Материал: Bovine Viral Diarrhea Virus Diagnosis, Management, and Control

of BVDV viremia suggests that the amount of virus shed in secretions and excretions and the duration of shedding might also be diminished for some period. The existence of some PI animals that could remain undetectable through intermittent or low shedding would be a logical explanation for the unexpectedly prolonged and continued transmission in herds thought not to have any PI animals, and for which transmission was thought to be solely from acutely infected animals (Barber et al., 1985; Moerman et al., 1993). In light of these findings and the intermittent viremia in PI animals (Brock et al., 1998), it would be prudent for programs aimed at detecting PI animals and at reducing BVDV transmission to consider the possibility that PI animals may shed the virus intermittently, rather than continuously.

TRANSMISSION FROM ACUTELY INFECTED

ANIMALS

The amount of virus shed and the duration of shedding by acutely infected animals very likely depend on the virulence of particular strains of BVDV and their propensity to replicate (Bolin and Ridpath, 1992), as well as on the presence and repertoire of SN antibodies (Bolin et al., 1991a). Animals infected acutely are believed to shed the virus briefly, perhaps for only a few days or weeks, depending on the strain of the virus (Duffell and Harkness, 1985; Niskanen et al., 2000). Under field conditions, where the vast majority of acute infections by indigenous strains in a herd are mild and subclinical, the duration of shedding may be only 1–2 days, or less. Failure to detect viremia by alternate-day PCR testing in unaffected dairy calves during a 4-week period preceding seroconversion suggests that viremia and shedding in acute infections can be very short-lived, perhaps less than 24 hours in calves experiencing no untoward signs of infection (Thurmond, unpublished observations).

In contrast to endemic BVDV infection and transmission of indigenous strains among cattle in a herd, the severity of disease and duration of shedding have been observed to be much different following the addition of new cattle to a herd. Clinical disease has been more severe and the virus has been shed for longer periods both in newly purchased, out-of-state cattle following week-long exposure to their new herdmates and in some of the herdmates in contact with the new replacements (Thurmond, unpublished observations). A possible explanation is that the animals became infected with new strains that were sufficiently different from those that directed their repertoire of cell-mediated and SN antibody immu-

nity such that the infecting viruses were able to produce severe disease, with viral shedding extending for several days or weeks. Similar observations were made in an experimental study where acute infection with virulent strains producing severe clinical disease resulted in more extensive shedding of virus for well over a week, compared to infection with strains that caused only mild disease (Kelling et al., 2002). Thus, the potential for shedding and transmission from acutely infected animals should be expected to vary considerably, depending on crossprotection acquired from other exposures, including vaccines (Bolin et al., 1991a), and the virulence of the virus.

As noted elsewhere (Brownlie, 1990), little attention has been given to the presumed latency of acute BVDV infections. A prevailing assumption has been that acute infections result only in transient shedding of the virus, perhaps as long as 14–21 days (Duffell and Harkness, 1985), but more typically between 1–2 days or less than a week, with no recrudescence of virus later on. Unfortunately, there are no reports of the requisite long-term follow-up studies necessary to confirm or deny such an assumption. The fact that SN titers persist long after viremia wanes indicates the virus is still replicating and being presented to the lymphoid tissue. Although seemingly unlikely, given the effect of SN antibodies in suppressing viral shedding, exacerbation of an acute infection with renewed shedding, perhaps from sequestered, protected sites, could explain transmission cycles and unusually long time (2–3 years) for all animals in some small herds to become infected (Barber et al., 1985; Moerman et al., 1993).

It is not known with confidence whether, under certain conditions, the latency of acute infections that follows an initial transient shedding can be broken, allowing the virus to be shed again. For example, in one study, a young bull that had acquired an acute BVDV infection, perhaps at 10–11 months of age when the blood testicle barrier was being developed, was observed to persistently shed the virus in his semen for at least 11 months thereafter. Virus could not be detected in serum, buffy coat, or other organs except the testicles, but high SN antibody titers were maintained to the homologous virus (Voges et al., 1998). The fact that this case resembles an acute infection, rather than persistent infection, would suggest the possibility that acutely infected bulls could persistently shed virus in the semen and yet maintain high antibody titers with no detectable virus in the serum or buffy coat. The authors recommend that as part of routine screening of

bulls for BVDV, semen should be tested for the presence of the virus.

The recurrence of shedding in acutely infected animals also may depend on the biotype of the virus. Intermittent nasal shedding of the virus was observed in animals experimentally infected with a cytopathic strain (Fray et al., 1998). Others have detected cytopathic virus in tissues and nasal secretions, after clearance of the virus from the blood, while SN antibodies were still present (Baule et al., 2001). Authors of this study suggested that sequestration of cyptopathic BVDV in acutely infected animals may present a source of virus for recurrent shedding.

EFFECT OF COLOSTRAL ANTIBODIES AND VACCINATIONINDUCED IMMUNITY ON TRANSMISSION

Passive or acquired immunity to BVDV can affect transmission in two ways. One is by potentially reducing the amount or duration of shedding in animals with an acute or PI infection, thus diminishing the amount and duration of infectiousness, as discussed above. Animals that shed lesser virus for a shorter period of time will enhance herd immunity by reducing the likelihood of susceptible animals acquiring the infection if they contact an infected animal. The other way immunity reduces transmission is by reducing the number of animals susceptible to infection where susceptibility exists on a continuous scale, rather than on an all-or-none scale. Animals with considerable immunity (i.e., high SN antibody titers to a diversity of strains) would be much less susceptible to a given infectious dose than animals with lesser immunity (i.e., low SN titers and limited antibody cross-reactivity). Immunity of varying degrees and duration can be acquired from colostral antibodies, vaccination, and infection with field strains.

PREVENTION OF TRANSMISSION BY

COLOSTRAL ANTIBODIES

Colostral antibodies can offer protection from BVDV infection for the first 2–4 months of life, depending on the quality and quantity of colostrum consumed, after which antibodies decay to levels that may no longer protect against infection. A large study of calves on two dairies practicing good colostrum management estimated that SN colostral antibody titers of half of the calves would decay to <1:16 by about 110 days of age for type 1 and 80 days of age for type 2 BVDV (Muñoz-Zanzi et al.,

2002). The rate of antibody decay determines the rate at which calves become susceptible to infection, and also to MLV BVDV vaccination, where SN antibody titers of 1:16–1:32 have been considered to offer protection against clinical signs associated with BVDV infection (Bolin and Ridpath, 1992; Howard et al., 1989).

Colostral protection depends on the quality of the colostrum management program, including ingestion of first milking colostrum with a high specific gravity from multiparous cows immediately after birth. Cows contributing colostrum should be well immunized against BVDV, preferably with both type 1 and type 2 strains. Because BVDV colostral antibodies can neutralize the BVDV in a MLV vaccine (Ellis et al., 2001), and thus diminish vaccine efficacy, it is necessary to know at what age colostral BVDV antibodies can be expected to decay to <1:16. At this age, calves become more susceptible both to infection with BVDV and to an effective immunization with a MLV vaccine. Because colostral antibodies of all calves do not decay at the same rate, the age of susceptibility that provides an early window for vaccination may vary by a month or more. Consequently, multiple vaccinations may be needed; an early vaccination to protect calves with little or no colostral antibody and a later vaccination for calves in which antibodies decay more slowly. Coupled with timing of vaccination to avoid colostral antibodies that may negate MLV vaccination is strategic vaccination before calves would begin to experience a risk of natural infection from herdmates.

The period of maximum transmission will vary from herd to herd, depending on the prevalence of PI calves and stocking rates. Transmission of BVDV in dairy calves in large, intensive dry-lot operations can begin at about 3 months of age and peak at about 7 months of age (Rush et al., 2001). Consequently, under conditions that foster such a high risk of infection, a narrow window for vaccination exists between the age when colostral antibodies decay to <1:16 and when the risk of infection begins to accelerate.

PREVENTION OF TRANSMISSION BY

VACCINATION

Most BVDV vaccination studies have targeted at how well vaccination might prevent or reduce disease caused by BVDV following experimental challenge with large doses of virus (van Oirschot et al., 1999) and not at how well vaccination might prevent infection. Because of the diversity of BVDV strains (Dubovi, 1992; Bolin et al., 1991a), the success of

vaccination aimed at increasing immunity and reducing the risk of infection and subsequent transmission will likely depend on the homology between the vaccine strain and the field strains present in the herd. The more homologous the strains, the more cross-protection will be achieved. One approach in using MLV vaccines is to preempt natural infections (Harkness, 1987), thereby controlling the timing of BVDV infection to strategically maximize immunity during critical periods of transmission. As discussed above, strategic vaccination of intensively managed calves attempts to induce an active immunity before transmission begins to increase at 3–4 months of age, but after colostral antibodies have decayed sufficiently so as not to neutralize the vaccine virus. Strategic vaccination also may be indicated before puberty and before heifers are bred to reduce conception failures associated with infection with field strains of BVDV (Muñoz-Zanzi et al., 2003b).

Another example of strategic vaccination is the common practice involving prebreeding vaccination with MLV vaccines or vaccination of pregnant cows early in gestation with killed vaccines. These strategies are intended to increase immunity, mainly by increasing the titer of SN antibodies, of the dams against acute infection and transmission of BVDV to the fetus before 120–150 days in gestation. The strategy of prebreeding vaccination with a MLV vaccine may reduce the risk of acquiring an acute infection in early pregnancy and subsequent development of PI in the fetus, depending on crossprotection between the strain of virus used in the vaccine and the wild strains circulating in the herd. However, complete protection against transplacental transmission to the fetus of cows given an MLV vaccine prebreeding may not be a realistic expectation. In one study, 2 of 12 (17%) antibody-negative heifers vaccinated 30 days before breeding with a MLV type 1 strain (NADL) gave birth to a PI calf after challenge exposure with a type 1 strain between 70 and 75 days in gestation (Cortese et al., 1998). In a similar study, PI was detected in 8 of 19 fetuses (42%) from cows vaccinated 45 days before breeding with a MLV type 1 strain (NADL) and challenged with a type 2 strain at 75 days of gestation (Brock and Cortese, 2001). The potential diversity and subsequent failure of cross-reactivity within genotypes also was observed whereby one or more of six calves vaccinated with either of two commercial vaccines, each containing a different BVDV type 1 strain, produced detectable antibodies against 10 cytopathic and 10 noncytopathic BVDV isolates,

but no calf produced antibodies to all 20 viruses (Bolin and Ridpath, 1989).

Further evidence for incomplete protection of the fetus by prebreeding vaccination was presented in a study in which transmission resulting in PI was not prevented in all cows vaccinated 3 weeks before breeding with a MLV type 1 vaccine; 7–15% of the fetuses of vaccinated cows challenged between 55 and 100 days in gestation with a selected heterologous type 1 strain developed PI (Dean et al, 2003). Use of a killed vaccine 2–3 weeks before breeding, also failed to prevent transmission of BVDV from cows to their fetus and establishment of PI (Bolin et al., 1991a). However, three reduced doses of killed vaccine prevented BVDV infection by respiratory challenge with the same vaccine strain (Howard et al., 1994), suggesting that killed vaccines may have a role in reducing transmission for some strains.

Further evidence for incomplete protection against congenital transmission was found in studies of newborn dairy calves from cows vaccinated with BVDV type 1 MLV vaccine. The study found that 10% of calves born alive had acquired a congenital BVDV infection, including infection with BVDV type 1 (Muñoz-Zanzi et al., 2003a), suggesting that considerable transplacental transmission can still take place in cows vaccinated once a year prior to breeding. These studies also illustrate the potential for genetic diversity of BVDV (Dubovi, 1992) for virus strains within a genotype and especially for strains with different genotypes (Ridpath, 2003). One should expect, therefore, that a vaccine containing only a type 1 strain of BVDV, for example, should not be expected to offer complete protection of the fetus against infection with other type 1 strains or necessarily any protection against infection with type 2 strains of the virus.

In a field study of natural horizontal transmission of wild strains of BVDV to dairy calves, vaccination with a MLV BVDV type 1 vaccine strain was estimated to prevent only 48% of infection with type 1 strains and effectiveness lasted only about 60 days. It is likely that presence of colostral antibodies was responsible for some of the incomplete protection. The vaccine, however, did not offer any detectable reduction in risk of infection with type 2 strains (Thurmond et al., 2001). In a similar study on another herd (Thurmond and Hietala, 2002), no difference in infection rates of BVDV type 1 was found between calves given a MLV vaccine containing both type 1 and type 2 strains and calves given a MLV vaccine containing only a type 1 strain. However, a significantly higher proportion of calves

given a vaccine with only one strain of BVDV type 1 became infected with BVDV type 2 than of those given the vaccine with both type 1 and type 2 strains. The efficacy in protecting against infection with BVDV type 2 was 64% for the vaccine with both type 1 and type 2 strains, compared with no protection offered by the vaccine with only a type 1 strain.

Field studies following the SN antibody titers of animals found considerable evidence for dual genotype natural infections, in which animals had evidence of infection with both type 1 and type 2 strains (Rush et al., 2001; Muñoz-Zanzi et al., 2002; Muñoz-Zanzi et al., 2003a), indicating that natural infection with one genotype does not necessarily protect against natural infection with the other genotype. Currently, most BVDV type 1 vaccines used in North America contain only type 1a strains, with the exception of one killed vaccine (Fulton et al., 2002). No vaccine trials have been reported that indicate the extent to which vaccines containing BVDV type 1a strains will protect against infection with type 1b strains, which have been found to be a predominant subtype in calves with respiratory disease (Fulton et al., 2002). These findings indicate that, under typical field conditions, the use of MLV vaccine with only a type 1 strain should not be expected to offer cross-protection against infection with type 2 strains, and suggest that vaccines employed to reduce transmission of both type 1 and type 2 strains of BVDV should contain both type 1 and type 2 strains.

INTRA-HERD TRANSMISSION

The rate at which BVDV is spread within a herd depends on the prevalence of PI animals, the rate of animal-to-animal contacts, the virulence of the virus strain(s), and the susceptibility of the cattle to new and indigenous strains in the herd. In studies of small, susceptible (unvaccinated) dairy herds with low density management involving seasonal grazing, transmission was reported to be slow, requiring 2–3 years for all adults to become infected (Barber et al., 1985), presumably in the absence of PI animal in the adult herd. In another similar herd, transmission was cyclical, with temporary increases in infection rates when acutely infected animals were mixed with susceptible animals, but also showing variation in transmission rates for cattle exposed to animals with PI (Moerman et al., 1993). Both studies concluded that transmission could be sustained for 2 or more years by shedding from acutely infected animals only, in the absence of PI animals, but that

transmission also can proceed unexpectedly slowly in a herd with PI animals. The R0 was estimated in the latter study to be only 3.9.

In contrast, another study of transmission among animals in an extensive management system, with a stocking density of 67 animals/km2, estimated R0 to be 2.3 if no PI animals were detected and R0 = 35 if the prevalence of PI in the herd was 1.2% (Cherry et al., 1998), suggesting a greater than tenfold increase in the transmission and risk of infection attributable to the presence of PI animals. To achieve eradication (R = 1), the authors concluded that if R0 = 32, PI animals would have to be removed from the herd before 11 days of age. In herds without PI, a 2–3-year cycle was projected for fetal loss attributable to variation in the rate of acute infections. For cattle managed under extensive grazing conditions, where stocking density ranged from 0.2–1 animal/acre, no transmission was observed from animals with acute infection, but the incidence of transmission from PI calves to cows ranged from 0.006–0.04 new cases per day (Radostits and Littlejohns, 1988).

Continual transmission over the course of 2–3 years, without exposure to a PI animal, however, would seem to require a much longer period of shedding from acutely infected animals than has been reported in the literature (Duffell and Harkness, 1985; Niskanen et al., 2000). If transmission in these herds was in fact due only to shedding by animals with an acute infection, the shedding period for acute infection would have to be considerably longer than the 1–3-week period typically quoted (Duffell and Harkness, 1985; Houe, 1995). The slow transmission in the presence of PI animals in one of the herds also was incompatible with the prevailing dogma about the high, continual risk of transmission from PI animals. One could reason that if a PI animal continuously shed large amounts of virus, all the animals in the herd, regardless of the animal density, would have come in contact with and been infected by a PI animal within 2 years. An alternative explanation for such a slow transmission would be intermittent shedding, either by acutely infected animals or by PI animals, as discussed above. Intermittent shedding by acutely infected or PI animals could explain both the cyclicity in new cases and the slow rate of spread in such small herds.

Under intensive management conditions, such as feedlot operations, in which animal density can be as high as 9.7 animals/100 m2 (Rush et al., 2001) and PI animals are invariably present, transmission of BVDV can be much more rapid. For a dry-lot dairy herd practicing MLV BVDV vaccination, 97%

of the heifers showed evidence of infection by the time they reached 13–14 months of age (Michel et al., 1993), and calves entering feedlots seroconverted to BVDV at the rate of 0.4% (Taylor et al., 1995) to 0.7% per day (Martin and Bohac, 1986). The highest incidence of seroconversion on two large dry-lot dairy calf operations was 0.5% and 1.3% per day for 7-month-old heifers (Rush et al., 2001). In another study, when a PI animal was yarded with susceptible cows in that same herd, the incidence rate increased from 0.006–0.04 new cases per day to 0.6 new cases in less than 12 hrs (Radostits and Littlejohns, 1988).

INTER-HERD TRANSMISSION

Considerable opportunity exists for transmission of BVDV from herd to herd, depending on the specific herd biosecurity policies, as reviewed elsewhere (Harkness, 1987). Introduction of new animals into a herd was found to be the main factor significantly associated with high herd seroprevalence of BVDV (Mainar-Jaime et al., 2001). Use of semen from an infected bull also was observed to spread the virus to a number of closed, BVDV-negative herds (Kirkland et al., 1994). Other means of inter-herd transmission include cross-fence contact with infectious animals, their secretions, or excretions; embryo transfer involving introduction of pregnant recipients carrying a fetus with PI, even if the recipient is uninfected; use of MLV BVDV vaccines; and feeding calves waste or “hospital” milk from dairies with lactating PI cows. Ruminants other than cattle can become infected with BVDV (Nettleton, 1990); however, the risk of transmission from these animals is not known.

MINIMIZING TRANSMISSION OF BVDV

REDUCING THE COEFFICIENT OF

INFECTIOUSNESS (ß)

One strategy in reducing transmission is to reduce the infectiousness of infected animals. The probability that an infected animal will transmit the infection to a susceptible animal may be reduced, under some conditions, by maximizing the SN antibody titer, either through colostral antibodies or vaccination. As discussed above, animals with SN antibodies at the time they acquire an acute BVDV infection may be expected to shed less virus, and for a shorter period of time, than if they had no antibodies. The fact that viremia in PI animals can cease in the presence of SN antibodies (Howard et al., 1989; Palfi et al.,

1993; Brock et al., 1998; Grooms et al., 2001) suggests that shedding also might be diminished following vaccination or ingestion of colostral antibodies. Certainly the main means of diminishing the overall herd infectiousness would be by removal of PI animals because PI animals shed considerably more virus than acutely infected animals.

REDUCING THE LIKELIHOOD OF ADEQUATE

CONTACT

As shown by the equations for R0 and incidence of BVDV infection, the rate of adequate animal-to- animal contact is very important in fostering transmission. In a prospective study (Rush et al., 2001), transmission was related not only to animal density but also to the absolute number of animals in a pen or corral (stocking rate). A small number of animals in a high-density corral or pasture would likely experience a lower risk of infection than a large number of animals in a low-density corral because a corral with many animals is more likely to contain a PI animal, regardless of the density. Strategies for control of BVDV transmission include limiting contact between animals by housing calves in individual isolation hutches (Rush et al., 2001), decreasing density, increasing feed bunk space and water troughs, reducing access to cross-fence sharing of feed and water troughs, double fencing to reduce cross-fence physical contact, and reducing the number of animals in a pasture or pen.

REDUCING THE DURATION OR PREVALENCE

OF INFECTIOUS ANIMALS

Duration of infectiousness may be reduced in acutely and perhaps in persistently infected animals by providing adequate SN antibody, as discussed above, for reducing the coefficient of infectiousness. A fundamental strategy in controlling transmission of BVDV is to identify and remove PI animals, thereby reducing the prevalence of infectious animals in a herd.

REDUCING THE PROPORTION OF

SUSCEPTIBLE ANIMALS

Because transmission cannot proceed without susceptible animals, a key concept in herd-based control of BVDV transmission is to maximize individual and herd immunity through high-quality colostrum management and strategic vaccination to maximize the repertoire of SN antibody diversity, which are aimed at reducing the probability of animals acquiring a BVDV infection at critical periods when BVDV infection could have important nega-