In vivo and in vitro effects of moderately virulent African swine fever virus on mitogenesis of pig lymphocytes

Six pigs were infected oro-nasally with a moderately virulent African swine fever (ASF) virus from the Dominican Republic (DR II). The effect of virus infection on the pig's immune system was tested by measuring peripheral leucocyte numbers and the ability of mononuclear leucocytes (MNL) to respond by lymphocyte proliferation (LP) to the mitogens phytohemagglutinin-P (PHA-P), concanavalin-A (Con-A), and pokeweed mitogen (PWM). All 6 pigs developed high viremias between 4 and 18 days post-inoculation (DPI) which became undetectable by 32 to 46 DPI. Virus was found in erythrocytes, plasma, and mononuclear leucocytes from peripheral blood. Overall, virus infection had only minor effects on the number of circulating leucocytes, lymphocytes, monocytes and granulocytes. At the early acute phase of infection slight neutrophilia and lymphocytopenia were observed with mildly elevated monocyte numbers and slightly depressed neutrophil numbers that continued from the time of evident reduction in viremia to beyond the period of viral clearance. The infected pigs readily produced high titers of ASF virus antibody shortly after the onset of viremia. No significant differences in LP responses of MNL from the 6 pigs to PHA-P, Con-A and PWM were observed after infection when compared to those obtained with MNL from normal pigs. The in vitro addition of infectious ASF virus to MNL from normal pigs did not affect LP responses to any of the three mitogens. These results do not support the hypothesis that immunosuppression is a consequence of ASFV infection of pigs.     

In vitro and in vivo association of African swine fever virus with swine erythrocytes

The association of African swine fever virus (ASFV) with swine erythrocytes in vivo, in high titers, was verified by inoculating 30 pigs with 17 ASFV isolates and assaying their plasma and washed erythrocyte fractions for residual virus. Viral antigens were specifically localized on the surface of in vitro and in vivo swine erythrocytes, using the fluorescent antibody technique and 3 monoclonal antibodies specific for ASFV. The same monoclonal antibodies immunoprecipitated virus-specific polypeptides of molecular weights 13 kd and 73 kd from ASFV-infected Vero cells. Erythrocytes from viremic swine infected with Lisbon-60, Dominican Republic, Badajoz-M98, or Cameroon isolates of ASFV were studied by transmission electron microscopy. Virus was found in membrane depressions at the surface of erythrocytes. These surface depressions resembled stages of smooth surfaced pits. Erythrocytes from viremic pigs were fragile osmotically.     

Distribution of bovine virus diarrhoea virus in tissues and white blood cells of cattle during acute infection

This study is performed to gain knowledge about the quantitative distribution of bovine virus diarrhoea virus (BVDV) in tissues and white blood cells (WBC) at different intervals after acute infection. Ten specific pathogen-free calves were intranasally inoculated with 10⁵ 50% tissue culture infective dose of the non-cytopathic BVDV strain 4800. Twelve hours after inoculation tonsil biopsies were taken and WBC were collected daily for virus isolation and titration. Each day one calf was killed and virus isolations and titrations were performed from a range of tissues. The results indicate that BVDV first replicates in nasal mucosa and to high titers in the tonsil. The virus then appeared to spread to the regional lymph nodes and then disseminates throughout the body. The virus titers were highest in tonsil, thymus and ileum and were low in the WBC. Also after in vitro infection virus titers in WBC were very low, whereas, they were high in epithelial cells. Although the WBC might not be as important as other cells for replication of BVDV, they may play a role in the spread of the virus throughout the body.     

In vitro immune serum-mediated protection of pig monocytes against African swine fever virus

Serum samples from pigs that had recovered from infection with a Dominican Republic isolate of African swine fever virus (ASFV) were mixed with dilutions of the virus, then assayed in microcultures of normal pig mononuclear leukocytes to determine whether the samples contained antibodies that protected monocytes against the virus. Protection was determined by the difference in titer (log10) between virus mixed with healthy pig serum and virus mixed with immune pig serum, using 50% cytopathogenic effect end points; protection was expressed as an immune serum-protection index. After addition of virus-serum mixtures to mononuclear leukocyte microcultures, a time-dependent decrease in protective index and production of infectious virus (determined by use of yield reduction assays) were observed. Protective effects were associated with the immunoglobulin fraction of serum, were rapidly lost on dilution, and were independent of complement. Antibody was most protective for the homologous Dominican Republic isolate of ASFV, with decreased protection against Lisbon '60 ASFV, and no protection against foot-and-mouth disease virus or bluetongue virus. Low concentrations of protective antibody were found during the acute viremic phase in infected pigs; antibody increased to maximal concentrations as the viremia decreased.     

Leukocyte-dependent platelet vasoactive amine release and immune complex deposition in African swine fever

Fifteen pigs were inoculated with African swine fever virus in a study of the pathogenesis of the disease. All pigs surviving the first two weeks developed high circulating antibody titers against African swine fever virus and persistent viremia. Hemolytic complement levels declined to 50 to 70 hemolytic complement 50 (CH50) units/ml from mean preinoculation levels of 120 CH50 units/ml. Immune deposits consisting of African swine fever antigen, host immunoglobulin G, and native C3 were found in the glomeruli of surviving pigs. All pigs surviving two weeks after inoculation developed leukocyte-bound antibody functionally characteristic of immunoglobulin E (IgE). Antigen-specific degranulation of antibody-coated leukocytes produced secondary platelet aggregation and vasoactive amine release. The results suggest that the IgE-basophil-platelet loop acting via amplification by leukocyte-derived platelet-activating factor participates in the immune complex deposition process in African swine fever.     

Comparison of two antigens for use in an enzyme-linked immunosorbent assay to detect African swine fever antibody

Two African swine fever virus (ASFV) antigens were tested for use in an ELISA to detect antibody to ASFV. Antigens used were the cytoplasmic soluble fraction (CS-P) of infected cells grown in the presence of porcine serum and the semipurified viral structural protein VP73 (SVP73). Both antigens were tested by ELISA against 72 sera obtained during several ASF field episodes and from ASFV-inapparent carriers. Of the 72 sera, only 2.8% has positive results by ELISA against CS-P antigen; 60% of positive-reacting sera (to both antigens) had higher ELISA values when the CS-P antigen was used. Samples (with positive results) that reacted only to CS-P antigen had results confirmed by immunoblot analysis. Such sera reacted against ASFV-infection proteins IP25, IP25.5, and IP30, but not against IP73. In time-course experiments to detect appearance of ASFV-antibodies in infected miniature pigs, antibodies were detected by immunoblot analysis on postinoculation day (PID) 8. At that time, only the polypeptides IP25, IP25.5 IP30, and IP31 were recognized; IP73 and IP12 were first detected 3 and 4 days later, respectively. In the same experiments, ASFV antibodies were detected by ELISA, using CS-P or SVP73 antigens, on PID 7 and 9, respectively. These results could explain the percentage of sera not having positive results by ELISA using SVP73 antigen, if the sera were obtained from ASFV-infected pigs during the first days of infection before induction of antibody response against the IP73 protein. This feature makes the use of CS-P antigen advantageous in early serologic detection of ASFV-infected pigs.     

Pathogenesis of African swine fever virus in Ornithodoros ticks

African swine fever virus (ASFV) is the only known DNA arbovirus and the sole member of the family Asfarviridae. It causes a lethal, hemorrhagic disease in domestic pigs. ASFV is enzootic in sub-Saharan Africa and is maintained in a sylvatic cycle by infecting both wild members of the Suidae (e.g. warthogs) and the argasid tick Ornithodoros porcinus porcinus. The pathogenesis of ASFV in O. porcinus porcinus ticks is characterized by a low infectious dose, lifelong infection, efficient transmission to both pigs and ticks, and low mortality until after the first oviposition. ASFV pathogenesis in warthogs is characterized by an inapparent infection with transient, low viremic titers. Thus O. porcinus porcinus ticks probably constitute the most important natural vector of ASFV, although both the mammalian and tick hosts are probably required for the maintenance of ASFV in the sylvatic cycle. The mechanism of ASFV transmission from the sylvatic cycle to domestic pigs is probably through infected ticks feeding on pigs. In addition to O. porcinus porcinus, a number of North American, Central American and Caribbean species of Ornithodoros have been shown to be potential vectors of ASFV.     

The effect of a killed porcine reproductive and respiratory syndrome virus (PRRSV) vaccine treatment on virus shedding in previously PRRSV infected pigs

Two experiments were conducted to investigate if virus shedding could be reduced following a killed porcine reproductive and respiratory syndrome virus (PRRSV) vaccination (KV) of PRRSV infected pigs. In experiment 1, PRRSV infected pigs were vaccinated with KV on days 14 and 28 following infection. Viremia and serum neutralizing (SN) antibody were compared to infected pigs with no KV. The second experiment was conducted in an identical manner. In addition to viremia and SN antibody, virus in oropharyngeal scrapings and interferon gamma (IFN-gamma) producing cells were monitored. Magnitude and duration of viremia were not different between KV vaccinated and non-vaccinated groups. No virus was detected in oropharyngeal scraping from any pig, nor was there a difference in the detection of viral RNA. In both experiments, however, increases in SN titer and number of IFN-gamma producing cells were observed. The SN titer was significantly higher in KV vaccinated groups than in non-vaccinated group on days 42 and 42-56 following infection in experiments 1 and 2, respectively. The number of IFN-gamma producing cells was slightly higher in KV vaccinated groups than in non-vaccinated group on days 42 and 63. These observations suggest that KV had no effect on virus shedding. However, previously infected pigs responded immunologically to KV, as demonstrated by increases in SN antibody titers and IFN-gamma producing cells.     

Experimental vesicular stomatitis virus infection of swine: Extent of infection and immunological response

Swine, a natural host species for infection by vesicular stomatitis virus (VSV), were infected with VSV-New Jersey (VSV-NJ) serotype virus obtained from a recent field isolate. Tissues collected from the infected pigs were examined for the presence of infective virus, for viral antigens, and/or for viral nucleic acid. Infective virus could be recovered from tissues near the site of infection for as long as 6 days after the primary infection with VSV. However, no infective virus was recovered following hypothermia induced 11 weeks after infection, or following a secondary challenge with virus 22 weeks after initial infection. Immunofluorescence tests for viral antigens and nucleic acid hybridization assays failed to detect viral antigens or nucleic acids in tissues from which no infective virus could be recovered. Titers of serum-neutralizing antibody peaked 3–5 weeks after infection and then fell slightly until the secondary infection which caused a rapid anamnestic response. Peripheral blood mononuclear cells (PBM) tested 3, 5, 8 or 18 weeks after primary infection all produced readily detectable antigen-specific proliferative responses when cultured with VSV. Thus, although direct tests failed to demonstrate persistence of virus after infection, the humoral and cellular immune response remained elevated for months. Infective VSV was not required to stimulate the proliferative response since UV-inactivated VSV was immunogenic in these in vitro tests. Following primary infection, antigen-specific proliferative responses could be stimulated by several strains of VSV-NJ, but not by VSV-Indiana (VSV-Ind) serotype virus. Secondary infection had relatively little effect on the proliferative response to VSV-NJ strains, but it did cause the PBM to gain responsiveness to VSV-Ind.     

Potential arthropod vectors of African swine fever virus in North America and the Caribbean basin

In an effort to identify arthropods that might serve as vectors and perhaps reservoirs of African swine fever virus (ASFV) if it were to enter the U.S.A., the blood-sucking insect Triatoma gerstaeckeri and four species of ticks of the genus Ornithodoros were established in colonies capable of reproducing in numbers sufficient to enable thorough studies to be made of their ASFV vector potentials. A nymphal stage of T. gerstaeckeri carried the virus for 41 days and retained it through one molt, but was unable to transmit it to susceptible pigs. Studies on O. coriaceus revealed that the species is able to harbor and transmit the virus for greater than 440 days, passing it trans-stadially from the first nymphal stage to the adult, sustaining it through at least four molts. Trans-ovarial passage was not demonstrated and nearly 40% of the ticks died, apparently, of the ASFV infection. O. turicata collected in Florida was also found to be capable of becoming infected with ASFV and transmitting it by bite to susceptible pigs. O. puertoricensis collected during the ASF eradication programs in the Dominican Republic and Haiti was not only readily infected experimentally, but it was also able to transmit the virus trans-stadially and trans-ovarially. However, ASFV was not isolated from any of the 350 O. puertoricensis collected in the Dominican Republic and Haiti. O. parkeri from a long-established laboratory colony were able to carry the virus through at least one molt, but they were unable to transmit it to susceptible pigs.     

Sequential changes in the humoral immune response of pigs to pseudorabies virus after vaccination, exposure to virulent virus, and reactivation of latent virus

Sequential changes in the humoral immune response of pigs to pseudorabies virus (PRV) after each of several exposures to the virus were evaluated by determining virus neutralization (VN) and radioimmunoprecipitation (RIP) activities of sera collected at selected intervals. Pigs were vaccinated intramuscularly with live attenuated virus (6 pigs), inactivated attenuated virus (6 pigs), or inactivated virulent virus (6 pigs). All pigs were challenged oronasally with virulent virus 3 weeks later and 12 (4 pigs of each vaccine group) were subsequently treated with dexamethasone in an attempt to reactivate latent virus. The relatively low serum titers of VN antibody that were raised by vaccination (titers ranged from 2 to 32) increased markedly (at least 16-fold) for all pigs after exposure to virulent virus. After dexamethasone treatment, the VN titers of 2 pigs increased 16-fold, whereas those of the other 10 dexamethasone-treated pigs and the 6 nontreated pigs either remained the same or increased only minimally (i.e., no more than 2-fold). The results of RIP using 35S-methionine-labeled viral proteins were initially similar to those of VN in that the low levels of serum RIP activity detected after vaccination increased markedly after subsequent exposure to virulent virus. In contrast to VN, however, most pigs (11 of 12) treated with dexamethasone had a clear increase in serum RIP activity. The increase was particularly striking for viral proteins of relatively low (less than 46K) molecular weight. Precipitating activity for 14C-glucosamine-labeled viral glycoproteins was not detected until after pigs were exposed to virulent virus. The increase in RIP activity detected after dexamethasone treatment was likely due to an additional antigenic stimulus associated with virus reactivation. However, virus was isolated from nasal swabs of only 4 of the 12 treated pigs. None of the results appeared to be affected appreciably by the type of vaccine used for initial immunization.     

Cytopathogenic effect of African swine fever virus for pig monocytes: characterization and use in microassay

A microculture assay is described for the titration of African swine fever virus (ASFV) using swine monocytes contained in mononuclear leucocyte (MNL) microcultures. Titration endpoints were determined by observing cytopathogenic effects (CPE) of ASFV infected monocytes with an inverted microscope at 40 X magnification. CPE was a late event following the detection of ASFV antigens in monocytes by radioimmune assay, immunofluorescence and hemadsorption. It began with the detachment, enlargement and rounding of monocytes which progressively formed into grape-like clusters of 3-20 or more cells which eventually lysed. The characteristic CPE was produced in monocyte microcultures by virulent, moderately virulent, Vero cell adapted, and nonhemadsorbing ASFV strains. The sensitivity and reproducibility of the CPE microassay was similar to that of the hemadsorption microassay.     

不同非洲猪瘟病毒株j5R基因及其编码蛋白的比较

根据非洲猪瘟病毒ＭａｌａｗｉＬＩＬ２０／１株ｊ５Ｒ阅读框Ｃ端抗原决定簇序列制备的合成肽能被不同毒株感染康复猪的免疫血清识别。多聚酶链反应研究表明,９个不同毒株的ｊ５Ｒ基因长度略有差异;蛋白转印杂交试验证明,这种基因长度的差异导致相应蛋白多肽的长度多样性。这些结果进一步证明,长度多样性是非洲猪瘟病毒基因及其编码蛋白较为普遍的特点。     

Experimentally induced bluetongue virus infection in white-tailed deer: coagulation, clinical pathologic, and gross pathologic changes

Ten yearling white-tailed deer (Odocoileus virginianus) were inoculated with bluetongue virus serotype 17. Two yearling white-tailed deer were inoculated with sonicated heparinized noninfected blood and served as controls. Clinical signs of bluetongue virus infection included increased rectal temperature, erythema, facial edema, coronitis, and stomatitis. By postinoculation day (PID) 8, excessive bleeding and hematoma formation at venipuncture sites, dehydration, and diarrhea developed. At necropsy, the most consistent findings were oral lesions and widespread hemorrhage, which ranged from petechia to massive hematoma formation. Bluetongue virus caused progressive prolongation of activated partial thromboplastin time and prothrombin time, and progressive reduction of Factors VIII and XII plasma activities beginning on PID 6. A progressive decrease in platelet numbers also developed on PID 6. Changes in platelet size were not detected. Mean thrombin time was shortened, but prolongation developed in 1 deer. Mean fibrinogen concentration and Factor V plasma activity initially increased and then decreased, but remained above preinoculation values. Factor V activity was low in a few deer. Results of screening tests for inhibitors of the intrinsic coagulation system were positive in 2 deer. High concentrations of fibrin(ogen) degradation products were first detected between PID 3 and 6. Hematologic changes included leukopenia, lymphopenia, neutrophilia, and low total plasma protein concentration. Differences in PCV, hemoglobin concentration, or RBC counts were not detected between infected and control deer. Serum total bilirubin concentration increased by PID 6, primarily because of increased unconjugated bilirubin concentration. Mild to severe increases in serum aspartate transaminase activity were accompanied by more marked increases in creatine kinase activity. Indirect Coombs test results were negative in all deer.(ABSTRACT TRUNCATED AT 250 WORDS)     

Multiplication and immunogenicity of a temperature-sensitive and thymidine kinase-deficient strain of Aujeszky's disease virus in pigs.

Hysterectomy-produced colostrum-deprived 5- and 27-day-old pigs were inoculated intramuscularly (IM) or intranasally (IN) with the temperature-sensitive and thymidine kinase-deficient ZHtsTK- strain of Aujeszky's disease virus (ADV), and the nasal swabs and organs of the pigs were periodically collected for virus isolation. No abnormal clinical signs were observed in these pigs, except for a mild febrile response. Viral shedding in the nasal swabs with low titers was detected in the pigs inoculated IN between postinoculation day (PID) 1 and 5, but not in those of the pigs inoculated IM. No contact infection, however, occurred in the cohabiting pigs. Viruses with low titers were isolated only from the muscles and lymph nodes at the site of inoculation in the pigs inoculated IM on PID 2 and 4, but not from any organs of the pigs inoculated IN. To investigate the ability of the ZHtsTK- strain to establish a latent infection in pigs, the pigs inoculated IM or IN with the ZHtsTK- strain were treated with prednisolone. No virus was detected in the trigeminal ganglia or the nasal swabs collected after prednisolone treatment by the cocultivation method. The immunological evaluation demonstrated that immunization of pigs with this strain was effective in preventing clinical signs caused by ADV infection. The duration of virus shedding was markedly shortened in immunized pigs, particularly in those immunized twice and the total quantity of virus recovered from immunized pigs was reduced in comparison with unimmunized pigs.     

Interferon status and white blood cells during infection with African swine fever virus in vivo

African swine fever virus (ASFV) is the causative agent of African swine fever that is the significant disease of domestic pigs, with high rates of mortality. ASFV is double-stranded DNA virus whose genes encode some proteins that are implicated in the suppression of host immune response. In this study, we have modeled in vivo infection of ASFV for determination of interferon (IFN) status in infected pigs. We measured the level of IFN-α, -β and -γ by enzyme-linked immunosorbent assay and showed that the level of IFN-α sharply decreased during infection. Unlike IFN-α, the level of IFN-β and -γ increased from the 2nd and 4th days post-infection, respectively. Also, we analyzed the population dynamics of peripheral white blood cells of infected pigs due to their important role in host immune system. We showed that the atypical lymphocytes appeared after short time of infection and this result is in accordance with our previous study done in vitro. At the last day of infection about 50% of the total white blood cells were destroyed, and the remaining cells were represented mainly by small-sized lymphocytes, reactive lymphocytes and lymphoblasts.     

Temporal relationships of viremia, interferon activity, and antibody responses of sheep infected with several bluetongue virus strains.

Sheep had viremias that were first detected on day 3 (+/- 1) after infection with several strains of bluetongue virus (BTV) representing United States serotypes 10, 11, 13, and 17. Diphasic peaks of infectivity were attained on days 6 and 10 (+/- 2). Interferon (IFN) was first detected in serum samples on day 5 (+/- 1), and reached greatest concentrations on day 6 (+/- 2), which coincided with the first viremic peak; IFN concentrations then decreased toward zero by day 10 (+/- 2). Interferon peak concentrations induced approximately a 90% decrease in virus titer. The decrease in IFN concentrations by day 9 (+/- 2) corresponded with the second viremic peak on day 10 (+/- 2). Onset of the decrease in detectable concentrations of virus after the second peak of viremia corresponded to the initial detection of serum antibody to BTV by day 10 (+/- 2). Virus titer decreased and antibody production increased until approximately days 21 to 28, when the titers plateaued and virus was not detected. Febrile responses peaked on day 7 (+/- 1) during the peak viremic period. The WBC count was depressed at the time the virus titer increased, but returned to normal values while the sheep were still viremic. Diphasic viremias in BTV-infected sheep were attributed to induction of high concentrations of IFN concurrent with the first virus titer peak, followed by production of antibody to specific BTV strains and a subsequent reduction in viremia at the second virus titer peak.     

Isolation and characterization of a Sagiyama virus from domestic pigs.

In 2002, a strain of Sagiyama virus (SAGV) designated ML/Taiwan/02 was isolated from farmed pigs in Taiwan. The nsP1 and E1 gene sequences of the ML/Taiwan/02 strain shared 98.6 and 96.7% homology, respectively, with corresponding genes of a Japanese strain of SAGV. Nucleotide and amino acid sequence comparison revealed this strain of SAGV to be most closely related to Getah virus, as opposed to its current classification as a subtype of Ross River virus. To investigate the seroprevalence of SAGV infection in Taiwan, a total of 586 pig sera collected from 11 of 17 Taiwanese districts were tested for serum neutralizing antibodies (SNA) against SAGV. Results indicated that 51% of the samples had SNA titer > or = 4, and 40% had SNA titer > or = 48, indicative of repeated exposure to SAGV in the field. To study the pathogenicity of the ML/Taiwan/02 strain, this strain was experimentally inoculated into 4-week-old specific-pathogen-free pigs that were seronegative for SAGV. Viremia was detected during postinoculation days (PID) 2-4, when the SNA titer was < or = 16. By PID 7, viremia was no longer detectable, coinciding with the increase of SNA titer to > or = 48. Clinical illnesses or remarkable lesions were not observed. To the authors' knowledge, this is the first reported isolation of a strain of SAGV from pigs in the field. The virus is experimentally nonpathogenic to pigs but is moderately widespread, most likely via repeated exposure to virus-carrying mosquitoes.     

Dynamics of African swine fever virus shedding and excretion in domestic pigs infected by intramuscular inoculation and contact transmission

African swine fever virus (ASFV) is a highly virulent swine pathogen that has spread across Eastern Europe since 2007 and for which there is no effective vaccine or treatment available. The dynamics of shedding and excretion is not well known for this currently circulating ASFV strain. Therefore, susceptible pigs were exposed to pigs intramuscularly infected with the Georgia 2007/1 ASFV strain to measure those dynamics through within- and between-pen transmission scenarios. Blood, oral, nasal and rectal fluid samples were tested for the presence of ASFV by virus titration (VT) and quantitative real-time polymerase chain reaction (qPCR). Serum was tested for the presence of ASFV-specific antibodies. Both intramuscular inoculation and contact transmission resulted in development of acute disease in all pigs although the experiments indicated that the pathogenesis of the disease might be different, depending on the route of infection. Infectious ASFV was first isolated in blood among the inoculated pigs by day 3, and then chronologically among the direct and indirect contact pigs, by day 10 and 13, respectively. Close to the onset of clinical signs, higher ASFV titres were found in blood compared with nasal and rectal fluid samples among all pigs. No infectious ASFV was isolated in oral fluid samples although ASFV genome copies were detected. Only one animal developed antibodies starting after 12 days post-inoculation. The results provide quantitative data on shedding and excretion of the Georgia 2007/1 ASFV strain among domestic pigs and suggest a limited potential of this isolate to cause persistent infection.     

A replication analysis of foot-and-mouth disease virus in swine lymphoid tissue might indicate a putative carrier stage in pigs

ABSTRACT: Foot-and-mouth disease virus (FMVD), one of the most contagious viruses of cloven-hoofed animals, may cause a prolonged, asymptomatic but persistent infection in ruminants, named the "carrier state". However, it remains an open question whether this carrier state occurs in pigs. Here we present quantitative analyses of the duration of FMDV RNA and infectivity in lymphoid and epithelial tissues in experimentally infected pigs with FMDV C-S8c1. The data indicated that although FMDV RNA remained in blood until day 14 post-infection (pi), viremia was cleared by day 7 pi. However, all tissues tested were positive for FMDV until day 14-17 pi. Interestingly, the specific infectivity of FMDV in these tissues was in some cases even higher than the FMDV C-S8c1. We therefore propose that a "pseudopersistent state" may occur in pigs in which virus replicates in lymphoid tissues for a prolonged period of time, thereby representing a potential source of virus.