Rai. Norval et al., FACTORS AFFECTING THE DISTRIBUTIONS OF THE TICKS AMBLYOMMA-HEBRAEUM AND A-VARIEGATUM IN ZIMBABWE - IMPLICATIONS OF REDUCED ACARICIDE USAGE, Experimental & applied acarology, 18(7), 1994, pp. 383-407
The ticks Amblyomma hebraeum and A. variegatum are the main vectors of
heartwater, a disease of ruminants caused by Cowdria ruminantium, in
the agricultural areas of Zimbabwe. At present, A. hebraeum is widely
distributed in the dry southern lowveld, and occurs in at least seven
foci in the higher rainfall highveld. Amblyomma variegatum occurs in t
he Zambezi valley and surrounding dry lowveld areas in the northwest.
The distribution of A. Habraeum has changed considerably over the past
70 years, while that of A. variegatum appears to have remained fairly
static. The distribution patterns of both species in Zimbabwe display
anomalous features; the ticks occur in areas of lowest predicted clim
atic suitability for survival and development and in areas where the d
ensities of cattle, the most important domestic host, are lowest. The
only factor favouring the survival of the species in the lowveld habit
ats in which they occur is the presence of alternative wildlife hosts
for the adult stage. Their absence from more climatically favourable h
ighveld habitats appears to have been the result of intensive acaricid
e treatment of cattle over a long period and a historic absence of sig
nificant numbers of wildlife hosts. Eradication of A. hebraeum and A.
variegatum by intensive acaricide treatment of cattle can be achieved
in the absence of significant numbers of alternative hosts, because of
the long attachment and feeding periods of the adults of these tick s
pecies. However, eradication becomes impossible when alternative hosts
for the adult stage are present, because a pheromone emitted by attac
hed males attracts the unfed nymphal and adult stages to infested host
s. The unfed ticks are not attracted to uninfested hosts, such as acar
icide-treated cattle. Regular acaricide treatment of cattle is expensi
ve and so, for economic reasons, the Government of Zimbabwe is no long
er enforcing a policy of strict tick control. It is likely that reduce
d tick control will result in the spread of Amblyomma ticks to previou
sly uninfested areas. Added to this, recent introductions of various w
ildlife species to highveld commercial farming areas have created cond
itions in which the ticks could become established in higher rainfall
areas. Amblyomma hebraeum is more likely to spread than A. variegatum,
because its adults parasitize a wider range of wildlife hosts (wartho
gs, medium to large-sized antelope, giraffe, buffalo and rhinoceros),
whereas adults of A. variegatum appear to be largely restricted to one
wildlife species (buffalo) in Zimbabwe, the distribution of which is
now confined to very limited areas of the country, as part of foot and
mouth disease control measures. A model to predict the rate of spread
of A. hebraeum through the highveld is described. Possible control op
tions for dealing with the spread of Amblyomma ticks and heartwater to
previous unaffected highveld areas, include (1) continuation of inten
sive acaricide treatment of cattle to prevent the spread, (2) establis
hment of a buffer zone of intensive tick control around affected areas
to contain the spread and (3) allow the spread to occur and control h
eartwater by means of immunization. An economic analysis to determine
the costs and benefits of the control options, which takes into accoun
t the development of Amblyomma - specific tick control technologies an
d improved heartwater vaccines, is recommended.