Laboratory examination is necessary to establish a diagnosis in cases of suspected Swine Fever and it was thought that an explanation of the procedure adopted at this laboratory would be of interest.
Swine Fever is caused by a very small virus, the size of the particles being estimated as 30 to 35 𝜇𝜇 (one 𝜇𝜇 (milli-micron) = one one-millionth of a millimetre). The virus multiplies rapidly when introduced into a susceptible pig but will infect only members of the pig family. Outside the body of the pig the virus can be propagated in minced swine testicle placed on agar slopes, or in similar tissue placed on the chorioallantoic membrane of developing fowl eggs. The growth cannot, of course, be seen, but is apparent by the increase in infective power of the inoculum.
In an infected pig the blood, organ suspensions, bile and urine are infective. Serum free of red cells appears to be infective but there is some difference of opinion on this point. The virus is fairly resistant but when collecting and handling material from a suspected case care should be taken to avoid putrefaction and to keep the specimen as cool as possible. In filtered blood it withstands 58ºC. for two hours but is destroyed by 78ºC. for one hour. Fluid material containing virus, when held at room temperature, retains its virulence for 10 to 14 weeks. The virus dies out fairly rapidly in putrid material.
Many cases in the field have a concomitant infection with Salmonella organisms, these being encountered in probably 50% of outbreaks. In the early stages of the outbreak the disease was transmitted by feeding tissue and by the subcutaneous injection of blood (free from Salmonella organisms) and filtrates of blood and serum. It appears probable that when subcutaneous injection is employed the incubation period is longer than when the injection is given intravenously. In order to establish a diagnosis of Swine Fever by a transmission experiment it is necessary to set up the disease with a bacteria-free filtrate. Blood from the suspected case is haemolysed by addition of distilled water, and spun in an angle centrifuge for twenty minutes (tissue suspension may be used). The supernatant fluid is then passed through an Elford collodion membrane. These filters are chosen because there is little absorption and the pore size is known (from 0.7. to 1.0 𝜇 A.P.D.—average pore diameter—have been used).
The filtrate is cultured to determine whether it is free from bacteria, and the remainder stored in the refrigerator. On the following day, if there is no growth from the filtrate, a pig is injected intraveneously (in the ear) with the filtrate, and maintained in an isolation house. Commencing on the fourth day after injection temperatures are taken daily for approximately 28 days. If neither temperature reaction occurs, nor signs of ill-health, the pig is subsequently tested for susceptibility by the injection of Swine Fever virus. The smallest amount of filtrate injected has been 0.25 ml. and this produced Swint Fever.
The maximum incubation period after injection of filtrates has been 13 days, the minimum four days, and the average six days. However, a pig injected intravenously with 0.3 ml. of filtrate showed lesions when killed 48 hours later.