Bovine leukosis virus transmission with mouthparts from Tabanus abactor after interrupted feeding

A successful attempt was made to mechanically transmit bovine leukosis virus (BLV) from a BLV-infected cow with a normal lymphocyte count to sheep by inoculation with horse fly (Tabanus abactor) mouthparts. After interrupted natural feeding, horse flies were anesthetized with CO2. Mouthparts were severed and pooled into a tissue grinder containing medium. Five inocula containing the mouthparts of 10 flies each, and 5 inocula containing the mouthparts of 20 flies each, were prepared and inoculated SC in the right axilla of 10 BLV antibody-negative sheep. Five additional sheep served as controls. Serum samples were collected at 2-week intervals and tested by agar gel immunodiffusion for BLV antibodies. One sheep injected with 20 mouthparts developed antibodies to BLV at 10 weeks after inoculation. Six months after inoculation with fly mouthparts, 1 BLV antibody-negative sheep was randomly selected from each treatment group and injected, in the left axilla, with 3 ml of blood from the donor cow to confirm susceptibility of the sheep. All 3 sheep developed antibodies to BLV within 4 weeks.     

Role of insects in the transmission of bovine leukosis virus: potential for transmission by mosquitoes

Bovine leukosis virus (BLV) was transmitted to sheep in a simulated mechanical transmission experiment, using the following species of mosquitoes; Anopheles freeborni, A stephensi, A quadrimaculatus, and A albimanus. Mosquitoes were fed on blood taken from a BLV-infected cow with persistent lymphocytosis. Mouthparts and heads of mosquitoes were removed immediately after feeding, placed in RPMI 1640 medium, and inoculated subcutaneously into sheep. Nine sheep were inoculated with mouthparts and heads from 37 to 122 mosquitoes. Infection was determined serologically. Three monthly serum samples were collected from the sheep and were tested for the presence of antibodies to BLV, using the agar-gel immunodiffusion (AGID) test. Sera that were negative by AGID at 3 months were tested by radioimmunoassay. Results from radioimmunoassay agreed with those obtained by AGID. Four of the 9 sheep developed antibody to BLV. Sheep that seroconverted were inoculated with mouthparts and heads from as few as 54 mosquitoes.     

Evaluation of the stable fly (Stomoxys calcitrans) as a vector of enzootic bovine leukosis

Experiments reported here were directed at 2 questions: (1) Can the stable fly (Stomoxys calcitrans) transmit enzootic bovine leukosis? (2) Could early viremia augment the probability of transmission by this insect? In one vector experiment, calves and bovine leukemia virus (BLV)-infected cows were housed with and without stable flies. The calves were monitored serologically during a 3-month postexposure period, using the agar gel immunodiffusion test. All fly-infested and fly-free calves remained BLV-seronegative. For a second vector experiment, donor calves, newly injected with blood from BLV-infected cows with high virus expression, were tethered alternately between uninoculated, weaned BLV-seronegative calves. These groups were housed with or without flies in 2 replicate trials. The inoculated calves from the first replicate seroconverted at 16 and 23 days after inoculation; the inoculated calves from the second replicate seroconverted at 11, 16, 16, and 37 days after inoculation. All uninoculated calves remained BLV-seronegative. In a manual transmission experiment, 50 unfed stable flies were allowed to complete a meal on each of 3 BLV-seronegative calves after feeding on a BLV-seropositive cow with high (42%) virus expression. One control calf was injected with blood from the cow. Seroconversion occurred in the control calf and 1 calf on which flies were given access. A scanning electron microscopic study was made of the everted and closed mouth parts of the stable fly. Given the lymphocyte count in blood from the cow used in the manual vector transmission experiment, it was calculated that 3,950 mouth part volumes would be necessary to transmit BLV.(ABSTRACT TRUNCATED AT 250 WORDS)     

Factors affecting the natural transmission of bovine leukaemia virus infection in Queensland dairy herds

Natural transmission of bovine leukaemia virus (BLV) infection in south-eastern Queensland dairy herds was slow in 2 herds with a low to moderate (13 to 22%) prevalence of infection. Infection spread much more rapidly in a herd that had a higher prevalence (42%) when first tested. In a 13 month study of this herd, the cumulative incidence of infection was 24%. In one herd new infections were confined almost entirely to calves of uninfected dams. Following the end of feeding bulk milk to calves, a common practice in dairy herds, no more calves in this herd became infected. In laboratory experiments, neither prolonged housing of susceptible calves with infected cattle, consumption of drinking water contaminated with infected blood, nor inoculation of sheep with saliva from infected cattle resulted in transmission of BLV infection. Sheep were infected by subcutaneous inoculation of a suspension of purified lymphocytes from an infected heifer. The minimum infective dose was 10(3) lymphocytes, equivalent to the number of lymphocytes in approximately 0.1 microliter blood. Thus, procedures involving the transfer of a very small volume of blood from animal-to-animal have the potential to transmit infection.     

Evaluation of transmission of bovine viral diarrhea virus (BVDV) between persistently infected and naive cattle by the horn fly <Emphasis Type="Italic">(Haematobia irritans)</Emphasis>

Identifying reservoirs and transmission routes for bovine viral diarrhea virus (BVDV) are important in developing biosecurity programs. The aim of this study was to evaluate BVDV transmission by the hematophagous horn fly (Haematobia irritans). Flies collected from four persistently infected cattle were placed in fly cages attached to principal (n?=?4) and control (n?=?4) BVDV-naïve calves housed individually in isolation rooms. Flies were able to feed on principal calves, but a barrier prevented fly feeding from control calves. Flies were tested for BVDV by RT-PCR and virus isolation at time of collection from PI cattle and after 48 h of exposure on BVDV-naïve calves. Blood samples were collected from calves and tested for BVDV infection. Virus was isolated from fly homogenates at collection from PI animals and at removal from control and principal calves. All calves remained negative for BVDV by virus isolation and serology throughout the study. Bovine viral diarrhea virus may be detected in horn flies collected from PI cattle, but horn flies do not appear to be an important vector for BVDV transmission.     

Investigation of the possible role of the tuberculin intradermal test in the spread of enzootic bovine leukosis

The single intradermal comparative test was used with both avian and bovine tuberculin. Three cattle infected with bovine leukosis virus (BLV) were used as a source of infection. BLV-positive and susceptible animals were tuberculin tested alternately. Fifteen susceptible calves and 15 susceptible sheep were tested. A further three calves and three sheep were used as controls; the needles of the tuberculin syringes were deliberately contaminated with blood from the BLV-infected cattle, before being used in the test. Whereas all three calves and the three sheep inoculated intradermally with contaminated needles developed BLV infections, all of the other 30 animals have remained serologically negative to BLV for 10 months. Transmission of BLV with needles contaminated with BLV-infected blood was prevented by wiping the needles with absorbent cotton wool.     

Vertical transmission of bovine leukemia virus and bovine immunodeficiency virus in dairy cattle herds.

Vertical transmission of bovine leukemia virus (BLV) and bovine immunodeficiency virus (BIV) was investigated in five dairy cattle herds in Hokkaido, where 36.1 and 17.0% of cattle were BLV and BIV seropositive, respectively, and 9.9% of dams were co-infected with both BIV and BLV. Twenty six cases of offspring born from dams infected with only BLV (17 cases) or with both BIV and BLV (9 cases) were examined for the presence of BLV and BIV before and after colostrum feeding by polymerase chain reaction (PCR) and syncytium assay. After birth, all calves were separated immediately from their dams. The offspring born from BLV-positive dams were BLV-negative before colostrum feeding, suggesting that no transplacental transmission had occurred. Thereafter, these offspring were fed colostrum or milk from their dams, but still remained BLV-negative. The other offspring born from BLV-positive dams were fed with BLV-negative colostrum, or with pasteurized BLV-positive colostrum. All these calves remained negative for BLV infection, suggesting that in utero transmission of BLV is negligible. In the case of offspring born from dams co-infected with BLV and BIV, calves were BIV-positive before colostrum feeding at 1 day after the birth, indicating in utero transmission of BIV. After colostrum feeding from their dams, newborn calves became BLV-positive. In addition, one calf was BLV-positive even before colostrum feeding. These results suggest that BIV can be transmitted to offspring in utero, and that BLV can be transmitted through colostrum or milk if dams are infected with both BIV and BLV.     

Development and serial passage of persistent lymphocytosis associated with bovine leukemia virus infection in cattle

Two calves each were inoculated with 1.5 x 10(8) or 5 x 10(9) lymphocytes collected from each one cow which had persistent lymphocytosis (PL) and antibodies to bovine leukemia virus (BLV). A sudden increase in the number of peripheral blood lymphocytes (PBL) was observed 14 and 23 days, respectively, after inoculation and the maximum number reached 29,000 and 52,000/microliters 72 and 57 days after inoculation. Although the degree of PL decreased gradually in these cattle, it continued until 14 and 44 months after inoculation when one animal was sacrificed and the other died of lymphosarcoma. The PL was passaged in cattle by inoculation of a large number of PBL obtained from cattle at the stage of PL (PLL). The degree of PL was severer in cattle inoculated with a larger number of PLL. PL was not caused by inoculation of PBL obtained from either BLV-infected non-PL cattle or cattle free of BLV. The PL was also caused by inoculation of PLL into BLV-infected non-PL cattle. On the other hand, it was not observed after inoculation of a large amount of cell-free virus obtained from short-term cultures of PLL. Antibodies to BLV developed earlier and to higher levels in cattle inoculated with PLL than in those inoculated with cell-free virus. These facts show that infection with BLV was established more effectively by PLL than by cell-free virus, the infection may occur by lymphocyte to lymphocyte interaction and the actual number of infected BLV may have an important role in development of PL.     

Experimental infection of sheep with bovine leukemia virus: infectivity of blood, nasal and saliva secretions

This study was designed to determine the relative infectivity of lymphocytes and secretions from BLV-infected cattle with and without persistent lymphocytosis (BLV+PL+ and BLV+PL-). Ninety-seven sheep of mixed sex and age were assembled into 21 experimental groups. The recipient sheep were inoculated intravenously with serial dilutions of whole blood, saliva or nasal secretions from BLV+PL+ and BLV+PL- donor cows. Between 200 to 20,000 cells from single and mixed BLV+PL+ or single and mixed BLV+PL- donor cattle were used for inoculation. A very small number of BLV-infected lymphocytes (200 cells) was sufficient to induce BLV infection in sheep inoculated with diluted whole blood from BLV+PL+ cattle. The inoculation of whole blood (containing up to 20,000 lymphocyte cells) from BLV+PL- cattle did not induce BLV infection in recipient sheep. Saliva and nasal secretions also failed to bring about BLV transmission.     

Transmission of bovine leukemia virus by Tabanus fuscicostatus

Bovine leukemia virus (BLV) was transmitted by horse flies, Tabanus fuscicostatus, from a cow with a lymphocyte count of 31,500/mm3 to goats and dairy calves. As few as 10 and 20 flies transmitted BLV to goats and calves respectively, but the minimal number of flies required to transmit the infection was not established. Groups of 150 and 100 T fuscicostatus transmitted BLV to beef calves from a cow with a lymphocyte count of 14,600/mm3. These results support a role for horse flies in the horizontal transmission of BLV.     

Role of horse flies in transmission of wquine infectious anemia from carrier ponies

Equine infectious anemia virus was transmitted from an acutely ill and an inapparently infected pony to uninfected ponies by the interrupted feeding of horse flies (tabanids). Transmission from acutely ill ponies was not accomplished following: (1) the interrupted feeding of a single horse fly, (2) bites of horse flies that had fed on an acutely affected pony 24 hours earlier, (3) bites of horse flies that had oviposited after feeding on an acutely affected pony, or (4) the inoculation of larval material derived from horse flies that had fed to repletion. It was concluded that horse fly transmission of equine infectious anemia virus is mechanical only and that infected horses that are free of clinical signs can be a source of virus for insect transmission.     

Exploratory trial to determine the efficacy of the PYthon and the PYthon Magnum slow-release insecticide ear tags for the control of midges (Culicoides spp.), attacking sheep and cattle and flies attacking cattle

This study investigated the prophylactic action of the chemical combination zeta-cypermethrin and piperonyl butoxide, administered by means of slow-release insecticide-impregnated ear tags, against biting midges (Culicoides spp) attacking sheep and against midges, horn flies (Haematobia irritant), stable flies (Stomoxys calcitrans), and houseflies (Musca domestica) attacking cattle. Treated sheep and cattle were protected 100 percent against blood-feeding midges for two months and there was a clear reduction in the number of midges collected from treated animals. Three days after the ear tags were attached to cattle, the number of horn flies on the cattle was reduced to practically zero and remained at a low level until the end of the trial (day 85). There was also a strong reduction in the numbers of stable flies and houseflies counted.     

In utero transmission of bovine leukemia virus

In an initial study, 18 calves born to cows persistently infected with bovine leukemia virus (BLV) were tested for infective virus and antibodies at birth, and no infected or seropositive animals were found. Four of these calves were maintained in quarters where infected animals were housed, and 3 of the 4 subsequently became infected. These were probably contact infections acquired during, or at some time after, birth. The remaining 14 calves were kept in isolation pens in a building housing no infected cattle. None of this group was found to be BLV infected during 1 year of observation. In further studies, 15 pregnant cows inoculated with BLV became infected. One abortion, considered to be unrelated to the BLV inoculation, occurred 38 days later. The remaining 14 cows gave birth to 1 dead and 14 live calves. The dead calf and its live twin were seropositive for BLV at birth, indicating that they had been infected in utero. The remaining 13 calves were negative for BLV antibodies at birth and remained so during 1 year of observation.     

An attempt to protect calves against experimental bovine leukosis using allografts

Six calves sensitised by implanting skin from a calf were later inoculated with lymphocytes from the same calf after the calf had been infected with bovine leukosis virus (BLV). Two out of 6 calves challenged did not develop BLV antibodies and BLV was not isolated from these animals, whereas all of the 5 control calves became infected with BLV.     

Studies on the feeding patterns of the sheep headfly Hydrotaea irritans (Diptera, Muscidae) in Great Britain

Serum-precipitin tests carried out on the stomach contents of laboratory maintained and field-caught Hydrotaea irritans showed that a positive result could be obtained for at least seven to 10 days after feeding. Eye, nasal and salivary secretions, even if undiluted, did not give a precipitin response in these tests. Field results from over 3000 flies thus indicate that 20 to 55% of the active fly population had taken a blood meal within the previous 10 days: 75 to 100% of these had fed on cattle blood and 15% or less on sheep blood. Significantly lower feeding levels were obtained from flies collected within plantations, indicating a protracted resting phase in woodland. From "fly feeding areas" high blood-feeding levels were shown from early July, as ovarian development begins. Early feeding rates were similar in both sexes.     

Sheep do not have a major role in bovine herpesvirus 1 transmission

With regard to BHV1 eradication programs in cattle it is important to know whether sheep can be a reservoir of BHV1. We therefore performed an experiment that consisted of three phases. In phase 1, 10 sheep were inoculated with high doses of BHV1 and kept in close contact with 5 sheep and 5 calves. All inoculated sheep excreted BHV1 between 8 and 15 days post inoculation and seroconverted. Although BHV1 was isolated from the nasal mucosa of 3 out of 5 sentinel sheep, none of the sentinel sheep produced antibodies against BHV1. One sentinel calf excreted BHV1 through days 12–17; the remaining 4 calves excreted BHV1 between days 18 and 24, suggesting that the first calf was infected by sheep and the remaining 4 sentinel calves were infected by that calf and not by sheep. The bacic reproduction ratio (R₀) of BHV1 between sheep and calves was estimated at 0.1, and among calves it was estimated at ≥9. In phase 2, all inoculated sheep were treated with dexamethasone and kept in close contact with 5 sheep and 5 calves. All dexamethasone treated sheep re-excreted BHV1 over a 6- to 9-day period. None of the sentinel animals seroconverted. In phase 3, the sentinel sheep and calves of phase 1 were kept in two groups and were treated with dexamethasone. None of the sentinel sheep re-excreted BHV1, whereas 3 out of 5 sentinel calves did. It is concluded that while BHV1 infection in sheep is possible, BHV1 does not spread from sheep easily to cattle.     

Indirect transmission of bovine viral diarrhoea virus at calving and during the postparturient period

Two trials were designed to investigate whether bovine viral diarrhoea virus (BVDV) could be transmitted after the birth of persistently infected calves, even if they were removed immediately after birth. In trial 1, 11 calves were actively exposed to fetal fluids and uterine lochia collected from cows that had delivered calves persistently infected with type 1 BVDV. One calf that was exposed to a sample taken on the day of calving seroconverted. In trial 2, six calves were housed in stables where persistently infected calves were being born and then removed immediately from their dams and from the stable unit within two to three hours. One of four calves in close contact with the cows after delivery seroconverted and one of two calves housed within the same stable unit became infected.     

Failure to demonstrate transmission of enzootic bovine leukemia virus infection from cows to sheep by use of common injection needles

Four bovine leukemia virus (BLV)-seropositive and 2 BLV-seronegative cows were used as donors in a study to provide evidence whether IM injection with common needles is a means of spreading bovine leukemia. Sheep were used as recipients. Of the 4 BLV-seropositive cows, 2 had high virus expression (VE; 43% and 28% of their lymphocyte thin sections had associated BLV-particles), whereas the other 2 cows did not have observed VE. After each of the 4 cows was given an injection of a 5-antigen Leptospira bacterin, a BLV-seronegative sheep was immediately given an injection of the same bacterin with the same needle. None of these sheep seroconverted, nor did either of 2 sheep given only the bacterin (with a previously unused needle). Sheep inoculated IM with 0.2 ml of whole blood from both of the cows with high VE and from 1 of the 2 BLV-seropositive cows that did not have observed VE did seroconvert. In contrast, the sheep inoculated with 0.2 ml of blood from the remaining BLV-seropositive (0% VE) cow and from the 2 BLV-seronegative cows remained seronegative. These results were interpreted to indicate that the quantity of infective lymphocytes passed during injection with common needles is too small to induce infection.     

Transmission of Escherichia coli O157:H7 to cattle by house flies

The main reservoir of Escherichia coli O157:H7 is the digestive tract of cattle; however, the ecology of this food-borne pathogen is poorly understood. House flies (Musca domestica L.) might play a role in dissemination of this pathogen in the cattle environment. In our study, eight calves were individually exposed to house flies that were orally inoculated with a mixture of four strains of nalidixic acid-resistant E. coli O157:H7 (Nal(R)EcO157) for 48h. Another eight calves were individually exposed to uninoculated flies and served as the control. Fresh cattle feces (rectal sampling) and drinking water were periodically sampled and screened for Nal(R)EcO157 up to 19 days after the exposure. At the end of the experiment, all calves were euthanized and the lumen contents of rumen, cecum, colon, and rectum as well as swab samples of gall-bladder mucosa and the recto-anal mucosa were screened for Nal(R)EcO157. On day 1 after the exposure, fecal samples of all eight calves and drinking-water samples of five of eight calves exposed to inoculated flies tested positive for Nal(R)EcO157. The concentration of Nal(R)EcO157 in feces ranged over time from detectable only by enrichment (<10(2)) to up to 1.1 x 10(6)CFU/g. Feces of all calves remained positive for Nal(R)EcO157 up to 11 days after the exposure and 62% were positive until the end of experiment. Contamination of drinking water was more variable and all samples were negative on day 19. At necropsy, the highest prevalence of Nal(R)EcO157 was in the recto-anal mucosa region, followed by rectal and colonic contents.     

Eradication of bovine leukemia virus infection from a high-prevalence herd, using radioimmunoassay for identification of infected animals

Radioimmunoassay (RIA), using the virion glycoprotein antigen, was applied in an attempt to eradicate bovine leukemia virus (BLV) infection from a herd in which virtually all the adult cattle are infected. Considering that most calves born to BLV-infected cows are negative for BLV at birth and remain negative for the first several months of life, the eradication program was based on the identification and isolation of the BLV-free calves born to infected cows. Twenty-five calves raised on colostrum and milk from their infected dams were classified as BLV-free on the basis of negative results in the RIA at 6 to 8 and 9 to 11 months of age. These animals were maintained in either complete (10 calves) or partial (15 calves) isolation from infected cattle and were examined at regular intervals for BLV and BLV antibodies. With the exception of 1 calf in the group raised in partial isolation, the animals have remained free of BLV up to the time of the last evaluation, when they were 32 to 35 months old. At these ages, more than 90% of the nonisolated cattle in the herd are BLV-positive. The data also show that this eradication trial would have failed if, in the initial procedure used to classify the calves as BLV-free, the agar gel immunodiffusion test instead of the RIA had been used. Inasmuch as the 25 calves in this study were fed colostrum and milk from their dams, the fact that only 1 of the calves became infected during the 26 to 29 months of observation provides further evidence that milk-borne transmission of BLV is infrequent and perhaps inconsequential.