Systemic and mucosal immune responses to H1N1 influenza virus infection in pigs

Influenza is a common respiratory disease in pigs, and since swine influenza viruses are zoonotic pathogens, they also pose human health risks. Pigs infected with influenza virus mount an effective immune response and are protected from subsequent challenge, whereas the currently available, inactivated-virus vaccine does not consistently confer complete protection to challenge. To develop and evaluate new vaccination strategies, it is imperative to fully understand the immune responses that are associated with protection following natural infection. Therefore, we have evaluated the phenotype and kinetics of immune responses to primary and re-challenge infection with H1N1 swine influenza virus in the pig. Through the use of isotype-specific antibody secreting cell ELISPOT assays, interferon-gamma ELISPOT assays and isotype-specific ELISAs on serum, nasal wash and bronchoalveolar lavage samples, we defined the humoral and cellular immune responses, both locally in the respiratory tract and systemically, to this viral infection. Virus-specific serum IgG, IgA, and HI titers all peaked 2-3 weeks after primary infection and did not substantially increase after re-challenge. The predominant virus-specific isotype in serum was IgG. Pigs responded with virus-specific IgG and IgA in both the upper (nasal washes) and lower (bronchoalveolar lavages) airways; IgA was the predominant isotype in both sites. Despite the fact that the pigs were completely protected from re-challenge, the antibody titers in the nasal washes increased. Results of the antibody-secreting cell ELISPOT assays demonstrated that the numbers of both IgG and IgA secreting cells in the nasal mucosa were dramatically higher than in any other tissue examined. In contrast, IFN-gamma secreting cells were predominantly localized to the spleen and tracheobronchial lymph nodes. These data will be helpful in the future development and evaluation of novel vaccines.     

Effect of maternally derived antibodies on the clinical signs and immune response in pigs after primary and secondary infection with an influenza H1N1 virus

The aim of this study was to determine the role of maternally derived antibodies (MDA) against an influenza H1N1 virus in the clinical protection of piglets and especially their effect on the development of the active immunity after an infection with a homologous influenza H1N1 virus. Twenty piglets with MDA and 10 piglets without MDA were housed together and inoculated twice with influenza H1N1 virus, at 7 and 15 weeks of age. Nine piglets without MDA were added to these groups at 12 weeks of age to be inoculated at 15 weeks of age only. Clinical signs, body temperature, growth performance, virus excretion, antibody responses, and influenza-specific T-cell response were monitored. It was shown that MDA protect piglets against the clinical consequences of a primary influenza infection, but that this protection is not complete. A short but significant rise in body temperature was observed and growth seemed to be inhibited due to the infection. Piglets with MDA shed virus for a longer period after an infection than piglets without MDA. Piglets with and without MDA were protected against the clinical consequences of a secondary infection. However, both after primary and secondary infection significant differences in immune responses were observed that indicated that pigs with MDA developed a weaker immunity than pigs without MDA. Furthermore, overall growth performances from weaning to slaughter show a trend in favour of pigs without maternal antibodies, compared to pigs with maternal antibodies, mainly caused by a significant better performance in the second half of the finishing period. The results of this study provide us insight in the role of MDA in clinical protection and their influence on active immunity after an influenza virus infection of pigs. Furthermore, it leads us to the discussion about the profitability of massive sow herd vaccinations in an attempt to increase MDA levels in piglets, taking into account the overall performance of these piglets and the possible effects on antigenic drift.     

Experimental infection with H1N1 European swine influenza virus protects pigs from an infection with the 2009 pandemic H1N1 human influenza virus

The recent pandemic caused by human influenza virus A(H1N1) 2009 contains ancestral gene segments from North American and Eurasian swine lineages as well as from avian and human influenza lineages. The emergence of this A(H1N1) 2009 poses a potential global threat for human health and the fact that it can infect other species, like pigs, favours a possible encounter with other influenza viruses circulating in swine herds. In Europe, H1N1, H1N2 and H3N2 subtypes of swine influenza virus currently have a high prevalence in commercial farms. To better assess the risk posed by the A(H1N1) 2009 in the actual situation of swine farms, we sought to analyze whether a previous infection with a circulating European avian-like swine A/Swine/Spain/53207/2004 (H1N1) influenza virus (hereafter referred to as SwH1N1) generated or not cross-protective immunity against a subsequent infection with the new human pandemic A/Catalonia/63/2009 (H1N1) influenza virus (hereafter referred to as pH1N1) 21 days apart. Pigs infected only with pH1N1 had mild to moderate pathological findings, consisting on broncho-interstitial pneumonia. However, pigs inoculated with SwH1N1 virus and subsequently infected with pH1N1 had very mild lung lesions, apparently attributed to the remaining lesions caused by SwH1N1 infection. These later pigs also exhibited boosted levels of specific antibodies. Finally, animals firstly infected with SwH1N1 virus and latter infected with pH1N1 exhibited undetectable viral RNA load in nasal swabs and lungs after challenge with pH1N1, indicating a cross-protective effect between both strains.     

Cellular immune responses of the rat to experimental infection with Dermatophilus congolensis

The host cell-mediated immune response was examined following experimentally-induced infection of rats with Dermatophilus congolensis, the causal agent of the skin disease dermatophilosis. Mononuclear cells (MC) isolated from Wistar rats 10 days following the induction of a third infection underwent a strong and specific proliferative response, as assessed by a [3H]thymidine incorporation assay, when cultured with various concentrations of inactivated D. congolensis cocci. Using specific monoclonal antibodies in an indirect fluorescent antibody test, this in vitro response was found to be characterised by a large expansion of the W3/25 (T-helper phenotype) population to form 56% of the total. Finally, the primed and stimulated MC were assessed for their ability to produce factors capable of inhibiting macrophage migration. The culture supernatants of D. congolensis-stimulated MC from infected rats caused significant migration inhibition of normal rat peritoneal exudate cells, whilst the supernatants of similarly-stimulated MC from naive rats failed to cause significant inhibition. The results show that a MC subpopulation becomes primed following experimentally-induced infection with D. congolensis and becomes activated after subsequent, in vitro, exposure.     

Pathology in dogs with experimental canine H3N2 influenza virus infection

Avian-lineage H3N2 canine influenza virus (CIV)-associated respiratory disease, which can be fatal, emerged in South Korean dogs in 2007. We show here that dogs experimentally infected with CIV only developed respiratory tract diseases, as no extrapulmonary lesions and virus antigens were detected. This differs from the multiorgan diseases that avian influenza H5N1 induces in small experimental animals. However, the CIV-infected dogs developed a distinctively severe, long-persistent bronchointerstitial pneumonia, which differs from the acute but short-term bronchopneumonia that human (H1N1 and H3N2) influenza cause in rodents and ferrets. Histopathology and in situ TUNEL assays revealed that the neutrophils infiltrating the lesions were undergoing apoptosis, which probably reflects the attempts by the body to maintain appropriate numbers of neutrophils for defense against secondary bacterial infections. Our observations suggest that neutrophils along with the related chemoattractant cytokines (TNF-α, IL-1 and IL-8, etc.) may play a key role in the pathogenesis of H3N2 CIV in dogs.     

Humoral and cellular immune responses to Fasciola gigantica experimental infection in buffaloes

Humoral and cellular immune responses to Fasciola gigantica experimental infection in buffaloes were studied. The results showed that 33.4+/-9.1% of the infection dose was recovered as adult flukes from infected animals at necropsy. Significant differences of weight gain between infected and non-infected buffaloes was observed at 4 MPI (months post-infection). Anti F. gigantica excretory-secretory products (FgESP)-IgG levels increased significantly from 3 WPI (weeks post-infection) and displayed a peak at 13 WPI. Western blot indicated that in FgESP six major bands of 11.5, 19.0, 23.4, 29.8, 47.5 and 53.2kDa were recognized by F. gigantica-infected buffaloes sera after 0 WPI. Eosinophil numbers increased significantly from 3 WPI in F. gigantica-infected buffaloes and displayed a peak at 8 WPI. Peripheral blood mononuclear cells (PBMC) proliferation induced by FgESP increased from 2 WPI with a peak at 5 WPI. IFNgamma secretion by FgESP-stimulated PBMC appeared early from 1 WPI with three peaks at 2, 5 and 8 WPI, respectively. IL-10 production was observed from 2 WPI with two peaks at 4 and 9 WPI, respectively. Our results suggested that buffaloes were highly susceptible to F. gigantica infection, and this susceptibility could be associated with the late and weak cellular immune response in the early phase of infection and the Th0-like response throughout the infection.     

Persistent immune responses in late infection and after treatment in experimental Schistosoma bovis infections in goats

This study explored host immune responses and their possible relationship to the anti-fecundity phenomenon in Schistosoma bovis-infected goats. The design comprised a primary infection with or without treatment at week (wk) 13, and with or without challenge at wk 36. Necropsy was performed at 36 or 52 wk. Serum levels of anti-egg IgG, and anti-worm IgG and IgM, were measured by ELISA. In chronic infection, anti-worm antibodies stayed high, reflecting persisting worm burdens, whereas anti-egg IgG remained high despite minimized egg excretion. After treatment, anti-worm IgM and anti-egg IgG were minimized, but anti-worm IgG remained above the values of the uninfected controls. Histopathology showed lowered numbers of perioval granulomas in chronic infection and resolution of liver fibrosis with time, but intestinal lymphoplasmacytic perivasculitis and hepatic eosinophilic infiltrates were maintained at wk 52. Significant splenic plasmacytosis persisted after treatment. The results indicated that persistent immune responses, in chronically infected and in treated goats, may explain sustained worm fecundity depression at challenge infection.     

Leukoencephalitis associated with selective viral replication in the brain of a pony with experimental chronic equine infectious anemia virus infection

Neurologic disease occurs sporadically in horses infected with the equine infectious anemia virus (EIAV). This report describes a case of clinically severe neurologic disease in a pony experimentally infected with EIAV. This pony did not have fever or anemia, which are the characteristic clinical signs of disease. The histopathologic changes were characterized as lymphohistiocytic periventricular leukoencephalitis. Polymerase chain reaction and in situ hybridization data showed that the brain lesions were directly associated with viral replication and that high-level viral replication occurred selectively within the lesion and not in other tissues. These findings suggest that EIAV-associated neurologic disease is the direct result of viral replication.     

Pre-infection of pigs with Mycoplasma hyopneumoniae modifies outcomes of infection with European swine influenza virus of H1N1, but not H1N2, subtype

Swine influenza virus (SIV) and Mycoplasma hyopneumoniae (Mhp) are widespread in farms and are major pathogens involved in the porcine respiratory disease complex (PRDC). The aim of this experiment was to compare the pathogenicity of European avian-like swine H1N1 and European human-like reassortant swine H1N2 viruses in na?ve pigs and in pigs previously infected with Mhp. Six groups of SPF pigs were inoculated intra-tracheally with either Mhp, or H1N1, or H1N2 or Mhp+H1N1 or Mhp+H1N2, both pathogens being inoculated at 21 days intervals in these two last groups. A mock-infected group was included. Although both SIV strains induced clinical signs when singly inoculated, results indicated that the H1N2 SIV was more pathogenic than the H1N1 virus, with an earlier shedding and a greater spread in lungs. Initial infection with Mhp before SIV inoculation increased flu clinical signs and pathogenesis (hyperthermia, loss of appetite, pneumonia lesions) due to the H1N1 virus but did not modify significantly outcomes of H1N2 infection. Thus, Mhp and SIV H1N1 appeared to act synergistically, whereas Mhp and SIV H1N2 would compete, as H1N2 infection led to the elimination of Mhp in lung diaphragmatic lobes. In conclusion, SIV would be a risk factor for the severity of respiratory disorders when associated with Mhp, depending on the viral subtype involved. This experimental model of coinfection with Mhp and avian-like swine H1N1 is a relevant tool for studying the pathogenesis of SIV-associated PRDC and testing intervention strategies for the control of the disease.     

H5N1亚型禽流感病毒对鸭的致病性研究

为评估H5N1亚型禽流感病毒(AIV)在实验室环境下对鸭的致病力,本研究以无特殊病原(SPF)鸭为模型,对我国近年分离的7株病毒进行了致病力分析。结果发现其中4株病毒对鸭致死率为100%,2株病毒对鸭的致死率分别为60%和80%,另外1株病毒,A/goose/Hubei/51/05(GS/HB/51/05),对鸭无致病力。本研究还发现,与高致病力毒株一样,GS/HB/51/05也可在鸭体内呈全身性复制,并且可通过喉头和泻殖腔向外排泄。我们推测GS/HB/51/05可能是中国南方出现的其他对鸭呈高致病力的H5N1病毒的祖先,对这些病毒的系统研究,可揭示H5N1亚型AIV对鸭的致病力遗传机制。     

The immune response and maternal antibody interference to a heterologous H1N1 swine influenza virus infection following vaccination

This study investigated the efficacy of a bivalent swine influenza virus (SIV) vaccine in piglets challenged with a heterologous H1N1 SIV isolate. The ability of maternally derived antibodies (MDA) to provide protection against a heterologous challenge and the impact MDA have on vaccine efficacy were also evaluated. Forty-eight MDA(+) pigs and 48 MDA(-) pigs were assigned to 8 different groups. Vaccinated pigs received two doses of a bivalent SIV vaccine at 3 and 5 weeks of age. The infected pigs were challenged at 7 weeks of age with an H1N1 SIV strain heterologous to the H1N1 vaccine strain. Clinical signs, rectal temperature, macroscopic and microscopic lesions, virus excretion, serum and local antibody responses, and influenza-specific T-cell responses were measured. The bivalent SIV vaccine induced a high serum hemagglutination-inhibition (HI) antibody titer against the vaccine virus, but antibodies cross-reacted at a lower level to the challenge virus. This study determined that low serum HI antibodies to a challenge virus induced by vaccination with a heterologous virus provided protection demonstrated by clinical protection and reduced pneumonia and viral excretion. The vaccine was able to prime the local SIV-specific antibody response in the lower respiratory tract as well as inducing a systemic SIV-specific memory T-cell response. MDA alone were capable of suppressing fever subsequent to infection, but other parameters showed reduced protection against infection compared to vaccination. The presence of MDA at vaccination negatively impacted vaccine efficacy as fever and clinical signs were prolonged, and unexpectedly, SIV-induced pneumonia was increased compared to pigs vaccinated in the absence of MDA. MDA also suppressed the serum antibody response and the induction of SIV-specific memory T-cells following vaccination. The results of this study question the effectiveness of the current practice of generating increased MDA levels through sow vaccination in protecting piglets against disease.     

Cytokine responses to EHV-1 infection in immune and non-immune ponies

Protecting equids against equine herpesvirus-1 (EHV-1) infection remains an elusive goal. Repeated infection with EHV-1 leads to protective immunity against clinical respiratory disease, and a study was conducted to measure the regulatory cytokine response (IFN-gamma and IL-4) in repeatedly infected immune ponies compared to non-immune ponies. Two groups of four ponies were established. Group 1 ponies had previously been infected on two occasions, and most recently 7 months before this study. Group 2 ponies had no history no vaccination or challenge infection prior to this study. Both groups were subjected to an intranasal challenge infection with EHV-1, and blood samples were collected pre-infection, and at 7 and 21 days post-infection for preparation of PBMCs. At each time point, the in vitro responses of PBMCs to stimulation with EHV-1 were measured, including IFN-gamma and IL-4 mRNA production, and lymphoproliferation. Group 1 ponies showed no signs of clinical disease or viral shedding after challenge infection. Group 2 ponies experienced a biphasic pyrexia, mucopurulent nasal discharge, and nasal shedding of virus after infection. Group 1 ponies had an immune response characterized both before and subsequent to challenge infection by an IFN-gamma response to EHV-1 in the absence of an IL-4 response, and demonstrated increased EHV-1-specific lymphoproliferation post-infection. Group 2 ponies had limited cytokine or lymphoproliferative responses to EHV-1 pre-challenge, and demonstrated increases in both IFN-gamma and IL-4 responses post-challenge, but without any lymphoproliferative response. Protective immunity to EHV-1 infection was therefore characterized by a polarized IFN-gamma dependent immunoregulatory cytokine response.     

Modulation of the cellular immune responses to T-cell-dependent and T cell-independent antigens in lambs with induced bovine viral diarrhea virus infection

Functional interaction between lymphoid cells and lymphotropic viruses is particularly evident for bovine viral diarrhea virus (BVDV) in cattle and its closely related virus, the border disease virus (BVDV) in sheep. The most important aspect of acute or chronic phases of BVDV or BDV infection was the host's increased susceptibility to secondary bacterial or viral infection. To study the ability of BVDV to alter the development of the cellular immune responses to concomitant inoculation with T cell-dependent and T cell-independent antigens, lambs were inoculated twice with rabbit RBC and Escherichia coli lipopolysacharide (LPS) and then were infected with a cytopathic strain of BVDV at postinoculation day 3. Leukopenia characterized by lymphopenia developed after BVDV infection. Increased [3H]thymidine incorporation was observed in resting or lectin-stimulated blood mononuclear cells in the first weeks after inoculation in BVDV-infected lambs, but was followed by decreased [3H]thymidine incorporation after the second inoculation for up to 8 weeks after initial inoculation. In contrast, transient decrease of blastogenic responses, associated with toxic effect of LPS, was detected in inoculated noninfected lambs, but was followed by stimulation of cellular immune responses. Inoculated noninfected lambs had good in vitro cellular immune response to rabbit RBC and LPS antigens, whereas lymphocytes from BVDV-infected lambs could not mount lasting cellular immune responses to antigens or BVDV. Results suggest that BVDV infection in lambs modulates the ability of lymphocytes to respond to lectins or antigenic stimuli according to the time after infection.(ABSTRACT TRUNCATED AT 250 WORDS)     

Potential role of viral surface glycoproteins in the replication of H3N2 triple reassortant influenza A viruses in swine and turkeys

The H3N2 triple reassortant (TR) influenza viruses emerged in swine in 1998 and then in turkeys in 2003. It was then hypothesized that these viruses crossed the species barrier and transmitted from pigs to turkeys. In previous work we identified viruses with different transmission behavior between the two species, of which A/turkey/Ohio/313053/04 (TK04) transmitted both ways between swine and turkeys, and A/swine/North Carolina/03 (SW03) did not transmit either way between the two species. Utilizing the 12-plasmid reverse genetics (RG) system, we rescued two viruses (TK04 and SW03) with potentially different transmission behavior between pigs and turkeys. Single gene reassortants (SGR) were generated by switching the hemagglutinin (HA) or the neuraminidase (NA) genes between both viruses, and were evaluated for replication in vitro (pig and turkey tracheal/bronchial epithelial cells) and in vivo (pigs and turkeys). RG-created TK04 replicated more efficiently than SW03 in vitro and in vivo. Additionally, TK04 exhibited better binding affinity to plasma membrane preparations (PMP) from pig and turkey tracheal/bronchial epithelial cells compared to SW03. In study with SGR viruses, the HA protein was found to be essential for TK04 virus transmission amongst turkeys, but not sole factor contributing to the efficient replication of virus in turkeys and pigs. Such findings further highlight the polygenic nature of influenza virus pathogenesis.     

Two novel HLA-A*0201 T-cell epitopes in avian H5N1 viral nucleoprotein induced specific immune responses in HHD mice

The influenza A nucleoprotein (NP) is an attractive target for avian flu vaccine development because of its high conversancy in the evolutionary chain of the virus. Here we identified two novel HLA-A*0201 restricted NP epitopes, named H5N1 NP373-381 AMDSNTLEL (NP373) and NP458-466 FQGRGVFEL (NP458), using computational bioinformatic analysis. The NP peptides showed a high binding affinity to HLA-A*0201 on T2 cells, and were able to induce the activation of the cytotoxic T cells in the human peripheral blood mononuclear cells. We examined the potential of using NP373 and NP458 peptide sequences supplemented with a single-chain trimer as potential DNA vaccine candidates in an HHD transgenic mouse model. A gene gun delivery system was used for administrating the vaccine candidates into the animals. The results from cytotoxicity and ELISPOT assays indicated that a significant amount of IFN-γ was secreted by the T cells of the vaccinated mice, and the T cells were able to eliminate the corresponding peptide-loaded T2 cells. The discovery of these novel immunogenic NP peptides provides valuable information for avian flu vaccine design and construction.     

Hepatic immune response in calves during acute subclinical infection with bovine viral diarrhoea virus type 1

Eight colostrum-deprived calves aged 8-12 weeks were inoculated intranasally with a non-cytopathic strain of bovine viral diarrhoea virus (BVDV) genotype-1 and the effects on the hepatic immune response were studied. Two calves were sacrificed at each of 3, 6, 9 and 14 days post-inoculation (dpi) and two uninoculated animals were used as negative controls. BVDV was detected in hepatic macrophages and monocytes from 3 to 14dpi and in Küpffer cells (KCs) from 6 to 14dpi. Increases in the numbers of MAC387(+) KCs and monocytes, but not interstitial macrophages, differentiated by morphological features, were evident in the liver following inoculation with BVDV. There was a substantial increase in the number of monocytes positive for tumour necrosis factor (TNF)-α, but only small increases in the numbers of TNF-α(+) KCs and interstitial macrophages and interleukin (IL)-6(+) monocytes, KCs and interstitial macrophages. There was an increase in the number of interstitial CD3(+) T lymphocytes in the liver, but no substantial changes in the numbers of circulating CD3(+) T lymphocytes, interstitial or circulating CD4(+) or CD8(+) T lymphocytes, or CD79αcy(+) B lymphocytes. Serum haptoglobin and serum amyloid A increased transiently at 12dpi. Upregulation of some pro-inflammatory cytokines by hepatic macrophages is evident in subclinical acute BVDV type 1 infection in calves.     

Molecular characterization of pandemic H1N1 influenza viruses isolated from turkeys and pathogenicity of a human pH1N1 isolate in turkeys

Suspected human-to-animal transmission of the 2009 pandemic H1N1 (pH1N1) virus has been reported in several animal species, including pigs, dogs, cats, ferrets, and turkeys. In this study we describe the genetic characterization of pH1N1 viruses isolated from breeder turkeys that was associated with a progressive drop in egg production. Sequence analysis of all eight gene segments from three viruses isolated from this outbreak demonstrated homology with other human and swine pH1N1 isolates. The susceptibility of turkeys to a human pH1N1 isolate was further evaluated experimentally. The 50% turkey infectious dose (TID50) for the human isolate A/Mexico/LnDRE/4487/2009 was determined by inoculating groups of 8-10-week-old turkeys with serial 10-fold dilutions of virus by oronasal and cloacal routes. We estimated the TID50 to be between 1 x 10(5) and 1 x 10(6) TCID50. The pathogenesis of pH1N1 in oronasally or cloacally inoculated juvenile turkeys was also examined. None of the turkeys exhibited clinical signs, and no significant difference in virus shedding or seroconversion was observed between the two inoculation groups. More than 50% of the turkeys in both oronasal and cloacal groups shed virus beginning at 2 days postinoculation (dpi). All birds that actively shed virus seroconverted by 14 dpi. Virus antigen was demonstrated by immunohistochemistry in the cecal tonsils and bursa of Fabricius in two of the birds that were infected by the cloacal route. Virus transmission to naive contact turkeys was at best doubtful. This report provides additional evidence that pH1N1 can cross the species barrier and cause disease outbreaks in domestic turkeys. However, it appears that the reproductive status of the host as well as environmental factors such as concurrent infections, stress, the presence or absence of litter, and stocking density may also contribute to efficient infection and transmission of this agent.     

Distribution of viral antigen and development of lesions after experimental infection with highly virulent bovine viral diarrhea virus type 2 in calves

OBJECTIVE: To correlate tissue distribution with development of lesions after experimental infection with a virulent strain of noncytopathic bovine viral diarrhea virus (BVDV) type 2 in calves. ANIMALS: Ten 14-day-old and two 2-month-old colostrum-deprived calves. PROCEDURE: Calves were intranasally inoculated with BVDV type-2 strain 1373 from an outbreak of clinically severe bovine viral diarrhea (BVD).Two 14-day-old calves served as noninfected controls. Two calves each were euthanatized on postinoculation days 3, 6, and 12, and 1 each on days 8, 9, 13, and 14. Tissues were collected for immunohistologic and histologic examination. RESULTS: Inoculated calves developed nonspecific clinical signs characterized by high fever and decreased numbers of leukocytes and thrombocytes. Viral antigen was detected focally in lymphoid tissues on day 3. On days 6, 8, 9, 12, and 14, viral antigen became increasingly widespread throughout organs and tissues. Viral antigen in lymphoid tissues was associated with severe depletion of all compartments. Lesions in other tissues were not well correlated with distribution of viral antigen. Depletion of lymphoid tissues was observed in a calf on day 13, but viral antigen had been cleared from most tissues and was detected in vascular walls only. CONCLUSIONS AND CLINICAL RELEVANCE: Infection with a virulent BVDV strain resulted in wide dissemination of viral antigen in host tissues. Severe lymphoid depletion developed in lymphoid tissues, whereas viral antigen was generally not associated with lesions in other tissues. Findings suggest that development of lesions in acute BVD is not solely a function of viral replication and is also attributable to host reaction to infection.