Bone marrow transplantation in a child with Wiskott-Aldrich syndrome latently infected with acyclovir-resistant (ACV(r)) herpes simplex virus type 1: emergence of foscarnet-resistant virus originating from the ACV(r) virus

DNA polymerases of herpesviruses and their inhibitors

Human herpesviruses are large double-stranded DNA viruses belonging to the Herpesviridae family. The main characteristics of these viruses are their ability to establish a lifelong latency into the host with a potential to reactivate periodically. Primary infections and reactivations with herpesviruses are responsible for a large spectrum of diseases and may result in severe complications in immunocompromised patients. The viral DNA polymerase is a key enzyme in the replicative cycle of herpesviruses, and the target of most antiviral agents (i.e., nucleoside, nucleotide and pyrophosphate analogs). However, long-term prophylaxis and treatment with these antivirals may lead to the emergence of drug-resistant isolates harboring mutations in genes encoding viral enzymes that phosphorylate drugs (nucleoside analogs) and/or DNA polymerases, with potential cross-resistance between the different analogs. Drug resistance mutations mainly arise in conserved regions of the polymerase and exonuclease functional domains of these enzymes. In the polymerase domain, mutations associated with resistance to nucleoside/nucleotide analogs may directly or indirectly affect drug binding or incorporation into the primer strand, or increase the rate of extension of DNA to overcome chain termination. In the exonuclease domain, mutations conferring resistance to nucleoside/nucleotide analogs may reduce the rate of excision of incorporated drug, or continue DNA elongation after drug incorporation without excision. Mutations associated with resistance to pyrophosphate analogs may alter drug binding or the conformational changes of the polymerase domain required for an efficient activity of the enzyme. Novel herpesvirus inhibitors with a potent antiviral activity against drug-resistant isolates are thus needed urgently.

Characteristics of Helicase-primase Inhibitor Amenamevir-resistant Herpes Simplex Virus

The antiherpetic drug amenamevir (AMNV) inhibits the helicase-primase complex of herpes simplex virus type 1 (HSV-1), HSV-2 and varicella-zoster virus directly as well as inhibiting the replication of these viruses. Although several mutated HSV viruses resistant to helicase-primase inhibitors have been reported, the mutations contributing to the resistance remain unclear as recombinant viruses containing a single mutation have not been analyzed. We obtained AMNV-resistant viruses with amino acid substitutions by several passages under AMNV-treatment. Twenty HSV-1 and 19 HSV-2 mutants with mutation(s) in UL5 helicase and/or UL52 primase, but not in co-factor UL8, were isolated. The mutations in UL5 were located downstream of motif IV, with UL5 K356N in HSV-1 and K355N in HSV-2, in particular, identified as having the highest frequency: 9/20 and 9/19, respectively. We generated recombinant AMNV-resistant HSV-1 with a single amino acid substitution using BAC mutagenesis. As a result, G352C in UL5 helicase and F360C/V and N902T in UL52 primase were identified as novel mutations. The virus with K356N in UL5 showed 10-fold higher AMNV resistance than did other mutants, and showed equivalent viral growth in vitro and virulence in vivo as the parent HSV-1, although other mutants showed attenuated virulence. All recombinant viruses were susceptible to the other antiherpetic drugs, acyclovir and foscarnet. In conclusion, based on BAC mutagenesis, this study identified for the first time mutations in UL5 and UL52 that contributed to AMNV resistance, and found that a mutant with the most frequent K356N mutation in HSV-1 maintained viral growth and virulence equivalent to the parent virus.

ACUTE RETINAL NECROSIS: Difference in Outcome by Viral Type and Options for Antiviral Therapy

PURPOSE

To investigate differences in outcomes of acute retinal necrosis with confirmed viral polymerase chain reaction between viral types and highlight different treatment options.

METHODS

The study evaluated 22 eyes in 18 patients of polymerase chain reaction-positive acute retinal necrosis at the University of Pittsburgh Medical Center from 2007 to 2018. Outcome measures included final visual acuity, treatment paradigms, and retinal detachment rate.

RESULTS

Eight eyes were polymerase chain reaction-positive for varicella zoster virus, two eyes for herpes simplex virus Type 1 (HSV-1), and 12 eyes for herpes simplex virus Type 2 (HSV-2). Final Snellen best-corrected visual acuity averaged 20/51 for varicella zoster virus, 20/25 for HSV-1, and 20/814 for HSV-2. Retinal detachment occurred in 2 (25%) of varicella zoster virus eyes and 8 (75%) of HSV-2 eyes. One eye with HSV-1 and three eyes with HSV-2 received cidofovir for treatment of refractory retinitis.

CONCLUSION

Acute retinal necrosis secondary to HSV-2 tended to have persistent active retinitis with a higher rate of retinal detachment despite similar treatment protocols, suggesting that in some cases combination intravenous acyclovir and adjuvant intravitreal foscarnet injections are not sufficient. Despite the risk of renal toxicity, intravenous cidofovir may be a consideration in select patients.

Impact of Amino Acid Substitutions in Region II and Helix K of Herpes Simplex Virus 1 and Human Cytomegalovirus DNA Polymerases on Resistance to Foscarnet

Amino acid substitutions conferring resistance of herpes simplex virus 1 (HSV-1) and human cytomegalovirus (HCMV) to foscarnet (PFA) are located in the genes UL30 and UL54, respectively, encoding the DNA polymerase (pol). In this study, we analyzed the impact of substitutions located in helix K and region II that are involved in the conformational changes of the DNA pol. Theoretical substitutions were identified by sequences alignment of the helix K and region II of human herpesviruses (susceptible to PFA) and bacteriophages (resistant to PFA) and introduced in viral genomes by recombinant phenotyping. We characterized the susceptibility of HSV-1 and HCMV mutants to PFA. In UL30, the substitutions I619K (helix K), V715S, and A719T (both in region II) increased mean PFA 50% effective concentrations (EC₅₀s) by 2.5-, 5.6-, and 2.0-fold, respectively, compared to the wild type (WT). In UL54, the substitution Q579I (helix K) conferred hypersusceptibility to PFA (0.17-fold change), whereas the substitutions Q697P, V715S, and A719T (all in region II) increased mean PFA EC₅₀s by 3.8-, 2.8- and 2.5-fold, respectively, compared to the WT. These results were confirmed by enzymatic assays using recombinant DNA pol harboring these substitutions. Three-dimensional modeling suggests that substitutions conferring resistance/hypersusceptibility to PFA located in helix K and region II of UL30 and UL54 DNA pol favor an open/closed conformation of these enzymes, resulting in a lower/higher drug affinity for the proteins. Thus, this study shows that both regions of UL30 and UL54 DNA pol are involved in the conformational changes of these proteins and can influence the susceptibility of both viruses to PFA.

Differential impact of various substitutions at codon 715 in region II of HSV-1 and HCMV DNA polymerases

This study aimed at understanding the impact of different substitutions at codon 715 localized in the region II of the palm domain of herpes simplex virus 1 (HSV-1) and human cytomegalovirus (HCMV) DNA polymerases (pol). Here, we report a new theoretical mutation V715S that confers resistance of HSV-1 to foscarnet/acyclovir (5.6- and 9.2-fold increases EC₅₀ values compared to wild type, respectively) and of HCMV to foscarnet/ganciclovir (2.8- and 2.9-fold increases in EC₅₀ values compared to wild type, respectively). To further analyze the importance of this amino acid, we investigated the impact of the already known mutations V715M and V715G on the replicative capacities and drug susceptibilities of both viruses as well as on the activity and drug inhibition of the DNA pol. The V715G recombinant HSV-1 mutant was resistant to foscarnet and acyclovir (3.4- and 4.6-fold EC₅₀ increase, respectively) whereas the V715M mutant was susceptible to foscarnet and resistant to acyclovir (3.4-fold EC₅₀ increase). The V715G recombinant HCMV mutant did not grow and the V715M mutant was resistant to foscarnet (3.7-fold EC₅₀ increase) and susceptible to ganciclovir. Finally, we showed by three-dimensional modeling that the differential impact of these mutations on the viral replicative capacity and drug resistance profile was related to different hydrophobic local environments for V715 in the DNA pol of the two viruses. Furthermore, we hypothesize that the DNA pol of HSV-1 is more tolerant to changes at this residue compared to that of HCMV because of a more hydrophobic environment stabilizing the region.

Database on natural polymorphisms and resistance-related non-synonymous mutations in thymidine kinase and DNA polymerase genes of herpes simplex virus types 1 and 2

The use of genotypic resistance testing of herpes simplex virus types 1 and 2 (HSV-1 and HSV-2) is increasing because the rapid availability of results significantly improves the treatment of severe infections, especially in immunocompromised patients. However, an essential precondition is a broad knowledge of natural polymorphisms and resistance-associated mutations in the thymidine kinase (TK) and DNA polymerase (pol) genes, of which the DNA polymerase (Pol) enzyme is targeted by the highly effective antiviral drugs in clinical use. Thus, this review presents a database of all non-synonymous mutations of TK and DNA pol genes of HSV-1 and HSV-2 whose association with resistance or natural gene polymorphism has been clarified by phenotypic and/or functional assays. In addition, the laboratory methods for verifying natural polymorphisms or resistance mutations are summarized. This database can help considerably to facilitate the interpretation of genotypic resistance findings in clinical HSV-1 and HSV-2 strains.

2-[4,5-Difluoro-2-(2-fluorobenzoylamino)-benzoylamino]benzoic acid, an antiviral compound with activity against acyclovir-resistant isolates of herpes simplex virus types 1 and 2

Herpes simplex viruses 1 and 2 (HSV-1 and HSV-2) are responsible for lifelong latent infections in humans, with periods of viral reactivation associated with recurring ulcerations in the orofacial and genital tracts. In immunosuppressed patients and neonates, HSV infections are associated with severe morbidity and, in some cases, even mortality. Today, acyclovir is the standard therapy for the management of HSV infections. However, the need for novel antiviral agents is apparent, since HSV isolates resistant to acyclovir therapy are frequently isolated in immunosuppressed patients. In this study, we assessed the anti-HSV activity of the antiadenoviral compounds 2-[2-(2-benzoylamino)-benzoylamino]benzoic acid (benzavir-1) and 2-[4,5-difluoro-2-(2-fluorobenzoylamino)-benzoylamino]benzoic acid (benzavir-2) on HSV-1 and HSV-2. Both compounds were active against both viruses. Importantly, benzavir-2 had potency similar to that of acyclovir against both HSV types, and it was active against clinical acyclovir-resistant HSV isolates.

Characterization of virus strains resistant to the herpes virus helicase-primase inhibitor ASP2151 (Amenamevir)

ASP2151 is an antiherpes agent targeting the helicase-primase complex of herpes simplex virus (HSV)-1, HSV-2, and varicella-zoster virus (VZV). We characterized the ASP2151-resistant HSV-1 and HSV-2 variants or mutants based on findings from sequencing analysis, growth, pathogenicity, and susceptibility testing, identifying several single base-pair substitutions resulting in amino acid changes in the helicase and primase subunit of ASP2151-resistant mutants. Amino acid alterations in the helicase subunit were clustered near helicase motif IV in the UL5 helicase gene of both HSV-1 and HSV-2, while the primase subunit substitution associated with reduced susceptibility, R367H, was found in ASP2151-resistant HSV-1 mutants. However, while susceptibility in the ASP2151-resistant HSV mutants to existing antiherpes agents was equivalent to that in wild-type HSV strains, ASP2151-resistant HSV mutants showed attenuated in vitro growth capability and in vivo pathogenicity compared with the parent strains. Taken together, our present findings demonstrated that important amino acid substitutions associated with reduced susceptibilities of HSV-1 and HSV-2 to ASP2151 exist in both the helicase and primase subunits of the helicase-primase complex, and that mutations in this complex against ASP2151 might confer defects in viral replication and pathogenicity.

Expression of herpes simplex virus type 1 recombinant thymidine kinase and its application to a rapid antiviral sensitivity assay

Antiviral-resistant herpesvirus infection has become a great concern for immunocompromised patients. Herpes simplex virus type 1 (HSV-1) infections are treated with viral thymidine kinase (vTK)-associated drugs such as acyclovir (ACV), and most ACV-resistance (ACV(r)) is due to mutations in the vTK. The standard drug sensitivity test is usually carried out by the plaque reduction assay-based method, which requires over 10 days. To shorten the time required, a novel system was developed by the concept, in which 293T cells transiently expressing recombinant vTK derived from the test sample by transfection of the cells with an expression vector were infected with vTK-deficient and ACV(r) HSV-1 (TAR), and then cultured in a maintenance medium with or without designated concentrations of ACV, ganciclovir (GCV) and brivudine (BVdU). The replication of TAR was strongly inhibited by ACV, GCV and BVdU in 293T cells expressing recombinant vTK of the ACV-sensitive HSV-1, whereas replication was not or slightly inhibited in cells expressing the recombinant vTK of highly resistant or intermediately resistant HSV-1, respectively. An inverse correlation was demonstrated in the 50% effective concentrations (EC(50)s) and inhibitory effects of these compounds on the replication of TAR among ACV(s) and ACV(r) HSV-1 clones. These results indicate that the EC(50)s of the vTK-associated drugs including ACV can be assumed by measuring the inhibitory effect of drugs in 293T cells expressing recombinant vTK of the target virus. The newly developed antiviral sensitivity assay system for HSV-1 makes it possible to estimate EC(50) for vTK-associated drugs, when whole vTK gene is available for use by gene amplification directly from lesion's samples or from virus isolates.

Resistance of herpes simplex viruses to nucleoside analogues: mechanisms, prevalence, and management

Herpes simplex viruses (HSV) type 1 and type 2 are responsible for recurrent orolabial and genital infections. The standard therapy for the management of HSV infections includes acyclovir (ACV) and penciclovir (PCV) with their respective prodrugs valacyclovir and famciclovir. These compounds are phosphorylated by the viral thymidine kinase (TK) and then by cellular kinases. The triphosphate forms selectively inhibit the viral DNA polymerase (DNA pol) activity. Drug-resistant HSV isolates are frequently recovered from immunocompromised patients but rarely found in immunocompetent subjects. The gold standard phenotypic method for evaluating the susceptibility of HSV isolates to antiviral drugs is the plaque reduction assay. Plaque autoradiography allows the associated phenotype to be distinguished (TK-wild-type, TK-negative, TK-low-producer, or TK-altered viruses or mixtures of wild-type and mutant viruses). Genotypic characterization of drug-resistant isolates can reveal mutations located in the viral TK and/or in the DNA pol genes. Recombinant HSV mutants can be generated to analyze the contribution of each specific mutation with regard to the drug resistance phenotype. Most ACV-resistant mutants exhibit some reduction in their capacity to establish latency and to reactivate, as well as in their degree of neurovirulence in animal models of HSV infection. For instance, TK-negative HSV mutants establish latency with a lower efficiency than wild-type strains and reactivate poorly. DNA pol HSV mutants exhibit different degrees of attenuation of neurovirulence. The management of ACV- or PCV-resistant HSV infections includes the use of the pyrophosphate analogue foscarnet and the nucleotide analogue cidofovir. There is a need to develop new antiherpetic compounds with different mechanisms of action.

Genital herpes due to acyclovir-sensitive herpes simplex virus caused secondary and recurrent herpetic whitlows due to thymidine kinase-deficient/temperature-sensitive virus

Herpes simplex virus (HSV)-2 caused a genital ulcer in a 40-year-old allogenic stem cell recipient, and a secondary herpetic whitlow appeared during 2 months of acyclovir (ACV) therapy. Both genital ulcer, and whitlow were cured 3 months later, but 6 months after recovery the whitlow alone recurred. DNA of the genital, first, and recurrent whitlow isolates showed similar endonuclease digestion fragment profiles. The genital virus was ACV-sensitive, and the two whitlow isolates were ACV-resistant/thymidine kinase (TK)-deficient. The TK gene of the whitlow isolates had the same frame shift from the 274th amino acid and termination at the 347th amino acid due to the deletion of a cytosine at the 819th nucleotide. Because the temperature of the thumb is 33/34 degrees C or lower, the temperature sensitivity of the isolates were compared, and both whitlow isolates were significantly more temperature-sensitive (ts) at 39 degrees C than the genital isolate. The two whitlow isolates showed cutaneous pathogenicity in mouse ear pinna but not midflank, while the genital isolate was pathogenic at both sites, suggesting that temperature adaptation was an important element of pathogenicity in the whitlow. The virus populations of isolates of the genital, and first whitlow were examined by 31, and 82 clones, respectively, and the clones from genital, and whitlow isolates were ACV-sensitive, and -resistant, respectively, showing their homogeneity. The acyclovir-sensitive genital lesion had spread as a TK-deficient/ts herpetic whitlow during ACV treatment, and an apparently TK-deficient virus adapted to the local temperature might have caused the whitlow recurrence.

Biocatalysed synthesis of beta-O-glucosides from 9-fluorenon-2-carbohydroxyesters. Part 3: IFN-inducing and anti-HSV-2 properties

In pursuing research on the antiviral, interferon (IFN)-inducing tilorone congeners, a new series of fluoren-carboxyhydroxyesters has been prepared and biologically explored. These esters have subsequently been used as sugar acceptors in the enzymatic transglycosylation reaction using the 'retaining' beta-glycosidase from the archaeon Sulfolobus solfataricus (Ssbeta-Gly). Both aglycones (1-6) and corresponding beta-glucosides (beta-glu 1-beta-glu 6) have been screened for cytotoxicity, interferon-stimulating and antiviral properties against HSV-2. It was found that the addition of compounds beta-glu 5, beta-glu 6 and beta-glu 4 to HSV-2 infected U937 cells downregulates viral replication and triggers cells to release IFN-alpha/beta. Taken together, the results showed improved pharmacological profiles as a consequence of glycosylation. A molecular modelling study carried out on this series of compounds completed the structural characterisation of the novel compounds.

Genotypic characterization of the DNA polymerase and sensitivity to antiviral compounds of foscarnet-resistant herpes simplex virus type 1 (HSV-1) derived from a foscarnet-sensitive HSV-1 strain

Foscarnet is widely used for the treatment of acyclovir-resistant herpesvirus infections, and foscarnet-resistant herpesvirus infections are a serious concern in immunocompromised patients. Twenty-seven single-plaque isolates of herpes simplex virus type 1 (HSV-1) resistant to foscarnet were selected from foscarnet- and acyclovir-sensitive HSV-1 strain TAS by exposure to foscarnet, and the DNA polymerase genes were analyzed. The sensitivities of these mutants to foscarnet, cidofovir, S2242, acyclovir, ganciclovir, and penciclovir were determined. A single amino acid substitution, double amino acid substitutions, and a combination of a single amino acid substitution with a deletion or insertion of amino acid residues in the viral DNA polymerase were demonstrated in 21, 4, and 2 isolates, respectively. Of the 27 isolates, an amino acid substitution of serine for asparagine at amino acid position 724 in the DNA polymerase (724 S-N) was detected in 8 isolates. An amino acid substitution in conserved region II was demonstrated in these eight isolates as well as four other isolates. The mutation in the DNA polymerase responsible for resistance to foscarnet was located between the pre-IV region and conserved region V, especially within conserved region II. All the isolates were sensitive or hypersensitive to cidofovir and ganciclovir. Seven, 5, and 15 of the 27 isolates were also sensitive to S2242, acyclovir, and penciclovir, respectively. Thus, most of the foscarnet-resistant HSV-1 isolates were sensitive or hypersensitive to cidofovir and ganciclovir.

Construction of recombinant herpes simplex virus type I expressing green fluorescent protein without loss of any viral genes

For use in various applications in research on herpes simplex virus type 1, we attempted to generate recombinant HSV-1 expressing green fluorescent protein (GFP) without any loss of viral genes. Our results were as follows. (i) A recombinant HSV-1 (YK333), in which a GFP expression cassette driven by the Egr-1 promoter was inserted into the intergenic region between UL3 and UL4, was constructed. (ii) YK333 replicated as well as wild-type HSV-1 F strain in Vero cells. (iii) As one application of the recombinant YK333 for research on HSV-1, we developed a system to detect anti-herpetic activity, termed a fluorescence-based anti-viral assay. The 50% inhibitory concentration of ganciclovir for YK333 determined using our newly developed assay was comparable to that determined using a plaque reduction assay. YK333 will be a convenient tool for herpes simplex virus research, including such applications as monitoring of viral replication in vitro and in vivo, and rapid screening of potential anti-herpetic agents.

Surveillance network for herpes simplex virus resistance to antiviral drugs: 3-year follow-up

Herpes simplex virus (HSV) infections are very common in the general population and among immunocompromised patients. Acyclovir (ACV) is an effective treatment which is widely used. We deemed it essential to conduct a wide and coordinated survey of the emergence of ACV-resistant HSV strains. We have formed a network of 15 virology laboratories which have isolated and identified, between May 1999 and April 2002, HSV type 1 (HSV-1) and HSV-2 strains among hospitalized subjects. The sensitivity of each isolate to ACV was evaluated by a colorimetric test (C. Danve, F. Morfin, D. Thouvenot, and M. Aymard, J. Virol. Methods 105:207-217, 2002). During this study, 3900 isolated strains among 3357 patients were collected; 55% of the patients were immunocompetent. Only six immunocompetent patients excreted ACV-resistant HSV strains (0.32%), including one female patient not treated with ACV who was infected primary by an ACV-resistant strain. Among the 54 immunocompromised patients from whom ACV-resistant HSV strains were isolated (3.5%), the bone marrow transplantation patients showed the highest prevalence of resistance (10.9%), whereas among patients infected by human immunodeficiency virus, the prevalence was 4.2%. In 38% of the cases, the patients who excreted the ACV-resistant strains were treated with foscarnet (PFA), and 61% of them developed resistance to PFA. The collection of a large number of isolates enabled an evaluation of the prevalence of resistance of HSV strains to antiviral drugs to be made. This prevalence has remained stable over the last 10 years, as much among immunocompetent patients as among immunocompromised patients.

Drug resistance patterns of recombinant herpes simplex virus DNA polymerase mutants generated with a set of overlapping cosmids and plasmids

Herpes simplex virus (HSV) DNA polymerase (Pol) mutations can confer resistance to all currently available antiherpetic drugs. However, discrimination between mutations responsible for drug resistance and those that are part of viral polymorphism can be difficult with current methodologies. A new system is reported for rapid generation of recombinant HSV type 1 (HSV-1) DNA Pol mutants based on transfection of a set of overlapping viral cosmids and plasmids. With this approach, twenty HSV-1 recombinants with single or dual mutations within the DNA pol gene were successfully generated and subsequently evaluated for their susceptibilities to acyclovir (ACV), foscarnet (FOS), cidofovir (CDV), and adefovir (ADV). Mutations within DNA Pol conserved regions II (A719T and S724N), VI (L778M, D780N, and L782I), and I (F891C) were shown to induce cross-resistance to ACV, FOS, and ADV, with two of these mutations (S724N and L778M) also conferring significant reduction in CDV susceptibility. Mutant F891C was associated with the highest levels of resistance towards ACV and FOS and was strongly impaired in its replication capacity. One mutation (D907V) lying outside of the conserved regions was also associated with this ACV-, FOS-, and ADV-resistant phenotype. Some mutations (K522E and Y577H) within the delta-region C were lethal, whereas others (P561S and V573M) induced no resistance to any of the drugs tested. Recombinants harboring mutations within conserved regions V (N961K) and VII (Y941H) were resistant to ACV but susceptible to FOS. Finally, mutations within conserved region III were associated with various susceptibility profiles. This new system allows a rapid and accurate evaluation of the functional role of various DNA Pol mutations, which should translate into improved management of drug-resistant HSV infections.

Genotypic and phenotypic characterization of the thymidine kinase of ACV-resistant HSV-1 derived from an acyclovir-sensitive herpes simplex virus type 1 strain

Twenty-four strains of acyclovir (ACV)-resistant (ACV(r)) herpes simplex virus type 1 (HSV-1) were generated from the HSV-1 TAS strain by exposure to ACV, and the genotype and phenotype of the thymidine kinase (TK) from these mutants were analyzed. The TK polypeptide of the ACV(r) HSV-1 strains was examined by Western blot using an anti-HSV-1 TK rabbit serum. The sensitivity of each strain to ACV, foscarnet and cidofovir (CDV) was also determined. A single guanine (G) insertion or a single cytosine (C) deletion was detected in 12 of the 24 ACV(r) strains at the G or C homopolymer stretches within the TK gene. Genotypic analysis predicted that two thirds of the ACV(r) HSV-1 strains expressed truncated TK polypeptides, while one third expressed viral TK polypeptide with a single amino acid substitution at various sites. Western blot abnormalities in the viral TK polypeptides were identified in 21 ACV(r) strains. There was an inverse correlation between the susceptibility of the HSV-1 mutant strains to ACV and that to CDV. Nucleotide sequencing of the TK gene and Western blot analysis of the viral TK polypeptides are considered to be one of the methods for predicting virus sensitivity to ACV and CDV.