Haemophilus agni/Histophilus ovis Infection in Sheep: Polyarthritis and Pleuritis

Etiology

Haemophilus agni and Histophilus ovis are gram-negative, pleomorphic coccobacilli belonging to the family Pasteurellaceae. Historical taxonomic confusion has resulted in these two names being used interchangeably for the same organism, with Histophilus ovis now the accepted species name. The bacterium is fastidious, requiring enriched media such as chocolate agar or blood agar supplemented with X and V factors (hemin and NAD), though some strains are V-factor dependent. Colonies appear small, grayish, and non-hemolytic after 24 to 48 hours of incubation under 5 to 10% carbon dioxide.

The organism possesses a polysaccharide capsule that contributes to virulence by inhibiting phagocytosis. Additional virulence factors include outer membrane proteins, lipopolysaccharide endotoxin, and iron acquisition systems that facilitate survival in host tissues. Histophilus ovis can survive within macrophages and epithelial cells, contributing to persistent infections and systemic dissemination.

Epidemiology

Histophilus ovis is a commensal of the upper respiratory tract and reproductive tract of healthy sheep. Carrier animals shed the organism in nasal secretions, saliva, and vaginal discharge. Transmission occurs primarily through direct contact, aerosol droplets, and fomites. Stress factors such as transport, crowding, poor ventilation, concurrent infections, and nutritional deficiencies predispose sheep to clinical disease.

The disease is reported worldwide, with higher incidence in lambs aged 2 to 6 months and in sheep under intensive management. Outbreaks often coincide with weaning, marketing, or adverse weather conditions. Morbidity can reach 30% in affected flocks, and mortality varies from 2 to 10% depending on the severity of systemic involvement.

Clinical Signs

The clinical syndrome combines polyarthritis and pleuritis, though one component may predominate. Incubation period ranges from 3 to 10 days after exposure.

In peracute cases, lambs may die suddenly without premonitory signs. Acute cases present with high fever, hot painful joints, and respiratory distress. Chronic cases develop fibrous joint thickening, muscle atrophy, and poor growth.

Pathology

Gross postmortem findings include fibrinous to fibrinopurulent polyarthritis with increased synovial fluid, joint capsule thickening, and erosion of articular cartilage. The pleural cavity contains fibrinous exudate with adhesions between visceral and parietal pleura. Lung consolidation and interlobular edema may be present. Pericarditis and meningitis are less frequent.

Histopathological examination reveals neutrophilic infiltration in synovial membranes, fibrin deposition, and necrosis. Pleural lesions show fibrin, neutrophils, macrophages, and hyperplasia of mesothelial cells. In the lungs, interstitial pneumonia with alveolar exudate is observed.

Diagnostic Approach

Definitive diagnosis requires isolation or molecular detection of Histophilus ovis from synovial fluid, pleural exudate, or lung tissue. A diagnostic workflow is presented below.

flowchart TD
    A[Clinical suspicion: lameness, respiratory distress, fever], > B[Sample collection]
    B, > C[Synovial fluid, pleural swab, lung tissue]
    C, > D[Gram stain: pleomorphic gram-negative rods]
    D, > E[Culture: chocolate agar + CO2]
    E, > F[Colony morphology: small grayish]
    F, > G[Biochemical tests: oxidase positive, catalase variable, V-factor requirement]
    G, > H[Molecular confirmation: PCR targeting 16S rRNA or specific genes]
    H, > I[Definitive diagnosis]
    C, > J[Blood culture in septicemic cases]
    J, > H
    B, > K[Serology: ELISA or AGID (limited sensitivity)]
    K, > L[Interpret with caution; cross-reactions with other Pasteurellaceae]

Differential Diagnosis

Histophilus ovis infection must be differentiated from other causes of polyarthritis and pleuritis in sheep:

• Erysipelothrix rhusiopathiae (swine erysipelas-like arthritis)
• Mycoplasma agalactiae (contagious agalactia)
• Mycoplasma capricolum subsp. capripneumoniae
• Streptococcus dysgalactiae (joint ill in lambs)
• Chlamydia abortus (polyarthritis in lambs)
• Borreliella (formerly Borrelia) species
• Trueperella pyogenes (supportative arthritis)

Respiratory involvement mimics pasteurellosis (Mannheimia haemolytica, Bibersteinia trehalosi) and Mycoplasma ovipneumoniae. Concurrent infections are common. A detailed comparison for Mycoplasma bovis in cattle is available in the article Mycoplasma bovis in Feedlot Cattle: Chronic Pneumonia, Arthritis, and the Challenge of Cultivation versus Molecular Detection.

Treatment

Antimicrobial therapy should be initiated promptly based on culture and susceptibility testing. Due to the intracellular nature of the infection, treatment duration of 7 to 14 days is recommended.

Supportive therapy includes nonsteroidal anti-inflammatory drugs for pain and inflammation, fluid therapy for dehydration, and nutritional support for recumbent animals. Severely affected joints may require arthrocentesis to remove purulent exudate.

Antimicrobial resistance has been documented, particularly to beta-lactam antibiotics. Judicious use of antimicrobials and routine resistance surveillance are advised.

Control and Prevention

Control measures focus on reducing stress, improving biosecurity, and minimizing exposure to infected carriers.

Management Strategies

1. Maintain clean, dry bedding and adequate ventilation in lambing pens and feedlots.
2. Reduce stocking density to minimize aerosol transmission.
3. Practice all-in/all-out management for feedlot groups.
4. Quarantine and test newly introduced animals.
5. Cull chronically infected sheep that may serve as reservoirs.

Vaccination

Autogenous vaccines (bacterins) have been used in some flocks with variable success. No commercial vaccine is widely available for Histophilus ovis. Development of a cross-protective vaccine is hindered by antigenic diversity among strains.

Antimicrobial Prophylaxis

Metaphylactic administration of long-acting oxytetracycline or tulathromycin at the time of high stress (e.g., weaning, transport) may reduce clinical outbreaks. This approach should be guided by susceptibility data and used sparingly to minimize resistance development.

Zoonotic Potential

There is no evidence that Histophilus ovis causes disease in humans. The organism is considered host-restricted to sheep and possibly goats. No public health measures are required.

References

The following references provide foundational information on the pathogenesis, diagnosis, and management of Histophilus ovis infection in sheep. No specific primary research articles were available from the provided literature search; therefore, standard veterinary microbiology texts and review sources are cited.

1. Quinn PJ, Markey BK, Leonard FC, FitzPatrick ES, Fanning S. Veterinary Microbiology and Microbial Disease. 2nd ed. Wiley-Blackwell; 2011.
2. Radostits OM, Gay CC, Hinchcliff KW, Constable PD. Veterinary Medicine: A Textbook of the Diseases of Cattle, Horses, Sheep, Pigs and Goats. 10th ed. Saunders Elsevier; 2007.
3. Aitken ID. Diseases of Sheep. 4th ed. Blackwell Publishing; 2007.