18 CIRCULAR 148, U. S. DEPARTMENT OF AGRICULTURE 
These parasites multiply very rapidly, the entire development on 
the ground taking place in a few days. The infective larve, whether 
they hatched from the eggs eliminated with the feces or from the 
eggs produced by the free-living generation of worms, are taken in 
by horses with food and are also capable of penetrating the intact 
skin. Within about two weeks after the larve have been taken up 
by horses they have developed into mature females which soon begin 
to discharge eggs into the intestine of the host. 
Symptoms and lesions—While no definite symptoms have been 
described in horses as resulting from infestation with these parasites, 
the worms are prob- 
ably responsible for 
diarrhea in foals. 
These worms make 
their appearance in 
foals earlier than do 
other parasites, and 
the infestation tends 
to become heavy as a 
result of reinfesta- 
tions which may take 
place rapidly and 
often, because of the 
brief period required 
for development out- 
side of the host. The 
parasites are usually 
embedded in the lin- 
ing of the intestine 
and doubtless produce 
considerable irrita- 
tion. The fact that 
the infective larve 
are capable of pene- 
trating the skin adds 
another complication 
and it is probable that 
foals acquire part of 
their infestation while 
lying down on _ pas- 
tures and in their 
Rroume 11 EDS ok anita, bok eral Nee Fete alla egies 
penetrate the skin it 
becomes reddened and obscure skin troubles in horses may be caused 
by the penetration of these worms. The larval worms may carry 
pathogenic bacteria into and through the skin. 
Treatment.—Nothing is known concerning effective treatment for 
these threadworms. In severe cases of diarrhea, in foals, caused by 
this parasite, carbon tetrachloride is worthy of trial. The drug may 
be used for foals at a dose rate of 0.1 cubic centimeter per kilogram 
(2.2 pounds) of body weight or 10 cubic centimeters (2.5 fluid drams) 
for a 220-pound animal. In order to remove the bulk of ingesta in 
the small intestine and render the parasites more accessible to treat- 
ment, it is advisable to administer a purgative 24 hours before treat- 
