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Herd Health and Management
Paratuberculosis (Johne's Disease) in Sheep and Goats
By S.M. Stehman and W.P. Shulaw2
Nov 5, 2002, 11:28am
United States Animal Health Association
1996Proceedings Committee on Sheep and Goats
Paratuberculosis (Johne's Disease) in Sheep and Goats
Recommendations for Diagnosis and Control
Report to the Sheep and Goat Committee
of the U.S. Animal Health Association
S.M. Stehman1 and W.P. Shulaw2
1 Diagnostic Laboratory, College of Veterinary Medicine, Cornell University, Ithaca, NY
2 Veterinary Extension, College of Veterinary Medicine, The Ohio State University, Columbus, OH
While many features of infection caused by M. paratuberculosis are similar across ruminant species, the clinical presentation, and the availability, performance, and interpretation of diagnostic tests differs between cattle and other ruminants such as sheep, goats, and farmed deer (30). Variations in Mycobacterium paratuberculosis strains, isolated from different host species, may contribute to some of the differences (7,11). Host variation in response to infection contribute to others. Because of these variations, diagnostic tests validated in cattle populations are not necessarily accurate in small ruminant populations and should be considered experimental until full validation is completed.
Clinical Signs and Epidemiology
The age of onset of clinical disease tends to be younger in sheep, goats, llamas, and deer than in cattle. Unlike cattle, which typically present with chronic diarrhea with weight loss, sheep, and goats present with chronic weight loss as the primary clinical sign of Johne's disease. Only 10-20 percent of clinical cases present with diarrhea or clumping of feces in the end stages of the disease. Hypoproteinemia with intermandibular edema and wool break and poor fleece condition have been reported in paratuberculous sheep (5,10). Anorexia, depression or clumping of feces may be present in the end stages of the disease in goats (16); however, diarrhea was an infrequent finding. Disseminated infections have been documented in llamas, deer, goats(9) (including mammary gland), and sheep (33). Infection is transmitted primarily by feces from infected animals. In utero and milk or colostral transmission has been reported in cattle and should be considered a possibility in other ruminants with disseminated infection.
Paratuberculosis must be differentiated from other causes of chronic weight loss including dental disease, parasitism, internal abscesses due to Corynebacterium pseudotuberculosis, caprine arthritis encephalitis virus (CAEV) or ovine progressive pneumonia virus (OPPV) infections in goats and sheep, respectively, and malnutrition (inadequate macro nutrients, energy and protein and/or micro nutrients, such as cobalt or copper).
Few large-prevalence-scale studies have been done; however, paratuberculosis can be an important infection in small ruminants in certain regions.
Investigations into prevalence in sheep and goats have been reported from Europe, Iceland, Britain, South Africa, New Zealand and Australia, where these industries are relatively more important than in North America. A Spanish study (2) found that 31.4 percent of flocks (n=226) and 2-5 percent of sheep (n=4504) tested by ELISA and AGID were positive for paratuberculosis. In this study, an association was found between ELISA seropositivity and decreased milk production in dairy sheep in sheep with subclinical paratuberculosis. Clinical paratuberculosis can be a significant cause of culling within individual flocks (10).
In Norway the infection rate in heavily infected goat herds approached 50 percent; annual slaughter surveys detected paratuberculosis in 4.7 percent to 9 percent of goats tested over a 15-year period (25). In North America, West reported a 3-percent prevalence of fecal-culture-positive goats in a survey of 238 goats on six farms in Central California (35). Paratuberculosis infection was identified in two of six herds tested by fecal culture; prevalence within the infected herds was 6 and 10 percent. The age of infected animals ranged from 1 to 5 years with highest infection rates detected in older animals.
Gross lesions associated with clinical Johne's disease in sheep include variable thickening of the distal small intestine, enlargement and edema of the associated lymph nodes, occasional loss of mesenteric fat, and enlarged mesenteric lymphatics in severe cases (5,6,24,26). In sheep with clinical disease, the gross intestinal lesions varied from mild thickening (36 percent) to severe corrugation of the mucosa (48 percent); the ileum was the most frequently and most severely affected site (5). Caseation and necrosis were not observed in sheep in a New Zealand study (5) and were found only infrequently in Shulaw's study (26) in the United States.
Gross lesions in goats are also variable. The most commonly reported lesions are thickening of the distal small intestine and enlargement of mesenteric lymph nodes (16,34). Corrugation of the ileal mucosa was observed in approximately 38 percent of cases in one study (16). Mesenteric and ileocecal lymph node enlargement with edema were relatively common lesions; occasional caseous necrosis and mineralization were also noted(14,34).
Histological lesions of two types were noted in sheep with clinical disease (5,6,23,26). The majority of sheep had extensive epithelioid cell infiltration into mucosa and submucosa with numerous acid-fast organisms. Fewer sheep had localized lymphocytic infiltration with few epithelioid cells and acid-fast organisms observed. Similar lesion types have been reported in goats (14,16,34). Granulomatous enteritis and lymphadenitis occur in more severely affected cases with infiltration of the submucosa and lymph nodes by large epithelioid and multinucleate giant cells containing acid-fast organisms.
Mesenteric and ileocecal lymph node changes included sinus histiocytosis, infiltration of epithelioid cells containing acid-fast organisms, occasional multinucleate giant cells and foci of caseous necrosis (26). Foci of epithelioid cells and periportal lymphocytic infiltration were changes noted in the livers of paratuberculosis infected sheep (7).
Granulomatous lesions were also noted in the liver and lungs of goats with advanced paratuberculosis (14,34).
The chronic, spectral nature of this infection has hampered development of a single, diagnostic test. Infected animals often shed organisms months prior to detectable immunologic responses to the disease. Both fecal shedding and the immune response to M. paratuberculosis can be intermittent. The existing immunologic and culture methods correlate better and more accurately predict disease during clinical stages when most animals shed large numbers of organisms than during sub-clinical stages when fecal shedding occurs less frequently and at lower levels. Current tests are more predictive of disease at the herd level than at the individual animal level.
Culture: M. paratuberculosis is found in clumps within macrophages in the intestinal lamina propria, intestinal lymph nodes, and in feces of infected ruminants. Sheep isolates have been extremely difficult to isolate using culture media (Herrold's egg yolk medium) and techniques routinely used to isolate bovine, goat and deer strains (5,7,8,11,26). In sheep flocks, the fecal culture detects less than 12 percent of clinical cases (8) and requires up to 12 months of incubation, making it an impractical diagnostic test. Paratuberculosis diagnosis in sheep is based upon compatible clinical signs and typical gross or histopathologic lesions with acid-fast organisms. The best tissues for diagnosis are the ileocecal junction and ileocecal and caudal mesenteric lymph nodes. Acid- fast smears of feces will identify approximately 33 percent of clinically affected sheep and 57 percent of sheep with histopathological lesions (8); but this provides only a presumptive diagnosis of mycobacteriosis. The PCR test, developed for detection of M. paratuberculosis in bovine feces, detected 64 percent of sheep with histopathologic lesions of paratuberculosis (8). Unlike the situation in cattle, the sensitivity of the PCR test in sheep was much greater than fecal culture(8).
Recent studies have reported improved isolation of sheep strains using Lowenstein-Jensen without pyruvate (LJP) (1,20) and Middlebrook 7H11/OADC media1. Colonies on Lowenstein-Jensen medium were difficult to visualize due to their small size and required up to 40 weeks to grow. Fecal and tissue samples from sheep confirmed to be infected by paratuberculosis were on average 54-percent positive by culture using LJP (20). The culture technique described by Aduriz (1) using Middlebrook 7H11 media resulted in visible colonies and was slightly improved by not supplementing the medium with mycobactin J. All isolates were confirmed to be M. paratuberculosis with polymerase chain reaction (PCR) using insertion sequence IS900.
Paratuberculosis was confirmed by histopathology or culture in 32/35 sheep showing clinical signs of disease from flocks with known infection(1). M. paratuberculosis was isolated within 8 to 16 weeks of culture from 25 of 32 (78 percent) sheep with confirmed infection; samples from three sheep were contaminated (9 percent). Fecal culture was less sensitive (70 percent) than combined cultures of feces, ileum and ileo-cecal lymph node (100 percent). By comparison, in this study, AGID and ELISA tests detected infection in 93.8 and 96.4 percent of infected animals respectively. While these are promising results, this technique has not been widely adapted for diagnostic use.
In goat herds, fecal culture detected 76-86 percent of goats with confirmed, clinical paratuberculosis (16,34). To the author's knowledge, no studies have reported the sensitivity of fecal culture for detection of subclinical paratuberculosis. Fecal culture will readily detect subclinically and clinically infected deer unless they are infected by the sheep type strain of M. paratuberculosis. When compared with culture of multiple tissues and histopathology as "gold standards", fecal culture detected 45-91 percent of subclinically and clinically infected fallow deer respectively (29).
Serological Tests in Sheep
Serologic tests used to screen for paratuberculosis in small ruminants include the agar-gel immunodiffusion (AGID), complement fixation (CF) tests, and enzyme-linked immunoassay (ELISA). Cell mediated immunity and fecal shedding typically precede humoral immunity; thus, serological assays detect animals in the later stages of infection. The AGID and CF tests are the most commonly used tests. Various ELISA methods have been used in sheep and goats; however, full validation in paratuberculosis-free and infected herds and flocks with high and low prevalence is currently lacking, making interpretation of the ELISA difficult.
Specificity is best measured in populations free of paratuberculosis. Using sera from SPF sheep and from a flock with no history of paratuberculosis, Hilbink18 reported specificities of 97.5 to 100 percent for the AGID test, 96 to 100 percent for the CF test, and 91 to 100 percent for the two ELISA tests. All positive reactions were considered to be weak positives in this group. However, ELISA and CF tests detect cross reacting antibodies to Corynebacterium pseudotuberculosis (3,12,18,24) resulting in reduced specificity in flocks infected with caseous lymphadenitis (CL). Specificity was 88-98 percent in groups of paratuberculosis-free sheep without CL and 64 percent in sheep with CL in a Canadian experiment32. Depending on the antigen used in ELISA tests, cross reacting antibodies to CL persisted from 45 to 90 days (18). A declining ELISA titer with repeated testing over time may suggest cross reacting antibodies rather than true infection. The AGID test for paratuberculosis does not appear to detect antibodies to CL (3,12,18,26). In sheep with CL, but without paratuberculosis, use of the AGID test on sera with high ELISA values increased specificity to 95 percent (32).
Determining test sensitivity for paratuberculosis is difficult due to our inability to diagnose early infection by any means. Reported test sensitivity varies and depends on the stages of disease represented within the herd or flock and the antigens used in the serologic test (3,18,32). The sensitivity of the AGID, CF and ELISA has been reported to be 87-100 percent for sheep showing clinical signs (18). However, the sensitivity of these tests may be less than 20 percent in individuals with subclinical disease with varying severity of lesions (18). In flocks with low prevalence of disease, the sensitivity of the ELISA was estimated to be 48 percent (12). In three studies, the AGID test sensitivity equaled or exceeded ELISA test sensitivity in subclinical and clinical sheep (12,18,32).
The AGID test appears to be the most specific serologic test with good sensitivity (80 percent or greater) for detection of individual animals in the later stages of disease (2,3,13,18,26). This test can be used to screen animals over 1 year of age to identify individuals most likely shedding organisms to herd mates and offspring. AGID test positive animals do not revert to negative status (2) and are usually culled from the flock within a year (2). Incomplete correlation between seropositivity by ELISA and AGID has been reported, suggesting that, in some animals, the two tests may be detecting different populations of antibody (3,26,31,32). Enzyme linked immunosorbent assays (ELISA) have been used in North America (12,31,32) and in New Zealand (14) on a research basis. At this time, we can only recommend the ELISA as a herd-screening tool. The adult herd's ELISA profile, i.e., frequency distribution of ELISA values, could be used to estimate the risk and possibly level of infection in the flock; but more research is needed to resolve issues of poor sensitivity in subclinical animals and more importantly, specificity issues.
Serological Tests In Goats
The AGID test has been reported to be as sensitive as fecal culture in goats (86 percent) (27,34); however, not all fecal-culture-positive animals were positive by AGID (16,27,34); nor were all AGID-positive goats consistently fecal-culture positive (27). The greatest sensitivity is achieved using both tests. A more economical testing option may be to screen goats 1 year of age or older with the AGID test followed by fecal cultures of the seronegative goats. In goats, the CF test has a reported sensitivity of 18 percent (34) to 80 percent (14) and often gives anticomplementary reactions due to non-specific consumption of complement resulting in neither a positive nor negative test.
A modified commercial ELISA test has been evaluated in goats with an apparent test sensitivity of 54 percent (4). Nineteen of 35 goats (54 percent) that had IS900 probe-positive feces also had antibody; 18 of 128 (14 percent) probe-negative goats in the same infected herd were antibody positive. In the population tested, specificity was 100 percent (0/128 goats were antibody positive) in a herd clinically free of paratuberculosis (4). Molina (22) reported on use of an ELISA in goat populations with different prevalences of paratuberculosis in Spain. He reported a sensitivity of 88 percent in a group of goats histopathologically and culture positive for paratuberculosis; specificity was 94 percent in a group of healthy culture-negative goats. The accuracy of this ELISA was high (90 percent) in herds with a high incidence of disease. In herds with low incidence of paratuberculosis, the predictive value of a positive test was less than 50 percent, requiring confirmatory culture in animals with high ELISA values. However, the predictive value of a negative test was high in low prevalence herds. Milner (21) reported the sensitivity of the absorbed ELISA (87 percent) to be higher than the AGID test (73 percent) in a group of goats with clinical disease. However, no confidence limits were presented so this finding may not be real. Test performance appears to be associated with the prevalence and severity of lesions (14). Caseous lymphadenitis, which causes decreased specificity of the paratuberculosis ELISA in sheep, is also common in goat herds. No reports have addressed specificity of the ELISA in goat herds with caseous lymphadenitis (CL) infection. There are no fully validated and licensed ELISA tests for paratuberculosis in sheep or goats in North America.
Assessing Infection in the herd or flock
The severity of infection or degree of spread within a herd or flock depends on management conditions, number of animals infected and duration of infection in the group. Because of the long incubation period, many animals in the group may be exposed or subclinically infected by the time each clinical case becomes apparent. This has been referred to as the iceberg effect of Johne's disease. The clinical case is often the tip of the iceberg in the herd or flock of animals. For each clinical case, up to 15 to 25 animals may be subclinically infected on the farm. The ratio of clinical to subclinical cases depends on several important factors:
1. How long has Johne's disease been present or suspected on the farm?
2. In that period, how many cases have been identified or suspected?
3. What was the age at onset of the first and subsequent cases?
4. Was the first clinical case home-reared or purchased?
5. How much contact did the young stock on the farm have with the clinical case, with other suspect or sick animals, or with adult manure?
6. What is the level of manure management and sanitation on the farm?
7. How open is the flock to animal movement from sources of unknown infection status?
Some understanding of disease prevalence in a herd is necessary for appropriate interpretation of serologic test results. Predictive value of a positive test will always be low in low-prevalence herds unless test specificity is 100 percent (fecal culture or IS900 probe). Testing has been used as a tool to evaluate the extent of infection on the farm, to identify infected individuals, to determine the level of intensity required of a control program and to monitor success of the program. However, because of the insensitivity of our current diagnostic tests, test and cull programs based on serology alone will not control the infection. In goat herds, whole herd fecal culture is more valuable in aggressive testing schemes because animals shedding the organism can be detected and isolated or culled. In sheep flocks where fecal culture is not an option, AGID testing will detect the most heavily infected animals in a flock. If only a subset of the flock is tested, animals with body condition score of less than 2 may provide the most information about Johne's status in the flock.
The frequency of testing will vary with each farm's goals and available funds for sample collection and testing costs. Farms choosing an aggressive program may elect to test on a 6-month or annual basis. Farms with limited resources may elect to test just once to determine their level of infection and then, with that information, develop an appropriate control program. If a single herd test is used, it is important to look for patterns of infection to focus the control program. For example, is infection more common in certain age groups, family lines, purchased animals or home-reared individuals?
Infection can also be monitored without testing by frequent assessment of body condition or weight and keeping records of numbers and causes of wasting on the farm. Animals unable to maintain mature body weight comparable to the rest of their group are suspect for infection if Johne's is present in the herd. This method of monitoring paratuberculosis is slightly less sensitive than serology if no other confounding causes of wasting such as inadequate nutrition, poor dentition, parasitism, OPPV infection or caseous lymphadenitis are present in the flock.
Management of Purchased Replacements
Johne's disease is most often introduced into a herd by purchasing infected animals. Unfortunately, there are no tests capable of detecting early infection in a purchased animal. The seller may not know they have infection on their farm so the buyer must know enough about the disease to ask the right questions. A buyer can manage the risk of introducing infection onto their farm by assessing risk of infection prior to purchase and by managing purchased animals on their own farms to prevent spread of infection. These management practices, referred to as biosecurity, are useful for Johne's disease and most other infectious diseases; however, where possible, a closed herd provides the best security.
A buyer should determine if the risk of infection in purchased animals is higher or lower than the risk of infection on their own farm and proceed accordingly. Determining the infection status of an individual prior to purchase depends on the degree of exposure to the disease. It is important to determine the status of the farm on which the animals being considered for purchase were born or spent their early months of life. Auction barns and dealer herds usually offer little to no history to assess the risk of exposure. When possible, buy directly from a breeder with a good reputation. Buyers should inquire about the infection status of potential source farms prior to purchase. Under Ohio law, an inquiry with a negative response establishes warranty.
The source herd should be ranked from lowest to highest risk for infection. Ideally, the best herds to purchase animals from are herds with repeated negative whole herd tests or closed herds with no history or suspicion of clinical disease on the farm. Few herds are certified test negative. Herds with or without test history which are practicing good management to control disease offer a lower risk than herds actively buying and selling animals with no management program in place. A single current negative whole herd test indicates a low risk of significant infection in that herd. Even herds with a low number of test positive animals, or only isolated cases in older or purchased animals, which are practicing good paratuberculosis management present a lower risk than herds with known infection and poor management, sale barns, or dealer herds.
Animals with a low-body-condition score should be considered at high risk of being infected with paratuberculosis or other common wasting diseases in small ruminants and should be handled accordingly. If the herd or flock history is unavailable or unknown, screening purchased animals with an AGID test will detect most animals with advanced paratuberculosis. If the test is negative, the animal may still be infected but has less of a chance of being in advanced stages of infection. If prepurchase screening is not possible, the buyer should segregate purchased animals and their manure and test prior to commingling with the home herd or flock. Given the general lack of sensitivity for detection of infection in individual animals, owners purchasing animals of unknown infection status should always use good sanitation and management of feces to prevent spread of infection from subclinically infected animals.
Unless specific regulations exist in a state, the seller is under no legal obligation to voluntarily disclose the infection status of an individual or herd so controlling the disease is up to the buyer. However, if sellers deny knowledge of a positive-infection status when specifically asked by the buyer and knowingly sells infected animals, they potentially could be sued in civil court by the buyer.
Management and Control on Infected Farms
Goat - Aggressive Elimination of Infection: Tests available for use in test-and-cull programs (fecal culture and serology) are more sensitive in goats than in sheep. An example of an aggressive program designed to eliminate infection from a farm was described by Gezon et al (16). Paratuberculosis was controlled and eliminated from a goat herd, in which replacements were frequently purchased, in 7 years by use of stringent management guidelines, frequent testing, and culling or segregation of test-positive animals (22). The testing protocol included quarterly screening of serum with an AGID test and semiannual fecal cultures. Body weights were monitored monthly. If progressive weight loss was noted, testing was performed on a monthly basis. Goats identified as positive by either test were strictly segregated in a separate building or culled. The goal of the detection program was early recognition and removal of infected animals to decrease buildup of M. paratuberculosis in the environment and to decrease the risk of transmission to kids reared on the farm and to uninfected herd mates. Purchased replacements were segregated and screened for infection before they entered the herd. Young stock were isolated away from adults and away from contact with their manure. Management changes focused on reducing fecal-oral transmission by using keyhole feeders, metal grain troughs and automatic waterers. Pasture use was discontinued. Goat manure was plowed under and pastures were reseeded before forage crops were harvested. Pens were cleaned several times a week rather than once weekly and personnel were instructed to disinfect footwear between pens and barns (16).
Management: Commercial herds or flocks may not find elimination of infection to be economically feasible or warranted. Programs designed to control the level of infection in herds or flocks by testing for and culling the heavy shedders alone have not controlled the shedder rate in subsequent years (25,26). Management of manure to control fecal-oral transmission, especially to young stock, is an important adjunct to test-and-cull programs. To limit fecal-oral transmission to offspring, care should be taken to avoid fecal buildup in birthing areas and contamination of replacement rearing areas by adult manure or runoff. Use of deeply bedded, and clean pens, to prevent exposure of neonates to fecal contaminated objects is recommended. The dams' udders and fleece should be free of fecal contamination. Ewes should be shorn or crutched prior to lambing. Removal of young at birth and feeding of colostrum and milk from a known Johne's-negative source will further limit transmission if a herd manager can afford the intensive labor involved in artificial rearing. If hand rearing is not an option, lambs should be weaned as early as possible and removed from contact with adult manure. Separation can be achieved using separate facilities, partitions, low traffic or buffer zones. Avoid contamination of mangers and feed with manure on boots, shovels, brooms, wheel barrow tires, and other cleaning implements.
In a rigorous control program, dam status needs to be taken into consideration. Dams showing signs of weight loss and subclinical dams shedding large amounts of organism are at risk for transmitting infection before birth in utero. Though the frequency of in utero transmission has not been measured in sheep and goats, disseminated infections do occur. Given the risk of in utero transmission, or, of infection via contaminated colostrum or milk, the risk of keeping offspring of infected animals must be weighed against the cost of having fewer replacements. Lambs from suspect ewes can be raised for marketing to slaughter but should not be raised as replacement stock for the home farm or for sale as replacement stock to other farms.
In grazing herds or flocks, control of infection is particularly problematic and few studies have investigated controlling paratuberculosis under this type of management. The decision to avoid grazing is not economical for many producers. On pasture systems, control will depend on identifying the most heavily shedding animals and removing them from pasture to prevent environmental contamination. Pasture rotation and avoidance of overgrazing will decrease contact with feces on pasture. When possible, young stock should be weaned as early as possible and removed to clean pasture without contact with adult manure. Commingling with other ruminant species should be avoided if their Johne's status is unknown. Water sources must also be kept free of contamination.
Thorough and frequent removal of manure is the foundation of paratuberculosis control. Manure should be spread on cropland--not on fields that will be grazed or harvested for forages in the same season. All animals should be managed as if they are infected. If testing and culling is not possible, good sanitation will always provide the safety net against spread of the infection.
Vaccination: Vaccination has been used to control paratuberculosis in sheep in Iceland (28) and Britain (10). In Iceland (28), lambs on 141 farms were divided in vaccinated (3273 lambs) and unvaccinated (3184 lambs) groups on each farm. Vaccination reduced mortality due to paratuberculosis by 93 percent (28). Cranwell (10) used vaccination and management changes to control paratuberculosis in a flock with a 9-percent premature culling rate due to clinical infection. All lambs kept as replacements were vaccinated at less than a month of age with the Weybridge modified-live vaccine. Management changes included immediate culling of clinical cases and their most recent lambs. Individual animal identification and improved record keeping were also instituted. Within three years, clinical cases were substantially reduced; vaccine failure rate was estimated at 0.6 percent (4/580). Based on a cost/benefit analysis and epidemiologic simulation model developed by Juste (19), vaccination of replacements appeared to be the most cost-effective strategy for controlling paratuberculosis in sheep flocks. However, assumptions of the model may not fit every management situation.
Vaccination has also been used successfully in goats herds in Norway to reduce the prevalence of paratuberculosis from 53 percent to less than 1 percent over a period of 15 years (25). The "test, cull and management" method of control used prior to the initiation of the vaccination program was considered ineffective due to the insensitivity of the tests available to diagnose paratuberculosis. The option for monitoring infection status with serologic tests is eliminated in vaccinated herds and flocks. Vaccination is not currently an option for control in the United States due to lack of an approved vaccine for small ruminants.
Certification of Flocks and Herds: Most states that offer paratuberculosis certification follow the National Paratuberculosis Certification Program Guidelines (NPCPG) adopted by the U.S. Animal Health Association in 1993 or a modification thereof. The National Certification Guidelines relate to paratuberculosis in cattle but with slight modification would also apply to goat herds. The program guidelines are not appropriate for sheep flock certification.
Basically, this voluntary program recommends whole herd testing of cattle over 20 months of age with either a serum antibody test or an organism detection test such as the fecal culture at least annually. Annual rotation between serum antibody tests and an organism detection test is recommended. Herds are certified at increasing levels reflecting the number of whole-herd negative tests achieved up to level 5. After five whole-herd negative tests, certification is maintained by annual testing with either test. Requirements for herd additions and the consequences of finding a positive animals in a certified herd or selling a positive animal from a certified herd are addressed. Testing must be performed at an accredited laboratory. Seropositive animals are only considered truly infected if confirmed by fecal culture or DNA probe. This program attempts to recognize herds for attaining a Johne's-test-negative status. Such herds will serve as an important source of paratuberculosis-free or low-risk replacements for herds purchasing animals in the future.
We recommend the following modifications of the NPCPG to make the program more applicable to goat herds:
(1) Require testing of all goats over 1 year of age in the herd; and
(2) At this time, the serology test of choice is the AGID test until the ELISA is fully validated for goats; however, fecal culture is the recommended test.
Each State veterinarian has a copy of the NPCPG and can provide interested producers with details of the state's position on paratuberculosis and details of the NPCPG.
Given the poor sensitivity of current diagnostic tests in sheep, certification that a flock is free of paratuberculosis would be very difficult if not impossible. Continuous monitoring of suspect (animals with a body-condition score less than 2) or cull sheep by histopathology would be required. An annual negative whole-flock AGID test with no history of clinical disease or evidence of infection in suspects examined by histopathology could be used to certify a flock as test negative indicating a low risk of infection. With each negative flock test, another level of test-negative certification could be added. With each increasing level, the flock would be considered at a lower risk of being infected.
Paratuberculosis in small ruminants is geographically widespread. In some herds and flocks, clinical paratuberculosis, primarily causing chronic weight loss, can be a significant cause of early culling. The effects of subclinical disease are less defined but may include decreased milk production in milking sheep and decreased weight gain in deer raised for slaughter. Paratuberculosis can also cause economic losses due to reduced in sales of breeding animals from pure bred flocks and herds.
Diagnosis in goats, deer and llamas is primarily based upon culture of feces or tissues and histopathology. Due to difficulty in growing sheep strains of M. paratuberculosis, diagnosis is based upon finding compatible histopathological lesions with acid-fast organisms. The AGID test is a highly specific serologic test for diagnosis of clinical paratuberculosis, and correlates with fecal shedding of the organism and degree of severity of the lesions, but is less sensitive than fecal culture at the herd or flock level for detection of subclinical disease in goats. The CF test detects clinically affected animals but suffers from poor specificity. ELISA methods are being evaluated for use as a herd screening test in sheep, goats. The test suffers from a lack of specificity in herds and flocks infected with caseous lymphadenitis. Absorption of sera with Corynebacterium pseudotuberculosis or M. phlei may improve specificity, but more studies are needed. Currently, ELISA testing for paratuberculosis should be considered experimental in small ruminants. If used, it should be considered a herd or flock screening tool only to evaluate infection on a whole herd basis in adult animals over one year of age. Use of the ELISA for diagnosis of individual cases requires confirmation with a more specific serologic test or organism detection test such as fecal culture or PCR. Larger studies are needed to evaluate the sensitivity of ELISA methods; relatively small studies have reported equal or lower sensitivity compared to AGID for diagnosis in sheep.
Paratuberculosis can be managed and eliminated (from goat herds) with stringent management combined with frequent testing and culling, or by vaccination (where permitted) combined with management of fecal-oral transmission. Management of fecal-oral transmission is particularly important to prevent exposure of young stock to the infection. Control programs vary with the goals and economics of each farm. The frequency of testing and level of management intervention will be determined by each farm's abilities, priorities, and finances.
Efforts should be made through State Departments of Agriculture or through breed associations to recognize flocks or herds with no history of infection or at low risk of being infected using standardized national certification guidelines. Test negative farms will be an important source of low risk replacements for flocks and herds purchasing animals.
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