Canine parainfluenza virus

Canine parainfluenza virus (CPIV) is one of the major causes of infectious respiratory diseases in dogs. In 1967, Binn and colleagues first isolated canine parainfluenza virus from the respiratory tract of affected dogs. Since then, reports about this virus have陆续 emerged worldwide, highlighting its widespread infection and posing a serious threat to the health of dog populations across the globe. The disease is prevalent in countries around the world and remains one of the most significant infectious threats to the global canine industry.

Canine parainfluenza virus exhibits diverse morphologies, most commonly appearing as spherical particles. The virus is synthesized in the cytoplasm and released via a budding mechanism. CPIV's envelope originates from lipids of the cell membrane and features a fragile lipid-protein coating that is easily disrupted. In 1972, it was officially classified into the Paramyxoviridae family, specifically within the genus Paramyxovirus. Parainfluenza viruses (PIVs) belong to the Paramyxoviridae family and currently have four distinct serotypes: PIV-1, PIV-2, PIV-3, and PIV-4. Among these, canine parainfluenza virus is a single-stranded, non-segmented negative-sense RNA virus, classified as PIV-2. The name "CPIV" can be confusing due to its varied usage—originally discovered in monkey kidney cells, CPIV is primarily associated with respiratory diseases in dogs, leading to its alternative designation as SV-5 (CPIV-5). However, studies have revealed that SV-5 shares significant antigenic similarities with parainfluenza virus type 2 (PIV-2), prompting its reclassification as CPIV-2. As a result, SV-5 is now generally considered to belong to the same serotype as PIV-2.

Canine parainfluenza virus is sensitive to formaldehyde, oxidizing agents, non-ionic detergents, and lipid-solvent substances such as ether and chloroform, causing it to become rapidly inactivated. CPIV is also heat-sensitive—when exposed at 50°C for 15 minutes, the virus loses its infectivity. Furthermore, canine parainfluenza virus is unstable under acidic or alkaline conditions, remaining relatively stable only in neutral solutions (pH 7.4–8.0). However, the virus quickly loses activity at a pH of 3.0 and 37°C.

The incidence of canine parainfluenza virus depends on the density of dog populations. In studies involving laboratory and military dogs, data showed that the prevalence of antibodies against canine parainfluenza virus increased significantly—from 3% upon arrival at kennels to 72% after six weeks. Susceptible animals are primarily dogs. While dogs of all ages can be affected, younger puppies tend to experience more severe illness, with high mortality rates observed in pups aged 11 to 12 weeks. In contrast, adult dogs usually exhibit milder symptoms and lower mortality rates. Research data from the Central China region, collected between September 2020 and January 2022, revealed that out of 418 disease samples tested via RT-PCR for CPIV detection, 15.8% were positive—a rate lower than the average positivity of 37.72% reported in a previous survey conducted in Lanzhou City from 2004 to 2009. The discrepancy may be attributed to differences in geographic location and time periods. While PCR proves highly sensitive in diagnosing CPIV infections, routine vaccination could potentially interfere with test results, particularly in young puppies, leading to occasional false-positive outcomes.

Modes of transmission

This virus can infect and cause respiratory illness in species such as guinea pigs, hamsters, tigers, wolves, foxes, and raccoon dogs. Infected dogs—and particularly those that are asymptomatic carriers—are the primary sources of infection, shedding the virus through saliva, respiratory secretions, and ocular discharge. Typically, viral particles spread via airborne droplets. During the acute phase, infected dogs pose the greatest risk of transmission, with respiratory routes being the main natural mode of infection.

The CPIV incubation period is typically less than one week, during which the virus can be shed even before clinical symptoms appear. The duration of shedding lasts about one week. This means that infected animals become contagious before showing any clinical signs, and animals exhibiting subclinical symptoms are also infectious. The virus can be released via respiratory secretions for 7 to 14 days, significantly increasing the pathogen load in the environment—and thereby greatly enhancing the likelihood of disease transmission.

Immune Response

Limited studies have examined the systemic antibody response kinetics in dogs seronegative for canine parainfluenza virus. HI and VN antibodies against parainfluenza virus isolates were detectable as early as 7 days post-infection, typically peaking by day 21 and remaining at that level until at least day 42. The immune response of dogs to canine parainfluenza virus is relatively slow, with some variability observed in the timing of the serum antibody response. Additionally, certain data indicate that VN titers may decline to undetectable levels 3 to 4 months after experimental infection.

Canine parainfluenza virus (CPIV) infection shares clinical symptoms with the common cold, including fever, cough, runny nose, and difficulty breathing. However, CPIV infections spread more rapidly than the common cold and often erupt suddenly. Affected dogs may exhibit signs of lethargy, dry coughing, and increased nasal discharge—ranging from serous to mucoid in nature. Research has shown that CPIV can also lead to acute encephalomyelitis and hydrocephalus in dogs, resulting in symptoms such as hind limb paralysis and ataxia. In most cases, respiratory disease initially presents without noticeable clinical signs or with very mild symptoms. Occasionally, after about 7 to 9 days, severe nasal discharge, mild pharyngitis, and tonsillitis may develop, persisting for 3 to 5 days. Notably, these dogs typically do not experience fever, though their body temperature may remain slightly elevated—about 1°C to 2°C above normal—for several days. Histological examinations of CPIV-infected tissues reveal characteristic lesions, including catarrhal rhinitis, characterized by mixed inflammatory cell infiltration in the mucosa and submucosal layers, as well as tracheobronchitis and bronchiolitis accompanied by ciliated cell loss, epithelial hyperplasia, and prominent goblet cell proliferation. These histological changes are most pronounced approximately 6 to 12 days post-infection. CPIV frequently occurs in co-infection with pathogens such as Bordetella bronchiseptica, canine adenovirus type 2, and Mycoplasma species, collectively contributing to upper respiratory tract inflammation in dogs.

During animal necropsies, serous or mucopurulent nasal discharge around the nostrils can be observed; conjunctivitis, tonsillitis, tracheitis, and bronchitis are also present, with occasional pinpoint hemorrhages in the lungs. In the neurological form, the primary manifestations include acute encephalomyelitis and hydrocephalus, with widespread lesions affecting both the central nervous system and spinal cord—most severely impacting the gray matter of the anterior horns.

Common laboratory detection methods include the serum neutralization test, hemagglutination inhibition assay, CPIV antigen detection card, RT-PCR method, enzyme-linked immunosorbent assay (ELISA), and gene chip-based detection techniques, among others.

The CPIV antigen test card can serve as an auxiliary tool for diagnosing CPIV. The rapid test card uses immunochromatography to qualitatively detect canine parainfluenza virus antigen in canine nasal fluid, offering simplicity, speed, and high sensitivity.

The RT-PCR method involves collecting feces from sick dogs, mucus from their nasal passages, pharyngeal secretions, and various tissues such as the lungs, lymph nodes, and tonsils. These samples are then used for virus isolation and identification. Although RT-PCR offers the advantages of simplicity, speed, and high efficiency in detecting viral samples, false-positive results can still occur in healthy dogs. Therefore, during disease diagnosis, it is essential to integrate clinical symptoms with other diagnostic approaches for a comprehensive evaluation.

Gene chip technology, also known as microarray technology, is a new detection method that has emerged in recent years alongside advancements in molecular biology. Thanks to its high sensitivity, excellent specificity, and reliable reproducibility, it allows for the simultaneous analysis of numerous samples, making it widely applicable. This technology is based on the principle of complementary base pairing, enabling the identification of target DNA sequences through the binding of target DNA to specific probes. As a result, it provides a rapid and accurate way to diagnose viral genomic information.

Serological assays: Collect paired serum samples from the early stage of illness and the recovery phase, and use serum neutralization and hemagglutination inhibition tests to determine whether antibody titers have increased. A rise of more than 2-fold in either test is sufficient to confirm canine parainfluenza virus infection. This method can be employed for retrospective diagnosis as well as epidemiological investigations.

Clinical signs of canine respiratory infectious diseases are remarkably similar and often difficult to distinguish. PCR testing is recommended, as it uses fluorescent quantitative PCR technology to directly detect pathogen genes, offering significantly higher sensitivity compared to ELISA kits and colloidal gold test strips.

Primarily, it involves strengthening feeding management—ensuring the density of the dog population is appropriately balanced—and maintaining excellent hygiene and facilities around the kennels, including proper ventilation and relative humidity levels. Environmental humidity is also strictly controlled. Regular, essential cleaning is carried out using disinfectants, while consistently upholding high standards of environmental sanitation.

Newly acquired dogs must undergo quarantine and be placed under isolated observation. They should also receive the parainfluenza virus vaccine—options include the hexavalent vaccine produced by U.S.-based Fort Dodge and Pfizer, as well as the one developed domestically by Xia Xianzhu and others, which covers canine parainfluenza. Once a sick dog is identified, immediate and stringent isolation and disinfection measures must be implemented. Additionally, emergency preventive treatment can be administered via injections of canine hyperimmune serum or immunoglobulin. Severely affected dogs should be promptly culled to prevent further spread of the disease.

References

1. Liu Chang: Isolation, Identification, and Biological Characterization of Canine Parainfluenza Virus, 2019

2. Wang Jun, Peng Cheng, Lu Wangyin. Epidemiological Investigation and Analysis of Canine Distemper, Canine Parvovirus Infection, and Canine Parainfluenza in Lanzhou City. *CNKI; WanFang*, 2011

3. Hui Xiaochen, Dai Peihua, Cao Longlong, Ye Jiawen, Chen Linwen, Wang Wenyuan, Zhou Dengyuan, Cao Shengbo, Li Qiuyan – Detection of Canine Parainfluenza Virus in Central China from 2020 to 2022 and Phylogenetic Analysis of Its F and HN Genes

4. John A. Ellis, DVM, PhD, DACVP, DACVM, and G. Steven Krakowka, DVM, PhD, DACVP A review of canine parainfluenza virus infection in dogs