Studies on the Immune Response to Amblyomma Variegatum in Cattle and the Effects of Haemoparasitism on the Acquisition of Tick Resistance
ABSTRACT
Acaricides in tick control, had been thought to be in Africa the panacea for over a century. Environmental and economic constraints, development of acaricide-resistant strains due to acaricide misuse, have brought about the need for an alternative tick control strategy. The use of tick resistant cattle has then been advocated and echoed in the USA and Australia where it got its full expansion. In Africa, few works have been undertaken on tick resistance despite the fact that many tick species are found on the continent. Studies on acquisition of immunity to tick and especially to A. variegatum in Africa are very scarce so several questions need to be answered before the use of such a method.
Therefore, an attempt was made to shed some lights on the tick-cattle interactions and on some of the factors which could influence the acquisition of resistance. We have chosen the Boran cattle-A variegatum model. That model was compared to those of A. variegatum-Ayrshire cattle and A. variegatum-crossbred Friesian x Boran. The resistance was artificially induced by applying simultaneously at one month interval, 100 nymphs on one ear and 20 males and 20 females on the other ear. At the beginning of the experiment the average age of the 20 animals was one year.
For the first time, the status of acquired resistance was assessed with a multivariate analysis method. The principal component analysis (PCA) and fastclus procedure (SAS/STAT) were performed on the variables with high loads. It was noticed after the first infestation that, all the biological
parameters favoured tick survival. The percentage engorged (PENGD), the percentage dead (PDEAD), the feeding period (FPR), the engorgement weight (ENGWT), the percentage that moulted (PMLTD) and the percentage that engorged above critical weight (PEACWT), with loads > 0.45 proved to be the indicators for the resistance to nymphs and adults. Three groups (high, moderate, low) of resistance have been defined using the fastclus procedure. Each group has been subdivided into three lots with two individuals each. The resistance dynamic was assessed using the ANOVA procedure. It was observed with the Boran cattle that resistance to the nymphs was already induced from the third infestation. Resistance to adults did not show a defined trend. A decline in the immunity level to nymphal and adult stage was observed from the 4th and 5th infestation.
In order to assess the effects of trypanosomosis or babesiosis on the acquired immunity, the tick-immune animals from the above lots were infected separately with either 1 ml of 107 blood stream T. congolense or with 150 ml of infective blood from an immunosuppressed donor calf bearing 10% B. bigemina parasites. The same number of ticks was applied, in the same manner at the height of parasitaemia. There was a significant decrease in the PENGD, ENGWT, PMLTD, PEACWT, and a significant increase in the PDEAD and the FPR which indicated an increase in the resistance status.
Acquired resistance to A. variegatum could also be demonstrated in the pure bred Bos taurus Ayrshire as well as in the crossbred Bos indicus (Boran) x Bos taurus (Friesian) cattle, after four to five successive infestations though some individual variations could also be noticed.
Comparison of the resistance acquired by the Boran bleed to the one
acquired by crossbred Friesian x Boran and Ayrshire revealed that, the crossbred acquired significantly higher immunity than the other two breeds after the first two infestations. Although relatively lower, the resistance acquired by the Ayrshire was not significantly different from that of the Boran.
The profile of gel filtration chromatography of the tick-immune serum obtained from the Boran cattle showed 3 peaks corresponding to IgM, IgGs, and albumins. The IgGs run through affinity chromatography using protein A- Sepharose 4BR, showed two peaks corresponding to the lgG1 and lgG2 fractions. The silver-stained profile revealed the heavy and light chains respectively around 54kDs and 29kDs while the IgM fraction showed three different bands of 77kDs, 69kDs and 29kDs.
SDS-PAGE gels of saliva and salivary gland homogenates, sequentially prepared from the ticks, showed that different molecules were injected into the host during attachment and feeding. Several molecules were recognized by the whole tick-immune serum as well as by the lgG1 and lgG2 obtained after fractionation and purification through gel filtration chromatography and gel affinity chromatography and the immunoblotting procedure.
Quantification of the antibodies implicated in the acquired immunity by indirect and direct ELISA showed that lgG1 was present and correlated with some of the parameters used to assess the acquisition of resistance. A higher host antibody concentration was observed after the sixth infestation when ticks were fed on the haemoprotozoan-infected animals. The IgM concentration remained unchanged during the infestations denoting its passive role in the
mechanism of acquired immunity.
CHAPTER ONE INTRODUCTION
Ticks have been known and recognized as living things since biblical times (Shaw, R.D.; Thorburn, J.A. and Wallace, H.G., 1976). They are widely distributed throughout the world and particularly in the tropical and subtropical countries. It has been estimated that 80% of the world’s 1,226 million cattle are infested with ticks (FAO, 1984). The ixodid ticks or hard ticks with 9 major genera {Amblyomma, Boophilus, Dermacentor, Haemaphysalis, Hyalomma, Ixodes, Margaropus, Nosomma and Rhipicephalus) supersede all other tick genera in economic importance. The majority of hard ticks are of veterinary importance. Among the soft ticks, one of the most veterinary important is Argas persicus parasite of man, bats and birds. Their importance as vectors of livestock debilitating-disease organisms was realized when it became necessary to increase the world livestock products to satisfy the needs of increasing human population living in the industrial areas. Cattle had to be moved into tick-infested areas where many died of tick burden and tick-borne diseases. Besides their importance as vectors of diseases, ticks cause other direct damages to livestock. Morel (1981) documented that a single Amblyomma variegatum tick can cause a loss of 2 ml of blood and a loss of productivity due to the diversion of nutrients and energy ingurgitated by the hosts. Sutherst et al. (1983), also documented that the daily loss by a cow due to an engorged Boophilus microplus female can reach 1,52g of weight. Due
to this double importance a lot of effort has been put into their control. Several methods have been used to fight the tick attacks.
Chemical control has been used since the turn of this century by cattle farmers in tick endemic areas as major means of control. These chemicals are often used in dipping tanks, spray machines (races or arches) and by hand spraying without any rigorous control measures. To this onslaught, the ticks have responded by developing resistance to different groups of chemicals (cross resistance phenomenon) (FAO, 1982; Matthewson, 1984). The cost of acaricide and labour, the increased cost involved in the development, testing and marketing of a new product, the rapidity with which resistance has developed against new acaricides in recent years and finally the chemical hazards in the environment, all create serious difficulties in the long term usage of chemicals to kill or control ticks (Latif, 1986). To avoid the tick resistance problem to whatever acaricide has been formulated alternative methods have been developed: pasture spelling in Australia, habitat modification, genetic control, use of chemical baited pheromones, anti-tick grasses and legumes such as Stytosanthes sp. and parasitoids (Ixodephagus hookeri) have been attempted. Among many of the control measures developed all over the world, the use of tick-resistant cattle has been given a considerable attention in Australia. A selection programme designed to develop a breed resistant to cattle tick Boophilus microplus was initiated in the sixties and seventies. Highly- resistant bulls were selected for production. In Africa, reports indicate that the
local breeds (Bos indicus) acquire a more effective natural resistance to tick infestation than the exotic ones (Bos taurus) (Latif, 1984a; 1984b; Norval, 1986).
An important question which is yet to be answered is whether, on a long term basis, natural resistance can control ticks absolutely. The question has arisen because it is felt that ticks should have been eliminated from Africa if the phenomenon of natural resistance was absolute. There has been no investigation whatsoever to answer this important question. ICIPE’s Livestock Tick Research Programme at Rusinga Island in Kenya (Latif, unpublished data) indicates that the eventual outcome of natural resistance on a long term basis is the stabilization of tick population on the animal and in the field. This in itself suggests a non-absolutism of natural resistance in controlling ticks and a need to study those factors which hinder natural resistance from achieving absolute control of ticks.
This information is important in Africa where indigenous cattle keepers had relied mostly on natural resistance to ticks for several decades, in Africa where multiple parasitic infections in livestock are the normal situation, it is necessary to ascertain whether the stress exerted by infections is a factor to the sustainability of natural resistance against ticks. Indeed, in many tropical countries, animals including domestic livestock are under constant challenge by pathogens, some of which are tick-borne, and by ixodid ticks themselves.
1.1 Justification of study objectives
Remarkable advances have been made within the last decade in research on the feeding habits and pattern of tick infestations as well as their diurnal and seasonal activities. However, little is known about tick embryogenesis, development and survivability under natural conditions, tick population modelling, pheromone and biochemical studies, field sampling techniques and the phenomenon of natural host resistance to tick infestation (Dipeolu, 1989). The justification of these following studies lies in the above statement.
1.2 Study objectives
1.2.1 Aim
The aim of this investigation was to elucidate the effects of infections with parasites such as Trypanosoma and Babesia on the sustainability of natural resistance against tick infestation. Knowing the effects of Trypanosoma congolense, and Babesia bigemina infections a correct timing of their treatment may notably influence the acquisition and the sustainability of natural resistance against ticks. A comprehensive knowledge of this phenomenon may throw some light on the factors which should be considered in cattle vaccination against ticks.
1.2.2 Objectives
To achieve our goals we were to pursue three objectives:
- To monitor the resistance in different breeds of cattle against Amblyomma variegatum after repeated infestations using the biological parameters of Dipeolu (1990 and Dipeolu et al. (1992). (See details of parameters in methodology)
- To establish the correlation between the resistance status and the immunoglobulin at the isotype
- To monitor the resistance dynamics in the congolense and B.
b/gem/na-infected resistant cattle.
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