Betaarterivirus suid 1
Betaarterivirus suid 1, formerly Porcine reproductive and respiratory syndrome virus, is a virus that causes a disease of pigs, called porcine reproductive and respiratory syndrome, also known as blue-ear pig disease. This economically important, panzootic disease causes reproductive failure in breeding stock and respiratory tract illness in young pigs. Initially referred to as "mystery swine disease" and "mystery reproductive syndrome", it was first reported in 1987 in North America and Central Europe. The disease costs the United States swine industry around $644 million annually, and recent estimates in Europe found that it costs almost 1.5b€ every year.
Classification
PRRSV is a small, enveloped RNA virus. It contains a single-stranded, positive-sense, RNA genome with a size of approximately 15 kilobases. The genome contains nine open reading frames.PRRSV is a member of the genus Arterivirus, family Arteriviridae, order Nidovirales. Other members of the genus Arterivirus include: equine arteritis virus, simian hemorrhagic fever virus, wobbly possum disease virus, and lactate dehydrogenase elevating virus.
Strains
PRRSV is subdivided in two major types, the European and the North American. Prototype sequences for each PRRSV type have been defined. For the European PRRSV, this is the Lelystad virus, while for the North American PRRSV, this is the VR-2332. The European and North American PRRSV strains cause similar clinical symptoms, but represent two distinct viral genotypes whose genomes diverge by approximately 40%, thus creating a veil of mystery about the origin of this virus. The genetic variation among the viruses isolated from different places increases the difficulty of developing vaccines against it. Similarly, maintaining diagnostic PCR detection assays is difficult due to the high mutation rate of this virus, see Risk of Missed PRRS PCR Detection.In the early 2000s a highly pathogenic strain of the North American genotype emerged in China. This strain, HP-PRRSV, is more virulent than all other strains, and causes great losses in Asian countries worldwide. Later a study showed that accelerated evolution of a group of strains in China.
Clinical signs
Subclinical infections are common, with clinical signs occurring sporadically in a herd. Clinical signs include reproductive failure in sows such as abortions and giving birth to stillborn or mummified fetuses, and cyanosis of the ear and vulva. In neonatal pigs, the disease causes respiratory distress, with increased susceptibility to respiratory infections such as Glasser's disease.Laboratory diagnosis
Laboratory-based diagnostic tests have evolved significantly since initial discovery of the PRRS virus in the late 1980s. Initially viral culture was used to confirm PRRSV in serum or tissue samples. This process involves growing the virus in-vitro on cell lines over a period of 3–14 days or longer. If cytopathic effect is observed during culture, the culture is confirmed as the PRRS virus by direct fluorescent antibody or other confirmation method prior to reporting the sample as positive for presence of PRRSV.In the late 1990s, nested PCR was used to the detect the virus as it showed improved sensitivity over non-nested PCR. Now, quantitative PCR assays offered as-good or better sensitivity than nested PCR, fast turnaround time in the lab, and lower rates of cross-contamination via closed-tube amplification.
As an RNA virus with a 15 kb genome, PRRS mutates at a relatively high rate as it is transmitted from pig-pig over time. The calculated rate of PRRSV nucleotide substitution is the highest reported so far for an RNA virus. It is estimated as 4.7-9.8 x 10−2 / site / year.
Though the quantitative PCR tests used now have high sensitivity and specificity, these improvements have come with some hazards as well. Quantitative PCR using Taq-man chemistry is prone to false-negative results when the virus mutates. A false negative result occurs when a test fails to detect the presence of the virus. Studies have found that even a single base-pair change in the viral RNA under the labeled probe can cause failure of detection. This specific source of the false-negative is not due to operator error on the part of the lab and is un-knowable at the time of testing.
The scenario that follows demonstrates how this hazard can result in risk to pork producers and laboratories:
→A strain of PRRS virus mutates during circulation within a herd. This strain spreads and becomes the predominant strain within the herd.
This series of events is a frustrating and expensive event for veterinarian, diagnostic lab, and animal owners. Many labs in the United States each use their own quantitative PCR method and communication of test failures due to new strains to other diagnostic labs is difficult. As a result, information learned about new strains is not leveraged across many diagnostic labs. Due to the cost of testing and rapid detection of new virus introduction, PCR alone is often relied on as the primary screening tool. This over-reliance on a single diagnostic assay lead to longer interval of virus spread while the problem is being resolved.
Veterinarian and producer
Veterinarians can reduce the impact of this risk by paying close attention to clinical signs and utilizing more than one PRRS diagnostic test. Early communication with the lab is essential as often other methods can quickly be employed on existing samples. Given the rate of mutation for the PRRS virus, contingency plans should be developed for false-negative events that include selection of alternative labs and tests.On the other hand, to reduce the impact of PRRSV infection in farms, genetic markers such as SGK1 and TAP1 can be implemented in crossbreeding and/or selection schemes favouring reproductive resilient phenotypes in sows, as they contribute to maintaining a stable number of piglets born alive and lost, particularly mummies, despite the outbreak; as described recently in .
Diagnostic laboratory
Some laboratories have moved to the use of commercially developed and maintained quantitative PCR assays, which transfers the work of assay updates to a 3rd party albeit at a significant extra cost over in-house developed assays. In recent years, this strategy has allowed quicker response to new variants than would have been previously possible. By commercial manufacturers leveraging assay updates across multiple labs, it is possible that detection capabilities for all client labs is improved. The flip-side of this approach is that if all labs run the same assay, there are limited options for veterinarians when an alternate assay is quickly needed.Earlier technologies such as nested PCR are often called on during an investigation if the lab has retained the capability to perform them. By using these earlier methods the laboratory staff are more quickly able to identify the new strain due to their more robust detection capabilities.