Veterinary Parasitology, 7 (1980) 11--18 11 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

THE IMMUNOSUPPRESSIVE EFFECTS OF EXPERIMENTAL T. CONGOLENSE INFECTIONS IN GOATS

LESLEY GRIFFIN**, S. WAGHELA and E.W. ALLONBY*

Veterinary Research Laboratories, Kabete (Kenya)

* UNDP/FAO Sheep and Goat Development Project, P.O. Box 30218, Nairobi (Kenya)

** Present address: P.O. Box 30020, Nairobi (Kenya)

(Accepted 6 November 1979)

ABSTRACT

Griffin, L., Waghela, S. and Allonby, E.W., 1980. The immunosuppressive effects of experimental T. congolense infections in goats. Vet. Parasitol., 7: 11--18.

The agglutinin response of four groups of goats inoculated with Brucella melitensis vaccine 0, 1, 2 and 4 weeks following experimental infection with Trypanosoma congo- /ense was compared with that in non-infected controls. Four weeks after vaccination the goats were treated with a trypanocidal drug and the recovery of the immune response observed. The results indicated that the majority of animals had a significantly but not completely suppressed antibody response. This was most marked in the group vaccinated 2 weeks post-infection, which corresponded with the onset of parasitaemia. Although the mortality rate in the infected goats was high the titre in those remaining animals that were treated with the trypanocidal drug increased immediately after treatment. The possible implications of trypanosome induced immunosuppression for vaccination programmes in goats are discussed briefly.

INTRODUCTION

It has been recognized in recent years that the pathogenicity of certain infectious diseases may arise from a suppression of the host's normal immune response to quite unrelated antigens. This phenomenon is particularly evi- dent in some parasitic diseases and, for example, it has been shown that laboratory animals infected with trypanosomiasis show a suppressed immune response to various foreign antigens (Goodwin, 1970; Goodwin et al., 1972; Urquhart et al., 1973; Hudson et al., 1976). More recently, studies of trypano- somiasis in domestic ruminants have indicated similar effects in the response of infected animals to commercially produced bacterial and viral vaccines (Holmes et al., 1974; MacKenzie et al., 1975; Scott et al., 1977).

In view of the increasing use of vaccines in domestic animals throughout Africa, including many areas of endemic trypanosomiasis, the possible im- munosuppressive effects of trypanosome infections may be important in

0304--401718010000--00001502.25 © 1980 Elsevier Scientific Publishing Company

12

the efficacy of the vaccines. However, there is some evidence that trypano- cidal drug t rea tment may rapidly be able to restore the immunological memory held in abeyance as a result of the t rypanosome infection (Murray et al., 1974). This may have a beneficial effect on the efficacy of vaccination in these areas. The following experiment was performed to investigate the rate and extent to which suppression of the ant ibody response to a Brucella vaccine occurred in goats following inoculation with T. congolense and the possible recovery of the ant ibody response following trypanocidal drug treatment.

MATERIALS AND METHODS

Study area and vegetation

The experiment was carried out at the Kiboko Tsetse Survey and Control Field Station in the Machakos District of Kenya. The Station, which is tsetse free, is an area of open savannah classified as Ecological Zone V (Pratt et al., 1966).

Experimental design

Adult male East African goats which had been born and reared in a trypa- nosome-free area without chemotherapeutic t reatment were used. The experimental animals were tested for evidence of Brucella melitensis anti- body before the start of the experiment and only goats which had a negative titre were used.

Six groups of ten animals per group were treated as follows: Group I: Subcutaneous inoculation of 2 ml of the killed B. melitensis H38

adjuvant vaccine 1 was performed on Day 0. As it was impractical to vaccinate non-infected animals at all stages of the study, and there was no evidence to suggest that the response altered with time using the same batch of vaccine, this group acted as a control to show the normal response to the vaccine.

Groups II to V: These animals were infected by syringe passage into the jugular vein with approximately 10 s T. congolense (T.c.) organisms from a donor goat carrying a naturally acquired infection.

Group II: These animals were vaccinated at the same time as being infected with the parasites.

Groups III, IV and V: Animals of these groups were vaccinated at various intervals after being infected as shown below.

All infected animals were treated with Berenil 2 (diminazine aceturate) at 3.5 g/kg 4 weeks after vaccination.

"Aborlane" Merieux (France). Berenil (Ber.)

1 3

D a y 0 W e e k l W e e k 2 W e e k 4 W e e k 5 W e e k 6 W e e k 8

Group I H38 -- -- Bet -- -- -- Group II H38/T.c. -- -- Ber -- -- -- Group III T.c. H38 -- -- Ber -- -- Group I V T.c. - - H38 -- -- Ber -- Group V T.c. - - -- H38 -- -- Bet Group VI . . . . . . .

Group VI: These animals were left uninfected and were used as controls to moni tor the incidence of naturally acquired trypanosomiasis and brucel- losis during the course of the experiment.

S e r u m c o l l e c t i o n

Serum collections f rom each animal were made at weekly intervals during the course of the experiment and for 4 weeks after Group V was t reated w i t h Bereni l . The serum was tested for Bruce l la m e l i t en s i s ant ibody t i tre using the Serum (tube) Agglutination Test (SAT) according to the method of Philpott and Auko (1972).

Diagnos i s o f i n f e c t i o n

Weekly thick blood smears f rom each animal were prepared and examined to determine whether the animals experimentally infected with T. c o n g o l e n s e

had detectable parasitaemia.

RESULTS

The mean ant ibody titres in international units for all groups except Group VI are shown in Table I. All the Group VI animals had negative titres to B. m e l i t e n s i s th roughout indicating that no endemic infection was present and that the positive titres of the other groups were entirely due to the vaccine.

Owing to the conditions prevailing at the field station during the course of the experiment it was not always possible to collect serum samples f rom every animal wi thout some degree of haemolysis. Where haemolysis was too great the sample could no t be tested serologically and accounts for the variations in the numbers of samples.

The Group I controls which had no T. c o n g o l e n s e infection showed a normal response to the vaccine with ant ibody titres which increased steadily reaching a mean of 2742 i.u. by the 4th week after vaccination and remained steady at about 800 i.u. th roughout the rest of the experimental period. Group II, those infected and simultaneously vaccinated, showed a normal, response for 2 weeks with a mean titre increasing to 1100 i.u., but this then decreased to 800 i.u. at Week 3 and increased to 1250 i.u. by Week 4 bo th

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15

of which were significantly lower at the 5% level than the response of the control group at the same time. By the 5th week after vaccination there was only one survivor, which had a titre of o n l y 1 0 0 i.u.

Group III, those vaccinated 1 week after infection, showed a similar result to Group II with a normal response for 2 weeks after vaccination but by Week 3 the ant ibody titre had dropped to a mean of 250 i.u., which was significantly lower than the controls at the 5% level of significance but by Week 4 only one animal survived.

Group IV, those vaccinated 2 weeks after T. congolense infection also showed a normal response initially with a mean titre of 600 i.u. by Week 2, but by the following week this had fallen to 580 i.u., which was significantly lower than that of the controls. The mean titre then decreased even further to only 50 i.u. a week later and this was also significantly lower than that of the controls (P < 0.01).

Group V, those vaccinated 4 weeks after T. congolense infection, showed an increase in titre to a mean of 933 i.u. by the 3rd week after vaccination but this was not significantly different from the controls. The following week, however, the titre had decreased to 533 i.u. which was significantly lower than the controls (P < 0.01). The titre continued to decrease to 207 i.u. by the 6th week and was still significantly lower than the controls.

Although the original plan was to treat infected animals 4 weeks after vaccination, the titre of this group was still decreasing so these animals were treated at 6 weeks after vaccination. Moreover as there was a high mortal i ty rate among the infected goats this group was the only one with sufficient animals to treat and make statistical comparisons of the ant ibody titre before and after treatment.

The mean ant ibody titres shown in Table I are of those animals in each group whose titre did not show a level of 3200 i.u., which was the level reached in uninfected animals. However, in each group there were two animals which responded normally to the vaccine, in spite of a patent T. congolense infection. These individuals apparently had a higher degree of tolerance, suffered little from the infection and survived in good health throughout the experiment. As the titres of these individuals were so dif- ferent from those of the others in the same group and the response to the infection uncharacteristic these individuals were not included in the statistical analysis of the results.

Table II indicates the degree of suppression of the immune response of each group at weekly intervals throughout the study, expressed as a percen- tage of the control group at the same time. The results indicate that Group II became suppressed by 55% at the 3rd week after vaccination whereas Groups III and IV were suppressed by up to 90% or more of the level in the controls. Group V, however, appeared to be less suppressed than Groups III and IV, reaching a maximum of 81%, and the degree of suppression was only 9% after diamazene aceturate treatment.

16

TABLE II

The degree of suppression in T. congolense-infected goats (as measured by SAT titres to BruceUa melitensis H38 adjuvant vaccine) expressed as percentage o f the titre in unin- fected controls

Weeks after inoculat ion wi th B. melitensis vaccine 1 2 3 4 5 6 7

Group 2 T.c./H38 s imul taneous ly 0 0 65 55 - - - - - -

Group 3 H38 1 w e e k after T.c. 0 0 90 . . . .

Group 4 H38 2 w e e k s after T.c. 0 0 75 98a - - - - - -

Group 5 H38 4 w e e k s after T.c. 0 0 60 81 48 74 9

- - No survivors. a Only one survivor.

DISCUSSION

The results indicate that the indigenous East African goats infected with T. congolense have a lower humoral response to killed B. melitensis H 3 8

adjuvant vaccine, than uninfected goats. MacKenzie et al. (1975) showed the humoral response to Vibrio fetus

antigen is lower in Blackhead Persian sheep infected with T. eongolense compared with uninfected controls, but only became significantly lower after 3 weeks following infection. The suppression in their work was not complete as occurs in T. brucei infected rodents (Goodwin et al., 1972; Murray et al., 1973). A similar mild degree of immunosuppression was found by Scott et al. (1977) in Zebu cattle infected with T. eongolense and vaccin- ated with FMD and clostridial vaccine.

In this study, the suppression, which reached a maximum of about 90% of the normal response, was not apparent until 3 or 4 weeks after inocula- tion with Brucella vaccine and was irrespective of the duration of T. congo- lense infection before vaccination. This slow response may be due to the vaccine being incorporated with an adjuvant and inoculated subcutaneously.

It was evident tha t two distinct groups of individuals occurred in this study; firstly those animals which had a chronic infection with only minor clinical symptoms and no suppression of the humoral response, and secondly those which had an acute fatal infection and showed a depressed response to vaccination. The former group of goats were not included in the statistical analysis of the results because with their titres the mean of each group was increased such tha t they showed no difference from the controls, suggesting that T. congolense unlike T. brucei does not cause immunosuppression. How-

17

ever, this is not a true representation of the case. Since the majority of animals showed a significant degree of antibody suppression, it was thought relevant to consider the two types of responses separately.

The mechanisms of suppression caused by trypanosomes has not yet been elucidated, but it has been suggested that trypanosomes may specifically inhibit reactive B lymphocytes from responding fully to antigen challenge and hence having a reduced antibody production (Murray et al., 1974). The degree of immunosuppression may be dependent on the presence of living trypanosomes (Freeman et al., 1974; Jennings et al., 1974; Scott et al., 1977). The results of this experiment may support this theory in so far as a T. congo- lense infection, which has a much lower parasitaemia, causes less suppression of the antibody response than T. brucei with a higher parasitaemia. This may account for the finding that animals vaccinated 4 weeks after infection showed a greater immune response to the vaccine than those vaccinated in the early stages of infection. It is possible that the disease had become chronic and the hosts supported a persistent but low level of parasites by the time vaccine was administered. Also, the antibody level to the vaccine increased after treatment with diamazene aceturate. However, this aspect is not conclusive because of high mortality prior to treatment.

The previous work done in laboratory animals with T. brucei has shown severe immunosuppression (Murray et al., 1973). Similar work in domestic animals but with T. congolense has shown low to moderate suppression (MacKenzie et al., 1975; Scott et al., 1977). The differences between the courses of T, congolense and T. brucei infection may be responsible for the variations in results obtained from this study compared to others using T. brucei.

Although in this experiment there was a high mortality, the results suggest that T. congolense is responsible for significant but by no means total sup- pressing of the humoral immune response. However, as Scott et al. (1977) suggest the response that does occur may be sufficient to protect against challenge infection. Clearly this aspect requires more detailed study, and further investigations on the possible role of immunosuppression in the pathogenesis of animal trypanosomiasis would be fully justified.

ACKNOWLEDGEMENTS

We would like to thank the staff of the Kiboko Range Research Station for assistance and daily co-operation during the study, the staff of the Serology Section, Kabete, for their technical help, and the UNDP/FAO Sheep and Goat Development Project for providing the experimental animals. The paper is published with the kind permission of the Director of Veterinary Services, Kenya.

18

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