The Basic Problem: A Continuing Need for Heterologous Antisera

It is possible to raise antibodies in avian eggs instead of horses. The advantages and limitations of antibody production in avian eggs were recently discussed in Bangkok in connection with a western firm's efforts to sell their technology to the Thai Red Cross Society. At the end of the day, however, the proposal generated little interest. It was felt that the chicken or duck egg, a heterologous biosystem similar to the horse but smaller and possibly less productive, offered no advantage over the time-proven system of equine or human antibody production. Horses can be bled every 4 weeks. One liter of horse serum yields 75 mL (15 000 IU) of equine rabies immune globulin (the usual dose for an adult victim of a dog bite is 2000 IU) or 100 mL of snake antivenin. The average "working" life expectancy of such a horse is approximately 6 years (according to the Thai Red Cross Society's experience using plasmapheresis). With the completion of a new facility at the Thai Red Cross Society, yields of heterologous antisera and antivenins are expected to increase as a result of more efficient production. Volunteer human blood donors with high titers of rabies antibody are having plasmapheresis every 2 to 3 months at the Thai Red Cross National Blood Center. One donor provides 300 to 400 mL of plasma, which yield 10 mL (1500 IU) of human rabies immune globulin (average adult dose, 1000 IU).

Snake venoms are complex substances; each is made of many protein and enzyme toxins [5]. Even the largest and most experienced manufacturer would find it difficult to produce monoclonal antivenins against the venoms of the many poisonous snakes (at least 200 species worldwide) that bite humans in the tropics. Experiments in Myanmar, Burma, have shown that it may be possible to immunize humans against Russell viper venom [6]. It is, however, unlikely that these interesting experiments will lead to production of antivenin from human donors or to other practical and effective measures, such as immunization of Burmese farmers against the venom of this snake.

The hope that every man, woman, and child will have been adequately immunized against tetanus and diphtheria by the beginning of the new century is, in our view, unrealistic. The World Health Organization and some health ministries provide optimistic projections for national immunization programs, but these projections appear to be inflated. The recent increase in the number of reported diphtheria cases in the former Union of Soviet Socialist Republics (USSR) may well have resulted from better surveillance and reporting instead of from the correction of previous politically motivated under-reporting [7, 8]. Diphtheria and tetanus have never really left us and could surface, even in western countries, whenever the opportunity arises [9, 10]. Mackay [11] has developed a map of the world that shows how expanded immunization programs for children have reached less than 50% of the population.

Rabies in particular remains a growing public health problem almost everywhere except Antarctica, Australia, New Zealand, and some island states, such as Hawaii. Each year, throughout the world, at least 34 000 persons die of rabies and more than 6 million receive prophylaxis after exposure [12, 13]. It was hoped that with the advent of potent tissue and avian culture rabies vaccines, administration of immune sera would no longer be required. However, it has been shown that these vaccines alone (without antiserum) do not protect all patients bitten by rabid animals [14, 15]. Human or equine rabies antisera are essential to neutralize virus at the inoculation site and provide protection during the first critical week after exposure until endogenous antibodies are produced, because the virus may be entering an immune-protected environment when it invades nerves [14-16]. In Mexico, a series of patients with rabies who received vaccine but no antiserum developed rabies encephalitis but survived with severe brain damage ([17], Baer GM. Personal communication). Similarly, treatment failures occurred in Asia when antisera were not properly administered [13], again emphasizing the therapeutic importance of rabies immune globulins. Nevertheless, more than 90% of worldwide prophylaxis series for rabies after exposure do not include antiserum or immune globulins, largely because these are not affordable and perhaps not even available [1, 13, 14]. There are no good estimates of the number of lives lost because of this lack of an essential biological, but we believe that the numbers are substantial.

Antivenins for hemotoxic and neurotoxic snake bites save lives and reduce the number and severity of such complications as clotting disorders and paralysis [18]. Only equine-origin antivenins are available and only in limited quantities [5, 18, 19]. Manufacturers of antivenins face additional problems. These products are usually species-specific. No antivenins are currently made against several dangerous venomous snakes [for example, the Malayan krait from Asia]. The efficacy and cross-protective potency of several so-called polyvalent snake antivenins have never been tested in prospective human studies, and well-controlled animal experiments are lacking for some products. For example, we lack good data on the efficacy of 1) the antivenin product against Siamese monocellate cobra venom when used for bites from the spitting cobras, recently reclassified as three different species [20]; and 2) the only monovalent sea snake antivenin that is being marketed (when available) for use in bites from at least 37 highly toxic species found in the Indian and Pacific Oceans [18]. Considerable research is required to sort out these problems in Asia and on other continents, and current equine snake antivenin production procedures may have to be revised when more data become available. Only a handful of scientists with limited resources work in this field. Many species of snakes are increasingly difficult to procure, and some are almost impossible to breed in captivity. They nevertheless continue to pose a substantial hazard to humans.

The Economics of Antiserum Therapy

Human antitetanus globulin costs $155 per average treatment course in Thailand. The equivalent equine product costs only $7. An average adult dose of human rabies immune globulin costs $180 in Thailand, with the exception of limited quantities that are made and distributed gratis by the Thai Red Cross National Blood Bank. Purified, pepsin-digested equine rabies antiserum costs only $28 per dose. Human-origin diphtheria immune globulin (not available in the United States) costs $1290 for an average treatment dose in this same region. The equine equivalent would cost $10. The minimum daily wage of a worker in Thailand is $5.50. (The salary of a recently trained physician at a government hospital is about $325 per month.) Treating a 50-kg patient for a bite from a rabid animal using the imported human rabies immune globulin would thus require the equivalent of at least 1 month of a laborer's wages (not counting the costs of vaccine, office visits, wound care, and transportation). Treatment would only cost the equivalent of 5 days of a laborer's wages if the equine products were used.

Minimum daily wages are much lower but drug costs are similar or higher in Vietnam, Myanmar, Bangladesh, Nepal, India, Pakistan, and Kampuchea than those in Thailand. The situation is the same or worse in most of the countries that were formerly part of the USSR, Africa, and parts of South America and Central America. Few private and almost no public hospitals in poor countries can afford to stock human rabies, tetanus, or diphtheria sera in their emergency departments; even snake antivenins are often not available when they are needed. In Thailand, human rabies immune globulin is usually ordered from a wholesale dealer in exchange for a cash payment for an individual patient. This process often results in substantial delays.

Biological and Logistic Issues

Modern equine antisera are purified, pepsin-digested products that carry an anaphylactic risk similar to or less than that of penicillin injections [21]. However, several developing countries in Africa and the Middle East and on the Indian subcontinent still manufacture crude equine serum products with a high protein content and high risk for serum sickness and anaphylaxis. Two major international manufacturers that used to produce purified equine antisera in Europe have discontinued in-house production during the past 4 years, adding to the worldwide shortage of these products. Although a few small producers (including the Queen Saovabha Memorial Institute in Thailand) remain scattered throughout the world, these producers cannot satisfy the requirements of their own countries, let alone export to less fortunate neighbors. Little if any attention is now paid to production of quality heterologous serum products in western countries, where molecular biology and advanced technology are kings.

Workers in developing countries who make out-dated equine antisera can expect little support from funding agencies; efforts to upgrade and increase production are often viewed as a waste of resources because "monoclonals will soon replace antisera." International equine antiserum and antivenin manufacturers that have closed their horse farms tell us that they had been harassed by animal rights activists who oppose using horses as blood donors. Furthermore, heterologous antiserum production is apparently not very profitable because the market for it is essentially limited to poor countries and prices cannot be increased.

What Can Be Done?

This problem cannot be ignored much longer. Some potential sources of heterologous antisera and antivenins are as follows. First, manufacturers should be encouraged by governments (perhaps through the use of tax incentives) to reverse the trend toward nonproduction and resume manufacturing purified and affordable heterologous antitetanus, antidiphtheria, and antirabies serum products. Several small-scale laboratories that still produce such antisera in developing countries need help to upgrade and enlarge their facilities. We understand that scientists from the American Centers for Communicable Diseases are engaged in such an effort in Ethiopia. Second, the International Red Cross and its local chapters have done outstanding work in many parts of the world in the fields of blood banking and manufacturing of blood products. Most of these centers also have the technology to produce or help in the production of heterologous immune globulins. This is now happening in Bangkok, where the Thai Red Cross Society is building a dedicated heterologous serum production facility that should eventually be able to satisfy Thailand's needs for infectious disease antisera and snake antivenins. This project has received the support of the royal family and will be paid for by private donations and a Thai government subsidy. Third, institutions that are able to operate horse farms successfully should form joint ventures with international pharmaceutical manufacturers to supply high antibody titer equine sera for purification and distribution. Several such small-scale ventures have been functioning in Europe, but these have been disrupted by the recent political conflicts in the Balkans, where most of the horse farms were located. Finally, efforts to rejuvenate old partnerships and create new ones must continue. Western physicians who counsel travelers must inform them that rabies antisera are in short supply or simply unavailable in many parts of the world. Travelers and expatriates relocating to such regions should be encouraged to update and maintain tetanus and diphtheria immunizations and should consider rabies pre-exposure vaccinations for rabies if their stay in a region in which canine rabies is endemic will be prolonged [22].