HERD HEALTH
PIH-44
pork
industry
handbook
COOPERATIVE EXTENSION SERVICE • PURDUE UNIVERSITY • WEST LAFAYETTE, INDIANA
Internal Parasites of Swine: Natural History and Control
Authors
Arlie Todd, University of Wisconsin
Robert Behlow, North Carolina State University
Ed Batte, North Carolina State University
Reviewers
Ralph F. Hall, University of Tennessee
Gerald M. Sandidge, Marshall, Missouri
It is well known and accepted that swine are damaged by many kinds of internal parasites (worms). Considerable financial loss results from retarded growth and the failure to convert feed efficiently, from increased susceptibility to other diseases and from organ and carcass condemnations.

These parasites or worms are the same all over the United States. There is no climatic barrier to parasitism. The most prevalent worm is the "large round worm," Ascaris suum; the next most prevalent is the nodular worm. These are followed by whipworms and stomach worms. Two types of lungworms are found throughout the U.S. Threadworms (Strongyloides) probably parasitize every pig. In some parts of the South the large kidney worm is prevalent. All of these worms are roundworms.

In the last 20 years worms that use beetles as intermediate hosts —certain stomach worms (roundworms) and the thornyheaded worm—have declined in the Midwest.

Natural History and Chronology
The swine breeding herd has its own population of internal worms, and these survive for months and years in the external environment, on the pastures and in lots, in the farrowing houses and gestation facilities. Older animals do not become immune to worm infection; rather, they redevelop parasitisms semiannually as they go back and forth from the farrowing house to their pastures and lots. Although sows and gilts have larger populations of nodular and stomach worms, it is a mistake to believe that they do not have large roundworms (Ascaris suum) and whipworms. Older animals (sows) have more kidney worms than gilts.

There is little evidence of prenatal worm infection in pigs. There is transcolostral transmission (in first milk) of the tiny little threadworm, Strongyloides. A pig’s first exposure to worms is to the infective larvae of Strongyloides in colostrum. This has been reported widely in the southeastern U.S. Second possible exposure to
worms occurs when pigs eat Ascaris (the large roundworm) eggs, and/or stomach and nodular worm larvae in old manure from sows, found in and around the farrowing crates and pens where pigs spend the first three or four weeks of their lives. Newly weaned pigs eat more infective Ascaris and whipworm eggs in the manure which may be present in the nursery or growing-finishing lots or pens.

There is a universal prevalence of worms in 20-30 lb. pigs. Usually the first Ascaris (large roundworm) infection in pigs is the largest one. This is followed by the major whipworm parasitisms. In some sections of the U.S., whipworms cause mortalities in pigs weighing less than 20 lb. When pigs reach 70-90 lb. there is a rise in the numbers of nodular worms and stomach worms.

Many pigs now are grown to 35-40 lb. with only a few Ascaris (large roundworms) or no worms at all, but when they are put into growing pens or dirt lots they encounter overwhelming exposures. The large roundworm (Ascaris) population increases along with that of the whipworm, the nodular worm, and the stomach worms. Weaned pigs placed on dirt lots for finishing rapidly acquire considerable populations of fvee or six different kinds of worms. Permanent pasture lots are where lungworms persist in their intermediate hosts, earthworms. Pigs have almost no ability to resist worm parasitisms until they are well past 100 lb.

Infective stages of worms are resistant to all kinds of weather conditions and external environments—where they are eaten by pigs. There are enormous populations of infective Ascaris (large roundworm) and whipworm eggs, and infective larvae of nodular and stomach worm larvae, on swine lots and pastures. Few swine herds are free of parasitisms.

Rationale for Treatment and Eventual Control
There is a widespread and unfortunate misimpression that worm parasitisms are similar to infectious diseases. Historically, much effort has been directed toward enabling
Cooperative Extension Work in Agriculture and Home Economics, State of Indiana, Purdue University and U. S. Department of Agriculture Cooperating. H. G. Diesslin, Director, West Lafayette, Ind. Issued in furtherance of the Acts of May 8 and June 30, 1914. It is the policy of the Cooperative Extension Service of Purdue University that all persons shall have equal opportunity and access to its programs and facilities without regard to race, religion, color, sex or national origin.