The practice of tick control by hand removal of ticks from their predilection sites on animals has remained the tick control method of choice for most local farmers even though it is labour-intensive, cannot be 100% efficient and is considered in the local context to permit disease transmission .This implies that the livestock owners allow ticks to infest their animals before they follow up to remove them. Thus, the time lapse between tick fixation and its removal by the herdsman is enough for the ticks to inoculate pathogen. However, this depends on two factors: the periodicity of tick removal and the pathogen species that eventually get inoculated. For example, if it is a Babesia or a Theileria, one needs three days approximately to get the pathogen transmitted. On the other hand, various forms of acaricides are available for the effective control of ticks in Cameroon but these drugs are well out of the reach of most small-scale livestock breeders mainly because of their high cost and irregularity in supply . Other constraints include the supply of fake and dangerous products by charlatans and the lack of technical knowhow in the appropriate use of the products .
The naive calves in this study quickly acquired three pathogens (B.bigemina, A. sp. Omatjenne and A. marginale) at a very early point in life (within the first two months (3-7weeks) while earliest contact with T. mutans pathogens was much longer (nine weeks). The age at first contact with the pathogens then increased gradually from five months onwards, recording the longest periods as 43–55 weeks in a few animals for B. bigemina and A. sp. Omatjenne and 55 weeks for A.marginale and T.mutans. This observation probably depicts the similarity in animal contact with vectors of the pathogens, the homogeneity in both the animal production system and ecosystem used for the study. The trends may also be associated with the less attractiveness of some calves to tick vectors in their early life and/or inherited maternal resistance to ticks that may be associated with breed difference to tick infestation and or transfer of maternal immunity through colostrum. It has been previously established in the same ecosystem that A.variegatum ticks infest lactating animals and older calves to a higher degree than non-lactating and younger calves . The established immunity may wane in old animals and disease state resurgence occurring in stressed animals could cause mortalities.
In the present study, the detection rate of A.marginale pathogen (in 15 out of 20 animals) was lower than that of B.bigemina and Ehrlichia spp. (A. sp. Omatjenne) both of occurred in 20 out of 20 animals exposed. This finding coroborates with that of Chollet  who reported a seroprevalence of antibodies to Anaplasma of between 40 and 76% in a cross-sectional study in the same region in which our study was conducted. In these highly endemic areas it would be logical to expect the development of a “pre-immune state” that is due to constant exposure of the animals to the pathogen causing clinical cases to occur mainly in extremely stressed animals. A common stress that occurs in the area is the dry season, characterized by fodder and water scarcity and frequent high cattle mortalities even for the local zebu (Bos indicus) breeds. The detection of T. mutans pathogens in four out of 20 animals in the present longitudinal study had not been previously reported in the area. Given our small sample size, the presence of other pathogenic strains in the region cannot be excluded and ought to be the subject of future investigations.
Despite the high density of A. variegatum ticks and their larvae that infested the animals, no clinical case of cowdriosis E.ruminantium- also commonly called heartwater disease) infection was recorded during the first 18 months of life of the animals. Heartwater disease has been previously diagnosed between 1988 and 1994 (unpublished data) using crushed brain smears stained with Giemsa in adult Gudali cattle and in Gudali X Holstein cross yearlings in the same experimental farm. The observation on Ehrlichia sp. detection rate is in line with the findings of Merlin et al.  who used microscopic examination of crushed brain tissue stained with Giemsa in the adjacent highland plateau of the north Western Cameroon and concluded that the rate of E.ruminantium infection was very low in village herds although A. variegatum infestations were very high. That all animals (20) under two years of age were infected with Ehrlichia sp. (A. sp. Omatjenne) indicates the endemicity of this pathogen in the region and one is poised to believe that such early infections without clinical disease occurrence may confer cross protecting immunity in the local animals against the other Ehrlichia spp. including E. ruminantium. Interestingly, a recent molecular screening of over 1500 adult cattle  in the three northern regions of Cameroon (including Adamawa where our study was undertaken) detected only one case of E.ruminantiumin Faro et Deo division of the Adamawa region. In other studies in Cameroon E. ruminantium DNA was detected in 142 (28.4%) of 500 un-engorged A. variegatum ticks collected from 182 cattle at, Société de Développement et d’Exploitation des Productions Animales (SODEPA) Dumbo ranch (SDR) in the North West Region and Upper Farms ranch (UFR) in the South West Region. A higher infection rate of E. ruminantium (40.9%) was observed in ticks from SDR than in the ticks (24.7%) collected from cattle at UFR . A serological study reported a high (61–67%) seroprevalence of E.ruminantium and demonstrated the association of heartwater with the presence of A.variegatumin the north region of Cameroon . In Benin, a study undertaken in 4 localities  showed that on the overall 10.8% of testedA. variegatum were infected by E. ruminantium (ranging: 8.9–15.6%), while in Burkina Faso the infection rate of ticks by this pathogen was higher, ranging from 9 to 20% .Taken together, these previous studies confirm the endemicity of E. ruminantiumin West and Central Africa in the tick vector and indicates that the tick also serves as the reservoir since the pathogen is maintained trans-stadially. On the contrary our study on Ehrlichia spp.corroborates with those of Vanegas et al.  who collected ticks at least five years later after our study from the same High Guinea Savannah zone of Cameroon from which they sequenced the ompB gene and three intergenic spacers (dksA-xerC, mppA-purC and rpmE-tRNAfMet) and detected Rickettsia africae in A. variegatum ticks, R. aeschlimannii in H. truncatum and H. rufipes ticks, R. massiliae in R. lunulatus ticks, R. sibrica in H. truncatum ticks and Candidatus R. barbariae in R. lunulatus ticks. That E. ruminantium was not found in our samples and those of Vanegas et al. and only one case reported recently in the same ecosystem , should be a matter of great concern for epizootiologists. Put together, this data disagrees with the general perception of the local animal breeders that heartwater caused by E. ruminantiumis one of the greatest killer of indigenous young and adult cattle in the High Guinea Savannah zone of Cameroon . This perception of the local cattle breeders may be unfounded. Observed mortalities may be due to other pathogens. It has been indicated that Ehrlichia spp. are maintained in nature through subclinical infections of ruminants (carriers) as well as ticks and have evolved mechanisms to persistently infect mammalian hosts by subverting the innate and adaptive immune responses . In the present study we found for first time in central Africa that the Ehrlichia sp. infecting calves and yearling cattle was A.sp. Omatjenne previously reported in goats in Mozambique as Ehrlichia sp. Bom Pastor  and later renamed E. sp. (Omatjenne). Ehrlichia sp. (Omatjenne), which is apparently apathogenic has also been detected in several ruminants in South Africa including Boer goats , in Uganda  and in Ethiopia [29, 30].The observed epizootiological picture with respect to Ehrlichia spp [E. sp. (Omatjenne)] and E.ruminantium from all the cited studies and ours suggests that other rickettsia or pathogens may be causing pathological or clinical signs similar to those of heartwater in the region since both show crossreactivity . Also, such immune cross-reactivity under natural conditions hinder, through cross-protection the expression of pathology by those otherwise pathogenic Ehrlichia spp. concurrently infecting the animals. Therefore, there is need for extensive genetic characterization, transmission, clinical and pathogenicity studies of Ehrlichia spp. from the major livestock raising regions of Cameroon and neighbouring countries to generate more data that could clarify the situation. Although A. sp. Omatjenne which we found in our study is apparently apathogenic [28, 31] in ruminants, a strain (E. sp. (Omatjenne) 1) of this parasite has been used experimentally to produce disease indistinguishable from cowdriosis in sheep  thereby not ruling out its pathogenicity in natural conditions. Anaplasma sp. Omatjenne may contribute to seropositivity to E. ruminantium with which it greatly cross reacts . More so, Anaplasma and Ehrlichia spp. are reported to cause significant economic losses to the livestock sector in Uganda .
Babesia bigemina was detected in all exposed animals implying that it is endemic in the study area. Semi-nested polymerase chain reaction (PCR) was undertaken with the same primers described by Devos & Geysen  based on the amplification of the 18S rRNAgene for the detection of Babesia spp. Thus Babesia species were perfectly differentiated based on their RFLP profiles. This technique has been described as being more sensitive than microscopic and serologic examinations [32, 33]. It has also been reported that in B.bigemina infection, calves from non-immune cows are as sensitive as adult animals [34, 35].
In the present study, calves that had their first contact with the pathogen within the first 18 months of life did not reveal any clinical disease condition during their first 18 months of life. This corroborates with findings from other studies which reported that cattle between 3 and 9 months of age have higher innate resistance to most tick-borne diseases and consequently disease incidence and corresponding mortality are typically lower for this stock class .
It has been reported that amongst many other farm management practices, the reduction of the tick vector population by use of acaricides or pasture management , increasing aridity  and intensive and prolonged tick control may be associated with breakdown in endemic instability to tick-borne diseases, which generally establishes in indigenous cattle following continuous contact with tick-borne pathogens [39, 40] A situation of endemic stability has been reported more than two decades ago in an A. variegatum-infested ecosystem inhabited by indigenous cattle . Maintaining the equilibrium state or balance between host, vector and pathogen is therefore crucial in ensuring endemic stability and thus avoiding economic loses.