Combined Active and Passive Immunization against Diphtheria: I. Studies of Antitoxin Response in Normal Students

A survey of veterinarians' practices, recommendations and perceptions associated with the prevention of tetanus in horses in Australia

OBJECTIVE

Identify veterinarians' practices, recommendations and perceptions when preventing tetanus in horses in Australia.

METHODS

Graduated members of Equine Veterinarians Australia participated in an online survey about the prevention of tetanus in horses in Australia.

RESULTS

Of 77 respondents (response rate ~ 8%), 50 (65%) reported that they had attended collectively 145-152 cases of tetanus in horses in Australia (1.1 cases of tetanus observed/10 veterinarian-years since qualification). The estimated case fatality rate (CFR) was at least 79%. Puncture wounds were most frequently suspected as the entry point (32%; n = 47). Five respondents (7%) reported cases of localised tetanus. Three respondents reported generalised tetanus within 2 weeks of tetanus antitoxin (TAT) administration. Respondents did not report any cases of tetanus in horses which had been vaccinated according to manufacturer's recommendations. All respondents recommended vaccination, but over 50% (N = 45) stated 'lack of veterinary recommendation' as often or sometimes a reason why clients did not vaccinate horses. Opinions varied on the use of TAT for peri-exposure prophylaxis; 67% of respondents dosed TAT independent of body weight, with the rest dosing according to body weight. Cases of Theiler's disease related to the use of TAT were not reported in Australia by respondents.

DISCUSSION

The caseload of equine tetanus appears relatively low among equine veterinarians in Australia. Consistent with the literature, estimated CFR was high. Respondents' recommendations and perceptions about tetanus vaccination were generally consistent but varied regarding TAT usage. Evidence for TAT usage is limited and we recommend studies of TAT efficacy in horses.

Combined active and passive immunization against diphtheria

Three sets of experiments were carried out on undergraduate medical students at Oxford, Cambridge and Liverpool during the years 1941–4 in order to supplement the information obtained previously (Downie et al. 1941) on the comparative antitoxin response of those given active immunization alone (Group A) and those given combined active and passive immunization (Group A+P). A summary of each of the experiments has already been given in the text, so that it is unnecessary here to do more than recapitulate briefly the main results.

1. The first experiment showed that in Group A the antitoxin response was not appreciably greater in students receiving doses of 0·3 and 0·3–0·5 ml. of A.P.T. at 4 weeks’ interval than in those receiving doses of only 0·1 and 0·3 ml. In Group A + P no difference was noticed in the antitoxin content of the serum 6–8 weeks after the second injection of A.P.T., but 10–12 weeks after the second injection there was a difference in favour of the students receiving the larger doses of A.P.T., though it was below the conventional level of statistical significance.

2. The second experiment showed that when the doses of A.P.T. were spaced by 2 instead of by 4 weeks the antitoxin response was much less in both the A and the A + P groups, though the difference was less in the latter group, particularly when the measurements were made 10–12 weeks after the second inoculation of A.P.T.

3. The third experiment showed that a dose of 5000 units of diphtheria antiserum given at the time of the first injection of A.P.T. inhibited antitoxin production to a greater extent than a dose of 400–500 units, though the difference was much less when the measurements were made at 12 weeks after the second inoculation than at 4 weeks.

A compilation of the results obtained during 1940–2 in groups of students receiving active and those receiving active plus passive immunization shows that the antitoxin production in the first group (Group A) was much higher than in the second group (Group A + P) 6–8 weeks after the second injection of A.P.T., but that 10–12 weeks after the second inoculation the difference, though still significant, was considerably less. The final Schick-test results at 10–12 weeks gave a Schick-conversion rate of 98.0% in Group A and of 90.9% in Group A + P.

A review of the results obtained during the years 1939–44 on about 450 students at Oxford, Sheffield and Liverpool leads to the conclusion that the effect of giving diphtheria antiserum at the time of the first injection of A.P.T. is to cause a delay and some degree of inhibition in the antitoxin response of the subject. The larger the amount of antiserum given, the greater is this effect. With a dose of 500 units, though the delay in antitoxin formation is very obvious 4 weeks after the second injection of A.P.T., the final degree of immunity attained, as judged by the antitoxin concentration of the blood serum and by the Schick-conversion rate, is not greatly inferior to that resulting from active immunization alone; and even with a dose of 5000 units, the Schick-conversion rate reaches 81 % 12 weeks after the second injection of A.P.T. It is clear, therefore, that the antiserum, even when given in a dose as large as 5000 units, does not neutralize more than a small part of the antigenic activity of the first dose of A.P.T. Its main effect is apparently to diminish the rate of sensitization of the tissues, so that when a second dose of A.P.T. is given 4 weeks later, the rise in the antitoxin content of the blood serum is considerably delayed. Our experiments suggest that by increasing the size of the first dose of A.P.T., some of this delay may be avoided.

The partial neutralization of the first dose of A.P.T. will result in a decrease in the total antigenic stimulus and a delay in the time at which ït comes into operation. It is suggested that, provided the tissues have not been previously sensitized to diphtheria toxin, the result may be that the two doses will act virtually as a single dose. Such an explanation, however, must remain unproven till further observations have been made (see p. 35).

The practical value of combined active and passive immunization, especially when joined with temporary segregation of healthy carriers, in combating outbreaks of diphtheria in schools and other institutions for children has been clearly shown by Fulton, Taylor, Wells & Wilson (1941). Our present experiments lead us to suggest that, when applying the method in practice, it would be wise to give an initial dose of 0·5 ml. of A.P.T., together with 500 units of diphtheria antiserum injected at a different site, followed 6 weeks later by a second dose of 0.5 ml. A.P.T. It is probable that children treated in this way will develop approximately the same ultimate degree of immunity as those actively immunized with doses of 0·3 and 0·5 ml. of A.P.T. at 4 weeks’ interval.

We should like to express our thanks to Prof. A. D. Gardner and Prof. H. R. Dean for permitting observations to be made on the students in the pathology classes at Oxford and Cambridge; and to the students themselves at Oxford, Cambridge and Liverpool, for their ready co-operation in the inquiry.

Antibody formation. II. The specific anamnestic antibody response

Diphtheria toxoid-antitoxin precipitates formed in antitoxin excess can prepare guinea pigs, rats, and rabbits for a secondary type of antitoxin response. Priming may occur without the development of detectable serum antibody. In rats, toxoid-antitoxin precipitates are more efficient than "free" toxoid in priming, whereas in guinea pigs, the magnitude of the anamnestic response varies with the precipitate employed. The possibility that priming is due to "free" antigen released from the specific precipitate rather than the precipitate itself is discussed. The anamnestic antitoxin response can be inhibited by passive antitoxin, but less efficiently than primary antitoxin formation. Partial suppression of the secondary antitoxin response was accomplished by injection of excess horse antitoxin as long as 4 days after reimmunization with toxoid. The importance of these findings for the understanding of passive-active immunization in the human is discussed.

SIMULTANEOUS ACTIVE AND PASSIVE IMMUNIZATION OF GUINEA-PIGS AGAINST TETANUS

1. Simultaneous active and passive immunization of guinea-pigs using tetanus toxoid adsorbed on to aluminium hydroxide produced a higher antitoxin response than simultaneous immunization using plain toxoid. The response was also better than that produced by plain toxoid alone.

2. A 24 hr. interval between the injection of plain toxoid and the subsequent injection of antitoxin gave no better results than when toxoid and antitoxin were given at the same time.

3. The response to simultaneous immunization, using adsorbed toxoid, of guinea-pigs sensitized to horse serum, was superior in the first 6 weeks to that of guinea-pigs which had not been sensitized.

4. The suggestion that simultaneous active and passive immunization of man should be performed with adsorbed toxoid rather than plain toxoid is supported by these results.

Immunization against tetanus of patients given injections of antitetanus serum

The authors report on an investigation conducted in the Netherlands into the best method of active immunization of patients treated with antitetanus serum; the active immunization is conferred by two intramuscular injections of tetanus phosphate toxoid at one month's interval.Trials were carried out on a total of 153 patients to determine the relative value of three treatment schemes: (a) the first vaccination and the serum injection given simultaneously but in different sites, (b) the first vaccination and the serum given in a single mixed inoculation, and (c) the first vaccination given one month after the serum injection. The results show that with simultaneous administration of vaccine and serum in different sites the patient acquires a reasonable antitoxin titre more rapidly than with either of the other schemes. Not only is he quickly protected, but he also receives his first vaccination at a psychologically opportune moment-when he is being treated for an accident.The authors lay special stress on the value of booster injections and discuss the difference in immune response between those under and over 20 years of age.