The text below each header is excerpted from the publication Helminth Parasites in the New Zealand Meat & Wool Pastoral Industries: A Review of Current Issues, by P.V. Rattray, September 2003. This is the most comprehensive study we have found on the issue of small ruminant parasitism.

Livestock Have Become Dependent on Wormers

"Prior to the introduction of modern anthelmintics, selection policies on most farms would have favoured animals that had the greatest ability to withstand internal parasite challenge and infection (Bisset et al., 2001). The intensive anthelmintic treatment programmes practiced over the last 2-3 decades have greatly reduced the influence of nematodes on animal performance and has encouraged keeping replacement animals that otherwise would have been culled."

Wormer Dependency is Unsustainable

"Serious consideration of breeding for such factors in NZ only became clear after recognition that our heavy reliance on anthelmintic use for worm control in livestock could not be sustained much longer, due to the growing anthelmintic-resistance problem and consumer demands to reduce chemical residues in meat."

New Zealand Is On The Cutting Edge

"Progress towards breeding sheep that are not affected by worms is much more advanced in NZ and Australia than in other countries (Vipond, 1998). This reflects the faith, particularly among NZ farmers in genetic improvements as permanent remedies to production problems."

Western Oregon Faces The Same Parasites as New Zealand Farmers

"The predominant nematode genera present in the NZ studies have generally been Trichostrongylus and Ostertagia, significant numbers of Nematodirus also being present in studies undertaken in southern regions of the South Island."

Lice and Worms Affect Goat Immune Systems Similarly

"Australian researchers have recently demonstrated significant phenotypic correlation between resistance to lice and resistance to both natural and artificial challenge with T. vitrinus (Kahn & Watson, 2001). In addition, the immunological mechanisms that mediate protection against arthropods and helminth parasites appear to be similar."

Resistance (Immunity) vs. Resilience (Tolerance)

"The concept of 'resilience' is confusing for the farmer. It is in fact 'tolerance' to infection (Bisset et al., 2001) and is the growth advantage under challenge compared with growth rate not under challenge (Vipond, 1998). The disadvantage of selection for resilience is that there is no overall effect on FECs, and individuals with lower resilience can receive a high challenge from contaminated pasture. In contrast selection for low FEC protects the individuals with lower resistance because the larval challenge is lower (Vipond, 1998; Sykes & Coop, 2001)."

"The main benefits derived from breeding for resistance (i.e. low FEC) in dual-purpose sheep are expected to be gained indirectly as a result of reduced pasture contamination, while the main benefits derived from breeding for resilience are likely to be gained through the improved ability of individuals to maintain health and productivity under challenge (Bisset et al., 2001)."

No Correlation Between Resistance and Resilience

"The genetic correlation between FEC and resilience as measured by total drenches is low (-0.17) and not significantly different from zero (Bisset et al., 1996b)."

"The relationship between resistance to roundworm infection and productivity in Romney sheep challenged with Ostertagia sp. and Trichostrongylus sp. in NZ is not simple (Bisset et al., 2001) and NZ section studies for improved resistance have not resulted in increased resilience (McEwan et al., 1995; Morris et al., 1997; Williamson et al., 1997)."

Resistance Does Not Lead to Increased Productivity

  'The key question is, 'will more resistant sheep necessarily be more productive sheep?' (Sykes & Coop, 2001). Highly resistant animals have a very effective immune system, which results in significant nutritional demand for the immune response, which takes priority over other productive functions (Sykes & Coop, 2001). The most resistant (immune) animals may not be the most resilient and most productive."

"Lambs with low FECs do not always perform better than lambs with high FECs and lambs which perform well when undrenched do not always have low FECs. The production benefit from selection of animals for improved resistance in the next generation may be due to reduced egg output by the current host population, reducing the larval challenge to the next generation (Bishop & Stear, 1997)."

"Unfavourable responses or side effects have been recorded in the resistant line compared with the susceptible line, for dag scores, fleece weight (in lambs, hoggets and ewes) and post-weaning weight gains (Bisset et al., 2001) (see next section).  It is probable that these differences reflect the cost to the host of resisting parasitic infection."

"Most NZ studies have indicated a slightly unfavourable genetic correlation between resistance and growth rate under challenge in lambs (Bisset et al., 2001; McEwan et al., 1992; McEwan et al., 1995; Morris et al., 2000, 2002; Heath, 2000).  Resistant genotypes in the selection lines also produce lower yearling and ewe fleece weights than their susceptible counterparts when grazed together (Morris et al., 1997; Morris et al., 2000).'

Another Option - Breeding For Resilience

"Some farmers in NZ were attempting to selectively breed from sheep which were left undrenched as lambs. This prompted AgResearch to reconsider resilience challenge as a breeding option (Bisset et al., 2001). The genetics of resilience to roundworm challenge has been under investigation in NZ since 1991."

"Sires whose male progeny required the fewest drench treatments to maintain acceptable body condition under roundworm challenge (i.e. were more resilient), had female progeny that had above average growth rates and below average dag-scores under challenge. Drench-requirements in male progeny showed no significant correlations with parasite resistance (based on FECs) in their paternal half-sisters, showing that breeding for increased resilience in Romney ram lambs exposed to a mixed nematode challenge in the field resulted in little or no genetic change in their FECs."

Increasing Level of Challenge Increases Resilience Heritability Values

"Overall, heritabilities for resilience traits based on the above drench requirement criteria were relatively low, h2 values ranging from 0.10-0.19 (Bisset & Morris, 1996). Heritability for resilience is low-moderate (Bisset, 1996) and lower than that for resistance. However, estimates depend to some extent on the severity of challenge experienced; considerably higher values (h2 = 0.24-0.53) were derived from one particular flock which had been subjected to much higher levels of cumulative challenge over three breeding seasons than the other flocks (Bisset et al., 2001). In breeding for disease resistance traits, greater genetic progress is likely to be achieved in years (or in flocks) where challenge is high (Campbell, 1986). However this will also result in a lower productivity and/or survival of the animals (Bisset et al., 2001)."

High Challenge Levels Are Impractical In Commercial Herds

"One area for concern over the application of selection on FECs in the UK is the ethical dilemma of having to expose an animal to parasitism in order to quantify its response (Vipond, 1998)."

"The method used by Albers et al. (1987) to assess resilience, i.e. measurement of growth rate in lambs while subjected to roundworm challenge, relative to their growth rate while not subjected to challenge, is generally considered to be impractical to implement under field conditions. In principle, an indication of resilience could be determined by exposing lambs to severe roundworm challenge for prolonged periods and identifying those least affected (Campbell, 1986), but in practice, this option is unacceptable on animal welfare and economic grounds in commercial flocks."