Consequences and management of pain in herpes zoster

Effect of CT-Guided Gasserian Ganglion Block with Local Anesthetics and Steroids on Acute/Subacute Zoster-Related Trigeminal Neuralgia: A Multicenter Retrospective Study

Purpose

To further evaluate the effects of gasserian ganglion block treatment with local anesthetics and steroids on patients with acute/subacute zoster-related trigeminal neuralgia.

Patients and Methods

This is a multicenteric retrospective study which included patients between the ages of 26–92 years, who suffered from acute/subacute zoster-related trigeminal neuralgia and had undergone CT-guided gasserian ganglion block. The medical records of these patients between January 13, 2016 and August 25, 2021 were retrieved, and the patients were followed up for 6 months. Follow-up outcomes were Numerical Rating Scale (NRS) scores, medications and dosage. The effective rate was defined as the number of cases with NRS score reduction of >50%/total number of cases×100% at 12 weeks postoperatively.

Results

The postoperative NRS scores were significantly decreased in all patients (P < 0.05), and NRS scores in acute zoster group were lower than those in subacute zoster group at different time points (P < 0.05). The percentage of patients who had a reduction of medication use value of >50% was 56.4% and the effective rate was 66.7% in all patients, at 3 months after the block treatment. There was no significant difference in the reduction of medication use value of >50% and the effective rates between the two groups. There were no intracranial hemorrhage, infection or other serious adverse effects in either groups.

Conclusion

CT-guided gasserian ganglion block with local anesthetics and steroids can be an effective and safe technique to relieve the pain of acute/subacute zoster-related trigeminal neuralgia.

Herpes Zoster in a Patient With Dengue Infection: A Case Report of a Rare Co-Infection

Dengue is an infectious disease that plays an essential role in morbidity and mortality in developing countries. Occasionally, it presents with rare presentations and co-infections. Co-infection of herpes zoster and dengue is possible in countries such as these where both infections are common. Varicella-zoster infection is one of the self-limiting viral infections, and dengue fever is an endemic infection in Sri Lanka. When there is suspicion in diagnosis due to a change of natural course or overlapping of clinical features, concurrent co-infections have to be strongly suspected. We present the case of a 46-year-old female who had herpes zoster and dengue infection and was managed with a multidisciplinary team approach. The patient improved without any complications.

Effect of Repetitive Intracutaneous Injections with Local Anesthetics and Steroids for Acute Thoracic Herpes Zoster and Incidence of Postherpetic Neuralgia

Background

Treatment of established postherpetic neuralgia (PHN) is difficult and often disappointing. In this study, we assessed the efficacy of repetitive intracutaneous injections with local anesthetics and steroids in acute thoracic herpes zoster (HZ) pain, herpetic eruption, and incidence of PHN.

Methods

Ninety-three patients with acute thoracic HZ were randomly assigned to receive a standard treatment of antiviral medication with p.o. analgesics or the standard treatment with the addition of repetitive intracutaneous injections of a local anesthetic and steroid mixture. Patients were permitted to take tramadol when the visual analog scale (VAS) ≥ 4. Pain assessment using VAS was conducted at the initial visit, as well as 1, 2, 4, 12, and 24 weeks after the end of the treatments.

Results

In comparison with the standard treatment group, the VAS scores of the intracutaneous injection group were significantly lower during the study. The intracutaneous injection group also reported shorter duration of pain and skin eruption than the control group ( P  = 0.005 vs P  < 0.001, respectively). At 1 month post-therapy, 12.8% patients in the intracutaneous injection group reported zoster-associated pain, compared with 47.8% in the standard treatment group ( P  < 0.001). At 3 and 6 months post-therapy, the incidence of PHN was still significantly lower in the intracutaneous injection group than the standard treatment group. EuroQol VAS scores were significantly higher in the intracutaneous injection group vs standard treatment group (P < 0.001).

Conclusion

Repetitive intracutaneous injections with local anesthetics and steroids along with standard treatment significantly reduce the duration of pain and herpetic eruption and incidence of PHN.

The use of Zostavax in Spain: the economic case for vaccination of individuals aged 50 years and older

Background Population aging brings up a number of health issues, one of which is an increased incidence of herpes zoster (HZ) and its complication, post-herpetic neuralgia (PHN). Zostavax vaccine has recently become available to prevent HZ and PHN. This study evaluates the cost-effectiveness of vaccination against HZ in Spain considering a vaccination of the population aged 50 years and older and comparing this to the current situation where no vaccination is being administered. Methods An existing, validated, and published economic model was adapted to Spain using relevant local input parameters and costs from 2013. Results Vaccinating 30% of the Spanish population aged 50 years and older resulted in €16,577/QALY gained, €2025/HZ case avoided, and €5594/PHN case avoided under the third-party payer perspective. From a societal perspective, the ICERs increased by 6%, due to the higher price of the vaccine. The number needed to vaccinate to prevent one case was 20 for HZ, and 63 for PHN3. Sensitivity analyses showed that the model was most sensitive to the HZ and PHN epidemiological data, the health state utilities values, and vaccine price used. Conclusion Considering an acceptable range of cost-effectiveness of €30,000-€50,000 per QALY gained, vaccination of the 50+ population in Spain against HZ with a new vaccine, Zostavax, is cost-effective and makes good use of the valuable healthcare budget.

Superficial cervical plexus block for management of herpes zoster neuralgia in the C3 dermatome: a case report

Introduction

Herpes zoster is a well-known reactivating viral disease that gives rise to painful skin lesions. Although this vesicular rash heals up within a few weeks, pain sometimes continues, becoming postherpetic neuralgia. In the case of those at high risk of developing postherpetic neuralgia, early interventional pain management is generally recommended as a preventive measure. Pain specialists usually do not see patients face-to-face for chronic refractory pain until the stage of postherpetic neuralgia. However, active and aggressive management, including antiviral treatment, of herpetic neuralgia during the acute stage of herpes zoster promises better results. In this respect, superficial cervical plexus block can help patients, such as the case reported here, by relieving the pain of herpes zoster involving the C3 dermatome.

Case presentation

A 65-year-old Korean man with severe pain in his left C3 dermatome due to herpes zoster was admitted to our hospital. His pain was so refractory to medication that he consulted our pain clinic for pain control. Due to the medication limitations imposed by his underlying diseases (hepatitis B, liver cirrhosis, atrial fibrillation, and asthma), early interventional therapy including stellate ganglion block was planned. In addition, because his painful C3 dermatome overlapped significantly with the superficial cervical plexus dermatome, ultrasound-guided superficial cervical plexus block was utilized for pain control of the intractable herpes zoster neuritis in his C3 dermatome. The result with respect to his sporadic neuralgia was satisfactory.

Conclusions

We found superficial cervical plexus block to be an effective interventional procedure for pain management of herpes zoster, particularly at the C3-dermatomal level.

A phase 2a, randomized, crossover trial of gabapentin enacarbil for the treatment of postherpetic neuralgia in gabapentin inadequate responders

OBJECTIVE

To compare the efficacy of high-dose (3,600 mg/day) vs low-dose (1,200 mg/day) oral gabapentin enacarbil (GEn) on pain intensity in adults with postherpetic neuralgia (PHN) and a history of inadequate response to ≥1,800 mg/day gabapentin.

DESIGN

Multicenter, randomized, double-blind, crossover study (NCT00617461).

SETTING

Thirty-five outpatient centers in Germany and the United States.

SUBJECTS

Subjects aged ≥18 years with a diagnosis of PHN.

METHODS

During a 2-week baseline period, subjects received open-label treatment with 1,800 mg/day gabapentin. Subjects who had a mean 24-hour average pain intensity score ≥4 during the last 7 days of the baseline period were randomized to receive GEn (1,200 or 3,600 mg/day) for treatment period 1 (28 days), followed by GEn 2,400 mg/day (4 days), and the alternate GEn dose for treatment period 2 (28 days).

RESULTS

There was a modest but significant improvement in pain intensity scores with GEn 3,600 mg vs 1,200 mg (adjusted mean [90% confidence interval] treatment difference, -0.29 [-0.48 to -0.10]; P = 0.013). The difference in efficacy between doses was observed primarily in subjects who received the higher dose during treatment period 2; certain aspects of the study design may have contributed to this outcome. Plasma steady-state gabapentin exposure during GEn treatment was as expected and consistent between treatment periods. No new safety signals or adverse event trends relating to GEn exposure were identified.

CONCLUSIONS

While the overall results demonstrated efficacy in a PHN population, the differences between treatment periods confound the interpretation. These findings could provide insight into future trial designs.

Measuring the burden of herpes zoster and post herpetic neuralgia within primary care in rural Crete, Greece

Background

Research has indicated that general practitioners (GPs) have good clinical judgment in regards to diagnosing and managing herpes zoster (HZ) within clinical practice in a country with limited resources for primary care and general practice. The objective of the current study was to assess the burden of HZ and post herpetic neuralgia (PHN) within rural general practices in Crete, Greece.

Methods

The current study took place within a rural setting in Crete, Greece during the period of November 2007 to November 2009 within the catchment area in which the Cretan Rural Practice-based Research Network is operating. In total 19 GP's from 14 health care units in rural Crete were invited to participate, covering a total turnover patient population of approximately 25, 000 subjects. For the purpose of this study an electronic record database was constructed and used as the main tool for monitoring HZ and PHN incidence. Stress related data was also collected with the use of the Short Anxiety Screening Test (SAST).

Results

The crude incidence rate of HZ was 1.4/1000 patients/year throughout the entire network of health centers and satellite practices, while among satellite practices alone it was calculated at 1.3/1000 patients/year. Additionally, the standardised incidence density within satellite practices was calculated at 1.6/1000 patients/year. In regards to the stress associated with HZ and PHN, the latter were found to have lower levels of anxiety, as assessed through the SAST score (17.4 ± 3.9 vs. 21.1 ± 5.7; p = 0.029).

Conclusions

The implementation of an electronic surveillance system was feasible so as to measure the burden of HZ and PHN within the rural general practice setting in Crete.

Persistence of varicella-zoster virus viraemia in patients with herpes zoster

BACKGROUND

Herpes zoster is caused by the reactivation of varicella-zoster virus from sensory neurons. The commonest complication following zoster is chronic pain termed post herpetic neuralgia.

OBJECTIVES

To investigate the dynamics of VZV viraemia and viral load following the resolution of zoster and its relationship to PHN development.

STUDY DESIGN

Blood samples were collected at baseline, 1 month, 3 months and 6 month from a prospective study of 63 patients with active zoster. Quantification of VZV DNA in whole blood was performed using a real-time PCR assay.

RESULTS

During acute zoster, all patients had detectable VZV DNA in their blood. VZV DNA remained detectable in the blood of 91% of patients at 6 months although levels declined significantly (p<0.0001). A history of prodromal symptoms (p=0.005) and severity of pain at baseline (p=0.038) as well as taking antivirals (p=0.046) and being immunocompromised (p=0.043) were associated, with longer time to recovery from PHN. Viral DNA loads were consistently higher in patients with risk factors for PHN and higher viral DNA loads over time were associated with longer time to recovery (p=0.058 overall and 0.038 in immunocompetent).

CONCLUSIONS

Based on these observations we hypothesise that VZV replication persists following acute shingles and that higher viral DNA loads contribute to the risk factors for PHN.

Economic burden of herpes zoster among skilled nursing facility residents in the United States

OBJECTIVES

To evaluate health care resource use and direct medical costs attributable to herpes zoster (HZ) among elderly residents of skilled nursing facilities (SNF).

METHODS

This was a retrospective matched cohort study using data from 300 SNF in the United States. A total of 404 patients with HZ were matched with 1616 patients with no documented HZ or post-herpetic neuralgia using propensity scores. The study period included a 1-month pre-index period and the 90-day acute/subacute phase following the HZ index date. Health care resource utilization was captured from the long term care minimum dataset (MDS) and SNF admission-discharge records. The direct medical costs consisted of the SNF net bed revenue and hospitalization cost.

RESULTS

Over the 4-month study period, significantly more patients were hospitalized among the HZ (20.5%) than non-HZ cohort (14.4%). Both the numbers of hospitalization episodes and hospitalization days were greater for HZ than for non-HZ patients. An average additional 0.09 hospitalization episodes and 0.55 days of hospitalization were estimated in a multivariate model for patients in the HZ compared with the non-HZ cohort. The incremental direct medical costs, which are composed of the incremental medical costs incurred in the SNF, and the incremental costs attributable to hospitalization, were estimated between $1079 and $1673 for patients with HZ.

CONCLUSION

In the SNF setting, the presence of HZ imposes significant health care resource utilization and direct medical costs.

Dementia following herpes zoster encephalitis

We studied the rare case of an older adult with dementia following herpes zoster encephalitis (HZE). This 71-year-old woman presented to us approximately 1 year following resolution of a rapid-onset episode of HZE, and subsequently underwent neuropsychological and neuroimaging examinations. Cognitive assessment revealed impairments in general cognitive functioning, verbal and nonverbal memory, executive functions, speed of information processing, attention/working memory, and motor skills. The patient's neuroimaging data, when compared to a demographically similar healthy control sample (n = 9), demonstrated moderate central and perisylvian brain volume loss, several subcortical lesions in the white matter, and resting state whole brain and hippocampal hypoperfusion. These findings highlight neuropsychological changes evident in a dementia syndrome of this type, and they suggest that early identification and treatment of HZE has implications for the preservation of long-term cognitive functioning.

Estimating the number needed to vaccinate to prevent herpes zoster-related disease, health care resource use and mortality

BACKGROUND

A clinical trial has shown that a live-attenuated varicella-zoster virus vaccine is effective against herpes zoster (HZ) and post-herpetic neuralgia (PHN). The aim of the study was to estimate the number needed to vaccinate (NNV) to prevent HZ-related outcomes.

METHODS

A cohort model of HZ associated disease, health care resource use and mortality was developed. Canadian population-based data were used to estimate age-specific incidence, hospitalization, quality-adjusted life-year (QALY) lost and mortality. NNV was calculated as the number of individuals needed to be vaccinated to prevent a specific HZ-related outcome during their lifetime. Different ages at vaccination were examined and probabilistic sensitivity analysis was performed.

RESULTS

For 65 year olds, the NNV (HZ vaccine efficacy=63%, PHN vaccine efficacy=67%, no waning) to prevent a case of HZ, a case of PHN, a HZ death, a life-year lost and a QALY lost is estimated to be 11 (90% Crl: 10-13), 43 (90% Crl: 33-53), 23,319 (90% Crl: 15,312-33,139), 3762 (90% Crl: 1650-4629) and 165 (90% Crl: 105-197), respectively. Results were most sensitive to the duration of vaccine protection and the age at vaccination.

DISCUSSION

The predicted NNV to prevent HZ and PHN are low even though vaccine efficacy is between 50-70%, which reflects the high incidence of these diseases among older adults. Results clearly show that the main benefit of HZ vaccination is prevention of morbidity caused by pain (as measured by QALYs lost) rather than mortality.

National survey of primary care physicians regarding herpes zoster and the herpes zoster vaccine

BACKGROUND

This study describes physicians' perception of burden associated with herpes zoster (HZ) and postherpetic neuralgia (PHN), intentions for recommending the HZ vaccine, and perceived barriers to vaccination.

METHODS

A national survey of 438 general internal medicine (GIM) and 433 family medicine (FM) physicians was conducted during November-December 2005.

RESULTS

The survey response rate was 69%. Approximately 35% of GIM and FM physicians strongly agreed that HZ and PHN caused a significant burden of disease. For patients 60-79 years of age, > or =80% of GIM and FM physicians were somewhat or very likely to recommend HZ vaccine. In multivariate analyses, physicians who strongly agreed that HZ and PHN cause significant burden were more likely to recommend the vaccine to patients 60-79 years of age (odds ratio [OR], 2.75 [95% confidence interval [CI], 1.85-4.09]), whereas those who felt there was insufficient information about duration of protection (OR, 0.40 [CI, 0.24-0.67]), that the need to store HZ vaccine in a freezer was a definite barrier (OR, 0.31 [CI, 0.13-0.75]), or that their patients would not pay for the vaccine if it was not covered by insurance (OR, 0.57 [CI, 0.38-0.86]) were less likely to recommend it.

CONCLUSIONS

Primary care physicians perceived a high level of burden from HZ and PHN and generally favored the HZ vaccine.

Effect of viral load on the outcome of herpes zoster

Varicella-zoster virus (VZV) is a member of the Herpesviridae family, primary infection with which causes varicella, more commonly known as chicken pox. Characteristic of members of the alphaherpesvirus subfamily, VZV is neurotropic and establishes latency in sensory neurons. Reactivation of VZV causes herpes zoster, also known as shingles. The most frequent complication following zoster is chronic and often debilitating pain called postherpetic neuralgia (PHN), which can last for months after the disappearance of a rash. During episodes of acute zoster, VZV viremia occurs in some, but not all, patients; however, the effect of the viral load on the disease outcome is not known. Here we describe the development of a highly specific, sensitive, and reproducible real-time PCR assay to investigate the factors that may contribute to the presence and levels of baseline viremia in patients with zoster and to determine the relationship between viremia and the development and persistence of PHN. VZV DNA was detected in the peripheral blood mononuclear cells (PBMCs) of 78% of patients with acute zoster and in 9% of healthy asymptomatic blood donors. The presence of VZV in the PBMCs of patients with acute zoster was independently associated with age and being on antivirals but not with gender, immune status, extent of rash, the age of the rash at the time of blood sampling, having a history of prodromal pain, or the extent of acute pain. Prodromal pain was significantly associated with higher baseline viral loads. Viral load levels were not associated with the development or persistence of PHN at 6, 12, or 26 weeks.

Clinical and psychosocial correlates of acute pain in herpes zoster

BACKGROUND

Acute and persistent pain are the most significant clinical manifestations of herpes zoster (HZ), but the characteristics of acute pain in HZ patients have been inadequately investigated.

OBJECTIVES

To correlate the severity of acute pain with clinical, demographic and psychosocial characteristics of HZ patients.

STUDY DESIGN

Five hundred thirty-three patients with acute HZ were recruited by 119 dermatologists who collected medical and demographic data at diagnosis, provided counselling and therapy where appropriate and asked the patients to complete the Short Italian Questionnaire designed for comprehensive evaluation of HZ patients.

RESULTS

In a univariate analysis, greater acute pain severity was significantly associated with female gender, number of dermatomes affected, presence of prodromal pain, abnormal sensations (dysesthesia), education level, anxiety and depression. Quality of life, even if greatly reduced, did not correlate with the intensity of pain. In a multivariate model, the intensity of pain was independently associated with the extent of rash (p=0.042), presence of prodromal pain (p=0.005), dysesthesia, education level (p=0.040), and depression (p<0.001), but not with gender, anxiety or quality of life.

CONCLUSIONS

This study suggests that in patients with acute HZ the severity of the disease and depression at presentation are the main correlates of pain intensity.

The incidence of herpes zoster in a United States administrative database

BACKGROUND

Few recent studies have reported data on the incidence of herpes zoster (HZ) in U.S. general clinical practice.

OBJECTIVE

To estimate the age- and sex-specific incidence of HZ among U.S. health plan enrollees.

DESIGN

Data for the years 2000 to 2001 were obtained from the Medstat MarketScan database, containing health insurance enrollment and claims data from over 4 million U.S. individuals. Incident HZ cases were identified through HZ diagnosis codes on health care claims. The burden of HZ among high-risk individuals with recent care for cancer, HIV, or transplantation was examined in sub-analyses. Overall incidence rates were age- and sex-adjusted to the 2000 U.S. population.

PARTICIPANTS

MarketScan U.S. health plan enrollees of all ages.

MEASUREMENTS AND MAIN RESULTS

We identified 9,152 incident cases of HZ (3.2 per 1,000 person-years) (95% confidence interval [CI], 3.1 to 3.2 per 1,000). Annual HZ rates per 1,000 person-years were higher among females (3.8) than males (2.6) (P<.0001). HZ rates rose sharply with age, and were highest among individuals over age 80 (10.9 per 1,000 person-years) (95% CI, 10.2 to 11.6). The incidence of HZ per 1,000 person-years among patients with evidence of recent care for transplantation, HIV infection, or cancer (10.3) was greater than for individuals without recent care for these conditions (3.0) (P<.0001).

CONCLUSIONS

The overall incidence of HZ reported in the present study was found to be similar to rates observed in U.S. analyses conducted 10 to 20 years earlier, after age- and sex-standardizing estimates from all studies to the 2000 U.S. population. The higher rate of HZ in females compared with males contrasts with prior U.S. studies.

Orofacial Pain—Part I

Orofacial pain is a common complaint, affecting the lives of millions of people around the world. Chronic orofacial pain often constitutes a challenging diagnostic problem that can be complicated by psychosocial factors and typically requires multidisciplinary treatment approaches. The fundamental prerequisite for successful management of orofacial pain is an accurate diagnosis. Generating a differential diagnosis, which will ultimately lead to a definite diagnosis, requires thorough knowledge of the diagnostic range of orofacial pain. There is a vast array of orofacial pain categories including: (1) musculoskeletal, (2) neuropathic, (3) vascular, (4) neurovascular, (5) idiopathic, (6) pain caused by local, distant, or systemic pathology, and (7) psychogenic. This article presents the salient clinical features and the therapeutic approaches for the various subtypes of musculoskeletal and neuropathic pain. Musculoskeletal pain is the most prevalent orofacial pain, with temporomandibular disorders and tension-type headache being the main examples. Neuropathic pain develops secondary to neural injury and/or irritation and can be distinguished into episodic, including trigeminal neuralgia and glossopharyngeal neuralgia, as well as continuous, such as herpetic and postherpetic neuralgia, traumatic neuralgia, and Eagle's syndrome.

Relief of pain in acute herpes zoster by nerve blocks and possible prevention of post-herpetic neuralgia

PURPOSE

This report describes two cases of acute herpes zoster (AHZ) treated by nerve block resulting in immediate pain relief and possible prevention of post-herpetic neuralgia (PHN).

CLINICAL FEATURES

Two elderly females with AHZ of cervical dermatomes and severe pain received deep cervical and greater occipital nerve blocks with a local anesthetic, epinephrine and steroid. In both patients, pain resolved immediately and permanently (one year follow-up) after a single treatment. Case #1: A 79-yr-old female with a mechanical mitral valve and anticoagulated with warfarin presented with AHZ of 17 days duration of the right C2, 3, 4 dermatomes and severe pain. A stellate ganglion block was not performed because of anticoagulation. Rather, a deep cervical root block at C3 and a greater occipital nerve block were performed with bupivacaine, epinephrine and methylprednisolone. No adverse events were evident. Case #2: A 73-yr-old female with a history of osteoarthritis and Meniere's disease presented with AHZ of seven days duration of the left C2, 3, 4 dermatomes and severe pain. Deep cervical root blocks at C3 and C4 and a greater occipital nerve block were performed with bupivacaine, epinephrine and methylprednisolone. Side effects of dizziness, hoarseness, hypertension and Horner's syndrome resolved in a few hours. A mild sensation of itching persisted for two weeks.

CONCLUSION

This report illustrates the potential of nerve blocks in severe AHZ to treat acute pain and possibly prevent PHN.

Oxcarbazepine monotherapy in postherpetic neuralgia unresponsive to carbamazepine and gabapentin

OBJECTIVES

We present the results of a preliminary, open-label trial to evaluate the efficacy and tolerability of oxcarbazepine in postherpetic neuralgia (PHN) unresponsive to treatment with antiepileptic drugs (carbamazepine and gabapentin) and local anesthetic blocks.

MATERIALS AND METHODS

Twenty-four patients were treated with oxcarbazepine monotherapy for 8 weeks. Starting dose was 150 mg/day, subsequently increased by 150 mg/day every 2 days until a maintenance dose of 900 mg/day. Pain was assessed using a visual analog scale (VAS).

RESULTS

There was a significant decrease in the mean VAS score following 8 weeks of treatment (Delta=5.33; paired t-test: P <0.0001) compared with baseline. Oxcarbazepine was effective from the first week of treatment. There was a significant reduction in allodynia, leading to improvements in patients' functioning and quality of life. Oxcarbazepine was generally well tolerated.

CONCLUSION

Oxcarbazepine appears to be a promising alternative monotherapeutic approach for patients affected by PHN.

Allodynia in rats infected with varicella zoster virus—a small animal model for post-herpetic neuralgia

The most common complication of herpes zoster is post-herpetic neuralgia (PHN), which has been defined as severe pain occurring 1 month after rash onset or persisting for greater than 3 months. PHN is classed as a neuropathic pain that is associated with mechanical allodynia where normally innocuous tactile stimuli are perceived as painful. The development of therapies to treat PHN has been hampered by the lack of animal models, which mimic the clinical situation. We have previously reported that varicella zoster virus (VZV) infection in the rat results in mechanical allodynia and thermal hyperalgesia. Here, we report that following VZV infection of the left footpad rats develop a chronic mechanical allodynia, which is present for longer than 60 days post-infection and which resolves by 100 days PI. The model is robust and reproducible with animals consistently developing allodynia by 3 days PI and continuing to present with symptoms for at least 30 days. The reproducible nature of the induction and course of the allodynia allows the use of this model to determine the effect of various compounds on, and to investigate the pathogenic mechanisms underlying the development of VZV-induced allodynia. Comparative studies using HSV-1 show that the induction of the chronic allodynia is VZV-specific and is not a result is of virus replication-induced tissue damage or accompanying inflammation. Therefore, we propose that the rat VZV infection model could prove useful in studying the mechanisms underlying post-herpetic neuralgia.