Genetic Resistance to Nematodes in Goats: Heritability and Breeding Strategies

Last Updated: January 22, 2026
Estimated reading time: 6 minutes
Word count: 1285

Sustainable livestock management is increasingly moving away from reliance on chemical drugs toward exploiting the innate biological potential of animals. In the fight against the parasitic nematode Haemonchus contortus, genetic resistance offers a permanent, cumulative solution. By identifying and breeding animals that naturally suppress parasite burdens, veterinarians and animal scientists can develop healthier herds. This post explores the genetic parameters, heritability estimates ($h^2$), and the critical role of sire selection in Indian goat breeds as detailed in the research findings.

Key Takeaways:

• Heritability: Resistance to worm infection (measured by egg counts) is a moderately heritable trait in Jamunapari goats ($h^2 \approx 0.20-0.30$), making selective breeding viable.
• Sire Effect: The father (sire) has a statistically significant impact on the resistance of the offspring, confirming that superior bucks can improve herd immunity.
• Breed Variation: While Sirohi goats are naturally more resistant, they showed lower heritability ($h^2 \approx 0.08$), suggesting less genetic variation available for selection compared to Jamunapari.
• Selection Marker: Log-transformed Faecal Egg Count (LFEC) is the most effective phenotypic marker for identifying genetically superior animals.
• Age Factor: Genetic expression of resistance varies with age; heritability estimates differ between kids (upto 12 months) and adults.

GENETIC RESISTANCE AND BREEDING STRATEGIES AGAINST HAEMONCHUS IN GOATS

Heritability Estimates of Resistance Traits

The core of any breeding program is determining how much of a trait is due to genetics versus the environment. This study calculated the heritability ($h^2$) of genetic resistance by analyzing Faecal Egg Counts (FEC) from 498 progenies of Jamunapari and 104 progenies of Sirohi goats. The data was transformed into Log scales (LFEC) to normalize the distribution. The results provided a promising outlook for genetic improvement, particularly in the Jamunapari breed.

“The heritability estimates for LFEC of Jamunapari kids (upto 12 months) and adults (>12 months) were observed separately… adults (0.20±0.07)” (Agrawal, 2009, p. 117).

For Jamunapari goats, the pooled heritability estimate was found to be 0.30 ± 0.14. In quantitative genetics, a heritability value between 0.20 and 0.40 is considered “moderate.” This implies that a significant portion of the variation in worm egg counts is passed down from parents to offspring. Therefore, if farmers select goats with the lowest egg counts as parents, the next generation will statistically have fewer worms. In contrast, the Sirohi breed showed a low heritability of 0.08 ± 0.07, indicating that in this specific herd, environmental factors played a larger role than genetics, or that the breed has already reached a “plateau” of natural resistance.

Student Note / Exam Tip: Heritability ($h^2$) is a ratio from 0 to 1. A value of 0.2 to 0.3 for parasite resistance is standard in literature; it means ~20-30% of the resistance is due to additive genetic effects.

Professor’s Insight: The low heritability in Sirohi doesn’t mean they aren’t resistant—it means they are uniformly resistant (low variation). Breeding progress is fastest where variation is highest (Jamunapari).

Breed	Category	No. of Observations	Heritability ($h^2$)
Jamunapari	Upto 12 months	221	0.18 ± 0.18
	Adults	350	0.20 ± 0.07
	Pooled data	498	0.30 ± 0.14
Sirohi	Pooled data	114	0.08 ± 0.07

Fig: Heritability estimates for LFEC in Jamunapari and Sirohi goats (Reformatted from Table 11, p. 88).

The “Sire Effect”: Selecting Superior Bucks

In livestock breeding, the male (sire) contributes 50% of the genetics to the herd but can produce hundreds of offspring, making his genetic value disproportionately important. The study utilized Harvey’s Mixed Model analysis to isolate the random effect of sires on the LFEC of their progeny. The analysis revealed a highly significant effect of sires in the Jamunapari breed ($P<0.01$), meaning some fathers consistently produced “worm-resistant” kids while others produced “susceptible” kids.

“The significant effect of sires on this trait indicated that superior bucks could be used effectively for enhancing the resistance to nematode infection” (Agrawal, 2009, p. 117).

Table 13 in the thesis displays the “Sire wise least squares mean” for Jamunapari goats. The Log FEC values for progenies of different sires ranged widely, from as low as 4.50 to as high as 7.06. This wide variation is the “raw material” for selection. A breeder using Sire #5287 (mean LFEC 4.50) would drastically reduce pasture contamination compared to using Sire #21 (mean LFEC 7.06). Conversely, the sire effect in Sirohi goats was non-significant ($P>0.05$), reinforcing the finding that genetic variation for this trait is narrower in the Sirohi population studied.

Student Note / Exam Tip: Sire Selection is the most efficient tool for genetic progress because selection intensity can be much higher in males (you need fewer males than females to reproduce).

Professor’s Insight: Practical application: If you can’t test every animal, test the bucks. A buck with a low Faecal Egg Count Breeding Value (EBV) is a valuable asset worth investing in.

Non-Genetic Factors Affecting Expression

While genetic resistance is the target, phenotypic expression is heavily influenced by non-genetic factors. The study extensively analyzed these “environmental” variables to ensure the genetic estimates were accurate. Significant factors included Age, Sex, and Season of Collection.

“The least squares means of LFEC at <3, >3-6, >9-12 and >12 months of age were significant (P<0.01) variation observed” (Agrawal, 2009, p. 84).

• Age: Kids aged 6–9 months showed the highest susceptibility (highest LFEC), likely due to weaning stress and the loss of maternal antibodies before their own acquired immunity fully developed. Adults were generally more resistant.
• Sex: Females were found to be significantly more susceptible than males (Mean LFEC 6.03 vs 5.72 in adults). This is often linked to the physiological stress of production (milk/pregnancy).
• Season: As established in previous sections, the rainy season spiked egg counts.
  Understanding these factors allows geneticists to “adjust” the data. For example, a female’s egg count might be mathematically lowered to compare it fairly with a male’s count when estimating breeding values.

Student Note / Exam Tip: Phenotype ($P$) = Genotype ($G$) + Environment ($E$). Statistical models “remove” $E$ (like age/season effects) to see the true $G$.

Professor’s Insight: The finding that females are more susceptible contradicts some sheep studies where males are more parasitized due to testosterone. In goats, the stress of high milk production in females seems to be the dominant factor.

Real-Life Applications

1. Buck Certification Programs: Breeding associations can issue “Worm Resistant” certificates for bucks based on the LFEC of their progeny (Progeny Testing). This adds market value to animals that require less deworming.
2. Culling Strategies: In Jamunapari herds, identifying and culling the offspring of “high-shedder” sires can rapidly lower the overall farm contamination rate.
3. Integrated Management: Since heritability is moderate (not high), genetics cannot replace hygiene. It must be combined with rotational grazing. However, over 5–10 years, genetic selection reduces the need for chemical dewormers, delaying the onset of anthelmintic resistance.
4. Breed Deployment: For low-input farms where veterinary care is scarce, recommending Sirohi goats is scientifically sound as they possess a naturally fixed high resistance, whereas Jamunapari require active genetic management to handle worm burdens.

Why this matters: Understanding quantitative genetics transforms a veterinarian from a simple “drug dispenser” into a “herd health architect,” capable of designing long-term solutions for sustainable food production.

Key Takeaways

• Selection Works: Breeding for lower egg counts is scientifically validated for Jamunapari goats ($h^2 = 0.30$).
• Buck Power: The genetics of the sire are the primary driver for herd improvement in resistance traits.
• Data Transformation: Raw parasite counts must be converted to Log scales to accurately measure genetic resistance.
• Environmental Noise: Age and sex significantly skew resistance data and must be statistically accounted for.
• Long-term Strategy: Genetic resistance is a slow but permanent fix, unlike dewormers which provide only temporary relief.

MCQs

1. What was the estimated heritability ($h^2$) of Faecal Egg Counts in the pooled data for Jamunapari goats?
   A. 0.08 ± 0.07
   B. 0.30 ± 0.14
   C. 0.55 ± 0.10
   D. 0.75 ± 0.05
   Correct: B
   Difficulty: Moderate
   Explanation: The study found a moderate heritability of 0.30 for the Jamunapari breed, indicating that selection would be effective (Agrawal, 2009, p. 88).
2. Which non-genetic factor was found to significantly influence the LFEC values in the study?
   A. Coat colour
   B. Horn length
   C. Age of the animal
   D. Ear shape
   Correct: C
   Difficulty: Easy
   Explanation: Age was a significant source of variation, with kids aged 6-9 months showing higher susceptibility compared to other age groups (Agrawal, 2009, p. 84).
3. Why were Faecal Egg Counts transformed to Loge (FEC+100) before genetic analysis?
   A. To make the numbers look bigger
   B. To correct for the skewed (non-normal) distribution of parasite data
   C. To account for different goat breeds
   D. To estimate the weight of the worms
   Correct: B
   Difficulty: Challenging
   Explanation: Parasite egg counts are highly aggregated (not normally distributed). Transformation normalizes the data, which is a prerequisite for the Least Squares Analysis used to estimate heritability (Agrawal, 2009, p. 71).

FAQs

Q: Can we breed goats that are completely immune to worms?
A: Unlikely. The goal is “resistance” (low worm burden) or “resilience” (productivity despite worms), not total immunity. Genetic selection reduces the burden but doesn’t eliminate it entirely.

Q: Which breed is better for a farmer who cannot afford dewormers?
A: Based on this study, Sirohi goats are better suited for low-input systems as they maintain naturally lower egg counts and are less dependent on genetic selection to achieve resistance.

Q: How long does it take to improve resistance through breeding?
A: With a heritability of 0.20-0.30, noticeable improvement can be seen in 1–2 generations (about 3–5 years), provided strict sire selection is maintained.

Lab / Practical Note

Ethics Note: Genetic studies often require keeping animals untreated to measure their natural worm burden (“challenge”). This raises ethical concerns. Researchers must balance the need for data with animal welfare, often by setting a “rescue treatment” threshold if an animal becomes severely anaemic (e.g., PCV < 15%).

External Resources

• ScienceDirect: Breeding for disease resistance in small ruminants
• NCBI: Genetics of host resistance to nematodes

Sources & Citations

Thesis: Comparative Study on Immune Response and Resistance Status in Indian Goat Breeds Against Haemonchus contortus Infection, Ms. Nimisha Agrawal, Supervisor: Dr. D.K. Sharma, Central Institute for Research on Goats (CIRG), Makhdoom, Mathura, 2009. Pages 71, 84, 88, 117.

Disclaimer: This summary was assisted by AI and verified by a human editor. It is intended for educational purposes only.

Author: Ms. Nimisha Agrawal (Ph.D. Candidate), Central Institute for Research on Goats.
Reviewer: Abubakar Siddiq