Ovine and Caprine Vaccination Programs

Amanda J. Kreuder • Paul J. Plummer

Factors Affecting Vaccine Availability, Choice, and Usage

Labeled vaccines for several important diseases of sheep and goats are currently available for use in the United States, but the small market share for small ruminant vaccines has negatively affected the funding available for vaccine research and development compared with other domestic species.

Most available vaccines are bacterins, so avoiding maternal antibody interference and ensuring timely booster administra­tion are key concepts for developing effective vaccine programs. Similar to other food animal species, proper vaccine administra­tion and adherence to meat quality assurance are important. Vaccines should be administered subcutaneously whenever possible, either in the neck or behind the shoulder in the axillary region. Confusion over vaccine reactions that can lead to swellings that resemble lymph nodes infected with C. pseudotuberculosis should be kept in mind when choosing a vaccination location. Compared with sheep, goats are known to exhibit more severe vaccine reactions. If vaccines labeled for other species are to be used off label in goats, it is recom­mended that a small number of animals be vaccinated first to observe for reactions before vaccinating the entire flock.

Clostridial Vaccines

The only vaccine currently recommended for routine use in all small ruminants is yearly administration of a Clostridium perfringens C and D plus tetanus toxoid. Vaccination or protec­tion via passive immunity (colostrum or antitoxin administration) is particularly important before common husbandry tasks like tail docking and castration (tetanus), as well as introduction to high-grain diets (C. perfringens type D). Ideally, all ewes and does should receive a booster vaccination 3 to 4 weeks before parturition; previous research in sheep indicates that passive immunity acquired from properly immunized ewes confers protection for up to 12 weeks in lambs.¹ Current recommendations for vaccination include an initial series of two doses 3 to 4 weeks apart, followed by yearly boosters. Appropriate timing of the booster in the first series of immuniza­tions is essential in the formation of an adequate immune response for the remainder of the animal’s life. When to initiate the first series of vaccinations is a topic of debate because there is little evidence-based literature on which to form a decision.

Multiple combination commercial products are available, including C. perfringens C/D only, stand-alone tetanus toxoid, and seven-way or eight-way clostridial combination products. There is little evidence that vaccination with multivalent clostridial products is routinely beneficial, but these may frequently be chosen to decrease cost or in herds with a history of disease caused by other Clostridial species. If multivalent clostridial vaccines are selected, care should be taken to ensure the vaccine contains a tetanus toxoid; in some vaccines, C. sordellii may be substituted in place of C. tetani.

Rabies Vaccines

Highly efficacious vaccines for the rabies virus are commercially available for sheep in the United States. Vaccination for rabies

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■ TABLE 48.5

Antigens and Antisera Currently Licensed in United States for Sheep and Goats

^aKilled or inactivated vaccine.

^bModified live vaccine.

^cVirulent live vaccine.

^dNot all available products labeled for use in goats.

^eSome or all products conditionally licensed.

^fProduct currently licensed but not being produced.

Information taken from North American Compendium of Veterinary Products.

■ TABLE 48.6

Sheep Vaccination Schedule for North America

■ TABLE 48.6

Sheep Vaccination Schedule for North America—cont'd

■ TABLE 48.6

Sheep Vaccination Schedule for North America—cont'd

FPT, Failure of passive transfer.

■ TABLE 48.7

Goat Vaccination Schedule for North America

■ TABLE 48.7

Goat Vaccination Schedule for North America—cont'd

■ TABLE 48.7

Goat Vaccination Schedule for North America—cont'd

FPT, Failure of passive transfer.

is recommended in endemic areas, especially in animals with increased human exposure, such as pets, show animals, and petting zoo exhibits. Vaccination of valuable animals is also recommended. Despite the availability of highly effective vaccines, administration to all small ruminants is not routinely recommended, owing to the high cost associated with vaccina­tion and the overall low rates of infection. In addition, unlike rabies vaccination in cats and dogs, previous vaccination of small ruminants does not necessarily constitute protection in regard to quarantine versus destruction of exposed animals. None of the currently available vaccines are labeled for goats, but they are considered relatively safe to use in this species in similar instances as with sheep. Initial vaccination after 3 months of age, followed by yearly revaccination, is recommended; after the initial yearly booster, vaccination frequency can be decreased to every 3 years in some cases, depending on vaccine label.

Foot Rot Vaccines

Commercial bacterin vaccines labeled for foot rot in sheep and targeting 10 strains of Dichelobacter nodosus and Fusobacterium necrophorum were once available in the United States. These vaccines are no longer commercially available in the United States, but the D. nodosus vaccine may be imported with USDA approval on a case-by-case basis. Localized vaccine reactions were common in both sheep and goats, and vaccine efficacy on a large scale has long been debated. Available evidence suggests that antigenic competition may reduce the effective­ness of simultaneous vaccination against multiple serogroups 2 3

of D. nodosus.

Current research performed in regions other than North America suggests newer approaches to vaccination for this disease may be more beneficial. In areas or flocks where only one or two strains may be present, targeted use of monovalent or bivalent autogenous vaccines in combination with culling of nonresponders may be of benefit in eradicating the disease.^4-6 However, it is considered uncommon for only one or two strains to be present; instead, multistrain infections are thought to more commonly occur.⁷ Use of multiple bivalent autogenous vaccines with sequential administration staggered every 3 months may help avoid antigenic competition while allowing for prolonged protection against multiple serotypes.⁸ Development of a “universal” vaccine through comparative genomics, transcriptional profiling, and serologic screening for potential candidate antigens has also shown promising results.⁹ When these newer technologies will be commercially available in the United States remains to be seen. Given the difficulties and limited availability of vaccines for foot rot, veterinary practitioners are encouraged to consider alternate management strategies that include antibiotic therapy and potentially footbaths.

Reproductive Disorder Vaccines

Commercial vaccines are currently available for sheep only to aid in control of some of the important infectious abortion diseases and contagious epididymitis in the United States. Again, many of these vaccines lack comparative or published field efficacy data, so blanket recommendations for use cannot be made. For additional information on vaccination strategies in reproductive diseases, two excellent reviews have recently been written with a more in-depth discussion of each of the following diseases.^10,11

An inactivated Brucella ovis vaccine is licensed in the United States for use in rams to aid in prevention of epididymitis, but at this time it is no longer being produced by the company other than on a special order basis. Inactivated vaccines are not believed to confer protective immunity to this organism but are known to interfere with serologic testing to detect carrier animals; therefore it is not recommended for routine use. In countries where Brucella melitensis is present, the Rev-1 live mutant vaccine may be useful in controlling both diseases; however, in B. melitensis-free countries like the United States and Canada, its use is prohibited.

There are currently two commercially available bivalent bacterin vaccines for abortion due to Campylobacter fetus subsp. fetus and C. jejuni in the United States. In general, there is minimal cross-protection between these organisms, and sheep appear to be much more susceptible than goats to abortions caused by these agents. Recommendations for use on farms with diagnosed abortions due to Campylobacter spp. include an initial series of two doses in ewe lambs, followed by administra­tion of an annual booster; exact timing varies on the basis of the vaccine chosen. Recently a new tetracycline-resistant strain of C. jejuni (sheep abortion clone IA3902) has emerged in the United States and is now thought to be the leading cause of Campylobacter-associated abortion in sheep.¹² Studies in guinea pig abortion models suggest that vaccination with certain commercial vaccines may be more beneficial than others in providing cross-protection to prevent abortion associated with this sheep abortion strain.¹³ Following an abortion storm, herd immunity may initially be high, and in future breeding seasons the extent of disease may be limited to naive animals. Over time, immunity may wane, and infections may reoccur.

Chlamydophila abortus (formerly Chlamydia psittaci) is a common cause of abortion in both sheep and goats worldwide. Following abortion, ewes and does will develop immunity to future abortion but will often continue to shed the organism and infect other naive animals. Currently, only an inactivated culture vaccine (bacterin) is available in North America for use in sheep. Some producers have used this vaccine in goats, and it appears to be generally safe in this species. Vaccination with this type of vaccine will not prevent shedding of the organism but may decrease the rate of abortions. Vaccine efficacy is unpredictable, owing to variation in the amount of antigen present and loss of antigenicity following in vitro culture during vaccine production. Strain variation from farm to farm may also play a role in perceived vaccine failures. An attenuated live vaccine is available in other countries and is thought to be more efficacious in preventing abortion and shedding but is not without drawbacks. Live attenuated C. abortus organisms can be infective to humans handling the vaccine, and the vaccine strain has recently been identified in aborted placentas in the United Kingdom.¹⁴ Newer vaccine technologies such as DNA vaccines or bacteriophage-delivered DNA may eventually offer a solution to the problems identified with current vaccine strategies.^15,16

Vaccines for other important reproductive diseases like Q fever (Coxiella burnetii) and toxoplasmosis (Toxoplasma gondii) are not available in the United States even though they remain important causes of abortion in sheep and goats. A phase I inactivated C. burnetii vaccine is commonly used in many areas of Europe, with observed reductions in shedding and decreased abortion rates. This vaccine is currently part of the mandatory vaccination program in The Netherlands.¹⁷ This vaccine is neither marketed nor approved for importation into the United States at present by the USDA. A modified live toxoplasmosis vaccine consisting of freeze-dried tachyzooites is available in Europe and New Zealand; it provides excellent long-term immunity. Both of these vaccines are potentially zoonotic and therefore must be handled carefully. Licensing of the C. burnetii vaccine has been considered in North America but may be hindered by licensing cost and market size. A vaccine for toxoplasmosis is not likely to become available in the United States in the near future, owing to the small market compared with overseas.

Caseous Lymphadenitis Vaccines

Vaccination for C. pseudotuberculosis has been attempted with a number of different vaccine modalities; all currently marketed products in the United States are bacterin-toxoids sold as either separate or combination products. Experimental trials have demonstrated a decrease in the occurrence of both internal and external abscesses following inoculation in vaccinated animals.¹⁸ On the basis of these and other data, the vaccine may have a place in some herds to decrease disease prevalence. Although a large amount of research into vaccines has been done overseas,^19,20 little other U.S. vaccine research for this disease has been done. The available vaccines will not prevent all animals from developing abscesses, and animals must be vaccinated before exposure. Vaccine reactions (lameness, lethargy, depression) may occur and can be significant. Despite frequent use in goats, the fully licensed vaccine is not labeled for use in this species and may result in severe injection site reactions that can lead to decreased milk production and poor appetite. Recently, a new conditionally licensed vaccine specific for goats has come on the market. At the time of this writing, little data are available regarding its clinical use. It is not approved for use in all states, so practitioners should consult their state veterinarian or the manufacturer for more informa­tion. Initial vaccination requires a two-dose series 2 to 4 weeks apart, with annual revaccination required. Producers must be aware that vaccination will lead to a positive result on serologic testing.

Contagious Ecthyma Vaccines

Virulent live virus vaccines are available in the United States and labeled for use in both sheep and goats to decrease the severity of disease caused by contagious ecthyma virus (soremouth, orf). Use of these vaccines should be limited to herds where endemic disease is present or in outbreak situations where the disease has been newly introduced to the farm. Use of the vaccine on a farm without previous evidence of infection will result in contamination of the farm with live virulent vaccine virus. The goal of vaccination is to induce mild disease while creating immunity to future exposure. An area free of wool or hair is selected (axilla, flank, inner thigh) and a 1-in² area is scarified with a provided instrument, followed by application of vaccine. Development of a scab in the region of scarification is an indication of successful vaccination. The vaccine is a virulent live virus, so care should be taken with administration as it poses a zoonotic risk of infection. Vaccination failures are known to occur, and it has been suggested that the increased occurrence of vaccine failures, particularly in goats, may be due to strain variation between sheep and goat isolates. Efforts are under way to develop goat-specific vaccine strains.²¹

Respiratory Vaccines

Despite the long list of bacterial and viral organisms thought to play a role in respiratory disease in small ruminants, the only available labeled vaccines for sheep and goats in the United States for respiratory disease are combination killed bacterins of Mannheimia haemolytica and Pasteurella multocida. Most vaccine strains of these organisms are derived from bovine isolates; small ruminant strains, particularly for M. haemolytica, are not the same as those found in bovine cases.²² Cross-reactivity among strains is variable, so the efficacy of these vaccines in small ruminants is inconsistent. In addition, many previously reported cases of P multocida may instead have been caused by the newly classified Bibersteinia trehalosi (formerly P. multocida type T). None of the currently labeled respiratory vaccines for sheep and goats (or cattle) contain any strain of B. trehalosi at this time, and experimental results investigating the efficacy of vaccination for M. haemolytica and/or P multocida in control­ling disease cause by B. trehalosi are mixed at best.^23,24 There are currently no labeled vaccines for viral agents of respiratory disease in small ruminants, and there are no published studies evaluating the safety and efficacy of the use of bovine vaccines in these species.

Other Vaccines

An MLV vaccine is available for bluetongue virus in the United States, but it is thought to be of limited efficacy in the field because it contains only one of the six serotypes known to circulate in North America. Cross-protection among serotypes is thought to be limited, so vaccination will only offer protection against the vaccine strain (serotype 10). The vaccine is known to cause fetal malformations in pregnant ewes and has the potential to be transmitted to unvaccinated animals during vector season.

An avirulent live culture anthrax vaccine is available for domestic livestock including sheep and goats in the United States. Although rare, sporadic outbreaks and epizootics do occur among U.S. livestock, and premises with previous diagnosis of anthrax are considered susceptible to future outbreaks when soil and weather conditions are appropriate, such as during the 2005 epizootic in North Dakota. In endemic areas, vaccination in the spring of all livestock within 3 miles of a diagnosed case is recommended for a minimum of 5 years following the last diagnosed case.²⁵

A conditionally approved (California only) endotoxin vaccine is labeled for use in milking goats to decrease the severity of toxic mastitis caused by E. coli. In other areas of the country, some of the endotoxin vaccines commonly administered in dairy cattle are used off label in milking goat herds. Because of the low incidence of toxic mastitis in this species, routine use is not recommended. If these vaccines are used, administration will not prevent infection but will decrease disease severity.

At present, there are no additional vaccines licensed for mastitis in milking sheep or goats in the United States. However, there is a vaccine currently licensed in multiple European countries for reduction of the incidence of subclinical mastitis cause by Staphylococcus aureus (sheep and goats) and coagulase-negative Staphylococcus (goats only) and the company is currently exploring approval for this vaccine in the United States. This vaccine is an inactivated product derived from a biofilm-expressing strain of S. aureus. Internal research reports by the company suggest efficacy in decreasing the incidence and severity of mastitis associated with staphylococcal infections, but no peer-reviewed research has been published to date. Previously published research has demonstrated that bacterins derived from strong biofilm producing strains of Staphylococcus provided the highest protection against intramammary infec­tion and subsequent mastitis in sheep.²⁶ As mastitis caused by Staphylococcus species remains a major cause for production loss in milking sheep and goat herds, approval of an efficacious vaccine for this purpose in the United States would be beneficial.