|
Lice are present on cattle all year
round, but their numbers increase dramatically during cold
weather. Cool temperatures and a long hair coat (giving them
more protection) create an ideal habitat for these parasites.
Heavy lice infestations are a common winter problem in cattle,
robbing them of vital nutrition just when they need it most. The
drain on the animal can cause weight loss, reduced milk
production and a general unthriftiness that can make the host
more susceptible to disease.
Lice are one of the most costly
and underrated parasites of cattle, accounting for millions of
dollars lost each year due to reduced feed conversion, weight
loss, anemia and deaths. Dr. Lee Townsend, University of
Kentucky extension entomologist, says that during the last cold
months of winter and into early spring, the main parasite of
cattle for ranchers to think about is lice. Irritation and
itching due to these pests puts additional stress on cattle as
they feed, draining energy reserves.
Both biting (chewing) and sucking
lice infest cattle. Biting lice feed on the skin itself, while
sucking lice penetrate the skin and suck blood. Biting lice (Damalinia
[Bovicola] bovis) are the most common, while sucking lice (Haematopinus
eurysternus, Linognathus vituli, and Solenopotes capillatus) are
most damaging. Both types cause severe irritation and itching.
The infested cattle rub on any
available surface, and chew or lick at themselves. Cattle that
are scratching and rubbing against feeders, gates, posts and
other objects are usually infested, says Greg Johnson, livestock
entomologist at Montana State University. The constant crawling
and biting (or piercing of the skin by sucking lice) causes
infested animals to be restless and nervous; normal feeding
activities are disrupted and appetite is hindered.
“The animal is concentrating more
on the irritation and discomfort, spending more time itching
than eating,” says Johnson. Chewing lice feed on the upper layer
of skin and can weaken the host animal by reducing amount of
time spent eating, making the animal more susceptible to
disease. “Biting and chewing lice prefer the back and tail
areas, but can be found all over the body. Different types of
lice find an area on the animal that meets their temperature
requirement, and usually stay in that area. Sucking lice usually
congregate around head, cheeks, neck and shoulders--the front
parts of the host animal. They are the most harmful because of
the blood loss and anemia they cause,” he says. They feed by
piercing the skin and sucking blood. Loss of blood can stunt
growth and reduce weight gain.
Continued heavy infestations can
weaken the animal to the point that stress from disease or
extremely cold weather can cause death. Sucking lice can extract
so much blood from an animal that red-cell blood counts can be
lowered as much as 75%. Heavy infestations of these lice can be
fatal to young calves. Any animal with a lot of lice may become
anemic, leading to poor feed conversion and vulnerability to
other diseases.
Dr. Townsend recommends Fall
treatment of the herd for lice control and says ranchers should
assume that any new animal brought into the herd is carrying
lice. “New animals should be isolated and treated, whatever time
of year they are brought in, before being put with the herd.
Some products for lice have a two-treatment protocol, and the
new animals should be kept isolated until they’ve had both
treatments,” he says.
Lice numbers are lowest in summer
because most of them are shed off with winter hair in spring.
Skin temperature also affects lice populations. In summer,
according to Jack Campbell (professor emeritus, University of
Nebraska entomology department), there is more heat on the
animal’s back, so lice tend to leave that area. They also don’t
reproduce as fast.
He’s done studies of summer
distribution of lice on cattle, checking for lice on cattle
hides at slaughter. He found that lice are present on all cattle
during summer, but just in lower numbers. “The lice increase
their reproductive rate in cold weather; the time spread between
egg laying and the adult stage decreases, so you get more
generations in a shorter time span,” says Campbell.
Dr. Doug Colwell (Lethbridge,
Alberta) says lice are temperature-sensitive and don’t thrive in
heat. “If the cow is standing in bright sunlight in summer, her
skin temperature may go up to 115 to 120 degrees F and this is
outside the thermal tolerance of a louse. Lice and their eggs
can’t survive at those temperatures. In summer the adult lice
are dying off and not reproducing, so their population crashes
when weather warms up,” says Colwell.
The populations build up as the
animal grows winter hair in the fall and reach a peak by winter.
Lice populations on cattle can multiply dramatically when
weather turns cold. Increased body contact between animals aids
spread of lice (as when cattle are congregated for feeding, or
brought into corrals for weaning calves, preg checking and
routine vaccinations). Winter hair coat gives lice increased
protection and an ideal environment for reproduction.
The life cycle is usually 20 to
30 days. The entire cycle takes place on the host animal, which
makes lice an easy parasite to kill. The eggs are attached to
the hair by the female louse and hatch in 5 to 14 days. The
nymphs emerge (looking just like adult lice, only smaller) and
go through three molts within about a week, becoming egg-laying
adults in about 14 days. Dislodged eggs, when hair is rubbed off
on a fence or feeder, can still hatch, and often keep a pen or
pasture infective for up to a week--for any animals that may
come in contact with them. If you put new cattle into a corral
where lousy animals have been recently been rubbing, lice can be
transmitted to the healthy cattle.
All three stages (eggs, nymphs
and adults) can be present at the same time on any given animal,
and though direct contact is the primary means of transmission,
eggs and nymphs can be transmitted by use of brushes or other
equipment, or contact with feeders and fences where infested
animals have rubbed.
Evidence of lice becomes obvious
after cattle have been infested awhile; they rub out patches of
hair and often rub to the point of injuring the skin. A close
look with the animal restrained will reveal the tiny parasites.
Part the hair on shoulders or head with your fingers, in good
light. You can often see them with the naked eye, but a
hand-held magnifying glass and a flashlight makes them easier to
see. Also check for eggs (nits)--small white, yellow or black
barrel-shaped specks attached to hairs. If one animal in a group
is carrying lice, then the entire herd is probably infested to
some degree.
Certain animals tend to harbor
abnormally high numbers of lice, even during summer. These
“carriers” serve as a continuing source of infestation for the
herd. Young animals and older undernourished cattle usually have
the heaviest infestations, and the latter can become “chronics”
or carriers that reinfest a herd every winter. Lice are spread
by contact from carriers to other cattle, and from cows to their
calves.
Carriers
Dr. Campbell says carrier animals usually have some
deficiency in the immune system that makes them more susceptible
to heavy lice infestations. Cattle normally develop some
resistance to lice after exposure. He says there is usually
quite a difference between the severity of lice infestation
between a young animal (that had never been exposed to lice) and
an older animal that has encountered them before. “The young,
naive animal will develop a high louse population upon first
exposure, whereas the older, more immune animal will have fewer
numbers.”
Dr. Jack Lloyd (University of
Wyoming, now retired) says young animals more readily become
heavily infested, with lice populations becoming established
rapidly and in large numbers. “Mature animals that have lower
levels of lice most likely have gotten some sort of antigen from
the lice, and create antibodies,” says Lloyd.
Campbell recommends that carrier
animals be culled from the herd. “A carrier is usually an older
cow that always harbors an unusually high number of lice. These
are cows that have an immune problem and are physiologically a
little different; they don’t develop resistance like the other
cattle, and the lice are not killed as readily by treatments.
Some lice on a carrier go ahead and lay eggs before they die.
Also, the cow is carrying so many that the treatment does not
kill them all. There is a swift reinfestation on that animal,
and she serves as a source of lice to reinfest the rest of the
herd.”
He says that if ranchers don’t
cull these carriers, lice population in the herd can build up
again quickly after treatment, necessitating further treatments
through winter. He cited a survey done on cattle in North
Dakota, Wyoming and Nebraska, in which some ranchers were giving
as many as 5 treatments for lice during winter, which means the
treatment was ineffective. This could be due to the presence of
carrier animals, or the type of treatment being used. For
instance, the injectable products kill sucking lice, but not
chewing lice.
Dr. Lloyd in Wyoming says carrier
animals that earlier had millions of lice, can suddenly become
louse-free. One of his research projects bought many carriers
from ranchers over the years, to use as a source of lice for the
study. “Every one we bought was very heavily infested, probably
due to an immune problem. But eventually every one of them
became louse-free, possibly because the immune system finally
kicks in. It may take one year, or several, but they all lose
their carrier status after awhile,” he says.
Dr. Lloyd did a lot of studies on
lice and grubs, using cattle with various levels of parasite
infestation for the trials. His university facility worked
cooperatively with drug companies and evaluated most of the
products on the market today. He says the cattle they received
for study had all four species of lice (some in more
concentration than others) and feels that these four species are
probably present on cattle in most regions. “Most trials and
data concentrate on only one species of louse but there are
usually three or four on cattle. For effective treatment, you
usually need something that gets both biting and sucking lice.”
“The new pour-ons are very good, but also expensive,” he says.
They are safer for humans than the old systemic organophosphates
(like Warbex) and can be used on dairy cattle, without any
withdrawal period for meat or milk.
Control of Lice
Any animal suspected of having lice should be treated in
early fall before lice populations build up (to help keep lice
from spreading to the rest of the herd) and all animals should
be treated in late fall before infestation becomes severe.
Effective control of lice requires two treatments two weeks
apart if using a product that only kills the lice and not the
eggs. The second treatment will kill the lice that hatch out in
between. It is very important to treat cattle for lice before
winter. Many stockmen now use the newer macrocyclic lactone
products that require only one treatment. These are systemic and
have enough residual effect to also kill the next batch of young
lice after they hatch.
There are a number of products
available for lice control. Dr. Colwell says the main ones used
today are the macrocyclic lactones which include ivermectin and
moxydectin. Some of these are pour-ons and some are injectable.
The injectable formulations kill sucking lice but not the
biting/chewing lice (which are usually most common). There is
still a fairly large market for the pyrethoid pour-on products
because they are less expensive. These generally have to be
repeated, however.
“Lice control is more crucial in
cold climates than warm ones,” says Dr. Lloyd. There is not as
much problem with lice in warm areas, and some southern stockmen
do not bother to treat for lice. The lice populations are never
as high in a warm winter climate than in a cold one.
Dr. Doug Colwell says most
northern stockmen treat for lice at the start of cold weather,
so the parasite population won’t build up. If you are going to
try to just delouse your cattle once, he recommends delaying
treatment as long as you can, so you can get the most good from
the treatment, since the lice will re-emerge later.
“We did some studies several
years ago on lice populations, with a standard counting
technique. I can follow the populations on untreated cows from
January on through winter, and by May you can’t find a louse,
using our counting method,” he says. There are probably a few
down underneath and between the hind legs in small areas where
they take refuge and can’t be rubbed or licked off, but the
animals are relatively lice-free during summer. By mid-October
you start seeing lice again, so treatment at that time is aimed
at knocking down the population growth curve so that by January
you don’t have a massive outbreak, explains Colwell.
Several methods and products are
available for treating lice, including insecticides applied by
spray, backrubbers, dust bags or pour-on, but the sprays will
not kill the eggs and a second spray 10 to 14 days later (no
later than 18 days afterward) will be necessary. Spraying has
the drawback of causing cold stress if cattle need to be treated
during cold weather, and should only be done on a day that
cattle can become dry by sundown.
If using back rubbers or dust
bags, do not locate them where spillage might contaminate water
supplies. Dust bags and oilers take some time to effectively
reduce a high louse population. They often work best as a
preventative rather than a control measure, and in this capacity
can help prevent population buildups. Follow label directions
whenever applying insecticides and do not use them in
conjunction with other insecticides at the same time (such as
ear tags). If using pour-ons, do not exceed the maximum
recommended dosage, and apply it as directed. Some just need to
be deposited on the back, while others should be applied all
along the topline of the animal from shoulders to hips. Some
pour-ons should have a portion of the dose applied to the top of
the neck and down the face.
Greg Johnson (Montana) explains
that oil-based pour-ons are formulated to travel through the
hair coat so the chemical travels over the whole body of the
animal. Other pour-ons are systemic and absorbed directly into
the body to kill sucking lice, grubs and internal parasites at
the same time.
Systemic insecticides must be
used before winter to avoid toxic reactions due to grubs being
killed while migrating through the esophagus or spinal nerve
canal. The dying grubs release toxins that cause swelling and
inflammation in the tissues. This could lead to death of the
animal (due to paralysis if the swelling occurs next to the
spinal cord, or choking if it’s next to the esophagus) unless
the reaction is reduced with prompt treatment. Check with your
vet for advice on insecticides and which products might be best
for your situation and climate.
Johnson says good control can be
obtained by using a systemic product (for both lice and grubs)
in the fall--before the cut-off date for grubs in your
particular climate--and then repeat the lice treatment using a
contact (non-systemic) product later in the winter if animals
start to rub. Lloyd says lice populations in Wyoming peak in
February and March. He recommends total herd treatment in the
Fall, and treatment of individual animals toward spring, if an
animal shows evidence of lice. Lice populations in the herd as a
whole will be decreasing toward spring.
Campbell says most ranchers treat
for lice in the Fall at weaning, and in some regions this
treatment will also get the last of the horn flies as well as
lice and grubs. He says the new ivermectin pour-on formulation,
though expensive, is effective against both chewing and biting
lice, whereas the injectable ivermectin was not. He also says
ivermectin seems to be beneficial in several ways. “Even if
cattle do not seem to have a significant load of internal
parasites (worms), they respond well to ivermectin; treated
cattle seem to do a little better than untreated ones.” He says
clinical parasitism, as detected by checking lice populations or
worm infestations, may not be the economic break point. “Cattle
with lower levels of parasites may still benefit immensely from
treatment.”
Colwell says the macrocyclic
lactone products are very effective for lice control if applied
properly, but won’t give winter-long control if applied too
early. “A few years back we had severe drought and ranchers
pulled cattle off pastures in late August and September. They
put them in drylots and did all their fall treatments at that
time, and then had huge louse outbreaks in January and
February,” he says. Winter-long control can only be obtained if
you treat late enough in the season when lice are already
starting to build up. Then you usually don’t need to re-treat if
a few lice show up in March or April because the population
won’t grow large at that late date. Even on untreated cattle,
the lice numbers drop rapidly as weather warms up.
Products must be used properly,
however, to give adequate lice control. Never underdose. If you
don’t kill all the lice on even one animal, that animal will
serve as a source of lice for the rest of the herd and spread
lice to the ones you treated. “When some of the newer products
first came out, a lot of the companies had a guarantee—stating
that just one treatment would last all winter and if you found
lice on any of your animals they would pay for retreatment,”
says Colwell. “This came back to bite them, but there were some
lessons learned.”
When treating for lice, use
proper dosage and don’t mix treated and untreated animals, or
the untreated ones soon reinfest the treated cattle and you’ll
have wasted your insecticide. Put treated cattle in a new,
separate pasture. For best louse control you may need to treat
cattle in late fall and again in mid-winter or early spring,
since lice populations may build up again before winter is over.
Resistance Issues
Underdosing with pesticides can lead to resistance in some types
of parasites, especially internal parasites (worms) and horn
flies. “Even though we haven’t seen resistance yet in lice to
the compounds we’re using, there is growing evidence that the
gastro-intestinal worms are becoming resistant to systemic
products like ivermectin,” says Colwell. The way cattle are
constantly moving around the country, resistant worms can be
spread readily from one area to another.
“With so much of our treatment
program today based on macrocyclic lactones, we need to pay
attention to this issue. In the short term, a producer might
think he’s saving money using a partial dose, but ends up paying
for it later. It’s also your responsibility to your fellow
cattlemen to help make these products remain effective as long
as you can, rather than have them become ineffective due to
improper use. If we lose some of these very effective products,
we’ll then be dependent upon the drug companies to come up with
something new. The macrocyclic lactones like ivermectin have
been in use for more than 25 years. This is a marvelous class of
compounds and there is probably nothing new coming on that will
work as well on so many different types of parasites,” says
Colwell.
There are a few new products
coming out, however, which may be helpful to add to the
available options for lice. One of these is called Clean Up. Dr.
Lloyd says the main chemical in this topical pour-on has been
around for quite awhile but was not made available for control
of ectoparasites (such as lice) until recently. “We did research
many years ago on this chemical for lice control and were very
impressed with it, but nothing happened with it. Then a few
years ago Boehringer Ingelheim made it available for lice and
horn fly control,” says Lloyd.
“It has 2 active ingredients. One
is diflubenzeron (dimilin), an insect growth regulator that has
been available in horn fly control for several years (as a
bolus—the ingredients of which end up in manure to kill fly
maggots by inhibiting their growth). The other ingredient in the
lice product is permethrin, which doesn’t kill sucking lice but
controls the chewing lice. The best thing about Clean Up is the
dimilin, which is effective against both types of lice,”
explains Lloyd. “The cattle product is a pour-on that moves
through the hair coat and tends to cover the whole animal but is
not absorbed at all. The dimilin seems to be quite residual and
probably lasts several months.”
Another new topical product for
lice is a pour-on (also used as a spray) called Elector. The
active ingredient, Spinosad, is an organic molecule derived from
a fermentation process and is in a different class than
pyrethroids or the organophosphates. This product has been
available in the U.S. for about 6 years. The battle against lice
is ongoing, and by rotating the use of some of these unrelated
products stockmen may be able to avoid or delay the development
of parasite resistance.
|
