Raymond W. Sweeney

Transmission of Paratuberculosis

Infection of the calf soon after birth by oral ingestion of M. paratuberculosis organisms from feces of infected cows is the most important method of transmission of paratuberculosis. The severity and rate of progression of the disease are dependent on the quantity of organisms in the exposure and the age of the animal. Only a small dose of organisms may be required to establish infection in a newborn calf, and overwhelming age-related resistance by introduction of a large dose of organisms to an adult cow is probably possible. Other means of transmission such as transplacental and direct excretion in the milk and colostrum should not be ignored and should be a particular concern primarily in advanced stages of infection when heavy fecal shedding is detected.

ELISA and fecal culture for paratuberculosis (Johne's disease): sensitivity and specificity of each method

The sensitivity and specificity of the ELISA and fecal culture tests for paratuberculosis in dairy cattle are examined. ELISA and fecal culture data from seven dairy herds where both fecal cultures and ELISA testing was done concurrently are included. A cohort of 954 cattle including 697 parturient adults, cultured every 6 months from 10 herds followed over 4 years served as the basis to determine fecal culture sensitivity. The fecal culture technique utilized a 2g sample with centrifugation and double incubation. Of the 954 cattle cohort of all ages (calf to adult) that were fecal sampled on the first herd visit, 79 were culture positive. An additional 131 animals were detected as culture positive over the next seven tests at 6-month intervals. The sensitivity of fecal culture to detect infected cattle on the first sampling was 38%. Of the 697 parturient cattle cohort, 67 were positive on the first fecal culture, while an additional 91 adult cattle were culture positive over the next seven tests, resulting in a sensitivity of 42% on the first culture of the total animals identified as culture positive. Animals culled from the herds prior to being detected as infected and animals always fecal culture negative with culture positive tissues at slaughter are not included in the calculations. Both groups of infected cattle will lower the apparent sensitivity of fecal culture. Infected dairy herds tested concurrently with both fecal culture and ELISA usually resulted in more than twofold positive animals by culture compared to ELISA. The classification of infected cattle by the extent of shedding of Mycobacterium paratuberculosis in the feces helps define the relative proportion of cattle in each group and therefore the likelihood of detection by the ELISA test. ELISA has a higher sensitivity in animals with a heavier bacterial load, i.e. high shedders (75%) compared to low shedders (15%). Repeated testing of infected herds identifies a higher proportion of low shedders which are more likely to be ELISA negative. Thus, the sensitivity of the ELISA test decreases with repeated herd testing over time, since heavy shedders will be culled first from the herds.

Evaluation of a Commercial Enzyme-Linked Immunosorbent Assay for the Diagnosis of Paratuberculosis in Dairy Cattle

The performance of a commercially available ELISA for detection of antibodies to Mycobacterium paratuberculosis was evaluated using sera from 1,146 cows. Samples were from uninfected cattle, infected subclinical cattle shedding low numbers of organism in feces, subclinical heavy shedders, clinical cases, and randomly selected cattle in a slaughterhouse survey for paratuberculosis. The overall sensitivity of the test, using the manufacturers recommended cutoff was 45% ± 4.8%, and the specificity was 99% ± 0.9%. The ELISA result was significantly correlated with the number of colonies of M. paratuberculosis detected by fecal culturing. The sensitivity of the test was highest for clinical cases of paratuberculosis (87% ± 8.4%), and lowest for subclinical, light-shedding cattle (15% ± 6.6%). Changing the cutoff point did not improve performance of the test. Evaluating ELISA results with a kinetics-based method reduced plate-to-plate variation in results but did not improve performance of the test based on receiver-operating characteristic curve analysis.

Rhodococcus equi pneumonia in 48 foals: Response to antimicrobial therapy

Case records of 48 foals with pneumonia due to Rhodococcus equi were reviewed. Twenty of the 48 foals survived and 28 died or were euthanized. There was no significant difference between the survivors and non-survivors in the age of onset of illness, duration of illness prior to admission, the mean white blood cell (WBC) count, or the mean plasma fibrinogen content. All foals had R. equi isolated from a tracheobronchial aspirate or lung specimens obtained at necropsy. All organisms were susceptible in vitro (Kirby-Bauer) to erythromycin and gentamicin. Susceptibilities to other drugs were: trimethoprim-sulfamethoxazole (88%), tetracycline (87%), chloramphenicol (83%); 97% were resistant to cephalothin and 83% to penicillin. Thirteen of the 20 surviving foals were treated with erythromycin and/or rifampin. A decline in mortality rate was observed with the introduction of the combination of erythromycin and rifampin. None of the 17 foals treated with penicillin and gentamicin survived. Chronic, active, non-septic synovitis was confirmed in 17 foals. These foals had joint distension with mild or no apparent lameness.

Pathogenesis of paratuberculosis.

Paratuberculosis in ruminants is characterized by oral ingestion of Mycobacterium avium subsp. paratuberculosis (MAP), followed by a long incubation period during which time MAP is able to survive within the host’s macrophages. Initially the infection is held in check by the host’s cell-mediated immune response, but gradually the host loses control of the infection. The infection incites a granulomatous inflammatory response in intestinal tissue and mesenteric lymph nodes, resulting in protein-losing enteropathy, malabsorption, diarrhea, weight loss, and edema.

Efficacy of commercial and field-strain Mycobacterium paratuberculosis vaccinations with recombinant IL-12 in a bovine experimental infection model

The efficacy of commercial (Strain 18) and field-isolate paratuberculosis vaccine preparations was investigated. The effect of prior exposure to Mycobacterium paratuberculosis and the adjuvant effect of rIL-12 on vaccine efficacy were also tested. Both Strain 18 and field-isolate vaccines induced strong local, systemic and enteric IFN-gamma responses. A significant reduction in mycobacterial colonization was observed when calves were vaccinated with the field-isolate prior to challenge, but not following vaccination with Strain 18 vaccine. Vaccination with rIL-12 prevented infection in some calves but its overall effect on IFN-gamma response and total mycobacterial load was not statistically significant. Efficacy of paratuberculosis vaccines may be enhanced if calves are vaccinated prior to M. paratuberculosis exposure with field-isolate vaccine instead of Strain 18 vaccine currently in use.

Paratuberculosis (Johne's Disease) in Cattle and Other Susceptible Species

Paratuberculosis (Johnes disease) is a widespread and costly disease. This consensus statement will summarize recommendations regarding diagnosis, control, and treatment of Johnes disease in cattle and other species. Each section of recommendations is followed by a statement that subjectively characterizes the strength of the supporting evidence. The role played by Mycobacterium avium subsp. paratuberculosis (MAP) in the pathogenesis has been a matter of controversy for many years. This statement concludes with an assessment of the evidence in favor of MAP as a potential zoonotic pathogen.

Longitudinal Study of ELISA Seroreactivity to Mycobacterium Avium subsp. Paratuberculosis in Infected Cattle and Culture-Negative Herd Mates

Two thousand nine hundred fifty-two serum samples, collected once or twice annually from 545 cows of known fecal culture status were tested for antibodies to Mycobacterium avium subsp. paratuberculosis using a commercially available enzyme-linked immunosorbent assay (ELISA) test. Overall, 13.5% of the samples from 282 infected cows had positive ELISA results, but when tested multiple times, 38.3% of the cows had at least 1 serum sample with positive results. Among 263 fecal culture–negative cows, 98.1% of the serum samples had negative ELISA results, but when tested multiple times, 7.8% of the cows had at least 1 positive ELISA sample. Fecal culture was positive on a test before the first positive ELISA in 50 cows, ELISA was positive before fecal culture in 12 cows, and in 38 cows, both tests became positive at the same testing time. An additional 174 cows were positive on fecal culture and always negative on ELISA until culled. For cows that had ELISA sample:positive (S/P) ratios below the cutoff point, the change in S/P between sequential tests was evaluated to determine whether a rise in S/P could predict infection status. In this study, change in S/P was not a useful predictor of infection status in seronegative cows.

Treatment of Potassium Balance Disorders

Potassium is the predominant intracellular cation and is critical for the maintenance of resting cellular membrane potential. Abnormalities of potassium balance can manifest as skeletal and cardiac muscle dysfunction. Abnormalities of potassium concentration in plasma can result from changes in external potassium balance (intake vs. excretion) or internal balance (intracellular to extracellular). Hyperkalemia can result from renal failure, uroperitoneum, or severe dehydration and acidosis in calves with diarrhea. Hypokalemia occurs due to reduced forage intake, when increased gastrointestinal losses occur as with diarrhea, due to increased renal losses as with metabolic alkalosis or exogenous corticosteroid administration which promote kaliuresis, or with redistribution of potassium into the intracellular compartment with alkalosis or in association with insulin-mediated glucose uptake. Aggressive intravenous and oral therapy are often necessary to correct potassium balance disorders, in addition to therapy aimed at correcting any underlying disorder contributing to the potassium imbalance.

Effect of subcutaneous administration of a killed Mycobacterium avium subsp paratuberculosis vaccine on colonization of tissues following oral exposure to the organism in calves

OBJECTIVE-To evaluate the effect of vaccination of calves with a killed Mycobacterium avium subsp paratuberculosis (MAP) vaccine on colonization of tissues following oral MAP exposure. ANIMALS-12 healthy Holstein calves. PROCEDURES-At 14 days after birth, calves received the MAP vaccine (1.0 mL, SC) or saline (0.9% NaCl) solution (1.0 mL, SC [control treatment]). Each calf received 1.2 x 10(9) CFUs of live MAP orally 21 and 22 days after vaccination. Prior to vaccination and at subsequent intervals, a blood sample was collected for ELISA detection of antibodies against MAP and for whole blood, antigen-specific, interferon (IFN)-gamma-release assay. Nine weeks after MAP challenge, calves were euthanized and various tissue samples were collected for mycobacterial culture. Interferon-gamma production in prescapular lymph node cells was measured following in vitro stimulation with MAP antigens. RESULTS-Calves were seronegative for anti-MAP antibodies at all times. Compared with the findings in control calves, antigen-specific IFN-gamma production in circulating lymphocytes and prescapular lymph node cells from vaccinated calves was significantly higher. Culture of tissues from vaccinated calves yielded significantly fewer CFUs of MAP (2,417 CFUs/g), compared with tissues from control calves (15,709 CFUs/g). Furthermore, significantly fewer tissue samples from vaccinated calves yielded MAP in culture (21.8 tissues/calf), compared with findings in control calves (27.6 tissues/calf). CONCLUSIONS AND CLINICAL RELEVANCE-Inoculation of calves with a killed MAP vaccine was associated with reduced colonization of intestinal tissues following experimental exposure to MAP. Use of the vaccine could potentially reduce transmission of MAP to calves in infected herds.