S.M. Mahan

An assessment of the economic impact of heartwater (Cowdria ruminantium infection) and its control in Zimbabwe.

Heartwater, caused by the rickettsial organism Cowdria ruminantium, is a serious constraint to livestock development in much of sub-Saharan Africa. Traditionally, the disease has been controlled by the use of chemical acaricides to control the vector tick. The University of Florida/USAID-supported heartwater research project (based in Zimbabwe) is developing a new inactivated vaccine to control the disease. In order that the vaccine is used effectively, the project has been studying the epidemiology of the disease in different livestock production systems of Zimbabwe, and evaluating the economic impact of the disease and of its future control using a vaccine such as the one under development. Initially, field studies were conducted to characterise the communal and commercial livestock-productions systems at risk from heartwater and to understand the epidemiology of the disease. The data from these studies were then applied to an infection-dynamics model of heartwater, which was used to provide estimates of disease incidence and impact under various scenarios over a period of 10 yr. Two principal outputs of the epidemiological model (cumulative annual heartwater incidence and infection-fatality proportion) were key inputs into an economics model. The estimated total annual national losses amount to Z

Variations in prevalence rates of tick-borne diseases in Zebu cattle by agroecological zone: implications for East Coast fever immunization

61.3 million (US

DNA vaccination with map1 gene followed by protein boost augments protection against challenge with Cowdria ruminantium, the agent of heartwater

5.6 million) in discounted value terms over 10 yr. Annual economic losses per animal in the commercial production system (Z

Reduction in mortality from heartwater in cattle, sheep and goats exposed to field challenge using an inactivated vaccine

56 discounted values) are 25 times greater than the losses in the communal system (Z

The inactivated Cowdria ruminantium vaccine for heartwater protects against heterologous strains and against laboratory and field tick challenge.

2.2). The greatest component of economic loss is acaricide cost (76%), followed by milk loss (18%) and treatment cost (5%). Losses in outputs other than milk (beef, traction and manure) appear to be minimal. A new vaccine has the promise of a benefit: cost ratio of about 2.4:1 in the communal and 7.6:1 in the commercial system. A control strategy based on a new vaccine would yield additional non-financial benefits to farmers and the government resulting from reductions in the use of chemical acaricides.

Ehrlichia ruminantium infection (heartwater) in wild animals

Abstract East Coast fever (ECF), caused by Theileria parva, is the most important tick-borne disease of cattle in much of eastern, central and southern Africa. In Kenya, immunization against ECF has been performed in many field trials using the infection and treatment method. The main targets for immunization are the Taurine and Taurine-cross cattle, due to their high susceptibility. However, indigenous Zebu cattle may also be targets for immunization where endemic instability to this parasite exists. For both cattle types, it is important to know the prevalence of other tick-borne diseases if the frequency of acaricide application is to be reduced following ECF immunization. In the Coast Province of Kenya there are four distinct agroecological zones (AEZs), classified on agroclimatic factors. The environmental conditions in these AEZs vary in their suitability for Rhipicephalus appendiculatus the main field vector of ECF. The abundance of this tick has a direct impact on the degree of endemic stability to T. parva and is hence the justification for immunization in the predominantly Zebu cattle population of the Province. A serological study was conducted on 316 Zebu calves on 30 farms in three of the four agroecological zones, and prevalence rates to Theileria parva, T. mutans, Babesia bigemina, Anaplasma marginale and Cowdria ruminantium were determined. Antibody prevalence rates to T. parva differed across AEZs (ranging from 22 to 85%) whereas to the other tick-borne diseases they were uniformly high (73–94%). It is concluded that the justification for immunization of Zebu cattle against ECF varies with AEZ, and that any reduction in acaricide usage following immunization is unlikely to be accompanied by an increased incidence of other tick-borne diseases. The sampling methodolohy used in the study was evaluated for its precision, and recommendations for sampling strategy in future studies of tick-borne disease prevalence are made.

Distributions of the vectors of heartwater, Amblyomma hebraeum and amblyomma variegatum (Acari: Ixodidae), in Zimbabwe

A DNA vaccine encoding the immunodominant MAP1 protein of Cowdria ruminantium (Crystal Springs (CS) strain) was shown to partially protect DBA/2 mice against homologous lethal challenge. To enhance the protective capacity of this DNA vaccine, the effects of length of interval between vaccinations and of prime-boost regimes were investigated. Increasing the interval between vaccinations from 2 to 12 weeks did not result in better protection (P=0.900). However, boosting DNA vaccine-primed mice with recombinant MAP1 protein significantly augmented protection on homologous challenge in various trials from 13-27 to 53-67% (P<0.050). The augmented protection by the prime-boost regimen correlated with augmented T(H1) type immune responses that were induced by the DNA vaccine. These responses were characterized by production of IFN-gamma, IL-2 and anti-MAP1 antibodies of predominantly IgG2a isotype, and were critical for protection against C. ruminantium infection. Cytokine analyses were done at 48h after in vitro stimulation of splenocytes with C. ruminantium or control antigens. In contrast, splenocytes of DNA vector control mice produced no cytokines and these mice were fully susceptible to challenge. In addition, DBA/2 mice immunized with the recombinant MAP1 protein without DNA vaccine priming produced non-protective T(H2) type immune responses which were characterized by production of IL-4, IL-5, IL-10 and IgG1 anti-MAP1 antibodies. A second DNA vaccine containing map1 gene from the Mbizi strain of C. ruminantium also delivered by a prime-boost regime, conferred less protection against heterologous challenge. Hence, in developing DNA vaccines against heartwater that contain map1 gene, a prime-boost regimen should be adopted and gene sequence heterogeneity of field isolates should also be considered.

Detection of Cowdria ruminantium by means of a DNA probe, pCS20 in infected bont ticks, Amblyomma hebraeum, the major vector of heartwater in Southern Africa

Inactivated vaccines for heartwater prepared with the commercially acceptable Montanide ISA 50 (ISA 50) adjuvant were field tested in Boer goats in Botswana, Angora goats in South Africa, and Merino sheep in Zambia and Zimbabwe. Two vaccines, one made using the Zimbabwean Mbizi isolate and the other using the respective local field isolate (Sunnyside in Botswana; Bathurst in South Africa; Lutale in Zambia), were tested at each site, except in Zimbabwe where only the Mbizi vaccine was tested. Compared with unvaccinated animals, the Mbizi vaccine significantly protected goats and sheep against field Amblyomma tick challenge in Botswana, Zambia and Zimbabwe (P = 0.018, 0.002 and 0.017, respectively), but failed to protect Angora goats in South Africa. However, in South Africa the vaccine prepared using the local field isolate Bathurst, induced significant protection (P=0.008). The vaccines containing the local isolates at all other sites were less protective than the Mbizi vaccine. The Mbizi inactivated vaccine also significantly protected 17 of 21 cattle (P = 0.05) against heartwater challenge from field ticks in Zimbabwe. Against the same challenge only 7 of 21 unvaccinated control cattle survived. This study demonstrates that heartwater is a major constraint to upgrading livestock in endemic areas, and caused an overall mortality of 77.6% in naive sheep and goats (97 of 125 died) and 67% in cattle (14 of 21 died). In contrast, the vaccine had a protective effect by reducing the overall mortality in sheep and goats to 54.3% (113 of 208 died) and to 19% in cattle (4 of 21 died).

CD8+ T cell knockout mice are less susceptible to Cowdria ruminantium infection than athymic, CD4+ T cell knockout, and normal C57BL/6 mice.

We previously described that an inactivated vaccine against heartwater prepared from Cowdria ruminantium (Crystal Springs strain) organisms and administered in complete Freunds adjuvant (cFA) protected sheep against homologous needle challenge. Further studies, described herein, demonstrated that this vaccine protected 100% of sheep against death on challenge with laboratory-infected ticks and with field ticks in a heartwater endemic farm, whereas a mortality rate of 44% and 62%, respectively, was recorded in the control sheep. Subsequently, the Mbizi strain of C. ruminantium was incorporated into the vaccine because of its wider cross-protective capacity, and trial data suggested that protection may be achieved against challenge with diverse geographical strains using this strain. The efficacy of five adjuvants with acceptable safety was compared with that of cFA. Against a homologous intravenous challenge, highest survival rates were observed in sheep vaccinated with inactivated C. ruminantium in either cFA, Montanide ISA 50 or Quil A. The vaccine prepared in Montanide ISA 50 protected six of seven sheep against natural challenge from field ticks on a farm in Zimbabwe where heartwater is endemic, whereas six of seven control sheep died (P = 0.029). These data support optimization of the vaccine prepared in Montanide ISA 50, followed by evaluation of its efficacy in all target domestic ruminant species and in other geographical regions where heartwater constrains livestock production.

Demonstration of vertical transmission of Cowdria ruminantium, the causative agent of heartwater, from cows to their calves

Several wild animal species have been implicated as hosts of Ehrlichia ruminantium (formerly Cowdria ruminantium), the rickettsial agent causing heartwater, a fatal disease of domestic ruminants in sub-Saharan Africa and eastern Caribbean. However, evidence for infection in most wild species is inconclusive because of inadequate diagnostic techniques. Infection has been proven only in 12 African ruminants, three non-African ruminants and two African rodents. A subclinical carrier state occurs in eight of the African ruminant species. Further studies on E. ruminantium infection in wild animal species are needed in order to determine the host range of this pathogen accurately. The host range of Ehrlichia ruminantium in wildlife is reviewed here and the role played by these species in the epidemiology and spread of heartwater is discussed.