Muellerius capillaris in Sheep and Goats: Protostrongylid Lungworm Diagnosis and Control

Etiology and Taxonomy

Muellerius capillaris is a nematode within the family Protostrongylidae, subfamily Muelleriinae. It is one of the most prevalent lungworm species infesting small ruminants worldwide. The adult worms are slender, threadlike parasites residing in the lung parenchyma, specifically within the bronchioles and alveoli. The male measures 12–14 mm in length, while the female reaches 19–23 mm. First-stage larvae (L1) are excreted in feces and are morphologically characterized by a dorsal spine and a wavy tail. Species composition and morphology of protostrongylids, including M. capillaris, have been documented in ruminants from Bulgaria, confirming its distinct morphometric features relative to other protostrongylids such as Protostrongylus rufescens and Cystocaulus ocreatus [1].

Epidemiology

Muellerius capillaris has a global distribution, with higher prevalence in temperate and Mediterranean regions. Epidemiological studies reveal infection rates exceeding 70% in managed goat flocks in Morocco, with young animals and those grazed on communal pastures at greatest risk [2]. The life cycle is indirect, requiring terrestrial gastropod intermediate hosts (e.g., snails and slugs). Sheep and goats acquire infection by ingesting gastropods containing third-stage larvae (L3). The prepatent period ranges from 5 to 8 weeks. Climatic factors such as humidity, temperature, and soil pH influence gastropod populations and larval development, thereby affecting transmission dynamics. Co-infection with other protostrongylids is common in endemic areas [1, 2].

Clinical Signs and Pathogenesis

In most immunocompetent adult sheep and goats, M. capillaris infection is subclinical. Pathogenesis is driven by the host's granulomatous inflammatory response to adult worms and eggs trapped in the pulmonary parenchyma. In heavy infections, especially in kids and lambs, clinical signs include chronic cough, reduced weight gain, dyspnea, and occasional nasal discharge. Secondary bacterial bronchopneumonia may occur due to impaired mucociliary clearance. Unlike Dictyocaulus filaria, which causes patent bronchial obstruction, M. capillaris induces interstitial pneumonia and nodular lesions that can be palpable as firm, grayish nodules on the lung surface at necropsy.

Pathology

Gross lesions are characterized by subpleural nodules 1–5 mm in diameter, often visible on the diaphragmatic lung lobes. Histopathologic examination reveals epithelioid macrophages, multinucleated giant cells, and eosinophilic infiltrates surrounding degenerating larvae and adult nematodes. Calcification of granulomas occurs in chronic infections. The pulmonary interstitium may show fibrosis and lymphoid hyperplasia. These changes can compromise lung function and predispose animals to secondary respiratory infections, a phenomenon also observed in other respiratory pathogens such as Mycoplasma bovis in cattle (see Mycoplasma bovis in Feedlot Cattle: Chronic Pneumonia, Arthritis, and the Challenge of Cultivation versus Molecular Detection).

Diagnostic Approaches

Fecal Examination and Larval Identification

The cornerstone of antemortem diagnosis is the detection of first-stage larvae (L1) in feces using the Baermann technique. Fresh fecal samples (10–15 g) are suspended in warm water (37–40°C) in a funnel apparatus. Larvae migrate downward and are collected after 6–24 hours. The L1 of M. capillaris are approximately 250–320 µm long, possess a dorsal spine located at 100–120 µm from the anterior end, and a distinctive S-shaped tail with a terminal spine. Morphometric keys differentiate M. capillaris from other protostrongylid larvae such as P. rufescens and C. ocreatus [1]. The sedimentation method and flotation techniques are less sensitive due to the low specific gravity of larvae and are not recommended.

Molecular Diagnostics

Polymerase chain reaction (PCR) assays targeting the internal transcribed spacer 2 (ITS-2) region of ribosomal DNA provide species-level identification directly from feces or lung tissue. Quantitative PCR (qPCR) can estimate larval burden. Molecular techniques are particularly useful when mixed infections are suspected or when morphological identification is ambiguous. These methods are analogous to those used in other parasite diagnostics, such as the pooled PCR approach for liver fluke described in Fasciolosis in Cattle and Sheep: Liver Fluke Diagnosis via Coproantigen ELISA, Pooled PCR, and Anthelmintic Resistance to Triclabendazole.

Serological Assays

Enzyme-linked immunosorbent assays (ELISAs) using crude or recombinant antigens derived from M. capillaris have been developed but are not widely adopted for field use due to cross-reactivity with other protostrongylids. They remain primarily research tools.

Necropsy and Histopathology

Definitive diagnosis relies on necropsy with detection of adult worms in lung parenchyma. Lungs should be examined carefully for subpleural nodules, which are incised to recover adult nematodes. Histopathology confirms granulomatous interstitial pneumonia with intralesional parasites.

Diagnostic Decision Flowchart

Below is a Mermaid diagram summarizing the diagnostic workflow for suspected M. capillaris infection.

flowchart TD
    A[Clinical suspicion: chronic cough, poor growth in sheep/goats], > B[Collect fresh feces]
    B, > C[Baermann technique]
    C, > D{L1 larvae detected?}
    D, Yes, > E[Morphometric identification: dorsal spine, wavy tail]
    D, No, > F[Consider other lungworms or bacterial etiology]
    E, > G[Confirm with ITS-2 PCR if mixed infection]
    F, > H[Check for Dictyocaulus, Protostrongylus, Cystocaulus]
    G, > I[Diagnosis confirmed: Muellerius capillaris]
    H, > I
    I, > J[Implement control measures]

Treatment

Effective anthelmintics against M. capillaris include benzimidazoles (e.g., fenbendazole, albendazole) at elevated doses (10–15 mg/kg for fenbendazole) repeated over 3–5 consecutive days. Macrocyclic lactones (e.g., ivermectin, doramectin) administered subcutaneously at 0.2 mg/kg have shown variable efficacy, with higher doses (0.3–0.5 mg/kg) often required for complete clearance. Levamisole is less effective against adult worms. Due to the intracellular location of larvae within granulomas, treatment regimens should be prolonged and may not eliminate all worms. Resistance to benzimidazoles has been reported in gastrointestinal nematodes of small ruminants, raising concern for potential cross-resistance in protostrongylids, though data for M. capillaris remain limited.

Control and Prevention

Control strategies must target both the definitive host and the intermediate host.

Pasture Management

Reducing exposure to gastropods is critical. This includes avoiding grazing during early morning and after rainfall when snails are most active, incorporating pasture rotation with prolonged rest periods (8–12 weeks), and topping or harrowing to reduce vegetative cover. Drainage of wet areas and removal of debris also lower snail habitat.

Targeted Deworming

Strategic anthelmintic treatments should be timed before the main grazing season and again in late summer or early autumn. Given the repeated dosing required, fecal egg count (or larval count) reduction tests should be used to monitor efficacy. This parallels the approach used for anthelmintic resistance in gastrointestinal nematodes, as discussed in Sheep Internal Parasites: Winter Management, Parasite Resistance in Dorpers, and Human Health Risks.

Biological Control

Grazing with alternative livestock species (e.g., cattle or horses) that are not competent hosts for M. capillaris can break the life cycle by removing infected sheep or goats from pasture. However, cattle are susceptible to other lungworm species, so this strategy must be tailored.

Molluscicides

Chemical control of intermediate hosts using molluscicides (e.g., metaldehyde or ferric phosphate) is rarely economical in extensive systems. Spot treatment of known snail aggregation sites (e.g., under water troughs) may reduce local populations.

Vaccination

No commercial vaccine exists for M. capillaris. Experimental trials using irradiated L3 or recombinant antigens have not progressed to field application.

Zoonotic Potential

Muellerius capillaris is not considered zoonotic. No human infections have been documented, in contrast to other small ruminant parasites such as Toxoplasma gondii (see Toxoplasmosis in Cats: Transmission Routes for Indoor Cats, Clinical Signs, Diagnostic Blood Testing, and Public Health Concerns). However, it is prudent to follow general hygiene when handling infected animals or fecal samples.

Conclusion

Muellerius capillaris remains a significant cause of subclinical respiratory disease in sheep and goats, with economic impact due to reduced weight gain and increased susceptibility to secondary infections. Accurate diagnosis relies on the Baermann technique and, where available, molecular confirmation. Control requires an integrated approach combining pasture management, strategic deworming, and monitoring of anthelmintic efficacy. Continued surveillance and research into parasite biology, diagnosis, and resistance are essential for sustainable management.

References

[1] Panayotova-Pencheva MS. Species composition and morphology of protostrongylids (Nematoda: Protostrongylidae) in ruminants from Bulgaria. Parasitol Res. https://pubmed.ncbi.nlm.nih.gov/21461727/

[2] Berrag B, Urquhart GM. Epidemiological aspects of lungworm infections of goats in Morocco. Vet Parasitol. https://pubmed.ncbi.nlm.nih.gov/8750686/