Neonatal Calf Enteric Disease Vaccines

Gerald E. Duhamel

Neonatal calf enteric diseases (NCEDs) can have a devastating impact on the profitability of beef cow-calf and dairy operations. In addition to mortality, medical, and labor costs, NCED can significantly reduce body weight of beef calves at weaning and performance of replacement dairy heifers.¹

Several well-characterized infectious causes of NCED have been described.² Some of the more common infectious agents are group A RVs, coronavirus (CV), enterotoxigenic E.

Because of the continuous calving and constant introduction of replacement heifers on dairy farms and the continuous flow of susceptible calves on veal calf operations, infections caused by more than one agent should be suspected in these types of operations.^3-9,^11-13 In addition to different infectious agents, mixed infections with different groups and serotypes or geno­types of viruses and bacteria can also occur. Consequently the severity, duration, and spread of clinical disease associated with complicated infections are usually greater than with a single agent, and control may be more challenging. When scours occurs in a herd that has been vaccinated against enteric diseases, possible contributing factors should be considered, including concurrent infection with pathogens not present in the vaccine (e.g., Cryptosporidium spp., salmonellae, nongroup A RVs, other pathogens), suboptimal management of colostrum feeding, and poor control of environmental contamination.

Management practices and risk factors associated with development of NCED are sufficiently different between dairy and beef herds that control measures can be tailored according to the type of production. For example, in dairy herds, high turnover of older cows and continuous introduction of replace­ment heifers pose a higher risk of introducing new enteric disease pathogens in a susceptible population than in closed beef herds. Similarly, continuous calving and proximity of susceptible calves on dairy farms creates an ideal environment for recirculation of enteric disease agents back into the adult population. This in turn may increase and broaden herd immunity such that the efficacy of a vaccine might be enhanced partly because of the booster effect vaccine can have on preexist­ing immunity. Beef cattle herds, in contrast, are generally relatively closed, and a susceptible calf population is present only for a relatively short period annually during the calving season. Introduction of new pathogens or new strains of enteric disease agents, particularly during the calving season, can have a devastating effect because of the potential for exposure of a high number of susceptible animals over a relatively short period of time. Conversely, after an outbreak, herd immunity might be more stable and the occurrence of NCED might be reduced during the following calving seasons.

Under both types of production systems, vaccination for NCED is rarely successful without reasonably good management programs aimed at providing adequate intake of high-quality colostrum and minimizing environmental contamination through effective sanitation.

1. Establish a sound biosecurity program, including purchase of replacement animals only from reputable sources and a complete ban on the introduction of foster calves, which pose a high risk of introducing new NCED agents.

2. Provide adequate protein and energy feed, including proper concentrations of vitamin and micronutrients (e.g., copper and selenium) and abundant clean drinking water throughout gestation and calving to ensure heifers and cows have optimal body condition scores at calving, produce adequate volume of high-quality colostrum and milk, and deliver healthy and vigorous calves at birth that have a fully developed and functional immune system.¹⁵

3. Avoid crowding and keep the calving area and equipment clean, dry, and protected from the elements, particularly cold winter wind or wet muddy conditions. Provide sufficient bedding in calving pens so that calves are not born in a wet, manure-contaminated environment. Separate feeding and bedding grounds should be available in the calving area. Similarly, to minimize infectious disease transmission, use separate equipment for feeding and manure handling.

4. Limit the spread of infectious disease agents from younger, more susceptible, and potentially shedding animals to the main herd by segregating first-calf heifer pairs from the mature cow herd during the calving season until the youngest calf is at least 4 weeks old. Because of potential problems associated with nursing in first-calf heifers, newborn calves in this group should be monitored closely for colostrum intake and early detection of failure of passive transfer so that rapid intervention can be implemented.^15-17 Because first-calf heifers are at a competitive disadvantage with mature cows, segregation of pregnant first-calf heifers also ensures free access to feed and water at all times.

5. Ensure that each calf ingests roughly 10% of its body weight (at least 4 L [1 gallon]) of colostrum divided into half as soon as possible and preferably within the first 2 to 3 hours of birth, then the rest 4 to 6 hours later, but always before the calf is 24 hours old. Consider heat-treating colostrum at 60° C for 60 minutes to reduce total coliform count, and thus, reduce risk of illness (including scours) as a function of improved passive transfer of maternal immunity.¹⁸ In dairy herds, do not rely on the calf nursing the dam for adequate colostrum intake; instead, routinely use nipple/ bottle or bucket feeding and esophageal tubing. If a calf is unable to suckle or is at risk for failure of passive transfer of maternal immunity, force-feed fresh or freshly thawed frozen colostrum from the calf's dam or from a mature cow in the same herd or a neighboring herd. First-milking colostrum is optimal for passive transfer of immunity to the calf; however, feeding calves colostrum from day 1 and day 2 milking can provide long-term health benefits.

6. Thoroughly rinse and soak with hot water, and vigorously scrub and disinfect all equipment with detergent and bleach, and let them completely dry between uses for feeding colostrum and treating sick calves. This includes such items as nipple bottles, esophageal tubes, buckets, balling guns, and calf carts. For more difficult pathogens to kill such as Cryptosporidium spp., consider using chlorine dioxide (ClO₂) disinfectant.

7. Move cow-calf pairs away from the calving area as soon as possible after birth in dairy herds. In beef herds, segregate calves by age by scheduled movement of dams that have not calved to new calving pastures—the so-called Sandhills Calving System (http://veterinaryextension.colostate.edu/ menu2∕Cattle∕Calf%20Scours%20101.pdf).

8. Develop a calf-care program including rapid identification and isolation protocols for scouring calves from healthy calves in dairy herds, or sick cow-calf pairs from healthy pairs in beef herds, to minimize environmental buildup of infectious disease agents.¹⁵

Giving calves a healthy start maximizes their genetic potential and reduces costs and labor associated with treating sick animals while minimizing production losses.

Rotavirus and Coronavirus Vaccines

RVs and CVs are ubiquitous in the cattle population; most adult cattle have VN serum antibodies.^19-23 RV infections are widespread in both dairy and beef cow-calf production systems, whereas infection with CV most often occurs as sporadic outbreaks of severe diarrhea in beef cow-calf herds or chronic low-grade diarrhea in dairy and veal calf operations. In addition to causing NCED, infection with CV causes winter dysentery in adult cattle²⁴ and respiratory tract infection in calves,²⁵ and it also contributes to BRD complex in feedlot cattle.²⁶ Fecal shedding of RV and CV is common among adult cattle,^27-30 providing a constant source of challenge for naive newborn calves and allowing persistence of these viral agents at the herd level.

Although only one type of CV is known to cause NCED,³¹ different subtypes of CVs can be identified on the basis of minor genomic and antigenic differences.^31-35 On a herd basis, however, affected calves can display a range of clinical signs from enteric only to mixed enteric and respiratory signs. A molecular epidemiologic investigation based on comparative analyses of a gene encoding a major structural protein of bovine CV revealed (1) identical CV strains in different animals from the same herd and from paired nasal and fecal samples from the same animals, suggesting herd outbreaks are associated with a single strain circulating among susceptible cattle; (2) identical CV strains in affected cattle from different herds in the same region, suggesting transmission between herds; and (3) different CV strains in cattle affected during different outbreaks occurring over several years in the same herd, sug­gesting herd outbreaks are associated with the introduction of new strains in a recovered herd.³⁶

Currently, RVs are classified into at least eight distinct groups, A through H.³⁷ Although RVs that belong to groups A, B, and C naturally infect cattle,^30,37-41 members of group A RV are by far the most prevalent.^39,41 The group A RVs are further classified according to sequence differences in two genes encoding the corresponding outer protein layer of the virus, namely viral protein 7 (VP7) or G (for glycoprotein) serotype and the protease-activated attachment protein VP4 or P (for protease sensitive) serotype.^42-44 Because both the G and the P proteins are involved in neutralization of virus infectivity in vitro and protection in vivo,^45-47 and each protein can segregate independently in progeny viruses during mixed infections leading to wide variations in relative frequency of individual G/P antigenic combinations according to geographic locations and individual farm, consideration of the G/P configuration of RVs is critical to development of effective vaccination programs for prevention of NCED.

Although a good correlation between the antigenicity of the G proteins and their corresponding gene sequences is well established, a similar relationship has not been found for the P proteins. Nevertheless, the binary G/P serotyping of group A RVs classification has largely been replaced by a G/P genotyp­ing system based on nucleotide sequence of the corresponding genes.⁴⁸ Globally, 27 different G genotypes (G1 to G27) and 35 P genotypes (P1 to P35) are found among group A RVs infecting animals and humans.⁴⁸ At least eight distinct G types (G1, G2, G3, G6, G8, G10, G11, and untypeable) are known to infect cattle in the United States^49,50; however, G6 appears to be the most widespread, while G8 and G10 account for fewer infections.^23,50-55 Similarly, RVs with any of four P types (P1, P5, P11, and untypeable) are known to infect cattle in the United States, but P5 appears most prevalent.^50,53,56

The genome of RVs is composed of 11 gene segments that can be exchanged among RV strains when animals are infected by more than one virus differing in G/P types at the same time.⁴³ Therefore mixed infections produce progeny RV with a genetic makeup derived from either parental strain by a mechanism called gene reassortment. Because the G and P proteins are independently involved in generation of specific VN antibodies, reassortment of the genes encoding these proteins during mixed infections can generate new progeny viruses that can evade what once was a protective immune response.⁵⁵ Genetic reassortment is a well-characterized mechanism of immune evasion that allows persistence of RVs in susceptible populations but can also promote interspecies transmission.⁵⁷ Some RV strains appear to be host specific; however, zoonotic spread of bovine RVs to humans is increasingly recognized.^57-60 In the case of bovine group A RVs, the potential for emer­gence of virus strains carrying any of 36 possible antigenic configurations (4 Ps ? 8 Gs = 32 potential types) underscores the limitation of vaccination programs and also the importance of sound management practices designed to minimize exposure by limiting environmental contamination.

Vaccination Programs

Two approaches are widely used in an attempt to protect calves against RV and CV infection and diarrhea. The most common approach involves passive protection of the suckling neonate by transfer of high levels of specific VN colostral antibodies induced by parenteral vaccination of the pregnant dam.^21,61-69 The mechanism of passive protection with this approach is due to the continuing presence of an amount of specific VN antibodies in the intestinal lumen sufficient to neutralize infectious virus before infection of the intestinal villous enterocytes.^19,66,70-74 The continuous presence of VN antibodies in the gut lumen may also reduce the severity of the disease if infection has already developed. In addition, some of the VN colostral antibodies that are absorbed into the bloodstream are secreted onto mucous membranes and may provide local immunity later in life.⁷⁵ However, colostral transfer of immunity is less efficient in ruminants than in other species; high con­centrations of maternal antibodies are present for less than 3 days postpartum, and these concentrations often fall below protective levels within 1 week of parturition.

A delicate balance exists between passive protection afforded by lactogenic immunity and development of the calf's own innate and adaptive local immunity to RV and CV Because most calves' level of resistance to the adverse clinical effects of RV and CV infection increases with age, disease caused by these agents can be controlled under certain circumstances by continuous hand-feeding of fresh, frozen, or fermented colostrum from vaccinated cows throughout the first 3 to 4 weeks of life? The goal with this approach is to allow development of subclinical infection while the milk VN antibody concentration is partially protective.^63,74,79 Conversely, lactogenic immunity might interfere with development of adaptive immunity, or some calves might not be exposed to RV and CV infection until after milk antibody concentrations have fallen below protective levels, leaving the calf fully susceptible to infection.^61,63,80

There are currently five safe and effective USDA-approved vaccines on the market for prevention of neonatal calf diarrhea caused by group A RV and CV. The first RV and CV vaccine for parenteral administration to pregnant cows (Calf-Guard [Zoetis]) became commercially available in 1979. This vaccine contains a single live attenuated G6:P1 strain of group A RV However, because precalving parenteral vaccination of cows with this vaccine resulted in low VN antibody concentrations in the colostrum and milk,^21,22,63,81 this product was later reformulated to a more potent parenteral preparation that contains inactivated G6 and G10 strains of group A RV and CV (ScourGuard 4KC [Zoetis]). Other inactivated vaccines that contain either three strains of group A RV and a strain of CV (Scour Bos 4 and 9 [Elanco U.S. Inc., Larchwood, Iowa]) or two strains of group A RV and two strains of CV (Guardian [Merck Animal Health, Madison, N.J.]) have also been licensed for use in pregnant cattle in the United States. The latter product can also be combined with a modified live CV vaccine (Bovilis [Merck]) for IN administration to healthy calves 3 days of age or older.

Vaccination of pregnant cows with inactivated RV and CV vaccine can raise the level of VN antibodies in the colostrum.^21,61-70,82 Although there are reports of successful protection of calves against NCED by parenteral vaccination of pregnant cows with RV and CV^: negative results have also been reported. Failure of this vaccination strategy is generally attributed to failure of passive transfer of colostral antibodies to the calf or overwhelming virus challenge. An alternative explanation might be that the serotypic specificity of the passively transferred maternal antibodies may affect the protective efficacy of the RV vaccine.⁹¹

Information on protective immunity against infection of calves with group A RVs having the same or a different G/P serotype configuration as the vaccine strain is incomplete.^51,82,92-96 Although immunity to RV appears to be G/P serotype specific with some 51829295

strains,^51,82,92,95 there is some indication that immunity directed against certain strains can neutralize in vitro⁵¹,⁹⁶ and protect challenge-exposed calves in vivo^51,95 against RV strains with different G/P configurations. Also, parenteral vaccination of seropositive cows with a single strain of RV can elicit serum VN antibodies to a broad spectrum of RV serotypes and genotypes, suggesting that this strategy may provide a means of enhancing passive protection against other potential RV serotype challenges.^83,93,95 However, a difference in the P protein of a vaccine RV and the infecting RV was suggested as the basis for failure of an inactivated monovalent RV and CV vaccine in beef cow-calf herds.^97,98

Although antigenic and genotypic variations are documented among bovine CVs,^31-35 reduced protection of nursing calves after challenge with a strain of bovine CV different from the vaccine strain has not been demonstrated. Consequently, addition of more than one strain of bovine CV in certain vaccines might not be justified at this time.

Another approach for preventing NCED caused by RV and CV involves oral vaccination of calves with an MLV vaccine that contain attenuated G6:P1 strain of group A RV (Calf-Guard) or IN vaccination with an MLV attenuated CV vaccine (Bovilis).^99-109 The mechanism of disease prevention with these vaccines is unknown, but interference with infection by virulent virus, followed by development of secretory IgM and IgA and/or CMI in the intestinal mucosa, has been proposed for the group A RV vaccine.^80,102 To achieve adequate protection, the manufacturer recommends that the group A RV vaccine be given immediately after birth, before the calf has nursed. This regimen might be applicable to calves whose dams have not been vaccinated. However, because the colostrum of most heifers and cows contains some level of VN antibodies arising from natural exposure, administration of colostrum should be delayed for several hours after vaccination to avoid inactivation of the vaccine virus. Under commercial conditions, it is nearly impossible to administer vaccine within minutes of birth or to effectively regulate colostrum intake in relation to the time of vaccination. Therefore infection before vaccination, neutralization of the vaccine virus by colostral antibodies, and overwhelming challenge with infectious RV shed by unvaccinated diseased calves might explain a lack of field efficacy with this approach.^64,104-107 This is evident from the data obtained in vaccine efficacy evaluation studies in which only a portion of the calves on a farm or a ranch are vaccinated in double-blind or odd/even-day vaccination trials.^76,108,109 When all calves are either vaccinated or not vaccinated in sequential comparisons, morbidity and mortality rates from NCED are significantly reduced by this vaccination 7099101108109

strategy,, ^101,108,109 but the design and statistical validity of the 90105

latter kinds of trials have been questioned.^90,105

Although not recommended by the manufacturer, the oral attenuated RV vaccine has been administered to calves with unknown immune status that were raised as veal calves or replacements in heifer development operations.⁶ Under these circumstances, vaccination might provide active immunity and protection against potential virus challenge when calves from several different sources are commingled.

The manufacturer recommends administration of the IN MLV attenuated CV vaccine in calves 3 days of age and older, after colostrum intake on the first day of age.

Rotavirus and Coronavirus Vaccination Products

ScourGuard 4KC

ScourGuard 4KC consists of inactivated bovine G6 and G10 strains of group A RV and a bovine CV strain combined with K99 E. coli bacterin and C. perfringens type C toxoid. For first-calf heifers and mature cows that have not previously been vaccinated, a two-dose series of vaccine should be administered by IM injection into the neck of pregnant cows and heifers 3 weeks apart, starting 6 to 9 weeks precalving. The second dose should be given 3 to 6 weeks before the expected calving date. A single annual booster dose should be administered 3 to 6 weeks before each subsequent calving.

Scour Bos 9

Scour Bos 9 consists of inactivated bovine group A RV strains G6, G8, and G10 and a CV strain combined with bacterins from four E. coli K99 serotypes and C. perfringens type C toxoid. The first vaccine should be administered by IM injection into the neck of pregnant cows and heifers 8 to 16 weeks before the expected calving date, with a booster dose of vaccine 4 weeks before the expected calving. Only one annual dose at 8 to 10 weeks before calving is required thereafter.

Guardian

Guardian consists of inactivated bovine group A RV G6 and G10 strains together with type 1 and type 3 bovine CV strains combined with cell-free extract of enriched K99 pili from E. coli and bacterin-toxoid from C. perfringens types C and D. Ihe vaccine should be administered by SC injection of pregnant cows and heifers up to 12 weeks before the expected calving date. A booster dose of vaccine is given approximately 3 to 6 weeks following initial vaccination. One annual dose admin­istered 5 to 7 weeks before expected calving is required thereafter.

Bovilis

Bovilis is a modified live bovine CV recommended for IN administration to calves 3 days of age and older.

Calf-Guard

Calf-Guard is a modified live bovine G6 strain of group A RV and bovine CV recommended for oral vaccination of newborn calves. The vaccine should be administered immediately after birth, before the calf has nursed. As indicated earlier, interfer­ence by maternal antibodies may limit the efficacy of the vaccine, so vaccination of calves older than 1 day may be ineffective.