Experimental transmission of enzootic bovine leukosis to sheep: latency period of the tumoral disease.

In the field of viral oncogenesis the latency period is the interval between detectable establishment of infection and appearance of a tumor. Between 1969 and 1985, a total of 60 sheep died with lymphosarcoma. They were inoculated with BLV-positive blood from various donor cows, by various routes, at various ages, etc. A statistical analysis was performed trying to find a correlation between the length of the latency period and, on the other hand, one or more factors, such as sex, family lineage, identity of the dam, age at inoculation, route of inoculation, or origin of the inoculum. None of the above mentioned parameters has a significant effect on the length of the latency period. In two series of sheep inoculated with decreasing number of lymphocytes from BLV-positive donor cows, hematological disorders and tumors appeared at first in recipient animals inoculated with the higher doses of infectious blood. Thus, the inoculated dose has an effect upon the length of the latency period; the higher the dose inoculated, the shorter the latency period. This finding suggests an explanation to the natural occurrence of multiple case herds as opposed to no-tumor case herds. A multiple case herd fulfills two conditions: the presence of a good donor and an efficient route of transmission allowing the transfer to the recipient of the optimal amount of infected blood.

Recombinant feline viruses containing the myc gene rapidly produce clonal tumours expressing T-cell antigen receptor gene transcripts.

We, and others, have recently shown that recombinant feline leukaemia viruses (FeLV) containing the myc gene (FeLV-myc) occur in up to 30% of naturally occurring cases of T-cell lymphosarcoma. Investigation of the disease spectrum of two FeLV-myc isolates showed that they induced clonal or oligoclonal T-cell tumours after a short latent period. The phenotypic pattern of the thymic tumours was restricted in that they all expressed the alpha and beta chains of the T-cell antigen receptor and could readily be established in culture in vitro without the addition of exogenous interleukin-2. Although helper FeLV was transmitted from infected cats to uninfected tracer cats, there was no evidence of horizontal transmission of FeLV-myc viruses, suggesting that these viruses arise de novo in individual cases of thymic lymphosarcoma.

[Experimental induction of lympholeukemia in calves by a purified preparation of bovine leukemia virus]. / Eksperimental'noe vosproizvedenie limfoleikoza u teliat preparatom ochishchennogo virusa leikoza krupnogo rogatogo skota.

The leukemogenic properties of bovine leukemia virus (BLV), isolated from blood plasma, milk and short-term cultures of blood-circulating leukocytes were studied. A complex life-time examination of 14 calves inoculated with a preparation of BLV established leukemia in 4 animals; it was suspected in 3 calves. By the present time leukemia has been morphologically established in 6 out of 13 calves and one was a suspect. Positive results point directly to the viral etiology of bovine leukemia.

A transmissible chicken tumour associated with reticuloendotheliosis virus infection.

Histiocytic lymphosarcomas of the intestine, liver, spleen and sciatic nerve were found at necropsy in a 36-week-old laying hen that was culled from a flock of 1800 birds because of emaciation. Type C particles were observed in ultrathin sections of liver and spleen. The serum of the hen contained reticuloendotheliosis virus (REV) antigen, and antibody against REV, but lacked antibodies reactive with Marek's disease virus or subgroups A and B of Rous sarcoma virus. The tumour was transmitted to chickens using a suspension of the initial tumours. These experimental tumours were then transmitted to further chickens, using cultured spleen cells, viable spleen cells that had been stored frozen, and disrupted spleen cells. The tumours, which developed after incubation periods as short as 2 weeks, were histologically similar to those in the original hen. A few chickens also developed feather abnormalities. The chickens with experimentally transmitted tumours developed antibody against REV and REV antigen was demonstrated in cultured cells from these chickens. The chickens failed to develop antibody against Rous sarcoma virus and only 1 of 29 developed antibody against Marek's disease virus.

Enzootic bovine leucosis.

Enzootic bovine leucosis is associated with infection by bovine leucosis virus. The incubation period is measured in years and a minority of infected animals develop clinical signs. The disease is widespread in Europe and elsewhere and can cause significant economic loss. The epidemiology is incompletely understood and findings from one cattle production system may not be directly applicable to another. Major control programmes exist in Denmark and West Germany and control schemes are being developed elsewhere. Eradication of enzootic bovine leucosis has been established as a goal in the EEC and research is revealing the ways in which this goal may be attained. To be effective, control and epidemiological monitoring must be interactive. Recently introduced serological tests, of improved sensitivity, provide a valuable tool.

Seroepidemiologic studies on the possible relationships of human and bovine leukemia: Brief communication.

A seroepidemiologic study was conducted in an attempt to identify antibodies against the bovine leukemia virus (BLV) in people exposed to cattle with lymphosarcoma. Farm families, farm employees, and veterinarians in contact with cow herds having documented cases of lymphosarcoma were tested for precipitating antibodies to the BLV with the agar gel immunodiffusion test. The cattle also were tested serologically. Information was collected form the farm families regarding consumption of unpasteurized milk from their dairy herd. Twenty-one dairy herds with documented cases of lymphosarcoma were identified. A total of 846 cows from these herds wre bled, of which 33% were serologically positive. No positive sera were found in the 45 dairy farmers, family members, and farm employees associated with the herds with lymphosarcoma. Consumption of raw milk was reported by 77% of the farm group. In addition, 83 veterinarians, 30 leukemia patients, and 200 control human sera were tested and found negative for antibodies to the BLV.

The epidemiology of the feline leukemia virus (FeLV).

Clustering of cases of feline lymphosarcoma (LSA) has been observed by veterinarians for many years. In 1964 it was discovered that feline LSA was caused by an oncornavirus, the feline leukemia virus (FeLV). In 1970, a simple, indirect immunoflourescent antibody (IFA) test for FeLV was developed which enabled large numbers of cats, living in their natural (household) environments, to be tested for the virus. In one study, over 2,000 cats were tested and the results showed conclusively that FeLV is a contagious agent for cats. This finding was independently confirmed by several other investigators using different testing procedures. After discovering the contagious nature of FeLV a test and removal program was devised which successfully prevents the spread of FeLV and the development of FeLV diseases in the pet cat population. There is, at present, no evidence that FeLV infects humans living with FeLV infected cats.

Biology of feline leukemia virus in the natural environment.

The feline leukemia virus (FeLV) was discovered in 1964 in a cluster of cats with lymphosarcoma. The observed clustering of cases of feline lymphosarcoma suggested that FeLV was an infectious agent for cats. The development of a simple immunofluorescent test for FeLV permitted a seroepidemiological study to be undertaken on the distribution of the virus in cats living in their natural environment. Over 2000 cats were tested, and the results showed conclusively that FeLV is an infectious agent for cats. This finding has now been independently confirmed using three different test procedures. After the infectious nature of FeLV was discovered, a simple FeLV test and removal program was devised to control the spread of the virus in the natural environment. The spread of FeLV was controlled in 45 households by removing the FeLV-infected cats, while in 25 households, where the infected cats were left in contact with the uninfected cats, 12% of the uninfected cats became infected. The ultimate control of FeLV awaits the development of an effective FeLV vaccine, which now seems feasible since we have already experimentally immunized some cats with attenuated FeLV. Although FeLV is infectious for cats there is no evidence that FeLV can infect humans.

[The incidence of lymphosarcoma (leukaemia) and feline leukaemia virus (FeLV) in cats in the Netherlands (author's transl)]. / Het voorkomen van lymfosarcoom/leukemie en katten-leukemievirus (FeLV) bij de kat in Nederland

Seven different groups of cats were examined to study the incidence and distribution of feline leukaemia virus (FeLV) in the Netherlands. The indirect fluorescent antibody (IFA) technique was used to detect FeLV antigen. Of the cats with lymphosarcoma (leukaemia), 73.2 per cent and of those with infectious peritonitis, 32.4 per cent were found to be positive for FeLV antigen. Of the sixty-six cats with other tumours, only one, a cat with carcinoma of the mammary gland; was positive for FeLV antigen. Of 557 cats with various lesions, forty-two (7.5 percent) were positive for FeLV antigen. The IFA-test was found to be a useful adjunct in establishing the correct diagnosis. Of all stud cats which had been in contact with FeLV-positive cats, 24.7 percent were positive for FeLV antigen, wheras all those which had not been in contact with these cats, were negative. There was a marked difference between the proportions of FeLV-positive cats in the groups of clinically normal cats which had (20.6 per cent) and which had not (0.4 per cent) been in contact with FeLV-positive cats. Follow-up studies showed that 67.8 percent of the clinically normal, FeLV-positive cats had died from or been sacrificed because of FeLV-associated diseases within twenty months.

Horizontal transmission of feline leukemia virus in cats.

Traditionally, cancer has not been considered an infectious disease although some multiple cases of leukemia in man and cattle have been reported. The discovery that feline lymphosarcoma was associated with an RNA virus (feline leukemia virus(FeLV)) meant that infectious transmission of the disease was a possibility. The critical question was whether the predominant method of transmission from one animal to another was 'vertical' (via the gametes) or 'horizontal' (via contagion or infection). A number of epidemiological studies have shown that the chances of healthy cats contracting lymphosarcoma are greatly increased when a cat with the disease lives in close proximity. It does not matter whether the healthy cats are related to the sick animal or not. It has also been established that viremic normal cats have an approximately 900 times greater chance of developing leukemia than cats whose FeLV status is unknown. Infectious FeLV is present in the excretions and blood of viremic animals. In the natural environment, feline lymphosarcoma occurs in clusters. The results in pet cats have been supported by experiments with cat colonies under controlled conditions and prove that horizontal transmission of FeLV occurs. This does not mean that epigenetic (infection in utero or via the milk) or vertical transmission cannot also occur. It should be possible to break the cycle of horizontal transmission of the virus by vaccination and thus control FeLV-related diseases.