CHAPTER ONE
1.0.0 INTRODUCTION
1.1.0 BACKGROUND
Livestock diseases transmitted by arthropod vectors impact a severe disease burden on small farmers in tropical countries where these diseases are endemic. In West Africa, the tick- borne diseases heartwater, babesiosis and anaplasmosis, together with the tick-associated skin disease dermatophilosis and tsetse-transmitted trypanosomiasis, have repeatedly frustrated attempts to increase animal productivity by genetic improvement of the indigenous cattle breeds through cross-breeding with exotic highly productive cattle (Aning, 1982; Uilenberg Gerrit, 1995; Bell-Sakyi et al., 1996; de Castro, 1997; Walker and Koney, 1999; Bell-Sakyi et al., 2004).
This disease burden has a disproportionate impact on resource-poor farmers due to the high cost of prevention using acaricides/insecticides and treatment of sick animals (De Castro, 1997; Minjauw and McLeod, 2003). However, the greatest impact results from the severity of disease among higher producing dairy and dual-purpose breeds and crosses between these higher productivity breeds and indigenous cattle breeds with consequent loss of capacity to improve productivity (Simuunza et al., 2011). The world cost of Tick–borne diseases, in relation to death of animals, losses in production, treatment and control of ticks are projected to be between $13.9 billion and $18.7 billion yearly.
Importantly, effective control of tick-borne diseases in endemic regions requires control of a complex of pathogens rather than a single pathogen-disease entity. A separate vaccine targeting each pathogen is not feasible for a variety of reasons. Therefore, the most effective means of control is to use one intervention to target multiple pathogens (Frisch et al., 2000; Graf et al., 2004; Estrada – Peña and Salman, 2013; de la Fuente et al., 2017). Multiple
pathogens infection is commonplace in most tropical regions. Here, the pathogen strains and the associated biological vector are widely prevalent. This condition allows for continuous pathogen challenge by intermittent feeding behaviour of tick species resulting in two or more pathogens circulating in individual cattle. However, not all cattle became infected with multiple pathogens. Notably the indigenous cattle have demonstrated high level of resistance to clinical disease without the need for treatment (Nadelman et al., 1997; Alekseev et al. 2001; Bock et al., 2004; Nyarko et al., 2006) and this could be due to the fact that, the innate immune responses of these calves have evolved to provide some sort of protection for the local animals. The exotic and cross breeds on the other hand appear to lack this innate capability and are therefore unable to withstand infectious diseases challenge. To prevent high mortality rates associated with the production of the crossbred in the endemic tropical requires intensive use of drugs and acaricides at a high cost to the farmer. This genetic difference between the indigenous and crossbred cattle raises a key question whether the innate immune responses of the exotic and cross breeds can be induced by stimulation to allow these breeds survive mortality and also withstand multiple pathogen infections.
One potential means to achieve this goal is through stimulation of the innate immune system using synthetic version of TLR7 and 8. It is predicted that when calves were stimulated at young age will reduce or prevent disease while allowing for infection with the pathogen and the subsequent development of long-term adaptive immunity required for lifetime protection against death.
Innate immunity is responsible for protection against severe disease upon hemoparasitic infection in indigenous cattle breeds (Aguilar–Delfin et al., 2001; Beutler et al., 2006; Ahmed et al., 2008; Bannerman et al., 2008a; Bannerman et al., 2008b). Babesia bovis is illustrative of this effect: in Friesian cattle, the parasite causes a high fever, severe anemia, neurologic disease (resembling cerebral malaria), and death within 10 days of infection, while
indigenous breeds show only mild disease. The time course (<10 days) points to innate immunity as the critical difference (Bock et al., 1997a; Bock et al; 1999a; Bock et al; 1999b; Goff et al., 2003; Bock et al; 2004; Brown et al., 2006; Bannerman et al., 2008a; Bannerman et al., 2008b; Carvalho et al., 2008;). This is supported by both laboratory studies showing induction of the innate immune response early in infection and, most importantly, by studies in which pre-treatment with mycobacterial-based adjuvants induce protection against severe disease (Aguilar–Delfin et al., 2001; Glass et al., 2005; Andersen et al., 2009). This innate immunity is reduced but not completely lost in exotic breeds, and that it can be induced is evident from studies with Babesia bovis (Goff et al., 2003). Cattle that survive the acute clinical disease phase remain persistently but asymptomatically infected and resistant to subsequent challenge with homologous or heterologous strains. The establishment of persistent infection actually functions as a live vaccine similar to that used to protect cattle in Israel, Australia and South Africa (Brown et al., 1999; Shkap et al., 2007). This pattern of asymptomatic persistent infection and protection holds for the other tick-borne pathogens (Peter et al., 1998; Barbour and Restrepo, 2000; Palmer et al., 2000).
Pathogens stimulate the innate immune response through Toll-like receptors that serve as sensors for pathogen molecules, including glycoproteins and carbohydrates, and therefore represent key initiators of innate immunity (Hajjar et al., 2002; Ibeagha et al., 2008; Ishii et al., 2008; Uematsu and Akira, 2008; Kumar et al., 2009). There is evidence that innate receptors can be stimulated by direct interaction with the agonists in humans, mice, and cattle. The strategy in the present study was to target the bovine TLR-7 and TLR-8 by activation with the cognate agonists. TLR7 and TLR8 typically recognize pathogen RNA and synthetic agonists which are small amines (Diebold et al., 2004; Kawai and Akira, 2007; Severa and Fitzgerald, 2007; Miller et al., 2008) and activation with agonists induces secretion of both anti-microbial effectors and inflammatory cytokines (Severa and Fitzgerald,
2007; Miller et al., 2008). There is strong precedent for innate immunity stimulation that resulted in significant protection as compared to untreated cohorts based on studies using mycobacterial extracts in Bos indicus x Bos taurus calves (Tewari et al., 1996). However, whether protection can be induced by synthetic TLR agonists in the Friesian x Sanga F1 calves has not been tested but requires empirical data to resolve. One of the key questions regarding the effectiveness of our approach is whether injection with the synthetic TLR 7/8 agonist emulsion can stimulate the innate immune response in young crossbred calves. To answer this question, we evaluated the temperature, swelling of the draining pre-scapular lymph node, and secreted cytokines. Any significant shifts in these biologic indicators in response to immune stimulation are easy to measure on the field. Success with this approach will provide a low-cost disease control strategy that would be globally scalable for improvement of livestock productivity in tropical and subtropical regions.
1.2.0 PROBLEM STATEMENT
Exotic breeds of cattle and their crosses (Friesian X Sanga) are known to grow faster and also produce more milk and meat than the low productivity indigenous cohorts. In the endemic region, farmers therefore desire the rearing of these high productivity crossbred cattle and exotic breeds but are frustrated by their high susceptibility to diseases. Interestingly, the indigenous cattle co-habiting the same endemic environment, on the other hand, have demonstrated unique capability to withstand severe diseases with no need for treatment. Vaccination remains the most effective and dependable approach to tick-borne disease control. However, this classical approach to control of tick–borne diseases of cattle by vaccination is essentially futile due primarily to occurrence of multiple tick–borne pathogens circulating in cattle in the endemic tropical region. It appears that, in the absence of vaccines,
indigenous animals are dependent on their innate immune response to survive. Innate immunity is responsible for protecting the indigenous but low productivity breeds against severe disease. As innate immunity is reduced but not completely lost in higher productivity crossbred cattle, this raises the question whether this perceived dormant innate immune response of the crossbred cattle can be triggered by injecting young crossbred calves with synthetic Toll-like receptor agonist emulsified in an adjuvant. If successful, then long-term activation of the innate cellular receptors in young calves will mitigate disease severity and restore full disease resistance in crossbred animals and thus provide the opportunity to improve livestock productivity to ensure food and economic security.
1.3.0 JUSTIFICATION OF THE STUDY
Cattle are sources of protein thus, making them economically significant but diseases-related mortality represents one of the major problems to profitable cattle production, food and economic security in Ghana (Saunsoucy, 1995; De Castro, 1997; Kaewthamasorn and Wongsamee, 2006, Rajput et al., 2006).
Throughout the world, tick –borne diseases have caused significant losses to cattle production in excess of $18.7 billion yearly. In Ghana, it is projected that 40% of livestock raised by farmers die of tick-borne diseases annually (Aryee et al., 1991; Bell – Sakyi et al., 2004; Jonsson et al., 2008; Estrada – Peña and Salman, 2013).
Exotic breeds are the most affected thereby making their production unsuccessful in resource- poor countries (Khan et al., 2004; Jongejan and Uilenberg, 2004). Currently, there is no effective vaccine for tick-borne infections and the near-term prospects for developing and deploying vaccines is not encouraging due to budgetary constraints. Disease control by way
of innate immune stimulation may provide the best alternative strategy for controlling animal pathogens that co-circulate in animals resident in the endemic tropical region. That this approach is applicable is supported by the evidence that TLR7/8 agonist does not target specific pathogen entity upon stimulation (Futse, unpublished). Subsequent encounter of the recipient cattle with challenges on the field from different pathogen strains will result in the development of pathogen-specific adaptive immune response. If this process is successful, it could be applied to the control of infections in other animals including small ruminants to reduce mortality and improve income of farmers. It could also provide research institutes/communities with substantial information on the normal temperature of cross bred (Friesian x Sanga) (F1) calves in West Africa and Ghana. It will be a novel discovery for researchers as the normal rectal temperature of Friesian x Sanga calves is not known. The present study therefore represents the first study establishing the baseline data of the rectal temperature of Friesian x Sanga (F1) calves.
Understanding how innate immune response can be induced and effectively stimulated to help host animals combat disease infections will provide evidence-based information critically needed by researchers to develop effective control against diseases of exotic and cross bred cattle in the endemic regions. Also, it will provide specific pathway to producing single vaccine to target multiple pathogens. The overall success of this approach will drive farmers’ interests to procure additional cross breed cattle to improve production of milk and meat.
1.4.0 HYPOTHESIS
Immunization with single dose of synthetic TLR7/8 agonist-fortified emulsions will trigger the innate immune response of the Friesian x Sanga F1 calves.
1.5.0 MAIN OBJECTIVES
The goal of this study is to determine whether the innate immune response of crossbred calves can be stimulated by injection with synthetic toll – like receptor7/8 agonist.
SPECIFIC OBJECTIVES
The specific objectives of the study are to determine:
- Whether crossbred calves (Friesian X Sanga) injected with TLR7/8 agonist will show significant variations in some measurable biomarkers for innate immune response in cattle.
These biomarkers include:
- Elevation of rectal temperature – fever.
- Increase in width of pre – scapular lymph node.
- Whether injection-induced secretion of plasma proteins, associated with innate immune response, will increase in calves stimulated with TLR7/8 agonist immunization. The following cytokine readouts were analysed in all calves:
- Tumor Necrosis Factor – α (TNF – α)
EFFECT OF TOLL–LIKE RECEPTOR7/8 AGONIST IMMUNISATION OF YOUNG CROSSBRED CATTLE ON INNATE IMMUNE STIMULATION AGAINST TICK– BORNE DISEASES
