Postmortem pathologic findings in dogs that underwent total body irradiation and hematopoietic cell transplant: A case series of five dogs with B-cell multicentric lymphoma.

Alternative therapies that can help achieve complete remission in dogs with lymphoma include total body irradiation and hematopoietic cell transplant, though there are few reports describing successes and pathologic sequelae of these procedures. During a 10-year period, 94 dogs with multicentric lymphoma received a hematopoietic cell transplant following total body irradiation at North Carolina State University College of Veterinary Medicine. Seven of these 94 dogs (7%) died prior to discharge, five (5%) of which presented for postmortem examination. Of these dogs, four received an autologous hematopoietic cell transplant, while one received a haploidentical allogeneic hematopoietic cell transplant. All five dogs had bone marrow depletion with all hematopoietic lines affected. Three had systemic candidiasis, while two had bacterial infections. To the authors' knowledge, this is the first report to document pathologic findings and development of systemic mycoses in dogs post total-body irradiation therapy and hematopoietic cell transplant.

VADER: a variable dose-rate external 137Cs irradiator for internal emitter and low dose rate studies.

In the long term, 137Cs is probably the most biologically important agent released in many accidental (or malicious) radiation disasters. It can enter the food chain, and be consumed, or, if present in the environment (e.g. from fallout), can provide external irradiation over prolonged times. In either case, due to the high penetration of the energetic γ rays emitted by 137Cs, the individual will be exposed to a low dose rate, uniform, whole body, irradiation. The VADER (VAriable Dose-rate External 137Cs irradiatoR) allows modeling these exposures, bypassing many of the problems inherent in internal emitter studies. Making use of discarded 137Cs brachytherapy seeds, the VADER can provide varying low dose rate irradiations at dose rates of 0.1 to 1.2 Gy/day. The VADER includes a mouse "hotel", designed to allow long term simultaneous residency of up to 15 mice. Two source platters containing ~ 250 mCi each of 137Cs brachytherapy seeds are mounted above and below the "hotel" and can be moved under computer control to provide constant low dose rate or a varying dose rate mimicking 137Cs biokinetics in mouse or man. We present the VADER design and characterization of its performance over 18 months of use.

Irradiation impairs mitochondrial function and skeletal muscle oxidative capacity: significance for metabolic complications in cancer survivors.

BACKGROUND: Metabolic complications are highly prevalent in cancer survivors treated with irradiation but the underlying mechanisms remain unknown. METHODS: Chow or high fat-fed C57Bl/6J mice were irradiated (6Gy) before investigating the impact on whole-body or skeletal muscle metabolism and profiling their lipidomic signature. Using a transgenic mouse model (Tg:Pax7-nGFP), we isolated muscle progenitor cells (satellite cells) and characterised their metabolic functions. We recruited childhood cancer survivors, grouped them based on the use of total body irradiation during their treatment and established their lipidomic profile. RESULTS: In mice, irradiation delayed body weight gain and impaired fat pads and muscle weights. These changes were associated with impaired whole-body fat oxidation in chow-fed mice and altered ex vivo skeletal muscle fatty acid oxidation, potentially due to a reduction in oxidative fibres and reduced mitochondrial enzyme activity. Irradiation led to fasting hyperglycaemia and impaired glucose uptake in isolated skeletal muscles. Cultured satellite cells from irradiated mice showed decreased fatty acid oxidation and reduced glucose uptake, recapitulating the host metabolic phenotype. Irradiation resulted in a remodelling of lipid species in skeletal muscles, with the extensor digitorum longus muscle being particularly affected. A large number of lipid species were reduced, with several of these species showing a positive correlation with mitochondrial enzymes activity. In cancer survivors exposed to irradiation, we found a similar decrease in systemic levels of most lipid species, and lipid species that increased were positively correlated with insulin resistance (HOMA-IR). CONCLUSION: Irradiation leads to long-term alterations in body composition, and lipid and carbohydrate metabolism in skeletal muscle, and affects muscle progenitor cells. Such changes result in persistent impairment of metabolic functions, providing a new mechanism for the increased prevalence of metabolic diseases reported in irradiated individuals. In this context, changes in the lipidomic signature in response to irradiation could be of diagnostic value.

Implementation of total body photon irradiation as part of an institutional bone marrow transplant program for the treatment of canine lymphoma and leukemias.

A total body irradiation (TBI) protocol was developed to support a bone marrow transplant (BMT) program for the treatment of canine hematologic malignancies. The purpose of this prospective study is to describe implementation of the protocol and resultant dosimetry. Nongraphic manual treatment planning using 6 MV photons, isocentric delivery, 40 × 40 cm field size, wall-mounted lasers to verify positioning, a lucite beam spoiler (without use of bolus material), a dose rate of 8.75 cGy/min at patient isocenter, and a source-to-axis distance of 338 cm were used for TBI. A monitor unit calculation formula was derived using ion chamber measurements and a solid water phantom. Five thermoluminescent dosimeters (TLDs) were used at various anatomic locations in each of four cadaver dogs, to verify fidelity of the monitor unit formula prior to clinical implementation. In vivo dosimetric data were then collected with five TLDs at various anatomic locations in six patients treated with TBI. A total dose of 10 Gy divided into two 5 Gy fractions was delivered approximately 16 h apart, immediately followed by autologous stem cell transplant. The mean difference between prescribed and delivered doses ranged from 99% to 109% for various sites in cadavers, and from 83% to 121% in clinical patients. The mean total body dose in cadavers and clinical patients when whole body dose was estimated by averaging doses measured by variably placed TLDs ranged from 98% to 108% and 93% to 102% of the prescribed dose, respectively, which was considered acceptable. This protocol could be used for institutional implementation of TBI.

Total skin electron therapy as treatment for epitheliotropic lymphoma in a dog.

BACKGROUND: Mycosis fungoides (MF) is an uncommon cutaneous neoplasm in dogs. Treatment options are limited. Total skin electron therapy (TSET) has been suggested as a possible therapy for canine MF. OBJECTIVE: To describe the use of TSET as palliative treatment for MF in a dog. RESULTS: An adult dog, previously diagnosed with nonepidermolytic ichthyosis, was presented with generalized erythroderma, alopecia and erosions. Histopathology revealed a densely cellular, well-demarcated, unencapsulated infiltrate extending from the epidermis to the mid-dermis compatible with MF. The infiltrate exhibited epitheliotropism multifocally for the epidermis, infundibula and adnexa. Due to a lack of response to chemotherapy, TSET was elected. Six megavoltage electrons were delivered using a 21EX Varian linear accelerator. A dose of 6 Gy was delivered to the skin surface and a 100 cm skin to surface distance was used for dog setup. The treatment time for the cranial half treatment was 3 h. The treatment was divided in two sessions (cranial and caudal halves of the body) 15 days apart. Clinical and histopathological complete remission was achieved and the dog was kept in remission with no additional treatments for 19 months before relapse and development of Sézary syndrome. CONCLUSION AND CLINICAL SIGNIFICANCE: To the best of the authors' knowledge, this is the first case reporting the use of TSET for medically refractory canine MF with post treatment follow-up. This case suggests that the use of TSET may be an effective palliative treatment for canine MF.

Characterization of an orthovoltage biological irradiator used for radiobiological research.

Orthovoltage irradiators are routinely used to irradiate specimens and small animals in biological research. There are several reports on the characteristics of these units for small field irradiations. However, there is limited knowledge about use of these units for large fields, which are essential for emerging large-field irregular shape irradiations, namely total marrow irradiation used as a conditioning regimen for hematological malignancies. This work describes characterization of a self-contained Orthovoltage biological irradiator for large fields using measurements and Monte Carlo simulations that could be used to compute the dose for in vivo or in vitro studies for large-field irradiation using this or a similar unit. Percentage depth dose, profiles, scatter factors, and half-value layers were measured and analyzed. A Monte Carlo model of the unit was created and used to generate depth dose and profiles, as well as scatter factors. An ion chamber array was also used for profile measurements of flatness and symmetry. The output was determined according to AAPM Task Group 61 guidelines. The depth dose measurements compare well with published data for similar beams. The Monte Carlo-generated depth dose and profiles match our measured doses to within 2%. Scatter factor measurements indicate gradual variation of these factors with field size. Dose rate measured by placing the ion chamber atop the unit's steel plate or solid water indicate enhanced readings of 5 to 28% compared with those measured in air. The stability of output over a 5-year period is within 2% of the 5-year average.

Design and characterization of an economical (192)Ir hemi-brain small animal irradiator.

PURPOSE: To describe the design and dosimetric characterization of a simple and economical small animal irradiator. MATERIALS AND METHODS: A high dose rate (HDR) (192)Ir brachytherapy source from a commercially available afterloader was used with a 1.3 cm thick tungsten collimator to provide sharp beam penumbra suitable for hemi-brain irradiation of mice. The unit was equipped with continuous gas anesthesia to allow robust animal immobilization. Dosimetric characterization of the device was performed with Gafchromic film measurements. RESULTS: The tungsten collimator provided a sharp penumbra suitable for hemi-brain irradiation, and dose rates on the order of 200 cGy/minute were achieved. The sharpness of the penumbra attainable with this device compares favorably to those measured experimentally for 6 MV photons, and 6 and 20 MeV electron beams from a linear accelerator, and was comparable to those measured for a 300 kVp orthovoltage beam and a Monte Carlo simulated 90 MeV proton beam. CONCLUSIONS: Due to its simplicity and low cost, the apparatus described is an attractive alternative for small animal irradiation experiments requiring steep dose gradients.

Pencil beam scanning dosimetry for large animal irradiation.

The space radiation environment imposes increased dangers of exposure to ionizing radiation, particularly during a solar particle event. These events consist primarily of low-energy protons that produce a highly inhomogeneous depth-dose distribution. Here we describe a novel technique that uses pencil beam scanning at extended source-to-surface distances and range shifter (RS) to provide robust but easily modifiable delivery of simulated solar particle event radiation to large animals. Thorough characterization of spot profiles as a function of energy, distance and RS position is critical to accurate treatment planning. At 105 MeV, the spot sigma is 234 mm at 4800 mm from the isocentre when the RS is installed at the nozzle. With the energy increased to 220 MeV, the spot sigma is 66 mm. At a distance of 1200 mm from the isocentre, the Gaussian sigma is 68 mm and 23 mm at 105 MeV and 220 MeV, respectively, when the RS is located on the nozzle. At lower energies, the spot sigma exhibits large differences as a function of distance and RS position. Scan areas of 1400 mm (superior-inferior) by 940 mm (anterior-posterior) and 580 mm by 320 mm are achieved at the extended distances of 4800 mm and 1200 mm, respectively, with dose inhomogeneity <2%. To treat large animals with a more sophisticated dose distribution, spot size can be reduced by placing the RS closer than 70 mm to the surface of the animals, producing spot sigmas below 6 mm.

Evaluation of thromboelastography for prediction of clinical bleeding in thrombocytopenic dogs after total body irradiation and hematopoietic cell transplantation.

OBJECTIVE: To determine whether thromboelastography is more accurate than conventional methods of evaluating hemostasis for the prediction of clinical bleeding in thrombocytopenic dogs following total body irradiation (TBI) and bone marrow transplantation (BMT). ANIMALS: 10 client-owned thrombocytopenic dogs with multicentric lymphoma. PROCEDURES: Results of a kaolin-activated thromboelastography assay, platelet count, and buccal mucosal bleeding time were evaluated for correlation to clinical bleeding. RESULTS: Maximum amplitude, derived via thromboelastography, was the only hemostatic variable with significant correlation to clinical bleeding. Buccal mucosal bleeding time had a high sensitivity but poor specificity for identifying dogs with clinical bleeding. CONCLUSIONS AND CLINICAL RELEVANCE: Compared with buccal mucosal bleeding time and platelet count, thromboelastography was more reliable at identifying thrombocytopenic dogs with a low risk of bleeding and could be considered to help guide the use of transfusion products in dogs undergoing TBI and BMT.

Autologous peripheral blood hematopoietic cell transplantation in dogs with T-cell lymphoma.

BACKGROUND: Peripheral blood hematopoietic cell transplantation (PBHCT) is a feasible treatment option for dogs with B-cell lymphoma. OBJECTIVE: To examine apheresis and PBHCT outcomes in dogs diagnosed with T-cell lymphoma (TCL). ANIMALS: Fifteen client-owned dogs diagnosed with high-grade TCL. METHODS: After high-dose cyclophosphamide and rhG-colony-stimulating (rhG-CSF) factor treatment, peripheral blood mononuclear cells were collected using cell separators. The harvested cells then were infused after varying doses of total body irradiation (TBI). Postirradiation adverse effects were managed symptomatically and dogs were discharged upon evidence of hematopoietic engraftment. RESULTS: More than 2 × 10(6) CD34+ cells/kg were harvested from 15/15 dogs. Thirteen of 15 (87%) dogs engrafted appropriately, whereas 2 (13%) of the dogs died in the hospital. One dog developed cutaneous B-cell lymphoma 120 days post-PBHCT. The median disease-free interval and overall survival (OS) of the 13 dogs transplanted in first remission from the time of PBHCT were 184 and 240 days, respectively. Stage and substage of disease at diagnosis had no effect on OS. Two of 13 (15%) dogs were alive 741 and 772 days post-PBHCT. CONCLUSIONS AND CLINICAL IMPORTANCE: PBHCT may be considered as a treatment option for dogs with TCL.

Autologous peripheral blood hematopoietic cell transplantation in dogs with B-cell lymphoma.

BACKGROUND: Peripheral blood CD34+ hematopoietic cell transplantation (PBHCT) is commonly used to treat human patients with relapsed non-Hodgkin diffuse, large B-cell lymphoma with cure rates approaching 50%. OBJECTIVE: To determine the safety and feasibility of performing PBHCT to treat canine B-cell lymphoma (LSA) patients in a clinical academic setting. ANIMALS: Twenty-four client-owned dogs diagnosed with B-cell LSA. METHODS: After high-dose cyclophosphamide and rhG-colony-stimulating factor treatment, peripheral blood mononuclear cells were collected using cell separator machines. The harvested cells then were infused after a 10 Gy dose of total body irradiation (TBI). Post-irradiation adverse effects were managed symptomatically and dogs were discharged upon evidence of engraftment. RESULTS: More than 2 × 10(6) CD34+ cells/kg were harvested in 23/24 dogs. Preapheresis peripheral blood monocyte count was correlated with the number of CD34+ cells/kg harvested. Twenty-one of 24 (87.5%) dogs engrafted appropriately, whereas 2 dogs (8.3%) died in the hospital. One (5%) dog exhibited delayed engraftment and died 45 days after PBHCT. One dog developed presumed TBI-induced pulmonary fibrosis approximately 8 months after PBHCT. The median disease-free interval and overall survival (OS) of all dogs from the time of PBHCT was 271 and 463 days, respectively. Five of 15 (33%) dogs transplanted before they relapsed remain in clinical remission for their disease at a median OS of 524 days (range, 361-665 days). CONCLUSIONS AND CLINICAL IMPORTANCE: In most cases, PBHCT led to complete hematologic reconstitution. Therefore, PBHCT may be considered as a treatment option for dogs with B-cell lymphoma.

Antiapoptotic and proliferative activity of curcumin on ovarian follicles in mice exposed to whole body ionizing radiation.

The aim of this study was to evaluate the antiapoptotic and proliferative activity of curcumin (Cur) on the ovarian follicles in mice exposed to whole body ionizing radiation (Rd). The mice were exposed to 8.3 gray whole body Rd, and Cur groups were given as a daily dose of 100 mg/kg of Cur for 10 days (10 days before Rd). The ovaries were collected 3 and 12 h after irradiation. To date, no such studies have been performed on antiapoptotic and proliferative activity of Cur on the ovarian follicles in mice exposed to whole body Rd. Analysis of mice ovary after exposure to Rd by terminal-deoxynucleotidyl-transferase-mediated dUTP nick end labeling showed that there were apoptotic cells both in the follicular wall and the antrum, and that the number of follicles showing early atresic features was high 3 h after Rd. On the other hand, analysis of mice ovary 12 h after exposure to Rd showed that the number of follicles containing apoptotic cells with advanced atresic features was significantly higher when compared to the 3-h Rd exposure group. The proliferating cell nuclear antigen -positive granulosa cells were decreased in association with follicular atresia. The groups given treatment were observed to have some benefit from Cur against the damage caused by Rd. The results of this study demonstrate that Cur prevents follicular atresia in Rd-induced apoptosis in ovarian follicles.

Hematologic changes after total body irradiation and autologous transplantation of hematopoietic peripheral blood progenitor cells in dogs with lymphoma.

Dogs with and without lymphoma have undergone hematopoietic cell transplantation in a research setting for decades. North Carolina State University is currently treating dogs with B- and T-cell lymphoma in a clinical setting with autologous peripheral blood progenitor cell transplants, using peripheral blood CD34+ progenitor cells harvested using an apheresis machine. Complete blood counts were performed daily for 15 to 19 days posttransplantation to monitor peripheral blood cell nadirs and subsequent CD34+ cell engraftment. This study documents the hematologic toxicities of total body irradiation in 10 dogs and the subsequent recovery of the affected cell lines after peripheral blood progenitor cell transplant, indicating successful CD34+ engraftment. All peripheral blood cell lines, excluding red blood cells, experienced grade 4 toxicities. All dogs had ≥ 500 neutrophils/µl by day 12, while thrombocytopenia persisted for many weeks. All dogs were clinically normal at discharge.

Total skin electron therapy technique for the canine patient.

Cutaneous epitheliotropic T-cell lymphoma in canine patients is a radiation sensitive tumor but total skin electron therapy is used only rarely. Our purpose was to evaluate dose distribution from a total skin electron therapy technique using 6MeV electron beams. The treatment was comprised of 12 fields, with the dog in lateral recumbency in a stride position at an extended distance from the source. Uniformity, flatness and symmetry were determined for each beam. The composite percent depth dose profile for all beams was measured in solid water phantoms and skin dose was determined on a canine cadaver using thermoluminescent dosimeters. The resulting d(max) of the composite beams was 1mm and dose variation over the skin was 6.8%, with the extremities having the most uneven dose distribution. Dimensions of the fields were adequate to obtain an effective treatment profile for the entire thickness of canine epidermis and the technique was feasible for clinical application. Individual tailoring of the protocol to deal with hot and cold spots may be necessary and set up will likely involve a significant time commitment for the therapy team.

Development and characterization of a novel variable low dose-rate irradiator for in vivo mouse studies.

Radiation exposure of humans generally results in low doses delivered at low dose rate. Our limited knowledge of the biological effects of low dose radiation is mainly based on data from the atomic bomb Life Span Study (LSS) cohort. However, the total doses and dose rates in the LSS cohort are still higher than most environmental and occupational exposures in humans. Importantly, the dose rate is a critical determinant of health risks stemming from radiation exposure. Understanding the shape of the dose-rate response curve for different biological outcomes is thus crucial for projecting the biological hazard from radiation in different environmental and man-made conditions. A significant barrier to performing low dose-rate studies is the difficulty in creating radiation source configurations compatible with long-term cellular or animal experiments. In this study the design and characterization of a large area, I-based irradiator is described. The irradiator allows continuous long-term exposure of mice at variable dose rates and can be sited in standard animal care facilities. The dose rate is determined by the level of I activity added to a large NaOH-filled rectangular phantom. The desired dose rate is maintained at essentially constant levels by weekly additions of I to compensate for decay. Dosimetry results for long-term animal irradiation at targeted dose rates of 0.00021 and 0.0021 cGy min are presented.

Gene expression profiles of mouse spermatogenesis during recovery from irradiation.

BACKGROUND: Irradiation or chemotherapy that suspend normal spermatogenesis is commonly used to treat various cancers. Fortunately, spermatogenesis in many cases can be restored after such treatments but knowledge is limited about the re-initiation process. Earlier studies have described the cellular changes that happen during recovery from irradiation by means of histology. We have earlier generated gene expression profiles during induction of spermatogenesis in mouse postnatal developing testes and found a correlation between profiles and the expressing cell types. The aim of the present work was to utilize the link between expression profile and cell types to follow the cellular changes that occur during post-irradiation recovery of spermatogenesis in order to describe recovery by means of gene expression. METHODS: Adult mouse testes were subjected to irradiation with 1 Gy or a fractionated radiation of two times 1 Gy. Testes were sampled every third or fourth day to follow the recovery of spermatogenesis and gene expression profiles generated by means of differential display RT-PCR. In situ hybridization was in addition performed to verify cell-type specific gene expression patterns. RESULTS: Irradiation of mice testis created a gap in spermatogenesis, which was initiated by loss of A1 to B-spermatogonia and lasted for approximately 10 days. Irradiation with 2 times 1 Gy showed a more pronounced effect on germ cell elimination than with 1 Gy, but spermatogenesis was in both cases completely reconstituted 42 days after irradiation. Comparison of expression profiles indicated that the cellular reconstitution appeared equivalent to what is observed during induction of normal spermatogenesis. CONCLUSION: The data indicates that recovery of spermatogenesis can be monitored by means of gene expression, which could aid in designing radiation treatment regimes for cancer patients leading to better restoration of spermatogenesis.

Principles of bone marrow transplantation (BMT): providing optimal veterinary and husbandry care to irradiated mice in BMT studies.

Bone marrow transplantation (BMT) is the treatment of choice for many leukemias, solid tumors, and metabolic diseases. The field of bone marrow research is highly dependent on in vivo experimentation, because in vitro techniques do not mimic these complicated in vivo systems. Therefore, understanding the medical and husbandry care needs of these transiently immunodeficient bone marrow recipient animals is crucial for researchers, veterinary and animal care personnel. Here we discuss the principles of bone marrow transplantation, mouse pathogens that can interfere with transplantation research, and important husbandry and veterinary practices for mice that may help to minimize unnecessary infections during the transplantation process. Whole-body irradiation is one of the most common tools for myeloablation of the recipient's bone marrow. We discuss the crucial role of the irradiator for BMT research and the importance of aseptic husbandry practices to lessen the possibility of the irradiator for being a source for disease transmission. Finally, we discuss some important guidelines for Institutional Animal Use and Care Committees reviewing irradiation and BMT protocols.

The radioprotective effects of the hexane and ethyl acetate extracts of Callophyllis japonica in mice that undergo whole body irradiation.

The radioprotective activity of extracts from the red seaweed Callophyllis (C.) japonica was investigated in mice that underwent whole-body exposure to gamma radiation. A methanol extract of C. japonica and its fractions [hexane, ethyl acetate (EtOAc), butanol and the remaining H2O] were used. Each fraction (100 mg/kg body weight) was administered intraperitoneally (i.p.) 2 times into the BALB/c mice, once at 1 and once at 24 h before exposure to 9 Gray (Gy) of gamma radiation. Pre-irradiation administration of the hexane and EtOAc fractions saved the mice, with their survival rates being greater than 80% at 30 days post-irradiation; the mice that were pretreated with the other fractions showed survival rates lower than 20% over the same time period. To examine the effect of each C. japonica fraction on the survival of intestinal and bone marrow stem cells, the number of intestinal crypts and bone marrow cells in the gamma-irradiated mice were examined. Pre-treatment of mice (i.p., 100 mg/kg body weight at 1 and 24 h before irradiation) with the hexane or EtOAc fraction prior to 6-Gy irradiation significantly protected the number of jejunal crypts and bone marrow cells at 9 days after irradiation. These findings suggest that certain extracts from C. japonica, when they are administered prior to irradiation, play an important role in the survival of irradiated mice, and this is possibly due to the extracts protecting the hematopoietic cells and intestinal stem cells against gamma irradiation.

Aminoguanidine ameliorates radiation-induced oxidative lung damage in rats.

PURPOSE: To investigate the possible protective effects of aminoguanidine (AG ) on lung damage in whole body irradiated rats. METHODS: To evaluate the biological damage of radiation on rat lung tissue, lipid peroxidation products were measured using biochemical parameters. Thirty Wistar albino rats were divided into three subgroups: control (C) , irradiation alone (RT), and RT + AG combined. After sacrificing the rats, antioxidant enzymes catalase (CAT), superoxide dismutase (SOD) and glutathione peroxidase (GSHPx) activities and malondiadehyde (MDA), nitric oxide (NO) levels were evaluated in lung tissue. RESULTS: Administration of AG resulted in an increase in the activities of CAT, SOD and GSHPx in the lungs. All were reduced after radiation. In addition, AG administration resulted in a decrease in both NO and MDA levels in lung compared with the irradiated group. CONCLUSION: Amnoguanidine increased the endogenous antioxidant defence mechanism in rats and protected the animals from radiation-induced lung toxicity. Moreover, AG may protect against ionizing radiation-induced lung damage because of its antioxidant effect.

Xenopus laevis: a possible vector of Ranavirus infection?

Frog virus 3 (FV3) or FV3-like viruses (Iridoviridae) infect a wide range of amphibian species, and they are becoming increasingly and causally associated with amphibian disease outbreaks worldwide. We have established the frog Xenopus laevis as an experimental model to study host defense and pathogenesis of FV3 infection. Although X. laevis adults usually clear FV3 infection within a few weeks, viral DNA has been detected in the kidneys several months after they had been experimentally infected; virus also has been detected in seemingly healthy nonexperimentally infected adults. Based on this information, we hypothesized that covert FV3 infection may occur in Xenopus. We first conducted a survey that detected FV3 by polymerase chain reaction (PCR) in the kidneys (the main site of FV3 infection) in a significant fraction of X. laevis raised in five different locations in the United States. Asymptomatic FV3 carriers also were detected by initiation of an acute systemic FV3 infection in frogs that had been immunosupressed by sublethal gamma-irradiation. Finally, we focused on macrophages as a potential site for viral persistence, and we showed that FV3 can infect peritoneal macrophages in vitro as determined by reverse transcriptase-PCR detection of viral mRNAs. Unlike kidney cell lines that are readily killed by FV3, infected macrophages, like uninfected macrophages, survived up to 12 days. Viral transcription also was detected in macrophages from animals up to 12 days after infection. These results suggest that FV3 can become quiescent in resistant species such as Xenopus, thereby making these species potential viral reservoirs.