Porcine Cytomegalovirus (PCMV), also known as Suid herpesvirus 2, is a widespread viral infection impacting swine populations across the globe. It belongs to the Herpesviridae family, specifically the Betaherpesvirinae subfamily, and is closely related to human herpesviruses 6 and 7. This enveloped DNA virus is endemic in nearly all swine populations, with seroprevalence reaching almost 100% in many regions. While often causing subclinical or mild disease, its ubiquity and potential implications for pig health and the swine industry warrant attention.
Impact on Swine Health
PCMV infection can manifest differently depending on the age of the pig and the immune status of the herd. In pregnant sows, particularly those experiencing their first infection late in gestation, PCMV can lead to reproductive losses, including stillbirths, mummified fetuses, and the delivery of weak or anemic piglets. These outcomes have significant economic implications for swine producers.
Neonatal piglets, especially those less than three weeks old, are particularly susceptible to severe symptoms if infected in utero or neonatally. They may display shivering, sneezing, respiratory distress, and poor weight gain. A characteristic sign in affected piglets is rhinitis, an inflammation of the nasal tissues, which can be severe enough to cause nasal hemorrhage and a black discoloration around the eyes from conjunctival exudate.
In weaned and growing pigs, PCMV infections are often subclinical or result in mild symptoms like sneezing. While typically not causing severe losses, PCMV has been associated with porcine respiratory disease complex (PRDC) and may predispose pigs to other respiratory ailments. Adult pigs may also show subclinical signs, though some can experience listlessness and mild anorexia.
Spread and Infection Mechanisms
PCMV primarily spreads through direct contact between pigs, with the virus shed in various bodily secretions. These include nasal and ocular secretions, saliva, urine, and cervical fluid from sows. Vertical transmission, where the virus is passed from an infected sow to her unborn fetuses across the placenta, is also a known mode of infection.
A notable characteristic of PCMV, like other herpesviruses, is its ability to establish latent infections. This means the virus can remain dormant within an infected pig for extended periods without causing active disease. However, under conditions of stress, the latent virus can reactivate, leading to renewed viral shedding and the potential for further transmission to other susceptible pigs. This latency makes control challenging, as apparently healthy carrier pigs can spread the virus.
PCMV is species-specific, meaning it only affects pigs. It is not considered a zoonotic risk to human health in general interactions. However, its implications in xenotransplantation, where pig organs are transplanted into humans, are still being investigated due to links with reduced xenograft survival.
Identification and Diagnosis
Diagnosing PCMV infection often involves a combination of laboratory techniques to confirm the presence of the virus or evidence of exposure. Molecular techniques, such as Polymerase Chain Reaction (PCR), are commonly used to detect viral DNA. PCR assays can be performed on various tissue samples, including nasal swabs, lung tissue, and lymph nodes, providing a sensitive method for direct virus detection. More sensitive PCR methods, like nested PCR and qPCR, can quantify viral load in blood, useful for detecting latent infections.
Serological tests, such as Enzyme-Linked Immunosorbent Assays (ELISA), are employed to detect antibodies against PCMV in blood samples. The presence of these antibodies indicates that an animal has been exposed to the virus. ELISA assays can differentiate between infected and non-infected animals and quantify maternal antibodies in neonates.
Histopathology, the microscopic examination of tissue, remains a traditional diagnostic method. This technique involves identifying characteristic basophilic intranuclear inclusion bodies within affected epithelial cells, particularly in the nasal turbinates, a hallmark of PCMV infection. Given the varied clinical signs and the possibility of subclinical infections, a definitive diagnosis frequently requires combining these methods.
Management and Biosecurity
Effective management of PCMV in swine herds primarily revolves around robust biosecurity measures, as there are currently no specific antiviral treatments or commercially available vaccines. Preventing the introduction of the virus into a herd is paramount, which includes establishing and maintaining a closed herd to minimize the risk of introducing infected animals from external sources.
Strict hygiene protocols are also fundamental in controlling PCMV spread within a farm. Regular cleaning and disinfection of facilities, along with proper ventilation, help reduce viral load in the environment. Managing stress factors in pigs is another important aspect, as stress can reactivate latent infections, leading to increased viral shedding and transmission.
While research continues into potential vaccines and treatments, current strategies focus on supportive care and preventing the disease’s impact through good husbandry practices. These practices aim to minimize the overall disease burden and maintain herd health, even in the presence of PCMV. Given PCMV’s widespread nature, consistent biosecurity and management remain the primary tools for swine producers.