Time Limit for Administering Equine Rabies Immune Globulin (ERIG)
Equine Rabies Immune Globulin (ERIG) should be administered as soon as possible after exposure, but can be given up to 7 days after the first dose of rabies vaccine if not immediately available.
Timing of ERIG Administration
The administration of rabies post-exposure prophylaxis (PEP), including ERIG, is considered a medical urgency, not an emergency. However, timely administration is critical for optimal effectiveness:
- ERIG should ideally be administered on day 0 (the same day as the first vaccine dose) 1
- If ERIG is not readily available at the time of initial treatment, it can still be given through the seventh day after administration of the first dose of vaccine 1
- ERIG is not indicated beyond the seventh day because an antibody response to the vaccine is presumed to have begun by that time 1
Rationale for the 7-Day Cutoff
The 7-day cutoff for ERIG administration is based on the immunological response to rabies vaccine:
- Vaccine-induced antibodies typically appear within 1 week of starting vaccination 1
- After this point, administering ERIG may potentially interfere with the developing immune response
- Research has shown that RIG can be administered with a delay of up to 5 days after the start of vaccine treatment without significant antibody suppression 2
Proper Administration of ERIG
When administering ERIG, follow these guidelines:
- The recommended dosage is 40 IU/kg body weight 3
- If anatomically feasible, the FULL dose of ERIG should be thoroughly infiltrated in the area around and into the wounds 1
- Any remaining volume should be injected intramuscularly at a site distant from vaccine administration 1
- ERIG should never be administered in the same syringe or in the same anatomical site as the rabies vaccine 3, 1
Important Considerations
- Even with substantial delays between exposure and PEP initiation, treatment should still be administered, as incubation periods of more than 1 year have been reported in humans 3
- The sooner treatment begins after exposure, the better the outcome 1
- ERIG has been used effectively worldwide with a low incidence of adverse reactions (0.8%-6.0%), most of which are minor 3, 4
- Purified ERIG is preferred over unpurified antirabies serum of equine origin, which is associated with higher rates of serious adverse reactions, including anaphylaxis 3
Potential Immunosuppressive Effect
It's worth noting that ERIG may have a suppressant effect on the immunogenicity of rabies vaccines:
- This effect appears to be less pronounced with ERIG than with Human Rabies Immune Globulin (HRIG) 5
- The standard 5-dose vaccination schedule (days 0,3,7,14, and 28) is recommended when using immunoglobulins 5
Clinical Efficacy
Properly administered PEP, including timely ERIG administration, is virtually 100% effective in preventing rabies when given appropriately 6. No PEP failures have been documented in the United States since current biologics have been licensed, provided they are administered according to guidelines 3.
Remember that while timing is important, even delayed administration of PEP is indicated if rabies exposure has occurred, as long as the patient is not already showing clinical signs of rabies.