Page 220 - The Veterinary Laboratory and Field Manual 3rd Edition

Parasitology 189

comparison for samples collected from sick ani-
mals where haemoparasites are suspected to be
the cause of disease.
Owing to the fact that each stage in the life
cycle of multiple host ticks (for example, Ixodes
sp.) spends some time in the environment their
distribution and abundance is determined by
environmental conditions, that is, the larval and
nymph stages require suitable levels of humidity
and ambient temperature to ensure survival to
Figure 3.42 Life cycle of a typical one-host tick (for infect the next host. In one-host ticks, typically
example, Rhipicephalus [Boophilus] microplus, R. only the larvae need to search for a host, the
annulatus). All stages of the life cycle, except for remainder of the life cycle occurs on the same
egg laying, occur on the same host. (A) Eggs laid by animal. Heavy infestation of ticks can result in
an engorged female off the host in the vegetation. ‘biting stress’ with resultant irritation and dam-
(B) the larvae hatch and cluster on long grass before age to the skin. In addition, ticks can transmit
attaching to the chosen host. (C) Larvae feed and a range of clinically important diseases. For
moult to form the nymph stage. Note that larvae example, Rhipicephalus (Boophilus) microplus and
have three pairs of legs as compared with four pairs
of legs in the nymph and adult stages. Some people
may confuse larval ticks with lice which is why it is
important to collect samples of ectoparasites for
correct identification in the veterinary laboratory.
(D) Nymphs feed and moult to form the adult. (E)
Mature male and female ticks mate and the female
engorges (often > 10× unfed size) and drops off
to lay her eggs in the vegetation. The eggs sur-
vive best in long vegetation and when the climate
is warm and fairly humid. It is for this reason that
ticks are more common in some regions and tend
to have different seasonal prevalence (see text).

For example, single host ticks (for example,
Rhipicephalus [Boophilus] annulatus) are easier to
control than two (Hyalomma sp.) or three host
ticks (Rhipicephalus sanguineus, Ixodes sp.) due
to the fact that the latter are present in large
numbers in the environment for much of their
life cycle. Where animals are hosting a number Figure 3.43 Photos of common ectoparasites: (A)
of ticks, even if they appear to be healthy, it is eggs of lice; (B) Dermanyssidae (mite) – cat; (C)
useful to collect blood smears to check for the Damalinia sp. (lice) – goat; (D) Ixodes scapularis
presence of haemoparasites (see Figure 3.36). (tick) – dog; (E) Chorioptes bovis (mite) – cattle;
Examination of blood smears from apparently (F) Demodex canis (mite) – dog. See also Plate 7.
healthy animals in an area provides a useful Photos: Dr Mani Lejeune, Cornell University, USA.

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