Internal Parasite Control

Over 20 species of parasites inhabit the digestive tract and lungs of cattle. Internal parasites can cause significant economic loss in a beef operation. Parasites compete for nutrients, reducing feed efficiency, milk production and average daily gain. They can also predispose the animal to disease by weakening the immune system as well as causing anemia, diarrhea and respiratory problems.

Lungworms live in the air passages leading to the lungs. Although mature cattle may develop a resistance to the parasite, calves may have difficulty breathing due to blockages of mucus and worms in the respiratory tract. Roundworms inhabit the abomasum (fourth stomach) and intestinal tract. Some abomasal worms attach to the stomach wall and suck blood, which can result in anemia. Others erode the stomach lining and interfere with the digestive process. Some intestinal worms consume nutrients after digestion and reduce feed efficiency. Although parasites are present throughout the pasture season, it is often not until late summer or fall that unthrifty or light calves become most obvious. Parasites often cause the biggest problem when the animal undergoes nutritional (ration changes or nutrient deficiencies) or physiological stress (such as pregnancy, calving or weaning). At these times, the immune system can be overwhelmed and illness results.

Although parasite problems are often treated in the fall, it may be more effective to address the issue in spring to reduce parasite shedding onto pasture by adult cattle. This will help to prevent the calves from becoming infected during the grazing season.

Parasites have complex life cycles. Some developmental stages take place in the animal itself, with others occurring in the manure. Parasite infections are spread by the consumption of feed or water that has been contaminated by manure. Some parasites are capable of surviving the winter in manure or can assume a dormant state in the animal. This allows the parasites to resume the cycle of infection the following grazing season.

The key to parasite control is to eliminate as many parasites as possible from infected animals and to reduce the risk of uninfected animals (such as calves) from becoming infected. There are several ways to accomplish this:

Stocking Rates

Since high stocking rates may force cattle to consume contaminated forage, grazing fewer animals on the same land base will reduce the spread of infection and make it easier to break the parasite cycle.

Pasture Rotation

Similarly, resting a pasture for a period of time will allow sunlight to dry, heat and help kill parasite larvae between grazing periods.

Sanitation

Using pasture watering systems that prevent cattle from drinking directly from streams, ponds or dugouts will prevent the cattle from contaminating the water source.

Anthelmintic Treatments

In cases of heavy parasite loads, it is important to treat the entire herd. A number of parasiticide products are given in the table below. Although there are many different products available on the market, they fall into two main classes. Products such as fenbendazole and levamisole have been around for many years. These products typically kill adult parasites or larvae that are present when the treatment is administered. These chemicals are not retained in the body, and have a short pre-slaughter withdrawal period. However, they do not provide much residual control over parasites that are not in the susceptible stage within a few days of treatment. Some larvae remain and mature into adult parasites and perpetuate the infection. Consequently, repeat treatments are recommended within two to four weeks.

The second class of products are the avermectins (eg. abamectin, doramectin, ivermectin, eprinomectin and moxidectin). These substances also kill parasites at a certain stage of development. However, these products are stored in the body and are released for a longer period of time. This allows the parasite cycle to be interrupted more effectively. These products are more costly but do not need to be readministered as frequently.

When to Treat Cattle for Grubs

Pour on avermectin class antiparisiticals effectively control all stages of cattle grubs. However, proper timing of treatment is important. For the most effective results, cattle should be treated as soon as possible after the end of the heel fly (warble fly) season. Destruction of Hypoderma larvae (cattle grubs) at the period when these grubs are in vital areas may cause undesirable host-parasite reactions. Killing Hypoderma lineatum (comon cattle grub) when it is in the esophageal tissues may cause bloat; killing H bovis (northern cattle grub) when it is in the vertebral canal may cause staggering or paralysis. Cattle should be treated either before or after these stages of grub development.

Treatment of cattle with organophosphate or avermectin compounds at levels toxic to migrating warble grubs should be avoided in Manitoba from November 1 to mid-February since severe reactions may occur due to the death of larvae in the wall of the esophagus or spinal canal.

For further information and advice on a planned parasite control program, consult your veterinarian.

Table 1. Products for internal parasite control in cattle on pasture

x = parasite killed; numbers refer to the licensed labelled claim for days of effective control from re-infection
¹Source: 1997 Compendium of Veterinary Products. Table 1 does not substitute reading of complete product guide
²Includes Levasol, Ripercol, topasol and Trmisol products
³Withdrawal time varies with the form of Safeguard used
⁴Includes Bannminth and Exhelm products
⁵Consult with your veterinarian to identify the most appropriate product(s) for your operation

Prepared by Melinda German and Dr. Terry Whiting, Manitoba Agriculture, Food and Rural Initiatives.

For More Information contact your Local Specialist or MAFRI GO Teams Office or Centre.