Field trials on a live bovine respiratory syncytial virus vaccine in calves.

Field trials were carried out in calves using a live bovine respiratory syncytial (BRS) virus vaccine prepared from the attenuated BRS virus, strain rs-52. Two hundred seventy-five and 353 calves were vaccinated intranasally and intramuscularly, respectively. No undesirable postvaccinal reactions were observed in the vaccinated calves. Of the serum neutralizing (SN) antibody negative calves 89.7% (26/29) and 92.8% (90/97) developed SN antibody 1 month after intranasal and intramuscular vaccination, respectively. Most of the calves having SN antibody titers of 1:1 or 1:2 at the time of vaccination showed a significant increase in SN antibody titer. About 70% and 90% of the calves vaccinated intranasally and intramuscularly, respectively, maintained SN antibody for 6 months after vaccination. In a field trial, a natural BRS virus infection occurred about 5 months after the start of the trial. Ten of the 16 unvaccinated control calves showed respiratory symptoms due to BRS virus infection. On the contrary, all of the 68 vaccinated calves exhibited no symptoms at all, indicating efficacy of the vaccine.     

A field trial of preshipment vaccination of calves

A field trial to investigate the efficacy of vitamins ADE, a Haemophilus somnus bacterin, a pasteurella bacterin, and two intranasal infectious bovine rhinotracheitis-parainfluenza type 3 vaccines administered to beef calves at least three weeks prior to weaning and shipment was conducted.

Over 1000 calves were vaccinated, but of the 692 calves shipped from the ranch of origin, only 276 calves were located in Ontario, or Quebec, feedlots. The average treatment rate was 30%. Neither vitamins ADE, H. somnus bacterin, pasteurella bacterin or the porcine tissue culture infectious bovine rhinotracheitis-parainfluenza type 3 vaccine had a significant effect on treatment rates for respiratory disease. Calves vaccinated with the temperature sensitive infectious bovine rhinotracheitis-parainfluenza type 3 vaccine had a significantly (p < 0.05) lower treatment rate than the nonvaccinated, and the porcine tissue culture infectious bovine rhinotracheitis-parainfluenza type 3 vaccinated, calves. Calves vaccinated with the temperature sensitive infectious bovine rhinotracheitis-parainfluenza type 3 vaccine did not have a significantly reduced treatment rate in comparison to nonvaccinated calves from the same source.

     

A bovine virus diarrhea calfhood vaccination trial in a persistently infected herd: effects on titres, health and growth.

A controlled calfhood vaccination trial to prevent bovine virus diarrhea was conducted in a 100 head cow-calf operation with a three year history of annual calf losses due to enteric bovine virus diarrhea (persistently infected herd). Approximately 50% of the calves were vaccinated at six, 12 and 24 weeks of age. Paired serum samples and growth data were collected on three occasions for comparison between vaccinates and controls. Three vaccinated calves died of enteric bovine virus diarrhea in the first year of the trial and one nonvaccinated calf died in the second year. Two of the three vaccinated calves had developed bovine virus diarrhea virus neutralization antibody titres of 2048 or greater before developing clinical signs. The control and third vaccinated calf failed to seroconvert before dying of enteric bovine virus diarrhea. Approximately 90% of the vaccinated calves seroconverted compared to approximately 40% of the controls. Paired serum samples collected from 75% of the cows in the spring, summer and fall of each year of the trial, showed persistent high bovine virus diarrhea virus neutralization titres in all samples. Calf vaccination before 12 weeks of age had little effect on seroconversion due to high levels of passive antibody to bovine virus diarrhea. Growth data showed that there was no improvement in weight gain or rate of growth in the vaccinated calves.     

Evaluation of efficacy of an inactivated vaccine against bovine respiratory syncytial virus in calves with maternal antibodies

OBJECTIVE: To assess short- and long-term efficacy of an inactivated bovine respiratory syncytial virus (BRSV) vaccine administered i.m. to calves with maternally derived antibodies. ANIMALS: 28 two-week-old calves with neutralizing, maternally derived antibodies against BRSV. PROCEDURE: For evaluation of short-term efficacy, 6 calves were vaccinated i.m. at 2 and 6 weeks of age and challenged intranasally and intratracheally along with a matched group of 4 unvaccinated control calves at 10 weeks of age. For evaluation of long-term efficacy, 2 groups of 6 calves each were vaccinated i.m. at 2, 6, and 18 weeks of age or 14 and 18 weeks of age; these calves were challenged intranasally and intratracheally along with 6 matched unvaccinated control calves at 43 weeks of age. Serum virus neutralizing antibody titer, clinical reactions, and virus shedding in nasal mucus and lung washings were assessed. RESULTS: None of the vaccination regimens resulted in a significant increase in serum virus neutralizing antibody titer. As judged by virus shedding in nasal mucus and lung washings, vaccinated calves were protected against challenge, compared with unvaccinated control groups. Clinical signs attributable to challenge were coughing (short-term efficacy study) and tachypnea and dyspnea (long-term efficacy study). The severity and incidence of disease were significantly lower in the vaccinated groups, compared with that in the unvaccinated groups. CONCLUSIONS AND CLINICAL RELEVANCE: Through vaccination, it is possible to protect vulnerable calves with maternal antibodies against BRSV infection and reduce respiratory tract disease.     

Vaccination of calves against Salmonella dublin with aromatic-dependent Salmonella typhimurium

Ten Holstein calves were divided into 2 groups. Five calves served as nonvaccinated controls, and 5 calves were vaccinated IM at 2 and 3 weeks of age with 10(9) aromatic-dependent (aro-) Salmonella typhimurium strain SL1479 containing O antigens 1, 4, 12. Serious adverse reactions to vaccination were not observed in the calves. Mean maximum rectal temperature increase in the vaccinated calves was 1.5 C. One calf had diarrhea and depressed appetite for 1 day after vaccination. At 5 weeks of age, all calves were challenge exposed orally with 1.5 X 10(11) virulent S dublin strain SL1367 (O antigens 9,12). After challenge-exposure inoculum was given, 1 of 5 vaccinated calves died and 4 of the 5 nonvaccinated calves died (P less than 0.05). Thus, some cross serotype protection against S dublin was induced by parenteral vaccination of calves with aro- S typhimurium strain SL1479, although protection was not complete.     

Studies on the efficacy of intranasal vaccination for the prevention of experimentally induced parainfluenza type 3 virus pneumonia in calves.

The efficacy of intranasal vaccination in preventing or limiting disease of the lower respiratory tract induced by parainfluenza 3 (PI3) virus was evaluated under experimental conditions, using a commercially available live vaccine containing a temperature-sensitive strain of PI3 virus. In a preliminary study four colostrum-deprived calves were vaccinated intranasally at one week and again at two months of age, and two similar calves were given an intranasal placebo. After the second vaccination serum antibodies to PI3 virus were detected in all four vaccinated calves, but not in the control animals. Seventeen days after the second vaccination all six calves were challenged with virulent PI3 virus, and they were killed six days later. The clinical scores and the extent of pulmonary consolidation were reduced in the vaccinated animals; PI3 virus was detected in the upper and lower respiratory tract of the control calves but in none of the vaccinated calves. In a larger scale study with 14 colostrum-fed calves, seven were vaccinated at one week and again at five weeks of age, and seven were given an intranasal placebo. Two weeks after the second vaccination all 14 calves were challenged with virulent PI3 virus. The clinical scores and lung consolidation were significantly reduced in the vaccinated calves in comparison with the controls. Six days after infection, 10 of the 14 calves were killed; PI3 virus was detectable in the nasal secretions of all seven control calves but in only one of the vaccinated animals, and PI3 viral antigen was detected in the lungs of the control calves but not in those of the vaccinated animals. One of the vaccinated calves had developed a severe clinical response after the challenge, but it had only minor lung consolidation when killed.     

A field trial to evaluate the efficacy of a combined rotavirus-coronavirus/Escherichia coli vaccine in dairy cattle.

A field trial was designed to determine the efficacy of a combination rotavirus-coronavirus/Escherichia coli vaccine on dairy farms in southwestern Ontario. In Part A of the trial, 321 cows on 15 farms were randomly assigned to either vaccination or placebo groups. On eight farms, 50% of the dams were vaccinated, while on the other seven farms, 80% of the dams were vaccinated. In Part B of the trial, 26 farms were randomly assigned to either a total vaccination program or to no vaccination program. Mortality, disease occurrence and weight gains were recorded on all calves for the first two weeks of life. In Part A, 23.5% of all calves were treated in the first two weeks of life, 20.9% were treated specifically for scours and 3.6% of live-born calves died. Enteropathogenic E. coli was identified on 13 of the 15 farms, rotavirus on 11 and coronavirus on ten. At least one of the three potential pathogens was found on every farm. There were no significant differences between calves from placebo-treated and vaccine-treated dams with regard to the proportion treated for all diseases, or for scours, or the proportion which died. Neither were there differences in days to first treatment for all diseases (seven days on average), days to first scour (6.7 days), duration of treatments (3.9 days for all diseases, 3.7 days for scours), or estimated weight gains (0.5 kg/day to 14 days). These results were not altered when the presence or absence of enteropathogenic E. coli, rotavirus or coronavirus on the premises was accounted for.(ABSTRACT TRUNCATED AT 250 WORDS)     

Foetal protection against bovine virus diarrhoea virus after two-step vaccination

In order to assess the efficacy of a two-step vaccination protocol with respect to foetal protection against transplacental infections with bovine virus diarrhoea virus (BVDV) with special attention to BVDV-2 seronegative heifers were vaccinated with an inactivated BVDV-1 vaccine and boostered with a modified live BVDV-1 vaccine after 4 weeks. A second group was left unvaccinated as control. Between days 30 and 120 of pregnancy the heifers of both groups were intranasally challenged with a mixture of BVDV-1 and -2. All heifers of the vaccinated group gave birth to nine clinically healthy, seronegative (precolostral) and BVDV-free calves. In contrast in the control group four BVDV viraemic underdeveloped calves were born. Additionally, one calf was stillborn and another viraemic calf was not viable and died 2 days after birth. All six calves of the control group were viraemic with BVDV-2. This study demonstrated for the first time that two-step vaccination of breeding cattle with a modified live BVDV vaccine 4 weeks after application of an inactivated BVDV vaccine was capable of providing a foetal protection against transplacental infection with BVDV-2.     

Antibody response of calves to immunoaffinity-purified bovine respiratory syncytial virus VP70 after vaccination and challenge exposure.

Immunoaffinity-purified bovine respiratory syncytial virus (BRSV) fusion (F) protein elicited anti-BRSV-specific antibody responses in BRSV-seronegative calves. After primary vaccination, all calves seroconverted to BRSV as determined by the virus neutralization (VN) test and developed anti-F protein antibodies detectable by protein immunoblot analyses. Subsequent vaccinations induced greater than twofold increase in VN titer in 3 of 9 (33%) calves, and 1 calf became VN-negative, but still had nonneutralizing antibody detectable by protein immunoblot analysis. This calf remained seronegative after challenge exposure. Two groups of calves were vaccinated IM with immunoaffinity-purified BRSV F protein. Each dose was 2 ml containing 20 micrograms of purified F protein. Freund's adjuvants were used for all vaccinations, with Freund's complete adjuvant used for the primary vaccination and Freund's incomplete adjuvant for subsequent vaccinations. The vaccine was administered to both groups at weeks 0 and 3; the first group received a third vaccination at weeks 21. Group-1 and -2 vaccinated calves and non-vaccinated contact controls were intranasally aerosol challenge-exposed with low cell culture-passage BRSV on weeks 22 and 9, respectively. Eight of 9 vaccinated calves did not develop a humoral anamnestic response following challenge exposure, as demonstrated by VN test and protein immunoblot analyses. Calf 14 from group 1 which had a 1:2 VN antibody titer prior to vaccination, was the only calf that developed an anamnestic response.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of age at the time of vaccination on antibody titers and feedlot performance in beef calves

OBJECTIVE: To compare antibody responses, feedlot morbidity and mortality rates, feedlot performance, and carcass value for calves vaccinated with 1 of 2 vaccination strategies and for unvaccinated control calves. DESIGN: Randomized controlled clinical trial. ANIMALS: 451 beef steers and heifers. PROCEDURES: Calves were vaccinated with a modified-live infectious bovine rhinotracheitis virus (IBRV), bovine viral diarrhea virus types 1 (BVDV1) and 2 (BVDV2), parainfluenza type 3 virus, and bovine respiratory syncytial virus vaccine and Mannheimia haemolytica and Pasteurella multocida bacterin-toxoid at approximately 67 and 190 days of age (group 1; n = 151) or at approximately 167 and 190 days of age (group 2; 150) or were not vaccinated (control; 150). Serum antibody titers were measured at approximately 2, 67, 167, 190, and 232 days of age. Morbidity and mortality rates, feedlot performance, and carcass value were recorded for 361 calves shipped to feedlots. RESULTS: Percentages of calves seroconverting to IBRV, BVDV1, and BVDV2 were significantly higher for groups 1 and 2 than for the control group. Mean treatment costs were significantly lower for vaccinated than for control calves, and mean mortality rate was significantly higher for control calves than for group 1 calves. Feedlot performance and carcass value did not vary significantly among groups. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggested that vaccination of beef calves with a 5-antigen modified-live virus vaccine at 67 and 190 days of age was as effective in terms of immunologic responses as was vaccination at 167 and 190 days of age.     

Comparative serological response in calves to eight commercial vaccines against infectious bovine rhinotracheitis, parainfluenza-3, bovine respiratory syncytial, and bovine viral diarrhea viruses

A field trial was conducted to compare the serological responses in calves to eight commercial vaccines against infectious bovine rhinotracheitis virus (IBRV), parainfluenza-3 virus (PI3V), bovine respiratory syncytial virus (BRSV), and/or bovine viral diarrhea virus (BVDV). Calves given IBRV, P13V, BRSV, and BVDV vaccines had significantly higher antibodies to these viruses than unvaccinated controls; however, serological responses to killed BVDV vaccines were low. Calves with preexisting antibodies to IBRV, PI3V, BRSV, and the Singer strain of BVDV had lower seroconversion rates following vaccination than calves that were seronegative initially.

Serological responses in calves to IBRV, PI3V, BRSV, and BVDV differed among various commercial vaccines. Antibody titers to IBRV were higher in calves vaccinated with modified-live IBRV vaccines than in those vaccinated with killed IBRV vaccines. Following double vaccination with modified-live IBRV and PI3V vaccines, seroconversion rates and antibody titers to IBRV and PI3V were higher in calves vaccinated intramuscularly than in those vaccinated intranasally. Calves given Cattlemaster 4 had significantly higher titers to BRSV and PI3V, and lower titers to BVDV, than calves given Cattlemaster 3, suggesting that the addition of BRSV to Cattlemaster 4 caused some interaction among antigens.

     

Foetal Protection against Bovine Virus Diarrhoea Virus after Two‐step Vaccination

In order to assess the efficacy of a two‐step vaccination protocol with respect to foetal protection against transplacental infections with bovine virus diarrhoea virus (BVDV) with special attention to BVDV‐2 seronegative heifers were vaccinated with an inactivated BVDV‐1 vaccine and boostered with a modified live BVDV‐1 vaccine after 4 weeks. A second group was left unvaccinated as control. Between days 30 and 120 of pregnancy the heifers of both groups were intranasally challenged with a mixture of BVDV‐1 and ‐2. All heifers of the vaccinated group gave birth to nine clinically healthy, seronegative (precolostral) and BVDV‐free calves. In contrast in the control group four BVDV viraemic underdeveloped calves were born. Additionally, one calf was stillborn and another viraemic calf was not viable and died 2 days after birth. All six calves of the control group were viraemic with BVDV‐2. This study demonstrated for the first time that two‐step vaccination of breeding cattle with a modified live BVDV vaccine 4 weeks after application of an inactivated BVDV vaccine was capable of providing a foetal protection against transplacental infection with BVDV‐2.     

Haemophilus somnus Infections II. A Canadian Field Trial of a Commercial Bacterin: Clinical and Serological Results

To evaluate the efficacy of a Haemophilus somnus bacterin a total of 1114 beef calves or yearling bulls were used in the province of Saskatchewan. The six herds were included in a vaccination trial in which the vaccine was administered subcutaneously in one or two doses. The prevalence of H. somnus in nasal swabs of these animals at the time of the initial vaccination was 0.35% Postvaccination information on morbidity and mortality for a four month period was requested from the five ranches where there were nonvaccinated control calves. Postvaccination outbreaks of infectious thromboembolic meningoencephalitis occurred in two of these herds and, although the numbers were limited, there was a trend to reduced morbidity and mortality in the vaccinated animals compared to controls. Seroconversion rates, as determined by the complement fixation test, in the six herds were 28.3% for control calves, 57% for animals vaccinated once and 80.3% for animals vaccinated twice. On the basis of these results the bacterin was considered to be sufficiently efficacious to warrant its further evaluation under field conditions.     

Escherichia coli O157:H7 vaccine field trial in 9 feedlots in Alberta and Saskatchewan

A feedlot trial was conducted to assess the efficacy of an Escherichia coli O157:H7 vaccine in reducing fecal shedding of E. coli O157:H7 in 218 pens of feedlot cattle in 9 feedlots in Alberta and Saskatchewan. Pens of cattle were vaccinated once at arrival processing and again at reimplanting with either the E. coli O157:H7 vaccine or a placebo. The E. coli O157:H7 vaccine included 50 microg of type III secreted proteins. Fecal samples were collected from 30 fresh manure patties within each feedlot pen at arrival processing, revaccination at reimplanting, and within 2 wk of slaughter. The mean pen prevalence of E. coli O157:H7 in feces was 5.0%; ranging in pens from 0% to 90%, and varying significantly (P < 0.001) among feedlots. There was no significant association (P > 0.20) between vaccination and pen prevalence of fecal E. coli O157:H7 following initial vaccination, at reimplanting, or prior to slaughter.     

Experimental Bovine Pneumonic Pasteurellosis II. Genesis and Prevention

Two experiments were conducted. In the first, 16 crossbred Hereford calves were divided into two equal groups. The first group was vaccinated intranasally with a commercial vaccine against bovid herpesvirus 1 and the second group was unvaccinated. The calves were later exposed to an aerosol of bovid herpesvirus 1 (strain 108) for five minutes. Four calves from each group were subjected to transportation and four calves from each group were kept in an environmental chamber for four days. Four days after viral aerosol all calves were exposed to an aerosol of Pasteurella haemolytica and the same subgroups were again transported or held in the chamber for a further four days.

The calves that did not die from pneumonia were necropsied ten days after the final day of transport. Pulmonary lesions were present in both vaccinated and control animals but were less extensive in the vaccinated calves. Six of eight vaccinated but none of the eight control calves survived.

In the second experiment, eight crossbred Hereford calves were divided into two equal groups. One group was vaccinated with bovid herpesvirus 1 (strain 108) and the other acted as controls. Four weeks later all calves were sequentially exposed to aerosols of bovid herpesvirus 1 (strain 108) and P. haemolytica four days apart. Three of the four controls but none of the vaccinates died from pneumonia. Every lobe of the lungs in all the controls was affected by pneumonia while no pulmonary lesions were found in the vaccinated calves. The differences in efficacy of the modes of vaccination and the possible role of transport stress are discussed.

     

Efficacy of a Neospora caninum killed tachyzoite vaccine in preventing abortion and vertical transmission in dairy cattle

A clinical trial was undertaken to assess the efficacy of Bovilis(?) Neoguard, a killed Neospora caninum tachyzoite vaccine on 5 commercial dairy farms in New Zealand with a history of Neospora-associated abortion. Cattle were enrolled in the trial at 30-60 days of gestation and randomly allocated to treatment or control groups. Treatment consisted of 5 mL doses of Bovilis Neoguard administered subcutaneously at enrolment then 4 weeks later. Isotonic saline was administered to the control group. Of 2246 cattle enrolled in the trial, 10.7% of cows and 12.6% of heifers were seropositive to N. caninum. Sampling of a randomly selected proportion of enrolled animals 6 weeks after the second treatment showed that 188/232 (81.0%) vaccinated with Bovilis(?) Neoguard had seroconverted, while 11/130 (8.5%) cows and 10/36 (27.8%) heifers in the control group had seroconverted. Forty-eight vaccinated and 63 control animals aborted. On one farm 12.5% of control animals and 6.1% of vaccinated animals aborted (vaccine efficacy 0.61; p=0.03). On another farm with a high level of abortion 8.4% of control animals and 8.7% of vaccinates aborted. On the remaining 3 farms fewer abortions occurred than expected. A modified Poisson regression approach was used to calculate relative risks for abortion and vertical transmission. Overall vaccine efficacy was 0.25 (p=0.12). Heifer replacement calves from the animals enrolled in the trial were sampled for antibodies to N. caninum at 6-9 months of age. Fourteen of 17 calves from vaccinated, seropositive cows were seropositive as were 13/23 calves from seropositive cows in the control group. The interaction between dam serostatus and treatment group was significant (p=0.05) with vaccination increasing the risk of vertical transmission. It was concluded that vaccination after conception prevented 61% abortions in one of five herds and that vaccination may have increased the risk of early embryonic death.     

Neural changes in vaccinated calves challenge exposed with virulent infectious bovine rhinotracheitis virus

Recurrent infection in calves vaccinated with infectious bovine rhinotracheitis-(IBR) modified live virus was induced by dexamethasone (DM) treatment given 49 days after challenge exposure with virulent IBR virus. Nonchallenge-exposed IM and intranasally vaccinated calves did not excrete the virus after DM treatment; however, IM and intranasally vaccinated and subsequently challenge-exposed calves excreted the challenge-exposure virus into the nasal secretions 5 to 11 days and 6 to 10 days after the DM treatment, respectively. The calves were killed 15 to 18 days (experiment 1) and 14 days (experiment 2) and DM treatment was started and then were examined by histopathologic and fluorescent antibody techniques. All DM-treated calves that were inoculated with the vaccinal virus and challenge exposed with the virulent virus developed nonsuppurative trigeminal ganglionitis and encephalitis. On the contrary, the DM-treated nonchallenge-exposed vaccinated calves did not have lesions in the peripheral nervous system and CNS. Infectious bovine rhinotracheitis virus antigens were not observed in tissues of any of the calves examined (experiments 1 and 2) by fluorescent antibody techniques. These observations indicated that the modified live IBR virus neither produced lesions nor induced latent infection and that modified live IBR virus vaccination did not protect the calves against the establishment of a latent infection after their exposure to large doses of the virulent IBR virus.     

Antibody Response in Calves Following Administration of Attenuated Infectious Bovine Rhinotracheitis (IBR) Vaccines

The serum antibody response of calves vaccinated against infectious bovine rhinotracheitis by the intramuscular route was compared to calves vaccinated subcutaneously. Immunological response in the calves as determined by serum neutralization tests was highly variable; however, a significantly greater percentage (87.5%) of the calves inoculated subcutaneously responded to vaccination by producing a four-week post-vaccinal serum titer of two or higher as compared to only 47.8% of the calves that were vaccinated intramuscularly. Of those calves that were vaccinated a second time, all maintained or had produced titers of two or higher within four weeks after the second immunization. However, the existing circulating serum antibody titers resulting from the first vaccination of nine of 22 calves were lowered by repeat vaccination.     

Vaccination of dairy calves with bovine adenovirus type 3

A field study was undertaken to determine the efficacy of vaccinating dairy calves with a killed bovine adenovirus type 3 (BA3) vaccine in a herd where pneumonia associated with BA3 infection had been a severe problem. Calves were first vaccinated when they were less than one week old; a second dose was given 10-14 days later. Efficacy of the vaccine was evaluated following natural exposure to the virus by comparing prevalence of pneumonia in control (n = 21) and vaccinated (n = 21) calves. Seroconversion to BA3 was shown in 9 of 12 control calves that developed pneumonia, 4 of these calves subsequently died as a result of the disease. Four additional control calves had a subclinical infection with the virus. All calves in the vaccinated group developed virus-neutralizing antibodies which averaged 1:20 eight weeks after the second vaccination. The serologic response to vaccination was not inhibited in calves possessing low levels of colostral antibodies. Two vaccinated calves developed pneumonia but they did not succumb to the disease. The prevalence of pneumonia in vaccinated calves was significantly reduced (P less than 0.05) when compared to control calves.     

The effect of oxytetracycline treatment on immunity induced by Anaplasma centrale.

Calves vaccinated with Anaplasma centrale were treated with 20 mg/kg of long-acting oxytetracycline (OTC/LA) before or simultaneously with vaccination or up to seven months later. Of 40 animals given one or two of OTC/LA from 3 to 13 days before vaccination, 23 become patent after vaccination, with an average prepatent period almost twice as long as that in non-treated vaccinated controls. Upon challenge with 2 x 10(8) A. centrale per dose all 17 previously non-patent calves showed average maximum parasitemias of 2 to 3.8%. Out of 30 calves treated with two to four doses of OTC/LA from one to four weeks after vaccination, 29 remained negative for A. centrale and reacted to challenge infection with average maximum parasitemias of 6.9-7.8%. Five out of 10 calves receiving OTC/LA simultaneously with the vaccination, and all of a separate group of 10 calves treated with a single dose seven days after vaccination, become patent an average of 51.6 and 63.5 d, respectively, after vaccination. Upon challenge, the five previously non-patent calves showed an average of 5.2% maximum parasitemia. In all groups, only rare parasites were seen in previously patent calves after challenge. Thirty calves treated with 2-4 doses of OTC/LA about six months after vaccination showed no or only a few parasites upon challenge. The above results show that treatment with single or multiple doses of OTC/LA a few weeks before or after administration of live A. centrale vaccine can interfere with elaboration of immunity.