aistand south~ern afrkca - (PDF, 101 mb) - USAID

control bmaks down, enormous losses can occur.
An extreme example was the breakdown of
dipping infrastructure during the war of
independence in Zimbabwe, where a compulsory
dipping policy had been in force since 1914.
Between 1974 and 1979 an estimated one million
cattle died, mainly of tick-borne diseases
(Lawrence et al, 1980). Research has shown that
game animals of varicus species maintain a tick
population and tick-borne diseases (Young et al,
1988).
Sustainable control methods
Stricttickcontrolisdifficulttomaintaininmany
countries and more rigorous methods for
controlling ticks and tick-borne diseases are
being investigated. The broad approach has been
to use integrated control measures which include
the natural exposure to Anaplasna,Babesia or
Cowdria organisms while animals are very
young. Other means include immunising older
animals with live vaccines; immunisation
against ECF by infection and treatment; chemotherapy;
and strategic acaricide application to
control overwhelming tick infestation or disease
challenge.
However, the most important element in this
control package is the use ofbreeds ofcattle that
are genetically resistant to tick infestations,
Resistant animals have a tendency towards light
tick burdens and require less dipping, making
control relatively easy and cheaper. In additior,
other biological control methods could be used in
support of control methods, e.g. anti-tick grasses
(Thompson et al, 1978; Sutherst et al, 1982;
Zimmerman et al, 1984) and application of
aggregation-attachment pheromone-baited
acaricide to single sites on bovine hosts (Rechav
et al, 1977; Sonenshine et al, 1979). These
approaches require an understanding of the
parasites that exist in a particular environment
and their epidemiology. It is also necessary to
have access to acaricides, vaccines and drugs and
to be able to monitor tick infestations and disease
outbreaks.
Host-resistance to ticks
It has long been recognised that some animals,
or whole breeds, consistently carry fewer ticks
than others kept in the same environment
(Roberts, 1968a; Wagland, 1975). Such differences
are caused by variation in the apimals'
abilities to respond immunologically to tick
infestation (Roberts, 1968b). The ability to
develop resistance is heritable (Hewetson, 1972;
Seifert, 1984) and the actual manifestation is
acquired (Rick, 1962; Roberts, 1968a). It is stable
over longer periods, although stresses such as
lactation or sickness cause a drop in resistance
(Wharton et al, 1970; Seifert, 1971; Utech et al,
1978). Different levels of resistance occur in all
breeds, but this is manifested more strongly in
zebu cattle and their crosses (Riek, 1962;
120
Wilkinson, 1962; Wharton et al, 1969; Seifert,
1971; Hewetson, 1979).
The improved tick control following the use of
tick-resistant cattle has been demonstrated in
various breeds of cattle and crossbreds (Riek,
1962). Kelly (1943) suggested that the tick
resistanre, of zehu cattle should be utilised for
tick control. Moreover, a cross between zebu and
taurine cattle was shown to carry fewer ticks and
required less dipping than temperate breeds of
cattle on similar pastures (Wharton et al, 1969).
The criteria for the assessment and
quantification of resistance to ticks in cattle are
given in many reviews (Willadsen, 1980;
Tatchell, 1986; Latif and Pegram, 1992).
Resistant animals consistently carry fewer ticks
than susceptible animals. Female ticks
completing engorgement are fewer and smaller
on resistant animals than on susceptible
animals.
Tick-resistant cattle in Africa
The earlier observations on cattle mortality due
to heartwater and its relation to the number of
ticks on animals of different breeds made by
Bonsma in 1940s (Bonsma, 1981) in South Africa
formed the basis of the studies on host-resistance
to tick infestationr. The study showed that
Africander cattle carried far fewer ticks than
British beef cattle and had a far lower mortality
rate than the British cattle (6% vs 60%). This
pioneer work by Bonsma was resurrected by
researchers in Africa 40 years later.Assessment
and quantification of host-resistance were
carried out on several African indigenous breeds
of cattle (Table 1). The majority of these breeds
are tick-resistant and of high productivity (Trail
and Gregory, 1982; Saeed et al, 1987).
The work shown in Table 1 comprises
short-term studies (up to two years) carried out
to assess the levels of host-resistance in different
breeds of cattle. There has been only one
long-term programme aimed at raising herd
resistance by selection, breeding or gene
alteration (de Castro, 1991); this was based at
the International Centre of Insect Physiology
and 7cology, Kenya. Although the progress and
results obtained were meaningful and in the
right direction, the programme was terminated
after three years due to lack of funding and
expertise. Over 100 hulls and 450 heifers at the
Boran National Stud were assessed for tick
resistance by repeated monthly exposure to
ticks. The proportion of bulls ranked in different
classes for total tick counts (Table 2) showed that
the majority (55%) of these animals had a high
level of resistance (rank 1) while a smaller
proportion (6%) were ranked as of low resistance
(rank 3). The repeatability of the observations
suggests a high degree of heritability of
resistance.