Immune response of Bos indicus cattle against the antigen Bm91 derived from local Rhipicephalus (Boophilus) microplus ticks and its effect on tick reproduction under natural infestation (English shop)

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In the present study two immunisation trials with the anti-tick antigen Bm91 using Bos indicus cattle were conducted in Northern Thailand. The antigen was derived from a Thai Rhipicephalus (Boophilus) microplus strain. The trials were conducted at different locations with similar environmental conditions. At both locations the animals were raised in an extensive grazing system with natural exposure to ticks.

Trial 1 was conducted at RJ Ranch, a private breeding farm about 120 km south of Chiang Mai. On the one hand White Lamphun cattle as an example of indigenous Thai cattle which are highly adapted to the local environmental conditions and on the other hand Brahman cattle as an example of an exotic breed were used. Six adult females of each of this Bos indicus breeds were immunised with the antigen Bm91 and the other 6 animals remained as control animals. Three immunisations with a 3-week interval in-between were given. Weekly blood samples over a period of 3 months were collected to measure the anti-Bm91 antibody level by ELISA. Additionally, the health status was monitored by the complete blood count.

Trial 2 was conducted at the Department of Animal and Aquatic Science of the Chiang Mai University. Eighteen adult female White Lamphun cattle were divided into 3 treatment groups with 6 animals each. One of the groups was immunised with the same antigen formulation as in trial 1. The other groups received saline and adjuvant, respectively. The immunisation scheme included immunisations in week 0, 3, and 6. After 6 months a 4th immunisation dose was applied. The antibody level was monitored for the 3 months following the primary dose and additionally for 1 month after the 4th treatment (week 26 until week 30 ppi). The health status was monitored as in trial 1. Standard engorged female ticks (4.5 to 8 mm) were collected daily from week 6 until week 12 ppi and from week 27 until week 30. After the engorged tick weight was recorded ticks were reared in individual containers. The egg mass weight was measured to calculate the reproductive efficiency index (egg mass/tick weight). Furthermore, the egg viability was checked.

In both trials Bm91 evoked a strong and long-lasting antibody response. A significant difference between the indigenous and the exotic animals was not found in trial 1. At both locations the antibody level showed a characteristic course. After a strong response to the primary immunisation a further increase was observed after the 2 subsequent immunisations. A stable level was reached after the 3rd treatment. This level was maintained until week 12 ppi in both trials. Until the 4th immunisation in trial 2 (week 26) the anti-Bm91 ELISA values decreased insignificantly. After this booster dose a moderate increase of the antibody level was recognised. The Western blot analysis confirmed that recombinant Bm91 elicited antibodies that bind to R. (B.) microplus proteins. An effect of Bm91 on the parameters of the complete blood count was observed in none of the trials. The difference between Brahman and White Lamphun of several blood parameters was significant and indicated a superior health status in the indigenous animals.

Regarding the different tick parameters the number of ticks was highly variable within the groups (trial 2). A difference between the 3 treatment groups was not found. The tick weight was as well not different between the groups. A significant effect (P < 0.05) of Bm91 was recorded for the reproductive efficiency index and the egg viability which were reduced by 6 and 8%, respectively.

In summary, the recombinant antigen Bm91 from Thai R. (B.) microplus ticks induced substantial antibody responses independent of the B. indicus breed. However, the effect on the reproductive tick performance measured under natural tick infestation was low.