The distribution of heartwater in the highveld of Zimbabwe, 1980-1997

Heartwater, the tick-borne disease caused by the rickettsia Cowdria ruminantium has historically been confined to the southern and western lowvelds of Zimbabwe. Since 1986, however, cases of heartwater have been diagnosed with increasing frequency in the central and eastern regions of the previously heartwater-free highveld plateau. During the same period, collections of the two major tick vectors of heartwater in Zimbabwe, Amblyomma hebraeum and Amblyomma variegatum, were made for the first time in these areas, suggesting that spread of these ticks was responsible for the changed distribution of the disease. The factors associated with this spread have not been determined, but increased cattle and wildlife movement and reduced intensity of dipping undoubtedly play important roles. Currently, the distribution of heartwater and its vectors in the highveld is still largely restricted to the central and eastern regions. The northern regions of the highveld appear to be predominantly uninfected, though it is likely that, eventually, heartwater will spread further with considerable impact on livestock production in Zimbabwe.The articles have been scanned in colour with a HP Scanjet 5590; 600dpi. Adobe Acrobat X Pro was used to OCR the text and also for the merging and conversion to the final presentation PDF-format.United States Agency tor International Development.mn201

Modelling the transmission dynamics of Cowdria ruminantium: Supporting the case for endemic stability

Heartwater, caused by the rickettsial organism Cowdria ruminantium and transmitted by ticks of the genus Amblyomma, is considered to be the most important tick-borne disease of cattle in southern Africa, and is second only in importance to East Coast fever (theileriosis) in eastern Africa. This paper describes the formulation of a transmission dynamic model for C. ruminantium in which the clinical manifestations of infection, namely heartwater disease and death, are the outcomes of interest and are Reported as functions of relative tick challenge. An outline of the model developed and the corresponding mathematical details are presented

Predicting the effect of vaccination on the transmission dynamics of heartwater (Cowdria ruminantium infection)

A mathematical description of the transmission dynamics of the tick-borne infection Cowdria ruminantium in commercial beef enterprises in Zimbabwe was used to consider the potential impact of a candidate vaccine to prevent heartwater. The important characteristics of the vaccine were (1) a delay in development of full protection, (2) prvention of clinical disease but not of infection and (3) a waning period of protection in the absence of challenge. Three different scenarios in which the vaccine might be used were considered: prophylactically in susceptible cattle prior to the introduction of infection into a herd; in susceptible cattle in the face of an epidemic (i.e., when the infection is introduced and disease is first noticed); and at equilibrium (i.e., when parasite, vector and host have been co-existing for some time). The epidemic rise in infection was modelled assuming two different patterns (i.e., resulting from slow and fast increases in tick challenge). Vaccination (administered both in the face of an epidemic and prophylactically) reduced and delayed the peak of the epidemic. With insufficiently frequent revaccination, this can result in the epidemic occurring during a period of susceptibility, so that the benefit derived from a more-efficacious vaccine is lower than that from a less-efficacious vaccine. A vaccine of only 30 percent or 50 percent efficacy (if given to the whole herd) can have important effects on both morbidity and mortality if administered with sufficient frequency. However, a highly efficacious vaccine (e.g., 90 percent) can have only minimal effect if revaccination occurs too infrequently - especially if the epidemic of disease occurs when tick challenge is high and vaccination-related immunity has waned. There was a fairly consistent pattern of decreasing returns on increasing protection, although this was reversed in the situation of annual vaccination undertaken prophylactically combined with an epidemic of infection that occurred when the tick challenge was relatively low. Vaccination in equilibrium situations was most beneficial at low and intermediate tick challenges. There was very little effect of vaccination in high-transmission areas regardless of vaccine efficacy and/or frequency of revaccination because most animals were infected during periods of innate or maternally derived immunity (i.e., under endemic stability). The results suggest that where relatively high tick challenge can be achieved and consistently maintained, vaccination may be used in susceptible herds to minimise losses in a policy of transition to endemic stability

Application of PCR in heartwater epidemiology

Heartwater is a severe tick-borne, non-contagious disease of ruminant livestock caused by the rickettsia Cowdria ruminantium, which is endemic in much of sub-Saharan Africa and in the Caribbean. Transmission occurs via ticks of the Amblyomma genus, primarily A. variegatum and A. hebraeum. Intense research efforts are presently directed towards developing new vaccines for heartwater which will provide safe, effective and less expensive control options to acaricide treatment and the blood-based vaccine currently in use (Mahan et al., 1995; Martinez et al., 1993). Essential to the successful implementation of these vaccines is a thorough understanding of heartwater epidemiology, in particular disease transmission dynamics in host and vector populations. Past studies on heartwater epidemiology have been limited by the difficulty in detecting C. ruminantium infection or exposure. The few antigen detection tests described, brain biopsy, ELISA, and xenodiagnosis with blood or ticks, cannot reliably detect infection past the clinical stage of disease, and are too cumbersome and lengthy for large scale use in field and experimental epidemiologic studies. Similarly, heartwater serological tests currently available lack the sensitivity and specificity required to be useful epidemiologic tools. The recent Development of a polymerase chain reaction (PCR) C. ruminantium detection assay of high sensitivity and specificity has permitted more rigorous analysis of infection dynamics (Mahan et al., 1992; Peter et al., 1995). The PCR assay can be applied quickly and inexpensively to large numbers of samples. However, while PCR performs well on infected ticks, it is not yet sensitive enough for single test detection of chronically infected ruminants, due to the very low levels of circulating rickesttsemia in such animals. Nevertheless, analysis of tick infections has allowed reasonably detailed investigation of transmission dynamics through the quantification of key variables such as field tick infection rate and the relative importance of adult and nymphal tick transmission. In addition, the importance of hosts as sources of infection at different stages of infection can be studied indirectly by anlaysis of infections established in ticks that feed on them. These analyses can provide valuable estimates of parameters required for mathematical models of heartwater transmission dynamics that evaluate the impact of current and alternate disease control strategies (O'Callaghahan et al., 1997)

Biological support for modeling the transmission dynamics of cowdriosis

Heartwater is a disease of high mortality in domestic ruminants (cattle, sheep and goats) in sub-Saharan Africa and in the Caribbean. An intracellular rickettsia, Cowdria ruminantium, is the cause of the disease and is transmitted by ticks of the genus Amblyomma. Losses due to heartwater are high and control has historically been by treatment of livestock with acaricides. More reliable, sustainable and cost-effective methods of control, in particular vaccines, have been the focus of intense research efforts in recent years. Prior to their use, the impact of new vaccines on transmission dynamics under different disease settings and on the economics of livestock production needs to be assessed and compared with current control strategies. To do this without lengthy and expensive field trials, transmission dynamic models have been developed and used to make quantitative predictions of disease impact under different control scenarios. The models attempt to capture the results of the biological processes underlying transmission and hence require reasonable estimates of the magnitudes of these parameters. When initially constructed, these models obtained data for parameter estimation from previously published literature. Subsequently, it was recognised that certain estimates needed to be improved through prospective studies. Initial model output also identified certain parameters as key to model performance and for which improved estimates were essential. We thus investigated four aspects of the transmission biology of C. ruminantium to provide new or improved information for the models. These included (i) the rate of decline in infectivity of hosts moving from the clinical to the carrier stage of infection; (ii) the relative importance of adult and nymph vector tick instars in transmission; (iii) the prevalence of infection in free-living vector instars; and (iv) the rate of attachment of vector ticks to hosts. These areas were investigated in laboratory and field studies

Predicting the effect of vaccination of the transmission dynamics of heartwater (Cowdria ruminantium infection)

Heartwater, an infectious disease of ruminants caused by the rickettsia Cowdria ruminantium (transmitted by ixodid ticks of the genus Amblyomma and vertically), severely constrains livestock production throughout sub-Saharan Africa. Its epidemiology varies from epidemics with high mortality (on introduction into a previously unexposed population) through to endemic stability (very low morbidity when the vector density is high and host infection is common and occurs early in life). A new generation of inactivated vaccines for heartwater is being developed and there is an urgent need to investigate the impact of their use on transmission dynamics and livestock production economics as an aid in the design of cost-effective, integrated, control programmes. Without conducting long-term, expensive field trials, the only way of exploring these aspects is through predictive, quantitative models. A mathematical description of the trransmission dynamics of the tick-borne infection Cowdria ruminantium in commercial beef enterprises in Zimbabwe was used to consider the potential epidemiological impact of a candidate vaccine to prevent heartwater with the quantitative results employed in an economic analysis