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Guidelines for Controlling Internal Parasites in Cattle
By Dr.
Tom Craig
Parasites of concern in beef operations vary with location, season, age
of cattle and management practices. Even though they can’t be seen,
internal parasites of cattle are an important source of lost profits.
Cattle may look fine, but they don’t produce to their potential.
Most infections are a combination of several worm species, but
Ostertagia ostertagi, the brown stomach worm, stands out. The most
significant aspect of Ostertagia infection is damage to the stomach
lining resulting in cattle with depressed appetites, decreased milk
production and low weaning weights.
In high numbers the parasite can cause severe clinical disease, weight
loss and diarrhea. Even when encountered in comparatively low numbers,
the parasite reduces cattle’s ability to efficiently convert forage into
milk or meat.
The economic importance of this parasite wasn’t fully appreciated until
effective dewormers, (anthelmintics) became available.
Control strategies for internal parasitism should be based on Ostertagia,
but timing varies with location.
Treating cattle for brown worms
Ostertagia larvae (L4) may cease development in the stomach lining
during periods when pasture conditions are unfavorable for survival.
When pasture conditions are favorable, these arrested larvae resume
development and begin laying eggs.
The pasture transmission of this parasite occurs during summer and
autumn in northern states, winter and spring in the south.
Control Ostertagia by treating cattle with anthelmintics that kill the
arrested larvae or have residual activity to kill incoming larvae for a
period of time. In the south, treatment in the summer using a
benzimidazole or macrolide anthelmintic may accomplish this goal. Beef
cows and calves are the primary thrust of the basic program.
Because Ostertagia is a common parasite, most cattle develop resistance
to clinical disease after repeated exposure. However, the goal of modern
production is optimal productivity, not just freedom from disease. In
some regions where cattle don’t get sufficient exposure to Ostertagia to
acquire natural resistance, disease can be seen in older cattle.
There are two basic reasons to administer an anthelmintic. One is to aid
an individual by saving its life or increasing production of meat, milk,
or possibly reproductive efficacy. The other is to lessen numbers of
infective parasites in the environment so “at-risk” animals won’t be
exposed to the level of infection that leads to disease.
With some parasites, treatment with an anthelmintic effective against
only adult worms won’t really help the individual. However, if the drug
is effective against larvae in arrested development, the individual may
benefit.
Using an anthelmintic effective against arrested larvae will aid the
individual and lower the pasture exposure to the next crop of calves.
Other parasites
The most common internal parasite encountered in calves is Cooperia.
Because eggs of this parasite are indistinguishable from Ostertagia,
they are often confused. From the standpoint of number of eggs, Cooperia
is usually the dominant internal parasite in calves. However, when
compared to Ostertagia, it’s less significant.
Calves will spontaneously rid themselves of this infection by 12 to 15
months of age. In general, Cooperia do not respond well to macrolides
and have become more evident in recent years as the more important
internal parasites are controlled.
In cattle, another worm, Haemonchus, is similar to or shared with a
parasite that is absolutely devastating to sheep and goats. Older cattle
become immune to disease caused by Haemonchus, but calves are fully
susceptible to the parasite.
Female Haemonchus produce thousands of eggs each day and are avid
bloodsuckers causing bottle jaw and anemia. Summer pastures grazed by
susceptible calves can have billions of infective larvae and become
killing grounds.
A recent report of Haemonchus resistant to macrolide and benzimidazole
anthelmintics in stocker calves is cause for concern. The resistance was
seen in calves on a warm season perennial pasture laden with larvae.
After a period of grazing, numbers of infective larvae in the pasture
rapidly escalated due to susceptibility of the calves and the tremendous
egg laying capacity of the parasites. Disease was seen late in the
grazing season when weather conditions were favorable for transmission
of the parasite.
Evidence strongly suggests cows and suckling calves more than two months
old will benefit from treatment with an anthelmintic entering the summer
in the south.
Fifteen to 50 additional pounds of calf at weaning or a cow breeding one
to two months earlier when treated at the right time for internal
parasites is the expected goal. Treatment can be done when cows are
pregnancy checked, the calves vaccinated, or other activities.
When calves are weaned and moved to another pasture, treatment will aid
in the transition to earning their own living. In addition cows,
especially first-calf heifers, in high rainfall areas benefit from
deworming entering the winter in the southern United States.
Stocker calves, especially on permanent pastures, are at high-risk for
all parasites. Therefore, it’s essential to treat when they are turned
out to pasture then repeat treatment one to three months later,
depending on the anthelmintic used.
Because these calves are at risk and pastures may be laden with
resistant worms, have your veterinarian check fecal samples two to four
weeks after treatment to aid in deciding if the anthelmintic used
actually worked on your farm or ranch.
Dr. Tom Craig is with the Texas A&M University department of veterinary
pathobiology.
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