Histopathological lesions following intramuscular administration of saline in laboratory rodents and rabbits

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Abstract

This review was undertaken to assess the nature and incidence of procedure-related changes in mice, rats and rabbits which received saline solution by intramuscular injection. Data were collected on the injection sites from 7 studies representing 152 animals. The original observations by the different study pathologists from both control and treated animals were evaluated in order to create a glossary of preferred terms to be used in toxicology studies.

These standardized terms were then applied to changes observed in the saline-treated animals. The review showed that the most severe of the procedure-related lesions were only of a slight level. Two days post-injection, the local reactions were mainly composed of minimal infiltration by mononuclear cells (lymphocytes and macrophages) with occasional degeneration of myofibres. From 10 to 42 days post-injection, lesions showed regeneration of myofibres and some fibrosis. In rats, the number of injections at each site influenced inflammatory infiltrate and degenerative lesions.

Introduction

The intramuscular route is a common route of administration for many pharmaceutical products, in particular, vaccines and veterinary medicines. This route of exposure is, therefore, also employed in toxicology studies on these products. During the course of a toxicology study, control animals receive the same injected volume as treated animals, but they receive either saline or vehicle. Histopathological changes at injection sites are often noted in controls secondarily to the administration procedure of saline (damage by the needle and effects of the volume of saline). Though their severity is usually minimal, the interpretation of histological changes seen in such studies relies heavily on the ability to distinguish between the changes induced by the administration procedures and a response to the test article.

The histopathological changes seen following the intramuscular administration of saline have not previously been well-documented. Most of the published studies report lesions following intramuscular injections of specific compounds like antibiotics or local anaesthetics or of vehicle or adjuvant (such as aluminium hydroxide) (Steiness et al., 1978; Svendsen and Blom, 1984; Diness, 1985; Manor and Sadeh, 1989; Svendsen et al., 1998). Several publications discuss the effects of volume, concentration, vehicle, pH, temperature and injection speed on muscular lesions in different species such as rabbits, pigs or mice (Svendsen and Blom, 1984; Svendsen et al., 1998). The aim of this study was to evaluate the histological changes recorded following intramuscular injections of saline, either after single or repeated administration in rats, rabbits and mice, using data collected from 7 toxicology studies. As the data were derived from previously conducted studies, it was essential to ensure that the terminology was standardized, particularly as the evaluation was across species.

Section snippets

Animals

Seven studies testing vaccine, adjuvanted vaccine or adjuvant alone were considered with mice, rats and rabbits obtained from Charles River Laboratory. Mice were Swiss OF1 strain of 8 weeks of age and 24–31 g body weight at initiation of treatment. Rats were Ico or Wistar strains of six weeks of age and 140–220 g body weight at initiation of treatment. Rabbits were New Zealand white KBL strain of 12–14 weeks of age and 2 and 3 kg body weight at initiation of treatment.

Mice were housed singly,

Terminology

The data from both control and treated animals was used to generate standardized terminology suitable for injection sites (Table 1). Details about this terminology regarding aspects of inflammation, degeneration and repair are described below. Lesions and their incidence for animals given saline are presented in Table 2.

Discussion

The intramuscular route is frequently used in toxicology studies, particularly on vaccines, and there is little published data reporting background lesions in intramuscular studies. The pathologist must have a good knowledge of the type and incidence of procedure-related lesions and should use consistent terminology across studies.

The data presented here represent a firm basis for a better interpretation of treatment-related changes in this test system. Influence of time between last injection

Acknowledgments

The authors wish to thank all of their colleagues who helped in the compilation of these data, particularly the archivists, Agnès Tortochaut and Annie Roche.

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