Effects of Internal Parasites on Replacement Heifers

Linda Benedict  |  4/27/2006 12:40:18 AM

Alvin F. Loyacano, J.C. Wiliams, Jeff Gurie and Andy DeRosa

Beef cattle in Louisiana are constantly infected with gastrointestinal nematodes, commonly referred to as roundworms. The brown stomach worm, Ostertagia ostertagi, is the most troublesome gastrointestinal nematode parasite. Also a problem in much of the Red River basin and the coastal marsh is the bovine liver fluke, Fasciola hepatica, a parasitic trematode. Between 50 percent and 100 percent of yearling and older cattle are infected with this pest. Young cattle (stockers and replacement heifers) are most susceptible to clinical parasitic disease and production loss.

This project was conducted to measure the production losses caused by roundworms and liver flukes in beef replacement heifers, to identify when these losses occur and the losses attributed to each type of parasite.

This study was conducted using 372 Angus- or Brangus-sired beef heifers from the 1993 through 1996 spring calving seasons. After weaning each year (approximately October 15), heifers were randomly allotted to three pasture groups and one of four treatments within each group. Parasite control treatments were as follows: Treatment 1 - no parasite control; Treatment 2 - roundworm control; Treatment 3 - liver fluke control; and Treatment 4 - roundworm and liver fluke control.

Roundworms were controlled with the injectable formulations of Ivomec (Merial) or Dectomax (Pfizer). Both products control the major nematode parasites for at least 21 days. Curatrem (Merial) drench was administered to control liver flukes. Heifers in this trial received no treatments for parasite control before the study. Treatments were initiated at allotment. Since the objective was to minimize the effects of the parasites, treatments were repeated at 28- to 84-day intervals, depending on time of year and risk of infection. The average of seven treatments per year was far in excess of a practical treatment regimen.

Weight and condition
Cattle were weighed and condition-scored at each treatment date. Fecal samples were collected per rectum from about half of each group. Parasite infection was monitored by fecal egg counts. Heifers remained in pasture groups until they were palpated for pregnancy diagnosis in October. Pregnant heifers were pooled at that time and pastured together through their first calving season and until their calves were weaned the next October. Calves were born during the spring calving seasons from 1995 through 1998. Calf weights were recorded at birth and at weaning.

Initial weights for the treatment groups were the same. By the beginning of the breeding season, heifers not treated for roundworms (Treatments 1 and 3) had gained significantly less and were significantly lighter than heifers treated for roundworms (Treatments 2 and 4). This relationship was maintained throughout the breeding season.

At palpation, heifers treated for roundworms and liver flukes (Treatment 4) had gained more than those heifers treated for only roundworms (Treatment 2), and heifers treated for only roundworms gained more than those not treated for roundworms. Condition scores also were affected by parasite infections. At the beginning of the breeding season, heifers that had not been treated for roundworms had lower scores than those treated for roundworms. By the end of the breeding season, heifers that had not been treated for either parasite (Treatment 1) had significantly lower scores than heifers that had been treated for flukes (Treatment 3). Heifers treated for only liver flukes had lower scores than heifers treated for roundworms. At palpation, heifers that received neither treatment had significantly lower scores than heifers treated for only one of the parasites. Heifers treated for both parasites had significantly higher condition scores than heifers treated for only one of the parasites.

Pregnancy rates
Parasite infections significantly affected pregnancy rates. Heifers repeatedly treated for both types of parasites had a 78 percent pregnancy rate. Heifers treated for flukes but not roundworms had a 66 percent pregnancy rate. Heifers treated for roundworms, but not liver flukes, had a 63 percent pregnancy rate, which was significantly lower than that of heifers treated for both parasites. Heifers not treated for either parasite had a 53 percent pregnancy rate, also significantly lower than that of heifers treated for both parasites. The parasites seemed to have a cumulative effect in that heifers treated for roundworms and liver flukes had higher pregnancy rates than heifers treated for only roundworms.

Treatment had a significant effect on calf birth weight. Calves from heifers treated for both parasites had heavier birth weights than those from heifers not treated for roundworms. At weaning, calves from heifers that had been treated for both parasites had the highest average weaning weights. Heifers treated for only one of the parasites produced calves with average weaning weights that were not significantly lower than those from heifers treated for both parasites. Calves from heifers that were not treated for either parasite were significantly lighter than calves from heifers that were treated for both parasites. Adjusted weaning weights followed the same pattern.

Initial nematode egg counts ranged from zero to 600 eggs per gram and were not different between treatments. At the beginning of the breeding season, an average of 46 days post treatment, heifers treated for nematodes had significantly lower average egg counts than heifers that were treated for flukes or that received no treatment. At the end of the breeding season and at palpation (60 to 84 days post treatment), there were no significant treatment effects on the nematode egg count. It must be emphasized that egg counts were not used to measure the efficacy of treatments. They were used to monitor infections and to demonstrate that heifers repeatedly reinfected with nematodes may have suffered some losses to nematode infections, despite repeated prophylactic treatments.

Initial liver fluke egg counts were not different between treatments and ranged from zero to 6.5 eggs per gram. The prevalence or rate of infection was 30 percent. At the initiation of the breeding season and throughout the rest of the study, heifers treated for flukes had significantly lower egg counts than untreated heifers. Fluke transmission is seasonal, occurring primarily in the spring and fall with a life cycle of up to 12 weeks. Animals that receive an effective treatment when transmission is not occurring can remain fluke-free for extended periods. Infection rates for heifers not treated for flukes were 41 percent at breeding and 80 percent at palpation. Infection rates for heifers treated for flukes were 7 percent at breeding and 35 percent at palpation.

In this study, roundworms reduced body weights and condition scores for yearling heifers throughout the test period. Fluke infections reduced body condition scores and weights by the end of the breeding season and reduced pregnancy rates. Heifers treated for both parasites had the highest pregnancy rates. Roundworm infections reduced calf birth weights. Treating heifers for roundworms increased the weaning weights of their calves. Treating for flukes resulted in weaning weights intermediate between those of heifers treated for roundworms and those receiving no treatment.

Roundworms, liver flukes affect growth
In the Red River region of central Louisiana, both roundworms and liver flukes significantly affect the growth and production of replacement heifers. Many heifers were obviously suffering from some parasitic disease before initiation of the trial each year. While treating for liver flukes did not increase heifer gains before the end of the breeding season, treating for roundworms resulted in increased gains during the first weigh period. Therefore, it must be assumed that roundworms were the major cause of the pre-trial effects of parasitism. This indicates that roundworms affected the heifers earlier than liver flukes did and that the optimum times for initiating treatment of the parasites is probably much earlier for roundworms. When both parasites are present, their effect seems to be cumulative.

In fluke-endemic areas, the strategic treatment of replacement heifers for both parasites is recommended. Timely treatments for roundworms should begin at least at weaning. Fluke control for spring-born heifers should be initiated by at least the beginning of their first breeding season. A product that controls both roundworms and liver flukes would be a good choice for use on younger heifers. Older animals might benefit more from timely treatments with a more effective flukacide and a broader spectrum endectocide.  

(This article was published in the winter 2000 issue of Louisiana Agriculture.)
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