Page 66 - The Welfare of Cattle

CattLe eCtoParasItes 43

blood meals on smaller mammals or birds during their immature life stages. Pastures may provide
the necessary range of alternate hosts needed for these ticks, while dry pen systems provide only
cattle leaving these ticks unable to feed during their immature stages. In contrast, 1-host ticks that
feed only on cattle (e.g., spinose ear tick, cattle fever tick) may be able to persist in a dry pen envi-
ronment if other aspects of the habitat are suitable. The spinose ear tick in particular seems to be
increasingly abundant in some types of intensive systems that include animal bedding where adult
ticks can survive to lay eggs and larval ticks can readily acquire a new host.
The opportunity for ectoparasites to acquire a new host is also critically important. Some
ectoparasites can be managed simply by removing their hosts from pens or pastures for a suitable
period of time to kill the ectoparasite from lack of a blood meal. Once the pen or pasture is ectopara-
site free, uninfested animals can be safely moved in. This technique works especially well for the
ectoparasites that cannot survive off their host for more than a few days (e.g., lice and mites). In an
intensive system, leaving pens vacant for a few days before rotating the next age-group of animals
into the pen will prevent transfer of lice and mites from one herd to the next. In pasture-based sys-
tems, cattle are often held in mixed-age herds with contact among animals allowing for transfer of
lice and mites from infested to uninfested animals. Ticks have also been managed by taking cattle
off a pasture (“pasture spelling”), but the time required to eliminate ticks can be very long as some
tick species can survive for months or even years without a blood meal, and deer or other wild ani-
mals may serve as substitute hosts in the absence of cattle. To acquire hosts, ticks climb grasses or
other vegetation and wave their front legs about until a suitable host brushes past allowing the tick
to grasp a few animal hairs. Where vegetation is lacking, ticks cannot easily acquire a host; unless
ticks and cattle encounter one another in animal bedding areas, as is the case for both the spinose
ear tick and the Pajaroello tick.
Increasing cattle density often also increases the opportunity for ectoparasites to acquire a new
host. When cattle density is high, contact among animals is greater, increasing transfer of lice and
mites among animals. Of course, ticks would also benefit from higher cattle density, decreasing
the time to acquire a new host and therefore increasing tick survival while they wait in the habitat
for a suitable host to wander past. Higher cattle density also increases the quantity of feces, feed,
and wastewater, so that biting flies that develop in these materials will be more abundant and cattle
will experience an increase in bites by these ectoparasites, perhaps resulting in increased oppor-
tunity for the transmission of disease agents (e.g., bovine pinkeye, bluetongue) among animals.
Paradoxically, increasing cattle density may actually decrease the impact of biting flies that develop
in non-manure-polluted aquatic habitats, since ectoparasite abundance would remain unchanged
leading to fewer bites per individual animal, thereby lowering production losses as bite avoidance
behaviors are reduced.

eCtOParaSIte LIFe hIStOrY CharaCterIStICS

Insects, ticks, and mites that harm cattle generally feed on blood, skin, hair, or exudates (tears
or mucus) at the external body surface of cattle, resulting in their common description as external
parasites or “ectoparasites.” Exceptions include the cattle grubs (Hypoderma lineatum and H. bovis)
and the New World screwworm fly (Cochliomyia hominivorax) that invade cattle tissues to feed dur-
ing their immature life stages, and are therefore more accurately described as internal parasites or
“endoparasites.” For simplicity, the term ectoparasite is used loosely in this review to include all of
the insects, ticks, and mites that negatively impact cattle in some way.
Ectoparasites often cause negative impacts to cattle production related to the type and duration
of their contact with a single host animal (Figure 6.1 and Table 6.1). Ectoparasites can complete all
life stages living and feeding on a single host animal (permanent ectoparasites), they can maintain
long-term contact with a single host animal during some portion of their life while also requiring