Overview of Paratuberculosis
Paratuberculosis is a chronic, contagious granulomatous enteritis characterized in cattle by persistent diarrhea, progressive weight loss, debilitation, and eventually death. It is considered a listed disease by the OIE, meaning it is a priority disease for international trade. The etiologic agent, Mycobacterium paratuberculosis, also known as Mycobacterium avium subsp paratuberculosis, is believed capable of infecting and causing disease in all other ruminants (eg, sheep, goats, llamas, deer) and in captive and free-ranging wildlife. The infection has also been recognized in omnivores and carnivores such as wild rabbits, foxes, weasels, pigs, and nonhuman primates. Distribution is worldwide. National control programs include those established in Australia, Norway, Iceland, Japan, The Netherlands, Denmark, Ontario, Canada, and the USA. The highest published prevalence is in dairy cattle, with 20%–80% of herds infected in many of the major dairy-producing countries. Limited information is available about the prevalence in other species. The disease is of economic importance for the goat industry in Spain and the sheep industry in Australia.
M paratuberculosis is excreted in large numbers in feces of infected animals and in lower numbers in their colostrum and milk. It is resistant to environmental factors and can survive on pasture for >1 yr; survival in water is longer than in soil. The infection is usually acquired through the fecal-oral route; the dose needed to infect an animal is not known. Introduction of the disease into a noninfected herd is usually through herd expansion or replacement purchases; the infection is introduced via subclinically infected carriers.
Infection is acquired early in life—often soon after birth—but clinical signs rarely develop in cattle <2 yr old, because progression to clinical disease occurs slowly. Resistance to infection increases with age, and cattle exposed as adults are much less likely to become infected. Infection is acquired by ingestion of the organism when nursing on contaminated teats; consumption of milk, solid feed, or water contaminated by the organism; or licking and grooming behavior in a contaminated environment. In the later, bacteremic stages of infection, intrauterine infections can be seen. After ingestion and uptake in the Peyer’s patches of the lower small intestine, this intracellular pathogen infects macrophages in the GI tract and associated lymph nodes. It is possible that some animals may eliminate infection through a cell-mediated immune response that encourages microbiocidal activity in macrophages, but the frequency with which this occurs is unknown. In most cases, the organisms multiply and eventually provoke a chronic granulomatous enteritis that interferes with nutrient uptake and processing, leading to the cachexia typical of advanced infections. This may take months to years to develop and is usually paralleled by a decline in cell-mediated immunity, a rise in serum antibody, and bacteremia with dissemination of the infection beyond the GI tract. Fecal shedding begins before clinical signs are apparent, and animals in this “silent” stage of infection are important sources of transmission.
Paratuberculosis in cattle is characterized by weight loss and diarrhea in the late phases of infection, but infected animals can appear healthy for months to years. In cattle, diarrhea may be constant or intermittent; in sheep, goats, and other ruminants, diarrhea may not be seen. It typically does not contain blood, mucus, or epithelial debris and is passed without tenesmus. Throughout weeks or months, the diarrhea becomes more severe, further weight loss occurs, coat color may fade, and ventral and intermandibular edema may develop due to a protein-losing enteropathy. This leads to low concentrations of total protein and albumin in plasma, although gamma globulin levels are normal. In dairy cattle and goats, milk yield may drop or fail to reach expected levels. Animals are alert, and temperature and appetite are usually normal, although thirst may be increased. The disease is progressive and ultimately terminates in emaciation and death. In infected herds, the mortality rate may be low for a number of years, but as many as 50% of animals may be infected subclinically with associated production losses. The disease in sheep and goats is similar, but diarrhea is not a common feature, and advanced cases may shed wool easily. In cervids (deer and elk), the course of the disease may be more rapid.
A diverse array of pathology may be seen in infected animals, ranging from a complete lack of gross lesions to a thickened and corrugated intestine with enlarged and edematous neighboring lymph nodes. Often, there is no correlation between clinical signs and the severity of lesions. Carcasses may be emaciated, with loss of pericardial and perirenal fat in more advanced, cachectic cases. Intestinal lesions can be mild, but typically the distal small-intestinal wall is diffusely thickened with a nonulcerated mucosa thrown into prominent transverse folds. Lesions may extend proximally and distally to the jejunum and colon. Serosal lymphangitis and enlargement of mesenteric and other regional lymph nodes are usually apparent. Histologically, there is a diffuse granulomatous enteritis characterized by the progressive accumulation of epithelioid macrophages and giant cells in the mucosa and submucosa of the gut. Sparse to myriad acid-fast organisms may be seen within the macrophages. Often, there is no correlation between clinical signs and the severity of lesions. Sheep, goats, and deer sometimes develop foci of caseation with calcification in the intestinal wall and lymph nodes.
There are many commercially available tests for paratuberculosis, each with their own advantages, disadvantages, and appropriate application. The assays focus on detecting the organism in feces or tissue (culture, PCR), on finding evidence of cellular immune response to infection (skin testing, interferon-γ), or on detecting antibody to M paratuberculosis antigens (ELISA). Use of different tests in combination can increase diagnostic sensitivity. Given the biology of the infection and the need to manage it on a herd basis, diagnostic information should be gathered for a group of animals rather than for an individual case. An animal showing signs of disease is more likely to provide diagnostic evidence of the infection (shedding, antibody production) than an animal at the preclinical stage of infection. Necropsy with culture and histopathology on multiple tissues is the gold standard for definitive diagnosis. Ziehl-Neelsen stains of tissue samples for acid-fast bacteria usually reveal abundant mycobacteria in lesions; however, in some cases, a careful search still may not reveal their presence. Acid-fast staining of an impression smear made from the ileum of a cow with typical pathology is a quick, low-cost (albeit insensitive) method to arrive at a preliminary diagnosis. Biopsy of full-thickness sections of ileum and regional lymph nodes for culture and histopathology may provide a definitive diagnosis; however, this approach is usually restricted to particularly valuable animals. M paratuberculosis has been isolated from a wide variety of tissue sites, but the mesenteric and ileocecal lymph nodes, ileum, and liver are most frequently recommended for diagnostic sampling.
Serologic tests are rapid, low-cost methods for antemortem confirmation of a clinical diagnosis; sensitivity is >85% in clinically affected animals. They are also useful to detect infection in clinically normal cattle in the later stages of infection that are shedding large numbers of M paratuberculosis; sensitivity is ~45%. Of the serologic tests, those based on ELISA technology offer the highest sensitivity and specificity and are best used to determine the infection prevalence in a herd. Quantitative use of ELISA to identify animals for selective culling or isolation in herds may be a cost-effective strategy for disease control; higher ELISA values are associated with higher probabilities of infection and higher rates of fecal shedding. Fecal culture is more sensitive and more specific than serology, but the organism grows very slowly (2–4 mo) and the assay is more costly than serology. Pooling of fecal samples (eg, five samples per pool) or culture of manure from farm sites where cattle commingle (environmental sampling) can establish a herd’s infection status at a lower cost, despite some reduction in test sensitivity. Proficiency at isolation of this pathogen varies significantly among laboratories. Use of a laboratory that has passed a proficiency test is recommended. Most strains infecting sheep will not grow on solid media but may be isolated using liquid culture media systems. Genetic probes for an element specific for M paratuberculosis DNA, such as IS900, can be used in conjunction with culture or directly on fecal samples. Commercial PCR kits are as sensitive and specific as fecal culture and much more rapid. Cost of culture and PCR is comparable.
Tests of cell-mediated immunity, such as the intradermal Johnin test, lymphocyte transformation test, and interferon-γ, are used more on a research basis and may be negative in advanced clinical cases. The genome of M paratuberculosis has been described and may provide the basis for new diagnostic approaches.
Tests that have fallen out of favor because of reports of low sensitivity and/or specificity are microscopic examination of Ziehl-Neelsen–stained fecal samples and the IV Johnin test. The complement fixation (CF) test also reportedly is less accurate than other serologic tests. The CF test is still required by many countries for importation of animals, although many of the reagents used in the CF test are made to different specifications in different countries, resulting in a lack of standardization.
No satisfactory treatment is known. Control requires good sanitation and management practices aimed at limiting the exposure of young animals to the organism. Calves, kids, or lambs should be birthed in areas free of manure, removed from the dam immediately after birth in the case of dairy cattle, bottle-fed colostrum that has been pasteurized or obtained from dams that test negative, and then reared segregated as much as possible from adults and their manure until >1 yr old. Use of milk replacer is recommended instead of waste milk unless the milk has been pasteurized. A routine testing program for adults can help focus efforts in controlling the disease. Low-cost tests (eg, ELISA) have the greatest cost benefit for commercial dairy herds that are confirmed infected by culture or PCR. Animals testing positive, particularly heavy shedders or that have strong-positive ELISA results, should be sent to slaughter as soon as economically feasible. Retesting at least annually should be continued until herd tests indicate a low (<5%) infection prevalence. Because intrauterine infection can occur, more aggressive control programs include culling of calves from dams that have or develop signs of the disease. Herd replacements should be obtained from herds believed to be free of the disease, and the replacements themselves should be tested before introduction to the new herd. More general procedures to minimize fecal contamination on the farm can also help, eg, elevating food and water troughs, providing piped water in preference to ponds, and harrowing frequently to disperse feces on pasture. Herd owners should be advised that paratuberculosis control takes at least 5 yr.
The formulation of M paratuberculosis vaccines varies by manufacturer. In many countries, their use is subject to approval by regulatory agencies and may be restricted to heavily infected herds. Vaccination of calves <1 mo old can reduce disease incidence but does not prevent shedding or new cases of infection in the herd. Vaccination thus does not eliminate the need for good management and sanitation. In the goat industry in Spain and Australia, vaccination has increased productive herd life. Cattle inoculated with an inactivated whole-cell, mineral-oil vaccine develop granulomas, one to several inches in diameter, at the site of inoculation (brisket) and may react positively on subsequent tuberculin tests. Accidental self-inoculation can result in severe acute reactions with sloughing and chronic synovitis and tendinitis.
There are conflicting data on the involvement of the causative organism in Crohn disease, a chronic granulomatous enteritis of unknown cause in people. However, M paratuberculosis is consistently detected by PCR in people with Crohn disease. This fact, coupled with its broad host range, including nonhuman primates, indicates that paratuberculosis should be considered a zoonotic risk until the situation is clarified.