In this article and in the one following in the next issue, we'll discuss some of the concepts and misconcepts of parasite control, both internal and external. I'd like to start by discussing some of the basics and what it is that we're contending with when we endeavor to control these critters.

Worms, Flukes and Other Bugs 101

There are many organisms which act as parasites in cattle. In the United States, veterinarians, producers and economists estimate annual parasite-related losses to the livestock industry at more than $100 million. Most parasite losses are subclinical, and losses go unnoticed, are not measurable and probably far exceed the estimates. University trials have shown paybacks from internal and external parasite control of $25 to $200 per head, which should make effective control one of the first goals of today's cattle producer. Although many types of parasites exist some of the more common are listed below:

� Stomach worms

� Intestinal Roundworms

� Lungworms

� Liver Flukes

� Grubs (heel fly larva)

� Lice, Horn flies, & ticks (ectoparasites)

� Cattle Tapeworms

� Coccidia (coccidiosis)

The existence and prevalence of a given parasite depends on region or location, environmental conditions (rainfall, drought conditions, etc) and so on.

Strickland (1996) discusses that a parasitic relationship exists when one organism (the parasite) profits at the expense of the other (the host). The parasite may harm the host enough to kill it if not controlled. This would end the relationship, so the parasite prefers that the host survive with decreased efficiency. Parasites are normally host-specific, and cattle serve as hosts for a variety of parasites. The major threat to cattle health and performance comes from internal parasitic nematodes (worms), especially those found in the stomach and intestines (gastrointestinal parasites). Today's cattle production is concentrated on heavily populated, permanent pastures where gastrointestinal parasites are abundant. This increases the probability that every worm will successfully reproduce. Most of the internal parasites of cattle are in the abomasum (true stomach) or small intestine.

Table 1 below illustrates many of the common internal parasites which should be treated for in cattle, as well as the most damaging stages.

The most common internal parasites of cattle are known as Brown Stomach Worms or Ostertagia. This type of worm can cause severe symptoms and may produce the highest economic loss. The stomach and intestinal linings, or mucosae, are damaged by irritation and inflammation. Therefore, cattle don't get the proper nutrients, due to the decreased digestive and absorptive ability of the mucosal surface.

The life cycles of the more common cattle internal parasites are direct. Infected cattle pass eggs in the manure, and with favorable weather conditions, the eggs hatch and develop into third-stage, infective larvae in about 14 days. These larvae move from the protective manure up moist grass blades and are eaten as the cattle graze. They then penetrate the gut lining, and mature into egg-laying adults two to four weeks after they're eaten.

The fourth stage in the life cycle of Ostertagia is different. Fourth-stage larvae are able to stay in the stomach glands for up to six months. These are called inhibited or arrested larvae. The ability to inhibit and then leave the stomach glands seems to be triggered by weather or nutritional factors.

The other internal parasites are normally a mixed infection with Ostertagia and cause similar symptoms. With Haemonchus, the Barberpole Worm, both the larvae and adults feed on the blood in the stomach. Small numbers can cause acute symptoms with blood and protein loss. Severe anemia or sudden death will be a noticeable symptom.

Diagnosis of Internal Parasite Infestation or Burden

Clinical signs, grazing history and season may cause us to automatically presume that an internal parasite infection in the herd exists. The diagnosis may be confirmed by finding worm eggs on a fecal exam. Eggs per gram (EPG) of manure is the numerical value used in egg counts. The EPG count is mainly made up of the Haemonchus, Ostertagia and Trichostrongylus complex (HOT complex). The numbers are not given but are rated low, moderate or high, depending on the EPG.

The EPG is not always an accurate indication of the number of adult worms present. EPG counts may be negative or low in the presence of large numbers of immature worms, and even when many adult worms are present the count may be low. An immunity produced by the cow or previous worm treatment at low dosage may lower egg production.

With the development of safe, effective de-wormers, treatment should be used where EPGs are absent or low and symptoms and history may suggest infection. Response after deworming would confirm the diagnosis. Deworming in most areas should be adopted as a standard practice when cattle are worked in the spring and fall.

Availability of larvae on pastures increases progressively from fall through winter to peak levels in spring, a time when temperature and moisture levels are at their best for survival and development of preparasitic larval stages. In many operations cows calve on pasture in spring and their calves stay with them throughout the spring and summer, being weaned in fall. These calves are either sent to feedlots at this stage or weaned and grazed as stocker cattle over the winter.

By the time suckling calves are eating grass, levels of pasture Stage 3 larvae (L3's) are beginning to decline. However, during their last two to three months of nursing, these calves may accumulate substantial worm burdens. At weaning, they should be treated again, although reality dictates that if they are going off to a feedlot they will not be treated at weaning on the farm since their parasites are soon to become someone else's problem. However, feedlots receiving weaned calves should treat these animals with an appropriate dewormer or anthelmintic, depending on origin of the cattle. Calves staying on the farm as replacement heifers or as stocker cattle should be treated at weaning and again once or twice during the subsequent winter and spring. The last treatment should be aimed at eliminating inhibited or arrested larvae that accumulate on late spring-early summer grasses.

Should an animal die from unexplained reasons, postmortem examinations of dead animals should always be performed by your veterinarian. Some larger worms, such as Haemonchus, may be easily seen, but Ostertagia or Trichostrongylus are hard to see unless they're alive and swimming in the stomach fluid. Stomach and intestinal contents can be scraped and washed to identify specific types of worms and get accurate counts. Where internal parasites are severe enough to be the cause of death, postmortems are invaluable in treatment and control methods. Other diseases and conditions may complicate the case, and a postmortem will be the only way to determine the primary cause. Other diseases such as shipping fever, digestive disturbances, salmonella infection and viral diarrhea will show similar symptoms as internal parasite infection.

Conclusions

Many parasites exist which can severely reduce productivity in a beef cattle operation. In the next issue we'll take a look at some of the external parasitic problems beef producers have and also examine treatment strategies for these challenges.