Porcine Reproductive and Respiratory Syndrome (PRRS) in pigs: Causes, Symptoms, and Effective Control Measures

The blue ear disease in pigs, scientifically known as PRRS (Porcine Reproductive and Respiratory Syndrome), is also referred to as the “Reproductive and Respiratory Disorder Syndrome in pigs.”
It is a dangerous infectious disease in pigs caused by the PRRSV virus, belonging to the Arteriviridae family. The disease causes pigs to lose appetite, develop high fever, bluish discoloration of the ears, cough, and experience difficulty breathing.
In particular, pregnant sows may suffer from abortion, premature farrowing, or stillbirths. PRRS is a major threat to pig production efficiency, causing serious economic losses for pig farmers worldwide, including in Vietnam.
Although PRRS in pigs has appeared for many years, it remains a nightmare for pig farms because of its rapid spread and difficulty in control. In this article, let’s join Fivevet to learn in detail about the causes, symptoms, and the most effective prevention and treatment methods for PRRS available today for pig farmers.
  1. Causes of PRRS in pigs
1.1. Classification of the virus causing blue ear disease in pigs
The cause of PRRS in pigs is a virus belonging to the Arteriviridae family. This name originates from a type of virus that causes equine arteritis (Equine arteritis virus).
In 1991, the PRRS virus was first successfully isolated at the Lelystad Veterinary Institute in the Netherlands, and later in the United States and Germany. To this day, the name “Lelystad virus” is still mentioned in research, but in practical pig farming, the name PRRS is more commonly used. 
PRRSV includes 2 genotypes:
• Genotype 1 (Prototype is the Lelystad virus strain) 
• Genotype 2 (Prototype is the VR-2332 strain)
The two prototype strains share approximately 44% nucleotide sequence similarity. Based on ORF5 gene sequencing, both genotypes exhibit extremely high genetic diversity. Even within the same strain, the virus shows differences in nucleotide sequences:
• For genotype 1, the variation can reach up to 30%
• For genotype 2, the variation exceeds 21%
Most PRRS outbreaks in Vietnam have been identified as being caused by highly pathogenic PRRSV strains, sharing up to 99% similarity with the PRRS strains circulating in China and 98% similarity with the North American strains. 
1.2. The resistance of the virus causing porcine reproductive and respiratory syndrome (PRRS) in pigs
PRRSV can survive for a long time in cold conditions but is sensitive to high temperatures. In addition, the virus can survive at a pH range of 6.5–7.5 and is inactivated under the following conditions:
• pH < 6 or > 7.5
• Exposure to sunlight or UV radiation
• Exposed to common disinfectants
In serum stored at 4°C or -20°C, PRRSV can still be isolated from 85% of samples after 7 hours.
2. Epidemiology of porcine reproductive and respiratory syndrome (PRRS)
2.1. Susceptible species
PRRSV only causes disease in pigs. All age groups can be affected, but piglets and pregnant sows are the most susceptible. In addition, wild boars can also be infected and are considered a natural reservoir of the virus. 
2.2. Virulence of PRRSV
PRRS virus exists in two forms of virulence:
• Classical form: Low virulence, causing a mortality rate of 1–5% of the herd.
• Highly pathogenic variant: Causes mass mortality and severe impact on pig herds.
2.3. Transmission routes of porcine reproductive and respiratory syndrome (PRRS)
PRRS can be transmitted both directly and indirectly:
• Direct transmission: Healthy pigs come into contact with sick or virus-carrying pigs.
• Indirect transmission: Through semen, artificial insemination, vertical transmission from sow to piglets, farm equipment, work clothing, insect vectors such as wild birds, flies, mosquitoes, arthropods, or via saliva, air, wind, and dust. 
3. Clinical signs of porcine reproductive and respiratory syndrome (PRRS)
3.1. Clinical signs in sows
Within the first month of viral infection, sows show anorexia for 7–14 days (affecting 10–15% of the herd) and high fever of 39–40°C. Late-term abortions are common (1–6%), and the ears may temporarily turn blue (2%), which is why the disease is called “porcine blue-ear disease.
Sows may give birth prematurely (10–15%), exhibit false estrus (3–5 weeks after insemination), or show no or delayed return to estrus after farrowing. Affected pigs may cough and show signs of pneumonia.
During the farrowing and lactation period, sows may exhibit anorexia, reduced water intake, agalactia, and mastitis. Farrowing may occur 2–3 days early, with discolored skin, lethargy, or even coma, and mummified fetuses (10–15% of fetuses die during the last 3–4 weeks of gestation). Piglets may die immediately after birth (30%), be weak, or have blue ears (under 5%) lasting for a few hours.
Notably, herd mortality can reach up to 70% in the 3rd–4th week after the onset of the first clinical signs. Additionally, approximately 5–80% of sows may experience abortion between days 100 and 118 of gestation.
Long-term impact on productivity:
• Reproductive disorders can persist for 4–8 months.
• Farrowing rates decrease by 10–15%, with increased postnatal mortality and reduced number of surviving piglets.
• Gilts often exhibit poor reproductive performance.
• Abortion rates increase by 2–3%, and anorexia during the reproductive period leads to significant reductions in production.
 3.2. Clinical signs in boars
• Anorexia, fever, lethargy, depression, and respiratory symptoms.
• Reduced libido or loss of sexual drive. Semen quality is poor, and semen volume is low, which negatively affects breeding performance and results in smaller piglets at birth.
 3.3. Clinical signs in suckling piglets
Piglets are weak and quickly develop hypoglycemia due to their inability to nurse. Eyes may have brown discharge, and the skin may show vesicular lesions. Severe diarrhea occurs, reducing survival rates. There is an increased risk of respiratory diseases. Piglets may exhibit splayed legs and unsteady walking. Mortality ranges from 12–15%, and can reach 60–100% if infected with highly pathogenic PRRS virus strains.
 3.4. Clinical signs in weaned piglets and growers
Affected pigs show anorexia, varying degrees of coughing, and rough, bristled hair. Some pigs may appear asymptomatic, while others, especially when co-infected with other diseases, may develop acute, diffuse pneumonia with multiple abscesses.
Piglets are weak with pale skin. They may have diarrhea, sneezing, watery eyes, rapid or labored breathing, and reduced weight gain. Mortality can reach 12–20%. PRRS often occurs in combination with other diseases such as swine fever, swine asthma, streptococci, salmonellosis, Glasser’s disease, and PCV2 infection.
Highly pathogenic strains cause prolonged high fever (40–42°C), rapid weight loss, and high mortality. Clinical signs may include cyanotic “blue” ears, vomiting, diarrhea or constipation, trembling, and conjunctivitis.
4. Pathological lesions of porcine reproductive and respiratory syndrome (PRRS)
In pigs infected with highly pathogenic PRRS Virus, severe lesions can be observed in multiple organs:
• Lungs: Hemorrhage and interstitial pneumonia characterized by firm, dense areas on the lung lobes. Affected lobes appear gray-red, with pus, firm, and elastic. On cross-section, affected lobes are raised and dry. In many cases, suppurative bronchopneumonia occurs on the underside of the apical lobes.
• Lymph nodes: Enlarged 2–10 times the normal size. Early in infection, nodes are edematous, brown to yellow, with hemorrhage and medium firmness; later, they become firm, hard, and white or brown in color.
• Other lesions: Skin necrosis, renal cortical hemorrhage, peritoneal and conjunctival hemorrhage, and thymic atrophy.
• Male pigs: 5–6 months old, 7–25 days post-infection, show seminiferous tubule atrophy, leading to reduced sperm count.
Aborted fetuses often present with perirenal edema, splenic ligament edema, edematous mesenteric lymph nodes, accumulation of fluid in the abdominal and thoracic cavities, and enlarged umbilical cords due to edema and band-like hemorrhages.
At PRRS outbreak sites in Vietnam, common lesions include: congested brain, lung congestion or hemorrhage, swollen and congested tonsils, enlarged congested liver, congested spleen, pin-point renal hemorrhages, hemorrhagic mesenteric lymph nodes, and ileocecal valve ulcers. Aborted piglets are often small and emaciated.
5. Diagnosis of porcine reproductive and respiratory syndrome (PRRS)
5.1. Clinical diagnosis
Diagnosis is based on epidemiological characteristics, herd history, clinical signs, and characteristic pathological lesions. Differential diagnosis should consider diseases caused by parvovirus, pseudorabies, Circovirus type 2, Enterovirus, swine influenza, classical swine fever, leptospirosis, meningitis, and others.
5.2. Laboratory diagnosis
If a herd is suspected of being infected, contact the Central Veterinary Diagnostic and Testing Center No. 5 (FiveLab) at the hotline +84 822 120 555 for timely consultation and support.
6. Prevention of porcine reproductive and respiratory syndrome (PRRS)
6.1. Hygiene and biosecurity
Improve the farming environment, implement “all-in, all-out” production, perform regular cleaning and disinfection, and provide adequate feed and water suitable for each age group. Avoid introducing pigs into the farm during disease outbreaks and cull emaciated or severely ill animals. Preventive vaccination against common pig diseases such as classical swine fever, FMD, Mycoplasma, pseudorabies, Glasser’s disease, Circovirus, PRRS, …
6.2. Vaccination for PRRS prevention - Five-PRRS gold 7. Treatment protocol for porcine reproductive and respiratory syndrome (PRRS)
- Isolate and separate sick pigs for treatment.
- Emergency fever reduction and anti-inflammatory treatment using one of the following products: Five-Gluco.KC namic, Five-MetaMax.50, Five-Ketofen,… for a course of 3–5 days.
- Supportive therapy and energy supplementation using Five-Butasal, Five-Cafein,... for a course of 3–5 days.
- Expectorant and cough relief: Inject Five-Bromhexin or administer Five-Extra Oil orally at 1 ml per 4 liters of water.
Inject Five-Tulaket or Five-Tulacin at 1 ml per 40 kg body weight as a single dose. Alternatively, use one of the following drugs: Five-Cefketo Plus, Five-Doflo@LA max, Five-PenStrep.LA,… for a course of 3–5 days.
- Whole herd:
+ Antibiotics in feed or water: Five-Doxyl 750, Five-Tylvasin, Five-Doflo,…
- Fever reduction: Five-Cam Cum, Five-Para.C, …
- Immune support and body supplementation: Five-Anti virus, Beta-Glucan.C, Five-Prozyme 5way, Five-Orgacid, Five-Aminovit super,... for a course of 3–5 days.
- Liver and kidney detoxification: Five-Orgamin, Five-Phosric, Five-Acemin.B12… for a course of 3–5 days.