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Definition

Local muscle inflammatory reactions are characterized by swelling and soreness after intramuscular injection of a substance. Severe local inflammations with infection show local accumulation of purulent material (abscess).

Risk factors

 The cervical and pectoral muscles appear to be more predisposed to muscle soreness, likely because these are smaller muscle groups compared to the gluteal or semimembranosus/semitendinosus muscles.

 Repeated injection into the same location.

 Some types of vaccines are anecdotally associated with a higher risk of injection site abscesses. Certain medications, typically acidic formulations or those with non‐aqueous carriers (gentamicin, tetracyclines, enrofloxacin, flunixin, phenylbutazone, etc.) are associated with increased tissue reactivity.

Pathogenesis

Local swelling and soreness are common complications, especially after repeated or large volume administrations or administration of irritating medications [2]. Abscess formation is a less common complication following intramuscular injection [1, 2] but may occur if the local inflammatory response is severe or if the injection site has been contaminated with bacteria. Abscesses may form even after intramuscular antimicrobial administration.

Prevention

Maximize aseptic technique for intramuscular injection or use alternate routes of administration for medication, if available. Rotation of injection sites is desired when frequent dosing is required and may delay development of muscle soreness by allowing time for the inflammation to resolve [2]. If a site becomes swollen or sore, it should no longer be used for injection [2]. Some practitioners advocate to avoid the gluteal muscles as this location is very difficult to drain if abscessation develops [2].

Diagnosis

Inflammation and/or soreness can be appreciated as raised, hardened and/or painful areas during normal clinical examination of the area. Abscess formation should be considered if the local muscle swelling appears severe or if the horse develops a fever.

Treatment

Avoid use of that location for further injections and apply warm compresses for analgesia. Warm compresses can also be used to help mature the abscess prior to establishing external drainage. In severe cases of muscle soreness, systemic non‐steroidal anti‐inflammatory treatment may be necessary. Drain abscess at a dependent location. The gluteal muscles are particularly difficult to drain if abscessation develops.

Expected outcome

Although most localized muscle soreness or abscessation resolves without long‐term consequence, fibrotic myopathy may develop after intramuscular injection [3]. This may be an additional consideration when administering intramuscular medications to performance horses.