Page 68 - The Welfare of Cattle

CattLe eCtoParasItes 45

infestation of cattle or other animals is achieved by treating with topical insecticides the infested
and uninfested wounds of all animals in the area. Injection of cattle with the parasiticide doramectin
is also effective.
Like the New World screwworm fly, the common cattle grub (H. lineatum) and the northern
cattle grub (H. bovis) live and feed on cattle during their immature life stages. Adult cattle grubs
are also called “heel flies” as adults of both species land on the legs and lower body of cattle where
they deposit eggs onto the hairs at these locations. While adult cattle grubs do not bite cattle, the
presence of an adult fly may cause cattle to run madly with their tail raised in the air in an apparent
effort to avoid these flies. This panicked running is called “gadding” and can result in cattle injuries
as cattle run into objects in their environment. However, it is the immature flies that cause the most
significant damage. Newly hatched fly larvae burrow into the skin and migrate through internal
body tissues until reaching the back, where larvae cut a breathing hole in the hide resulting in a
swelling (“warble”) within which the larvae feed on exudates to complete immature development
before dropping to the ground to become adult flies. Cattle grubs are effectively controlled using
systemic insecticides applied once each year in late summer when the younger larvae are just begin-
ning to migrate through cattle tissues.
Ticks are intermittent ectoparasites that typically remain on their animal host for days to weeks
during each blood feeding period. Ticks generally require a bloodmeal for each of their three active
life stages (larva, nymph, and adult), though there are some exceptions to this feeding pattern. The
majority of tick species (3-host ticks) feed on different individual animals during each life stage,
dropping off the host between bloodmeals to molt to the next stage or for adult ticks to lay eggs.
Many of these 3-host ticks will feed on cattle mainly during the adult stage, feeding on smaller
mammals or even birds during their immature life stages. In contrast, a few tick species attach as
larvae to cattle and then remain on the same host animal through all feeding stages, dropping off
only when feeding is no longer required (1-host ticks). Adult ticks will deposit up to thousands of
eggs on the ground where the tick dropped from its last host. Unfed ticks can survive off the host
animal for months or even years depending upon the tick species, making these pests very difficult
to control.
Cattle fever ticks (Rhipicephalus annulatus and R. microplus) are 1-host ticks that feed on the
same animal during all life stages, dropping off the host after a final bloodmeal during the adult
stage. Cattle fever ticks commonly feed on cattle, antelope, and related bovids, in addition to several
species of deer, making control of these ticks difficult when alternate hosts are available. These
ticks can be infected with Babesia parasites passed from female ticks to their offspring and then to
cattle during feeding, resulting in bovine babesiosis or “Texas cattle fever” which presents as ane-
mia, wasting, and eventually death of cattle. Due to their significant impacts to the cattle industry,
the U.S. Department of Agriculture in 1906 initiated a cattle fever tick eradication program that
coupled an aggressive tick surveillance program with mandatory animal treatments using topically
applied acaricides. This highly successful program resulted in eradication of these ticks from the
U.S. in 1943. Cattle fever ticks remain common in northern Mexico, and the U.S. maintains an
active quarantine program at the southern U.S. border to prevent the reintroduction of these ticks.
Another 1-host tick, the spinose ear tick (Otobius megnini), feeds on a single host animal only
during the immature stages, then drops off the host to complete development to a non-feeding adult.
These ticks attach and feed within the folds of the cattle ear. Spines on the tick body help hold
these ticks in place within the ear. Spinose ear ticks are known to cause restlessness and head shak-
ing behavior in infested animals, leading to possible decreases in weight gain or milk yield when
infestation is heavy. Spinose ear ticks are common to the southwestern U.S., particularly in pasture
settings and increasingly in freestall barns where animal bedding may provide a refuge for adult
ticks and host-seeking larvae.
Other ticks of concern to cattle are 3-host ticks. For these ticks, cattle are simply one of several
(or many!) suitable hosts from which they can obtain a bloodmeal. Common 3-host ticks that are