Antiviral activity of clevudine [L-FMAU, (1-(2-fluoro-5-methyl-beta, L-arabinofuranosyl) uracil)] against woodchuck hepatitis virus replication and gene expression in chronically infected woodchucks (Marmota monax).

L: -FMAU [1-(2-fluoro-5-methyl-beta,L-arabinofuranosyl) uracil] has been shown to be an effective inhibitor of hepatitis B virus (HBV) and duck hepatitis B virus replication in cell culture and duck hepatitis B virus replication in acutely infected Peking ducks. The woodchuck hepatitis virus (WHV) and its natural host, the Eastern woodchuck (Marmota monax), have been established as a predictive model for the evaluation of antiviral therapies against chronic HBV infection. In this report, the antiviral activity of l-FMAU against WHV replication in chronically infected woodchucks is described. Four weeks of once-daily oral administration of L-FMAU significantly reduced viremia, antigenemia, intrahepatic WHV replication, and intrahepatic expression of woodchuck hepatitis virus core antigen (WHcAg) in a dose-dependent manner. At the highest dose administered (10 mg/kg/d), significant reductions of intrahepatic WHV RNA and covalently closed circular (ccc)WHV-DNA levels also were observed. The reduction in viremia was remarkably rapid at the higher doses of L-FMAU, with greater than 1,000-fold reductions in WHV-DNA serum levels observed after as little as 2 to 3 days of therapy. Following the withdrawal of therapy, a dose-related delay in viremia rebound was observed. At the highest doses used, viremia remained significantly suppressed in at least one half of the treated animals for 10 to 12 weeks' posttreatment. No evidence of drug-related toxicity was observed in the treated animals. L-FMAU is an exceptionally potent antihepadnaviral agent in vitro and in vivo, and is a suitable candidate for antiviral therapy of chronic HBV infection.

Antiviral activities of oral 1-O-hexadecylpropanediol-3-phosphoacyclovir and acyclovir in woodchucks with chronic woodchuck hepatitis virus infection.

Acyclovir triphosphate is a potent inhibitor of hepatitis B virus DNA polymerase, but acyclovir treatment provides no benefit in patients with hepatitis B virus infection. This is due in part to the fact that hepatitis B virus, unlike herpes simplex virus, does not code for a viral thymidine kinase which catalyzes the initial phosphorylation of acyclovir. We synthesized 1-O-octadecyl-sn-glycero-3-phospho (3-P)-acyclovir and found that it was highly active in reducing hepatitis B virus replication in 2.2. 15 cells, while acyclovir was inactive. The greater antiviral activity of 1-O-octadecyl-sn-glycero-3-P-acyclovir appeared to be due to liver cell metabolism of the compound to acyclovir monophosphate (K. Y. Hostetler et al., Biochem. Pharmacol. 53:1815-1822, 1997). However, a closely related compound without a hydroxyl group at the sn-2 position of glycerol, 1-O-hexadecylpropanediol-3-P-acyclovir, was more active and selective in 2.2.15 cells in vitro. In this study, we treated woodchucks chronically infected with woodchuck hepatitis virus with increasing oral doses of 1-O-hexadecylpropanediol-3-P-acyclovir and assessed the response to therapy versus acyclovir or a placebo. At a dosage of 10 mg/kg of body weight twice a day, the test compound significantly inhibited viral replication in vivo, as indicated by a 95% reduction in serum woodchuck hepatitis virus DNA levels and by a 54% reduction in levels of woodchuck hepatitis virus replicative intermediates in the liver. Higher doses were somewhat less effective. In contrast, 20 mg of acyclovir/kg twice daily, a 5. 3-fold-higher molar dosage, had no demonstrable activity against woodchuck hepatitis virus. Oral 1-O-hexadecylpropanediol-3-P-acyclovir appeared to be safe and effective in chronic woodchuck hepatitis virus infection.

Effects of age and viral determinants on chronicity as an outcome of experimental woodchuck hepatitis virus infection.

Acute hepadnavirus infections either resolve or progress to chronicity. Factors that influence chronicity as an outcome of hepatitis B virus (HBV) infection in humans can be studied experimentally in the woodchuck model. Accordingly, several woodchuck hepatitis virus (WHV) inocula were characterized. Representative inocula had high titers of infectious virus (approximately 10(7.7)-10(9.5) woodchuck 50% infectious doses per milliliter [WID(50%)/mL] by subcutaneous inoculation), with 1 WID(50%) ranging between 21 and 357 physical virion particles. WHV7P1 (standard high dose, 5 x 10(6) WID(50%)) produced a 72% chronicity rate (i.e., percent chronic of total infected) in neonatal woodchucks (1-3 days old). Comparable doses of WHV8P1 resulted in a lower chronicity rate in neonates (34% chronic) indicating that it represented a strain different from WHV7P1. Neonatal woodchucks were more susceptible to chronic infection by high doses of WHV7P1 (range, 65%-75% chronic) compared with 8-week-old weanlings (33% chronic) and adult woodchucks (0% chronic; i.e., all resolved). High doses of cloned wild-type viruses also induced high rates of chronicity in neonates (70%-80% chronic). Chronicity rates in neonates were decreased for low doses of WHV7P1 (500 WID(50%), 9% chronic) and for high doses of a precore WHeAg-minus mutant WHV8 clone (17% chronic). Thus, both age and viral determinants can influence chronicity as an outcome of experimental WHV infection. Standardized inocula will enable the study of mechanisms that initiate and maintain chronic hepadnavirus infection and also provide a means for developing WHV carriers for therapeutic studies.

Antiviral activity and toxicity of fialuridine in the woodchuck model of hepatitis B virus infection.

Woodchucks were used to study the antiviral activity and toxicity of fialuridine (FIAU; 1,-2'deoxy-2'fluoro-1-beta-D-arabinofuranosyl-5-iodo-uracil). In an initial experiment, groups of six chronic woodchuck hepatitis virus (WHV) carrier woodchucks received daily doses of FIAU by intraperitoneal injection for 4 weeks. At 0.3 mg/kg/d, the antiviral effect was equivocal, but at 1.5 mg/kg/d, FIAU had significant antiviral activity. No evidence of drug toxicity was observed during the 4-week period of treatment or during posttreatment follow-up. In a second experiment, groups of nine WHV carriers or uninfected woodchucks were given 1.5 mg/kg/d of FIAU orally for 12 weeks, and the results compared with placebo-treated controls. After 4 weeks, the serum WHV-DNA concentration in the FIAU-treated carrier group was two to three logs lower than that in the placebo-treated group. After 12 weeks of FIAU treatment, serum WHV DNA was not detectable by conventional dot-blot analysis, hepatic WHV-DNA replicative intermediates (RI) had decreased 100-fold, and hepatic expression of WHV core antigen was remarkably decreased. No evidence of toxicity was observed after 4 weeks, but, after 6 to 7 weeks, food intake decreased and, after 8 weeks, the mean body weights of woodchucks treated with FIAU were significantly lower than controls. Anorexia, weight loss, muscle wasting, and lethargy became progressively severe, and all FIAU-treated woodchucks died or were euthanized 78 to 111 days after treatment began. Hepatic insufficiency (hyperbilirubinemia, decreased serum fibrinogen, elevated prothrombin time), lactic acidosis, and hepatic steatosis were characteristic findings in the final stages of FIAU toxicity in woodchucks. The syndrome of delayed toxicity in woodchucks was similar to that observed previously in humans treated with FIAU, suggesting that the woodchuck should be valuable in future investigations of the molecular mechanisms of FIAU toxicity in vivo and for preclinical toxicological evaluation of other nucleoside analogs before use in patients.

Titration of recombinant woodchuck hepatitis virus DNA in adult woodchucks.

In vivo transfection of Eastern woodchucks (Marmota monax) with recombinant woodchuck hepatitis virus (WHV) DNA is effective in inducing virus infection for the study of replication, pathogenicity, and oncogenicity of wild-type and mutated WHV. The one drawback to this procedure is the need for preparation of large amounts of WHV DNA. Reduction of the amount of WHV DNA in the transfection protocol necessary to induce infection would save considerable time and resources. Therefore, we conducted a titration of WHV DNA, ranging from 50 micrograms to 50 pg of DNA, in adult woodchucks to determine the minimum infectious dose of recombinant WHV DNA. As little as 50 ng of transfected WHV DNA induced productive infection in adult woodchucks. Thus, transfection with large amounts of recombinant WHV DNA appears to be unnecessary.

Depletion of mitochondrial DNA, destruction of mitochondria, and accumulation of lipid droplets result from fialuridine treatment in woodchucks (Marmota monax).

Fialuridine (FIAU, 1-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)-5-iodouracil) is toxic to liver, heart, muscle, and nerve in clinical trials for chronic viral hepatitis (CH). Mitochondrial toxicity was hypothesized. To address pathophysiologic mechanisms, we examined mitochondrial changes in FIAU-treated woodchucks (WC) with CH from woodchuck hepatitis virus infection. WC (with and without CH from woodchuck hepatitis virus infection) were treated with FIAU (1.5 mg/kg/day) for 12 weeks. WC were killed. Liver, heart, skeletal muscle, and kidney samples underwent DNA extraction and were analyzed ultrastructurally (transmission electron microscopy). Myocardium, skeletal muscles, and liver samples were analyzed histologically. Abundance of hepatic, myocardial, muscle, and kidney mtDNA decreased in FIAU-treated WC, but the magnitude varied. mtDNA decreased 55% in heart, 65% in kidney, 74% in liver, and 87% in muscle (p < 0.02 for each tissue: FIAU-treated versus FIAU-untreated). Cellular damage was characterized ultrastructurally by mitochondrial enlargement, cristae dissolution, and lipid droplets. Lipid droplets found in the heart, diaphragm, biceps, and liver were sufficient to identify FIAU-treated WC (p < 0.05 each). Widespread mitochondrial damage to many tissues resulted from chronic FIAU treatment and occurred irrespective of CH. It manifested with mtDNA depletion, intracytoplasmic lipid droplets, and destroyed mitochondrial cristae. Defective mtDNA replication with mtDNA depletion seems central to the subcellular pathophysiology of altered energy metabolism and multiorgan failure in FIAU toxicity.

Liver-targeted antiviral nucleosides: enhanced antiviral activity of phosphatidyl-dideoxyguanosine versus dideoxyguanosine in woodchuck hepatitis virus infection in vivo.

It would be desirable to develop antiviral agents that can be targeted to liver to enhance their antiviral effects and reduce nonhepatic toxicity. 2',3'-Dideoxyguanosine (ddG) has been found to be a potent and selective antihepatitis B agent both in vitro and in vivo. To evaluate ddG and its liver-targeted analog, we synthesized a series of phosphatidyl-ddGs and incubated them with 2.2.15 cells, which chronically produce hepatitis B virus. 1,2-Dipalmitoylphosphatidyl-dideoxyguanosine (DPP-ddG) inhibited the production of hepatitis B virus (HBV) DNA in the culture medium by 90% at 4.5 mumol/L versus 9.1 mumol/L for ddG, while the liposome vehicle itself had no effect. To compare the efficacy of free ddG with its lipid prodrug in vivo, we treated woodchucks that were experimentally infected with woodchuck hepatitis virus (WHV) for 4 weeks by intraperitoneal injection of 2.6 mumol/kg/d of free ddG or liposomes containing 2.6 mumol/kg/d of DPP-ddG. Liposomal DPP-ddG reduced serum WHV DNA by 23- to 46-fold at the end of the fourth week, while free ddG reduced serum WHV DNA by 2.2- to 10.4-fold. Treatment with small unilamellar liposomes containing DPP-ddG is substantially more effective than free ddG in reducing WHV-DNA levels in serum in WHV-infected woodchucks. The data suggest that the use of lipid prodrugs to target the liver may be useful in enhancing antiviral therapy of hepatitis.

Use of paromomycin for treatment of cryptosporidiosis in a cat.

Cryptosporidium oocysts were found in the feces of a 6-month-old female cat with persistent diarrhea. The oocysts disappeared from the feces immediately after treatment with paromomycin (165 mg/kg of body weight, PO, for 5 days), and the diarrhea eventually resolved.

DNA ploidy analysis of hepatic preneoplastic and neoplastic lesions in woodchucks experimentally infected with woodchuck hepatitis virus.

We analyzed the DNA ploidy and the nuclear size of hepatocytes within hepatocellular carcinoma, putative preneoplastic (clear cell and basophilic foci) and adjacent non-neoplastic liver in 30 woodchucks neonatally infected with the woodchuck hepatitis virus. In livers from control woodchucks, in clear cell foci and in most chronic portal hepatitis, the hepatocytes were diploid, with less than 10% tetraploid cells. Aneuploid peaks were found in 50% of the livers with chronic active hepatitis, in 63% of basophilic foci and in 90% of hepatocellular carcinoma. Within the same tumor, aneuploid peaks with different DNA indices were observed frequently, indicating heterogeneity of tumor. S-phase was always elevated, indicating an increased rate of proliferation. Aneuploid cells had nuclei that were larger than those of control liver cells. In some basophilic foci and in some livers with chronic active hepatitis, abnormal DNA was demonstrated before the development of hepatocellular carcinoma, suggesting that these may be populations of hepatocytes at risk of neoplastic transformation.

Heterogeneity of the woodchuck hepatitis virus genome in a chronically infected woodchuck.

The nucleotide sequence of an isolate of woodchuck hepatitis virus (WHV) from the serum of a woodchuck trapped in New York state (WHVNY) was compared with the sequences of previously published isolates. The nucleotide sequence of WHVNY was closest to that of an isolate originating from New Jersey: the two genomes shared a 15 nucleotide in-frame deletion in the region where the presurface and polymerase genes overlap (nucleotides 3260-3274) and differed by 54 point mutations (1.6% of genome). Amino acid differences ranged from 0.4% in the surface gene to 5.7% in the X gene. Three isolates from woodchucks that originated in Pennsylvania and Maryland did not contain the deletion and differed from WHVNY by 102 to 106 point mutations (3.0% to 3.2% of nucleotides). Amino acid changes ranged from 0.5% in the core gene to 5.7% in the X-gene. Thus, WHVNY differed little from previous isolates. Next, the genomes from 102 independent clones of WHVNY were compared to ascertain the extent of sequence variation among WHV genomes in a chronically infected animal. A total of 98 clones had genomes of unit length while 2 clones had genomes shorter than unit length and 2 clones had genomes longer than unit length. The clones not of unit length possessed deletions or inverted duplications of sequence. The rate of mutation in the viral genes was 2.65 mutations per 10,000 nucleotides in the precore domain, 1.27 per 10,000 in the X-gene, 0.98 per 10,000 in the presurface gene, and 3.77 per 10,000 at the 5' end of the core gene.(ABSTRACT TRUNCATED AT 250 WORDS)

The woodchuck hepatitis virus X gene is important for establishment of virus infection in woodchucks.

All mammalian hepadnaviruses possess a gene, termed X, that encodes a protein capable of transactivating virus gene expression. The X gene overlaps the polymerase and precore genes as well as two newly identified open reading frames (ORFs) termed ORF5 and ORF6. In this investigation, we examined whether ORF5, ORF6, and the X gene were important for the replication of woodchuck hepatitis virus (WHV) in susceptible woodchucks. First, we investigated whether proteins were produced from ORF5 and ORF6 by in vitro translation of appropriate viral transcripts, searched for antibodies against the putative proteins in the sera of animals infected with wild-type virus, and looked for an antisense WHV transcript, necessary for expression of a protein from ORF6, in the livers of acutely or chronically infected woodchucks. All such experiments yielded negative results. Next, we used oligonucleotide-directed mutagenesis to introduce termination codons into ORF5 and ORF6 at two locations within each ORF. Adult woodchucks in groups of three were transfected with one of the four mutant genomes. All of these woodchucks developed WHV infections that were indistinguishable from those of animals transfected with the wild-type WHV recombinant. Polymerase chain reaction amplification and direct DNA sequencing confirmed that reversion of the mutants to a wild-type genotype did not occur. Taken together, these data indicate that ORF5 and ORF6 are not essential for virus replication and are unlikely to represent authentic genes. Finally, we generated five WHV X-gene mutants that either removed the initiation codon for protein synthesis or truncated the carboxyl terminus of the protein by 3, 16, 31, or 52 amino acids. Groups of three adult woodchucks were transfected with one of the five X-gene mutants. Only the mutant that possessed an X gene lacking 3 amino acids from the carboxyl terminus was capable of replication within the 6-month time frame of the experiment. In contrast, all seven woodchucks transfected with wild-type WHV DNA developed markers consistent with viral infection. Thus, it is likely (P < 0.01) that the WHV X gene is important for virus replication in the natural host.

The precore gene of the woodchuck hepatitis virus genome is not essential for viral replication in the natural host.

A number of naturally occurring hepatitis B virus mutants that cannot synthesize the virus precore protein have been identified. Such mutants have been associated with more severe forms of hepatitis, including fulminant hepatitis. The most common mutation observed is a substitution of G to A in the distal precore gene that converts a codon specifying Trp (TGG) to a termination codon (TAG). Using oligonucleotide-directed mutagenesis, we have produced the same point mutation in the precore gene of an infectious clone of woodchuck hepatitis virus (WHV). Transfection of mutant WHV DNA into the livers of adult woodchucks resulted in replication of the mutant in three of three susceptible animals. Levels of virus replication and transient elevations in liver enzymes in serum were similar to those of adult animals infected with wild-type WHV. Virions, found to possess mutant precore genes by polymerase chain reaction amplification and DNA sequencing, were recovered from the serum of one of the animals and inoculated subcutaneously into neonatal woodchucks. They produced infection in all five animals studied. The level of virus replication in neonatal animals infected with this mutant virus was comparable to that found in neonatal woodchucks infected with wild-type WHV, but none of five woodchucks infected with the precore mutant virus as neonates became chronic virus carriers. It was concluded that the precore gene of the WHV genome is not essential for virus replication in the natural host but may be important for chronic infection.

Immunosuppression with cyclosporine during the incubation period of experimental woodchuck hepatitis virus infection increases the frequency of chronic infection in adult woodchucks.

The immunosuppressive agent cyclosporine was given to adult woodchucks during acute experimental infection with woodchuck hepatitis virus (WHV). All 17 woodchucks given WHV alone or with a vehicle resolved the infection (i.e., zero chronicity), but when cyclosporine was given throughout the incubation and acute phases of infection (0-12 or 14 weeks; n = 12), the rate of chronic infection increased to 92%. When cyclosporine was given only during the incubation period (0-4 weeks; n = 10) or only during the acute phase of infection (2-12 weeks; n = 9), the rates increased to 50% and 55%, respectively. However, when the drug was given after the acute phase (8-18 weeks; n = 9), the chronic infection rate (11%) did not differ from that in untreated and vehicle controls. Immune responses inhibited by cyclosporine are important in resolution of acute WHV infection and occur mainly during the first 8 weeks. Immunosuppression of these responses for even short intervals during incubation (e.g., 0-4 weeks) increases the risk of chronicity.

Immunopathology of glomerulonephritis associated with chronic woodchuck hepatitis virus infection in woodchucks (Marmota monax).

Retrospective analysis of necropsy findings of 705 woodchucks was performed to determine the prevalence and morphology of immune-mediated glomerulonephritis, its relationship to woodchuck hepatitis virus (WHV) infection, and the presence of major WHV antigens. Twenty-six woodchucks had glomerular lesions. Renal tissue of the 26 animals was evaluated histologically and immunohistochemically for immune-mediated glomerulonephritis. Of these 26 animals, immune-mediated glomerulonephritis was diagnosed in six, all of which were chronic WHV carriers. Membranous glomerulonephritis was identified in three animals, two of which also had mesangial proliferation. Host immunoglobulin was present within the mesangium and along capillary loops in all three. Woodchuck hepatitis virus core antigen (WHcAg) was present along capillary loops of two of these animals, one membranous and one mixed, and in the mesangium of all three. Woodchuck hepatitis virus surface antigen (WHsAg) deposition was similar to WHcAg deposition but was only present along capillaries in those animals with mixed nephritis. The remaining three animals had mesangial proliferation. WHsAg and host immunoglobulin deposition were predominately mesangial; WHcAg was not detected. Transmission electron microscopy showed thickening of the capillary loop basement membranes and subepithelial electron-dense deposits in animal one, and deposits in the mesangium in animal six.

Cyclosporin A modulates the course of woodchuck hepatitis virus infection and induces chronicity.

Immunosuppression is known to influence the state of chronic hepatitis B virus infection, and is thought to increase the risk of developing chronic infection in newly exposed individuals. Cyclosporin A (CsA), an immunosuppressive agent that inhibits Th cell function, was administered to woodchucks chronically infected with woodchuck hepatitis virus (WHV), and resulted in a decreased severity of chronic hepatitis and an increased viremia during the treatment. Adult woodchucks inoculated with WHV and given CsA for 14 wk had increased viremias, decreased acute phase liver injury, and developed chronic infections at a higher rate compared with immunocompetent woodchucks given virus alone (chronicity in seven of seven WHV + CsA + vs zero of nine WHV + CsA-; p less than 0.001). These results in a relevant animal model of hepatitis B virus infection indicate: 1) that liver injury in acute hepadnavirus infections is immune-mediated and not a direct cytopathic effect of virus replication; 2) that Th cells function in the inflammatory response and in the immunologic control of hepadnavirus infection; and 3) that suppression of Th cell function in acute hepadnavirus infection decreases liver injury but alters the outcome of infection in favor of chronicity. These results also suggest continued challenges in the application of CsA in liver transplantation for hepatitis B virus-induced diseases.

Woodchuck hepatitis virus is a more efficient oncogenic agent than ground squirrel hepatitis virus in a common host.

Chronic infection with hepatitis B viruses (hepadnaviruses) is a major cause of hepatocellular carcinoma (HCC), but the incubation time varies from 1 to 2 years to several decades in different host species infected with indigenous viruses. To discern the influence of viral and host factors on the kinetics of induction of HCC, we exploited the recent observation that ground squirrel hepatitis virus (GSHV) is infectious in woodchucks (C. Seeger, P. L. Marion, D. Ganem, and H. E. Varmus, J. Virol. 61:3241-3247, 1987) to compare the pathogenic potential of GSHV and woodchuck hepatitis virus (WHV) in chronically infected woodchucks. Chronic GSHV infection in woodchucks produces mild to moderate portal hepatitis, similar to that observed in woodchucks chronically infected with WHV. However, HCC developed in GSHV carriers about 18 months later than in WHV carriers. Thus, although both viruses are oncogenic in woodchucks, GSHV and WHV differ in oncogenic determinants that can affect the kinetics of appearance of HCC in chronically infected animals.

Evaluation of praziquantel for treatment of experimentally induced paragonimiasis in dogs and cats.

Praziquantel was used successfully for treatment of a small number of dogs and 1 cat infected with Paragonimus kellicotti. To further evaluate the usefulness of this drug in treating such infections, 7 cats and 7 dogs were inoculated orally with metacercariae (12 and 20 to 22, respectively) obtained from crayfish, then were treated after the infections became patent; 2 cats and 2 dogs served as noninfected controls. Beginning 1 week before infection, and continuing weekly thereafter, physical, hematologic, and fecal examinations were performed on each animal; thoracic radiography was performed every other week. By postinoculation week 6, all dogs given metacercariae had patent infection diagnosed on the basis of positive results of fecal examination. By postinoculation week 7, 5 cats had confirmed patent infection, but 2 cats given metacercariae never had patent infection or had signs of infection. Clinical signs of infection were minor and included increased respiratory tract noise, slight inducible cough, or mild dyspnea. Transient eosinophilia was detected in dogs around postinoculation week 3. Pretreatment radiography revealed cavitated lesions in cats only; pleural lines and patchy infiltrates in cats and dogs; or pneumothorax in dogs only. The treatment regimen consisted of 23 mg of praziquantel/kg of body weight given every 8 hours for 3 days; 1 infected cat and dog were not treated. By 11 days after treatment, eggs had disappeared from the feces of infected animals, and marked resolution of lung lesions was evident radiographically.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence against a requisite role for defective virus in the establishment of persistent hepadnavirus infections.

The factors involved in the establishment of persistent hepadnavirus infection are poorly understood. Recent findings demonstrate that the sequence of the genome of hepatitis B virus (HBV) is variable in infected individuals and that, in some cases, virus mutants predominate. Our objectives in the present study were to analyze the variability of woodchuck hepatitis virus (WHV) genomes in an infected animal and to determine whether sequence heterogeneity played a critical role in the ability of WHV to induce chronic infection. We cloned and determined the complete nucleotide sequence of three supercoiled genomes from an animal that became infected after inoculation with a standardized WHV serum pool (i.e., the WHV7 virus pool). We found that there were four nucleotide substitutions among the three genome sequences as well as a 73-nucleotide deletion in one of the recombinants. DNA transfection experiments revealed that only one of the three recombinants was capable of independent replication. These data suggest that a significant proportion of replicative templates in woodchucks that are infected with WHV are defective virus genomes. Next, we compared the outcome of acute infection after inoculation with a serum pool containing a uniform population of replication competent virus (i.e., the WHV7R pool) with a serum pool composed of WHV genomes of variable sequence. The WHV7R serum pool originated from a woodchuck that became a chronic carrier after in vivo transfection of the liver with the infectious WHV7 recombinant. Neonatal woodchucks were inoculated with 5 x 10(6) WHV genome equivalents of either the WHV7 pool or the WHV7R pool. All animals in the study became acutely infected with WHV. Of the animals infected with the WHV7 serum pool, 65% became chronic carriers, while 80% of the animals infected with the WHV7R serum pool developed chronic infection. Thus, infection of woodchucks with a serum pool containing defective virus resulted in a rate of chronic WHV infection that was similar to, or even lower than, a rate from a pool containing only wild-type virus. This suggests that the presence of defective virus in the inoculum is not a prerequisite for the establishment of persistent hepadnavirus infections.