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Appendix D-18
The Prospects for Immunizing Against Vibrio cholerae

DISEASE DESCRIPTION

Cholera is a diarrheal disease of the small bowel of humans. The causative bacteria is Vibrio cholerae, serogroup O1; only two biotypes, El Tor and classical composed of immunologically related serotypes, Inaba and Ogawa, cause epidemic cholera. The bacteria colonize the surface of the intestinal epithelium, but do not invade tissue. They elaborate a two-part protein enterotoxin that causes the diarrhea. This diarrhea is the cardinal, if not the sole, manifestation of the disease.

Recognized cases of cholera are, in general, more severe than other diarrheal diseases, but cholera has become the prototype of an increasingly recognized group of “enterotoxic enteropathies” (Finkelstein, 1984; Levine et al., 1983). These are far more significant causes of morbidity and mortality on a global scale than is cholera alone. Some of these agents also cause significant diarrheal disease in food supply animals. Many work through enterotoxins that are immunologically related to the cholera enterotoxin. At present, treatment depends on the vigorous replacement of fluid and electrolytic losses by oral or intravenous solutions of appropriate composition. Antibiotics serve an adjunctive role.

Vaccines against cholera, consisting of killed whole cells administered parenterally, have been used since the turn of the century. Extensive scientifically controlled field studies conducted in the 1960s and 1970s revealed that the protective effect of these vaccines is limited (Khan and Greenough, 1985). Many of the affected countries now have eliminated the requirement for cholera vaccine. The immunologically dominant component of the killed bacteria, the lipopolysaccharide somatic antigen, has been shown to have similar limitations as a vaccine (Finkelstein, 1984; Levine et al., 1983).

The committee gratefully acknowledges the efforts of R.A.Finkelstein, who prepared major portions of this appendix, and the advice and assistance of R.E.Black and J.B.Kaper. The committee assumes full responsibility for all judgments and assumptions.



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New Vaccine Development: Establishing Priorities, Volume II, Diseases of Importance in Developing Countries Appendix D-18 The Prospects for Immunizing Against Vibrio cholerae DISEASE DESCRIPTION Cholera is a diarrheal disease of the small bowel of humans. The causative bacteria is Vibrio cholerae, serogroup O1; only two biotypes, El Tor and classical composed of immunologically related serotypes, Inaba and Ogawa, cause epidemic cholera. The bacteria colonize the surface of the intestinal epithelium, but do not invade tissue. They elaborate a two-part protein enterotoxin that causes the diarrhea. This diarrhea is the cardinal, if not the sole, manifestation of the disease. Recognized cases of cholera are, in general, more severe than other diarrheal diseases, but cholera has become the prototype of an increasingly recognized group of “enterotoxic enteropathies” (Finkelstein, 1984; Levine et al., 1983). These are far more significant causes of morbidity and mortality on a global scale than is cholera alone. Some of these agents also cause significant diarrheal disease in food supply animals. Many work through enterotoxins that are immunologically related to the cholera enterotoxin. At present, treatment depends on the vigorous replacement of fluid and electrolytic losses by oral or intravenous solutions of appropriate composition. Antibiotics serve an adjunctive role. Vaccines against cholera, consisting of killed whole cells administered parenterally, have been used since the turn of the century. Extensive scientifically controlled field studies conducted in the 1960s and 1970s revealed that the protective effect of these vaccines is limited (Khan and Greenough, 1985). Many of the affected countries now have eliminated the requirement for cholera vaccine. The immunologically dominant component of the killed bacteria, the lipopolysaccharide somatic antigen, has been shown to have similar limitations as a vaccine (Finkelstein, 1984; Levine et al., 1983). The committee gratefully acknowledges the efforts of R.A.Finkelstein, who prepared major portions of this appendix, and the advice and assistance of R.E.Black and J.B.Kaper. The committee assumes full responsibility for all judgments and assumptions.

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New Vaccine Development: Establishing Priorities, Volume II, Diseases of Importance in Developing Countries Formalin and glutaraldehyde-treated toxoids, administered parenterally, also have been evaluated and found to be relatively ineffective (Finkelstein, 1984; Levine et al., 1983). Studies in American volunteers and in endemic areas have established conclusively that the disease itself is an immunizing process (Finkelstein, 1984; Levine et al., 1983). The human host, presented with all the products of the cholera vibrio at the local level, undergoes maximal stimulation of secretory antibody. This finding has contributed to the notion that the best way to produce immunity against cholera would be to use a living, attenuated strain of V. cholerae that would colonize the gut and stimulate immunity, but not cause cholera. PATHOGEN DESCRIPTION The causative agent of epidemic cholera is Vibrio cholerae, serogroup 01. This group includes the El Tor biotype (the cause of the present pandemic) and the classical biotype, both of which occur as two serotypes, Inaba and Ogawa. The bacterium is gram-negative, comma-shaped, oxidase positive, and motile by means of a single polar flagellum. It is identified by its agglutinability in 0 group 1 and type-specific, Inaba or Ogawa, antisera. Other ancillary characteristics, such as growth and reaction on selective media (e.g., TCBS agar), and particular biochemical reactions (e.g., fermentations) can be useful in suggesting its identity, but confirmation depends on serologic reactions. The definition of the Vibrio cholerae species recently has been expanded to include a diverse variety of other vibrios that are known collectively as non-0 group 1 V. cholerae (because they do not agglutinate in O group 1 antisera). Occasional rare strains of non-O group 1 vibrios, formerly called NAG (nonagglutinable) or NCV (noncholera vibrios) produce a cholera-related enterotoxin and have been associated with diarrheal disease. HOST IMMUNE RESPONSE Despite the noninvasive nature of cholera, a vigorous immune response is induced in the host. This response is manifested both by circulating IgM and IgG antibodies and by secretory IgA antibodies against the lipopolysaccharide (LPS) somatic antigen, the enterotoxin, and other less well studied antigens. The antibacterial antibodies are vibriocidal (in the presence of complement) and agglutinating, and the anti-enterotoxin is neutralizing. Either or both are protective in animal models, and there is some evidence that they may interact synergistically. The disease is an immunizing process, and it has been demonstrated that volunteer convalescents are resistant to rechallenge for at least 3 years. In endemic areas, cholera is less frequent in adults who have circulating antibody.

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New Vaccine Development: Establishing Priorities, Volume II, Diseases of Importance in Developing Countries DISTRIBUTION OF DISEASE Geographic Distribution Cholera has swept the world in seven great pandemic waves. From the early 1900s to 1961, cholera was virtually restricted to its endemic focus in the Indo-Pakistani subcontinent. The present pandemic, caused by V. cholerae of the El Tor biotype, began in 1958 in the Celebes Islands, Indonesia. It subsequently swept through and became endemic in the Philippines, Southeast Asia, and Africa. Outbreaks that have been more or less self-limited have occurred in the Soviet Union, Japan, Italy, Spain, northern Europe, and North America (including the United States). Cholera vibrios of the classical biotype appear to be reemerging in India and Bangladesh. Disease Burden Estimates The burden of disease caused by V. cholerae has been calculated only for areas in Africa and Asia where cholera is endemic. Table D-18.1 shows the estimated number of cases of the disease in Asia; Table D-18.2 presents the same information for Africa. The combined endemic disease burden for the two continents is shown in Table D-18.3. No attempt has been made to estimate the disease burden produced by cholera epidemics or pandemics because it is very difficult to predict where and to what extent they will occur. This inability to identify a vaccine target population prevents calculation of potential health benefits that could be obtained from a vaccine. However, any vaccine developed to prevent endemic cholera could play a major role in curtailing epidemic cholera. Traditionally, it has been assumed that cholera does not have any long-term sequelae. A recent epidemiological study in India suggests, however, that a strong association may exist between cataract development and episodes of cholera and other severe diarrheal diseases (Minassian et al., 1984). Because of the preliminary nature of this evidence, the committee chose not to include visual disability in the current disease burden estimates for cholera. Further research on this topic is warranted. Also omitted from disease burden calculations are possible adverse effects of cholera during pregnancy. PROBABLE VACCINE TARGET POPULATION In endemic areas, cholera occurs with greatest frequency in children between 2 and 15 years of age and in adult females. Children less than 2 years old have a relatively low incidence of the disease, particularly when breast-fed. Thus, vaccination in infancy would be appropriate, and a suitable vaccine could be incorporated into the World Health Organization Expanded Program on immunization (WHO-EPI). In areas in which the introduction of cholera is recent (neoepidemic areas), the rates of disease are more uniform across the

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New Vaccine Development: Establishing Priorities, Volume II, Diseases of Importance in Developing Countries TABLE D-18.1 Estimated Cholera Cases in Asia by Age Group Age Group (years) Total Population (millions) Cholera Endemic Populationa (millions) Annual Incidence of Hospital Cases/1,000 Number of Hospital (Severe) Cases (8%) Number of Moderate Cases (20%) Number of Mild Cases (72%) Number of Deaths (20% of severe) Under 5 373 93 2.0b 186,000 465,000 1,674,000 37,200 5–14 639 160 0.75c 120,000 300,000 1,080,000 24,000 15–59 1,517 379 0.50d 189,500 473,750 1,705,500 37,900 60 and over 133 33 0.50d 16,500 41,250 148,500 3,300 aTwenty-five percent of total population, based on World Health Organization (1984). bBased on Matlab, Bangladesh (3.9); Calcutta, India (1.0); and Surabaya, Indonesia (1.5). cBased on Matlab, Bangladesh (4.0); Calcutta, India (0.65); and Surabaya, Indonesia (0.42). dBased on Matlab, Bangladesh (2.0); Calcutta, India (0.65); and Surabaya, Indonesia (0.42).

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New Vaccine Development: Establishing Priorities, Volume II, Diseases of Importance in Developing Countries TABLE D-18.2 Estimated Cholera Cases in Africa by Age Group Age Group (years) Total Population (millions) Cholera Endemic or Neo-Endemic Populationa (millions) Number of Hospital Cases (8%) Number of Moderate Cases (20%) Number of Mild Cases (72%) Number of Deaths (20% of severe) Under 5 90 9.0 5,000b 12,500 45,000 1,000 5–14 149 14.9 15,000b 37,500 135,000 3,000 15–59 226 22.6 75,000b 187,500 675,000 15,000 60 and over 26 2.6 5,000b 12,500 45,000 1,000 aTen percent of total population, based on Stock (1976) and World Health Organization (1984). bTotal cases and proportion by age group estimated from references in a proportion of cases: under 5 years, 5 percent; 5–14 years, 15 percent; 15–59 years, 75 percent; 60 years and over, 5 percent.

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New Vaccine Development: Establishing Priorities, Volume II, Diseases of Importance in Developing Countries TABLE D-18.3 Disease Burden: Vibrio cholerae       Under 5 Years 5–14 Years 15–59 Years 60 Years and Over Morbidity Category Description Condition Number of Cases Duration Number of Cases Duration Number of Cases Duration Number of Cases Duration A Moderate localized pain and/or mild systemic reaction, or impairment requiring minor change in normal activities, and associated with some restriction of work activity Mild diarrhea 1,719,000 3 1,215,000 3 2,380,500 3 193,500 3 B Moderate pain and/or moderate impairment requiring moderate change in normal activities, e.g., housebound or in bed, and associated with temporary loss of ability to work Moderate diarrhea 477,500 5 337,500 5 661,250 5 53,750 5 C Severe pain, severe short-term impairment, or hospitalization Severe diarrhea, dehydration 191,000 7 135,000 7 264,500 7 21,500 7 D Mild chronic disability (not requiring hospitalization, institutionalization, or other major limitation of normal activity, and resulting in minor limitation of ability to work)     n.a.   n.a.   n.a.   n.a. E Moderate to severe chronic disability (requiring hospitalization, special care, or other major limitation of normal activity, and seriously restricting ability to work)     n.a.   n.a.   n.a.   n.a. F Total impairment     n.a.   n.a.   n.a.   n.a. G Reproductive impairment resulting in infertility     n.a.   n.a.   n.a.   n.a. H Death   38,200 n.a. 27,000 n.a. 52,900 n.a. 4,300 n.a.

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New Vaccine Development: Establishing Priorities, Volume II, Diseases of Importance in Developing Countries age groups. To avert a widespread epidemic in these areas, it may be necessary to vaccinate both adults and children. In developed countries, the potential vaccine target population consists of travelers and military personnel; however, the incidence of cholera among the former is very low and may not justify vaccination. Vaccine Preventable Illness* The target population for a cholera vaccine in endemic areas would be all children under 2 years of age (vaccinated through the WHO-EPI). Because the vast majority of cholera cases in these areas occur after age 2, the committee assumed that the entire disease burden could be prevented by the hypothetical vaccine. The apparent nonhuman reservoirs of cholera found recently in the United States and Australia and suggested for other regions (Miller et al., 1985) make it unlikely that cholera could be eradicated completely, but this issue remains controversial. SUITABILITY FOR VACCINE CONTROL Because cholera itself is a highly effective immunizing process, it should be possible to duplicate this immunity artificially and without unacceptable side effects. Cholera is also a highly suitable candidate for control by vaccines because all of the epidemic cholera in the world is caused by only two serotypes (representing both biotypes) of V. cholerae and a single enterotoxin (or series of closely related enterotoxins). Alternative Control Measures and Treatments The incidence of cholera could be markedly reduced, if not totally eliminated, by sanitary measures: appropriate disposal of human feces and chlorination of water. However, achieving these goals is unlikely in the next few decades in areas where cholera occurs most commonly. Oral rehydration or intravenous electrolyte replacement therapy prevents mortality from cholera, if it is used. However, current high mortality figures attest to inadequate treatment delivery systems. Treatment also represents a major expense to the developing nations. Epidemic cholera may overwhelm available medical services in some areas. *   Vaccine preventable illness is defined as that portion of the disease burden that could be prevented by immunization of the entire target population (at the anticipated age of administration) with a hypothetical vaccine that is 100 percent effective (see Chapter 7).

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New Vaccine Development: Establishing Priorities, Volume II, Diseases of Importance in Developing Countries PROSPECTS FOR VACCINE DEVELOPMENT Previous Work Several studies have established that parenterally administered, killed whole cell vaccines are inadequate and are not cost-effective (Finkelstein, 1984; Levine et al., 1983). The evidence also suggests that the parenterally administered toxoids do not provide adequate protection, although relevant trials have been criticized on technical grounds (Finkelstein, 1984; Levine et al., 1983). Current Activities Current activities focus primarily on the development of orally administered cholera vaccines, containing either live attenuated strains or nonviable antigens. Other approaches also are discussed below. Live Attenuated Vaccines The most promising approach to cholera vaccines in terms of cost and efficacy are live attenuated vaccines. Attenuated bacterial strains for oral administration have been derived by chemical mutagenesis and by recombinant DNA techniques. Two attenuated strains derived by chemical mutagenesis have been studied in man and have been demonstrated to produce significant protective immunity in American volunteers although not equivalent to that resulting from the disease. A hypotoxinogenic mutant, M-13, was unstable and some isolates from volunteers regained toxigenicity (Finkelstein, 1984; Levine et al., 1983; Woodward et al., 1976). Another mutant known as Texas Star-SR produces only the nontoxic immunodominant B region of the cholera enterotoxin (choleragenoid) and not the A or active portion of the molecule, and thus is incapable of causing cholera (Honda and Finkelstein, 1979). While the precise genetic lesion in the cholera toxin gene is unknown, the mutation appears to be stable. The vaccine efficacy was 61 percent, but approximately 25 percent of the recipients manifested one to a few loose stools following administration of the vaccine (Levine et al., 1984). There are no current plans to further evaluate this strain as a vaccine. More recent attenuated strains have been derived using recombinant DNA techniques. Genes encoding the cholera enterotoxin have been cloned and sequenced, and substantial, non-reverting deletion mutations have been constructed in vitro. The mutated genes were then recombined into the chromosome of virulent V. cholerae strains to derive well-characterized, attenuated vaccine candidates (Kaper et al., 1984a,b; Mekalanos et al., 1983). The first two candidates to be evaluated in volunteers, JBK70 and CVD101, contain deletions of both the A and B toxin subunits (JBK70) or of the A subunit only (CVD101). Both strains

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New Vaccine Development: Establishing Priorities, Volume II, Diseases of Importance in Developing Countries induced excellent immunity and JBK70, in the absence of any portion of the cholera toxin, conferred protective immunity equivalent to that resulting from the disease (Kaper et al., 1985). However, both of these strains were even more reactogenic than Texas Star-SR, inducing mild to moderate diarrhea in approximately 50 percent of volunteers upon immunization. Additional vaccine strains have been constructed in an attempt to decrease the side effects seen with JBK70 and CVD101. One very promising vaccine candidate is CVD103 which is a derivative of the highly toxinogenic classical strain 569B Inaba which lacks the A subunit (Kaper et al., 1986). Minimal reactogenicity is seen with this vaccine strain with very mild diarrhea seen in only 2 of 37 (5.4 percent) volunteers so far tested. The strain confers excellent immunity and protection against homologous challenge with 569B. Additional clinical trials are underway with this strain and, if successful, should lead directly to a field trial in the very near future. Nonviable Antigens for Oral Administration Killed bacteria administered orally have been advocated as vaccines since the late 1800s. More recent studies (Cash et al., 1974) have shown that while repeated oral administration of massive doses of killed V. cholerae offers some protection against challenge, it is less effective than the vaccine administered parenterally (which has been discarded because of its lack of efficacy). More research is needed on the use of oral adjuvants to improve the immunogenicity of these preparations before their potential can be accurately evaluated. Inactivated cholera enterotoxin also has been considered for oral administration. Inactivation can be accomplished by treatment with formalin or glutaraldehyde, by the removal of the A subunit from the pentameric B subunits (choleragenoid), or by controlled heating, which results in the formation of a relatively nontoxic, large molecular weight polymer called procholeragenoid (Finkelstein et al., 1971). Of these, only the glutaraldehyde-toxoid has been tested by itself for efficacy in volunteers, and it failed to induce demonstrable protection (Levine et al., 1983). Many studies in animal models and man, however, have shown that various forms of the toxin administered orally can induce an immune response, either alone or in conjunction with parenterally administered antigen (Finkelstein, 1984; Levine et al., 1983). The toxin itself and procholeragenoid have been shown to be the most effective (Pierce et al., 1983). Although it has been shown in many laboratories that choleragenoid (pentameric B subunit), when separated from the holotoxin, is a less effective antigen, “B-subunit” oral vaccine elicited a measurable antitoxic response in Bangladeshi recipients (perhaps already primed) who were given single or multiple oral doses of milligram quantities (Svennerholm et al., 1983).

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New Vaccine Development: Establishing Priorities, Volume II, Diseases of Importance in Developing Countries Researchers also have assessed the efficacy of different combinations of killed bacteria and inactivated enterotoxin. Glutaraldehydetoxgid (2 mg weekly for 4 weeks) combined with killed V. cholerae (1010 vibrios twice weekly for 4 weeks) was found to provide 67 percent protection against subsequent challenge in small groups of volunteers (Levine et al., 1983). “B-subunit” (choleragenoid) in three oral doses of 5 mg each, together with three doses totaling 6×1011 of killed V. cholerae, also provided significant, but not absolute, protection to volunteers (Levine et al., 1983). In contrast, procholeragenoid, administered in three doses totaling 300 µg, combined with a different preparation of 1011 heat- and formalin-killed bacteria, only marginally reduced the incidence of the disease in challenge volunteers, although the disease was significantly attenuated (Levine et al., 1983). A large-scale field trial of “B-subunit” combined with killed vibrios given orally is being conducted in Bangladesh (Svennerholm, in press). Preliminary results are encouraging (Kaper, 1986, personal communication), but the duration of protection must be established in order to justify the expense and effort (three doses) of this vaccine. One other oral, nonviable preparation that has recently been field-tested is a V. cholerae cell-wall fraction prepared by the Institut Pasteur. This vaccine was tested in Zaire and appeared to provide significant protection (Dodin et. al., 1983). Additional evaluation of this vaccine, including volunteer studies, is required to adequately compare it to other vaccine candidates. The major drawback to nonviable vaccines is the much higher manufacturing cost relative to live attenuated strains. In addition, the live replicating antigen is significantly more immunogenic than non-replicating antigen vaccines. Whereas the nonviable preparations would require more than one dose (probably three doses), an attenuated strain should require only a single dose. In volunteer trials, the only cholera vaccines that have conferred protection equivalent to that conferred by the disease are the attenuated strains JBK70 and CVD103. It is certainly possible that vaccination of a population in a country endemic for cholera and perhaps already primed would provide better protection than that seen in U.S. volunteers. Currently, the leading candidates for cholera vaccines are the B subunit-whole cell vaccine and the attenuated strains. The issues of cost, immunogenicity, and reactogenicity will be significant considerations in deciding which type of vaccine would be more practical for developing countries where cholera is endemic. Other Approaches Four additional approaches appear to merit further study. These are the use of carrier bacteria containing cloned V. cholerae genes, the development of new nonviable preparations administered parenterally, the use of synthetic antigens, and the possible use of passively administered antibody, per os, to susceptible populations.

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New Vaccine Development: Establishing Priorities, Volume II, Diseases of Importance in Developing Countries Carrier Bacteria This approach utilizes the live, oral typhoid vaccine strain Ty21a into which genes encoding V. cholerae antigens have been cloned. Thus, the S. typhi strain serves as a “carrier” bacterial strain which can present foreign antigens to the local immune system. Details of this vaccine have not been published as of this writing, but clinical trials will soon be underway in Australia (New Scientist, 1986). Ty21a has proven to be extremely safe and non-reactogenic in field trials and it is thought that by adding genes for the LPS and outer membrane proteins of V. cholerae to Ty21a, it will be possible to protect against both typhoid and cholera with a single vaccine. This approach may prove useful for other vaccines in addition to cholera and clearly merits further study. Nonviable Preparations The limited efficacy of the previously evaluated parenterally administered products led many researchers to believe that no parenteral vaccine would merit further consideration. However, procholeragenoid has been shown to be superior to other forms of the toxin antigen in stimulating gut immunity following parenteral administration of moderate doses (Fujita and Finkelstein, 1972). This observation has been confirmed in many laboratories (Finkelstein, 1984). Procholeragenoid has been shown to protect piglets against diarrheal disease due to E. coli strains that produce an immunologically related, but not identical, enterotoxin. It appears to be enriched in the A-subunit of the cholera enterotoxin in a relatively innocuous form. The cholera holotoxin is a potent immunologic modulator; it is conceivable that this property resides in the A-subunit, which is not highly immunogenic by itself. Thus, further consideration should be given to the evaluation of procholeragenoid, administered parenterally either by itself or, for possible synergistic effects, in combination with bacterial antigens. A recent study has shown that procholeragenoid-like products can be produced from the related E. coli heat labile enterotoxins, LTs (Finkelstein et al., 1984). Synthetic Vaccines Recent studies have shown that the cholera-related family of enterotoxins (which includes the E. coli LTs, Salmonella LT, some non-0–1 V. cholerae enterotoxins, and perhaps others) share common or conserved amino acid sequences that appear to be important to their function and immunogenicity. Furthermore, preliminary evidence indicates that certain synthetic small peptides that duplicate some of these sequences, when coupled to carrier proteins, elicit neutralizing antibody and some protection in animal models (Jacob et al., 1983). It is conceivable that appropriate synthetic antigens administered either parenterally or orally (perhaps in protective microspheres as

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New Vaccine Development: Establishing Priorities, Volume II, Diseases of Importance in Developing Countries suggested recently by Klipstein et al., 1983) could provide broad spectrum protection against a large proportion of diarrhea-producing agents. However, neither volunteer studies nor epidemiological data provide support for this concept. The possibility cannot be completely dismissed but considerably more basic research is required to clarify the prospects of success with this approach. Passive Protection Bovine colostrum and milk contain significant amounts of an immunoglobulin G class 1 antibody that retains immunologic reactivity after exposure to intestinal enzymes (McClead and Gregory, 1984). Conceivably, specific bovine colostral antibodies could be a source of passive immune protection for human infants and adults at risk for cholera and other diarrheal diseases. Promotion of breast-feeding may also reduce cholera but the mechanisms involved are not fully understood. PROBLEMS TO BE OVERCOME Several problems markedly inhibit further progress. The first, as noted above, is the slow development of vaccine candidates in the laboratory. This results partly from the lack of suitable animal models of cholera. Although a large number of different systems have been proposed and applied, it is doubtful whether any of them realistically reproduce the disease as it occurs in humans. Younger animals, such as mice and rabbits, are susceptible to moderate doses of V. cholerae, but are difficult to use in protection studies. Other major limitations are the cost and organizational problems associated with conducting large-scale cholera vaccine efficacy trials, once suitable candidates have been developed and evaluated in volunteers. There are only a few places in the world where these trials can be conducted (e.g., the National Institute of Allergy and Infectious Diseases’ vaccine development centers), and such facilities must devote time and resources to many different pathogens. REFERENCES Cash, R.A., S.I. Music, J.P.Libonati, J.P.Craig, N.F.Pierce, and R.N.Hornick. 1974. Response of man to infection with Vibrio cholerae. II. Protection from illness afforded by previous disease and vaccine. J. Infect. Dis. 130(4):325–333. Dodin, A., B.Masengo, and C.Loucq. 1984. Shaba-Zaire Choleric Epidemy, 1983: Field-trial data on the activity of Institut Pasteur anticholeric oral vaccine. C.R. Acad. Sci. Paris 299:205–207. Finkelstein, R.A. 1984. Cholera. Pp. 107–136 in Bacterial Vaccines, R.Germanier, ed. New York: Academic Press.

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New Vaccine Development: Establishing Priorities, Volume II, Diseases of Importance in Developing Countries Finkelstein, R.A., K.Fujita, and J.J.La Spalluto. 1971. Procholeragenoid: An aggregated intermediate in the formation of choleragenoid. J.Immunol. 107:1043–1051. Finkelstein, R.A., C.V.Sciortino, L.C.Rieker M.F.Burks, and M. M.Boesman-Finkelstein. 1984. Preparation of procoligenoids from Escherichia coli heat-labile enterotoxins. Infect. Immun. 45(2): 518–521. Fujita, K., and R.A.Finkelstein. 1972. Antitoxic immunity in experimental cholera: Comparison of immunity induced perorally and parenterally in mice. J. Infect. Dis. 125:647–655. Honda, T., and R.A.Finkelstein. 1979. Selection and characterization of a Vibrio cholerae mutant lacking the A (ADP-ribosylating) portion of the cholera enterotoxin. Proc. Natl. Acad. Sci. USA 76:2052–2056. Jacob, C.D., M.Sela, and R.Arnon. 1983. Antibodies against synthetic peptides of the B subunit of cholera toxin: Cross-reaction and neutralization of the toxin. Proc. Natl. Acad. Sci. USA 80:7611–7615. Kaper, J.B. 1986. Personal communication, University of Maryland School of Medicine, Baltimore. Kaper, J.B., H.Lockman, M.M.Baldini, and M.M.Levine. 1984a. Recombinant nontoxinogenic Vibrio cholerae strains as attenuated cholera vaccine candidates. Nature 308(5960):655–658. Kaper, J.B., H.Lockman, M.M.Baldini, and M.M.Levine. 1984b. A recombinant live oral cholera vaccine. Biotechnology (April): 345–349. Kaper, J.B., M.M.Levine, H.A.Lockman, M.M.Baldini, R.E.Black, M.L.Clements, and J.G.Morris. 1985. Development and testing of a recombinant live oral cholera vaccine. Pp. 107–111 in Vaccines 85, R.A.Lerner, R.M.Chanock, and F.Brown, eds. Cold Spring Harbor, N.Y.: Cold Spring Harbor Laboratory. Kaper, J.B., M.M.Levine, D.A.Herrington, J.Michalski, J.Galen, R. Hall, and J.Ketley. 1986. Recent advances in developing a safe and effective live oral attenuated Vibrio cholerae vaccine. Abstracts of the U.S.-Japan Cooperative Medical Science Program, 22nd Joint Conference on Cholera, Toyama, Japan. Khan, M.U., and W.B.Greenough, III. 1985. Pp. 37–52 in Bacterial Diarrheal Diseases, T.Takeda and T.Miwatani, eds. Tokyo: Scientific Publishers. Klipstein, F.A., R.F.Engert, and R.A.Houghten. 1983. Protection in rabbits immunized with a vaccine of Escherichia coli heat-stabile toxin cross-linked to the heat-labile toxin B subunit. Infect. Immun. 40:888–893. Levine, M.M., J.B.Kaper, R.E.Black, and M.L.Clements. 1983. New knowledge on the pathogenesis of bacterial enteric infections as applied to vaccine development. Microbiol. Rev. 47:510–550. Levine, M.M., R.E.Black, M.L.Clements, C.Lanata, S.Sears, T.Honda, C.R.Young, and R.A.Finkelstein. 1984. Evaluation in humans of attenuated Vibrio cholerae El Jor Ogawa strain Texas Star-SR as a live oral vaccine. Infect. Immun. 43(2):515–522.

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New Vaccine Development: Establishing Priorities, Volume II, Diseases of Importance in Developing Countries McClead, R.E., and S.A.Gregory. 1984. Resistance of bovine colostral anti-cholera toxin antibody to in vitro and in vivo proteolysis. Infect. Immun. 44(2):474–478. Mekalanos, J.J., D.J.Swartz, G.D.N.Pearson, N.Harford, F.Groyne, and M. de Wilde. 1983. Cholera toxin genes: Nucleotide sequence, deletion analysis, and vaccine development. Nature 306:551–557. Miller, C.J., R.G.Feachem, and B.S.Drasar. 1985. Cholera epidemiology in developed and developing countries: New thoughts on transmission, seasonality, and control. Lancet I:261–263. Minassian, D.C., V.Mehra, and B.R.Jones. 1984. Dehydrational crises from severe diarrhoea or heatstroke and risk of cataract. Lancet I:751–753. New Scientist. 1986. New cholera vaccine will be tested on volunteers. New Scientist 109(1500):23. Pierce, N.F., W.C.Cray, Jr., J.B.Sacci, Jr., J.P.Craig, R. Germanier, and E.Furer. 1983. Procholeragenoid: A safe and effective antigen for oral immunization against experimental cholera. Infect. Immun. 40(3):1112–1118. Stock, R.F. 1976. Cholera in Africa. Diffusion of the Disease 1970–1975 with Particular Emphasis on West Africa. African Environmental Special Report 3. Plymouth, U.K.: Clarke, Doble & Brendon, Ltd. Svennerholm, A.M. In press. Proceedings of Nobel Conference 11, Recent Advances in Vaccines and Drugs Against Diarrhoeal Diseases, Saltsjobaden, Stockholm, Sweden, June 3–6, 1985. Svennerholm, A.M., M.Jertbon, L.Gothefors, A.Kavim, D.A.Sack, and J.Holmgren. 1983. Current status of an oral B subunit whole cell cholera vaccine. Dev. Biol. Stand. 53:73–79. Woodward, W.E., R.Gilman, R.Hornick, J.Libonati, and R.Cash. 1976. Efficacy of a live oral cholera vaccine in human volunteers. Dev. Biol. Stand. 33:108–112. World Health Organization. 1984. Cholera. Week. Epidemiol. Rec. 19: 141–142.