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To study the effect of a previous porcine respiratory and reproductive syndrome-infection (PRRS) of the respiratory tract on influenza virus and Actinobacillus pleuropneumoniae (App) infections, 3-week-old specific-pathogen-free (spf) piglets were intranasally infected with PRRS virus. One week later, when the lung alveolar macrophages of PRRSV infected pigs were lowest in number, a second infection was applied by intranasal aerosol of influenza virus H3N2 or by endobronchial instillation of a mildly virulent App. The first experiment consisted of two groups (only influenza infection or dual PRRSV/influenza infection). A second experiment consisted of 4 groups (only influenza infection, only PRRSV infection, dual PRRSV/influenza infection and uninfected controls). At day 2, 4, 14 and 21 after influenza infection, two pigs were killed and sampled for virological and histopathological examination. Influenza H3N2 virus caused only a mild inflammation of the smaller bronchioli. Previous PRRSV infection did not influence clinical signs during influenza infection. Next, we studied in two experiments the effect of dual PRRSV/App infection during the acute stage at two days after App infection. In a third experiment, the influence of PRRSV on more chronic stages of App infection was studied at two weeks after the App infection. At the end of the experiments, the pigs were killed. Lungs were ranked according to size and kind of the lesions. Lesions were cut and measured, samples were taken for virological and histopathological examination. Statistical analysis of the ranked lung-lesions in the first experiment showed a distinct but small effect of previous PRRSV infection on the development of App-lesions. In PRRSV infected pigs, App produced a more severe disease. The second and third experiment however failed to show any influence of the previous PRRSV infection on the App infection. We conclude that previous PRRSV infection of the respiratory tract of spf pigs does not necessarily enhance the severity of secondary infections of the respiratory tract. Author Keywords: Porcine reproductive and respiratory syndrome virus; Swine influenza virus; Actinobacillus pleuropneumoniae; Dual infections; Pigs

Delivery of drugs via the buccal mucosa is an alternative for the low oral absorption and inconvenient parenteral administration of hydrophilic macromolecular drugs. Due to the low permeability of the buccal epithelium the use of absorption enhancers is a prerequisite. In this study, buccal delivery of fluorescein isothiocyanate-labelled dextran 4400 (FD4) and the peptide drug buserelin was investigated in vivo, in pigs. The delivery device consisted of an application chamber with a solution of FD4 or buserelin, and was attached to the buccal mucosa for 4 h using an adhesive patch. A randomized cross-over study including intravenous administration and buccal delivery without and with 10 mM sodium glycodeoxycholate (GDC) as absorption enhancer was performed in pigs. After buccal administration steady-state plasma levels were rapidly achieved. Co-administration of 10 mM GDC increased the absolute bioavailability from 1.8 ± 0.5% to 12.7 ± 2.0% for FD4, and from 1.0 ± 0.3% to 5.3 ± 1.1% for buserelin. From the present studies it is concluded that buccal administration is a suitable route of delivery for macromolecules and hydrophilic compounds such as peptide drugs.

Clostridium difficile is emerging as pathogen in both humans and animals. In 2000 it was described as one of the causes of neonatal enteritis in piglets, and it is now the most common cause of neonatal diarrhea in the United States. In Europe, C. difficile infection (CDI) in both neonatal piglets and adult sows has also been reported. Diagnosis of this infection is based on detection of the bacterium C. difficile or its toxins A and B. Most detection methods, however, are only validated for diagnosing human infections. In this study three commercially available enzyme immunoassays (EIAs) and a commercial real-time-PCR (Becton, Dickinson, and Company) were evaluated by testing 172 pig fecal specimens (139 diarrheic and 33 nondiarrheic piglets). The results of each test were compared to those of cytotoxicity assays (CTAs) and toxigenic culture as the “gold standards.” Compared to CTAs, the sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) were, respectively, as follows: for real-time PCR, 91.6, 37.1, 57.6, and 82.5%; for Premier Toxins A&B (Meridian), 83.1, 31.5, 53.1, and 66.7%; for ImmunoCard Toxins A&B kit (ICTAB; Meridian), 86.6, 56.8, 66.9, and 80.7%; and for VIDAS (bioMérieux), 54.8, 92.6, 85.0, and 72.8%. Compared to toxigenic culture, the sensitivity, specificity, PPV, and NPV were, respectively, as follows: for real-time PCR, 93.0, 34.7, 50.0, and 87.5%; for Premier Toxins A&B, 80.3, 27.7, 43.8, and 66.7%; and for ICTAB, 80.0, 46.2, 52.8, and 75.4%; and for VIDAS, 56.4, 89.8, 77.5, and 76.7%. We conclude that all tests had an unacceptably low performance as a single test for the detection of C. difficile in pig herds and that a two-step algorithm is necessary, similar to that in cases of human CDI. Of all of the assays, the real-time PCR had the highest NPV compared to both reference methods and is therefore the most appropriate test to screen for the absence of C. difficile in pigs as a first step in the algorithm. The second step would be a confirmation of the positive results by toxigenic culture.

Clostridium difficile is emerging as pathogen in man as well as in animals. In 2000 it was described as a cause of neonatal enteritis in piglets and it is now the most common cause of neonatal diarrhoea in the USA. In Europe, C. difficile infection (CDI) in neonatal piglets has also been reported. Diagnosis of this infection is based on detection of the bacterium or its toxins A and B. Most detection methods, however, are only validated for diagnosing human infections. In this study three commercially available Enzyme Immuno Assays and a commercial RT-PCR were evaluated by testing 172 pig faecal specimens. The results of each test were compared with cytotoxicity assays (CTA) and toxigenic culture as gold standards. Compared with CTA, sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) were for RT-PCR respectively 91.6%, 37.1%, 57.6%, and 82.5%, and for Premier Toxin A+B (Meridian): 83.1%, 31.5%, 53.1% and 66.7%, and for Immunocard tox A/B (Meridian) 86.6%, 56.8%, 66.9%, and 80.7%, and for VIDAS (BioMerieux): 54.8%, 92.6%, 85.0%, and 72.8%. Compared with toxigenic culture sensitivity, specificity, PPV and NPV were; for RT-PCR 93.0%, 34.7%, 50.0%, and 87.5%, and for Premier Toxin A+B: 80.3%, 27.7%, 43.8%, and 66.7%, and for Immunocard tox A/B: 80.0%, 46.2%, 52.8% and 75.4%, and for VIDAS: 56.4%, 89.8%, 77.5%, and 76.7%. We conclude that all tests had an unacceptable low performance as a single test for detection of C. difficile in pig herds and that a two step algorithm is necessary. Of all assays, the RT-PCR had the highest NPV compared to both reference methods and is therefore the most appropriate test to screen for absence of C. difficile in pigs as a first step in the algorithm. The second step would be a confirmation of the positive results by toxigenic culture.

Clostridium difficile is recognized as an important cause of nosocomial diarrhoea in humans especially in association with administration of antibiotics. In pigs, C. difficile can cause neonatal enteritis and can be isolated from faeces from both diseased and healthy animals. The presented prospective study describes how soon C. difficile can be isolated from newborn piglets after normal parturition and how C difficile spreads within a pig farm. Six sows, their farrowing crates and their litters at one farm were sampled until C. difficile was found in all piglets. Within 48 h after birth, all 71 piglets became positive for C. difficile (two piglets were already positive within 1 h post partum), all sows became positive within 11311 after parturition and the farrowing crates were found intermittently positive. C. difficile could also be detected in air samples and in samples of teats of the sows. All isolates belonged to PCR ribotype 078. Twenty-one C. difficile ribotype 078 isolates, found at the farm, were further analyzed by MLVA (multiple-locus variable-number tandem repeat analysis) and belonged to one clonal complex, except one isolate. To be sure that piglets were not born already infected with C. difficile ribotype 078, 38 caesarean derived piglets were sampled immediately after surgery. All piglets tested negative at delivery and stayed negative for C difficile ribotype 078 during the 21 days in which they were kept in sterile incubators. This study shows that C. difficile ribotype 078 spreads easily between sows, piglets and the environment. Vertical transmission of C. difficile ribotype 078 was not found and is very unlikely to occur. (C) 2010 Elsevier B.V. All rights reserved.

Two outbreaks of porcine parvovirus occurred in a unit of 380 sows, with a subsequent decrease in the size of gilts' litters. Of the 203 gilts and 64 primiparous sows which were seronegative at the time of insemination, 134 gilts and 55 primiparous sows seroconverted during pregnancy. Of the second parity sows nine of 271 were still seronegative at the time of insemination but all nine seroconverted during their third gestation. The gilts that seroconverted during their first pregnancy produced on average 0.9 fewer live piglets than the gilts that did not seroconvert. Second litter sows which seroconverted produced 0.3 fewer piglets than those which did not seroconvert. There were no significant differences between sows which seroconverted or did not seroconvert in the numbers of returns to service, or in the percentages of piglets which were born dead or died before weaning.

SWINE influenza ([Olsen and others 2006][1]) and Actinobacillus pleuropneumoniae infection ([Gottschalk and Taylor 2006][2]) are described in textbooks as having well defined clinical signs, but signs of the two diseases prove to be indistinguishable in practice. Apathy, reluctance to rise, anorexia

The pharmacokinetics of oxytetracycline (OTC) were studied in healthy pigs and in pigs endobronchially inoculated with Actinobacillus pleuropneumoniae toxins. In two groups of seven pigs OTC was administered intravenously in a single dose of 10 or 50 mg/kg, respectively. OTC was administered to clinically healthy pigs and 7 days later at 3 h after a challenge with A. pleuropneumoniae toxins. Pneumonia developed in toxin-treated pigs. In the challenged pigs there was a decreased distribution-rate constant (alpha) and a significantly increased elimination-rate constant (beta) (P less than 0.05). Moreover, the apparent volume of distribution (Vd beta) was decreased. The elimination half-lives (t1/2 beta) were approximately 6 h in the healthy pigs and 5 h in the diseased animals. There was no difference in the pharmacokinetic profile of OTC following administration of 50 mg/kg compared to 10 mg/kg.

Atrophic rhinitis in pigs is rarely reported in Southern Africa. To determine the relationship between Pasteurella multocida clones from clinical cases of atrophic rhinitis, twenty-one strains were characterised by selected phenotypic and genotypic methods. Biochemical analysis classified 18 strains as P. multocida subspecies multocida, whilst the remainder were grouped into separate unassigned biotypes. Capsular groups A (16/21) and D (l/21) were found among the isolates by PCR. Four ribotype patterns were obtained following HpaII ribotyping, whilst random amplification of polymorphic DNA (RAPD) revealed three main clusters. However, subclusters were also noted for each RAPD cluster. Our results indicate that RAPD offers a better discrimination of strains than ribotyping and that none of the phenotypic characters were directly related to the genotypic clusters.

A cross-sectional study was carried out on 32 Dutch breeding herds to estimate the incidence of influenza-virus infections in piglets before the start of the finishing period, at the age of approximately 10 weeks. Longitudinal studies on two herds (8 and 10 litters, respectively) were done to obtain an average decay function for maternal antibodies. Each participating farm in the cross-sectional study was visited twice within 5 months; each time, blood samples were taken randomly from one compartment (a separate room with separate air flow) of 4–5-week-old piglets and one compartment of 8–9-week-old piglets. These blood samples (a total of 2598; 16–23 per compartment, depending on its size) were tested in a haemagglutination inhibition test for antibodies against influenza-virus subtypes H1 and H3. Samples from 8–9-week-old piglets from the first sampling period (n=660) were also tested in an IgM ELISA. For each individual herd and each influenza-virus subtype separately, the decay function derived from the longitudinal studies was used to calculate an expected seroprevalence in 8–9-week-old piglets, which was then compared to the observed seroprevalence. Depending on subtype and sampling period, between 10 and 15 of the 32 herds were suspected of virus circulation during the weaning period because the observed seroprevalence was significantly higher than the expected seroprevalence (P

The objective of this study was to investigate the dynamics of PRRSV infection and to quantify transmission within a breeding herd, and its impact on herd performance. For this purpose a longitudinal study was performed in a closed breeding herd of 115 sows. Statistical methods and Monte Carlo simulations based on stochastic SIR models were used to analyse the observational data. Moreover, a case-control study was performed to determine whether seroconversion of sows during gestation was associated with aberrant litters. The transmission parameter R was estimated to be 3·0 (95% confidence interval 1·5–6·0) for the model version based on the most plausible assumptions that the infectious period lasts 56 days and no lifelong immunity exists after infection. Based on simulations using a breeding herd of equal size the average time-to-extinction was estimated to be 6 years; using a herd of twice the size, it was 80 years. Furthermore, in contrast to the epidemic phase of the disease, the endemic phase was not detrimental to herd performance.

Drug disposition, including hepatic drug metabolism, is markedly affected by infection, inflammation and other conditions that invoke the acute phase response. In the present study, an Escherichia coli lipopolysaccharide (LPS)-induced acute phase response model was developed in pigs. This model was used to study the effects of the acute phase response on drug disposition and hepatic drug metabolism in vivo and in microsomal preparations. The results obtained were compared with those from Actinobacillus pleuropneumoniae-infected pigs. Intermittent intravenous administration of LPS induced a mild acute phase response as evidenced by increased rectal body temperatures, anorexia and increased cytokine (TNF-alpha and IL-6) serum levels within 1-2 h after the first LPS injection. The acute phase response is associated with a pronounced decrease of antipyrine plasma clearance (control 8.5 +/- 0.8 vs. LPS 2.2 +/- 0.7 mL/min.kg). Furthermore, total cytochrome P450 content and microsomal cytochrome P450-dependent activities were significantly decreased after 24 h. The decrease in cytochrome P450 activities was accompanied by losses of cytochrome P4501A and P4503A apoproteins. The microsomal glucuronidation rate of 1-naphthol was not affected in LPS-treated pigs. Comparing the LPS model with our previous findings in the Actinobacillus pleuropneumoniae model showed a remarkable similarity with regard to the effects on hepatic drug metabolism.

Buccal delivery of fluorescein isothiocyanate labeled dextran 4400 (FD4) was investigated in-vivo in pigs. The delivery device consisted of an application chamber with a solution of FD4 and was adhered to the buccal mucosa for 4 h using an adhesive patch. A randomized crossover study including intravenous administration and buccal delivery without and with 10 mM sodium glycodeoxycholate (GDC) as absorption enhancer was performed in five pigs. After buccal administration, steady state plasma levels were rapidly reached. Coadministration of 10 mM GDC increased the absolute bioavailability of FD4 from 1.8 +/- 0.5% to 12.7 +/- 2.0%. Since FD4 is a macromolecular and hydrophilic compound such as peptide and protein drugs, buccal delivery would provide an adequate alternative to the parenteral administration of these drugs.

A commercially available enzyme-linked immunosorbent assay (ELISA) for the detection of antibodies against porcine respiratory and reproductive syndrome virus (PRRSV) was compared to an immunoperoxidase monolayer assay (IPMA). Serum samples used were collected from pigs experimentally infected with either the American or European antigenic type of PRRSV, and also from piglets born to sows that had been experimentally infected with the European antigenic type of PRRSV. In addition, three sets of European field sera (n = 275, n = 68, n = 349) were tested and evaluated using the IPMA as the gold standard. Results showed that both the IPMA and the ELISA were able to detect antibodies against the two antigenic types of PRRSV. When sera of experimentally infected pigs were tested, the IPMA with homologous antigen detected antibodies 2 to 3 days earlier than the ELISA, and was more sensitive in detecting maternal antibodies. The ELISA was slightly more sensitive for detecting antibodies against the American type than for the European type. When sets of field sera were tested, the relative sensitivity of the ELISA ranged between 0.68 and 0.91, and the relative specificity ranged between 0.75 and 0.97. However, in two of these sets (n = 275, n = 349) we determined that a decrease of the threshold value of ELISA (from 0.4 to 0.3) increased sensitivity without loss of specificity. We concluded that the ELISA is an easy, quick and reliable test to diagnose PRRSV infection in swine herds.

To study the effects of various doses of Anguillicola crassus in primary infections, parasite-free European eels were orally infected with doses of 0, 1, 5, 10, 20 or 40 third-stage (L3) larvae. The eels were either killed and examined for parasites and lesions after 56 days, or reinfected with 20 L3 larvae to study the effect of primary infection on resistance. Reinfected eels were killed and examined on Day 112. Blood samples were collected weekly and examined in an enzyme-linked immunosorbent assay (ELISA) to detect antibodies against adult parasite cuticula antigen. The mean percentage of A. crassus recovered from eel swimbladders ranged from 14 to 20% on Day 56 and from 9 to 26% on Day 112. There was no significant relation between the proportion of recovered parasites and the primary and/or secondary infection dose. Furthermore, the higher the dose, the more severe were the haemorrhages and pigment spots seen in the swimbladder. Reinfected eels had significantly more severely thickened swimbladders, haemorrhages in the swimbladders and congestion of blood vessels in the swimbladders. Results of the ELISA showed that the eels developed no detectable antibody response against A. crassus. We concluded that under the given stressful experimental conditions, although eels develop pathological signs after primary and secondary infection with A. crassus, they do not develop an antibody response or resistance.

Our objective was to identify pigs of an endemically infected herd that were susceptible to pleuropneumonia due to Actinobacillus pleuropneumoniae. The presence of toxin-neutralizing antibodies was studied in serum of 36 pigs from birth until 24 weeks of age. Titers gradually declined during the first twelve weeks of life and increased thereafter. Sera from one-hundred 3-weeks-old piglets and one-hundred 20-weeks-old pigs were sampled and neutralization titers were determined. From each group we selected 5 pigs with the lowest titers and 5 pigs with the highest titers. These selected pigs (n = 20) were inoculated endobronchially with A. pleuropneumoniae. Pigs that survived from infection were necropsied after 48 h. Pigs with low neutralization titers had severe lung lesions, whereas pigs with high titers had no or minor lung lesion. These differences were significant (P < 0.05). From this field study we conclude that susceptibility to Actinobacillus pleuropneumoniae can be predicted by absence of toxin-neutralizing antibodies.

To study whether Actinobacillus pleuropneumoniae induces a species-specific immunity, we infected pigs in the left lung with serotype 3 or 9 and after 3 weeks we infected their right lungs with serotype 9. Convalescent pigs were protected against homologous strain reinfection, but after heterologous strain reinfection the degree of protection varied. Neutralizing antibodies were not essential for protection.

To study the cellular response of phagocytic cells to Actinobacillus pleuropneumoniae, we investigated whether porcine alveolar macrophages (AM) and polymorphonuclear leukocytes (PMN) are able to phagocytize and intracellularly kill A. pleuropneumoniae in vitro. Bacterial cultivation methods of A. pleuropneumoniae were used to assess in vitro phagocytosis and the ability to kill. A specific-pathogen-free pig was killed, blood was collected, and PMN were isolated and counted. The AM were isolated by lung lavage of the same animal and counted. In addition, convalescent-stage serum was collected from a specific-pathogen-free pig that was infected with A. pleuropneumoniae. Both porcine AM and porcine PMN effectively phagocytized A. pleuropneumoniae in the presence of convalescent-stage pig serum. PMN killed 90 to 99% of the bacteria intracellularly, whereas AM did not. Because A. pleuropneumoniae produces exotoxins that kill porcine AM and porcine PMN, we incubated equal amounts of bacteria and phagocytic cells and tested the viability of the cells 120 min later. In the presence of convalescent-stage pig serum, A. pleuropneumoniae was toxic to AM but not to PMN. Probably in porcine AM, intracellular released toxins of A. pleuropneumoniae lessen the ability of the cell to kill the bacterium. Consecutive lysis of AM and release of viable A. pleuropneumoniae may initiate the characteristic porcine pleuropneumonia.

This study assessed the influence of continuous high volume hemofiltration on right ventricular function of pigs with endotoxin induced shock. Eighteen anesthetized and ventilated pigs were studied for 240 min after the start of infusion of 0.5 mg/kg endotoxin over 30 min. Right ventricular ejection fraction (RVEF) was measured by rapid response thermodilution technique. After endotoxin infusion, the pigs were randomly divided into 3 groups: group 1 as a control group, receiving endotoxin only, group 2 to observe the effects of zero balance high volume veno-venous hemofiltration with removal of ultrafiltrate at a rate of 6000 ml/h, and group 3 to evaluate the effect of the extracorporeal circuit itself on RVEF. The decline of RVEF in group 2 was less than in group 1 (0.04 +/- 0.02 vs 0.21 +/- 0.03 (mean +/- SEM); p less than 0.001). The decline of RVEF in group 3 (0.24 +/- 0.02) was more pronounced than that in group 1 (p less than 0.05). The differences in the course of RVEF between group 1 and group 2 could not be explained by differences in heart rate, preload or afterload. Cardiac output and mean arterial pressure were significantly higher in group 2 than in group 1 (p less than 0.01). It is concluded that in this model, high volume hemofiltration improves RVEF and cardiac performance by removal of vasoactive mediators, responsible for myocardial depression.

In the present study the feed and water consumption and pharmacokinetic parameters of orally administered oxytetracycline were compared in clinically healthy pigs and in the same pigs following a challenge with Actinobacillus (Haemophilus) pleuropneumoniae toxins. Endobronchial challenge with A. pleuropneumniae toxins was accompanied by anorexia, increased lassitude, labored breathing, fever, and increased white blood cell counts. Pleuropneumonia was evident in all pigs on autopsy. Following the challenge, both feed and water consumption were markedly reduced. In contrast to recommendations in the literature, it is concluded that drugs should not be administered to pneumonic pigs via water. In healthy pigs the oral bioavailability of oxytetracycline (50 mg/kg), given on an empty stomach, was 4.8% and the elimination half-life (t1/2 beta) was 5.92 h. After challenge, the pigs showed great variation in oxytetracycline plasma concentrations. In addition, the mean computed elimination rate constant (beta), t1/2 beta, the area under the plasma concentration-time curve (AUC), and clearance in pneumonic pigs differed significantly (P less than .05) from the values found in healthy pigs. The elimination half-life (t1/2 beta), AUC, and volume of distribution (Vd area) were increased. In diseased pigs the mean of maximum plasma concentrations (.87 micrograms/ml) was reached after 7 h, in contrast to 1.74 h (1.87 micrograms/ml) in the healthy pigs.

Reference strains of serotypes 1 to 12 of Actinobacillus (Haemophilus) pleuropneumoniae were cultured in Eagle minimal essential medium with 10% Serum Plus. Culture supernatants were examined for cytotoxicity to alveolar macrophages and for the ability to hemolyze sheep erythrocytes. All strains except the reference strain of serotype 6 produced cytotoxin, whereas only serotypes 1, 5, 9, 10, and 11 produced hemolysin. Both cytotoxin and hemolysin appeared to be heat labile. Antisera were raised against cytotoxin- and hemolysin-containing culture supernatants of serotypes 1 to 11. Cross-neutralization studies revealed that the hemolysins were serologically homogeneous. In contrast, four serologically different cytotoxins were distinguished. One cytotoxin was produced by serotypes 1, 5, 9, and 11, and a second was produced by serotypes 2, 3, 4, and 8. A third cytotoxin was produced by serotypes 7 and 12; this cytotoxin was related to the cytotoxins of serotypes 1, 2, 4, 5, 9, and 11. A fourth cytotoxin, produced by serotype 10, was related to the cytotoxin of serotypes 1, 5, 9, and 11. Seventy field strains belonging to serotypes 2, 3, 7, 8, 9, and 11 were also tested for production of cytotoxin and hemolysin. All strains belonging to serotypes 9 and 11 produced hemolysin and cytotoxin, whereas all strains of serotypes 2, 3, 7, and 8 produced only cytotoxin. Hemolysins and cytotoxins of both the field strains and the corresponding serotype reference strains were comparably neutralized. These findings strongly suggest that the observed differences in production and type of hemolysin and cytotoxin were related to serotype and not to strain.

Viable Haemophilus pleuropneumoniae bacteria were toxic for porcine alveolar macrophages in vitro. This cytotoxic effect proved to be dose-related. A cell-free extract of H. pleuropneumoniae , heat-killed bacteria, and a Pasteurella multocida field strain were nontoxic. When macrophages were cultured with H. pleuropneumoniae bacteria in a ratio of 100 macrophages to six bacteria, ultrastructural signs of cellular degeneration were observed within 1 h. This degeneration was observed in macrophages with or without phagosomes containing H. pleuropneumoniae . A cytotoxic substance was filtered from a H. pleuropneumoniae culture in Eagle's minimal essential medium supplemented with Earle's salts (EMEM) and 10% foetal calf serum that was incubated for 10 h at 37°C. This substance was destroyed by heating at 65°C for 30 min. Macrophages were less susceptible to the toxic effect of H. pleuropneumoniae when serum of convalescent pigs was added.

The interaction between enrofloxacin and porcine phagocytes was studied with clinically relevant concentrations of enrofloxacin. Enrofloxacin accumulated in phagocytes, with cellular concentration/extracellular concentration ratios of 9 for polymorphonuclear leukocytes (PMNs) and 5 for alveolar macrophages (AMs). Cells with accumulated enrofloxacin brought into enrofloxacin-free medium released approximately 80% (AMs) to 90% (PMNs) of their enrofloxacin within the first 10 min, after which no further release was seen. Enrofloxacin affected neither the viability of PMNs and AMs nor the chemotaxis of PMNs at concentrations ranging from 0 to 10 μg/ml. Enrofloxacin (0.5 μg/ml) did not alter the capability of PMNs and AMs to phagocytize fluorescent microparticles or Actinobacillus pleuropneumoniae , Pasteurella multocida , and Staphylococcus aureus . Significant differences in intracellular killing were seen with enrofloxacin at 5× the MIC compared with that for controls not treated with enrofloxacin. PMNs killed all S. aureus isolates in 3 h with or without enrofloxacin. Intracellular S. aureus isolates in AMs were less susceptible than extracellular S. aureus isolates to the bactericidal effect of enrofloxacin. P. multocida was not phagocytosed by PMNs. AMs did not kill P. multocida , and similar intra- and extracellular reductions of P. multocida isolates by enrofloxacin were found. Intraphagocytic killing of A. pleuropneumoniae was significantly enhanced by enrofloxacin at 5× the MIC in both PMNs and AMs. AMs are very susceptible to the A. pleuropneumoniae cytotoxin. This suggests that in serologically naive pigs the enhancing effect of enrofloxacin on the bactericidal action of PMNs may have clinical relevance.