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May 2017

The impact of parasite infections on small ruminant productivity

Dahlia O'Brien

Virginia State University


Clearly, clinical and sub-clinical worm infections can significantly impact animal productivity.  This loss is greater when a drug with reduced efficacy is used to deworm infected animals.

Worm infections can impact the productivity of grazing animals significantly.  For many years, the small ruminant industry has relied heavily on broad-spectrum dewormers to control worms, especially the blood-sucking parasite Haemonchus contortus (barber pole worm). However, the effectiveness of these drugs has been threatened by the development of drug resistance in worm populations on many farms.  While we’re aware of the potentially devastating clinical effects that result from infection with the barber pole worm and others, most of us fail to realize that sub-clinical effects are much more common and could result in a greater loss in production.


There are a number of factors that impact how susceptible grazing sheep and goats are to worms.  For instance, younger animals tend to have more worms, animals grazing a heavily- infested pasture have more worms, animals located in the southern U.S. with its warmer temperatures promoting an optimal environment for worm development will have more worms, some breeds of sheep and goats are more susceptible to infection and will have more worms, and below optimal nutrition can predispose animals to infection and they will have more worms.  Regardless of these factors, parasitism remains the single most significant health issue affecting small ruminant production.

So how exactly does worm infection affect productivity? Worm infections not only reduce feed intake, but affect feed utilization, resulting in devastating effects on growth rates. Consider the blood-sucking barber pole worm. This parasite infects the true stomach or abomasum, pierces the wall, causing blood loss which depletes the animal’s supply of red blood cells and plasma protein.  This leads to the clinical signs of anemia and edema (sub-mandibular) usually observed in animals heavily infected with this worm.

Research studies have shown that chronic infections affect feed intake in lambs and that moderate infections can have a significant effect on milk production in ewes and does leading to slow growth rates in their offspring. These impacts are due to the irreversible loss of protein brought about by blood loss, as well as decreased digestion of organic matter throughout the entire digestive tract resulting from infection. 

Two other parasites of importance in sheep and goats include the brown stomach worm (Teladorsagia circumcincta) and the bankrupt worm (Tricostrongylus colubriformis). The brown stomach worm also infects the abomasum and reduces the availability of nutrients by depressing food intake in sheep and goats.  In addition, this worm damages the wall of the abomasum causing the formation of lesions and leakage of plasma proteins from the blood into the gut. On the other hand, the bankrupt worm, which infects the small intestine, affects the animal by reducing the amount of nutrients it absorbs by impacting the surface area of the small intestinal wall making absorption less efficient. It also causes protein loss primarily as sloughed cells and mucin rather than as blood. Infections with either of these parasites leads to damage and inflammation in the gut resulting in scouring. 

In addition to red blood cell loss, nutrient loss, and reduced nutrient intake caused directly by worm infections described above, nutrients are also diverted from production in order to fight infection intensifying the overall effect. Clinically parasitized animals might appear weak, lose weight/condition, present with loss of appetite, anemia and sub-mandibular edema (bottle jaw), or have scouring. In severe cases if not treated can lead to death. The impact of animal suffering or animal loss on production is very clear.  On the other hand, sub-clinical parasitized animals do not present with clinical


signs of infection, but loss in productivity, even though less obvious, still occurs.  This type of infection can have the greatest impact on small ruminant productivity.  This is because sub-clinical worm infections have been shown to reduce wool production, live weights, body condition scores, and carcass weights in lambs.


With the emergence of drug resistance on many farms, sustainable integrated parasite management techniques provide farmers with tools/strategies that can be employed to minimize the effect of worm infections.  An important strategy in this toolbox is the use of the FAMACHA© system for targeted selective deworming, where animals are only treated if needed (depending on their eyelid anemia score).  It is recommended that farmers know the status of drug resistance on their farms so that they are utilizing a drug that is highly effective when the need to deworm arise.  However, testing for drug resistance can be costly and/or time consuming and due to this, many producers forego this testing.


There are two tests available for determining drug resistance; the fecal egg count reduction test (FECRT) and the Larval Development Assay (LDA; DrenchRite®). Routine testing (every 2 years) is recommended and there is evidence that suggests that the cost associated with this is much less than that associated with using a drug with reduced efficacy to treat infected sheep/goats. 


The economic impact of drug/dewormer resistance has been investigated in lambs in New Zealand.  Two separate studies compared various production traits in a group of lambs treated monthly with either a highly effective dewormer (>99% efficacy) or a dewormer to which resistance was present (<50% efficacy). Following initial treatment, fecal egg counts remained elevated and were always higher in the group treated with the drug with reduced efficacy.  It is important to note that fecal egg counts never went above 1500 eggs per gram for either group. There were also differences noted in fleece weight, live weight, body condition scores and fecal soiling with each more positive when the high efficacy drug was used. 


Another interesting result is that the time taken for animals to reach market weight was 17 days shorter when animals were treated with the highly effective drug, and at slaughter, carcass weights were as much as 10 lbs. heavier on average for the group treated with a highly effective drug.  All data were used to calculate the effect of dewormer resistance on production which was estimated to be a 14% reduction in carcass value in one study.  Even though we never recommend treating all lambs on a monthly basis as done in these studies, their results demonstrated the impact of deworming with a drug to which parasite resistance is present. In addition, since fecal egg counts were relatively low throughout these studies, the results also demonstrate the impact of subclinical parasite infection on animal productivity. 


Clearly, clinical and subclinical worm infections can significantly impact animal productivity.  This loss is greater when a drug with reduced efficacy is used to deworm infected animals. Therefore, it is important to have a parasite control strategy to minimize the impact of worm infections on production and to include a highly effective dewormer/dewormer combination in this program. Profitability and sustainability of the industry depends on this. 


I.A. Sutherland, J. Shaw and R.J. Shaw. 2010. The production cost of anthelmintic resistance in sheep managed within a monthly preventive drench program. Veterinary Parasitlogy (171) 300 – 304.


C.M. Miller, T.S. Waghorn, D.M. Leathwick, P.M. Candy, A-M.B. Oliver and T.G. Watson. The production cost of anthelmintic resistance in lambs. Veterinary Parasitlogy (186) 376 – 381.

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