Sheep and Goat Parasites: Comprehensive Guide to Gastrointestinal and External Parasites

Introduction: The Parasitic Burden in Small Ruminants

Parasitic infections represent a primary constraint to health, welfare, and productivity in sheep and goat operations globally. The term sheep and goat parasites encompasses a diverse array of endoparasites (gastrointestinal nematodes, protozoa, cestodes, trematodes) and ectoparasites (mites, lice, keds, ticks, flies) that cause subclinical production losses, clinical disease, and mortality. The economic impact of parasitism in small ruminants is substantial, driven by reduced weight gain, decreased milk yield, impaired wool and mohair quality, reproductive failure, and costs of anthelmintic and acaricide treatments [1]. The epidemiology of these parasites is profoundly influenced by climate, grazing management, host immunity, and the emergence of drug resistance. Understanding the biology, diagnostics, and integrated control of these pathogens is essential for the veterinary practitioner.

Gastrointestinal Nematodes (GINs)

The most significant endoparasites of small ruminants are the gastrointestinal nematodes (GINs). The principal genera affecting sheep and goats include Haemonchus, Teladorsagia (formerly Ostertagia), Trichostrongylus, Cooperia, Nematodirus, and Oesophagostomum [2]. These parasites inhabit the abomasum and small intestine, causing a spectrum of pathology from anemia and hypoproteinemia to diarrhea and ill-thrift. The term sheep parasites predominantly refers to these GINs due to their ubiquity and pathogenic potential.

Abomasal Nematodes

Haemonchus contortus (Haemonchus contortus in Sheep: Anthelmintic Resistance and FAMACHA-Based Control) is the most pathogenic GIN in warm, humid climates. This blood-feeding nematode causes acute or chronic hemorrhagic anemia, submandibular edema (bottle jaw), and death in heavy burdens [3]. Adult females are recognized by their "barber's pole" appearance (white ovaries spiraled around a red blood-filled intestine). Infective third-stage larvae (L3) are ingested from pasture; they exsheath in the rumen, molt to L4 in the abomasal mucosa, and emerge as adults to feed on blood [4]. The prepatent period is 18–21 days. Periparturient ewes and lambs are most susceptible due to immunological relaxation.

Teladorsagia circumcincta (Teladorsagia circumcincta in Sheep: Abomasal Parasitism, Anthelmintic Resistance, and Integrated Control in Temperate Regions) is the dominant abomasal parasite in cooler temperate regions. Larvae penetrate the gastric glands, causing hyperplasia and loss of parietal cells, leading to elevated abomasal pH, reduced protein digestion, and diarrhea [5]. Type I disease occurs in grazing lambs; Type II (hypobiosis reactivation) can cause winter outbreaks in adults.

Intestinal Nematodes

Trichostrongylus colubriformis and Trichostrongylus axei (Trichostrongylus colubriformis: The Bankrupt Worm of Sheep and Cattle; Trichostrongylus axei: Abomasal Hairworm of Cattle and Sheep) are slender, non-blood-feeding worms that burrow between enterocytes. T. colubriformis in the small intestine causes villous atrophy, reduced nutrient absorption, and profuse watery diarrhea [6]. T. axei is unusual in that it can infect the abomasum and the proximal duodenum, causing gastritis. Cooperia curticei (Cooperia curticei in Sheep: Small Intestine Nematode and Periparturient Rise) is a small intestinal nematode that contributes to the periparturient rise in fecal egg counts in ewes [7]. Nematodirus battus (Nematodirus battus in Sheep Lambs: Spring Outbreak Epidemiology, D-Value Forecasting, and Anthelmintic Control) is unique in that the egg must overwinter and hatch en masse after a cold period, causing sudden spring outbreaks of severe diarrhea and dehydration in lambs [8]. The pre-infective larval stage (L1/L2) develops within the egg, and hatching is triggered by accumulated degree-days above a threshold (D-value).

Large Intestinal Nematodes

Oesophagostomum columbianum (nodular worm) causes granulomatous inflammation in the large intestinal wall. Larvae provoke a host immune response that encysts them, forming caseous nodules that can calcify and rupture, leading to peritonitis in heavy infections [9]. Chabertia ovina is a large, stout worm that attaches to the colonic mucosa, causing hemorrhagic colitis and tenesmus.

Protozoal Parasites: Coccidia and Cryptosporidia

Coccidiosis, caused by apicomplexan parasites of the genus Eimeria, is a major enteric disease of lambs and kids worldwide. In sheep, the most pathogenic species are Eimeria crandallis and Eimeria ovinoidalis [10]. In goats, Eimeria arloingi and Eimeria ninakohlyakimovae are of primary concern [11].

The life cycle is direct. Animals ingest sporulated oocysts from contaminated feed or water. Sporozoites excyst, invade enterocytes, and undergo multiple generations of merogony (asexual reproduction) followed by gametogony (sexual reproduction) and oocyst shedding [10]. The endogenous cycle destroys intestinal epithelium, causing villous atrophy, crypt hyperplasia, and hemorrhage. Clinical signs include profuse, watery diarrhea (often with blood or mucus), tenesmus, dehydration, and weight loss [11]. Subclinical infections cause significant growth retardation. Eimeria crandallis in lambs produces a watery, non-hemorrhagic diarrhea, while E. ovinoidalis is more pathogenic and hemorrhagic [10]. Diagnosis relies on quantitative fecal flotation and speciation of oocysts; counts above 50,000 oocysts per gram in diarrheic lambs are considered significant [11].

Cryptosporidium parvum is a zoonotic protozoan that causes neonatal diarrhea in lambs and kids. The parasite invades the microvillus border of enterocytes, leading to malabsorptive diarrhea. Diagnosis is via modified Ziehl-Neelsen staining or antigen-capture ELISA on fecal samples.

Trematodes: Liver Flukes

Trematode infections are regionally important, particularly on wet pastures with suitable snail intermediate hosts.

Fasciola hepatica (Liver Fluke (Fasciola hepatica) in Sheep: Anthelmintic Resistance Diagnosis and Herd-Level Management; Fasciolosis in Cattle and Sheep: Liver Fluke Diagnosis via Coproantigen ELISA, Pooled PCR, and Anthelmintic Resistance to Triclabendazole) causes fasciolosis, a chronic, debilitating disease of the liver. The life cycle requires the snail Galba truncatula [12]. Metacercariae are ingested on herbage; the juvenile fluke excysts, penetrates the intestinal wall, and migrates through the liver parenchyma, causing traumatic hepatitis and hemorrhagic tracts. Adult flukes in the bile ducts cause cholangitis, epithelial hyperplasia, and fibrosis [12]. Chronic fasciolosis is characterized by anemia, hypoproteinemia, and submandibular edema. Acute fasciolosis (due to massive migration of immature flukes) can cause sudden death due to liver rupture and hemorrhage. Diagnosis is by fecal sedimentation (egg detection) or coproantigen ELISA, which detects fluke antigens even in the pre-patent period [13]. Triclabendazole resistance is an emerging global problem.

Dicrocoelium dendriticum (Dicrocoelium dendriticum (Lancet Fluke) in Sheep and Cattle: Bile Duct Pathology and Ant-Based Lifecycle) is the lancet fluke. Its complex lifecycle uses land snails as first intermediate hosts and ants as second intermediate hosts [14]. Infection causes cholangitis and biliary fibrosis but is often subclinical. Diagnosis is by egg detection on fecal flotation or sedimentation. The eggs are distinctive, operculated, and brown.

Cestodes: Tapeworms

Moniezia expansa (Moniezia expansa in Sheep and Cattle: Oribatid Mite Lifecycle, Clinical Signs, and Control) is a large, non-segmented tapeworm of the small intestine. The intermediate host is a free-living oribatid mite [15]. Gravid proglottids are shed in feces. Most infections in lambs are subclinical, but heavy burdens can cause intestinal obstruction or unthriftiness. Diagnosis is by detection of proglottids or characteristic square, operculated eggs.

Taenia multiceps (larval stage: Coenurus cerebralis) (Coenurus cerebralis in Sheep: Gid or Sturdy, Taenia multiceps Larval Cestode Infection, and Nervous Signs) causes gid or sturdy. Dogs are the definitive host; sheep ingest eggs from canine feces. The oncosphere migrates to the brain, where it develops into a fluid-filled cyst (coenurus) [16]. Clinical signs include circling, blindness, head pressing, ataxia, and death.

External Parasites (Ectoparasites)

Mites

Psoroptes ovis (Psoroptes ovis in Sheep: Sheep Scab Mange, Highly Contagious Notifiable Ectoparasitosis, and Dipping Protocols) is the cause of sheep scab, a highly contagious and notifiable disease in many countries. P. ovis is a non-burrowing mite that feeds on skin surface debris and exudate, causing intense pruritus, wool loss, and severe dermatitis [17]. Transmission is by direct contact or via fomites. Diagnosis is by skin scrapings and microscopic identification of the mite. Treatment historically relies on plunge dipping with organophosphates or synthetic pyrethroids, but injectable macrocyclic lactones (e.g., moxidectin) are also effective [17]. Chorioptic mange (caused by Chorioptes bovis) affects the lower legs and scrotum, causing pruritus and stamping.

Lice

Sheep and goats are infested by two types of lice: biting (chewing) lice and sucking lice.

Damalinia ovis (Damalinia ovis in Sheep: Biting Louse Infestation, Fleece Damage, Wool Loss, and Control Measures) is a biting louse that feeds on skin debris and wool fibers, causing severe fleece damage, matting, and wool break [18]. Infestation causes pruritus and self-trauma.

Linognathus ovillus (Linognathus ovillus in Sheep: Face Louse, Sucking Louse Infestation, Anemia, and Pruritus) is a blood-sucking louse found predominantly on the face and head. Heavy infestations can cause anemia, especially in lambs [19]. Linognathus pedalis (the foot louse) infests the legs and can cause lameness. Linognathus africanus is a sucking louse of goats.

Diagnosis of pediculosis is by visual inspection and microscopic examination of hair/wool samples and the lice themselves. Treatment is by topical application of synthetic pyrethroids or macrocyclic lactones.

Keds

Melophagus ovinus (Melophagus ovinus in Sheep: Sheep Ked, Wingless Fly Ectoparasite Infestation, Fleece Damage, and Anemia) is a wingless, blood-feeding fly (sheep ked). It is a permanent ectoparasite, spending its entire lifecycle on the host [20]. Keds cause fleece staining, hide damage (cockle), and anemia in heavy burdens. The pupa is attached to the wool. Control is similar to lice control, using synthetic pyrethroids or macrocyclic lactones.

Myiasis (Fly Strike)

Fly strike (cutaneous myiasis) is a major welfare and production problem. The primary agent is the blowfly Lucilia cuprina in warm climates. Flies are attracted to soiled, moist fleece (e.g., urine, feces, or wound exudate). Larvae hatch and feed on living tissue, causing a severe, often fatal, toxemia [21]. Prevention involves crutching (dagging), tail docking, and the use of insect growth regulators (e.g., dicyclanil, cyromazine).

Ticks

Ticks (Ixodes, Rhipicephalus, Amblyomma spp.) infest sheep and goats in many regions. They cause anemia, irritation, and paralysis (via salivary toxins). More critically, they are vectors for viral and bacterial pathogens, including louping ill virus and Anaplasma phagocytophilum. Control relies on acaricide application.

Diagnostic Approaches

Effective parasitological diagnosis is the cornerstone of targeted treatment and resistance management.

Fecal Egg Count (FEC)

The modified McMaster technique is the standard quantitative method for FEC. It provides eggs per gram (EPG) of feces, which correlates with adult worm burden for Haemonchus and Trichostrongylus [22]. For Nematodirus, FEC must be performed with a specific counting technique due to the larger egg size. Pooled FEC from groups of animals is used for monitoring flock-level burdens.

Larval Culture

Larval culture is essential for genus-level identification when multiple GIN species are present. Feces are incubated at 22–27°C for 7–14 days, and third-stage larvae are recovered and identified using morphometric keys [23]. This is critical for determining which species are driving the FEC and for detecting resistance.

FAMACHA System

The FAMACHA system (Haemonchus contortus in Sheep: Anthelmintic Resistance and FAMACHA-Based Control) is a clinical scoring method for anemia, correlating with H. contortus burden. Ocular mucous membranes are scored from 1 (red, non-anemic) to 5 (white, severely anemic) [3]. This allows targeted treatment of only anemic animals, reducing selection pressure for anthelmintic resistance.

Coproantigen ELISA

For Fasciola hepatica, coproantigen ELISA detection (immunochemical capture of fluke excretory/secretory products) has high sensitivity and specificity, detecting infections earlier than fecal sedimentation [13].

Skin Scrapings and Ectoparasite Examination

For ectoparasites, deep skin scrapings from the lesional edge (mites), or collection of wool/hair samples (lice, keds) are used. Mites are cleared with potassium hydroxide and examined by light microscopy.

Clinical Signs and Herd-Level Indicators

The clinical presentation of parasitism varies by parasite burden, host age, and nutritional status.

• Anemia: Pale mucous membranes, bottle jaw, weakness. Highly suggestive of H. contortus or fasciolosis.
• Diarrhea: Profuse, watery feces common in trichostrongylosis, coccidiosis, and Nematodirus infection.
• Weight loss, ill-thrift: Reduced growth rates despite adequate nutrition, typical of subclinical GIN burdens.
• Pruritus and wool/hair loss: Seen in scab, lice, and ked infestations.
• Mortality: Acute death can occur in haemonchosis, acute fasciolosis, and fly strike.

Treatment and Anthelmintic Resistance

Treatment of sheep and goat parasites must be tailored to the parasite species, the class of animal, and the resistance status of the flock.

Anthelmintic Classes

The major anthelmintic classes include:

• Benzimidazoles (e.g., albendazole, fenbendazole): Inhibit tubulin polymerization in nematode cells.
• Macrocyclic lactones (e.g., ivermectin, moxidectin): Glutamate-gated chloride channel agonists, causing paralysis.
• Imidazothiazoles (e.g., levamisole): Nicotinic acetylcholine receptor agonists.
• Amino-acetonitrile derivatives (e.g., monepantel): A newer class with a unique mode of action at the nematode nAChR.
• Salicylanilides (e.g., closantel): Effective against blood-feeding parasites (Haemonchus, Fasciola).

Anthelmintic Resistance

Widespread resistance, particularly in H. contortus, T. circumcincta, and T. colubriformis, is the primary challenge in small ruminant parasite control [24]. Resistance to benzimidazoles, macrocyclic lactones, and levamisole is prevalent. Diagnosis of resistance is by the fecal egg count reduction test (FECRT), where FEC is measured before and 10–14 days after treatment [24]. A reduction of less than 95% indicates resistance.

Treatment Protocols

A targeted selective treatment (TST) approach, using the FAMACHA score for H. contortus and FEC thresholds for other GINs, is recommended to maintain a refugia of susceptible worms [3]. For coccidiosis, treatment is with toltrazuril or diclazuril, often administered prophylactically to lambs at a critical age [10]. Fasciolosis requires a flukicide with activity against immature stages (triclabendazole) for acute disease, or an adulticide for chronic disease.

Integrated Parasite Control Strategies

Integrated parasite management (IPM) combines multiple approaches to reduce parasite burdens while minimizing drug use.

Pasture Management

• Rotational grazing: Moving animals to clean pasture every 3–4 weeks reduces L3 density.
• Mixed or alternate grazing: Cattle and sheep share few GIN species; alternating species breaks the parasite lifecycle.
• Resting pastures: Pasturing with no grazing for 6–12 months during summer (drought) or winter (frost) reduces L3 survival.

Nutritional Management

High-protein diets improve host immunity to GINs, particularly during the periparturient period [25]. Supplementation with protein-rich concentrates or forage can reduce FEC and improve resilience.

Genetic Selection

Breeding for parasite resistance is feasible. Breeds such as the Gulf Coast Native sheep and some hair sheep breeds (e.g., Katahdin) demonstrate lower FEC and improved resilience to H. contortus [26]. Quantitative trait loci (QTL) for resistance have been identified on ovine chromosomes.

Biological Control

Nematophagous fungi, such as Duddingtonia flagrans, can be fed to animals; spores survive passage through the gut and trap larval nematodes on pasture. This is an emerging area of interest but is not yet widely deployed.

Biosecurity

Quarantine drenching of incoming stock with a combination of anthelmintic classes (e.g., monepantel + abamectin) prevents introduction of resistant nematode populations [27].

Mermaid Diagram: Parasite Diagnostic Workflow

flowchart TD
    A[Examination: Clinical Signs, History], > B{Anemia Present?}
    B, Yes, > C[FAMACHA Score 3-5]
    C, > D[FEC + Larval Culture]
    D, > E{High H. contortus burden?}
    E, Yes, > F[Targeted Treatment with Closantel or ML]
    E, No, > G[Broad-spectrum Anthelmintic]
    B, No, > H{Diarrhea?}
    H, Yes, > I[Fecal Flotation + Oocyst Count]
    I, > J{Coccidia > 50k OPG?}
    J, Yes, > K[Toltrazuril Treatment]
    J, No, > L[FEC for GINs]
    L, > M[Genus ID via Larval Culture]
    M, > N[Select Anthelmintic Based on Resistance History]
    H, No, > O[Monitor Weight Gain, FEC Surveillance]

Conclusion

The successful management of sheep and goat parasites requires a comprehensive, integrated approach that combines diagnostic surveillance, targeted treatment, pasture management, and genetic selection. Anthelmintic resistance is the defining challenge of modern small ruminant parasitology, demanding a shift from calendar-based mass treatments to evidence-based, targeted interventions. Veterinary practitioners must be proficient in fecal egg counting, larval identification, FAMACHA scoring, and the interpretation of resistance tests. Control of external parasites relies on regular inspection, husbandry improvements, and careful use of acaricides. By integrating these strategies, it is possible to maintain productivity, ensure animal welfare, and sustain drug efficacy for future generations.

References

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[27] Leathwick, D. M., & Hosking, B. C. Managing the risk of introduction of drug-resistant nematodes. New Zealand Veterinary Journal. *** Disclaimer: This article is for educational and informational purposes only. It is not intended to substitute for professional veterinary advice, diagnosis, treatment, or regulatory guidance. Always consult a licensed veterinarian or qualified specialist regarding animal health, disease diagnosis, and therapeutic decisions.