Peste des Petits Ruminants (PPR)

Peste des petits ruminants (PPR) is a highly contagious, acute to subacute, viral disease of domestic and wild small ruminants, characterized by severe pyrexia, erosive stomatitis, mucopurulent ocular and nasal discharges, gastroenteritis, and pneumonia. It is caused by the peste des petits ruminants virus (PPRV), a morbillivirus within the family Paramyxoviridae, genus Morbillivirus. PPR is clinically and pathologically analogous to rinderpest in cattle but affects sheep and goats, with goats generally more severely affected. The disease is endemic in much of Africa, the Middle East, and Asia, and has been targeted for global eradication by 2030 by the FAO and OIE. In affected flocks, morbidity can reach 100% and mortality ranges from 20% to 90%, causing devastating economic losses through death, reduced productivity, and trade restrictions. The disease is a major constraint to small ruminant production in developing countries, impacting food security and rural livelihoods. PPR is a notifiable disease to the World Organisation for Animal Health (OIE) and is subject to strict international trade regulations.

The causative agent is peste des petits ruminants virus (PPRV), a single-stranded, negative-sense RNA virus belonging to the genus Morbillivirus in the family Paramyxoviridae. PPRV is closely related to rinderpest virus (RPV), measles virus, canine distemper virus, and phocine distemper virus. The virus has a lipid envelope and is sensitive to heat, ultraviolet light, and common disinfectants. There are four distinct genetic lineages (I, II, III, IV) based on the partial sequence of the fusion (F) protein gene, but all lineages are serologically indistinguishable and confer cross-protection. The virus primarily infects epithelial cells and cells of the immune system, particularly lymphocytes and macrophages, leading to profound immunosuppression. The virus is shed in all secretions and excretions during the acute phase, including ocular, nasal, oral, and fecal material. The virus can survive in the environment for short periods but is readily inactivated by sunlight, heat, and lipid solvents.

PPR affects domestic sheep (Ovis aries) and goats (Capra hircus), with goats generally more susceptible and exhibiting more severe clinical signs. Wild small ruminants, including various species of gazelles, ibex, and deer, can also be infected and may serve as reservoirs. The disease is endemic in sub-Saharan Africa, North Africa, the Middle East, Turkey, Central and South Asia, and parts of China. In endemic areas, the disease is often maintained in goat populations, with periodic outbreaks in sheep. Transmission occurs primarily through direct contact with infected animals via aerosol droplets, ocular and nasal discharges, and contaminated feed and water. The incubation period is typically 2-6 days. Morbidity can reach 100% in susceptible flocks, and mortality varies from 20% to 90%, depending on the virus strain, host species, and immune status. In naïve populations, the disease can cause explosive outbreaks with high case fatality rates. The disease is more severe in young animals (3-12 months) and in goats compared to sheep. In endemic regions, the disease is often enzootic, with outbreaks occurring seasonally, often associated with introduction of new animals or stress factors such as transportation, parturition, or adverse weather. Economic losses arise from mortality, reduced milk production, weight loss, abortion, and trade restrictions.

PPRV enters the host via the respiratory or oral route and initially replicates in the regional lymph nodes, particularly the tonsils and retropharyngeal lymph nodes. The virus then spreads via the bloodstream (viremia) to secondary lymphoid organs, including the spleen, lymph nodes, and bone marrow, and to epithelial tissues of the respiratory, gastrointestinal, and oral mucosa. The virus has a strong tropism for epithelial cells and lymphocytes. In epithelial cells, viral replication causes necrosis and erosion, leading to the characteristic oral lesions, diarrhea, and pneumonia. In lymphocytes, the virus induces apoptosis and necrosis, resulting in severe lymphopenia and immunosuppression, which predisposes to secondary bacterial infections, particularly pneumonia. The virus also damages the vascular endothelium, leading to increased vascular permeability, edema, and hemorrhage. The gastrointestinal tract shows severe necrotic enteritis, particularly in the ileum and colon, with villous atrophy and crypt necrosis, leading to malabsorption and profuse diarrhea. The respiratory tract shows interstitial pneumonia, with necrosis of bronchial and alveolar epithelium, and secondary bacterial bronchopneumonia is common. The immunosuppression also leads to increased susceptibility to other pathogens, such as Pasteurella multocida and Mannheimia haemolytica, exacerbating the respiratory disease.

Several factors increase the risk of PPR outbreaks and severity of disease. Intrinsic factors include species (goats more susceptible than sheep), age (young animals 3-12 months are most susceptible), and breed (some indigenous breeds may have some resistance). Extrinsic factors include overcrowding, poor biosecurity, introduction of new animals without quarantine, and stress from transportation, parturition, or adverse weather. In endemic areas, the disease is often maintained in goat populations, and outbreaks occur when susceptible animals are introduced or when immunity wanes. Malnutrition and concurrent infections, such as gastrointestinal parasitism or other viral infections, can exacerbate the severity of PPR. Lack of vaccination or inadequate vaccination coverage is a major risk factor. Trade and movement of animals across borders can introduce the virus into previously free areas.

The clinical signs of PPR vary from peracute to subacute, depending on the virus strain and host susceptibility. The incubation period is 2-6 days. The peracute form is seen mainly in goats and is characterized by sudden death with few premonitory signs. The acute form is the most common and presents with a sudden onset of high fever (40-41°C), depression, anorexia, and serous ocular and nasal discharges that become mucopurulent within 2-3 days. Oral lesions appear as hyperemia, small necrotic foci, and erosions on the gums, dental pad, hard palate, and tongue. These lesions may coalesce to form extensive necrotic areas. Diarrhea develops 2-3 days after the onset of fever, often with blood and mucus, and can be profuse and dehydrating. Respiratory signs include coughing, dyspnea, and nasal discharge, which may become purulent due to secondary bacterial infection. Pregnant animals may abort. In severe cases, death occurs within 5-10 days. The subacute form is milder, with less severe clinical signs and lower mortality, and is more common in sheep. In endemic areas, the disease may be inapparent or mild in some animals. On flock examination, multiple animals may be affected simultaneously, with a high morbidity rate.

Differential diagnoses for PPR include: 1) Foot-and-mouth disease (FMD): Caused by aphthovirus, affects cattle, sheep, goats, and pigs; characterized by vesicular lesions on the feet, mouth, and teats; vesicles rupture to form erosions; but diarrhea and pneumonia are not prominent. 2) Bluetongue (BT): Caused by orbivirus, transmitted by Culicoides midges; affects sheep and goats; characterized by fever, oral lesions, facial edema, coronitis, and lameness; but diarrhea is less common. 3) Contagious ecthyma (orf): Caused by parapoxvirus; characterized by proliferative, scabby lesions on the lips, muzzle, and sometimes udder; lesions are more proliferative than erosive. 4) Sheep pox and goat pox: Caused by capripoxviruses; characterized by generalized skin nodules, fever, and respiratory signs; but oral erosions and diarrhea are less common. 5) Pasteurellosis (pneumonic pasteurellosis): Caused by Mannheimia haemolytica or Pasteurella multocida; characterized by pneumonia, fever, and nasal discharge; but oral lesions and diarrhea are absent. 6) Rinderpest: Caused by rinderpest virus, which is now eradicated; clinically similar but affects cattle and buffalo, not sheep and goats. 7) Malignant catarrhal fever (MCF): Caused by ovine herpesvirus-2 or alcelaphine herpesvirus-1; affects cattle and deer, but can affect sheep and goats; characterized by fever, oral erosions, corneal opacity, and neurological signs. 8) Salmonellosis: Caused by Salmonella spp.; characterized by fever, diarrhea, and septicemia; but oral lesions are absent. 9) Coccidiosis: Caused by Eimeria spp.; affects young animals; characterized by diarrhea, sometimes with blood; but fever and oral lesions are absent. 10) Nutritional deficiencies or toxicities: e.g., copper deficiency or selenium deficiency, which can cause diarrhea and poor growth, but not the acute febrile disease.

The diagnostic approach for PPR involves: 1) Flock history: Assess morbidity and mortality rates, recent introduction of animals, vaccination status, and presence of similar cases in neighboring flocks. 2) Clinical examination: Look for characteristic signs such as fever, ocular and nasal discharges, oral erosions, diarrhea, and pneumonia. 3) Differential diagnosis: Rule out other diseases with similar clinical signs, such as FMD, bluetongue, orf, and pasteurellosis. 4) Laboratory confirmation: Collect samples from acutely affected animals, including whole blood (EDTA) for PCR, serum for serology, and swabs from ocular, nasal, and oral lesions for virus isolation or PCR. Also collect tissue samples (lymph nodes, spleen, lungs, and intestines) at necropsy for histopathology and PCR. 5) PCR: Reverse transcription polymerase chain reaction (RT-PCR) is the preferred method for rapid and sensitive detection of PPRV RNA. 6) Virus isolation: Can be performed on Vero cells or other susceptible cell lines, but is time-consuming and requires specialized facilities. 7) Serology: ELISA can detect antibodies to PPRV, which is useful for surveillance and confirming exposure. 8) Necropsy: Perform a thorough necropsy to identify characteristic lesions, such as erosive stomatitis, necrotic enteritis, and pneumonia. 9) Histopathology: Examine tissues for characteristic lesions, such as syncytial cells, intracytoplasmic and intranuclear inclusion bodies, and lymphoid necrosis. 10) Confirmatory diagnosis: Based on positive PCR or virus isolation, along with compatible clinical signs and pathology.

Laboratory findings in PPR include: 1) Hematology: Leukopenia, particularly lymphopenia, due to viral-induced lymphocyte destruction. 2) Blood chemistry: May show elevated liver enzymes (AST, GGT) and muscle enzymes (CK) due to tissue damage. 3) Serology: Detection of antibodies to PPRV by ELISA (cELISA or sELISA) indicates exposure or vaccination. 4) Molecular diagnostics: RT-PCR is highly sensitive and specific for detecting PPRV RNA in blood, swabs, and tissues. 5) Virus isolation: Can be attempted on cell cultures, but is not routinely performed. 6) Histopathology: Characteristic lesions include necrosis of lymphoid tissues, syncytial cell formation, and intranuclear and intracytoplasmic eosinophilic inclusion bodies in epithelial cells. 7) Immunohistochemistry: Can detect PPRV antigen in formalin-fixed tissues. 8) Fecal examination: May reveal secondary parasitic infections, but is not specific for PPR.

Imaging is not commonly used in the diagnosis of PPR, but thoracic radiography or ultrasonography may reveal lung consolidation and pneumonia in affected animals. Ultrasonography of the thorax can show B-lines and consolidation, indicating interstitial or bronchopneumonia. Abdominal ultrasonography may show thickened intestinal walls and fluid-filled loops, consistent with enteritis. However, imaging is not a primary diagnostic tool for PPR and is mainly used to assess the severity of respiratory complications.

Histopathological examination of tissues from animals with PPR reveals characteristic lesions. In the oral mucosa, there is necrosis and erosion of the epithelium, with ballooning degeneration and syncytial cell formation. Intranuclear and intracytoplasmic eosinophilic inclusion bodies may be present in epithelial cells. In the lymphoid tissues (lymph nodes, spleen, tonsils), there is severe lymphoid depletion and necrosis, with the presence of syncytial cells. In the lungs, there is interstitial pneumonia with necrosis of bronchial and alveolar epithelium, and secondary bacterial bronchopneumonia may be present. In the intestines, there is necrotic enteritis with villous atrophy and crypt necrosis. Immunohistochemistry can demonstrate PPRV antigen in affected tissues. Cytological examination of impression smears from oral lesions may show syncytial cells and inclusion bodies, but this is not a reliable diagnostic method.

There is no specific antiviral treatment for PPR. Treatment is supportive and aims to reduce mortality and secondary infections. Supportive care includes: 1) Fluid therapy: Administer oral or intravenous fluids to correct dehydration and electrolyte imbalances. 2) Nutritional support: Provide easily digestible feed and ensure adequate nutrition. 3) Antibiotics: Administer broad-spectrum antibiotics (e.g., oxytetracycline, amoxicillin, or sulfonamides) to prevent or treat secondary bacterial infections, particularly pneumonia. 4) Anti-inflammatory drugs: Non-steroidal anti-inflammatory drugs (NSAIDs) such as flunixin meglumine or meloxicam can reduce fever and inflammation. 5) Vitamin and mineral supplementation: Provide vitamins A, D, and E, and minerals to support immune function. 6) Isolation: Isolate affected animals to prevent spread. 7) Vaccination: In the face of an outbreak, emergency vaccination with a live attenuated PPR vaccine can be used to control spread. The vaccine is effective and provides long-lasting immunity. 8) Biosecurity: Implement strict biosecurity measures, including disinfection of premises and equipment, and restriction of animal movement.

The prognosis for individual animals with PPR is guarded to poor, especially in goats and young animals. Mortality rates can be high, particularly in peracute cases. In flocks, the prognosis depends on the immune status of the population, the virus strain, and the quality of supportive care. With prompt supportive treatment and control measures, mortality can be reduced. However, the disease can have long-term economic impacts due to reduced productivity and trade restrictions. In endemic areas, the disease is often controlled by vaccination, and the prognosis for flocks with good vaccination coverage is better.

After an outbreak of PPR, follow-up measures include: 1) Monitoring: Continue to monitor the flock for new cases and assess recovery. 2) Vaccination: Implement a regular vaccination program to prevent future outbreaks. 3) Biosecurity: Maintain strict biosecurity measures to prevent introduction of the virus. 4) Quarantine: Quarantine new animals before introducing them to the flock. 5) Disinfection: Thoroughly clean and disinfect premises and equipment. 6) Reporting: Report the outbreak to the relevant veterinary authorities as required by law. 7) Surveillance: Conduct serological surveillance to monitor the immune status of the flock. 8) Necropsy: Perform necropsies on any animals that die to confirm the diagnosis and rule out other diseases.

Clinical pearls: 1) PPR should be considered in any outbreak of febrile disease with oral erosions, diarrhea, and pneumonia in sheep and goats. 2) Goats are more severely affected than sheep. 3) The presence of oral lesions and diarrhea in a goat with high fever is highly suggestive of PPR. 4) Vaccination is the most effective control measure. 5) Supportive care can reduce mortality. Pitfalls: 1) Confusing PPR with FMD or bluetongue, which have different control measures. 2) Failing to report the disease to authorities, leading to spread. 3) Using antibiotics alone without supportive care, which may not reduce mortality. 4) Not implementing biosecurity measures, leading to spread within the flock and to neighboring flocks. 5) Vaccinating animals that are already incubating the disease, which may not provide protection.

There is no specific antiviral drug for PPR. Treatment is supportive and includes: 1) Fluid therapy: Oral rehydration solutions (e.g., electrolytes with glucose) or intravenous fluids (e.g., lactated Ringer's solution) at 50-100 ml/kg/day. 2) Antibiotics: Oxytetracycline (10 mg/kg IM or SC, q24h) or amoxicillin (10 mg/kg IM, q24h) or sulfadimethoxine (50 mg/kg PO, q24h) for 3-5 days to prevent secondary bacterial infections. 3) NSAIDs: Flunixin meglumine (1.1-2.2 mg/kg IV or IM, q24h) or meloxicam (0.5 mg/kg PO or SC, q24h) to reduce fever and inflammation. 4) Vitamins: Vitamin A (10,000-20,000 IU/kg IM) and vitamin C (100-200 mg/kg IM) may be given. 5) Vaccination: Live attenuated PPR vaccine (e.g., Nigeria 75/1 strain) at a dose of 10^3 TCID50 per animal, administered SC or IM. The vaccine provides immunity for at least 3 years. 6) Withdrawal times: For meat, oxytetracycline has a withdrawal time of 28 days; for milk, 96 hours. For flunixin, meat withdrawal is 7 days; milk withdrawal is 36 hours. Always follow label instructions and local regulations.

PPR is a well-studied disease, and the literature supports the following: 1) The disease is caused by PPRV, a morbillivirus, and is closely related to rinderpest virus. 2) The disease is endemic in many parts of Africa, the Middle East, and Asia, and has been targeted for global eradication by 2030. 3) Vaccination with live attenuated vaccines is highly effective in controlling the disease. 4) The disease causes significant economic losses in small ruminant production systems. 5) Diagnosis is based on clinical signs, PCR, and serology. 6) Supportive care can reduce mortality, but prevention through vaccination is the most effective strategy. 7) The disease is notifiable to the OIE, and control measures include quarantine, movement restrictions, and vaccination. 8) Research is ongoing to develop new vaccines and diagnostic tools. Key references include: OIE Terrestrial Manual, FAO PPR Global Eradication Programme, and peer-reviewed articles in journals such as Veterinary Microbiology and Transboundary and Emerging Diseases.