G. DUVALLET, A. OUEDRAOGA, M. PINDER and A. VAN MELICK
Introduction
Materials and methods
Results
Conclusion
References
Since the beginning of the century it has been known that humpless African cattle suffer less from trypanosome infection than zebus (Pierre, 1906; Cazablou, 1906). These West African taurine breeds manage to live and multiply in tsetse-infested areas where trypanosomiasis rapidly kills zebus. This ability which is also found in species of wildlife, is called trypanotolerance (IEMVT, 1977).
A better knowledge of trypanotolerance is an essential component for livestock development in humid savanna regions. It is for this reason that the CRTA in Bobo-Dioulasso is carrying out research on this natural resistance.
Observations within the Baoule breed (phenotypically selected animals) have shown that this trypano-resistance is a variable trait and that although some animals were very resistant others died very quickly under high tsetse challenge (Roelants, 1986).
In order to enable selection of susceptible and resistant animals they were exposed to natural high challenge at the experimental station of Samandeni (Roelants et al., 1983). Pinder et al. (1987) have demonstrated, by comparing a natural infection under high tsetse challenge with an experimental cyclical infection with a T. congolense clone derived from an East African strain, that there is no correlation between these two types of infection and that in a stable it is not possible to reproduce the same observations obtained under natural challenge.
We have tried to make the selection by exposing the animals to cyclical experimental infection of a T. congolense clone derived from a West African strain in a tsetse-free building.
Trypanosomes
The T. congolense clone used in this experiment is a Karankasso/83/CRTA/57 strain stabilate 12 of 24/1/86 isolated near Bobo-Dioulasso. This strain has already been used at the CRTA in a mouse model to study trypano-resistance (Pinder et al., 1985; Pinder et al., 1986; Roelants and Pinder, 1987).
Cattle
The 5 zebus and 17 Baoules used in the experiment were born at the CRTA and raised in a tsetse-free stable. At the beginning of the experiment, the animals were between 10 and 12 months old.
Tsetse
Cyclical infection was carried out with female Glossina morsitans submorsitans from the CRTA colony. Newly hatched tsetse took their first three blood meals from ears of rabbits which had been infected three days earlier. Subsequent meals were membrane feeds. Twenty days following the first blood meal, salivary probe samples were collected on microscope slides warmed to a temperature of 37°C, and, out of 1089 live tsetse, 11 were found to be infected (10.1%).
Infection in cattle
Five zebus and 11 Baoules were bitten once by an infected tsetse which was allowed to feed on the flank of the previously tranquillized animal (Rompun2% (R),0.2ml/100 kg). The remaining six Baoules were kept as controls.
The animals were monitored by regularly checking weight, body temperature, skin reactions, PCV and parasitaemia as well as specific immune responses by the complement lysis test. One zebu (No. 698) was treated with Berenil (R) on day 31 following infection due to a poor general condition and a PCV of 20%. All the other animals, including the controls, were treated with Berenil (R) at the end of the experiment on 10/7/86. This date, chosen for logistic reasons, corresponds with days 80 and 93 following infection, depending on the animals.
Complement lysis test
For each parasitaemic bovine, the first peak parasitaemia was isolated in irradiated mice (650 rd, Cs 0.8 Krad. mn-1) and preserved in liquid nitrogen for future use as antigens in the lysis test. The immune response of each bovine was studied individually using as antigen the first parasitaemia peak isolated in each animal.
The test was done by micromethod (Terasaki plates). Guinea pig serum preserved in liquid nitrogen was used as a source of complement. The stabilates were inoculated into irradiated mice to allow multiplication and collection of trypanosomes. For each bovine, six sera were chosen between day 0 and day 130 following infection in order to determine the titre producing 100% lysis.
Infection
In 14 out of 19 cases, a single bite from an infected fly (sometimes an attempt only, i.e. penetration of the buccal parts without blood suckling) infected the cattle. In two Baoule cattle no infection was observed after being bitten by infected tsetse, even though the two flies involved had infected other cattle in the group. In total, five zebus and nine Baoules showed patent parasitaemia.
Skin reactions - prepatent periods
In Table 1, we have shown the presence or absence of skin reactions (chancres) to infected fly bites for each animal, the period of observation of the chancre and the prepatent period. For the latter we have indicated the interval between the last negative observation and the first positive observation. Results do not show any difference between zebus and Baoules.
Table 1. Skin reactions and prepatent periods.
|
Cattle |
Chancre |
Prepatent periods |
||
|
+/- |
Dates |
|||
|
Z 693 |
+ |
9/16a |
12/16b |
|
|
E 698 |
+ |
9/16 |
12/16 |
|
|
B |
||||
|
U 703 |
+ |
9/12 |
16/18 |
|
|
S 708 |
- |
. |
16/18 |
|
|
|
709 |
- |
. |
18/21 |
|
815 |
+ |
6/15 |
15/17 |
|
|
B 850 |
+ |
13/16 |
16/20 |
|
|
A |
||||
|
O 824 |
+ |
9/20 |
16/20 |
|
|
U 859 |
+ |
9/12 |
13/15 |
|
|
L |
||||
|
E 827 |
- |
. |
14/16 |
|
|
S 848 |
- |
. |
15/19 |
|
|
|
842 |
- |
. |
16/19 |
|
|
840 |
- |
. |
16/20 |
|
|
849 |
- |
. |
15/18 |
a Observed from day 9 to day 16 after infection.
b Parasitaemias negative on day 12 and positive on day 16 after infection.
Body temperature
The average body temperatures were calculated for each group of animals every five days. The infected group did not show any differences (Figure 1) and the temperature curves closely followed the parasitaemia curves.
Weight
Weight change after infection was expressed as a percentage of weight at infection. Results for each group of animals are shown in Figure 2.
Statistical analysis shows that the difference between Baoule controls and infected Baoules (which is not significant up to day 70 following infection) becomes significant from day 80 onwards.
Figure 2. Weight change of three groups of animals after infection.
PCV
PCV changes were calculated for each group of animals every five days and are shown on Figure 3.
The four zebus which were not treated during the experiment had PCVs of 38.5% at the beginning of the infection and 23.1% around the 55th day. After the 55th day, PCV rose gradually and reached the same level as that of the infected Baoules (29.1%) on the 80th day.
Infected Baoules had a minimum PCV level of 27% on the 55th day, after which PCV rose gradually and reached a normal level at the time of treatment.
Baoule controls had PCV levels varying from 32 to 35.4% during the period of observations.
Statistical analysis was based on the average variations for each animal, between the highest and the lowest values observed during the period. Results are summarized in Table 2.
Table 2. PCVs: drop and minimum level during the period of observation.
|
|
Number |
Drop |
Minimum |
|
Baoule |
|
9.8 |
8.3 |
|
Controls |
6 |
(1.72) |
(1.37) |
|
Infected |
|
11.8 |
25.1 |
|
Baoules |
9 |
(2.92) |
(1.9) |
|
Infected |
|
17.5 |
21.7 |
|
Zebus |
4 |
(3.7) |
(2.8) |
() = standard deviation
The difference between infected zebus and Baoules is very significant, but the difference between Baoule controls and infected Baoules is not significant. There is a significant difference between infected Baoules and zebus for minimum levels observed (p-2.8% for 11 df).
Parasitaemia
Results of parasitaemias (level of first peak and duration of detectable parasitaemia) are shown on Figure 4 and summarized in Table 3.
Figure 4. Parasitaemia scores.
Table 3. Parasitaemias: level of first peak and duration of detectable parasitaemia.
|
|
Number |
Level of 1st peak |
Duration of detectable parasitaemia |
|
Baoules |
|
5.67 |
|
|
9 |
(0.23) |
62 |
|
|
Zebus |
|
5.82 |
|
|
5 |
(0.16) |
82* |
|
|
Comparison of averages
|
|
NS |
S |
|
|
P>0.20 |
P<0.01 |
*Number = 4
No difference was observed between Baoules and zebus for the first parasitaemia peak. But the duration of detectable parasitaemia was significantly higher in zebus.
Figure 5. Immune response: Complement lysis test
Immune response
Results are shown in Figure 5. We have shown serum titres producing 100% lysis with complement. The response of zebus appears to be as high (titre) as that of the Baoules, but appears later (80th day vs 60th day). These are just preliminary results and immune responses of animals in this monitored experiment should be studied in greater depth.
In brief, this is an experiment on the cyclical infection of previously uninfected Baoule and zebu cattle with a T. congolense strain isolated in the Bobo-Dioulasso area.
The PCV drop in zebus is significantly higher than in Baoule cattle. With the exception of one zebu which had to be treated prematurely, all infected animals began to regulate their PCV from the 55th day following infection, before treatment. Baoule cattle also controlled their parasitaemia faster than zebus although the levels of first peak parasitaemias were not significantly different.
No marked differences were observed within the group of infected Baoules to enable selection of resistant and susceptible animals. As Pinder et al. (1987) have demonstrated, it would therefore be difficult to select susceptible and resistant animals in the Baoule breed in tsetse-free stables using classic parameters.
After this experiment, the animals were exposed in the field to allow comparison under high natural challenge. Analyses of results are presently underway.
Cazalbou, L. 1906. La Souma. Rev. gent Med. vet. 8: 240.
IEMVT. 1977. La Trypanotolerance, synthese des connaissances actualles. IEMVT, Maisons-Alfort.
Pierre, C. 1906. L'elevage dans l'Afrique Occidentale Francaise. Paris, Gouvernement general de l'A.O.F. (Inspection de l'Agriculture).
Pinder, M., F. Fumoux and G.E. Roelants. 1985. Immune mechanisms and genetic control of natural resistance to Trypanosoma congolense. In: Genetic Control of Host Resistance to Infection and Malignancy. E. Skamene, ed. New York: Alan R. Liss. pp. 495-500.
Pinder, M. P. Chassin and F. Fumoux. 1986. Mechanisms of self-cure from Trypanosoma congolense infection in mice. J. Immunol. 136 (5): 1427-1434.
Pinder, M., J. Bauer, F. Fumoux and G.E. Roelants. 1987. Trypanosoma congolense: lack of correlation between the resistance of cattle subjected to experimental cyclic infection or to field challenge. Exp. Parasitol. 64: 410-417.
Roelants, G.E., I. Tamboura, D.B. Sidiki, A. Basinga and M. Pinder. 1983. Trypanotolerance. An individual, not a breed character. Acta Trop. 40 (2): 99-104.
Roelants, G.E. 1986. Natural resistance to African trypanosomiasis. A viewpoint. Parasite Immunol. 8 (1): 110.
Roelants, G.E. and M. Pinder. 1987. The virulence of Trypanosoma congolense can be determined by the antibody response of inbred strains of mice. Parasite Immunol. 9: 379-388.
