Ticks of Domestic Animals in Africa - Alan R Walker - Science Writer

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Hyalomma anatolicum excavatum Koch, 1844.
General.
Hyalomma anatolicum excavatum is significant because it is
very similar to Hyalomma anatolicum anatolicum and the two
sub-species overlap in distribution. However, because of the
feeding preferences of Hy. a. excavatum its status as a vector of
Theileria protozoa is in doubt. Epidemiological studies on theileriosis
therefore require these two sub-species to be distinguished
wherever they occur together.
Differential diagnosis.
Hyalomma anatolicum excavatum is closely similar to Hy. a.
anatolicum. Hyalomma a. excavatum is a robust and dark coloured
tick with the ringed legs and large size typical of the genus
Hyalomma. It is about 25% larger than Hy. a. anatolicum.
The females are differentiated by the shape of their cervical
fields, in Hy. a. excavatum these have steep sides and curved
margins. It is important to compare the males to separate these
two sub-species because the females are very similar. Males of
Hy. a. excavatum have a distinctive depressed area in the posterior
scutum, with steep sides. They have a distinctive formation
of the festoons such that the paracentral festoons are joined
anteriorly to form an arch, also the central festoon is pale. The
close similarity of the two sub-species has caused some confusion
in the naming of the anatolicum complex and binominal
names like Hy. anatolicum and Hy. excavatum occur in the literature.
Hoogstraal (1956) used the name Hy. excavatum for a
species that was later revised to Hy. a. anatolicum (Hoogstraal
& Kaiser, 1959). The taxonomic status of these sub-species needs
to be confirmed.
Hosts.
Cattle, sheep, goats, camels, horses and donkeys are the hosts
of adult Hy. a. excavatum. The adults attach on the hindquarters
of their hosts (perineum, udder). Larvae and nymphs feed on
hares, hedgehogs, and rodents particularly gerbils and jirds. Hyalomma
a. excavatum is a ditropic tick and in contrast to Hy. a.
anatolicum the larvae and nymphs of Hy. a. excavatum do not
naturally feed on cattle and other large hosts.
Life cycle and seasonal occurrence.
This tick can feed as a two-host or three-host tick depending on
availability of hosts. It is a robust tick able to feed during most
of the year, even in climates with a distinct winter. This is not a
domestic tick; unlike Hy. a. anatolicum it is not closely associated
with livestock housing. It may coexist in the same region
with its sub-species Hy. a. anatolicum but in this case it usually
occupies the more ecologically marginal areas. Where these two
sub-species coexist the Hy. a. anatolicum population becomes
more numerous and uniformly distributed than Hy. a. excavatum.
In North Africa adult Hy. a. excavatum are found on livestock
throughout the year with a peak in spring and a reduction in
numbers in winter.
Disease.
The disease relationships of Hy. a. excavatum are poorly known.
The immature stages under natural conditions characteristically
feed on rodents and small vertebrates and not large animals.
Thus the ability of this tick to act as a vector of pathogens such
as Theileria, which are transmitted transstadially, is uncertain.
Hyalomma a. excavatum has not been reported as an important
field vector of Theileria but can transmit it under experimental
conditions.
Habitat and distribution.
Hyalomma a. excavatum is adapted to the Mediterranean and
steppe climatic regions of North Africa and to steppe climatic
regions elsewhere in its wide range. This tick is often less commonly
found on livestock than Hy. a. anatolicum but it has a
wider geographical distribution and is found in much of North
Africa from Mauritania to Egypt and also into Sudan, Ethiopia
and Eritrea. It is not found commonly south of the Sahara but it
is found eastwards through to Iran and Turkmenistan.
Distribution of Hyalomma anatolicum excavatum.