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

ever, infected animals become immune and the number of susceptible animals decline. As the number of susceptible animals decline, the likelihood that an infectious animal will come in contact with a susceptible animal will decrease, and the R-value will decline with each newly infected animal. Eventually, the number of susceptible animals becomes so small that the transmission rate will decline to R = 1, referred to as the endemic state, in which each new infection creates only one more new infection. As the number of susceptible animals declines further, R will become less than 1 and new infections will stop because transmission is no longer supported by a sufficient number of susceptible animals. Transmission will not continue unless additional susceptible animals are added to the herd, the immunity wanes, the contact rate is increased, the duration of infectiousness increases (i.e., a recrudescence or extension of shedding), or pathogenicity of the virus changes (i.e., mutation).

Because ß is often unknown, an alternative approximation for R0 is R0 = 1 + L/A, where L is the average or median life span of animals in a herd and A is the average or median age of animals at the time of infection. Suppose, for example, the median lifetime of a cow on dry-lot dairies is 40 months and half (median) of the animals are infected with BVDV by 8 months of age (Rush et al., 2001), then R0 = 1 + 40/8 = 6. Thus, one would expect that in a fully susceptible population, introduction of 1 infected animal would result in transmission of infection to 6 susceptible animals, which in turn would transmit infection to other susceptible animals, and so on. This somewhat hypothetical example does not distinguish between persistent and acute infections.

Alternatively, ß can be estimated using the formula for incidence of BVDV infection, II, along with available data and reasonable assumptions. Incidence provides a measure of transmission by estimating the rate at which new BVDV infections occur, or the proportion of susceptible animals that become infected during a specified time period. The formula for II is II = ßPk, where ß is the unknown probability of infection given a contact (coefficient of infectiousness), P is the assumed prevalence of infectious animals for a given time period, and k is the contact rate for the given time period (Haloran, 1998). Suppose incidence is 0.5% per day for weaned calves in a feedlot environment (Taylor et al., 1995; Rush et al., 2001), k is about 25 contacts per day, and the prevalence of PI animals is 1% (assuming that all PI animals are infectious and ignoring the prevalence of acutely infectious animals),

then ß = (0.005)/(25)(0.01) = 0.02. After substituting this value for ß into the formula for R0, and assuming an infectious period of at least 100 days for animals with PI, R0 = (0.02)(25)(100) = 50. Because these formulas assume random mixing, constant duration of infectiousness, and a constant coefficient of infectiousness, they should be used to obtain a general sense for the rate of transmission. The main benefit of these equations is that they provide a conceptual context for key elements in the transmission of BVDV.

MEASURING THE EFFECT OF VACCINATION

ON TRANSMISSION

In addition to characterizing key forces influencing BVDV transmission (i.e., probability of infection, contact rate, and duration of infectiousness), estimates of R0 and II can be used as a benchmark for assessing vaccine efficacy required to control infection. Suppose, for example, that R0 = 4, whereby introduction of an acutely infected animal into a completely susceptible herd or group of animals will result in 4 new infections. If the proportion of the animals, p, represents those animals that have been effectively immunized and that are not susceptible is 0.5, 1 new BVDV infection will result in only 2 new infections rather than 4, because half of the contacts will be immune. In planning a vaccination program, we can use R0 to estimate p, the proportion of the herd that must be immunized to prevent transmission within the herd. If p R0 animals will not become infected because of vaccination, the average number of animals that will become infected by the primary case is R0 (p R0 ).

In order to prevent transmission of infection, the number of secondary cases resulting from the primary case will need to be less than 1, or R0 (p R0 ) = <1. This is equivalent to R0 1 <(p R0 ) or p >1 1/R0, which is the formula for vaccination protection (Giesecke, 2002). In other words, if R0 = 4, then the proportion of the herd that will need to be effectively immunized by vaccination (assuming the vaccine is 100% effective in immunizing an animal against the particular virus strain) to prevent transmission from acutely infected animals is 1 1/R0 = 1 1/4, or 75%. If the R0 for the strain causing the acute infections was 2, rather than 4, only 50% of the herd would need to be immunized.

TRANSMISSION UNDER THE INFLUENCE OF

HERD IMMUNITY

Herd immunity implies that the overall immunity of the herd is greater than the sum of immunity enjoyed

by individual animals in the herd. In other words, a nonimmune, completely susceptible animal enjoys some immunity from infection if it is in a herd with animals that possess immunity, where immunity may have been acquired by natural infection with a field strain, through vaccination, or from colostral antibodies. Several factors can contribute to herd immunity. Transmission within a herd or group of animals with a high proportion of immune animals is slower, as shown above using R0 or II, than for herds with a low proportion of immune animals. Thus, a susceptible animal in a herd with a high proportion of immune animals will have a lower risk of infection than if it were in a herd with a low proportion of immune animals. Individual animal immunity also can affect the dynamics of transition from one disease state (i.e., latent, infectious, noninfectious) to the other and the infectious dose needed to produce infection. For example, animals with little or no immunity can be expected to have a shorter latent period and to require a lower dose of virus to become infected, compared with animals with immunity.

Although specific data are not available for BVDV herd immunity, some generalizations are possible by applying what is known about the infectiousness of the disease. For example, depending on strain variation, cell-mediated or humoral immunity acquired from exposure to one strain of BVDV may reduce the amount of virus shed and/or duration of shedding, and thus reduce ß and d, if the animal is infected with a different strain of the virus (Howard et al., 1989; Brock et al., 1998: Grooms et al., 2001). Generally, immunity to most infectious agents is not likely to be absolute; infection may be acquired by animals with some marginal or incomplete spectrum of immunity if exposed to a sufficiently large dose of the agent. This is particularly true in the case of BVDV because of the presence of high antigenic diversity among BVDV strains (Dubovi, 1992; Ridpath, 2003). Low levels of immunity might protect against low doses of a reasonably homologous strain, but higher levels of immunity would be required to protect against high doses of the virus, particularly if it is a heterologous strain. If an animal with partial immunity becomes infected, however, it may not necessarily shed as much virus and not for as long, when compared to a nonimmune animal (Howard et al., 1989; Brock et al., 1998; Grooms et al., 2001). Generally, acute BVDV infection of animals with some immunity can be expected to manifest in shorter periods of shedding and lower doses of the agent being shed, which subsequently would reduce the probability of transmission to a fully sus-

ceptible animal. The extent to which viral shedding is reduced in PI animals with SN antibodies is not known. However, the transient cessation of viremia in some PI animals after consuming BVDV colostral antibodies or with active SN antibodies could suggest that viral shedding might also be reduced, at least temporarily.

When herd immunity is present, virus transmission is less likely. This is because herd immunity results in a reduction in the number of animals that are susceptible to viral infection. Thus, the probability that the agent will be transmitted to a susceptible animal during an “adequate” contact is reduced, as indicated by a diminished value for ß in the equations above for R0 and incidence. In addition, the reduced duration of shedding (d), or reduced prevalence of infectious animals (P), will further diminish the force of transmission through a herd, as indicated by the equations for R0, and the diminished incidence would not only reduce disease caused by BVDV but would also reduce virus transmission by limiting the amount of virus shed from infected animals.

HORIZONTAL AND VERTICAL TRANSMISSION OF BVDV

Transmission of BVDV can occur vertically, resulting in congenital infection of the fetus, or horizontally after birth, sometimes referred to as postnatal transmission. Transmission of BVDV to the fetus before 120–150 days in gestation may result in abortion, resorbtion, or stillbirth. Surviving fetuses will be born as BVDV-infected calves. The BVDV infection will persist for the life of the animal and will result, presumably, in the continuous shedding of the virus into the environment. Thus, PI animals are believed to serve as a reservoir of BVDV. In utero transmission after about 120–150 days of gestation, when fetus becomes immune-competent, will result in a congenital infection characterized by abortion, stillbirth, congenital defects, or the birth of a live, normal-appearing calf. Horizontal (postnatal) transmission results in an acute infection sometimes referred to as transient infection (TI), varying in severity from inapparent or mild signs to life-threatening disease. Although animals that have acquired an acute BVDV infection may remain infected for some time, as indicated by prolonged duration of SN antibodies for a year or more, it is generally believed that they shed the virus only transiently for a week or less. Unfortunately, the requisite long-term data are not available to confirm or deny subsequent recrudescence of shedding. Several reviews of BVDV have addressed specific aspects of BVDV infection

(Duffell and Harkness, 1985; Radostits and Littlejohns, 1988; Harkness, 1987; Brownlie, 1990; Houe, 1995; Radostits et al., 1999).

CONGENITAL TRANSMISSION RESULTING IN PI

There are two ways in which a fetus can acquire a persistent infection. One is by transmission of the virus from a PI cow to her fetus, which very likely will result in PI calf. The other way is by acute infection of the dam in the first 120–150 days of gestation during which it can transmit infection to the fetus. The rate at which PI animals are created in a herd depends on the prevalence of PI among pregnant cows and on the proportion of cows acquiring an acute BVDV infection during the first 120–150 days of gestation. The ability of BVDV to infect embryos prior to attachment may be limited, as suggested by studies showing only one of six fetuses exposed before 60 days developed into PI calves (Moerman et al., 1993). In contrast the generation rate of PI animals was significantly higher when fetuses were exposed after 45–60 days gestation (Liess et al., 1984).

CONGENITAL TRANSMISSION AFTER

120–150 DAYS IN GESTATION

Little information exists on the rates of congenital transmission in fetuses after they become immunecompetent. Two large studies of midwestern slaughterhouses found that 20% of the fetuses from culled cows had evidence of BVDV infection (Bolin et al., 1991b; Bolin and Ridpath, 1998), some of which appeared to be attributable to cytopathic vaccine strains. As shown in a recent field study of large dairies, congenital transmission after 120–150 days gestation may occur in 10% or more of vaccinated dairy cows (Muñoz-Zanzi et al., 2003a). This estimate represents only those calves that are born alive and probably would be considerably higher if congenital infections in aborted fetuses and stillborn calves are also included. Congenital transmission of BVDV after 120–150 days in gestation, which is indicated by the presence of precolostral BVDV-neu- tralizing antibodies in calves at birth, has recently taken on new importance. Calves congenitally infected with BVDV were more likely to develop severe diseases in the first 4–5 months of life (MuñozZanzi et al., 2003a) and to experience breeding difficulties as heifers (Muñoz-Zanzi et al., 2003b).

HORIZONTAL TRANSMISSION AND ACUTE

INFECTION

Acute infection results from horizontal transmission of the virus to a susceptible animal following ade-

quate contact either with an acutely infected animal that is in the infectious (shedding) phase of the disease or with a PI animal that continuously sheds the virus. Acute infection can be acquired through various routes of transmission, including direct nose-to- nose contact, fomites contaminated with the virus, and vaccination with a modified live virus (MLV) (Duffell and Harkness, 1985; Harkness, 1987; Brownlie, 1990; Houe, 1995; Radostits and Littlejohns, 1988; Radostits et al., 1999).

ROUTES AND MEANS OF BVDV TRANSMISSION

Generally, one can expect BVDV to be shed in most excretions and secretions, including tears, milk, saliva, urine, feces, nasal discharge, and semen of acutely infected animals during the transient, infectious phase of the disease, as well as those of most if not all PI animals (Houe, 1995). Some key routes of transmission pertaining to current management conditions are discussed in the following sections.

TRANSMISSION VIA SEMEN

Semen from PI and acutely infected bulls contains large amounts of BVDV, although semen from acutely infected bulls typically would have much less virus than semen from PI bulls (Brock et al., 1991; Voges et al., 1998). Transmission of BVDV via semen can result in an acute infection or in the development of PI, or both. The potential for establishment of persistent infection in calves sired by a PI bull was illustrated in a study in which all 12 seronegative heifers developed acute BVDV infection following insemination with semen from a PI bull (Meyling and Mikel Jensen, 1988). One of the 12 calves (8%) developed persistent infection, indicating a significant potential not only for acute transmission of BVDV from PI bulls but also for the establishment of PI animals. Even though only 1 of 12 calves developed PI, a transmission rate of 8% would suggest that natural breeding by PI bulls could contribute significantly to an accumulation of PI animals in large herds.

The risk of calves developing persistent infection may be high in herds using a large number of bulls in natural breeding, particularly in large dairy herds where each bull may service several hundred cows per year and in herds with a significant proportion of susceptible, antibody-negative females. In one case, a single PI bull was found to have transmitted BVDV to 55 cows, resulting in at least 2 PI animals, and was responsible for spreading BVDV to several clean herds (Kirkland et al., 1994). In contrast, in-

semination of 73 heifers with semen of a bull with acute BVDV infection resulted in virus transmission to only 3 of the animals, but a second cycle of transmission appearing 29 days after insemination resulted in 8 of 15 of the heifers that were subsequently followed becoming infected.

Bulls that acquire an acute BVDV infection also can shed the virus in the semen for several days after cessation of viremia, perhaps up to 14 days after infection, even though semen quality may remain normal (Kirkland et al., 1991; Kirkland et al., 1994). The much lower amount of virus found in the semen of acutely infected bulls, as compared with that in PI bulls, and the low rate of shedding of BVDV in acutely infected bulls (Whitmore et al., 1978), would suggest a low risk of BVDV transmission via semen from acutely infected bulls, compared with that from PI bulls. In one reported case, however, BVDV was shed persistently for at least 11 months in an apparently acutely infected young bull (Voges et al., 1998), suggesting that some acutely infected bulls may pose the same threat as PI bulls. Thus, use of semen from bulls with acute BVDV infection could pose a substantial risk of transmission to susceptible cows, with possible development of PI in their fetuses, as previously illustrated (Kirkland, et al., 1997).

The high prevalence of PI (0.8%) found among well-screened bulls at artificial insemination centers (Howard et al., 1990) and the high rate of PI in embryo transfer (ET) animals (>2%) (Hietala et al., 2000) suggest that bulls selected for breeding, particularly ET bulls, can be expected to have a higher probability of PI than the general population of cattle. Coupled with the high rate of contact with females, therefore, bulls can pose a significant risk of infection, either as a PI animal or temporarily as an acutely infected animal. Recommendations for reducing bull-related transmission include routine testing of bulls for PI, testing the semen for virus, and vaccination (Brock et al., 1991; Voges et al., 1998).

TRANSMISSION VIA EMBRYO TRANSFER

Embryo transfer can constitute an important means of BVDV transmission to valuable genetic stock. The virus can be transmitted to the fetus if the donor has acute or persistent infection and the embryo is not adequately washed, if the recipient has acute infection or PI, or if the fetal calf serum (FCS) used to wash the embryo contains BVDV. The replication and persistence of BVDV in in vitro embryonic production systems appears to be strain-specific

(Givens et al., 2000). Large amounts of virus have been found in serum (106 TCID50/ml), vaginal mucous (106 TCID50/ml), urine (104 TCID50/ml), uterine flush medium (102 TCID50/ml), and feces (104 TCID50/ml) of a PI donor heifer (Brock et al., 1991). In addition, several studies have documented BVDV contamination of commercial FCS. In one study, 10% of irradiated lots and 62% of nonirradiated lots of FCS were contaminated. Heat inactivation of serum at 56°C for 30 min was not considered to be a reliable method to eliminate BVDV from sera (Rossi et al., 1980). The virus also was found in 20.6% of slaughterhouse fetuses, which constitute the main source of FCS (Bolin et al., 1991b). BVDV was found in 20.3% of 1000 lots of FCS harvested by a commercial supplier from an abattoir that killed beef and dairy cattle from three states (Bolin and Ridpath, 1998). Thus, even if donors and recipients are not infected with BVDV, use of untested FCS may result in transmission of BVDV to an embryo and establishment of PI. Embryo transfer also can result in back-transmission of BVDV from an infected or contaminated embryo to a susceptible recipient animal. It has been proposed that maternal families of animals with PI may be established if future breeder animals acquire PI as a result of BVDV transmission through embryo transfer (Brock et al., 1991).

TRANSRECTAL TRANSMISSION

Rectal transmission of BVDV was demonstrated following the palpation of eight seronegative heifers, all of which seroconverted after a single rectal palpation of the uterus and ovaries, using the same glove that had been used to palpate an animal with PI. Virus was isolated from five of the heifers, and all eight showed mild signs of clinical BVD. The results suggest that transmission to seronegative animals following rectal palpation with the same glove as used for a PI animal could be an important means of transmitting BVDV (Lang-Ree et al., 1994), and of establishing PI if cows are palpated before 120 days in gestation.

IATROGENIC, FOMITE, ENVIRONMENTAL,

AND INSECT TRANSMISSION

The BVD virus can survive in a cool, protected environment for several days or even weeks (Houe, 1995). Consequently, fomite or iatrogenic transmission can occur if susceptible animals are exposed to feed or equipment (e.g., nose tongs, halters, milk bottle nipples, balling guns, etc.) previously used on animals with PI or acute infection (Radostits and

Littlejohns, 1988; Gunn, 1993; Houe 1995; Radostits et al., 1999). Transmission to seronegative animals also is possible by reusing the same 19 gauge needle within a few minutes after it has been used on a PI animal, but viable virus may remain in the needle for at least 3 days (Gunn, 1993). Environmental contamination of shared calving pens by uterine discharges from PI or acutely infected cows may create a common source of exposure for other cows and for newborn calves that lack colostral protection (Radostits and Littlejohns, 1988).

Environmental conditions that favor crowding and aerosol transmission would increase the likelihood of BVDV transmission from acutely infected, coughing calves with a respiratory form of BVDV infection, particularly that caused by BVDV type 1b (Baule et al., 2001). It also is possible for flies to mechanically transmit the virus to susceptible animals. The BVD virus was isolated from face flies (Musca autumnalis) feeding on the nose and face of a PI animal (Gunn, 1993). In another study, stable flies (Stomoxys calcitrans), horseflies (Haematopota pluvialis), and head flies (Hydrotaea irritans) force-fed for 10–15 minutes on a PI animal, were able to transmit infection to susceptible animals (Tarry et al., 1991).

TRANSMISSION VIA MILK AND COLOSTRUM

Milk from PI cows can be expected to contain BVDV and thus will serve as a source of virus for calves fed unpasteurized waste milk from sick or treated cows, as is practiced commonly on large calf-raising operations. Colostrum from cows with persistent or acute infection may or may not contain infectious BVDV, depending on whether crossreacting neutralizing antibodies are present.

TRANSMISSION VIA MODIFIED LIVE VIRUS

VACCINE

Iatrogenic transmission of BVDV to the fetus, resulting in CNS lesions, teratogenic defects, and death, can occur if susceptible females are vaccinated with a MLV BVDV vaccine during pregnancy (Orban et al., 1983; Liess et al., 1984). A survey of fetuses harvested from midwestern slaughterhouses found cytopathic strains of BVDV identical to those used in some MLV vaccines (Bolin et al., 1991b), suggesting that fetal infection following BVDV MLV vaccination may not be uncommon. Vaccination with MLV BVDV also can be a highly efficient means of transmitting BVDV to the fetus and producing animals with persistent infection. An experimental study of congenital infection following MLV

vaccination found evidence of PI in all live-born calves of 9 seronegative cows vaccinated before 120 days in gestation, while fetuses of 12 other vaccinated seronegative cows were aborted, had congenital defects, or were stillborn (Liess et al., 1984).

Shedding of virus after vaccination can be minimal for some strains. In one study, a transient viremia lasted 3–10 days after vaccination, but no shedding was detected in nasal secretions or by exposing susceptible contact animals (Kleiboeker et al., 2003). In contrast, PI calves infected with BVDV type 1b and given a BVDV type 1a MLV vaccine shed the Ia vaccine strain of virus in nasal secretions for up to 28 days after vaccination (Fulton et al., 2003). These findings suggest that transient shedding of vaccine strains is possible and may represent a means of secondary transmission to pregnant animals in contact with vaccinated animals.

TRANSMISSION OF BVDV FROM INFECTED ANIMALS

Contagiousness, or the degree to which BVDVinfected animals can transmit infection following contact with a susceptible animal, can depend on multiple factors, which may include type of infection (PI or acute), the stage of an acute infection, presence of neutralizing antibodies, and the strain of virus and its associated virulence.

TRANSMISSION FROM PI ANIMALS

PI animals can be assumed to shed large amounts of BVDV in secretions and excretions most, if not all, of the time. Pregnant PI cows invariably transmit the virus to the fetus in the first 120–150 days of gestation, resulting in persistent infection of the fetus. Bulls with persistent infection will shed large amounts of the virus in their semen, while maintaining acceptable semen quality. An understanding that PI is transmitted from generation to generation along maternal lines (Radostits and Littlejohn, 1988; Brock et al., 1991) can be used in identification of PI animals by testing the dam, daughters, and sons of PI females.

In contrast to acute infection, PI animals are believed to continuously shed large amounts of virus, thereby posing a constant threat of exposure to susceptible animals. However, considerable data have accumulated to indicate that viremia in PI animals with SN antibodies that cross-react with the PI strain will cease for some period of time until antibodies wane (Howard et al., 1989; Palfi et al., 1993; Brock et al., 1998; Grooms et al., 2001). Although data on virus shedding are not readily available, the cessation