Classical Swine Fever
Posted May 3, 2009on:
Classical swine fever (CSF) is a highly contagious and economically significant viral disease of pigs. The severity of this disease varies with the strain of the virus, the age of the pig, and the immune status of the herd. Acute infections, which are caused by highly virulent isolates and have a high mortality rate, are likely to be diagnosed rapidly. However, infections with less virulent isolates can be more difficult to recognize, particularly in older pigs. These infections may be relatively mild, and can resemble septicemias caused by other agents, as well as other diseases. In some herds, the only symptom may be poor reproductive performance or the failure of some pigs to thrive. The wide range of clinical signs and similarity to other diseases can make classical swine fever challenging to diagnose. Although classical swine fever was once widespread, many countries have eradicated this disease from domesticated swine. Reintroduction of the virus can be devastating. In 1997-1998, an outbreak in the Netherlands spread to more than 400 herds and cost $2.3 billion to eradicate. Approximately 12 million pigs were killed, some in eradication efforts but most for welfare reasons associated with the epidemic. The United Kingdom experienced a CSF epizootic in 2000, and minor outbreaks were reported in Romania, Slovakia, Spain and Germany in 2001. North America is also at risk for the introduction of this disease, which is still endemic in much of South and Central America, including parts of Mexico. Because intensive swine production practices are used in the U.S., there may be extensive movements of pigs at different phases of production. This increases the potential for direct or indirect contact between pigs from different sources. Both factors increase the risk of virus spread. In addition, trade has become globalized, and international passenger travel and immigration have grown, increasing the risk of accidental introduction.
Classical swine fever (hog cholera) results from infection by classical swine fever virus (CSFV), a member of the genus Pestivirus and family Flaviviridae. Only one CSFV serotype has been found, but minor antigenic variability has been demonstrated between viral strains. This virus is closely related to the ruminant pestiviruses that cause bovine virus diarrhea and border disease. Other pestiviruses have also been described recently.
Classical swine fever affects domesticated and wild pigs. All feral and wild pigs, including European wild boar and collared peccaries, are thought to be susceptible.
Classical swine fever is found in much of Asia, some Caribbean islands, the African countries of Madagascar and Mauritius, and much of South and Central America. This disease has been eradicated from the United States, Canada, New Zealand, Australia and most of western and central Europe. CSFV is endemic in wild boar in parts of Europe; the significance for domesticated pigs is controversial.
Classical swine fever is highly contagious. Infected pigs are the only reservoir of virus. Blood, secretions and excretions (including oronasal and lacrimal secretions, urine, feces and semen) and tissues contain infectious virus. Virus shedding can begin before the onset of clinical signs, and occurs throughout the course of acute or subclinical disease. Chronically or persistently infected pigs can shed virus continuously or intermittently for months. Transmission between pigs occurs mainly by the oral or oronasal routes, via direct or indirect contact. CSFV is often spread by feeding uncooked contaminated garbage. Animals can also be infected through the mucous membranes, conjunctiva and skin abrasions. CSFV can be spread by genital transmission or artificial insemination. Infected carrier sows may give birth to persistently infected pigsThe virus can also be spread on fomites, and mechanical spread by insects, birds and other wild or domesticated animals may occur. Airborne transmission seems to be possible over short distances; however, the maximum distance the virus can spread is unclear. While aerosol transmission occurred only within a radius of 250 meters (820 feet) in one study, transmission could occur up to 1 km (0.62 miles) in another. CSFV is moderately fragile in the environment; this virus is reported to survive for three days at 50ºC (122 ºF) and 7 to 15 days at 37ºC (98.6ºF). Estimates of its survival in pens and on fomites under field conditions vary. Some studies suggest that virus inactivation occurs within a few days, while others describe survival, under winter conditions, for up to four weeks. CSFV can remain infectious for nearly three months in refrigerated meat and for more than four years in frozen meat. In this proteinaceous environment, this virus does not appear to be inactivated by smoking or salt curing. Reported virus survival times in cured and smoked meats vary with the technique, and range from 17 to more than 180 days.
The incubation period can range from 2 to 15 days, depending on the virulence of the strain, the route of inoculation and the dose. Under field conditions, disease may not become evident in a herd for 2 to 4 weeks or longer.
The signs of classical swine fever vary with the strain of virus, and the age and susceptibility of the pigs. More virulent strains cause acute disease; less virulent strains can result in a high percentage of chronic, mild or asymptomatic infections. Although highly virulent strains were once more prevalent, most epizootics are now caused by moderately virulent strains. Older animals are less likely to show severe symptoms than younger pigs. Some breed-specific differences have also been reported. Acute swine fever is the most severe form of the disease. In this form, common symptoms include a high fever [41o C (105o F)], huddling, weakness, drowsiness, anorexia, conjunctivitis, and constipation followed by diarrhea. Pigs may be in coordinated or exhibit an unsteady, weaving or staggering gait, which progresses to posterior paresis. Some pigs may vomit yellow, bile- containing fluid, or develop respiratory signs. The abdomen, inner thighs, ears and tail may develop a purple cyanotic discoloration. Hemorrhages can also occur in the skin. Severe leukopenia usually occurs soon after disease onset, and convulsions may be seen in the terminal stages. Pigs with acute classical swine fever often die within one to three weeks.
Classical swine fever should be suspected in pigs with signs of septicemia and a high fever, particularly if uncooked scraps have been fedor new animals have been added to the herd. This disease may also be considered in herds with other symptoms, including breeding herds with poor reproductive performance and disease in piglets. It can be difficult to differentiate classical swine fever from other diseases without laboratory testing. Differential diagnosis
The differential diagnosis varies with the form of the disease, and includes African swine fever, porcine dermatitis and nephropathy syndrome, porcine circovirus associated disease (especially porcine dermatitis nephritis syndrome), hemolytic disease of the newborn, porcine reproductive and respiratory syndrome, thrombocytopenic purpura, anticoagulant (e.g. warfarin) poisoning, salt poisoning, Aujeszky’s disease (pseudorabies) and parvovirus infections. Septicemic diseases such as erysipelas, eperythrozoonosis, salmonellosis, pasteur-ellosis, actinobacillosis, and Haemophilus parasuis infections must also be considered. Congenital infection with the pestiviruses that cause bovine virus diarrhea or border disease can resemble classical swine fever.
Classical swine fever can be diagnosed by detecting the virus, its antigens or nucleic acids in whole blood or tissue samples. Viral antigens are detected by direct immunofluorescence (FAT or FATST test) or enzyme-linked immunosorbent assays (ELISAs). The virus can also be isolated in several cell lines including PK-15 cells; it is identified by direct immunofluorescence or by immunoperoxidase staining. Reverse transcriptasepolymerase chain reaction (RT-PCR) tests are used in some laboratories. The ruminant pestiviruses that cause bovine virus diarrhea and border disease can occasionally infect pigs nSerum neutralization tests, or immunoperoxidase procedures that use monoclonal antibodies, can differentiate CSFV from these viruses. They can also be distinguished using genetic methods such as RT-PCR. Serology is used for diagnosis and surveillance Antibodies develop after 2 to 3 weeks, and persis lifelong. For this reason, serology is most useful in herds thought to have been infected 30 or more days previously It is particularly helpful in herds infected with less virulent strains, where viral antigens may be more
difficult to find. The most commonly used tests are virus neutralization tests, which include the fluoresce antibody virus neutralization (FAVN) test and the neutralizing peroxidase-linked assay (NPLA), and various ELISAs. Antibodies against ruminant pestiviruses may be found in breeding animals; only tests that use monoclonal antibodies can differentiate between these viruses and CSFV. The definitive test for differentiation is the comparative neutralization test. Congenitally infected pigs are immunotolerant and are negative on serology. Companion ELISAs have been developed for marke vaccines, but have limitations in their sensitivity and ospecificity.