Neonatal diarrhea, or baby calf scours, continues to pose a major health and economic threat to the calf-rearing segments of the beef and dairy industries. Affected calves develop severe diarrhea, leading to dehydration, electrolyte imbalance, and acidosis. Death, impaired performance, retarded growth, and the added time and expense of treating sick animals all contribute to economic loss. Infectious causes of calf scours include enteric strains of rotavirus and coronavirus, Escherichia coli, salmonellae, strains of clostridia, and coccidia and cryptosporidia. Noninfectious causes include environmental conditions (wet, drafty quarters, dirty pens, stress) and nutritional factors (over-feeding, poor quality milk replacer).

Unlike the bovine respiratory disease complex, which has a "progression of disease" etiology, neonatal calf scours can have as its primary etiology nutrition, viruses, bacteria, or protozoa independent of one another. Due to its rapid onset, lack of consistent response to available treatment, and its costly nature, baby calf scours offers up no simple solution. The disease can best be managed through a comprehensive control program that incorporates measures for minimizing environmental stress, optimizing nutrition, and protecting against infectious agents through vaccination.

Since the period of time from vaccination of the neonate to onset of immunity is longer that the incubation period for many of the enteric pathogens, vaccination of the dam during pregnancy so she will develop and transfer maternal antibodies in her colostrum and milk is regarded as the most efficient means of protecting the newborn calf.

One of the first commercial vaccines for the prevention of calf scours was ScourvaxReo®, which was introduced by Norden Laboratories in 1973 to protect against rotavirus scours. Additional viral and bacterial antigens have since been added to this monovalent product to form the current line of Pfizer Animal Health calf scours vaccines:

• ScourGuard 3® (K)/C for protecting calves against scours caused by rotavirus, coronavirus, E. coli, and Clostridium perfringens type C
• ScourGuard 3® (K) for protection against rotavirus, coronavirus, and E. coli scours
• CalfGuard® for protection against rotavirus and coronavirus scours

The ScourGuard products have served the livestock industry since the early 1980s as the only viral-bacteria combination vaccines for the prevention of neonatal calf scours.

A second line of viral-bacterial calf scours products (Scour Bos™) was introduced in the fall of 1999 by Grand Laboratories, Inc. In the promotion of Scour Bos, Grand Laboratories has made various comparisons with the ScourGuard line of vaccines, each of which will be addressed in this article. The intent of this article is to provide relevant scientific information to consider when selecting a scours product, allowing producers to make an informed decision on choice of a calf scours vaccine.

Number of Doses to Establish Protection
In one of its promotional pieces, Grand Laboratories notes that primary vaccination with Pfizer's ScourGuard 3(K)/C requires two doses, whereas Scour Bos requires only one dose.¹ A review of product labels provides the following vaccination recommendations:

• All ScourGuard vaccines require that two doses be given the first year. Each year thereafter cows should be vaccinated before calving with just a single dose.
• Both Scour Bos 4 (rotavirus-coronavirus) and Scour Bos 9 (rotavirus-coronavirus-E. coli-Cl. perfringens) require a booster for rotavirus scours during the first year. Of the products in the Scour Bos line, only Scour Bos 6 (coronavirus-E. coli-Cl. perfringens) requires a single dose for primary vaccination.

Thus, the convenience of single dosing with Scour Bos products exists only when producers choose to provide either no protection against rotavirus scours or a level of protection that is less than that recommended in the product label. A vaccination decision such as this disregards epizootiological data showing that rotavirus is the most frequently diagnosed cause of viral calf scours and is often found in combination with other scours pathogens.^{2 3} Rotavirus typically affects calves between 5 and 18 days of age and causes scours by infecting and blunting villi that line the small intestine. Once damaged, the intestinal villi are unable to absorb fluids and nutrients from the intestines. The physical damage not only causes severe diarrhea and dehydration, but also creates opportunities for other pathogens to invade and in severe cases to produce a system-wide infection. Uncomplicated rotavirus infection can be comparatively mild, but calf growth rates and performance can be affected for an extended period.

Grand Laboratories' claims also say that Scour Bos vaccines can be given as early as 10 weeks before calving, noting that a booster dose of ScourGuard is recommended for cows that have not calved within 40 days of the last dose.¹ This is accurate reporting of label information, but a scientific explanation for the ScourGuard recommendation is warranted.

The booster dose of ScourGuard is recommended at 40 days after the last dose because the concentration of antibodies in cow serum and colostrum dissipates over time. By 40 days after vaccination, the level of antibodies may be insufficient to provide protection. The oil-based adjuvant in Scour Bos may delay the peak of the antibody curve by slowly releasing vaccine antigens to prolong duration of effect; however, the height of the antibody peak also needs to be considered. Here's why. In field situations when the level of pathogenic challenge is low, a low level of antibodies will in all likelihood provide protection. At the end of the calving season, though, when the level of pasture contamination has increased and the level of antibodies has decreased, scours breaks sometimes occur in late-born calves. For this reason, it is critical that calves receive an adequate concentration of antibodies in their colostrum, and that is why Pfizer recommends revaccination of pregnant cows 40 days after the last vaccination.

Number of Vaccine Strains and Scope of Protection
A table in the Scour Bos promotional literature compares the number of vaccine strains in ScourGuard 3(K)/C and Scour Bos 9, showing that ScourGuard has a total of four strains and Scour Bos has a total of nine.¹ As the discussion in this section points out, the number of strains in a vaccine has less bearing on its immunologic capabilities than does the cross-protective or cross-neutralizing properties of those strains. What follows is a summation of the immunologic characteristics of each of the ScourGuard antigens.

E. coli K99
ScourGuard vaccines contain one strain of E. coli K99, whereas Scour Bos vaccines contain four. Whether the additional strains provide more comprehensive protection against E. coli-induced scours is scientifically disputable. This is because the key to neutralizing the pathogenic effects of enterotoxigenic E. coli, or ETEC, is to neutralize the K99 pilus, a cell wall structure shared by ETEC strains. The finger-like K99 pili located on the external surface of ETEC function to attach the bacteria to enterocytes lining the intestinal wall. Only after becoming attached do E. coli release enterotoxin that triggers the rapid flow of fluids and electrolytes into the intestines, resulting in the clinical signs of scours. Unattached, ETEC strains pass harmlessly through the intestinal tract.

When a calf absorbs colostrum from a cow vaccinated with K99 antigen, the intestines of that calf are bathed with K99 antibodies. These antibodies bind with K99 pili, therein preventing attachment of ETEC to the cells of the intestinal lining. The number of K99 strains included in a neonatal calf scours vaccine given to the dam is irrelevant because the K99 antibodies that the cow produces against one strain of ETEC cross-protect against other ETEC strains sharing the same K99 pili.

Cl. perfringens type C
All Cl. Perfringens type C bacterin-toxoids are held to the same potency release standards that are codified and published in the Code of Federal Regulations. The practical upshot of the manufacturing regulation is that there is no difference in the antigen content of the Cl. perfringens fraction of ScourGuard and Scour Bos vaccines.

Coronavirus
Scour Bos promotional literature notes that the product line's coronavirus strain protected against a second heterologous strain during challenge studies.¹ The scientific literature reveals that few studies have been conducted to assess cross-reactivity among strains of bovine coronavirus. One study published in 1999 did identify 6 strains of coronavirus isolated from the nares of feedlot cattle that were antigenically similar to two enteric strains of bovine coronavirus obtained from neonatal calves with diarrhea. The authors of this study theorized that vaccines developed to protect against enteric strains also might protect against respiratory tract strains.⁴ Clearly, though, additional research needs to be undertaken to further define the antigenic and biological properties of bovine coronaviruses, including the vaccinal strains used in ScourGuard and Scour Bos. In vivo cross-protection studies might then be conducted to assess whether the differences and similarities between strains are immunologically significant.

Rotavirus
Scour Bos promotional literature states that its rotavirus vaccine fraction contains three field isolates and that ScourGuard contains only one.¹ For the following reasons, this difference in antigenic content is of little clinical relevance:

• Group A rotaviruses, which are generally the only rotaviruses isolated from calves, have two structural capsular proteins that induce immunity in the host-the G protein and the P protein.
• The larger G protein has two genotypes-G6 and G10-and stimulates production of the majority of antibodies when incorporated into a killed-virus vaccine. Antibodies against the G protein are thought to initiate the immune response that controls degree of rotavirus infection. The P protein has three genotypes-P1, P5, and P11. Antibodies against the P protein are regarded as necessary for clearing rotavirus from the body after the infection has been controlled.
• More than 85% of the rotavirus strains isolated from the field have the G6 protein.
• The rotavirus strain in ScourGuard contains both G6 and P1 protein. Recent studies conducted at the University of Nebraska show that the vaccinal strain in ScourGuard provides good cross-neutralization of rotavirus B641, which has the P5 and G6 proteins.⁵ The G6 protein does not cross-neutralize the G10 protein; however, G10 is identified in fewer than 20% of field isolates.^{2 3}
• The University of Nebraska findings correlate well with the field use of ScourGuard in that failure of protection has seldom been associated with an inability to cross-protect against a heterologous strain of rotavirus. Typically, follow-up investigationsof these cases have attributed calf scours to inadequate protein and/or energy during the dry period, poor colostrum transfer, and high challenge conditions.

Thus, the only substantive difference between the antigenic content of the rotavirus fraction in ScourGuard and Scour Bos is the inclusion of the G10 protein in Scour Bos, field strains of which are currently associated with fewer than 20% of the laboratory-confirmed cases of rotavirus scours.

This is the first half of the article entitled "Comparing neonatal calf scours vaccine: Some additional considerations." Please watch for the second half that will be posted in January of 2001.

For More Information
As a company devoted to the dairy industry, Pfizer can help you assist clients in protecting their dairy herds through sound biosecurity management practices and vaccination programs designed with their needs in mind. For reprints of this article, additional HerdSecure™ biosecurity management information and information on Pfizer Animal vaccines, like CattleMaster®, ScourGuard 3™ (K)/C, and Bovi-Shield™ please call 1.800.829.5528.

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