Abstract
The efficacy of currently available washed whole cell Streptococcus suis bacterins is generally poor. We developed and tested the efficacy of a novel ceftiofur-washed whole cell bacterin. Sixty-six, 2-week-old specific pathogen free (SPF) pigs were randomly divided into 5 groups. Three groups were vaccinated 28 and 14 d prior to challenge. The 3 ceftiofur-washed whole cell bacterins each contained 1 of 3 different adjuvants (Montanide ISA 25, Montanide ISA 50, and Saponin). Pigs exhibiting severe central nervous system disease or severe joint swelling and lameness were euthanized immediately and necropsied. All remaining pigs were necropsied at 14 d post inoculation. The ceftiofur-washed whole cell S. suis bacterin with Montanide ISA 50 adjuvant significantly (P < 0.05) reduced bacteremia, meningitis, pneumonia, and mortality associated with S. suis challenge. Further work on this novel approach to bacterin production is warranted.
Résumé
Les bactérines à base de cellules entières lavées actuellement disponibles pour l’immunisation contre Streptococcus suis sont généralement peu efficaces. Nous avons développé et testé l’efficacité d’une nouvelle bactérine constituée de cellules entières lavées avec du ceftiofur. Un total de 66 porcelets exempts d’agents pathogènes spécifiques (SPF) âgés de 2 semaines ont été divisés de manière aléatoire en cinq groupes. Trois groupes ont été vaccinés 28 et 14 j avant une infection défi. Les 3 bactérines à base de cellules entières lavées au ceftiofur étaient combinées à 1 de 3 adjuvants différents (Montanide ISA 25, Montanide ISA 50, et Saponin). Les animaux montrant des signes d’une atteinte sévère du système nerveux central ou d’inflammation articulaire et de boiterie furent euthanasiés immédiatement et une nécropsie effectuée. Tous les autres animaux furent soumis à une nécropsie 14 j post-inoculation. La bactérine constituée de cellules entières de S. suis lavées avec du ceftiofur combinées au Montanide ISA 50 a réduit de manière significative (P < 0,05) la bactériémie, la méningite, la pneumonie et la mortalité associées à l’infection défi avec S. suis. Des travaux supplémentaires basés sur cette nouvelle approche de production de bactérine sont justifiés.
(Traduit par Docteur Serge Messier)
Streptococcus suis has been associated with meningitis, arthritis, bronchopneumonia, septicemia, and high mortality in nursery pigs (1,2). A protective immune response against S. suis serotype 2 has been experimentally induced by inoculation of pigs with live (3,4) and washed whole cell (5) cultures of S. suis. Efficacy of the currently available washed whole cell S. suis bacterins is highly variable and generally poor (6,7). For this reason, the use of autogenous washed whole cell bacterins for controlling S. suis infections is common but results are also generally poor and variable (2). The washed whole cell bacterins that are more commonly used for vaccinating against S. suis are inactivated by formalin (5–7) or heat (5). Subunit vaccines (7) and purified hemolysin vaccines (8) have also demonstrated variable efficacy against experimental challenge with virulent S. suis serotype 2 strains. Formalin inactivation acts on proteins and nucleic acids producing cross-links and structural rigidity (9), and heat inactivation causes protein denaturation. Ceftiofur is a cephalosporin that suppresses synthesis of the bacterial cell wall by competitive inhibition of the enzymes responsible for cross-linking the cell wall glycopeptide polymer units (10,11).
Different kinds of adjuvants, such as water in oil emulsion adjuvants (5,7), oil adjuvants (12), or aluminum hydroxide-based adjuvants (5–7,12) are commonly used for S. suis bacterins. Water in oil emulsion adjuvants have been shown to be superior to aluminum hydroxide-based adjuvants in their capacity to stimulate an immune response and in reducing mortality associated with S. suis serotype 2 strains (7). Oil adjuvants also induced a significantly higher serum antibody response against S. suis serotype 2 compared to S. suis vaccines with aluminum hydroxide-based adjuvants (12).
The objectives of this research were to test the efficacy and safety of a novel ceftiofur-washed whole cell S. suis bacterin adjuvanted with 3 different adjuvants (Montanide ISA 25, Montanide ISA 50, and Saponin).
The study was approved by the Iowa State University Committee on Animal Care and Use. Sixty-six specific pathogen free (SPF) pigs from a sow unit free of porcine reproductive and respiratory syndrome virus (PRRSV), Mycoplasma hyopneumoniae, and transmissible gastroenteritis virus based on periodic serologic monitoring of the breeding herd were used in the experiment. The pigs were weaned at 12 d of age and transported to the Iowa State University Livestock Infectious Disease Isolation Facility. The pigs were negative for S. suis type 2, Bordetella bronchiseptica, and Haemophilus parasuis by preinoculation culture of tonsil and nasal swabs.
For the homologous challenge, the S. suis serotype 2 isolate ISU-VDL #40634/94 was used. This isolate was obtained in 1994 from a field case of meningitis in a pig submitted to the Iowa State University Veterinary Diagnostic Laboratory (ISU-VDL). The inoculum was prepared as previously described (6).
Three S. suis vaccines were prepared from the challenge isolate and inactivated with ceftiofur hydrochloride (0.5 mg/mL final concentration). A 500 mL culture of S. suis isolate ISU-VDL #40634/94 was prepared similar to that of the challenge inoculum (6). Briefly, 3 blood agar plates were streaked with 1 swabful each of S. suis brain homogenate and incubated for 17 h at 37°C and 5% CO2. The colonies were then transferred via a sterile Dacron tipped swab to 6 mL of sterile, prewarmed Todd-Hewitt broth (THB), and incubated for 2 h at 37°C and 5% CO2. After the incubation, 24 mL of sterile, prewarmed THB was added to the 6 mL culture. This was incubated for 2 h at 37°C and 5% CO2. After the incubation, 470 mL of sterile, prewarmed THB was added to the 30 mL culture and incubated for 3 h at 37°C and 5% CO2. A 1 mL aliquot was then taken to determine culture purity, viability, and concentration by the spread plate dilution method. The spread plate dilution method revealed a final concentration of 8 × 108 colony forming units (CFU)/mL. Then 5 mL of 50 mg/mL ceftiofur hydrochloride was added to the 500 mL of culture and incubated at 37°C and shaken (125 RPM) in an incubator for 24 h. The inactivated culture was checked for viability by washing a 1 mL aliquot of the product 3 times with an equal volume of phosphate buffered saline solution (PBSS), and then spread plated onto a sheep blood agar plate. No growth was observed after 48 h of incubation at 37°C and 5% CO2. The inactivated culture was then concentrated by centrifugation at 3000 × g for 30 min and half of the supernatant was discarded.
Three different adjuvants were used to prepare the final vaccine products: Montanide ISA 25 (oil in water emulsion; Seppic, Paris, France), 1:3 adjuvant to antigen ratio; Montanide ISA 50 (oil in water emulsion; Seppic) 6:5 adjuvant to antigen ratio; and Saponin (Quest International, Naarden, The Netherlands) 1:1 adjuvant to antigen ratio.
The 66 pigs were randomly divided into 5 groups in 2 different rooms in 1.2 × 2.4 m2 pens. They were ear-tagged and fed ad libitum. Group 1 (n = 15), group 2 (n = 13), group 3 (n = 16), and group 4 (n = 14) were randomly assigned to 7 pens in 1 room. Group 5 (n = 8) was housed in 1 pen in a second room. The air flow into the rooms was 10 to 15 air changes per hour and the temperature was 26.5°C. Groups 2, 3, and 4 were given a 2 mL intramuscular dose of S. suis vaccine with either the Montanide ISA 25, Montanide ISA 50, or Saponin adjuvants, respectively, at 28 and 14 d prechallenge with S. suis. One hour before inoculation, all 66 pigs were given 5 mL of 1% acetic acid (AA) (pH 2.9) intranasally (IN) to enhance the severity of the S. suis challenge (13). Groups 1, 2, 3, and 4 were inoculated IN with 2 mL of 1.05 × 109 CFU/mL of the isolate ISU-VDL #40634/94 of S. suis. Group 5 remained as the uninoculated negative control group. After challenge, pigs from groups 1, 2, 3, and 4 were mixed so that 2 to 3 pigs from each group were placed in each of the 7 raised wire deck pens in room 1.
The pigs were monitored twice daily. Rectal temperatures, central nervous system (CNS) signs, joint swelling and lameness (0 = normal, 3 = severe), and clinical respiratory disease (0 = normal, 6 = severe) were scored, as previously described (14,15). Other clinical signs, such as inappetence and lethargy, were also noted. Pigs exhibiting severe CNS disease (ataxia, prostration, or opisthotonus) or severe joint swelling and lameness resulting in recumbency were euthanized immediately and necropsied. Complete necropsies were performed on all remaining pigs at 14 d post inoculation (pi). Brain; cerebellum; lung; joint tissue; heart; tonsil; spleen; inguinal; tracheobronchial; and mediastinal lymph nodes, liver, kidney, ileum, and turbinates were examined grossly, and samples were collected in 10% neutral buffered formalin for histopathology. Whole blood for bacteriology was collected from the jugular vein using a single-use blood collection system containing ethylenediaminetetraacetic acid (EDTA) (Vacutainer; Becton Dickinson, Franklin Lakes, New Jersey, USA), prior to inoculation; and at 0, 2, 4, and 7 d pi. Blood, meningeal swabs, serosa swabs, and joint swabs were collected for bacteriology at necropsy. All samples for bacteriology were immediately streaked onto sheep blood agar plates. All cultures were incubated at 37°C in 5% CO2 for 24 to 48 h. Alpha-hemolytic Streptococcus-like colonies were tested for growth in 6.5% NaCl and production of amylase (16). Representative colonies that did not grow in NaCl and were positive for production of amylase were checked by coagglutination to determine if they were S. suis type 2 (17).
For statistical analysis, mortality, mortality delay, gross lesions, microscopic lesions, and isolation of S. suis from samples collected at necropsy were first analyzed using an overall χ2 test to guard against type 1 error inflation for group differences at α = 0.05. When the overall test was significant, indicating at least one group is different from the others, pairwise tests of group values were performed to determine which values differed from the others. For comparison of mortality, a Fisher’s Exact test was performed and for the rest of the parameters the Wilcoxon non-parametric test was performed.
Clinical signs are summarized in Table I. The only clinical signs observed in the negative control group were mild transient respiratory signs between 3 to 10 d pi. In the positive control group, the first clinical signs of high temperature (> 41°C) and lameness appeared by 2 d pi and high fevers persisted through 7 d pi. The highest respiratory scores were recorded in this group between 3 to 5 d pi. Central nervous system signs, characterized by ataxia, recumbency, and opisthotonus, were followed by mortality that occurred between 3 to 9 d pi with most of the deaths occurring between 4 to 6 d pi. Lameness associated with swollen joints started at 1 d pi and persisted through to the conclusion of the experiment at 14 d pi.
Table I.
Clinical parameters at each day post inoculation (pi) in pigs vaccinated (Montanide ISA 25, Montanide ISA 50, and Saponin groups) and unvaccinated (positive and negative control groups) against Streptococcus suis
| GROUP | Parameter | 1a | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Positive control | Temperatureb | 6.6% | 53.3% | 57.1% | 27.3% | 22.2% | 40% | 0% | 0% | 0% | 0% | 0% | 0% | 0% |
| n = 15 | Respiratoryc | 0.27 | 0.27 | 0.92 | 1.1 | 0.9 | 0.2 | 0.25 | 0.75 | 0 | 0.33 | 0 | 0 | 0.67 |
| CNSd | 0 | 0.13 | 0.64 | 0.9 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | |
| Lamenesse | 0.13 | 0.73 | 0.57 | 0.82 | 0.89 | 0.6 | 0.75 | 1 | 1 | 1.33 | 1.33 | 1 | 0.67 | |
| Cumulative mortality | 0% | 0% | 6.7% | 26.7% | 40% | 66.7% | 73.3% | 73.3% | 80% | 80% | 80% | 80% | 80% | |
| Montanide ISA 25 | Temperature | 15.4% | 0% | 15.4% | 15.4% | 27.3% | 12.5% | 0% | 0% | 0% | 0% | 0% | 0% | 0% |
| n = 13 | Respiratory | 0.23 | 0.23 | 0.84 | 0.38 | 0.9 | 0.57 | 0.33 | 0.66 | 0.33 | 0.33 | 0.17 | 0.17 | 0.33 |
| CNS | 0 | 0 | 0 | 0.85 | 0.81 | 0.43 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | |
| Lameness | 0.23 | 0.53 | 0.23 | 0.54 | 0.54 | 0 | 0 | 0.17 | 0 | 0 | 0.17 | 0.17 | 0 | |
| Cumulative mortality | 0% | 0% | 0% | 0% | 15.4% | 38.5% | 53.8% | 53.8% | 53.8% | 53.8% | 53.8% | 53.8% | 53.8% | |
| Montanide ISA 50 | Temperature | 12.5% | 18.7% | 12.5% | 20% | 7.7% | 0% | 8.3% | 0% | 0% | 0% | 0% | 0% | 0% |
| n = 16 | Respiratory | 0.37 | 0.62 | 0.75 | 0.8 | 0.45 | 0.5 | 0.5 | 0.36 | 0.1 | 0.27 | 0.18 | 0.18 | 0.36 |
| CNS | 0 | 0.31 | 0.37 | 0.53 | 0.23 | 0 | 0.25 | 0 | 0 | 0 | 0 | 0 | 0 | |
| Lameness | 0.12 | 0.37 | 0.31 | 0.33 | 0.23 | 0 | 0.1 | 0.27 | 0 | 0 | 0.1 | 0 | 0 | |
| Cumulative mortality | 0% | 0% | 0% | 6.2% | 18.7% | 25% | 31.2% | 31.2% | 31.2% | 31.2% | 31.2% | 31.2% | 31.2% | |
| Saponin | Temperature | 0% | 14.3% | 7.1% | 0% | 15.4% | 10% | 0% | 0% | 11.1% | 0% | 0% | 0% | 0% |
| Respiratory | 0.14 | 0.14 | 0.14 | 0.46 | 0.38 | 0.55 | 0 | 0.22 | 0.11 | 0.12 | 0 | 0.12 | 0.12 | |
| CNS | 0 | 0 | 0.21 | 0 | 0.69 | 0 | 0 | 0 | 0.33 | 0 | 0 | 0 | 0 | |
| Lameness | 0 | 0.14 | 0.28 | 0.31 | 0.61 | 0.11 | 0.22 | 0.11 | 0.11 | 0.12 | 0.37 | 0.5 | 0.57 | |
| Cumulative mortality | 0% | 0% | 0% | 7.1% | 7.1% | 35.7% | 35.7% | 35.7% | 35.7% | 42.8% | 42.8% | 42.8% | 42.8% | |
| Negative control | Temperature | 12.5% | 0% | 0% | 0% | 0% | 0% | 0% | 0% | 0% | 0% | 0% | 0% | 0% |
| n = 8 | Respiratory | 0 | 0 | 0.25 | 0.25 | 0.25 | 0.37 | 0.25 | 0.25 | 0.25 | 0.37 | 0 | 0 | 0 |
| CNS | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | |
| Lameness | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | |
| Cumulative mortality | 0% | 0% | 0% | 0% | 0% | 0% | 0% | 0% | 0% | 0% | 0% | 0% | 0% |
CNS — central nervous system
Days post inoculation
Percentage of pigs with greater than 41°C rectal temperature
Mean respiratory score (0 = absence, 6 = severe)
Mean central nervous system score (0 = absence, 3 = severe)
Mean lameness score (0 = absence, 3 = severe)
In the vaccinated groups (Montanide ISA 25, Montanide ISA 50, and Saponin) a delay in the appearance of the clinical signs and a decrease in their severity was observed. The length of time between inoculation and mortality was significantly (P < 0.05) longer in the Montanide ISA 50 and Saponin vaccinated groups. The number of pigs with high temperatures (> 41°C) was significantly less (P < 0.05) in all vaccinated groups compared to the positive controls, and fevers disappeared sooner in the vaccinated pigs (by 7 to 8 d pi). Central nervous system signs and the period of time that mortality occurred were shorter in duration (4 to 7 d pi) in the vaccinated pigs. Lameness was less severe in vaccinated groups than in the positive controls. The Montanide ISA 50 group had the lowest lameness scores among the vaccinated groups. The Saponin adjuvanted vaccine caused injection site reactions characterized by red and swollen areas 3 to 5 cm in diameter for 3 to 4 d duration following both doses at 28 and 14 d preinoculation. No injection site reactions were observed with the other 2 vaccines. Overall mortality was significantly lower (P < 0.05) in the Montanide ISA 50 group (Table I).
Gross and microscopic lesions are summarized in Table II. The main gross lesions observed in the inoculated groups were meningitis; lung lesions, characterized by well demarcated red-purple consolidation involving 2% to 20% of the lung, particularly in the cranioventral lobes; polyserositis; enlargement of the lymph nodes, between 2 to 4 times the normal size; presence of fibrin tags in the abdominal cavity; and increased fluid in the joints. There were significantly less (P < 0.05) pigs with meningitis in the Montanide ISA 50 and Saponin vaccinated groups, and there were significantly more (P < 0.05) pigs with enlarged lymph nodes in the Saponin vaccinated group. No gross lesions were observed in pigs in the negative control group.
Table II.
Gross and microscopic lesions in vaccinated and unvaccinated pigs inoculated with Streptococcus suis
| Gross lesions
|
Microscopic lesions
|
|||||||
|---|---|---|---|---|---|---|---|---|
| Group | Meningitis | Pneumonia | Polyserositis | Enlarged lymph nodes | Meningitis | Pneumonia | Synovitis | Lymphadenitis |
| Positive control | 10/15a | 7/15a | 0/11a | 1/15a | 11/15a | 12/15a | 2/15a | 6/15a |
| Montanide ISA 25 | 5/13ac | 2/13a | 0/13a | 0/13a | 7/13ac | 8/13ac | 1/13a | 2/13ac |
| Montanide ISA 50 | 3/16bc | 4/16a | 0/16a | 1/16a | 4/16bc | 6/16bcd | 1/16a | 3/16ac |
| Saponin | 3/14bc | 3/14a | 2/14a | 9/14b | 3/14bc | 6/14bc | 3/14a | 11/14d |
| Negative control | 0/8bc | 0/8a | 0/8a | 0/8a | 0/8b | 0/8d | 0/8a | 0/8bc |
Values in each column with different superscripts are significantly different (P < 0.05)
Microscopically, the lesions observed in the S. suis inoculated groups were fibrinosuppurative meningitis, multifocal suppurative interstitial pneumonia, fibrinosuppurative and lymphohistiocytic synovitis, and multifocal suppurative lymphadenitis. There were significantly less (P < 0.05) pigs with meningitis and the lung lesions were significantly less (P < 0.05) severe in the Montanide ISA 50 and Saponin vaccinated groups. No microscopic lesions were observed in pigs in the negative control group.
Table III summarizes the isolation of S. suis from blood at 0, 2, 4, and 7 d pi; and from the tissues and blood of the animals from each group at the time of necropsy. Streptococcus suis serotype 2 was isolated from the blood of 0 out of 58 pigs at 0 d pi, from 4 out of 58 pigs at 2 d pi, from 9 out of 58 at 4 d pi, and from 2 out of 58 pigs at 7 d pi. Streptococcus suis serotype 2 was isolated at the time of necropsy from blood of 16 out of 58 pigs, meninges of 20 out of 58 pigs, joints of 2 out of 58 pigs, and serosal surfaces of 4 out of 58 pigs. There were significantly fewer (P < 0.05) pigs with S. suis bacteremia in the Montanide ISA 50 and Saponin vaccinated groups. No growth of S. suis was detected from any of the pigs in the negative control group.
Table III.
Isolation of Streptococcus suis type 2 in blood at 0, 2, 4, and 7 d post inoculation, and in blood, brain, serosal surfaces, and joints at the time of the necropsy in vaccinated and unvaccinated pigs inoculated with S. suis
| Blood collected prior to necropsy from live pigs
|
Samples collected at necropsy
|
|||||||
|---|---|---|---|---|---|---|---|---|
| Group | 0a | 2 | 4 | 7 | Blood | Brain | Serosab | Joints |
| Positive control | 0/15 | 2/15 | 4/11 | 1/4 | 9/15c | 7/15c | 1/15c | 1/15c |
| Montanide ISA 25 | 0/13 | 0/13 | 1/13 | 0/6 | 4/13ce | 5/13c | 0/13c | 0/13c |
| Montanide ISA 50 | 0/16 | 1/16 | 2/15 | 0/12 | 1/16de | 4/16c | 2/16c | 0/16c |
| Saponin | 0/14 | 1/14 | 2/13 | 1/9 | 2/14de | 4/14c | 1/14c | 1/14c |
| Negative control | 0/8 | 0/8 | 0/8 | 0/8 | 0/8de | 0/8c | 0/8c | 0/8c |
Days post inoculation
Swab from pleura, pericardium and peritoneum
Values in each column with different superscripts are significantly different (P < 0.05)
The high mortality and high incidence of clinical disease observed in the positive control group validates the ability of the intranasal acetic acid and S. suis inoculation model (13) to reproduce S. suis-associated disease. Since transmission of S. suis is thought to be by inhalation, our model adequately reproduces S. suis-associated pneumonia and meningitis, which are the most common signs associated with S. suis in nursery pigs in the field. However, the high level of mortality observed (80%) in our challenge model is considerably higher than reported in S. suis outbreaks in the field and would certainly represent the extreme in field case exposure scenarios.
The ceftiofur-washed whole cell Montanide ISA 50 adjuvanted S. suis bacterin significantly reduced mortality associated with challenge by a homologous strain of S. suis. The Montanide ISA 50 and Saponin adjuvanted vaccines appeared to be more efficacious in delaying onset of mortality and decreasing clinical signs and lesions associated with S. suis than the Montanide ISA 25 adjuvanted product. The significant delay in onset of mortality in the Montanide ISA 50 and Saponin vaccinated groups may be enough to allow time for producers to detect the sick animals and begin appropriate anti-microbial treatment. However, the Saponin vaccine induced unacceptable injection site reactions and significantly more severe gross and microscopic lesions in lymph nodes making it an unacceptable choice for commercialization.
Inactivation with ceftiofur was chosen in hopes that protective antigens would be better preserved. Damage to the epitopes by fixation with formalin (cross-links and structural rigidity of proteins and nucleic acids) or heat treatment (protein denaturation) may decrease the effectiveness of the bacterins in providing protection against S. suis and may explain the inconsistent results with these types of inactivated vaccines. Mortality rates of 0% to 80% in vaccinated pigs compared to 75% to 80% in unvaccinated pigs (7), 25% to 75% in vaccinated versus 100% in unvaccinated pigs (5), and 40% in vaccinated compared to 63% in unvaccinated pigs (6) have been reported after using different formalin and heat inactivated vaccines prior to S. suis challenge. It is difficult to compare results from these studies with those obtained in this experiment since different S. suis strains, challenge models, inactivation methods, and adjuvants were used.
In a previous experiment (6), we used the same S. suis challenge strain as in the current experiment. In the previous experiment the pigs were coinfected with S. suis and PRRSV and were not treated with acetic acid prior to challenge. We observed 40% mortality in coinfected pigs vaccinated with the formalin inactivated aluminum hydroxide adjuvanted bacterin prior to the homologous challenge. In comparison, mortality rates were 13.8% lower than that obtained using our Montanide ISA 25 vaccine, similar to mortality rates obtained using the Saponin vaccine, but almost 9% higher than that obtained using Montanide ISA 50 vaccine in our study. We also had considerably higher mortality rates (80% compared to 63%) in the unvaccinated positive controls in the current study.
The mechanism of bacterial inactivation of ceftiofur is by suppression of synthesis of the bacterial cell wall by competitive inhibition of the enzymes responsible for cross-linking of the cell wall glycopeptide polymer units. Several studies have confirmed that some bacterial protein fractions present in different virulent strains of S. suis, such as the 44-kDa protein (18), 110-kDa protein (19), or 128-kDa protein (20), induce protective immunity. Unlike other forms of inactivation, ceftiofur treatment may better preserve the antigenicity of the glycopeptide polymer units and the ceftiofur-washed whole cell vaccines may contain both external cell wall surface proteins and internal cell wall surface proteins. Ceftiofur-washed whole cell vaccines may be at least as good, or better, at inducing homologous and heterologous protection against S. suis, as well as other pathogenic bacteria. Further studies comparing this product to current commercial products with homologous and heterologous challenge are warranted.
Acknowledgments
This study was funded by an Iowa Healthy Livestock Initiative Grant and Pork Check Off Dollars from the National Pork Board. The authors appreciate the technical assistance of Dee Murphy, Tim Klinefelter, and expert advice from Dr. Lorraine Hoffman.
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