title: "Primary Cell Cultures and Continuous Cell Lines in Veterinary Virology" category: "microbiology" metaDescription: "A detailed technical guide on the use of primary cell cultures and continuous cell lines for virus isolation, propagation, and diagnostics in veterinary virology, covering biophysical mechanisms, cell line characteristics, media requirements, and cytopathology." primaryKeyword: "cell culture veterinary virology" secondaryKeywords: ["primary cell culture", "continuous cell line", "virus isolation", "MDCK", "MARC-145", "CEF", "SAM"] lastUpdated: "2026-05-26"

Primary Cell Cultures and Continuous Cell Lines in Veterinary Virology

Cell culture systems remain the cornerstone of diagnostic virology for the isolation, propagation, and quantification of animal viruses. The choice between primary cell cultures and continuous (established) cell lines directly influences the sensitivity, specificity, and reproducibility of viral detection. This article provides a comprehensive technical review of both systems, emphasizing the biophysical principles underpinning cell-virus interactions, the specific culture characteristics of commonly used veterinary cell substrates, and the practical diagnostic applications that mandate careful cell line selection.

Biophysical Principles of Cell Culture in Viral Isolation

Cell Adhesion and Monolayer Formation

Most adherent cells require a substratum for anchorage-dependent growth. Attachment is mediated by integrin receptors binding to extracellular matrix proteins (e.g., fibronectin, vitronectin, collagen) adsorbed onto the culture surface from serum or secreted by the cells. The strength of adhesion depends on the density of receptor-ligand bonds and the surface charge of the plastic (typically tissue culture treated polystyrene with negative charges introduced by corona discharge). For primary cells, a higher surface charge is often needed to overcome electrostatic repulsion from cell surface glycocalyx moieties.

Contact Inhibition

Normal diploid cells exhibit contact inhibition: upon reaching a confluent monolayer, cell proliferation ceases due to cyclin-dependent kinase inhibition and Hippo pathway signaling. This property is lost in many continuous cell lines that have undergone transformation, allowing them to pile up and form foci. For virus isolation, contact inhibition is critical because it maintains a stable, nondividing monolayer that can be infected synchronously. Overconfluent cultures may become senescent or detach, reducing viral yield.

Viral Adsorption Kinetics

The adsorption of virus particles to permissive cells follows first order kinetics: the number of attached virions per cell is proportional to the inoculum concentration and time, with a rate constant dependent on diffusion, electrostatic attraction, and receptor abundance. At 37 degrees Celsius, most enveloped viruses complete adsorption within 45 to 90 minutes. The efficiency is enhanced by gentle rocking to maximize particle-cell collisions but reduced by the presence of serum proteins that can bind viral glycoproteins or compete for attachment sites. For this reason, maintenance medium during the adsorption phase is often serum-free.

Primary Cell Cultures

Primary cultures are derived directly from animal tissues, dissociated enzymatically (typically with 0.25% trypsin-EDTA) and seeded into culture vessels. They exhibit a finite replicative lifespan (usually 10 to 20 passages) and retain a diploid karyotype. Their major advantage is a close replication of the in vivo cellular environment, making them indispensable for fastidious viruses that cannot grow in continuous lines.

Common Primary Systems in Veterinary Virology

Swine Alveolar Macrophages (SAM) SAM are obtained by bronchoalveolar lavage from specific-pathogen-free pigs. They are nonadherent or weakly adherent and must be maintained in suspension culture using RPMI 1640 supplemented with 10% fetal bovine serum (FBS) and 2 mM L-glutamine. SAM are the gold standard for isolation of Porcine Reproductive And Respiratory Syndrome Virus (PRRSV), which requires the CD163 receptor highly expressed on porcine macrophages [1]. PRRSV replicates exclusively in SAM, causing syncytia formation and cell lysis.

Swine Testicular (ST) Cells ST cells are primary cultures derived from fetal pig testes. They are fibroblastic monolayers used for the isolation of Classical Swine Fever Virus, Transmissible Gastroenteritis Virus, and porcine enteroviruses. ST cells grow in MEM with 10% FBS and are passaged at a 1:2 split ratio.

Chicken Embryo Fibroblasts (CEF) CEF are prepared from 9- to 11-day-old embryonated specific-pathogen-free chicken eggs. The embryos are minced, trypsinized, and seeded in Medium 199 or DMEM with 4% FBS. CEF are widely used for Avian Encephalomyelitis Virus, Avian Influenza Virus (plaque assays), Newcastle Disease Virus, and Mareks Disease Virus. Their susceptibility relies on the presence of sialic acid receptors for influenza and the lack of endogenous avian retroviruses in specific-pathogen-free eggs.

Chicken Embryo Kidney (CEK) Cells CEK are prepared from the kidneys of 19-day-old chicken embryos. They are epithelial in morphology and used primarily for Infectious Bronchitis Virus and Avian Nephritis Virus. CEK require DMEM/F12 with 5% FBS and are passaged no more than five times to avoid loss of susceptibility.

Enzymatic Dissociation and Perfusion

Tissue dissociation involves 0.25% trypsin at 37 degrees Celsius for 20 to 30 minutes with constant stirring. Trypsin cleaves cadherins and integrins, releasing single cells. After filtration through gauze, the cells are washed to remove trypsin and debris. Perfusion techniques (used for larger organs like liver or kidney) involve continuous flow of trypsin solution through the organ vasculature to maximize viable cell yield. Primary cultures require careful control of pH (7.2 to 7.4) and oxygen tension; some, like SAM, are sensitive to shear forces and require gentle handling.

Continuous (Established) Cell Lines

Continuous cell lines are derived from primary cultures that have undergone spontaneous or induced immortalization. They exhibit an aneuploid karyotype with altered chromosome numbers, can be passaged indefinitely, and often lose contact inhibition. Their homogeneity, ease of cultivation, and reproducibility make them the workhorses of diagnostic virology, although they may lack specific receptors for some fastidious viruses.

Important Continuous Cell Lines in Veterinary Virology

MDBK (Madin Darby Bovine Kidney) Origin: bovine kidney. Epithelial morphology. Grown in MEM with 5% to 10% FBS. Susceptible to Bovine Herpesvirus 1, Bovine Viral Diarrhea Virus, and bovine adenoviruses. MDBK cells support high titers of BoHV-1 and are the standard for plaque reduction neutralization tests.

MDCK (Madin Darby Canine Kidney) Origin: canine kidney. Epithelial monolayers. Grown in MEM or DMEM with 10% FBS. Critical for isolation of Canine Influenza A Virus and equine influenza. MDCK cells express alpha-2,3 and alpha-2,6 linked sialic acid receptors, making them permissive for influenza A viruses from multiple host species [2]. Plaque assays in MDCK cells are used to quantify infectious virus.

Vero (Vervet Monkey Kidney) Origin: African green monkey kidney. Epithelial morphology. Grown in DMEM with 5% FBS. Vero cells are interferon-deficient due to a deletion in the type I interferon gene cluster, allowing robust replication of many viruses. They are used for Rabies Lyssavirus, Canine Distemper Virus, and various arboviruses.

MARC-145 (Monkey Kidney Clone) Origin: derivative of MA-104 (fetal rhesus monkey kidney). Epithelial monolayers. Grown in DMEM with 10% FBS. MARC-145 are the standard continuous line for PRRSV isolation and propagation. They express CD163 and are permissive for European and North American PRRSV genotypes [3].

RK13 (Rabbit Kidney) Origin: rabbit kidney. Epithelial morphology. Grown in MEM with 10% FBS. Used for Equine Arteritis Virus and some rabbit herpesviruses.

PK15 (Porcine Kidney) Origin: porcine kidney. Epithelial morphology. Grown in MEM with 5% FBS. Susceptible to Porcine Circovirus 2, Porcine Parvovirus, and Pseudorabies Virus. PK15 are commonly used for virus neutralization tests.

Fish Cell Lines: EPC and RTG-2 EPC (Epithelioma Papulosum Cyprini) derived from common carp skin, and RTG-2 (Rainbow Trout Gonad) derived from rainbow trout gonad, are fibroblastic or epithelial lines grown in L-15 or MEM at 15 to 20 degrees Celsius (no CO2 required for L-15). EPC is the cell line of choice for Viral Hemorrhagic Septicemia Virus and Spring Viremia Of Carp Virus. RTG-2 is used for Infectious Pancreatic Necrosis Virus and Infectious Hematopoietic Necrosis Virus. These lines are cold-adapted and require controlled temperature and osmolarity.

Cell Culture Media, Adjuvants, and Growth Conditions

Formulations

Minimal Essential Medium (MEM) and Dulbecco's Modified Eagle Medium (DMEM) are the most widely used basal media. MEM contains balanced salt solution, amino acids, and vitamins at low glucose (1 g/L). DMEM has higher amino acid and vitamin concentrations and glucose at 4.5 g/L, supporting faster cell growth. RPMI 1640 is preferred for suspension cells like lymphocytes and macrophages. For fish cell lines, Leibovitz L-15 medium is used because it uses galactose and pyruvate instead of bicarbonate as an energy source, allowing atmospheric CO2.

Serum Requirements

Fetal bovine serum (FBS) provides growth factors (IGF, EGF, FGF), attachment proteins (fibronectin), and hormones. Routine concentration is 5% to 10% for continuous lines and 10% to 20% for primary cultures. Donor horse serum is sometimes substituted for certain cell lines (e.g., MDBK) to reduce batch variability. Heat inactivation at 56 degrees Celsius for 30 minutes destroys complement proteins and reduces the risk of mycoplasma and viral contamination. All sera must be tested for bovine viral diarrhea virus and bovine herpesvirus 1 by PCR before use.

Environmental Controls

Incubators maintain 5% CO2 in air at 37 degrees Celsius for mammalian cells and 15 to 20 degrees Celsius for piscine lines. The CO2/bicarbonate system buffers pH at 7.2 to 7.4. HEPES (10 to 25 mM) can be added as an additional buffer during virus adsorption or transport. Antibiotics (penicillin 100 U/mL, streptomycin 100 micrograms/mL, amphotericin B 0.25 micrograms/mL) are routinely added to prevent bacterial and fungal contamination but should not be relied upon for sterile technique.

Diagnostic Applications and Cytopathology

Seeding and Confluence

Monolayers are seeded at a density of 2 to 5 x 10^4 cells per cm^2 to reach 70% to 90% confluence within 24 to 48 hours. Overconfluent monolayers may show increased cell death and reduced virus susceptibility. For viral isolation, 24-well or 96-well plates are preferred to allow multiple dilutions.

Adsorption Phase

The clinical sample (tissue homogenate or swab eluate) is inoculated onto washed cell monolayers. Inoculum volume should just cover the monolayer (0.1 mL per cm^2). Adsorption is performed at 37 degrees Celsius for 1 hour with intermittent rocking. After adsorption, the inoculum is removed and replaced with serum-free maintenance medium (typically MEM with 0.5% FBS, plus antibiotics). Serum-free conditions reduce nonspecific inhibitors and prevent serum-induced cell overgrowth.

Cytopathic Effect Observation

Monolayers are examined daily for cytopathic effect (CPE). The type of CPE can be indicative of the virus family. Examples:

• Syncytia: paramyxoviruses, PRRSV.
• Rounding and detachment: picornaviruses, caliciviruses.
• Focal necrosis: herpesviruses.
• Foamy degeneration: retroviruses.

When no CPE is observed after 5 to 7 days, a blind passage of freeze-thawed lysate onto fresh monolayers is performed. Up to three passages may be required for slow-growing viruses.

Plaque Assays

For quantification, serial dilutions of virus are adsorbed onto confluent monolayers and overlayed with semisolid medium (0.5% to 1% agarose or methylcellulose) to limit spread. After 2 to 5 days, plaques are visualized by staining with neutral red or crystal violet. MDCK cells are used for Avian Influenza Virus plaque assays.

Cell Line Utility Comparison

Table 1. Comparative Characteristics of Cell Culture Systems in Veterinary Virology

| Cell System | Host/Tissue Origin | Growth Medium | Cell Type | Target Viruses | | :-, | :-, | :-, | :-, | :-, | | SAM | Porcine lung (alveolar macrophages) | RPMI 1640 + 10% FBS | Suspension (nonadherent) | PRRSV | | ST | Porcine testes | MEM + 10% FBS | Monolayer (fibroblast) | CSFV, TGEV | | CEF | Chicken embryo (whole body) | M199/DMEM + 4% FBS | Monolayer (fibroblast) | AE, AIV, NDV | | CEK | Chicken embryo kidney | DMEM/F12 + 5% FBS | Monolayer (epithelial) | IBV, ANV | | MDBK | Bovine kidney | MEM + 5% FBS | Monolayer (epithelial) | BoHV-1, BVDV | | MDCK | Canine kidney | MEM/DMEM + 10% FBS | Monolayer (epithelial) | Canine Influenza A, equine influenza | | Vero | African green monkey kidney | DMEM + 5% FBS | Monolayer (epithelial) | Rabies, CDV, arboviruses | | MARC-145 | Monkey kidney (MA-104 clone) | DMEM + 10% FBS | Monolayer (epithelial) | PRRSV | | PK15 | Porcine kidney | MEM + 5% FBS | Monolayer (epithelial) | PCV2, PPV, PrV | | EPC | Common carp skin | L-15 + 10% FBS | Monolayer (epithelial) | VHSV, SVCV | | RTG-2 | Rainbow trout gonad | L-15 + 10% FBS | Monolayer (fibroblast) | IPNV, IHNV |

Mermaid Flowchart: Cell Culture Propagation and Viral Infection Protocol

flowchart TD
    A[Clinical Sample Collection], > B{Tissue or Swab?}
    B, >|Tissue| C[Homogenize in transport medium]
    B, >|Swab| D[Elute in medium with antibiotics]
    C, > E[Centrifuge at 2000g for 10 min]
    D, > E
    E, > F[Clarify supernatant; filter 0.45 micron]
    F, > G{Cell System Selection}
    G, >|Primary| H[Thaw primary cells or prepare fresh]
    G, >|Continuous| I[Thaw cryovial of continuous line]
    H, > J[Seed culture flasks with cells]
    I, > J
    J, > K[Incubate at 37°C, 5% CO2]
    K, > L{Confluence 70-90%?}
    L, >|No| M[Change medium; continue incubation]
    M, > K
    L, >|Yes| N[Wash monolayer with PBS]
    N, > O[Inoculate clarified sample]
    O, > P[Adsorb at 37°C, 1 hour, rock]
    P, > Q[Remove inoculum; add serum-free maintenance medium]
    Q, > R[Incubate and observe daily for CPE]
    R, > S{CPE observed?}
    S, >|Yes| T[Harvest supernatant and cells for further testing]
    S, >|No after 5-7 days| U[Freeze-thaw lysate; blind passage]
    U, > V[Inoculate fresh

References

1. Wensvoort, G., Terpstra, C., Pol, J. M. A., ter Laak, E. A., Bloemraad, M., de Kluyver, E. P., ... & van Buiten, L. (1991). Mystery swine disease in The Netherlands: the isolation of Lelystad virus. Veterinary Quarterly, 13(3), 121-130.
2. Matrosovich, M., Matrosovich, T., Garten, W., & Klenk, H. D. (2006). New low-viscosity overlay medium for viral plaque assays. Virology Journal, 3, 63.
3. Kim, H. S., Kwang, J., Yoon, I. J., Joo, H. S., & Frey, M. L. (1993). Enhanced replication of porcine reproductive and respiratory syndrome (PRRS) virus in a homogeneous subpopulation of MA-104 cell line. Archives of Virology, 133(3-4), 477-483.