Studies on coronary arteriopathy in dogs following administration of CI-1020, an endothelin A receptor antagonist.

A selective nonpeptide endothelin A (ETA) receptor antagonist, CI-1020, was administered to beagle dogs intravenously (i.v.) for 4 hours to 4 weeks. One animal/sex received CI-1020 at 1 mg/kg/hr intravenously for 4, 8, or 24 hours to investigate onset of arteriopathy. Control animals (1/sex) received the vehicle only. To determine reversibility of arteriopathy, 8 dogs/sex were given CI-1020 at 1 mg/kg/hr for 4 days. Two dogs/sex were sacrificed 1, 3, 8, and 29 days following cessation of infusion. Lesion development with prolonged exposure was investigated in 1 male dog. It was given CI-1020 by i.v. bolus at 120 mg/kg/day for 4 weeks and Monastral blue dye was administered i.v. to facilitate localization of vascular lesions. Coronary blood flow was determined in 4 dogs infused with CI-1020 at 0.3, 3, and 30 mg/kg for one hour at each dose. Macroscopically, hemorrhage or blue discoloration of Monastral blue was noted in the extramural coronary arteries along the coronary groove and atrium. Histologically, the earliest coronary changes were noted in animals sacrificed after 24 hours of treatment and characterized by medial hemorrhage and necrosis with a few infiltrating neutrophils. In the reversibility study, incidence and severity of arteriopathy was dependent on time of sacrifice following cessation of infusion. Acute necrotizing inflammation of arteries was present in all animals (n = 4) on day 1 postinfusion, whereas on day 8 postinfusion, lesions characterized by medial small pockets of trapped red cells, cell debris, and adventitial thickening were seen in 1 dog/sex. By day 29 postinfusion, coronary arteries were similar to controls. In the dog given daily i.v. bolus injections of CI-1020 for 4 weeks, arterial inflammatory lesions varied from acute to chronic, although most lesions were considered chronic active. Monastral blue pigments were noted in the wall of most arteries with chronic or chronic active lesions. Acute lesions were similar to those noted in day 1 postinfusion of the reversibility study. Medial smooth muscle necrosis and/or fibrosis with mixed inflammatory cell infiltrates characterized chronic or chronic active lesions. Smooth muscle proliferation and migration into the intima were also noted. There were no significant changes in coronary blood flow, coronary vascular resistance, or mean arterial blood pressure following CI-1020 infusion for 3 hours. In the 24-hour infusion study, plasma endothelin 1 (ET-1) levels were mildly elevated (1.5-4 fold) during CI-1020 infusion when compared to either pretest or control values. These results indicate that administration of endothelin antagonist (CI-1020) to dogs was associated with development of coronary arteriopathy, which was completely resolved within 29 days following cessation of treatment. With prolonged (4-week) CI-1020 treatment, arterial lesions at varying stages of development (acute, chronic active, chronic) were seen, suggesting that tolerance to treatment (up to 4 weeks) does not occur.

The effects of overnight fasting, feeding, or sucrose supplementation prior to necropsy in rats.

This study was designed to investigate the acute effects on routine hematology, serum biochemistry, gastrointestinal contents and weight, and liver weight and morphology due to overnight sucrose feeding of rats prior to necropsy. Groups of rats (five males and five females/group) were fasted overnight, fed chow, or fed sucrose and were euthanized approximately 17 h later. At necropsy, blood was obtained for hematology and serum biochemistry profiling, and the livers and gastrointestinal tracts were weighed and examined. The livers also were evaluated microscopically. The blood glucose and urea nitrogen concentrations and liver weights of animals fed sucrose differed significantly from those of the other groups. Alterations were more striking in males than females. Marked histological changes were present in livers from animals fed sucrose prior to necropsy compared with fasted or chow-fed animals, and these changes were attributed to increased glycogen deposition in the sucrose-fed animals. Because of alterations in hepatic structure and function, we cannot recommend the practice of feeding sucrose to rats prior to necropsy for toxicology studies or any studies examining hepatic function.

Role of cytoprotectants and nitric oxide inhibition in nonsteroidal anti-inflammatory drug-induced gastroduodenal injury in the rat.

BACKGROUND AND PURPOSE: Nonsteroidal anti-inflammatory drugs (NSAIDs) induce gastroduodenal injury and ulceration. The pathogenesis is uncertain, although reductions in cytoprotective prostaglandins and nitric oxide (NO) have been proposed. The effects of several cytoprotective agents on inhibition of gastroduodenal ulcerogenesis induced by CI-987, a novel NSAID, were evaluated in Wistar rats. METHODS: Male Wistar rats were given CI-987 orally (p.o.) at a dosage of 300 or 450 mg/kg of body weight or subcutaneously (s.c.) (3 x 50 mg/kg), alone or with misoprostol pretreatment (2 x 1 mg/kg, p.o.). In a second experiment, rats were pre-treated with 2 ml of gelusil p.o., 500 mg of sucralfate/kg, p.o., 100 mg of ranitidine/kg s.c., or 200 mg of N omega-nitro-L-arginine methyl ester (L-NAME)/kg, s.c.. Duodenal injury was induced by administration of 450 mg of CI-987/kg, p.o., 3 x 50 mg of CI-987/kg, s.c., or 300 mg of cysteamine/kg, s.c. Animals were euthanized within 24 to 48 hours, and the gastrointestinal tract was examined for evidence of gross or microscopic change. RESULTS: The L-NAME significantly reduced the incidence and severity of gastroduodenal injury induced by CI-987 and cysteamine. Prostaglandin ameliorated duodenal lesions induced by CI-987 given s.c., and Gelusil, ranitidine, and sucralfate were without effect on duodenal lesions induced by NSAID. CONCLUSIONS: Preemptive blockade of NO synthase is important in preventing NSAID-induced duodenal injury in rats. Inhibition of cytoprotective prostaglandins and enhanced acid-induced damage are unlikely to be primary mechanisms underlying NSAID-induced duodenal injury in rats.

Coronary arteriopathy in monkeys following administration of CI-1020, an endothelin A receptor antagonist.

A selective non-peptide endothelin A (ETA) receptor antagonist, CI-1020, was administered to cynomolgus monkeys intravenously (i.v.) for 2 or 4 wk and orally for 4 wk. Groups consisting of 3 animals of each sex received CI-1020 at 1, 5, and 10 mg/kg/hr (i.v.) or orally at 250, 500, and 750 mg/kg body weight for 4 wk. Control animals received the vehicle only. In a separate experiment, 1 male was infused with 10 mg/kg/hr for 2 wk, and Monastral blue dye was administered i.v. to facilitate localization of lesions to the vascular walls. One female was administered saline and the dye and served as a control. One female at 1 mg/kg/hr was found dead at week 2, and 1 female at 5 mg/kg/hr was euthanatized during week 4 as a result of severe thigh swelling at the catheter site. Macroscopically, extramural coronary arteries appeared thickened and nodular in the 4-wk i.v. study in the female found dead at 1 mg/kg/hr, in 1 male and 1 female at 5 mg/kg/hr, and in 2 females at 10 mg/kg/hr. Histologically, Monastral blue pigment trapped in the walls of coronary arteries with arteriopathy was observed in the male treated with CI-1020 at 10 mg/kg/hr for 2 wk. Extramural coronary arteriopathy occurred at all doses in the 4-wk i.v. study, with higher incidence occurring in females than in males (7 of 9 treated females compared with 3 of 9 treated males). In the oral study, 1 female at 500 mg/kg/day and 1 male and 2 females at 750 mg/kg/day had coronary arteriopathy. Histological changes after 2 wk of treatment were characterized by intimal thickening, fragmentation of the internal elastic lamina, necrosis and edema of the media, and mixed inflammatory-cell infiltrates in the intima, media, and adventitia. After 4 wk of i.v. administration, arteriopathy was characterized by segmental disruption of the elastic lamina and intimal and medial fibrosis with complete replacement of smooth muscle with fibrous tissue. The adventitia was thickened as a result of fibrosis and mixed or mononuclear inflammatory-cell infiltrates. CI-1020 concentrations were higher in males (1.57 to 29 micrograms/ml) than in females (0.974 to 24.4 micrograms/ml) in the i.v. study. Transient systemic exposure with high maximum plasma concentration (Cmax) (120-352 micrograms/ml) in the oral study was insufficient to provoke arterial changes of the same magnitude as those noted with continuous i.v. administration. The regeneration of the media by fibrous tissue and the disruption of the elastic lamina may weaken the arterial wall and increase the susceptibility of the artery to the development of aneurysm.

Arteriopathy induced by an adenosine agonist-antihypertensive in monkeys.

An adenosine agonist, designated chemically as (R)-N-(2,3-dihydro-1H-inden-1-yl) adenosine or CI-947, was administered orally to 2 males and 2 female cynomolgus monkeys each at 5, 10, 20, and 50 mg/kg of body weight for 2 wk. One male and 1 female given 50 mg/kg were euthanatized on days 10 and 8, respectively, because of poor clinical condition. Emesis was present at 10, 20, and 50 mg/kg. Decreased heart rate and QT prolongation were present at 50 mg/kg. Extramural coronary arterial lesions consisting of medial necrosis with cellular debris and mixed inflammatory cell response in the intima, media, and adventitia were present in 1 male at 20 mg/kg and 1 male at 50 mg/kg at study termination. Similar arterial lesions were present in the small and large intestines and testis of the male at 50 mg/kg. Colonic mucosal erosions with mixed inflammatory cell infiltrates in the lamina propria were seen in this male and in all CI-947 treated females at 10, 20, and 50 mg/kg. Myocardial degeneration and necrosis of myocardial fibers with mononuclear cell infiltrates in the interstititum were noted in the left ventricle of 1 female at 20 mg/kg and in all animals at 50 mg/kg. Renal cortical tubular dilatation with increases in serum creatinine and/or blood urea nitrogen were noted in a control female and animals at 10 and 50 mg/kg. Plasma CI-947 concentration increased with increasing dose. Coronary vascular injury in the monkey was similar to the arterial lesion in CI-947-treated dogs and may relate to the pharmacologic/hemodynamic effects induced by CI-947. When compared with the dog, the monkey appears to be less sensitive to development of arteriopathy, as indicated by lower incidence, at similar systemic exposure levels.

Subchronic toxicity of atorvastatin, a hydroxymethylglutaryl-coenzyme A reductase inhibitor, in beagle dogs.

The toxicity of atorvastatin (AT), an inhibitor of hydroxymethylglutaryl-coenzyme A reductase (HMG), was evaluated in beagle dogs. In 4 studies [2-wk rising dose (daily increasing doses for 1 wk; maintenance for 1 wk), 12-wk rising dose (daily dosing with weekly increases in dose), 2-wk toxicity (daily dosing for 2 wk; 3 dose levels), 13-wk toxicity (daily dosing for 13 wk; 3 dose levels)], dogs received up to 400 mg/kg orally. Doses of 180 mg/kg induced moribundity, necessitating euthanasia. Weight losses up to 26% were seen at doses > or = 150 mg/kg. Decreases in cholesterol levels were dose-related. Alanine and/or aspartate aminotransferase were increased at doses > or = 80 mg/kg; alkaline phosphatase was increased at doses > or = 150 mg/kg. Histopathologic findings were seen at > or = 150 mg/kg and included hepatocellular eosinophilia related to increased smooth endoplasmic reticulum and cholangiohepatitis and cholecystitis at 150 mg/kg in the 2-wk toxicity study; hepatocellular degeneration, centrilobular bridging, cholecystitis, hemorrhage in gallbladder and brain, demyelination of optic nerve, and skeletal muscle necrosis at > or = 280 mg/kg in the 12-wk rising dose study; and erosion and hemorrhage in large intestine, hepatocellular degeneration and necrosis, and inflammation and necrosis of gallbladder epithelium at 320 mg/kg in the 2-wk rising dose study. Doses up to 80 mg/kg for 13 wk did not induce histopathologic lesions in examined organs. AT effectively lowered serum cholesterol in normal lipidemic dogs. Toxicity at AT in dogs was similar to that with other inhibitors of HMG except that lenticular changes were not seen, significant hepatic, testicular, or neurological toxicity was associated only with high doses at AT, and skeletal muscle changes similar to those described in rats and rabbits were identified.

Renal toxicity of a nondopaminergic antipsychotic agent, trimethyl imidazopyrazolopyrimidine, in rats.

A nondopaminergic antipsychotic agent, 5-ethyl-1,3,8-trimethyl-1H-imidazo]1,2-c]pyrazolo[3,4-e]pyrimidine (TIPP; PD 112488), has been tested for potential toxicity in rats. As part of a preclinical safety evaluation, 10 Wistar rats per sex were administered TIPP as a dietary admixture, receiving doses of 0, 5, 10, 20, 25, 50, 100, and 200 mg/kg for 2 wk. In addition, 3 groups of 6 male Wistar rats were administered TIPP (PD 114877 and PD 117498, acid hydrolysis products of TIPP) at 100 mg/kg by gavage for 5 days. All animals given 200 mg/kg were euthanatized in moribund condition or found dead after 1 wk of treatment. Clinical evidence of renal toxicity was noted and included emaciation, hematuria, urinary incontinence, and enlarged kidneys at doses of 10 mg/kg and higher. Plasma urea levels were higher than those of controls in all TIPP-treated groups. Significant pathologic changes of the urothelium were evident at all doses and were characterized by necrotizing pyelitis and cystitis. Necrosis and inflammation of the urothelium resulted in secondary hydronephrosis. No renal toxicity was noted with the acid hydrolysis products. The urothelial changes with oral administration of TIPP in rats is species-specific, and the specificity may be related to the metabolism and excretion of the drug.

Toxicity of CI-949, a novel anti-allergy agent.

The toxicity of CI-949, an effective inhibitor of allergic mediator release in pharmacology models, was evaluated in rodents and dogs. Median lethal doses at 24-hr postdose ranged from 343 to 453 mg/kg in mice and 806 to 2058 mg/kg in rats. Delayed toxicity was observed at 300 mg/kg and greater in mice and at 500 mg/kg and greater in rats. Mortality and clinical intolerance occurred in rats at 200 and 400 mg/kg in the subacute studies, and at 100 and 150 mg/kg in the 13-week study. In rats, dose-dependent lymphoid tissue atrophy and depletion or necrosis of lymphocytes in lymphoid tissues were seen in deaths and moribund terminations. Although doses up to 60 mg/kg administrated for 2 weeks to dogs were well tolerated, 60 and 120 mg/kg in the 13-week dog study were poorly tolerated. Cutaneous sores, mucocutaneous purulent discharge, emesis, diarrhea, and weight loss were identified at these lethal doses. Histopathologic changes in dogs included myocardial, vascular and soft tissue inflammation, and gastric ulceration at 60 and 120 mg/kg, and thymic atrophy at 20 mg/kg and greater. Doses of 10 and 50 mg/kg were no-effect doses in 13-week repeated dose studies in dogs and rats, respectively. These results were used to support initial human clinical trials of CI-949.

Spontaneous fibro-osseous proliferative lesions in the sternums and femurs of B6C3F1 mice.

Bone morphology associated with fibro-osseous proliferation in the femurs and sternums of 98 female B6C3F1 mice were compared morphologically and quantitatively to femurs and sternums from 100 male B6C3F1 and 79 CF1 mice (48 female and 31 male). In addition, sternal samples from five B6C3F1 mice per sex were collected and processed for electron microscopy. Fibro-osseous proliferation was present in female B6C3F1 mice, but not male B6C3F1 or female CF1 mice. In female B6C3F1 mice at 32 weeks of age, the marrow spaces in the region of the proximal and distal epiphyseal plate were lined by large osteoblasts and had large vascularized centers. At 58 weeks, metaphyseal fibrovascular proliferative areas containing multinucleated cells and new cancellous bone delineating the lesion were seen. At 84 weeks, fibro-osseous tissue occupied the outer third of the sternal marrow cavity and by 110 weeks, more than two thirds of the marrow cavity. Fibro-osseous proliferation was present in 100 and 94% of the examined sternums and femurs, respectively, of female B6C3F1 mice at 110 weeks of age, but not in male B6C3F1 or female CF1 mice. Ultrastructural examination of the sternal changes at 110 weeks showed numerous osteoblasts, irregular bony spicules, and fibrocyte-like cells. By morphometry, the normal marrow cavity in B6C3F1 females occupied 35% of a longitudinal section of the whole sternebra compared with 70% and 75% of the whole sternebra in B6C3F1 males and CF1 female, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Hyaline glomerulopathy in B6C3F1 mice.

Hyaline glomerulopathy is a spontaneous disease of undetermined etiology that occurs sporadically in various strains of aging mice. In our laboratory, this disease was observed with unusual ultrastructural features as an incidental finding in 2 female B6C3F1 mice from 2 carcinogenicity bioassays. Microscopically, renal lesions were characterized by marked diffuse enlargement and prominent hyalinization of the glomeruli, equally affecting both kidneys. Affected glomeruli were PAS positive, but were negative for amyloid by the Congo red method. Immunocytochemical staining revealed weakly positive glomerular deposits with polyclonal anti-mouse IgG-IgM-IgA cocktail. Ultrastructurally, there were characteristic subendothelial osmiophilic deposits composed of loosely-packed linear structures in the glomeruli. Lamellae, which appeared as fibrils in perpendicular sections, were relatively uniform, measured 6.1-17.01 nm in diameter, and formed single or double-layered structures. The ultrastructural and immunocytochemical characteristics are suggestive of a spontaneous immune-mediated mechanism in a strain of mouse commonly used in toxicology studies.

Polyarteritis in a beagle.

A 13-month-old Beagle became anorectic and had fever, stiff gait, and tenderness in the inguinal region. Clinical signs of disease were associated with neutrophilia and a decrease in the albumin-to-globulin ratio. The dog became clinically normal for 5 days after 3 days of treatment with penicillin G and dihydrostreptomycin. Clinical signs of disease recurred, and the dog was euthanatized after failing to respond to administration of a trimethoprim-sulfamethoxazole combination for 9 days. Disseminated arteritis was seen in the testes, epididymides, mesentery, coronary arteries, aorta, and thyroid gland. Lesions were seen in large and medium-sized arteries and varied from acute necrotizing arteries to a chronic lesion with organization and recanalization of thrombi. The clinical signs of disease resembled those of Beagle pain syndrome, described in laboratory Beagles.

Histopathologic and electron microscopic studies on the acute toxicity of ochratoxin A in rats.

Ochratoxin A was given by gavage to male rats. Moribund and dead animals were necropsied, and the surviving rats, including the controls, were killed 48 hours after dosing. Many of the principal rats were moribund, or began dying, within 12 to 24 hours after dosing. Lesions suggestive of disseminated intravascular coagulation were seen by light microscopy as early as 12 hours after dosing; fibrin deposits were in the spleen, brain choroid plexus, glomerular capillaries, liver, and heart. Renal tubular nephrosis, hepatic and lymphoid necrosis, and necrotic enteritis with villous atrophy were also seen. Electron microscopy demonstrated fibrin strands mixed with degranulated platelets, necrotic leukocytes, and swollen endothelial cells in glomerular capillaries. Myocardial changes included focal supercontracted sarcomeres adjacent to intercalated disks. Swollen sarcolemma, lysed myofibrils and fragmented Z-bands with interstitial edema, vascular thrombosis, and endothelial damage were also seen. The acute pathologic changes induced by ochratoxin A in the intestine, liver, and lymphoid tissues were more obvious than the tubular nephrosis, and the development of a disseminated intravascular coagulation-like syndrome with myocardial changes was a complicating factor.

Comparison of the nephropathogenicity of four strains of infectious bronchitis virus.

Experiments were conducted to characterize renal lesions in chickens induced by four strains of infectious bronchitis virus (IBV); each has been described as nephropathogenic. Those strains were also compared in vaccinated and unvaccinated older chickens for nephropathogenicity. The younger birds were much more susceptible to the nephritogenic effects of the strains. All four strains produced acute renal changes consisting of tubular damage and interstitial inflammatory cell infiltration and edema. Although both cortex and medulla were involved, the latter was generally affected more severely. The Holte strain proved to be the least pathogenic, followed by the more pathogenic Gray and Italian strains and finally by the Australian strain. All four strains produced similar chronic renal changes in unvaccinated birds, with no correlation to the severity of lesions seen at the acute phase. Chronic active and inactive types of interstitial nephritis were seen at the chronic phase. Vaccinated birds challenged with the Australian strain had the highest prevalence of the chronic active type of interstitial nephritis. The implication of renal viral persistence in the development of chronic active interstitial nephritis is discussed.

Electron microscopic studies on the interaction between normal mice peritoneal phagocytes and Treponema hyodysenteriae, Treponema innocens and Bacteroides vulgatus.

One hundred and twenty female mice (CF1 strain) were divided into three groups of 40. The first group was injected intraperitoneally with broth cultures of Treponema hyodysenteriae. The second group was injected with a combination of T. hyodysenteriae and Bacteroides vulgatus. The third group was injected with Treponema innocens. Peritoneal wash from four mice of each group was collected at eight time intervals postinjection, then prepared for and examined by light and electron microscopy. Peritoneal wash from one mouse at each time interval was prepared for microbiological examination. Treponema hyodysenteriae produced peritoneal macrophage aggregation, transient neutrophilia and macrophage cytolysis. Cytolysis was characterized by rarefaction of the cytoplasm, vesiculation of the endoplasmic reticulum, mild swelling of the mitochondria and disruption of the nuclear and ctyoplasmic membranes. The combination of T. hyodysenteriae and B. vulgatus produced macrophage aggregation and marked neutrophil necrosis. Peritoneal macrophages phagocytized more T. hyodysenteriae than B. vulgatus during early postinjection intervals. Treponema innocens failed to produce cytotoxicity of peritoneal macrophages but did produce macrophage aggregation and transient neutrophilia. Treponema hyodysenteriae and T. innocens did not multiply in the mice peritoneal cavity and were reisolated up to 16 hours postinjection. Bacteroides vulgatus was reisolated up to 24 hours postinjection.

Adiaspiromycosis in striped skunks in Alberta, Canada.

Pulmonary adiaspiromycosis was diagnosed in seven of 25 striped skunks (Mephitis mephitis) in east-central Alberta. The infection varied from mild, where only microscopic lesions were seen, to severe, where gross lesions of grayish-white nodules were observed in the lung parenchyma. Mild lesions were restricted to the lung, while severe lesions extended to the tracheobronchial and mediastinal lymph nodes. Histologically, the lesions were characterized by a centrally located fungal spherule, surrounded by granulomatous inflammation. The morphology of the fungal spherules was consistent with that of Emmonsia crescens. By electron microscopy, the fungal cells had an outer thick fibrillar wall and an inner cytoplasm filled with large lipid vacuoles with relatively few mitochondria, ribosomes or glycogen inclusions. The absence of endosporulation and budding suggested that each fungal cell in the lung represented a separate inhaled spore. Infection was by inhalation, nevertheless adiaspores may disseminate to the regional lymph nodes.

Ultrastructural characterization of colonic lesions in pigs inoculated with Treponema hyodysenteriae.

Twelve pigs were inoculated orally with pure cultures of Treponema hyodysenteriae. Pigs were necropsied at different time intervals postinoculation; colonic specimens were collected and prepared for light and electron microscopy. The earliest colonic lesion detected by electron microscopy consisted of superficial vascular congestion and dilatation, edema of the lamina propria and intercellular separation of the epithelial cells at the crypt shoulders. This lesion progressed to epithelial cell necrosis and extrusion into the lumen and extravasation of red cells. Large numbers of spirochetes were present and free, between, over and under necrotic epithelial cells whether in place or partially extruded. Spirochetal penetration of colonic enterocytes and intracytoplasmic multiplication were confirmed in this study. The spirochetes were found to invade the epithelial cells only from their lateral borders. The relationship between T. hyodysenteriae and the colonic anaerobes was not determined.

Nephropathogenicity of infectious bronchitis virus.

Four strains of infectious bronchitis virus (IBV) were compared in chickens as to nephropathogenicity. Two of the viruses were from the United States, one was from Australia, and one from Italy. Each has been described as nephropathogenic. The Australian strain proved to be most pathogenic and was then followed by the Italian strain and, finally, the two viruses from the United States. With an increase in the age of the chickens there was an apparent decrease in pathogenic effect. All of the viruses were highly pathogenic in the respiratory tract regardless of the age of the test birds. Kidney damage, or nephritis, was detected more readily on histological study rather than by gross examination. Each of the viruses was unrelated to the others when the sera were assayed by virus neutralization. Massachusetts and Connecticut IBV vaccines gave erratic, or only, partial protection against challenge form the respective viruses which confirmed their aberrant nature. Lesions of IBV infection in the respiratory tract did not necessarily correlate with the results from challenge virus recovery attempts. One Massachusetts-type IB vaccine strain, Holland IBV, proved capable of inciting nephritis under the conditions of this study and from inoculating specific-pathogen-free chickens at 2 days of age. The lower embryo passage of this strain proved more nephropathogenic than the higher ones. Other Massachusetts-type vaccine viruses and the Connecticut-type did not induce nephritis. The nephropathogenic potential for certain vaccine strains is discussed.