T.F. Peter

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

Responses of the ticksAmblyomma hebraeum andA. variegatum to known or potential components of the aggregation-attachment pheromone. I. Long-range attraction

61.3 million (US

Investigating the epidemiology of heartwater ( Cowdria ruminantium infection) by means of a transmission dynamics model

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

COMPARISON OF EFFICACY OF AMERICAN AND AFRICAN AMBLYOMMA TICKS AS VECTORS OF HEARTWATER (COWDRIA RUMINANTIUM) INFECTION BY MOLECULAR ANALYSES AND TRANSMISSION TRIALS

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

Olfactory responses of adult Amblyomma hebraeum and A. variegatum (Acari : Ixodidae) to attractant chemicals in laboratory tests

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

Ten known or potential components of the aggregation-attachment pheromone (AAP) of the ticksAmblyomma hebraeum andA. variegatum, as well as mixtures of these components, extracts of prefed males and live prefed males, were tested as aggregation stimulants. In field assays, laboratory-reared unfed male and female ticks were released 20 cm downwind of CO2/pheromone release sites; the numbers of ticks that aggregated at the release sites were recorded after 30 min. InA. variegatum, aggregation was induced by methyl salicylate,o-nitrophenol, 2,6-dichlorophenol, phenylacetaldehyde and some mixtures containing these compounds; a strong aggregation response was induced by an extract of five prefed malesA. variegatum and a weak response was induced by an extract of 50 prefed males ofA. hebraeum. InA. hebraeum, aggregation was induced by phenylacetaldehyde, mixtures of compounds that included phenylacetaldehyde, extracts of 50 prefed males ofA. hebraeum orA. variegatum and 50 live prefed males ofA. hebraeum. InA. variegatum, aggregation was inhibited if compounds that do not occur naturally in the AAP of the species were included in mixtures. InA. hebraeum, phenylacetaldehyde appeared to act as an arrestant for ticks that had been attracted to release sites by other compounds.

Demonstration of a carrier state for Cowdria ruminantium in wild ruminants from Africa.

A mathematical model of the transmission dynamics of Cowdria ruminantium by the ixodid tick Amblyomma hebraeum in the bovine host is developed and used to investigate the epidemiology of heartwater across a range of vector challenge. The processes described are supported by empirical data. The pattern of outcome measures (incidence, case-fatality and proportion of infected hosts) predicted agrees with those described anecdotally from field experience and empirical observation, and demonstrates the concept of endemic stability. The underlying theory is explored and it is shown that endemic stability may be due principally to the protection of calves against disease by either innate or maternally derived factors. The role of vertical infection in the establishment and maintenance of endemic stability is also investigated. Although increasing the vertical infection proportion results in endemic stability occurring at progressively lower levels of tick challenge, the concomitant reduction in incidence and case-fatality predictions across the range of tick challenge means the endemically stable state simultaneously becomes less discernible. Model limitations and future developments are discussed. The essential role of a transmission dynamics model in assessing the impact of new vaccines in conjunction with vector control programmes is highlighted.

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

The ability of Amblyomma americanum, Amblyomma cajennense, Amblyomma maculatum, and Amblyomma variegatum to acquire and transmit Cowdria ruminantium infection was investigated. Uninfected nymphs were fed on clinically reacting C. ruminantium-infected sheep and then analyzed for infection by specific DNA detection assays and by tick transmission trials. By polymerase chain reaction (PCR), the mean infection prevalence of A. maculatum ticks (50.7%) was similar to that of A. variegatum, Elevage strain (43.5%; P = 0.83) and Petit Bourg strain (45.9%; P = 0.26) ticks. Though Amblyomma hebraeum were not tested by PCR, by DNA probe their infection prevalence was 94%. In contrast, A. americanum and A. cajennense ticks demonstrated very low susceptibility to C. ruminantium, and the prevalence of infection by PCR was approximately 1%. The higher susceptibility of A. maculatum and A. variegatum to C. ruminantium correlated with superior vector efficiency, depicted by similar prepatent periods and severity of disease transmissions to sheep. Amblyomma americanum and A. cajennense failed to transmit infection, confirming that low susceptibility to C. ruminantium correlates with the poor vector status of these species. These results highlight the importance of A. maculatum as a potential vector that is likely to play a major role in the establishment and maintenance of heartwater, if the disease were to be introduced to the U.S.A., Central, and South America.

Competence of the African tortoise tick, Amblyomma marmoreum (Acari: Ixodidae), as a vector of the agent of heartwater (Cowdria ruminantium).

Unfed adults of the African ticks,Amblyomma hebraeum Koch andA. variegatum (Fabricius), important vectors of human and animal diseases, were exposed to volatile compounds in an olfactometer in efforts to identify both tick-produced or synthetic chemicals capable of eliciting an attraction response. A formula, relative efficacy of attraction, was devised for comparison of responses between species and sexes to a particular test stimulus, or within a homogeneous population to different stimuli. Adults of both species responded strongly to known tick-pheromone constituents, nonanoic acid, methyl salicylate, 2.6-dichlorophenol and benzyl alcohol, as well as to a commercially produced antiseptic, TCP (Pfizer), and its major components, chlorinated and iodinated phenols. Benzaldehyde, a proposed tick-pheromone component, and heptadecane, not known from ticks, were markedly attractive to adults ofA. hebraeum but not to those ofA. variegatum. Males of the former species, but neither conspecific females not either sex of the latter species, responded significantly to salicylaldehyde (known from males of four species of ticks, includingA. variegatum).o-nitrophenol, a major component of the aggregation-attachment pheromone of males of bothA. variegatum andA hebraeum and a proven long-range attractant for them in the field, was only partially attractive to either species in the olfactometer. Neither species was attracted to 2-methylpropanoic acid, previously identified in volatile effluents form feeding maleA. hebraeum. It is concluded that these important disease vectors respond positively to a variety of volatile chemicals, which may conceivably be used to attract them to traps, animals or acaricides in efforts to control ticks or the diseases they transmit.

Pheromone/acaricide mixtures in the control of the tickAmblyomma hebraeum: Effects of acaricides on attraction and attachment

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.