Update on Classical Swine Fever (Hog Cholera)
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Non refereed Diagnostic notes Update on classical swine fever (hog cholera) Douglas Gregg DVM, PhD Summary peccaries, but is not known to infect cattle days in cured smoked hams, and up to 180 Classical swine fever (CSF), commonly or sheep. days in salted and dried Sorrono hams, known in the United States as hog cholera, loins, or sausages.5 The virus is quite resis- is a highly contagious viral disease of swine Geographic distribution tant to pH changes between 3 and 11, and caused by a Pestivirus related to bovine vi- According to the 1996 Office International can survive in the environment for months rus diarrhea (BVD) and border disease vi- des Epizooties Manual, classical swine fever in contaminated soil of barnyards, particu- rus (BDV) of sheep. Virulence varies from has a nearly worldwide distribution involv- larly in temperate climates.6 Scraps of mild to severe. Most current outbreaks are ing 44 swine-producing countries on all CSFV-infected fresh, cured, or associated with moderately virulent continents except North America and Aus- insufficiently cooked meat fed to pigs can strains.1 The classic virulent disease is now tralia.4 In Europe, CSF is endemic in wild transmit the virus. Some international air- rather uncommon. Classic highly virulent boar in Italy, Germany, and parts of France lines and ships serve meals containing spe- disease characteristically presents with high and Switzerland. Countries of the Euro- cialty pork products from their country of fever, extreme lethargy, hemorrhages in pean Economic Community no longer vac- origin, and the protein-rich scraps may be numerous organs, neurological signs, leu- cinate for CSF but experience periodic out- fed to pigs at the destination. This com- kocytopenia, and high mortality. Moder- breaks, often originating from wild boar. mon but risky practice of feeding to pigs ately virulent disease causes high fever, Reports from Asia are incomplete, but the garbage from air and sea ports should be mild lethargy, mild hemorrhages in lym- disease is believed to be present in swine- prohibited, or at least carefully monitored phoid organs, transient leukocytopenia (or producing areas and controlled by vaccina- cooking should be required. none), and low mortality.2 Recovered pigs tion. Many countries of South America Once introduced, CSFV usually spreads may have a chronic disease with intermit- and Asia vaccinate for CSF. The most re- through livestock production systems ei- tent fever and poor reproductive perfor- cent outbreaks in the Netherlands and En- ther by direct movement of pigs or by fo- mance. Pigs born of infected sows may be gland were both of Asian origin. mites, farm workers, veterinarians, farm persistently infected and carry the virus for equipment, and vehicles.7 Infected pigs life, but with no immune response. The CSF: A disease on the move shed virus in all excretions including feces, virus can also be transmitted in semen to Historically, CSF has moved with the urine, saliva, semen, and blood. Chroni- sows. An outbreak of moderately virulent transport of infected pork products. Since cally or persistently infected sows may in- CSF may go undiagnosed for months due the earliest reports of hog cholera in Ten- fect piglets in utero, and boars may shed to its mild nature and few lesions. nessee, this disease has been spread widely virus in semen.1 Transmission is usually in cured hams and sausage. The recent out- through ingestion, but also through skin break in England in 2000 was suspected to Etiology abrasions, bites (fighting), and contami- Classical swine fever virus (CSFV) is a have been introduced in ham that may nated needles and instruments. Airborne, lipid-enveloped virus in the family have come from southeast Asia. Despite rodent, or insect transmission is possible Flaviviridae, genus Pestivirus. The virus has strict animal importation regulations, CSF- over short distances but is not likely to one serotype, but many strains have anti- infected swine products are likely to be in- cause distant spread. There is no evidence genic similarity to BVD and BDV. troduced into the United States by air, sea, that CSFV replicates in insect vectors. Pestiviruses comprise a spectrum of strains or ground transportation in garbage or the that are difficult to distinguish serologi- baggage of travelers from endemic areas. Introduction of CSF into a breeding herd cally. Virus neutralization tests or mono- Specialty ethnic pork products are highly is particularly devastating for several rea- clonal antibodies can differentiate CSFV valued and are regularly confiscated from sons. Once introduced, moderately viru- from these related viruses, but multiple international travelers at US airports. lent CSFV may cause only mild disease in tests may be required. Both BVD and Smuggled pork products confiscated at the mature sows and result in reproductive BDV have been isolated from swine in Eu- borders amount to hundreds of tons per problems that may be overlooked or attrib- rope. Bovine viral diarrhea occasionally year. The CSF virus can survive in fresh uted to other causes. Sows may remain causes a syndrome in pigs similar to that meat for 45 days at 4˚C and for years at - chronically infected for months and may caused by CSF.3 Classical swine fever virus 20˚C, and is quite resistant to inactivation farrow persistently infected litters.8,9 Some infects all swine including wild boar and by salting or curing. Virus survives for 85 of these offspring are immunologically tol- erant to CSFV and may carry high titers of Foreign Animal Disease Diagnostic Laboratory, NVSL, APHIS, USDA. virus for life. If CSFV enters a large agribusiness operation, persistently infected This article is available online at http://www.aasv.org/shap.html. pigs would likely be moved hundreds and Gregg D. Update on classical swine fever (hog cholera). J Swine Health Prod. 2002;10(1):33–37. even thousands of miles between farrowing Journal of Swine Health and Production — Volume 10, Number 1 33 and finishing, spreading the disease widely The initial clinical signs of moderately may be mild constipation. Fever persists for before it is discovered. virulent CSF are similar but milder than 2 to 3 weeks with only trembling noted those of highly virulent CSF. There is a clinically. A few pigs develop moderate di- The increasing popularity of wild boar longer incubation period (6 to 7 days) fol- arrhea. Remarkably, most pigs do not die, meat has resulted in commercial trade in lowed by a slight fever and mild lethargy. and the fever decreases or fluctuates wild and feral swine carcasses. It is sus- Fever rises slowly for several days and usu- through the fourth week. There may be pected that a number of European out- ally peaks at 40.5 to 41.1˚C. Pigs prefer to conjunctivitis and poor weight gain, but breaks have been caused by wild boar hunt- huddle together but will rise, drink, and pigs are typically not gaunt. There are few ing, particularly in Germany, or eat when disturbed. Appetite is reduced. or no hemorrhages or areas of skin discol- importation of Asian wild boar meat. Pigs appear weak upon rising, but there is oration. A few pigs may die suddenly with CSF: Many faces of the same little staggering and no convulsions. There pneumonia due to enteric bacteremia. Re- disease The incubation period of CSF varies with Figure 1: Conjunctivitis in a pig infected with moderately virulent classical the virulence of the virus, and ranges from swine fever virus, 10 days post inoculation. 3 to 15 days. Under experimental condi- tions, 4 to 7 days is the average time from exposure to the onset of fever. Clinical features of CSF vary widely, not only with virus virulence, but also with the age, breed, and condition of the host. Younger pigs are generally more susceptible than market weight hogs. Immunocom- promised pigs experience more severe dis- ease. Pregnant sows are more susceptible than boars of the same age. Sows may have stillbirths or abortions or farrow persis- tently infected piglets. Pigs prone to por- cine stress syndrome are paradoxically more resistant to CSFV.10 Since the late 1960’s, moderate-virulence and low-virulence strains have become more predominant in Europe and Central America. In Central America, this may be the result of intermittent vaccination used as a sole control measure to eliminate the Figure 2: more obvious virulent CSF. Ulcers with a yellowish pseudomembrane (button ulcers) over the lymphoid tissue at the ileocecal junction (cecal tonsil) in a pig infected with The classic disease is severe, acute, and moderately virulent classical swine fever virus. nearly always fatal. This form of CSF has received the most attention in veterinary textbooks but is now relatively uncommon. The initial signs are high fever and leth- argy, trembling, and huddling or piling of pigs. They rapidly lose appetite and may vomit. Fever may reach 42.2˚C for several days. Pigs have constipation initially and yellowish diarrhea in the last days. They continue to drink but only when aroused. Conjunctivitis may be marked and eyes may be heavily crusted with exudates (Fig- ure 1). Pigs are reluctant to rise, stand with arched backs, and move with a swaying or staggering gait. Terminally, there may be purple discoloration of the skin of the ab- domen, thighs, and ears. Convulsions may be seen at any time after onset of fever, and are often initiated by handling. 34 Journal of Swine Health and Production — January and February, 2002 covered pigs may appear normal and gain scesses. Changes in the small and large in- raised, dark, firm infarcts along its margin. weight, but at a slower rate. Some recov- testine may include catarrhal exudate, Though splenic infarcts are uncommon ered pigs are chronically infected for hemorrhage, erosion, and ulceration.