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Definition

Acute hepatic necrosis (serum hepatitis, Theiler’s disease, post vaccination hepatitis) is a disease associated with administration of biological products of equine origin. Clinical manifestations of serum hepatitis usually occur with rapid onset 2–3 months (up to 6 months) after administration of a biological product.

Risk factors

Risk factors for developing serum hepatitis after administration of tetanus antitoxin include pregnancy or lactation. It is unknown whether this is also true for serum hepatitis following plasma transfusion [33].

Pathogenesis

Serum hepatitis has been reported after administration of plasma in horses [33]. The plasma was reported to come from the same commercial source but from different batches. The incidence of serum hepatitis following plasma administration was low (<0.4%) in one study, although there is currently a lack of long‐term follow‐up reports after plasma transfusion [33]. However, outbreaks after plasma transfusion have also been reported, with morbidity rates ranging from 1% to 18% [34].

The disease shares many similarities with human post transfusion hepatitis, which was associated with administration of blood products before the detection of the Hepatitis B and C viruses. Based on these observations an infectious agent has been suggested to play a role. Recently three members of the Flaviviridae, close relatives of the human hepaciviruses, have been identified in horses (equine hepacivirus (EHVC), equine Pegivirus, and Theiler’s disease associated virus, TDAV). The later has been associated with an outbreak of equine serum hepatitis [34]. The virus has since been experimentally inoculated into healthy horses of which one developed elevated liver enzymes further strengthening the theory that TDAV might be a causative agent of Theiler’s disease [34]. Equine hepacivirus has also been found in horses with serum hepatitis and has been shown to cause clinical disease in experimentally infected horses [35]. Evidence is mounting that Theiler’s disease is associated with equine hepatotropic viruses potentially transmitted via contaminated blood products.

Prevention

Avoid unnecessary plasma transfusions (see also complications of colloid use).

Diagnosis

Clinical signs include lethargy, anorexia, profound icterus, decreased gastrointestinal tract activity and various nervous system signs due to hepatoencephalopathy and liver failure. Diagnosis is based on a history of administration of plasma or another biological product, elevated liver enzymes, bile acids and bilirubin. Subclinical cases with elevations of liver enzymes but without overt clinical signs have also been reported.

Treatment

There is no specific treatment for Theiler’s disease. Supportive treatment with intravenous fluids and treatment for hepatoencephalopathy can be attempted.

Expected outcome

The mortality rate among symptomatic horses ranges between 50% and 90%.