Worried about disease striking your herd? You're not alone. The uncertainty of when and where disease might strike has made vaccination increasingly important. Learn more in this month's Health Focus about why, when and how cattle should be vaccinated.

	Cattle Vaccines and Their Use
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	Immunizing Beef Calves: A Preconditioning Immunization Concept
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Immunizing Beef Calves: A Preconditioning Immunization Concept
Infectious diseases cause sickness and death in calves, before or after they are born. Unborn and nursing calves are at high risk to fatal diseases during the time of year when a beef rancher is calving cows, moving and mixing these cows, and bringing in bulls to them.

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Johne's disease is a chronic, infectious, usually fatal, intestinal disease of cattle that also occurs in sheep, goats, and deer. It is widely distributed throughout the world and across all regions of the United States.

Johne's is a bacterial disease caused by a bacillus, Mycobacterium paratuberculosis. The disease is spread by the shedding of the organism in the feces of infected animals. Since the organism is resistant to temperature changes and drying and is well protected in fecal material, it persists in the environment for long periods of time. Cattle in infected herds usually fit into four categories: (1) infected animals that are clinically sick, usually one or two at any given time, shedding large numbers of bacteria; (2) infected animals with no signs of disease but shedding bacteria; (3) infected and not shedding; (4) noninfected animals. The proportion of the herd in each category is influenced by the level of exposure of young animals to the organism. Poor husbandry and sanitation encourage continued spread of the disease. The unknown infection status of most herds, lack of control measures within infected herds, and difficulty of diagnosing carrier animals makes Johne's disease a threat to all cattle.

The economic significance of Johne's disease to the cattle industry is not well documented. Estimates are that from 5 to 20 percent of cattle in the U.S. are infected.

Johne's is usually introduced into the herd through replacement cattle that are infected but appear healthy. Once introduced, herds will remain infected for years, and farmers can suffer severe financial losses due to the disease. At any given time, only a small percentage of infected animals deteriorate or will be culled due to clinical disease.

The larger economic losses may be associated with the effects of infection on milk production, reproductive efficiency, and susceptibility to mastitis. When infected and noninfected cows in the same herd were compared, some studies revealed that infected cows had lower milk production, longer calving intervals, and tended to be culled from the herd at an earlier age than noninfected herd mates. Cows with subclinical M. paratuberculosis infection (infection without signs of disease) were culled at a much higher rate for problems of mastitis and infertility than noninfected cows that were culled.

Clinical disease, though highly variable, usually appears in animals between 2 and 5 years of age. Because of the long incubation period (1 year or more), animals may remain infected for years without showing evidence of the disease. Usually symptoms first appear following calving, a severe stress, or poor management. A persistent diarrhea without straining is commonly the first clinical sign, followed by a gradual loss of flesh. The disease develops so slowly that by the time the owner becomes concerned, the herd may be in serious trouble. Infected animals continue to eat, but have constant or recurring bouts of diarrhea. Cattle can become emaciated in a short period of time, such as a few weeks, but often the wasting progresses over a few months. Some cattle may appear to recover temporarily, but relapse later, such as after calving. Infected cows may shed organisms for up to 12 to 18 months after showing signs of infection. Even in herds with widespread infection, at any one time only a few animals may be showing signs of disease.

Infection results in invasion of the terminal portion of the small intestine. The result is a reduced ability to absorb and utilize nutrients. The intestinal wall may be thickened, with a reddened surface. Although the early signs are similar, Johne's must be differentiated from coccidiosis, winter dysentery, and parasitism. Malnutrition increases the severity of the disease signs.

Diagnosis

Diagnosis from necropsied or slaughtered animals is made by histopathologic examination of the ileocecal portion of intestine and the regional lymph nodes. If an animal is infected, intracellular, acid-fast organisms and characteristic granulomatous cellular changes are present. The organism can also be cultured from ileum, lymph node, or fecal material. In the individual live animal, the best diagnosis is made by a culture of the manure, although the animal must be shedding the organism at the time. The fecal culture is specific and does not give false positive results. In earlier stages of the disease, infected animals may shed very low numbers of organisms, sometimes intermittently. These may not be detected on a single fecal culture. When whole herd fecal cultures are performed, approximately one third of the infected animals will be shedding enough organisms (100 organisms per gram of manure) to be positive. Repeated cultures at 6 month intervals, however, are effective in identifying the significant shedders in the herd that are contaminating the environment. Three negative whole herd cultures give reasonable assurance the herd is clean. Three to 4 months are required to confirm negative fecal cultures due to the slow growth of M. paratuberculosis in the laboratory and its requirement for a growth enhancing substance, mycobactin.

Culling of a infected animal should be based on fecal culture, but serologic and allergic tests can be used to screen the infection status of a herd, as in a control program or a herd from which individual animals might be purchased. The occurrence of false positive and negative results, however, makes these tests unsuitable for an individual diagnosis.

Animals in the early stages of infection or animals that have merely been exposed to M. paratuberculosis will react to intradermal skin (allergy) testing. Skin testing of young animals can provide an indication of the presence or level of herd exposure to M. paratuberculosis. Exposed animals that have not become infected and animals exposed to other antigenically similar organisms often give false positive responses. The skin test frequently becomes negative as immune reactivity declines in more advanced stages.

AGID (agar gel immunodiffusion) and ELISA (enzyme linked immunoabsorbent assay) are newer, quicker serologic tests being used to detect serum antibody to M. paratuberculosis.

s of the disease and often disappears in later stages. AGID and ELISA can be useful for screening, but may fail to detect some infected animals (false negative results). The ACID or ELISA can be used as a quick diagnostic test in clinically affected cattle. A positive AGID or ELISA test is very reliable; however, a negative is inconclusive. By the time the quick serologic tests become positive, animals are usually shedding enough organisms to be detected by fecal culture. ACID and ELISA are available through some private and state laboratories.

The serum complement fixation (CF) test currently available uses a crude antigen and is unreliable due to many false positives and negatives.

Two new tests for Johne's have been recently introduced: a gene-probe for testing feces and a gamma interferon blood test. Both tests may be useful in supplementing information from other diagnostic tests.

The IV Johnin test and staining of fecal smears or rectal scrapings for clumps of intracellular acid-fast bacteria can be helpful in the diagnosis of clinical cases. Smears require careful and thorough examination, and interpretation can be difficult. In advanced clinical cases, rectal biopsies may be diagnostic.

Control

Control and eradication of Johne's disease depends upon reducing contamination of the environment and eliminating exposure to the organism, especially for young calves. This requires identification and culling of infected shedders and necessary changes in management practices for youngstock. Fecal culture at 6-month intervals is used to identify infected shedders so that they can be removed as soon as possible. Any animals showing diarrhea and weight loss should be culled immediately or tested and isolated pending test results. Newer AGID or ELISA tests are quick and fairly accurate when used in the advanced clinical cases. Manure from infected adults should be applied only on cropland to be tilled, and youngstock must be isolated from contaminated barnyards, pastures, and water.

Young animals are the most susceptible and acquire Johne's infection at a very young age (as early as 1 day) even though they show no signs of the disease until 2 or more years later. Halting the infection cycle in an infected herd requires that calves be removed from dams immediately after birth and be raised as replacements completely separated from the adults and the manure contaminated environment. Once a control program is initiated, intradermal skin testing may be useful in screening young calves. A positive reaction in these animals would indicate likely exposure and the need for further testing.

Indications are that calves can become infected immediately after birth. Infection in the uterus before birth has been documented in clinical Johne's cows, but is apparently rare in subclinically infected cows. A higher percentage of offspring from infected dams become infected due to increased exposure of the calf to contaminated feces. The organism has been isolated in the semen of bulls, although spread of infection has not been demonstrated by artificial insemination.

If it is necessary to buy replacement cattle, they should be purchased from known negative Johne's herds or herds without a history of the disease that have been screened using one of the quicker, less sensitive tests. This may become more routine as support grows for a voluntary program certifying that cattle have disease free status.

For herds with high levels of infection, a killed vaccine is available through cooperation with an accredited veterinarian and the Pennsylvania Department of Agriculture. The vaccine is administered to calves under 35 days of age. While vaccination will reduce the incidence of clinical disease in the herd and decrease the degree of shedding, it does not prevent infection. Vaccination is not a solution, but must be coupled with the same stringent management practices used to control the disease in non-vaccinated herds. Large persistent lumps at the vaccination site and cross reaction with the tuberculin caudal fold test are disadvantages to its use. Caution must be used in administering the vaccine because chronic granulomatous infections in humans can result from self-injection of an extremity. The decision to vaccinate needs to be carefully weighed since Johne's disease can be controlled without vaccination.

Dairy producers wishing to use the vaccine in their herds must sign a memorandum of understanding with their veterinarian and the Pennsylvania Department of Agriculture.

Guidelines for Johne's Disease Eradication

In known infected herds, it is possible to eradicate Johne's disease if the following program is carefully implemented. Eradication has been achieved in both Johne's vaccinated and non- vaccinated herds. Most procedures are designed to prevent spread from adult carriers to susceptible calves.

1. Remove calves to clean quarters immediately after birth. Natural nursing must not be permitted.
2. Thoroughly wash dam's udder to remove all possibly contaminated manure before drawing colostrum to be used for initial calf feedings.
3. Use only clean utensils for feeding calves. Use cleaning tools maintained for noninfected group only. Don't contaminate calves with manure from footwear or clothing.
4. Raise calves separately from all contact with adults or manure. Individual hutches or pens are ideal. Use only clean, uncontaminated bedding.
5. Be sure that feed is not contaminated by manure. Pasteurized milk or replacer for calves may be worth the effort. Do not give leftover feed from adults to youngstock.
6. Pasture calves on clean, uncontaminated areas and maintain in winter quarters separately from adults until at least 1 year of age or until necessary to add to the milking line. If no clean area is available, land can be tilled and covered with 8 inches of clean topsoil to prevent exposure to the organism.
7. Protect young animals from all drainage that may come from area occupied by adult cattle. Fence or remove any stagnant pools. Allow animals to drink from uncontaminated tanks or free flowing streams only.
8. Culture all mature animals in infected herds at 6-month intervals.
9. Remove all culture positive animals from the herd.
10. Test and remove immediately for slaughter any animal showing signs of recurrent diarrhea. These may be shedding billions of organisms per day into the environment.
11. Test and confine or cull from the herd any unthrifty animals until condition is diagnosed. Handle these animals after clean animals in routine chores. Do not return these animals to the herd unless possibility of Johne's disease is eliminated.
12. Use semen from noninfected bulls.
13. Cresylic and sodium orthophenylphenate disinfectants kill exposed mycobacteria. To clean and disinfect exposed areas, all traces of manure must be removed first. Bacteria are well protected by organic matter.
14. If necessary to purchase replacement animals, obtain mature individuals from known negative herds or at least herds with no history of Johne's disease. Some of the quicker tests can be used to screen the herd before animals are considered for purchase. If no testing is done, isolate purchased animals until proven negative by fecal culture.
15. Clean and disinfect footwear when going from adult housing areas to the youngstock housing areas.

Dr. Barry Hutchinson, Extension Veterinarian, PSU
Dr. Robert Whitlock, Professor of Medicine, University of Pennsylvania
Dr. Christine Rossiter, Field Investigator, PSU

Issued in furtherance of Cooperative Extension work, Acts of Congress May 8 and June 30,1914 in cooperation with the US Department of Agriculture and the Pennsylvania legislature. L. F. Hood, Director of Cooperative Extension, The Pennsylvania State University.