Mycoplasma capricolum Infection in Sheep and Goats: Pleuropneumonia and Mastitis

Introduction

Mycoplasma capricolum is a member of the "Mycoplasma mycoides cluster," a group of phylogenetically related mollicutes that cause significant diseases in ruminants [1]. Within this species, two subspecies are recognized: Mycoplasma capricolum subsp. capripneumoniae (Mccp), the primary agent of contagious caprine pleuropneumonia (CCPP) in goats and occasionally sheep, and Mycoplasma capricolum subsp. capricolum (Mcc), associated with arthritis, mastitis, and pneumonia in both sheep and goats [2, 3]. The clinical syndromes produced by these organisms range from acute fibrinous pleuropneumonia to mastitis and polyarthritis, causing substantial economic losses in small ruminant production systems across Africa, the Middle East, and Asia [4, 5]. This article provides an exhaustive clinical and diagnostic reference for veterinary professionals and computational biologists, integrating molecular diagnostic strategies with traditional pathological assessment.

Etiology and Taxonomy

M. capricolum belongs to the class Mollicutes, order Mycoplasmatales, and family Mycoplasmataceae. The species is characterized by its small genome (approximately 1.1 Mb), lack of a cell wall, and absolute dependence on host-derived cholesterol for membrane stability. The M. mycoides cluster includes M. mycoides subsp. mycoides (small colony type), M. mycoides subsp. capri, M. capricolum subsp. capripneumoniae, and M. capricolum subsp. capricolum [1]. Differentiation between Mccp and Mcc is critical for epidemiological and control purposes, as Mccp is a notifiable pathogen under the World Organisation for Animal Health (WOAH) guidelines for CCPP.

Biochemically, both subspecies ferment glucose, reduce tetrazolium, and produce phosphatase. They are distinguished from other cluster members by serological methods, DNA-DNA hybridization [6], and molecular techniques such as PCR-RFLP [7] and high-resolution melting (HRM) curve analysis [8]. A dot immunobinding assay on membrane filtration (MF dot) has also been used to study antigenic relationships within the cluster [9]. The membrane proteins of M. capricolum strains can be analyzed by SDS-PAGE and immunoblotting to reveal strain-specific and cross-reactive antigens [10].

Epidemiology

Geographic Distribution and Host Range

CCPP caused by Mccp is endemic in many parts of Africa (e.g., Ethiopia, Tanzania, Uganda, Mauritania, Egypt) and the Middle East, with sporadic outbreaks in Asia and Europe [4, 5]. A systematic review and meta-analysis of CCPP prevalence in sheep and goats estimated a pooled prevalence of approximately 30% in goats and 15% in sheep across affected regions [4]. Sheep are less susceptible to clinical CCPP but can become infected and serve as carriers [11, 12]. In Egypt, CCPP was identified for the first time in both sheep and goats, with pathological and molecular characterization confirming Mccp infection [12]. Similarly, in Uganda, Mccp was isolated from goats and sheep during a CCPP outbreak [13]. In northern Mauritania, a 2023 study detected Mccp co-infections with peste des petits ruminants virus (PPRV) in small ruminants, highlighting the complexity of respiratory disease in the region [14].

The Mcc subspecies has been isolated from sheep flocks in Iran [15], Great Britain [16], and Jordan [17], where it is associated with mastitis, arthritis, and pneumonia. Mcc has also been linked to arthritis in sheep [18] and to outbreaks of contagious agalactia in small ruminants in Northwest Iran [19].

Risk Factors

Several studies have identified risk factors for CCPP. In Ethiopia, herd-level risk factors include animal movement, introduction of new animals, large herd size, and contact with wildlife or other ruminants [20, 21]. In the Borana zone of southern Ethiopia, seroprevalence studies highlighted the role of mixed-species grazing (sheep and goats) and poor biosecurity practices [22]. In Tanzania, risk factors for Mccp and morbillivirus infection included agro-ecological zone, age, and husbandry practices [23]. A meta-analysis of CCPP in Ethiopia found that the disease is more prevalent in the highland regions and during the rainy season [24]. In the Derashe zone of southern Ethiopia, seroprevalence was associated with animal age, sex, and previous contact with diseased animals [25].

Transmission

M. capricolum is transmitted primarily via direct contact through respiratory droplets. Aerosol transmission over short distances is the main route for CCPP. Carrier animals, including those with subclinical or chronic infections, are an important source of Mccp [2]. For Mcc, the organism can also be shed in milk, leading to mastitis transmission to suckling kids and lambs. The ability of Mycoplasma species to survive in the environment is limited, but survival in fomites (e.g., contaminated needles, milking equipment) can occur for several hours.

Clinical Signs

Contagious Caprine Pleuropneumonia (Mccp Infection)

In goats, CCPP presents as an acute, subacute, or chronic fibrinous pleuropneumonia. Clinical signs include high fever (40-42°C), severe dyspnea, open-mouth breathing, productive cough, nasal discharge, and thoracic pain. Morbidity can reach 80-100% and mortality 60-80% in naïve herds [2]. In sheep, clinical signs are often milder, with low-grade fever, cough, and reduced productivity [11]. Thoracic ultrasonography has been used to characterize lung lesions antemortem in both species [26]. In a retrospective study of ovine CCPP cases, clinical, sonographic, and pathological findings confirmed that sheep can develop severe pleuropneumonia indistinguishable from that in goats [11].

Mastitis, Arthritis, and Other Syndromes (Mcc Infection)

M. capricolum subsp. capricolum is a cause of mastitis in sheep and goats, often presenting as acute gangrenous mastitis or chronic indurative mastitis. Affected animals exhibit swollen, painful udders, with a serous to purulent secretion that may become bloody. Agalactia is common, leading to poor growth of offspring. Arthritis, typically polyarticular, is also seen, especially in lambs and kids, with lameness, joint swelling, and fever [3, 18]. Experimental infection of sheep with Mcc produced pyrexia, arthritis, and in some cases pneumonia [3].

Pathology

Consistent gross pathological findings include severe unilateral pleuropneumonia with massive fibrinous exudate in the pleural cavity, hepatization of lung lobes, and fibrin deposition on pleural surfaces. The characteristic "marbled" appearance is due to interlobular septal edema and fibrin. Histopathological examination reveals necrotic alveolitis, vasculitis, and intense inflammatory cell infiltration with mononuclear cells and neutrophils. In mastitis, the mammary parenchyma is firm, with a thickened interstitium and multiple abscesses.

Importantly, CCPP lesions must be differentiated from those caused by Pasteurella multocida, Mannheimia haemolytica, and Mycoplasma mycoides subsp. capri [27]. Co-infections with PPRV and Pasteurella multocida are frequent and complicate pathological interpretation [14, 28].

Diagnosis

A definitive diagnosis of M. capricolum infection requires a combination of clinical, epidemiological, pathological, and laboratory methods. The following diagnostic workflow is recommended.

Mermaid Diagnostic Workflow

graph TD
    A[Clinical suspect: fever, dyspnea, cough, mastitis], > B[Thoracic ultrasonography / necropsy]
    B, > C[Thoracic fluid / milk / joint fluid / lung tissue]
    C, > D[Microbiological culture in SP4 or Hayflick medium]
    D, > E{Colony growth within 3-7 days}
    E, >|Positive| F[Biochemical & serological typing]
    E, >|Negative| G[PCR / HRM / Multiplex qPCR]
    F, > H[Subspecies confirmation: PCR-RFLP / HRM]
    G, > H
    H, > I[Report & control actions]
    C, > J[Direct PCR (without culture)]
    J, > H
    B, > K[Serology: cELISA, MF dot]
    K, > I

Molecular Diagnostics

Polymerase chain reaction (PCR) targeting the 16S rRNA gene or species-specific genes (e.g., the Mccp-specific region) is highly sensitive. High-resolution melting (HRM) curve analysis can differentiate Mccp from Mycoplasma mycoides subsp. capri in a single tube [8]. A multiplex RT-qPCR assay that simultaneously detects PPRV, capripoxvirus, Pasteurella multocida, and Mccp has been developed for rapid differential diagnosis [28]. PCR followed by restriction endonuclease digestion (PCR-RFLP) is useful for distinguishing Mcc from Mccp and other cluster members [7, 29]. In Iran, Mcc was detected in sheep flocks using PCR and culture [15]. In Tanzania, field strains of Mccp were characterized by molecular analysis [30].

Serological Assays

Serological tools include the complement fixation test (CFT), competitive ELISA (cELISA), and dot immunobinding assays [9]. A cELISA is recommended by WOAH for CCPP serosurveillance. Seroprevalence studies in Ethiopia [22, 25, 31] and Turkey [32] have relied on ELISA to estimate exposure. However, serology cannot differentiate between Mccp and Mcc due to antigenic cross-reactivity; a positive result must be interpreted in conjunction with clinical signs and, where possible, confirmed by molecular methods.

Differential Diagnosis

A comprehensive differential diagnosis list for CCPP-like respiratory disease includes Pasteurella multocida, Mannheimia haemolytica, Mycoplasma ovipneumoniae, PPRV, and lungworms. Mastitis caused by Mcc must be distinguished from staphylococcal, streptococcal, and Trueperella pyogenes infections (see [Trueperella pyogenes in Cattle] for comparative mastitis pathogenesis).

Antimicrobial Treatment and Resistance

Mycoplasmas lack a cell wall, rendering beta-lactams and other cell wall-active antibiotics ineffective. The drugs of choice are macrolides (e.g., tylosin, tilmicosin), tetracyclines (e.g., oxytetracycline), fluoroquinolones (e.g., enrofloxacin, danofloxacin), and pleuromutilins (e.g., tiamulin). In vitro susceptibility testing of Jordanian isolates of Mcc showed sensitivity to enrofloxacin, oxytetracycline, and tylosin [17]. In a study evaluating an inactivated vaccine against Mcc, the authors also assessed antimicrobial susceptibility and found varying MIC values [33].

In practice, treatment success with antimicrobials is often limited because of the rapid progression of disease in acute CCPP and the development of antimicrobial resistance (AMR). Empirical therapy should be guided by in vitro susceptibility testing when possible. The commonly used antimicrobials and their concentrations for in vitro testing in Mycoplasma cultures follow CLSI guidelines for mollicutes.

Control and Prevention

Vaccination

Inactivated vaccines against Mcc have been developed and evaluated in Morocco, showing promising immunogenicity and safety in goats [33]. Vaccines against CCPP (Mccp) are also used in some endemic countries, though they are not always effective due to strain variation and the need for adjuvant formulations. No commercial vaccine is universally available for all M. capricolum subspecies.

Biosecurity

Control strategies focus on preventing the introduction of infected animals into clean herds, quarantine of new arrivals, and culling of clinically affected animals. In outbreak situations, strict isolation is mandatory. Improved husbandry practices, such as reducing stocking density and separating age groups, can reduce transmission. The role of wildlife as a reservoir is important; CCPP outbreaks have been documented in captive wild ungulates, such as at the Al Wabra Wildlife Preservation in Qatar [34], suggesting that spillover to wildlife can occur.

Eradication of CCPP has been achieved in some countries through test-and-slaughter programs coupled with movement restrictions. In endemic regions, control relies on vaccination, antimicrobial metaphylaxis, and improved biosecurity.

Conclusion

Mycoplasma capricolum infection represents a major health challenge for small ruminant production systems worldwide. The two subspecies, Mccp and Mcc, cause distinct but overlapping clinical syndromes: pleuropneumonia (CCPP) and mastitis/arthritis, respectively. Accurate diagnostic differentiation is essential for appropriate control actions. Molecular tools such as HRM, multiplex PCR, and PCR-RFLP have enhanced the speed and accuracy of diagnosis. While antimicrobial therapy can be attempted, prevention through vaccination and biosecurity remains the cornerstone of control. Continued surveillance, particularly in regions where co-infections with PPRV are common, is necessary to understand the full epidemiological picture. The development of novel vaccines and point-of-care diagnostics will be critical for reducing the global burden of this disease.

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