Free living larvae cannot survive the cold temperatures encountered during
winters in the northern U.S. As a result, kidney worm infections are more common in the
southern United States.
Infected older sows and boars are the usual source of infection for
younger animals and the longevity of adult worms in their host (2-3 years) means that
adult infected pigs may be constant sources of infections for young animals.
Batte and his colleagues demonstrated transplacental transmission in
pregnant gilts but it is uncertain how important this is as a factor in transmission.
The dependency of the life cycle on free living preparasitic larvae and
earthworms as transport hosts means that this is a parasite primarily seen in pigs raised
outdoors. Raising pigs indoors will significantly reduce the prevalence of kidney worms.
The "gilts only" system of breeding and rearing pigs was devised
as an effective method to control swine kidney worms. In this system, breeding is confined
to gilts which are then marketed immediately after weaning their first litters. The long
prepatent period of 9-16 months means that pigs are raised , bred and marketed before
infections become patent and this system was shown to be an effective method of
controlling transmission of Stephanurus dentatus. The problem is that the system
fails economically since gilts have smaller litters than older sows and the economic
return from these smaller litters is not enough for the system to be viable.
Overt clinical signs are unusual in
Stephanurus infections. Aberrant migrations of larvae to the spinal cord may cause a
posterior paralysis but these cases are not common.
The usual picture is one of failure to thrive with the primary effects on
weight gains. Heavy infections in pigs raised outdoors may cause enough liver damage to
result in emaciation and death. The image to the right is of a pig showing
chronic effects of Stephanurus dentatus infections.
were present in the urine and at necropsy there were extensive liver lesions and adult
worms in the perirenal areas.
Image courtesy of Merial Inc.
|The long prepatent period of Stephanurus means that patent infections are
unusual in pigs less than two years old. Infections in adult sows and boars are easily
diagnosed by finding the distinctive strongyle-type egg in urine of infected animals - as
shown in the accompanying image. Infections often go undiagnosed in live animals,
Image courtesy of Dr. Edward Batte
| pigs less than a year old. Therefore infections are more often diagnosed at
necropsies or at meat inspections when the characteristic liver lesions are found.