Porcine Diseases Special: Pseudorabies

Pathogen

The pseudorabies virus (ADV or PRV) belongs to the Herpesviridae family, subfamily Alphaherpesvirinae, and is classified as Suid herpesvirus 1. It is a herpesvirus capable of causing fever, severe itching, and meningoencephalitis as its primary symptoms in various animals. Pigs serve as the reservoir host and source of infection for PRV, while humans are not susceptible to the virus. PRV is highly sensitive to lipid solvents such as ether, acetone, chloroform, and alcohol and is not resistant to disinfectants. The virus can be inactivated using Roxycide disinfectant solution.

Epidemiology

Pigs are the primary reservoir host for PRV, but other livestock, such as cattle, sheep, cats, and dogs, can also be naturally infected. Many wild animals, carnivores, and wild rodents are also susceptible. In all susceptible species other than pigs, infection with PRV invariably results in death. Pseudorabies shows a certain seasonal prevalence, occurring more frequently in colder seasons, although it can also appear at other times due to the virus's enhanced survival in low temperatures.

In pigs, PRV is primarily transmitted via nasal secretions, but milk and semen may also serve as transmission routes. On pig farms, infected pigs shedding the virus are the main source of infection for healthy pigs. Additionally, contaminated personnel and equipment play significant roles in spreading the virus. Airborne transmission is the most important mode of PRV dissemination.

Clinical Symptoms

The clinical manifestations of PRV depend primarily on the virulence of the infecting strain, the viral load, and the age of the infected pigs, with age being the most critical factor. Similar to other herpesviruses, PRV infections are most severe in young pigs.

●Neonatal Piglets: Infection in neonatal piglets causes high mortality rates. Clinically, piglets appear normal on the first day of life, but symptoms begin on the second day, with peak mortality occurring within 3–5 days. Entire litters may succumb to the disease. Affected piglets exhibit severe neurological symptoms, lethargy, abnormal vocalizations, vomiting, and diarrhea. Death usually occurs within 1–2 days of symptom onset. Postmortem examinations reveal pinpoint hemorrhages on the kidneys, and in some cases, pulmonary edema and meningeal congestion or hemorrhage. For piglets under 15 days of age, the disease is extremely severe, with mortality rates reaching 100%. Symptoms include sudden onset, high fever exceeding 41°C, extreme lethargy, tremors, incoordination, convulsions, vomiting, and diarrhea, with recovery being rare.

●Weaned Piglets: Infection results in a morbidity rate of approximately 20–40% and a mortality rate of 10–20%, characterized by neurological symptoms, diarrhea, and vomiting.

●Adult Pigs: Generally experience subclinical infections, with mild symptoms if present. Affected pigs may display fever, lethargy, vomiting, and coughing, typically recovering within 4–8 days.

●Pregnant Sows: Infection can lead to abortion, mummified fetuses, or stillbirths, with stillbirths being the most common outcome. Symptoms of intense itching, previously rare, are now frequently observed.

Another notable manifestation of pseudorabies is infertility in breeding pigs. In recent years, outbreaks in spring have led to stillbirths or pseudorabies cases in weaned piglets, followed by infertility in sows, with return-to-estrus rates as high as 90% and repeated unsuccessful matings. Infected boars may exhibit testicular swelling, atrophy, and loss of breeding capability.

Comprehensive Control and Prevention

1.Treatment: There is currently no specific treatment for pseudorabies. In emergencies, high-immune serum therapy can reduce mortality rates. Vaccination is the primary measure for preventing and controlling pseudorabies. Available vaccines include conventional attenuated vaccines, inactivated vaccines, and gene-deleted vaccines (both live attenuated and inactivated). These vaccines effectively reduce or prevent clinical symptoms, minimizing economic losses caused by the disease.

2.Rodent Control: Eliminating rodents from farms is crucial for preventing pseudorabies. Additionally, strict control of access by dogs, cats, birds, and other animals to pig farms, along with regulating human movement, thorough disinfection, and serological monitoring, plays an active role in disease prevention and control.

Immunization Protocol

●Inactivated Vaccine:

    ○Breeding pigs (including boars): Administer the first dose, followed by a booster 4–6 weeks later, and then every six months. Administer a booster approximately one month before farrowing to ensure strong immunity, protecting nursing piglets until weaning.

    ○Replacement breeding piglets: Administer one dose at weaning, followed by a booster 4–6 weeks later, then follow the breeding pig immunization schedule.

    ○Fattening piglets: Administer one dose at weaning, with no further vaccination needed before slaughter.

●Live Attenuated Vaccine:

    ○Breeding pigs: Administer the first dose, followed by a booster 4–6 weeks later, and then every six months.

    ○Fattening pigs: Administer one dose at weaning, with no further vaccination needed before slaughter.