POSSIBLE CHANGES IN HEARTWATER DEMOGRAPHICS AND CONTRIBUTING FACTORS.

Heartwater is caused by the proteobacterium Ehrlichia ruminantium, which is transmitted by the tick Amblyomma hebraeum. It is one of the most economically important tick-borne diseases of livestock in South Africa.  It is endemic to the north-eastern parts of the country, from the north-east of the North West province, through Limpopo and the north-eastern parts of Mpumalanga, along the coastal belt of KwaZulu-Natal and the Eastern Cape. It has been estimated that up to 35%, 54% and 12% of total cattle, goat and sheep populations, respectively, are at risk of infection. Control is difficult as the methods presently used to control heartwater are only partially effective. For example, there is no safe vaccine available. Use of prophylactic antibiotics is costly and may contribute to resistance in the organism. The same could be said about the use of acaricides to control the tick. There is anecdotal evidence from veterinarians, indicating that heartwater distribution is changing, but this has not been formally investigated before. Therefore, the study of the researchers cited below aimed to provide an indication of whether or not there has been a change in the distribution and incidence of heartwater and possible reasons for the change.

A survey was conducted to scrutinise these factors using both veterinarians and farmers working in the known areas of the endemic provinces in which heartwater had previously been confirmed.

The results suggest changes to the spatial distribution of heartwater in many areas, with serious expansion in some instances of up to 150 km. In total, 48% of veterinarians and 42% of farmers reported increases in the number of farms affected by heartwater. Climate change as a causative factor indicated by observations of increased average temperatures, milder frosts, less rain and shorter rainy seasons was identified by the majority of farmers but not by as many veterinarians. Respondents in both groups considered vegetation change an important factor. The increasing number of wildlife, especially antelope, was seen as a major factor by most veterinarians and also by many farmers. Both groups identified the movement of livestock and wildlife as an increasingly important factor that should be of major concern for both industries because it leads to the avoidable spread of many diseases, apart from heartwater. Control achieved by routine and regular block treatments of entire flocks or herds was also seen as a major factor and as increasing in use for both respondent groups, each giving it a high ranking.

Discussion and conclusions: The future impact of relying on tick control is of concern because more frequent dipping of whole herds or flocks to control heartwater will eventually lead to acaricide resistance. Also, reliance on regular block treatments could increase the risk of developing antimicrobial resistance against the one drug group (tetracyclines) that is cheap and currently effective in treating heartwater. There is anecdotal support that resistance to this drug has already begun to emerge. It is clear that satisfactory heartwater control could only be achieved by concerted, balanced and epidemiologically sound management and not by unsustainable and ultimately dangerous reliance on total acaricidal tick control and routine suppression of infection by antibiotics. Development of a true vaccine that is practical, effective, safe and affordable should be of the highest concern and priority. After decades of trials, researchers at Onderstepoort Veterinary Institute have developed a very promising candidate vaccine; its further development to the commercial stage by Onderstepoort Biological Products must be a priority. As a long term solution, the role of genetically determined resistance or resilience to heartwater infection should be investigated. Breeding better adapted animals could provide part of a sustainable approach to better control.     

Reference:

Leask, R. & Bath, G.F., 2020. Observations and perceptions of veterinarians and farmers on heartwater distribution, occurrence and associated factors in South Africa. Journal of the South African Veterinary Association 91(0), a1763. https://doi.org/10.4102/ jsava.v91i0.1763