There are numerous dairy studies that suggest that milk production is decreased in the parasitized cow/heifer, but there are also some studies that found little if any impact of parasites on lactation. In a study1 published by Bert Stromberg, PhD, University of Minnesota College of Veterinary Medicine, with Bob Corwin, they found there was a significant increase in milk production in parasite-free beef cattle. “The study used the weigh-suckle-weigh technique and found that anthelmintic-treated cows produced 3.49 lbs. of milk vs. 2.76 lbs. for untreated cows per suckling,” Stromberg says.

The only parasite that we expect to see in cattle being passed from the dam to the calf through milk is Strongyloides spp. “We’ve actually had calves that were days old shedding Strongyloides eggs,” he says. “Fortunately, this is a relatively benign parasite and usually is quite short lived.” Stromberg adds that he has seen 99% of these eggs gone in one month. 

Contamination by the dam

There is always the risk of infection via contamination by the dam to the young calf. The issue becomes one of the time required for the egg to hatch and develop to the infective stage. “These larvae may also contaminate the udder and thus provide another opportunity for ingestion/infection,” Stromberg notes. This is also true for coccidia and Cryptosporidium

“We’ve actually seen Strongyloides in about four days,” he says. “Most other nematode parasites require about the standard three week prepatent period.”

Calves are also amplifiers of parasite eggs into the environment and the greater the distribution the greater the opportunity for spreading the parasites.

Effect on the calf

Worms may be a problem when present in the calf. The very young calf has a developing immune system and is thus very susceptible to most parasites. Remove the worms and the animal recovers quickly. However, this is not always true for coccidiosis. “These parasites particularlyEimeria bovis and E. zuernii can cause so much damage that the animal will never catch up to its cohorts,” Stromberg says. “There are some studies by Fox that would suggest that there is a bit of a compensatory gain when the parasites are removed.”

Parasites can be fatal; however, Stromberg says these are under very poor conditions and with reasonable (perhaps even poor) care this should never happen.   


The best diagnostic test for parasites is still the fecal exam. “The numbers can be important and we recommend a semi-quantitative fecal exam,” Stromberg suggests. “Take a routine fecal sample the size of the last digit of the thumb, run a flotation, centrifugal if you can, and evaluate the numbers. “You can actually count or estimate negative, + = 1 to 10 eggs per cover slip and so on up to ++++.  It offers a means of comparing.”


Even though parasites can be a problem in young calves, Stromberg cautions not to overdo deworming. “Where we used to recommend routine deworming, we now suggest that we only deworm when needed because of the anthelmintic resistance issue.” 

1Stromberg, B.E. & Corwin, R.M. 1993. Epizootiology of Ostertagia ostertagi in cow-calf production systems in the American Midwest. Veterinary Parasitology 46:297-302.