Laboratory-scale inactivation of African swine fever virus and swine vesicular disease virus in pig slurry

Two methods were evaluated for the inactivation of African swine fever (ASV) and swine vesicular disease (SVD) viruses in pig slurry: chemical treatment and heat treatment. The addition of NaOH or Ca(OH)2 at different concentration/time combinations at 4 degrees C and 22 degrees C was examined, as was virus stability at different temperature/time combinations. ASF virus (ASFV) was less resistant to both methods than SVD virus (SVDV). In slurry from one source, ASFV was inactivated at 65 degrees C within 1 min, whereas SVDV required at least 2 min at 65 degrees C. However, it was found that thermal inactivation depended on the characteristics of the slurry used. Addition of 1% (w/v) of NaOH or Ca(OH)2 caused the inactivation of ASFV within 150 s at 4 degrees C; 0.5% (w/v) NaOH or Ca(OH)2 required 30 min for inactivation. NaOH or Ca(OH)2 (1% (w/v)) was not effective against SVDV at 22 degrees C after 30 min, and 1.5% (w/v) NaOH or Ca(OH)2 caused inactivation of SVDV at both 4 degrees C and 22 degrees C. At higher chemical concentrations or temperatures, ASFV and SVDV inactivation was faster in slurry than in buffered medium.     

猪瘟病毒C株共感染口蹄疫病毒影响口蹄疫病毒复制

【背景】猪瘟(Classical Swine Fever)是由猪瘟病毒(Classical Swine Fever Virus,CSFV)引起的猪高度接触性传染病,致死率极高。在临床中存在着CSFV与猪其他病原菌共感染的情况,例如CSFV与口蹄疫病毒(Foot-and-Mouth Disease Virus,FMDV)的共感染。【目的】利用CSFV与FMDV共感染猪源宿主细胞,研究CSFV与FMDV共感染对FMDV病毒复制的影响。【方法】构建体外共感染细胞模型,在正常PK-15细胞上进行CSFV共感染FMDV实验,通过观察细胞病变效应(Cytopathic Effect,CPE)、实时荧光定量PCR(RT-qPCR)、Western Blot、间接免疫荧光检测CSFV和FMDV共感染及FMDV单独感染情况下FMDV复制水平的差异。利用RT-qPCR筛选鉴定能够影响FMDV复制的CSFV蛋白。【结果】CSFVC株共感染FMDV能够抑制FMDV的复制,而且灭活的CSFV同样抑制FMDV的复制。通过筛选鉴定出CSFV的C蛋白能够抑制FMDV复制。【结论】研究发现CSFV C株共感染FMDV能够抑制FMDV复制,而其C蛋白具有抑制FMDV复制的能力。     

Chemical disinfection of high-consequence transboundary animal disease viruses on nonporous surfaces

Disinfection is a critical part of the response to transboundary animal disease virus (TADV) outbreaks by inactivating viruses on fomites to help control infection. To model the inactivation of TADV on fomites, we tested selected chemicals to inactivate Foot and Mouth Disease virus (FMDV), African Swine Fever virus (ASFV), and Classical Swine Fever virus (CSFV) dried on steel and plastic surfaces. For each of these viruses, we observed a 2 to 3 log reduction of infectivity due to drying alone. We applied a modified surface disinfection method to determine the efficacy of selected disinfectants to inactivate surface-dried high-titer stocks of these three structurally different TADV. ASFV and FMDV were susceptible to sodium hypochlorite (500 and 1000 ppm, respectively) and citric acid (1%) resulting in complete disinfection. Sodium carbonate (4%), while able to reduce FMDV infectivity by greater than 4-log units, only reduced ASFV by 3 logs. Citric acid (2%) did not totally inactivate dried CSFV, suggesting it may not be completely effective for disinfection in the field. Based on these data we recommend disinfectants be formulated with a minimum of 1000 ppm sodium hypochlorite for ASFV and CSFV disinfection, and a minimum of 1% citric acid for FMDV disinfection.     

Efficacy of vaporized hydrogen peroxide against exotic animal viruses.

The efficacy of vapor-phase hydrogen peroxide in a pass-through box for the decontamination of equipment and inanimate materials potentially contaminated with exotic animal viruses was evaluated. Tests were conducted with a variety of viral agents, which included representatives of several virus families (Orthomyxoviridae, Reoviridae, Flaviviridae, Paramyxoviridae, Herpesviridae, Picornaviridae, Caliciviridae, and Rhabdoviridae) from both avian and mammalian species, with particular emphasis on animal viruses exotic to Canada. The effects of the gas on a variety of laboratory equipment were also studied. Virus suspensions in cell culture media, egg fluid, or blood were dried onto glass and stainless steel. Virus viability was assessed after exposure to vaporphase hydrogen peroxide for 30 min. For all viruses tested and under all conditions (except one), the decontamination process reduced the virus titer to 0 embryo-lethal doses for the avian viruses (avian influenza and Newcastle disease viruses) or less than 10 tissue culture infective doses for the mammalian viruses (African swine fever, bluetongue, hog cholera, pseudorabies, swine vesicular disease, vesicular exanthema, and vesicular stomatitis viruses). The laboratory equipment exposed to the gas appeared to suffer no adverse effects. Vaporphase hydrogen peroxide decontamination can be recommended as a safe and efficacious way of removing potentially virus-contaminated objects from biocontainment level III laboratories in which exotic animal disease virus agents are handled.     

Capsid functions of inactivated human picornaviruses and feline calicivirus

The exceptional stability of enteric viruses probably resides in their capsids. The capsid functions of inactivated human picornaviruses and feline calicivirus (FCV) were determined. Viruses were inactivated by UV, hypochlorite, high temperature (72 degrees C), and physiological temperature (37 degrees C), all of which are pertinent to transmission via food and water. Poliovirus (PV) and hepatitis A virus (HAV) are transmissible via water and food, and FCV is the best available surrogate for the Norwalk-like viruses, which are leading causes of food-borne and waterborne disease in the United States. The capsids of all 37 degrees C-inactivated viruses still protected the viral RNA against RNase, even in the presence of proteinase K, which contrasted with findings with viruses inactivated at 72 degrees C. The loss of ability of the virus to attach to homologous cell receptors was universal, regardless of virus type and inactivation method, except for UV-inactivated HAV, and so virus inactivation was almost always accompanied by the loss of virus attachment. Inactivated HAV and FCV were captured by homologous antibodies. However, inactivated PV type 1 (PV-1) was not captured by homologous antibody and 37 degrees C-inactivated PV-1 was only partially captured. The epitopes on the capsids of HAV and FCV are evidently discrete from the receptor attachment sites, unlike those of PV-1. These findings indicate that the primary target of UV, hypochlorite, and 72 degrees C inactivation is the capsid and that the target of thermal inactivation (37 degrees C versus 72 degrees C) is temperature dependent.     

Vaccines prepared from chimeras of foot-and-mouth disease virus (FMDV) induce neutralizing antibodies and protective immunity to multiple serotypes of FMDV.

The G-H loop of VP1 (residues 132 to 159) of foot-and-mouth disease virus (FMDV) is a prominent feature on the virion surface and has an important role in vaccine efficacy, generation of antigenic variants, and cell binding. Using an infectious cDNA of FMDV, we have constructed serotype A viruses in which the G-H loop has been substituted with the homologous sequences from serotype O or C. These chimeric viruses replicated to high titer and displayed plaque morphologies similar to those of wild-type viruses, demonstrating that the functions provided by the loop can be readily exchanged between serotypes. Monoclonal antibody analyses showed that epitopes contained within the loop were transferred to the chimeras and that epitopes encoded by the type A backbone were maintained. Chemically inactivated vaccines prepared from chimeric viruses induced antibodies in guinea pigs that neutralized both type A and either type O or type C viruses. Swine inoculated with the A/C chimera vaccine also produced cross-reactive antibodies, were protected from challenge with the type A virus, and partially protected against challenge with type C. These studies emphasize the importance of epitopes outside of the G-H loop in protective immunity in swine, which is a natural host of FMDV.     

Guinea pig-adapted foot-and-mouth disease virus with altered receptor recognition can productively infect a natural host

We report that adaptation to infect the guinea pig did not modify the capacity of foot-and-mouth disease virus (FMDV) to kill suckling mice and to cause an acute and transmissible disease in the pig, an important natural host for this pathogen. Adaptive amino acid replacements (I(248)-->T in 2C, Q(44)-->R in 3A, and L(147)-->P in VP1), selected upon serial passages of a type C FMDV isolated from swine (biological clone C-S8c1) in the guinea pig, were maintained after virus multiplication in swine and suckling mice. However, the adaptive replacement L(147)-->P, next to the integrin-binding RGD motif at the GH loop in VP1, abolished growth of the virus in different established cell lines and modified its antigenicity. In contrast, primary bovine thyroid cell cultures could be productively infected by viruses with replacement L(147)-->P, and this infection was inhibited by antibodies to alphavbeta6 and by an FMDV-derived RGD-containing peptide, suggesting that integrin alphavbeta6 may be used as a receptor for these mutants in the animal (porcine, guinea pig, and suckling mice) host. Substitution T(248)-->N in 2C was not detectable in C-S8c1 but was present in a low proportion of the guinea pig-adapted virus. This substitution became rapidly dominant in the viral population after the reintroduction of the guinea pig-adapted virus into pigs. These observations illustrate how the appearance of minority variant viruses in an unnatural host can result in the dominance of these viruses on reinfection of the original host species.     

Viricidal effects of Lactobacillus and yeast fermentation

The survival of selected viruses in Lactobacillus- and yeast-fermented edible waste material was studied to determine the feasibility of using this material as a livestock feed ingredient. Five viruses, including Newcastle disease virus, infectious canine hepatitis virus, a porcine picornavirus, frog virus 3, and bovine virus diarrhea, were inoculated into a mixture of ground food waste (collected from a school lunch program) containing Lactobacillus acidophilus. Mixtures were incubated at 20, 30, and 40 degrees C for 216 h. In a second trial, four viruses, including Newcastle disease virus, infectious canine hepatitis virus, frog virus 3, and a porcine picornavirus, were inoculated into similar edible waste material containing Saccharomyces cerevisiae. Mixtures were incubated at 20 and 30 degrees C for 216 h. Samples were obtained daily for quantitative (trial 1) and qualitative (trial 2) virus isolation. Temperature, pH, and redox potential were monitored. Controlled pH and temperature studies were also done and compared with the inactivation rates in the fermentation processes. In trial 1 (Lactobacillus fermentation), infectious canine hepatitis virus survived the entire test period in the fermentation process but was inactivated below pH 4.5 in the controlled studies. Newcastle disease virus was inactivated by day 8 in the fermentation process and appeared to be primarily heat sensitive and secondarily pH sensitive in the controlled studies. The porcine picornavirus survived the fermentation process for 8 days at 20 degrees C but was inactivated more rapidly at 30 and 40 degrees C. The controlled studies verified these findings. Frog virus 3 was inactivated by day 3 in the fermentation process and appeared to be sensitive to low pH in the controlled studies. Bovine virus diarrhea was rapidly inactivated in the fermentation process (less than 2 h) and was pH and temperature sensitive. In trial 2 (yeast fermentation), infectious hepatitis virus survived the entire test period in the fermentation process. Newcastle disease virus was inactivated by day 7 at 20 degrees C and day 6 at 30 degrees C. The porcine picornavirus was inactivated by day 7 at 30 degrees C but survived the entire test period at 20 degrees C. Frog virus 3 was inactivated by day 3 at 20 degrees C and day 2 at 30 degrees C.     

Inactivation kinetics of model and relevant blood-borne viruses by treatment with sodium hydroxide and heat.

To determine the efficacy of a clean-in-place system for the inactivation of viruses present in human plasma, the effect of 0.1 M sodium hydroxide at 60 degrees C on viral infectivity was investigated. Inactivation of the following model and relevant viruses were followed as a function of time: human hepatitis A virus (HAV), canine parvovirus (CPV; a model for human parvovirus B-19) pseudorabies virus (PRV, a model for hepatitis B virus), and bovine viral diarrhoea virus (BVDV, a model for hepatitis C virus and human immunodeficiency virus). Infectivity of CPV was determined by a novel in situ EIA method which will prove useful for studies to validate parvovirus inactivation or removal. Infectivity of BVDV, PRV and CPV were shown to be reproducibly inactivated below the limit of detection by 0.1 M NaOH at 60 degrees C within 30 s. HAV was inactivated to below the limit of detection within 2 min. Treatment with heat alone also resulted in some log reduction for all viruses tested except for CPV which remained unaffected after heating at 60 degrees C for 16 min. Treatment of HAV with hydroxide alone (up to 1.0 m) at 15 degrees C did not lead to rapid inactivation. Collectively, these data suggest that 0.1 M NaOH at 60 degrees C for two min should be sufficient to inactivate viruses present in process residues.     

猪源胰酶中特异病毒和非特异病毒安全检测简

目的：对猪源胰酶样品进行病毒检测,以评价其病毒安全性。方法：非特异性病毒检测采用致细胞病变、血凝吸附试验和形态学检测方法;特异性病毒检测包括猪繁殖与呼吸障碍综合征病毒（PRRSV）、猪瘟病毒（CSFV）、猪圆环病毒（PCV）、猪细小病毒（PPV）、猪口蹄疫病毒（FMDV）和猪伪狂犬病病毒（PRV）特异性核酸检测,以及CSFV、PPV、PRV特定性抗原蛋白直接免疫荧光检测。结果：受检样品非特异性病毒检测中,未见可观察到的细胞病变产生和病毒粒子,对豚鼠、鸡和人的0型红细胞无凝集现象。特异性病毒检测中,PRRSV、CSFV、PCV、PPV、FMDV和PRV核酸检测均为阴性,CSFV、PPV、PRV免疫荧光检测均为阴性。结论：猪源胰酶样品经非特异性病毒检测和特异性病毒检测均无可检出的病毒存在。     

Genetic and phenotypic variation of foot-and-mouth disease virus during serial passages in a natural host

Foot-and-mouth disease virus (FMDV), like other RNA viruses, exhibits high mutation rates during replication that have been suggested to be of adaptive value. However, even though genetic variation in RNA viruses and, more specifically, FMDV has been extensively examined during virus replication in a wide variety of in vitro cell cultures, very little is known regarding the generation and effects of genetic variability of virus replication in the natural host under experimental conditions and no genetic data are available regarding the effects of serial passage in natural hosts. Here, we present the results of 20 serial contact transmissions of the highly pathogenic, pig-adapted O Taiwan 97 (O Tw97) isolate of FMDV in swine. We examined the virus genomic consensus sequences for a total of 37 full-length viral genomes recovered from 20 in vivo passages. The characteristics and distributions of changes in the sequences during the series of pig infections were analyzed in comparison to the O Tw97 genomes recovered from serially infected BHK-21 cell cultures. Unexpectedly, a significant reduction of virulence upon pig passages was observed, and finally, interruption of the viral transmission chain occurred after the14th pig passage (T14). Virus was, however, isolated from the tonsils and nasal swabs of the asymptomatic T15 pigs at 26 days postcontact, consistent with a natural establishment of the carrier state previously described only for ruminants. Surprisingly, the region encoding the capsid protein VP1 (1D) did not show amino acid changes during in vivo passages. These data demonstrate that contact transmission of FMDV O Tw97 in pigs mimics the fitness loss induced by the bottleneck effect, which was previously observed by others during plaque-to-plaque FMDV passage in vitro, suggesting that unknown mechanisms of virulence recovery might be necessary during the evolution and perpetuation of FMDV in nature.     

Evaluation of a genetically modified foot-and-mouth disease virus vaccine candidate generated by reverse genetics

ABSTRACT: BACKGROUND: Foot-and-mouth disease (FMD) is the most economically important and highly contagious disease of cloven-hoofed animals worldwide. Control of the disease has been mainly based on large-scale vaccinations with whole-virus inactivated vaccines. In recent years, a series of outbreaks of type O FMD occurred in China (including Chinese Taipei, Chinese Hong Kong) posed a tremendous threat to Chinese animal husbandry. Its causative agent, type O FMDV, has evolved into three topotypes (East-South Asia (ME-SA), Southeast Asia (SEA), Cathay (CHY)) in these regions, which represents an important obstacle to disease control. The available FMD vaccine in China shows generally good protection against ME-SA and SEA topotype viruses infection, but affords insufficient protection against some variants of the CHY topotype. Therefore, the choice of a new vaccine strain is of fundamental importance. RESULTS: The present study describes the generation of a full-length infectious cDNA clone of FMDV vaccine strain and a genetically modified virus with some amino acid substitutions in antigenic sites 1, 3, and 4, based on the established infectious clone. The recombinant viruses had similar growth properties to the wild O/HN/CHA/93 virus. All swine immunized with inactivated vaccine prepared from the O/HN/CHA/93 were fully protected from challenge with the viruses of ME-SA and SEA topotypes and partially protected against challenge with the virus of CHA topotype at 28 days post-immunization. In contrast, the swine inoculated with the genetically modified vaccine were completely protected from the infection of viruses of the three topotypes. CONCLUSIONS: Some amino acid substitutions in the FMDV vaccine strain genome did not have an effect on the ability of viral replication in vitro. The vaccine prepared from genetically modified FMDV by reverse genetics significantly improved the protective efficacy to the variant of the CHA topotype, compared with the wild O/HN/CHA/93 virus. Thus, the full-length cDNA clone of FMDV can be a useful tool to develop genetically engineered FMDV vaccine candidates to help control porcinophilic FMD epidemics in China.     

型内嵌合口蹄疫病毒全长感染性cDNA克隆的构建

【目的】近年来,O型口蹄疫的不断暴发严重危害了我国畜牧业的发展,其病原——O型口蹄疫病毒已演化出3种谱系:中国型猪毒系、泛亚系和缅甸98系。其中中国型猪毒系病毒高度嗜猪,对养猪业危害最大。目前应用的疫苗已不能有效保护中国型猪毒系变异株的流行,这给我国猪口蹄疫的防控带来了极大的困难。为了进一步发展免疫原性好、抗原谱广的猪O型口蹄疫疫苗候选株,本研究以O/HN/93现用疫苗毒株的感染性克隆为骨架,用流行的新猪毒系病毒的部分VP3和VP1基因(主要是替换VP1蛋白上的B-C环和G-H环)替换疫苗毒株的相应部分,构建了嵌合的FMDV全长cDNA克隆。【方法】线化的嵌合全长质粒和表达T7 RNA聚合酶的真核质粒pcDNAT7P共转染BHK-21细胞,体内转录拯救嵌合病毒。【结果】嵌合全长质粒转染BHK-21细胞36h后,出现明显的FMDV致细胞病变效应。对收获的病毒分别用RT-PCR、间接免疫荧光、电子显微镜观察结果证实成功拯救到嵌合的FMDV。拯救的病毒乳鼠致病性试验结果表明该拯救病毒对乳鼠的致病力减弱。该嵌合病毒的成功拯救为研制口蹄疫新型疫苗等奠定了基础。     

Foot-and-mouth disease virus and poliovirus particles contain proteins of the replication complex.

Nonstructural proteins 2C, 3CD, 3C, and 3D, and the cellular protein actin, are present in highly purified preparations of foot-and-mouth disease virus (FMDV) and poliovirus. They remain bound in variable amounts to the RNAs when the RNAs are extracted from the viruses with phenol or phenol-sodium dodecyl sulfate (SDS) and, for FMDV, when the RNA is released from the particles by a lowering of the pH below 7. RNA prepared by these methods is rapidly degraded at 37 degrees C, particularly in the presence of NH4+ ions, but hydrolysis can be prevented by antibody against Escherichia coli-expressed 3D, indicating that it is the RNA polymerase that has nuclease activity. In contrast, virion RNA from which the nonstructural proteins and actin have been removed by extraction with guanidine thiocyanate-phenol-chloroform or proteinase K-phenol is stable at 37 degrees C, although its specific infectivity is lower than that of the RNA extracted with phenol or phenol-SDS. The possible implications of the close association of replication complex proteins with the RNA in virus particles are discussed.     

Skin as a potential source of infectious foot and mouth disease aerosols

This review examines whether exfoliated, virus-infected animal skin cells could be an important source of infectious foot and mouth disease virus (FMDV) aerosols. Infectious material rafting on skin cell aerosols is an established means of transmitting other diseases. The evidence for a similar mechanism for FMDV is: (i) FMDV is trophic for animal skin and FMDV epidermis titres are high, even in macroscopically normal skin; (ii) estimates for FMDV skin cell aerosol emissions appear consistent with measured aerosol emission rates and are orders of magnitude larger than the minimum infectious dose; (iii) the timing of infectious FMDV aerosol emissions is consistent with the timing of high FMDV skin concentrations; (iv) measured FMDV aerosol sizes are consistent with skin cell aerosols; and (v) FMDV stability in natural aerosols is consistent with that expected for skin cell aerosols. While these findings support the hypothesis, this review is insufficient, in and of itself, to prove the hypothesis and specific follow-on experiments are proposed. If this hypothesis is validated, (i) new FMDV detection, management and decontamination approaches could be developed and (ii) the relevance of skin cells to the spread of viral disease may need to be reassessed as skin cells may protect viruses against otherwise adverse environmental conditions.     

Antibodies to selected viral disease agents in wild boars from the Czech Republic

Blood samples were collected from wild boar (Sus scrofa) shot during the hunting season from 1999 to 2005 in the Czech Republic. Sera were tested by enzyme-linked immunosorbent assay for the presence of antibodies against classical swine fever virus (CSFV), swine vesicular disease virus (SVDV), Aujeszky's disease virus (ADV), and bovine viral diarrhea virus (BVDV). Indirect fluorescence antibody test was used for detection of antibodies against porcine circovirus type 2 (PCV-2) and transmissible gastroenteritis virus (TGEV). Antibodies against ADV, BVDV, PCV-2, and TGEV were detected in 30% (101 of 338), 1% (2 of 352), 43% (57 of 134), and 1% (1 of 134) of wild boars, respectively. Sera of 6,471 and 362 tested wild boars were negative for the presence of antibodies against CSFV and SVDV, respectively. This is the first survey of TGEV antibodies in wild boars and the first serologic survey of viral diseases in wild boars in the Czech Republic. Wild boars in the Czech Republic may act as a potential reservoir of ADV and thus have a role in the epidemiology of this disease.     

Thermal inactivation of foot-and-mouth disease viruses in suspension

The heat resistance of foot-and-mouth disease virus (FMDV) strains isolated from outbreaks in Thailand was investigated in phosphate-buffered saline (PBS) at 50, 60, 70, 80, 90, and 100 degrees C. The first-order kinetic model fitted most of the observed linear inactivation curves. The ranges of decimal-reduction time (D value) of FMDV strains at 50, 60, 70, 80, 90, and 100 degrees C were 732 to 1,275 s, 16.37 to 42.00 s, 6.06 to 10.87 s, 2.84 to 5.99 s, 1.65 to 3.18 s, and 1.90 to 2.94 s, respectively. The heat resistances of FMDV strains at lower temperature (50 degrees C) were not serotype specific. The effective inactivating temperature is approximately 60 degrees C. Heat resistances of FMDV strains at 90 and 100 degrees C were not statistically different (P > 0.05), while the FMDV serotype O (OPN) appeared to be the most heat resistant at 60 to 80 degrees C. The other observed inactivation curves were linear with shoulder or tailing (biphasic curves). The shoulder effect was mostly observed at 90 and 100 degrees C, while the tailing effect was mostly observed at 50 to 80 degrees C. The adjusted D values in the case of shoulder and tailing effects did not affect the overall estimated heat resistance of these FMDV strains, so even unadjusted D values of deviant inactivation curves were legitimate. The z values of FMDV serotypes O, A, and Asia 1 were 21.78 to 23.26, 20.75 to 22.79, and 19.87 degrees C, respectively. The z values of FMDV strains studied were not statistically significantly different (P > 0.05). The results of this study indicated that the heat resistance in PBS of FMDV strains from Thailand was much less than had been reported for foreign epidemic FMDV strains.     

Extensive cell heterogeneity during persistent infection with foot-and-mouth disease virus.

Coevolution of viruses and the host cells occurred in BHK-21 cell cultures persistently infected with foot-and-mouth disease virus (FMDV) (J. C. de la Torre, E. Martínez-Salas, J. Diez, A. Villaverde, F. Gebauer, E. Rocha, M. Dávila, and E. Domingo, J. Virol. 62:2050-2058, 1988). In the present report we provide evidence of an extreme phenotypic heterogeneity of the cells, which was generated in the course of persistence. A total of 248 stable cell clones isolated from FMDV carrier cultures at early or late passages were analyzed. At least six distinct cell phenotypes were distinguished with regard to cell morphology, resistance to FMDV strain C-S8c1, and cell growth characteristics. No infectious FMDV or viral RNA was detected in variant cell clones, suggesting that the altered phenotypes were caused by inheritable cell modifications, selected in the course of persistence. Thus, the FMDV-BHK-21 carrier cell system must be described as a dynamic interaction between an evolving heterogeneous population of virus and multiple cell variants. We suggest that cell heterogeneity confers a selective advantage for long-term virus and cell survival by providing the cell population with a range of responses toward FMDV.