Presently used control methods, consisting of antibiotic treatment, vaccination and arthropod control, have not significantly changed for many years and depend on the geographical area, availability, cost and feasibility of application (Kocan et al., 2000).
1.8.1. Antibiotics
Three types of antibiotics are used for the treatment of anaplasmosis: tetracyclines,
fluoroquinolones and imidocarb dipropionate (Aubry and Geale, 2011).
Chemotherapeutic treatment is effective in decreasing bacterial numbers, but the efficacy in clearing infection and thus preventing the establishment of a pathogen reservoir is variable (Coetzee et al., 2005; Reinbold et al., 2010; Wallace et al., 2007). Recently, treatment of persistently infected steers with oral chlortetracycline for 80 days cleared A. marginale infections (Reinbold et al., 2010). Nevertheless, treatment of clinically affected animals is expensive. Moreover, it is becoming less acceptable, as antibiotic resistance rises in pathogens. In addition, some countries restricted the use of imidocarb due to its prolonged retention in the edible tissues of animals for slaughter (Kocan et al., 2010b).
1.8.2. Arthropod control
Treatment of animals with acaricides reduces the number of ticks, thus indirectly decreasing anaplasmosis transmission. However, the use of acaricides for vector
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control is becoming a concern due to increasing acaricide-resistance among tick populations (George et al., 2004), the pollution of the environment and the contamination of milk and meat products (Graf et al., 2004).
The modern approach for arthropod control is based on the use of anti-tick vaccines,
which have the benefits of being cost-effective, reducing environmental
contamination and preventing the development of acaricide-resistant ticks. Two vaccines Gavac (Vargas et al., 2010) and TickGARD (Odongo et al., 2007) containing recombinant R. (B.) microplus gut antigens Bm86 and Bm95 are currently being used.
Another promising antigenic protein involved in the modulation of tick feeding and reproduction, subolesin, has been tested (de la Fuente et al., 2005a). Preliminary experiments have shown, that the number of ticks infected with Anaplasma spp. were reduced when ticks were injected with subolesin double- stranded RNA before being fed on cattle with ascending rickettsemia (de la Fuente et al., 2006). Furthermore, cattle immunized with recombinant subolesin were protected against R.
(B.) microplus infestations due to a decrease in tick survival and reproduction rates
(Almazan et al., 2003; Merino et al., 2011).
1.8.3. Vaccination
A long-lasting immunity induced by vaccination is an economical and effective way to prevent and control bovine anaplasmosis. Mass vaccination programs can significantly reduce the use of acaricides and antibiotics thus preventing an emergence of resistant ticks or pathogens. At present, two types of vaccines are used and are the vaccines of choice: live vaccines and inactivated formulations. They induce protection from severe clinical symptoms, but do not prevent infection, so that cattle after infection may remain carriers of A. marginale (Kocan et al., 2003).
1.8.3.1. Live vaccines
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isolation of A. centrale (Theiler, 1911). They consist of less pathogenic A. centrale or attenuated strains of A. marginale.
The immune response induced by such vaccines is similar to a natural infection and
animals develop persistent infections with the vaccine strain. However,
preimmunization with one strain has been shown not to provide cross-protection in widely separated geographic areas (Kenneil et al., 2013; Kuttler et al., 1984). Live vaccines consist of infected blood, taken from splenectomized, quarantined calves inoculated with the selected vaccine strain. These vaccines carry the risk of
transmitting other “silent” pathogens and despite the global impact of anaplasmosis,
their use is forbidden in the US (Rogers et al., 1988).
Immunization of cattle with less pathogenic subspecies i.e. A. centrale is in routine use in several countries: South Africa, Zimbabwe, Malawi, Australia, Israel, Uruguay and Argentina (Shkap et al., 2009).
It is noteworthy that some African and Latin American isolates of A. marginale can overcome an A. centrale induced immunity (Bock et al., 2003; Brizuela et al., 1998).
The second type of live vaccine consists of attenuated A. marginale strains. These vaccines were used in South America and California, although severe reactions have been observed in adult cattle after vaccination (Henry et al., 1983). In calves, however, these vaccines produce mild infections and lead to immunity against clinical anaplasmosis, although not in widely separated geographic areas (Kocan et al., 2003). The attenuation of A. marginale can be achieved by two methods. The first involves irradiation and subsequent multiple passages through deer and sheep (Ristic and Carson, 1977). The second consists of numerous passages through splenectomized calves followed by passages through splenectomized sheep (Jorgensen et al., 1993). Yet, it has been reported that attenuated A. marginale vaccines reverted to virulence after successive passages through cattle or ticks (Kocan et al., 2000).
1.8.3.2. Inactivated vaccines
An inactivated vaccine comprising o f non-living A. marginale was developed in the United States and was used effectively till its withdrawal from the market in 1999
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(Kocan et al., 2003). Although inactivated vaccines are also produced in splenectomized animals, it is less likely that any other pathogens contaminating the vaccine will remain viable and infectious after the inactivation process. However, extensive purification is required to remove bovine cell stroma as only partial purification resulted in the development of erythrocytic isoantibodies in vaccinated cattle. The inactivated vaccines reduced clinical disease and mortality, yet did not always provide cross-protection (Kuttler and Winward, 1984). For this reason inactivated vaccines are most likely to be useful when produced from locally isolated strains.
1.8.3.3. Culture-derived vaccines
Bacteria grown in tick cell cultures are being investigated as an alternative source of
A. marginale for live vaccine production. This technique has the advantage of allowing
the inclusion of multiple strains, ease of standardization, freedom from bovine red blood cells and pathogens and does not require the use of expensive, splenectomized calves (Kocan et al., 2003). Cattle immunized with a cell culture- derived A. marginale strains develop protective immunity and do not develop clinical signs of anaplasmosis after challenge. However, as with most anaplasmosis vaccines, infection with the challenge strain is not prevented (Bastos et al., 2010; de la Fuente et al., 2002b; Kocan et al., 2001).