Other rabies biological products
Category 3 exposures defined as single or multiple transdermal bite/scratchs or contamination of mucous membranes with saliva, call, in most instances in a rabies infected area, for passive immunization in association with vaccination. Two types of products are available for passive immunization: human and animal (mainly equine) immunoglobulin. In developing countries the use of highly purified horse immunoglobulin, that are safer than the heterologous products of the previous generation, should provide at least a partial solution to the problems of access to human immunoglobulin and the cost involved.
Production techniques for both products are provided below as well as detailed purification techniques for heterologous immunoglobulins.
However, the current situation in developing countries is characterized by the following:
- less than 1% of all PET are comprised of vaccine and serum
- human RIG is available in confidential quantities on specific markets and is too expensive for most people (about $250 per adult patient, approximately five times more expensive than purified horse serum)
- cheaper and safe (purified pepsin digested horse serum) equine immunoglobulin (ERIG) is available in limited quantities and in most situations is inaccessible to those that need it most
In addition this situation is getting worse as:
- more and more international manufacturers are discontinuing ERIG production
- where production of purified equine products has been initiated (e.g. Thailand) it remains limited and hardly satisfies national needs
- animal protection groups that are becoming more and more influential in developing countries, condemn animal rearing for serum production
In this context accelerating research and development of alternative products is very timely and using a limited number of carefully selected MAbs for therapeutic purpose is an attractive possibility. MAbs have demonstrated their activity in certain animal models and with the progress of technology their potential ease of production in large quantities at low cost and ease of quality control compared to polyclonal serum is attractive. In 2002 considering the chronic shortage of immunoglobulin in developing countries WHO convened a meeting of its network of WHO Collaborating Centres on rabies to discuss the feasibility of producing a monoclonal antibody cocktail for rabies post-exposure treatment (full report, web only [pdf, 26,2 kb])-
Production of antirabies serum of equine origin
Different types of equine antirabies immunoglobulin (ERIG) have been produced using various immunogenic preparations, consisting usually of a combination of inactivated and fixed strains of rabies virus. A therapeutic antirabies immunoglobulin for human use is produced at the Queen Saovabha Memorial Institute (QMSI), Bangkok, Thailand, by immunizing horses with a purified Vero cell rabies (PVR) vaccine. The animals are given a series of injections of the vaccine in increasing volumes. All the injections are given subcutaneously into the lateral aspect of the neck. The immunization period lasts 105 days and the first bleeding is made 14 days later. For more information, please click on the link.
Purification techniques for heterologous rabies antiserum
In view of the high costs of rabies immunoglobulin of human origin (HRIG), heterologous (mainly equine) immunoglobulins are still frequently prescribed for the prevention of rabies in persons who have been severely exposed (category III) to the virus, even though they may cause sensitization and are eliminated more rapidly than HRIG.
Purification techniques can be used to reduce the risk of sensitization to ERIG. Their objective is to maximize the specific activity and to minimize the allergenic substances in the product. When these techniques are implemented, it is advisable to adhere to the recommendations of the WHO Expert Committee on Biological Standardization.
The purification of immunoglobulins from human plasma is carried out according to the technique of Cohn et al, based on the selective precipitation of proteins by chilled ethanol. This technique has been adapted for purifying heterologous immunoglobulins and was described in the third edition of Laboratory techniques in rabies. Chapter 45 of the fourth edition (see link below) describes two other techniques for the purification of ERIG, which may be applied to clarified equine serum or plasma.
Production of human rabies immunoglobulin
The combination of local treatment of the wound, passive immunization with rabies immunoglobulins and vaccination is recommended for all severe (category III) exposures to rabies. The use of homologous immunoglobulins for human post-exposure treatment virtually eliminates the risk of anaphylaxis and serum sickness associated with heterologous serum products. In 1965, approximately 16% of persons treated with antirabies serum of equine origin were reported to have developed serum sickness; among persons over 15 years of age, the incidence was 46%. In the past few years, purified equine immunoglobulins have become available, and in recent studies, the incidence of serum sickness among recipients was reported to be <1-6.2%.
To avoid such reactions, human rabies immunoglobulin (HRIG) preparations have been developed and used for post-exposure treatment in most industrialized countries. HRIG is well tolerated, but it is expensive and available in only limited quantities. Since 1975, this product has been administered to more than 250 000 people in the United States and no cases of serum sickness have been reported. For more information, please click on the link.
International standard for rabies immunoglobulin
The second International Standard for Rabies Immunoglobulin was established in 1993, with a potency of 30 IU per ampoule.
See WHO Expert Comittee on Biological Standardization, Forty-fourth report, TRS 848, WHO,1994, pp 8-9. Attention is drawn to the fact that the potency was defined in terms of International Units of Rabies Antibodies rather than of Rabies Immunoglobulin.
The International Standard for Anti-Rabies Serum, Equine, which had been established in 1955 was formelly discontinued in 1993. Since 1985, the human immunoglobulin standard had been used for the standardization of equine as well as human antibody preparations without complications.
The quantitative assay of anti-rabies antibody preparations is described in Laboratory techniques in rabies, 4th ed. It should be noted that, whereas some laboratories have had problems with the virus-neutralization test in mice (MNT), fewer difficulties have been reported with the rapid fluorescent focus-inhibition test (RFFIT) in cell cultures.
National control laboratories should establish reference sera of specified potency, in terms of IU per ml, to be used routinely for in vitro and in vivo neutralization tests. It is suggested that the potency of anti-rabies reference immunoglobulin be determined in comparative studies carried out at the national reference centre and at least one of the WHO Collaborating Centres concerned with rabies.