic enteropathies - rare, but can cause similar signs; histopathology. 9) Copper deficiency - causes poor growth and wool abnormalities; liver copper levels. 10) Chronic salmonellosis - causes diarrhea and weight loss; fecal culture. Definitive diagnosis requires specific testing for MAP, such as PCR or culture, and histopathology of intestinal biopsies or necropsy.

The diagnostic approach for paratuberculosis in sheep and goats involves a stepwise process: 1) Flock history: Assess for chronic weight loss, diarrhea, and premature culling in adult animals. 2) Physical examination: Identify emaciation, poor coat condition, and submandibular edema. 3) Fecal examination: Collect fecal samples from suspect animals for direct smear (acid-fast staining) or PCR for MAP DNA. Fecal culture is the gold standard but is slow (up to 12 weeks) and expensive. 4) Serology: ELISA for antibodies against MAP is available, but sensitivity is low in subclinical cases; it is more useful at the flock level. 5) Molecular testing: PCR on feces or tissues is rapid and specific, but may have variable sensitivity. 6) Necropsy: If animals die, perform a thorough necropsy, examining the ileum and associated lymph nodes for thickening and granulomatous lesions. Collect tissue samples for histopathology (acid-fast staining) and PCR. 7) Histopathology: Characteristic lesions include granulomatous enteritis with acid-fast bacilli in macrophages. 8) Differential diagnosis: Rule out other causes of chronic weight loss, such as parasitism, CLA, CAE, and OPP. 9) Flock-level testing: If the disease is suspected, test a representative sample of adult animals (e.g., 30-50) using fecal PCR and ELISA to estimate prevalence. 10) Control measures: Based on the results, implement a control plan, including culling of positive animals, improved hygiene, and vaccination if available.

Laboratory findings in paratuberculosis are often nonspecific but can support the diagnosis. Hematology may reveal mild anemia and lymphopenia. Serum biochemistry may show decreased total protein and albumin due to protein-losing enteropathy, and elevated globulins due to chronic inflammation. Liver enzymes may be mildly elevated. Fecal examination: Direct fecal smears stained with Ziehl-Neelsen may show acid-fast bacilli, but this is insensitive. Fecal culture on Herrold's egg yolk medium with mycobactin is the gold standard, but takes 8-16 weeks. PCR on fecal samples is rapid and specific, with sensitivity varying from 50-80% in clinical cases. Serology: ELISA for antibodies to MAP is commercially available; sensitivity is low in subclinical cases (15-30%) but higher in clinical cases (70-90%). The absorbed ELISA is more specific. Interferon-gamma release assays (IGRA) are used in research settings. In advanced cases, fecal culture or PCR is often positive. It is important to note that a negative test does not rule out infection, especially in early stages. Therefore, repeated testing over time is recommended. In a flock setting, testing multiple animals is more informative than testing a single animal.

Imaging modalities are not commonly used for the diagnosis of paratuberculosis in sheep and goats, but they can be helpful in certain situations. Ultrasonography may be used to assess the thickness of the intestinal wall, particularly the ileum, which may be thickened in affected animals. Transabdominal ultrasound can also detect mesenteric lymphadenopathy. However, these findings are not specific and can be seen in other chronic enteropathies. Thoracic radiography may be performed to rule out respiratory diseases such as OPP, but is not diagnostic for paratuberculosis. In research settings, advanced imaging such as computed tomography (CT) or magnetic resonance imaging (MRI) may be used to evaluate intestinal changes, but these are not practical in clinical practice. In summary, imaging is of limited value in the diagnosis of paratuberculosis, and the diagnosis relies primarily on laboratory testing and necropsy findings.

Histopathology is a key diagnostic tool for paratuberculosis. On necropsy, the ileum and jejunum may appear thickened, with corrugated mucosa and enlarged mesenteric lymph nodes. Microscopic examination reveals granulomatous enteritis, with infiltration of the lamina propria by macrophages, epithelioid cells, and multinucleated giant cells. Acid-fast staining (Ziehl-Neelsen) demonstrates numerous acid-fast bacilli within macrophages (often referred to as 'clubs' or 'globi'). The lesions are typically diffuse, affecting the entire thickness of the mucosa, and may extend into the submucosa. In sheep, the lesions may be more paucibacillary, with fewer organisms, making diagnosis more challenging. In goats, the lesions are often multibacillary, with abundant organisms. Cytology of intestinal scrapings or lymph node aspirates may also reveal acid-fast bacilli. Immunohistochemistry using anti-MAP antibodies can enhance detection. PCR on formalin-fixed, paraffin-embedded tissues can confirm the diagnosis. It is important to differentiate paratuberculosis from other granulomatous diseases, such as intestinal tuberculosis (caused by Mycobacterium bovis) or fungal infections, which are rare in small ruminants.

There is no effective treatment for paratuberculosis in sheep and goats, and treatment is generally not recommended due to the chronic nature of the disease, the potential for zoonotic concerns, and the risk of antimicrobial resistance. Antibiotics such as rifampicin, isoniazid, and streptomycin have been used experimentally, but they do not eliminate the infection and are not approved for food animals. Supportive care, including nutritional support and treatment of concurrent infections, may temporarily improve the condition of valuable animals, but they remain infected and can shed bacteria. Therefore, the focus is on control and prevention. Control measures include: 1) Culling of clinically affected animals and positive test results. 2) Improving hygiene and sanitation to reduce fecal-oral transmission. 3) Ensuring adequate colostrum intake from negative dams, and avoiding pooled colostrum or milk from potentially infected animals. 4) Implementing a vaccination program, where available. A killed vaccine (Gudair) is available in some countries for sheep and goats, and has been shown to reduce clinical disease and shedding, but it does not prevent infection and can interfere with tuberculosis testing. 5) Maintaining a closed flock or testing new animals before introduction. 6) Pasture management, such as resting pastures for several months to reduce environmental contamination. 7) Regular monitoring and testing of the flock to identify and remove infected animals. In dairy goat herds, milk hygiene is important to prevent potential zoonotic transmission, although the risk is considered low.

The prognosis for individual animals with clinical paratuberculosis is poor, as the disease is progressive and ultimately fatal. Most clinically affected animals will die or be culled within 6 to 12 months of diagnosis. Even with supportive care, the intestinal damage is irreversible, and the animal will continue to lose condition. For subclinically infected animals, the prognosis is variable; some may remain subclinical for years, but they are at risk of developing clinical disease, especially under stress. The prognosis for the flock is guarded, as the disease is difficult to eradicate once established. With rigorous control measures, including culling of positive animals and improved management, the prevalence can be reduced over time, but eradication is challenging. The economic impact can be significant, with losses due to premature culling, reduced production, and increased veterinary costs. Therefore, early detection and implementation of control measures are crucial to minimize the impact on the flock.

Follow-up for paratuberculosis involves both individual animal monitoring and flock-level surveillance. For individual animals that are suspected or confirmed infected, regular monitoring of body condition, weight, and fecal shedding is recommended. However, since there is no treatment, the main goal is to prevent transmission. For the flock, a comprehensive control program should be implemented, including: 1) Regular testing of adult animals (e.g., annually) using fecal PCR and ELISA to identify infected animals. 2) Culling of positive animals, especially those with clinical signs or high shedding. 3) Improving biosecurity to prevent introduction of new infections. 4) Implementing hygiene measures, such as cleaning and disinfecting calving/lambing areas, and providing clean water and feed. 5) Vaccination, if available, as part of a comprehensive control program. 6) Pasture management, such as rotational grazing and resting pastures for 6-12 months to reduce environmental contamination. 7) Monitoring the effectiveness of the control program by tracking the prevalence of infection over time. 8) Educating farm personnel about the disease and control measures. In dairy goat herds, milk hygiene is important, and pasteurization of milk for human consumption is recommended. Regular veterinary involvement is essential to adjust the control program based on the flock's status.

Clinical pearls: 1) Paratuberculosis should be considered in any adult sheep or goat with chronic weight loss and diarrhea, especially if the animal has a good appetite. 2) In sheep, diarrhea may be absent, so weight loss is the primary sign. 3) Submandibular edema (bottle jaw) can occur due to hypoproteinemia, but is less common than in cattle. 4) The disease is often introduced into a flock through purchased animals, so quarantine and testing of new animals is critical. 5) Fecal PCR is the most sensitive test for detecting shedding animals, but may be negative in early stages. 6) ELISA is useful for flock-level screening, but has low sensitivity in subclinical cases. 7) Necropsy and histopathology are the most reliable methods for confirming the diagnosis. 8) Vaccination can reduce clinical disease and shedding, but does not prevent infection. Pitfalls: 1) Misdiagnosing paratuberculosis as parasitic gastroenteritis, as both cause weight loss and diarrhea; always perform fecal egg counts and response to anthelmintics. 2) Relying solely on serology for diagnosis, as many infected animals are seronegative. 3) Failing to consider the disease in young animals, as they can be infected but show no signs. 4) Assuming that a negative fecal PCR rules out infection, as shedding can be intermittent. 5) Treating affected animals with antibiotics, which is ineffective and may contribute to antimicrobial resistance. 6) Neglecting biosecurity measures, leading to continued spread within the flock. 7) Not implementing a comprehensive control program, which is essential for reducing the impact of the disease.

There are no approved drug protocols for the treatment of paratuberculosis in sheep and goats. Antibiotics such as rifampicin (10-20 mg/kg PO q24h), isoniazid (10-20 mg/kg PO q24h), and streptomycin (10-15 mg/kg IM q24h) have been used experimentally, but they are not effective in eliminating the infection and are not recommended due to the risk of antimicrobial resistance and the lack of withdrawal times. Supportive care may include fluid therapy for dehydrated animals, nutritional support with high-quality feed, and treatment of concurrent infections. For example, if a secondary bacterial infection is present, oxytetracycline (10 mg/kg IV or IM q24h) or penicillin G (22,000 IU/kg IM q24h) may be used, but these do not affect MAP. The use of anti-inflammatory drugs, such as flunixin meglumine (1.1-2.2 mg/kg IV or IM q24h), may be considered to improve appetite and reduce inflammation, but they do not alter the course of the disease. It is important to emphasize that treatment is not recommended, and the focus should be on control and prevention. Vaccination with a killed vaccine (Gudair) is available in some countries; the recommended dose is 1 mL subcutaneously in sheep and goats, given to animals over 4 weeks of age, with a booster after 4 weeks, and annual revaccination. The vaccine can cause injection site reactions and may interfere with tuberculosis testing. Withdrawal times for meat and milk must be considered if any drugs are used, but since treatment is not recommended, this is less relevant.

Evidence-based literature on paratuberculosis in sheep and goats is extensive, with key studies focusing on diagnosis, control, and vaccination. A landmark study by Windsor et al. (2005) evaluated the efficacy of the Gudair vaccine in sheep and found a significant reduction in clinical disease and fecal shedding, but not in infection prevalence. Another study by Juste et al. (2009) in dairy goats showed that vaccination reduced the incidence of clinical disease and improved milk production. Diagnostic test performance has been evaluated in multiple studies; a meta-analysis by Nielsen and Toft (2008) found that fecal culture and PCR have high specificity but variable sensitivity, while ELISA has lower sensitivity, especially in subclinical cases. A study by Dhand et al. (2007) in sheep flocks in Australia demonstrated that a combination of testing and culling, along with improved hygiene, could reduce the prevalence of infection over time. The economic impact of paratuberculosis was assessed by Bennett et al. (2010), who estimated significant losses due to premature culling and reduced production. Consensus guidelines from the American Association of Small Ruminant Practitioners (AASRP) and the European College of Small Ruminant Health Management (ECSRHM) recommend a comprehensive control program, including biosecurity, testing, culling, and vaccination where appropriate. The zoonotic potential of MAP remains controversial, but a systematic review by Feller et al. (2007) found no conclusive evidence linking MAP to Crohn's disease. Overall, the literature supports the use of vaccination as a tool to reduce clinical disease, but emphasizes that it should be part of a multifaceted control strategy.