Nematodirus

Nematodirus is a parasitic infection caused by thread-necked strongyle worms of the genus Nematodirus, with Nematodirus battus being the most pathogenic and economically significant species affecting sheep. This parasitic condition primarily targets the small intestine of young lambs, causing severe damage to the intestinal lining and resulting in profuse watery diarrhea, rapid weight loss, and potentially death if left untreated. The disease is particularly devastating because it can strike suddenly in naive lambs that have had no previous exposure to the parasite, overwhelming their undeveloped immune systems before clinical signs become apparent to producers.

Nematodirus infections predominantly affect lambs between six and twelve weeks of age, though calves and goat kids can also become infected with various Nematodirus species. The condition occurs worldwide in temperate climates where sheep production is prevalent, with particularly high incidence rates reported in the United Kingdom, Ireland, northern Europe, and parts of North America and Australasia. The unique lifecycle of Nematodirus battus, which involves a prolonged development period within the egg before hatching, creates predictable seasonal patterns of disease that typically coincide with spring turnout of lambs onto contaminated pastures.

The economic and welfare impact of nematodirus infection on sheep operations can be substantial, with outbreaks causing significant mortality rates in untreated flocks, sometimes exceeding thirty percent of affected lamb crops. Beyond direct mortality losses, subclinical infections reduce growth rates, decrease feed conversion efficiency, and delay lambs reaching market weight, all of which translate into reduced profitability for producers. The welfare implications are equally concerning, as affected lambs experience severe abdominal discomfort, dehydration, weakness, and distress that compromises their quality of life during the acute phase of infection.

Early detection and prompt treatment of nematodirus infection are critical for minimizing losses and improving outcomes in affected lambs. Unlike many other gastrointestinal parasites, nematodirus causes severe clinical disease before significant numbers of eggs appear in fecal samples, making traditional fecal egg count monitoring less reliable for prediction. Producers and veterinarians must therefore rely on forecasting models, pasture history, and clinical vigilance to identify at-risk animals and implement timely treatment protocols. With appropriate anthelmintic therapy administered at the correct time, prognosis for affected lambs is generally good, though prevention through strategic pasture management and targeted treatment remains the cornerstone of effective nematodirus control programs.

The primary cause of nematodirus infection is ingestion of infective third-stage larvae (L3) of Nematodirus species, most commonly Nematodirus battus in sheep and Nematodirus helvetianus in cattle. These parasitic roundworms belong to the family Molineidae and are characterized by their distinctive thread-like anterior end, which gives rise to their common name of thread-necked strongyles. Infection occurs when susceptible young animals graze on pastures contaminated with infective larvae that have developed within eggs shed by previously infected animals during the prior grazing season, creating a cyclical pattern of environmental contamination and host infection.

Unlike most gastrointestinal nematodes, Nematodirus battus has a unique lifecycle that contributes significantly to the explosive nature of disease outbreaks. The eggs passed in feces of infected animals require an extended period of cold exposure followed by warming temperatures to trigger synchronous hatching, a process that can take up to twelve months. This means that larvae consumed by lambs in spring originated from eggs deposited by the previous year's lamb crop, and the simultaneous hatching of massive numbers of larvae creates conditions for overwhelming infections in naive lambs. The requirement for specific temperature triggers means that nematodirus risk can be predicted based on weather patterns, leading to the development of regional forecasting systems.

Environmental and management factors play crucial roles in determining the severity of nematodirus challenge on individual farms. Pastures that were grazed by lambs during the previous season will harbor overwintered eggs that pose a risk to the current lamb crop, with contamination levels directly related to stocking density and grazing duration. Wet, mild springs following cold winters create optimal conditions for mass larval hatching, while unusually warm winters or dry springs may reduce or delay the parasite challenge. Permanent sheep pastures and fields used repeatedly for lambing carry higher contamination loads than rotational grazing systems that incorporate rest periods or alternation with cattle.

Young lambs between six and twelve weeks of age face the highest risk of clinical nematodirus disease due to their immunological naivety and the timing of pasture exposure relative to larval hatching. Lambs born earlier in the season may encounter larvae before their immune systems have matured sufficiently to mount protective responses, while those born later may be exposed to higher larval challenges as hatching peaks. Nutritional status influences susceptibility, with poorly nourished lambs from twin or triplet births or those nursing ewes with inadequate milk production showing increased vulnerability to parasitic challenge and reduced ability to withstand the metabolic demands of infection.

The pathophysiology of nematodirus infection involves direct damage to the small intestinal mucosa as larvae penetrate and develop within the intestinal lining. Unlike adult worms of most gastrointestinal parasites, the larval stages of Nematodirus cause the most severe pathology, burrowing into the intestinal villi and causing extensive destruction of absorptive surfaces. This mucosal damage results in malabsorption of nutrients, protein loss into the intestinal lumen, fluid and electrolyte imbalances, and bacterial translocation that can lead to secondary septicemia. The rapid development of large numbers of larvae simultaneously overwhelms the host's compensatory mechanisms, leading to the acute clinical syndrome characteristic of nematodirus infection.

Early warning signs of nematodirus infection in lambs are often subtle and can be easily overlooked until the disease has progressed to a more severe stage. Affected lambs may initially appear slightly dull or less vigorous than their cohorts, spending more time lying down and showing reduced interest in nursing or grazing. Producers who observe their flocks closely may notice that certain lambs are falling behind in growth compared to their peers, appearing slightly tucked up in the abdomen or showing early signs of dehydration around the eyes. These prodromal signs typically precede the onset of diarrhea by one to two days and represent the period during which larval penetration and intestinal damage are occurring.

The most characteristic and commonly recognized symptom of nematodirus infection is the sudden onset of profuse, watery, dark-colored diarrhea that rapidly leads to soiling of the perineum and hindquarters. This diarrhea is often described as having a distinctive dark green or black coloration and an unusually offensive odor compared to other causes of lamb scours. The fluid loss through diarrhea quickly leads to visible dehydration, with affected lambs showing sunken eyes, dry mucous membranes, and loss of skin elasticity when the skin is tented. Weight loss occurs rapidly, and lambs that were thriving just days earlier can become severely emaciated within a very short timeframe.

Behavioral changes associated with nematodirus infection reflect the systemic effects of intestinal damage, fluid loss, and metabolic derangement. Affected lambs typically separate themselves from the flock, standing hunched with lowered heads and showing reluctance to move when approached. They stop nursing from their dams and show no interest in grass or supplemental feed, leading to further nutritional compromise. Weakness progresses as the condition worsens, with lambs becoming recumbent and unable to rise, lying in lateral recumbency with paddling movements of the limbs in terminal cases.

Physical examination of affected lambs reveals a constellation of findings consistent with severe dehydration and intestinal disease. Abdominal palpation may elicit signs of discomfort, and fluid-filled intestinal loops can sometimes be appreciated in thin lambs. Rectal temperature may be normal, subnormal in severely affected animals due to circulatory compromise, or elevated if secondary bacterial infection has occurred. Mucous membranes appear pale due to anemia from blood loss into the intestine, and capillary refill time is prolonged reflecting poor peripheral perfusion. Auscultation of the abdomen typically reveals increased intestinal sounds during active diarrhea or decreased sounds as ileus develops in terminal stages.

Symptom progression in untreated nematodirus cases follows a predictable but rapid course from initial dullness to death within as few as two to five days. The speed of deterioration is a hallmark of this condition and distinguishes it from more chronic parasitic conditions that develop over weeks to months. Lambs that survive the acute phase without treatment often suffer from lasting damage to intestinal absorptive capacity, resulting in poor doers that fail to thrive despite adequate nutrition and continued antiparasitic treatment. The synchronous nature of larval hatching means that multiple lambs within a group typically become affected simultaneously, creating the characteristic outbreak pattern.

Emergency symptoms requiring immediate veterinary intervention include severe dehydration with prolonged skin tenting, complete recumbency with inability to rise, convulsions or neurological signs suggesting metabolic derangement, and signs of septicemia including high fever, rapid weak pulse, and cold extremities. Lambs showing these severe signs require aggressive supportive care including intravenous fluid therapy in addition to anthelmintic treatment, and even with optimal care, mortality rates in severely affected individuals remain high. Producers should be advised that any lamb showing profuse watery diarrhea during the spring risk period should be treated as a potential nematodirus case requiring immediate attention rather than observation.

Clinical examination for suspected nematodirus infection begins with a thorough assessment of affected individuals combined with evaluation of flock-level patterns that may suggest parasitic disease. The veterinarian will note the age of affected lambs, timing of disease onset relative to turnout and weather patterns, and the number and distribution of cases within the flock. Physical examination findings including dehydration status, body condition, fecal character, and signs of anemia help establish disease severity and guide treatment intensity. The combination of acute onset watery diarrhea in young lambs during spring, particularly following a period of warming weather after cold temperatures, creates a clinical picture highly suggestive of nematodirus infection.

Diagnostic testing for nematodirus presents unique challenges because severe clinical disease often develops before patent infection is established and eggs appear in feces. Traditional fecal egg counts may be low or negative in lambs showing severe clinical signs, as the damaging larval stages have not yet matured to egg-producing adults. When eggs are present, they are distinctively large compared to other strongylid parasites and can be identified based on size and morphological characteristics. Post-mortem examination of lambs that have died from the condition reveals characteristic findings including thickened, inflamed small intestinal mucosa with visible larval stages and adult worms, along with evidence of dehydration and cachexia.

Differential diagnosis of watery diarrhea in young lambs includes several infectious and non-infectious conditions that must be considered. Coccidiosis causes similar clinical signs but typically affects slightly older lambs and produces bloody rather than dark watery feces. Cryptosporidiosis primarily affects lambs under three weeks of age and is often associated with environmental contamination around lambing areas. Enterotoxemia caused by Clostridium perfringens produces acute death often without premonitory diarrhea, while nutritional scours from milk replacer or sudden dietary changes typically affects younger lambs. Salmonellosis and colibacillosis should be considered in lambs showing systemic signs of septicemia alongside diarrhea.

Herd-level diagnostics play an important role in confirming nematodirus as the cause of flock outbreaks and in developing preventive strategies for future seasons. Pasture larval counts can quantify the environmental challenge, though these are rarely performed in routine practice due to the specialized techniques required. Analysis of grazing history, stocking rates, and previous parasite control practices helps identify risk factors contributing to the current outbreak. Regional nematodirus forecasting services, where available, provide valuable information about predicted hatching dates and risk levels based on accumulated temperature data. Strategic fecal egg count monitoring of tracer lambs or sentinel animals can help predict rising infection pressure before clinical disease appears, though timing is critical given the rapid onset of clinical signs relative to patent infection.

Emergency treatment of clinically affected lambs with nematodirus infection must be initiated immediately upon recognition of clinical signs to maximize survival rates and minimize lasting intestinal damage. The cornerstone of treatment is administration of an effective anthelmintic drug at the appropriate dose for the lamb's body weight, with benzimidazoles such as fenbendazole or albendazole traditionally considered first-line treatments due to their efficacy against both larval and adult stages of Nematodirus. Accurate weight estimation or ideally weighing of lambs is essential to ensure adequate dosing, as underdosing reduces efficacy and contributes to resistance development while overdosing wastes product and may cause toxicity in debilitated animals.

Medical management of nematodirus infection extends beyond anthelmintic treatment to address the metabolic consequences of severe diarrhea and intestinal damage. Oral or intravenous fluid therapy is indicated for dehydrated lambs, with severely affected individuals requiring aggressive intravenous rehydration with balanced electrolyte solutions. Non-steroidal anti-inflammatory drugs may help reduce intestinal inflammation and provide analgesia for abdominal discomfort. Vitamin B complex supplementation supports recovery by replacing vitamins lost through diarrhea and addressing deficiencies resulting from malabsorption. When treating food-producing animals, all drug withdrawal times must be carefully observed and documented to ensure that any treated animal entering the food chain is free from violative residues.

Surgical intervention is not applicable to nematodirus infection, as the condition involves diffuse intestinal parasitism rather than localized lesions amenable to surgical correction. However, supportive nursing care plays a vital role in recovery and should not be underestimated. Affected lambs should be removed from contaminated pastures and housed in clean, dry, well-bedded areas where they can be closely monitored and provided supportive care. Protection from adverse weather conditions reduces metabolic demands during recovery, and ensuring access to clean water and palatable feed encourages voluntary intake as intestinal function improves.

Supportive care measures for recovering lambs include provision of high-quality nutrition to support intestinal repair and compensatory growth. Milk from the dam remains the ideal nutrition source for young lambs, and affected individuals should be allowed to nurse on demand or supplemented with milk replacer if the dam's production is inadequate. As lambs recover and begin to graze, offering fresh, clean pasture with low parasite contamination supports continued recovery while minimizing reinfection risk. Probiotics may help restore normal intestinal microbiota disrupted by the infection and antiparasitic treatment, though evidence for their efficacy in nematodirus cases specifically is limited.

Herd treatment protocols should be implemented when nematodirus is diagnosed in any flock members, as the synchronous nature of larval hatching means that other lambs in the group have likely been exposed to similar infection pressure. Metaphylactic treatment of the entire lamb cohort with an effective anthelmintic reduces the risk of clinical disease in animals that are infected but not yet showing signs. The choice of anthelmintic drug class should take into account regional resistance patterns and the farm's anthelmintic usage history. Group treatments provide an opportunity to weigh a sample of lambs to ensure accurate dosing across the flock.

Treatment decisions on sheep farms must balance individual animal welfare against economic realities of commercial production, and nematodirus outbreaks frequently require producers and veterinarians to make difficult choices. Lambs showing severe clinical signs with poor prognosis despite treatment may be candidates for euthanasia on welfare grounds rather than prolonged treatment attempts. The cost of intensive individual treatment including intravenous fluids and nursing care must be weighed against the value of recovered animals and the likelihood of full productive recovery. Prevention of future outbreaks through strategic pasture management and targeted anthelmintic use represents a more cost-effective approach than repeated treatment of clinical cases each season.

Recovery timeline for lambs treated promptly for nematodirus infection is generally favorable, with clinical improvement often visible within twenty-four to forty-eight hours of anthelmintic treatment in uncomplicated cases. Diarrhea typically begins to resolve within the first day or two, with feces gradually returning to normal consistency over the following week. Appetite and nursing behavior improve as intestinal inflammation subsides and fluid balance is restored through supportive care. However, the intestinal damage caused by larval penetration requires extended time to heal fully, and affected lambs may continue to show evidence of impaired absorptive function for several weeks after clinical resolution.

Post-treatment care and monitoring of recovered lambs involves ongoing assessment of growth rates, fecal consistency, and general thrift compared to unaffected cohorts. Lambs should be weighed regularly following recovery to track compensatory growth and identify individuals that may benefit from enhanced nutrition or additional interventions. Fecal egg count monitoring two to three weeks post-treatment confirms treatment efficacy and helps identify any animals requiring retreatment. Careful observation for signs of recurrence is important, particularly if lambs are returned to contaminated pastures before the seasonal larval challenge has passed.

Prognosis factors for individual lambs recovering from nematodirus infection include the severity of initial infection, promptness of treatment initiation, adequacy of supportive care, and the animal's underlying nutritional and immune status. Lambs that received early treatment before severe dehydration developed typically make full recoveries with minimal lasting effects on growth potential. Those that survived severe infections despite delayed treatment may experience permanent damage to intestinal absorptive capacity, resulting in lifelong poor doing that limits their productivity. Twin and triplet lambs or those from ewes with poor milk production enter infection at a nutritional disadvantage and may have reduced resilience and poorer outcomes.

Return to production considerations for recovered lambs involve decisions about continued feeding, market timing, and breeding stock selection. Lambs that fully recover and demonstrate normal compensatory growth can progress through normal marketing channels once any withdrawal periods from treatment have elapsed. Those that fail to thrive following recovery may require extended feeding periods to reach market weight, with associated additional costs that impact profitability. Breeding stock selection should exclude animals that experienced severe nematodirus infection, as resilience to parasitic challenge appears to have heritable components and selecting survivors of clinical disease may perpetuate susceptibility in future generations. Recovered lambs should ideally be grazed on clean pastures or pastures not used by lambs in the previous season to minimize reinfection risk during the remainder of the grazing season.

Vaccination protocols for nematodirus prevention are not currently available, as no commercial vaccines exist for this or most other gastrointestinal nematode parasites of livestock. Research into parasite vaccines continues, but the complex lifecycle and immune evasion mechanisms of these organisms have thus far prevented development of effective commercial products. In the absence of vaccination, prevention relies on integrated approaches combining strategic anthelmintic use, pasture management, and monitoring to reduce infection pressure and protect susceptible young stock during high-risk periods.

Biosecurity measures for nematodirus focus on managing environmental contamination levels rather than preventing introduction, as the parasite is ubiquitous on sheep farms worldwide. However, producers should be aware that purchasing sheep from other properties may introduce parasite populations with different drug resistance profiles, and quarantine treatment with multiple drug classes is recommended for all incoming animals. New stock should be held on designated quarantine pastures rather than immediately mixed with resident animals, and fecal egg count reduction testing can identify animals carrying resistant parasites that could compromise control programs on the receiving farm.

Nutritional prevention strategies support the lamb's immune system and resilience to parasitic challenge rather than directly preventing infection. Ensuring adequate colostrum intake by newborn lambs establishes passive immune protection and supports healthy gut development. Maintaining ewes in good body condition during late pregnancy and lactation ensures adequate milk production to support rapid lamb growth and immune development. Trace mineral supplementation, particularly selenium and cobalt in deficient areas, supports immune function and helps lambs mount effective responses to parasitic challenge. Well-nourished lambs can tolerate higher parasite burdens before developing clinical disease compared to their poorly nourished counterparts.

Management practices form the cornerstone of nematodirus prevention on sheep farms and center on reducing the contact between susceptible young lambs and infective larvae on pasture. The most effective strategy involves grazing lambs on pastures that were not used by lambs during the previous grazing season, breaking the annual cycle of contamination and infection. Cattle grazing can be used to clean up sheep pastures, as most nematode species show strong host specificity and cannot complete their lifecycle in alternative host species. Delaying turnout of lambs until after the peak larval hatching period has passed reduces exposure, though this must be balanced against the nutritional benefits of early access to spring grass.

Quarantine and testing protocols for nematodirus specifically are less relevant than for contagious diseases, but general parasite monitoring programs support preventive management. Fecal egg count monitoring of ewes prior to lambing identifies heavily contaminated individuals that may warrant targeted treatment to reduce pasture contamination during the lambing period. Regional nematodirus forecasting services provide advance warning of high-risk periods based on temperature accumulation models, allowing producers to implement prophylactic treatment before larval ingestion occurs. Strategic use of anthelmintics based on forecasts and pasture history rather than routine calendar-based treatments helps preserve drug efficacy while protecting vulnerable lambs during genuine risk periods.

Daily management and monitoring of sheep flocks during the nematodirus risk period requires heightened vigilance and regular close observation of all young lambs. Producers should walk through their flocks daily during spring months, observing lamb behavior, body condition, and fecal consistency to identify early signs of disease. Any lamb showing depression, reduced nursing activity, or early signs of scouring should be examined closely and considered for immediate treatment if nematodirus is suspected. Recording the number and identity of affected animals helps track the progression of any outbreak and evaluate the effectiveness of control measures implemented.

Housing and environmental management for nematodirus prevention focuses primarily on pasture allocation and grazing strategies rather than building-based housing. Most sheep in temperate climates are managed on pasture during the spring risk period, making field selection and grazing rotation the primary management tools. Pastures designated for lambing and early lamb grazing should ideally have been rested from sheep or grazed only by cattle during the previous season. If permanent sheep pastures must be used, avoiding the same fields for lamb grazing in consecutive years reduces contamination buildup. Providing supplementary feeding in dedicated areas rather than throughout the grazing field concentrates contamination and allows lambs to access cleaner grazing areas.

Herd health programs for sheep farms should incorporate nematodirus risk assessment and prevention as core components of annual parasite control planning. Working with a veterinarian to develop farm-specific protocols based on local conditions, grazing system, and historical disease patterns ensures that prevention strategies are appropriately targeted. Identifying high-risk fields based on previous grazing history helps prioritize pasture allocation decisions, and integrating nematodirus monitoring with broader parasite surveillance programs provides comprehensive information to guide treatment decisions. Regular review and updating of control strategies based on treatment outcomes, resistance testing, and changing farm circumstances keeps programs effective over time.

Record keeping and monitoring systems support effective nematodirus management by providing the information needed for evidence-based decision making. Essential records include pasture grazing histories showing which fields were used by which stock classes in previous seasons, dates of lamb turnout and any strategic treatments administered, and records of clinical cases including number affected, treatment given, and outcomes achieved. Fecal egg count results and any resistance testing data should be maintained as part of the parasite control record. Tracking weather patterns and consulting regional forecast information during spring helps predict risk levels and time interventions appropriately.

Economic considerations for nematodirus management involve balancing prevention costs against potential losses from clinical disease outbreaks. The cost of strategic anthelmintic treatments during high-risk periods is typically far less than the value of lamb losses that can occur in untreated outbreaks, making preventive treatment economically justified in most situations. However, over-reliance on anthelmintics without attention to pasture management contributes to resistance development that threatens long-term control efficacy. Investing in infrastructure such as handling facilities and weighing equipment that enable accurate dosing and efficient flock treatment improves both welfare outcomes and cost-effectiveness of parasite control programs. Producers should work with their veterinarians to develop economically sustainable control strategies that balance short-term costs against long-term productivity and drug efficacy preservation.

High-risk breeds and populations for nematodirus infection are defined more by management system, age, and immune status than by genetic breed differences in susceptibility. All breeds of sheep produce lambs that are susceptible to nematodirus during their first grazing season, before acquired immunity develops through exposure. However, intensively managed flocks with high stocking densities, permanent pasture systems, and synchronized lambing dates that align with spring larval hatching face higher challenge levels than extensively managed flocks on large range areas. Purebred flocks maintaining high-value breeding stock may warrant more aggressive prevention programs given the greater economic value at risk.

Production type considerations influence nematodirus risk through their effects on lambing timing, grazing management, and lamb growth rates. Early lambing flocks that turn lambs out to pasture in late March or April in the Northern Hemisphere may expose lambs to peak larval hatching, while later lambing flocks may miss the main risk period. Intensive finishing systems that aim for rapid lamb growth may inadvertently select for susceptibility by marketing the fastest-growing lambs before immunity develops, leaving slower-growing but potentially more resistant animals in the breeding population. Dairy sheep operations with early weaning may see different disease patterns than maternal breeds where lambs remain with ewes throughout the grazing season.

Genetic selection and testing for nematodirus resistance specifically is not currently practiced, though broader selection for parasite resistance based on fecal egg counts is incorporated into breeding programs in some countries. Estimated breeding values for worm resistance are available for certain breed societies and provide a tool for selecting rams that pass on enhanced immune responsiveness to their offspring. Within-flock selection against animals that repeatedly require treatment for parasitic disease may gradually increase flock resistance over time. However, genetic progress is slow and must be balanced against other production traits, meaning that management-based prevention remains the primary control strategy for the foreseeable future. Crossbreeding programs that incorporate hardy hill breeds known for resilience under extensive conditions may confer some advantage in systems where parasite challenge is high.

Commonly co-occurring conditions with nematodirus infection include other gastrointestinal parasitoses, as lambs exposed to Nematodirus larvae on contaminated pastures are typically simultaneously exposed to larvae of other strongylid species. Mixed infections with Teladorsagia, Trichostrongylus, and Cooperia species are common and may contribute to overall parasitic gastroenteritis syndrome. Coccidiosis frequently affects the same age group of lambs and may occur concurrently with nematodirus, complicating diagnosis and requiring treatment with both anthelmintics and anticoccidials. Secondary bacterial enteritis can develop when intestinal barrier function is compromised by parasitic damage, leading to more severe systemic illness than either condition alone.

Conditions with similar clinical presentations to nematodirus include several causes of acute diarrhea in young lambs that must be considered in differential diagnosis. Coccidiosis produces diarrhea that is often darker or bloody compared to nematodirus and typically affects slightly older lambs, but the conditions overlap sufficiently that laboratory testing may be required for differentiation. Cryptosporidiosis and rotavirus infection cause diarrhea in younger lambs, typically under three weeks of age, before the nematodirus risk period. Salmonellosis can affect lambs of any age and produces systemic signs of septicemia alongside diarrhea. Clostridial enterotoxemia causes sudden death that may be preceded by diarrhea but often occurs without premonitory signs.

Complications and sequelae of nematodirus infection include both immediate and long-term consequences for affected lambs. Acute complications include severe dehydration and electrolyte imbalances that can cause cardiovascular collapse and death if untreated. Secondary bacterial septicemia may develop when damaged intestinal mucosa allows translocation of gut bacteria into the bloodstream. Long-term sequelae in recovered lambs include stunted growth and reduced feed conversion efficiency due to permanent intestinal damage affecting nutrient absorption. Rectal prolapse occasionally occurs in lambs with severe straining from diarrhea, requiring surgical correction. Poor doers that survive severe infection may never achieve their genetic growth potential and may be more susceptible to other health challenges throughout their productive lives.