Specific pathogen prevention program for the tough diseases of sheep and goats (Proceedings)

Several insidious diseases cause significant economic loss to sheep and goat producers through decreased longevity, growth rate, milk production and animal sales, and they are often purchased through inapparent carriers. Direct contact can transmit these diseases between adults over time, but they can be transferred quickly from the adults to the young through colostrum, milk and direct contact. There is no effective treatment for any of these diseases, and a specific pathogen prevention program has been developed to decrease their incidence. Infected herds remove the newborns from the adult population at birth, feed heat-treated colostrum and pasteurized milk or milk replacer, and permanently separate the new clean replacements from the infected adult population. These insidious diseases include, but are not limited to, Johnes disease, caprine arthritis-encephalitis, ovine progressive pneumonia, mycoplasma, caseous lymphadenitis, and scrapie.

Johne's disease

Johne's disease or paratuberculosis affects both wild and domestic ruminants. Mycobacterium avium subspecies paratuberculosis (MAP) can survive for extended periods in contaminated environments or in manure spread on pasture for over one year. Contaminated soil and bacteria-laden feces coat the teats and udder of the dam and are nursed by the newborn, while infected animals shed mycobacteria in both milk and colostrum. In utero transmission of MAP has been documented in both sheep and cattle, but the percentage of goats born with infection has not been determined. The incubation period is very long and clinical symptoms are more common in animals between 2 and 4 years of age. The severe stress associated with pregnancy, parasitism, or environmental change may trigger clinical disease. The most common presentation of Johne's disease in sheep and goats is severe, progressive weight loss leading to emaciation in spite of good nutrition. Affected animals exhibit lethargy, anemia, rough haircoat, flaky skin and occasionally bottle jaw or pendulant edema. Diarrhea is uncommon in small ruminants with Johnes disease but may appear terminally. Fecal culture and AGID or ELISA testing of blood samples may be used for antemortem diagnosis, but most cases are confirmed with histopathology at necropsy.

Caprine arthritis-encephalitis

Caprine arthritis-encephalitis (CAE) is caused by a retrovirus that lives in macrophages, and the virus is shed by inapparent carriers and clinically ill animals in colostrum, milk, lochia, aerosol droplets and other body secretions. Any method that moves white blood cells from one goat to another can transmit CAE: colostrum, milk, close contact, shared housing, feeding equipment and water sources, injection needles and equipment. The virus does not survive long outside the host, so environmental contamination has minimal importance in transmission or maintenance of the infection in a herd. The percentage of infected animals that demonstrate clinical disease is low, but stress and poor management increase the appearance of clinical symptoms. Chronic progressive arthritis affecting multiple joints, chronic progressive interstitial pneumonia, and weight loss associated with chronic disease are the most common presentations. The least common expression of CAE is an ascending paralysis in otherwise healthy afebrile kids. An unusual form of udder edema that is non-responsive to diuretic therapy may be associated with CAE. There is no difference in prevalence between breeds or sex, but the incidence of infection increases with age in herds with virus-positive animals. Both tests for antibody (AGID and ELISA) and virus (PCR) are commercially available. CAE does not kill goats but decreases productivity, life span and quality of life.

Ovine progressive pneumonia

The virus that causes ovine progressive pneumonia (OPP) or maedi-visna (MV) is a retrovirus affecting sheep that is closely related to the caprine arthritis-encephalitis virus. These viruses are similar enough that either virus may cause disease in the other specie and both diseases can be passed back and forth between the two species through prolonged direct contact. OPPV persists within macrophages similar to CAEV and can be transmitted through direct contact, milk or colostrum. The rate at which OPP spreads through a flock may be markedly influenced by whether the sheep are on extensive pasture or closely confined, and the incidence of seropositive animals increases with age in infected flocks. The onset of clinical disease may follow stress, exertion, severe weather, or change of environment and is insidious in nature. The most common presentation would be chronic progressive interstitial pneumonia leading to open mouth breathing, flaring of the nostrils, expiratory dyspnea, and coughing with chronic weight loss. Affected females exhibit "hardbag" or a large firm udder due to macrophage infiltration of the mammary tissue, and their lambs have slower growth rates due to decreased milk production. Chronic arthritis and ascending paresis are occasionally observed. Affected animals eventually die due to wasting or chronic pneumonia or are culled for low production. Serologic tests using AGID, ELISA or PCR are available but most cases are confirmed at necropsy.

Mycoplasma

Many different strains of mycoplasma affect goats causing pneumonia, arthritis, septicemia, abortion, mastitis, and/or conjunctivitis. Mycoplasma mycoides subspecies mycoides large colony type presents a complex sequence of symptoms that can have devastating impact on different age groups at different times. Young stock may develop high fever, pneumonia, inflamed joints and meningitis, while adult does may demonstrate fever, abortion, chronic degenerative arthritis, chronic weight loss and severe mastitis. Morbidity and mortality are very high, and surviving animals become carriers that shed mycoplasma in their colostrum and milk. Special transport media are required for culturing mycoplasma and not all diagnostic laboratories are capable of isolating these difficult bacteria. Clinically normal carriers may be identified by culturing ear canal swabs or first-milking colostrum. Other species of mycoplasma such as M. putrefaciens, M. capricolum and M. agalactiae may cause conjunctivitis or mastitis, but these infections are usually self-limiting and have less economic impact than M.m.m large colony type.

Caseous lymphadenitis

Corynebacterium pseudotuberculosis is the causative agent of contagious abscesses known as caseous lymphadenitis or "boils" in both sheep and goats. This organism is found in the thick greenish discharge from ruptured abscesses and can survive for many years in contaminated soil or barns and on equipment or instruments. These bacteria are capable of penetrating intact skin but presence of skin wounds increases the infection rate. The bacterial toxin phospholipase D allows the organism to spread from lymph node to lymph node throughout the body even though the immune system attempts to encapsulate it with connective tissue. Most lesions begin in the lymph nodes about the head and neck and then spread to internal nodes around the lungs, heart, liver, kidneys, intestine and mammary glands. Lactating adults with abscesses in the mammary glands transmit large numbers of bacteria to their offspring in the milk and colostrum. Corynebacterium abscesses frequently increase in size with age and interfere with body function. Necropsy studies identify this organism as the most common cause of wasting in both sheep and goats. Animals infected with C. pseudotuberculosis are permanently infected and shed the organism in body fluids, abscess contents and coughed aerosol droplets.

The disease is most commonly confirmed by bacterial culture and accurate serologic tests are not readily available for screening herds to identify infected individuals. The California Animal Health and Food Safety Laboratory at the University of California at Davis offers a synergistic hemolysin inhibition (SHI) test for detection of antibodies to C. pseudotuberculosis. In their experience, SHI titers less than 1:8 are not considered significant and probably represent cross-reactions with common environmental organisms, while SHI titers greater than 1:256 have a high association with internal abscess formation. The SHI test cannot discriminate between vaccinated animals and those with infection. Use of autogenous bacterins or of the two commercial vaccines against C. pseudotuberculosis currently marketed in the United States combined with good management practices leads to a decreased incidence of abscesses in infected herds.

Development of the specific pathogen prevention program

Beginning in 1979, Mycoplasma mycoides subspecies mycoides large colony type (Mmm) wreaked havoc in commercial goat dairies in California. Young kids between 2 and 8 weeks of age developed hot swollen joints, high fevers of 105-107 degrees F, pneumonia, and meningitis, and died within one or two days. Adult does demonstrated septicemia, mastitis, chronic progressive arthritis and abortion. Mmm was isolated from organs and body fluids during the febrile phase and from the first-milking colostrum of parturient does. Drs. Brook, DaMassa and Adler from the University of California at Davis discovered that the morbidity and mortality of Mmm could be reduced from 90% to 5% in these commercial herds by culturing the colostrum and feeding only mycoplasma-free colostrum and milk or milk replacer to the kids. Does shedding mycoplasma in their milk were culled for slaughter. Adoption of this early mycoplasma prevention program markedly decreased the incidence of mycoplasma in participating herds.

In 1980, Drs. Crawford, Adams, Cheever and Cork from Washington State University published the first reports of viral-induced progressive arthritis and demyelinating encephalomyelitis in goats and designated the causative virus CAEV. Their research led to the development of an ELISA test for CAEV and demonstrated that heat-treating colostrum to 135 degrees F for one hour and pasteurizing milk killed the virus. Between 1982 and 1988, Dr. Randall Cutlip and his staff at the National Veterinary Services Laboratory tested 3,790 goats from 196 herds in 28 states for antibodies to caprine arthritis-encephalitis virus using AGID. In 1984, Dr. Cutlip began limiting the free testing program to those herds that were willing to separate newborns from the adult population, feed heat-treated colostrum and pasteurized milk or milk replacer, and cull test-positive animals or permanently separate them from the rest of the herd. Drs. East and Rowe from the University of California at Davis demonstrated that separation of the kids permanently from the adult population along with feeding heat-treated colostrum and pasteurized milk combined with periodic testing and removal of positive goats markedly lowered the incidence of CAE and prevented seroconversion in adults. Parallel research projects at Washington State University, University of California at Davis and the National Veterinary Services Laboratory demonstrated that use of this CAE-prevention program would decrease the prevalence of CAEV in herds as well as decrease the incidence of clinical disease. Many goat producers adopted the CAEV prevention program and markedly decreased the incidence of CAE in their herds.

During the same time period, Dr. Cutlip also tested 16,827 sheep from 164 flocks in 29 states for presence of antibodies to ovine progressive pneumonia virus using an AGID test. 26% of all sheep tested were seropositive for OPP and 48% of the flocks had one or more positive sheep. Seropositive sheep were not evenly distributed between flocks but were clustered in a few flocks of sheep, and a high number of flocks had no or only a few seropositive sheep. The percentage of positive sheep within infected flocks increased with age, and there was no association with breed, sex or place of origin. In 1990, the OPP Concerned Sheep Breeders Society was formed to educate sheep producers and veterinarians about OPP and to provide a supportive network to eradicate and control this disease. They maintain an interactive and highly educational website at www.oppsociety.org which describes in detail the disease, testing methods, and control and eradication programs.

An unexpected by-product of the CAE and mycoplasma prevention programs was a decreased incidence of caseous lymphadenitis and Johnes disease in participating herds. Unlike the other diseases discussed here, Mycobacterium avium subspecies paratuberculosis can also be transmitted in utero and this prevention program cannot prevent that method of transmission. However, separation of the newborns from infected adults and feeding of heat-treated colostrum and pasteurized milk or milk replacer will still decrease the incidence of clinical Johnes disease.

The specific pathogen prevention program

With the exception of scrapie, the incidence of these serious insidious diseases of sheep and goats can be decreased through use of a specific pathogen prevention program. All of these diseases are readily passed from the infected adult population to newborns. Newborn lambs and kids should be removed from the affected adult population at birth and be fed heat-treated colostrum (heated to 135 degrees F and held at that temperature for 1 hour) and pasteurized milk or milk replacer. Raw cow colostrum or cow milk should not be substituted due to the high incidence of mycoplasma and paratuberculosis in dairy cattle. If cow colostrum or milk must be used, then it should be heat-treated as described above. Newborn lambs and kids should receive 1 ounce of colostrum per pound of body weight three times within the first 24 hours of life. The new clean population may not share feeders, water sources, or housing with infected animals, and there should be at least 10 foot wide alleys between status groups. Clean animals should be handled, fed and cared for first, and equipment must be cleaned after each use in the infected population. Sheep producers may find it more difficult or less appealing to hand raise lambs separately from the ewe flock. Testing the adult population for Johnes disease or OPP every 6 months and culling test-positive ewes and their lambs less than one year of age may speed removal of the infected adults from the herd if the sheep producer chooses not to hand raise the lambs.

Sufficient space for feeding and drinking should be provided for the number of animals housed, and efforts must be made to prevent fecal contamination of feed and water. Separate equipment should be used for feeding and cleaning to prevent manure contamination of feed. Animals exhibiting chronic weight loss of unknown origin should be isolated, examined and tested for these diseases, and animals that die of unknown causes should be necropsied. Affected animals and asymptomatic carriers should be separated from the main herd and should be milked and handled last. There is no effective treatment for any of these diseases, but owners of pet ruminants may choose to keep clinically ill animals and seek palliative care. Reduction of stress, provision of good quality feed and good nursing care combined with pain medication for symptoms of arthritis may allow affected sheep and goats to live a somewhat normal life.

The last component of the specific pathogen prevention program concerns routine blood or fecal sampling as part of a test and cull program. Affected animals and asymptomatic carriers should be separated from the adult herd as they are identified and should be culled for slaughter or euthanized as it becomes economically feasible to remove them from the herd. Animals should be tested at 6 month intervals until the whole adult population has two negative tests, and then annual tests are recommended to maintain surveillance.

Purchasing new stock

Producers purchasing new stock should carefully question the seller regarding that herd's status for caseous lymphadenitis, CAE, OPP, mycoplasma, Johne's disease and scrapie, the methods used for raising offspring, and the health of the oldest animals in the herd. Purchased additions should be tested prior to arrival at the new herd and should remain separate from the established herd for at least 30 days or until they have obtained a negative test in the new herd. If possible, young stock which have not been bred should be purchased to decrease exposure to pathogens transmitted through breeding. Milk from lactating does should be cultured prior to purchase to prevent introduction of new mastitis pathogens such as mycoplasma or coagulase negative Staphylococcus species. No vaccine or drug can replace keeping the environment clean. Removing dirty bedding and opening a closed barn prevent parasitism, pneumonia and paratuberculosis. Adequate amounts of nutritious feeds appropriate for the type and number of animals present prevents nutrient deficiencies and allows livestock to maximize productivity. Well-fed sheep and goats with adequate trace minerals appropriate to the locally available feeds are more likely to mount an effective immune response when challenged. It is very easy to purchase these diseases through infected colostrum, facilities or herd additions, but once in the herd, these diseases are very difficult to eliminate. Diligent use of a specific pathogen prevention program will decrease the incidence of all of these diseases in affected herds.

Attending shows

Many producers cannot elude the appeal of the show ring. Producers should arrive early and pick pens as far away from other herds as possible. Tack pens, walls, fences or inexpensive tarps can be used as barriers between herds, and producers should avoid penning back to back with other herds. Herds should be grouped by health status or prevention programs. Pens should be cleaned on arrival and bedded heavily to prevent transmission of soil-borne organisms. Pens should be cleaned frequently, and the animals should not be over-crowded. Upon return to the home herd, the animals should be treated as if they were new additions and quarantined separately for 30 days. If they have been commingled with animals that are not of the same test status, then they should be retested prior to housing with the established herd.

Fomites

People, equipment and animals can transmit disease, and infectious agents can be carried on hands, clothing, boots and pets. Producers should not wear their barn clothes to another producer's facility, and they should leave their dogs at home. If a visitor arrives with a dog in tow, the dog should be returned to the vehicle. Visitors should wear disposable plastic boot covers and should wash their hands before and after visiting livestock. Herds should not exchange equipment unless absolutely necessary, and then it should be thoroughly cleaned before using it and again before returning it. Producers selling stock should have the buyer park on the road and walk the animals off the property to the vehicle. Likewise, dead animals should be placed at the edge of the property so that the renderer will not enter the pens or come in contact with the livestock.

While many producers see the benefit of developing herds free of diseases such as CAE, OPP, mycoplasma, infectious abscesses or Johne's disease, not all are convinced that the effort involved in developing a clean herd will provide an increased economic return. Sheep or goats with chronic wasting diseases do not live as long and do not produce as much milk or pounds offspring on a yearly basis as do animals under the same management without chronic disease. Animals infected with these chronic diseases are permanently infected and serve as a source of contagion to other livestock. Many progressive commercial herds enjoy greater productivity, longevity and sales of healthy stock after adopting a prevention program and eliminating these diseases.

Scrapie

Scrapie is an infectious disease of sheep and goats characterized by intense pruiritis, varying neurologic symptoms and long incubation period. This transmissible spongiform encephalopathy is thought to be caused by an abnormal prion protein transmitted in the placenta and uterine fluids during the first 4 to 6 weeks after parturition. The abnormal prion protein has been found in brain, spinal cord, lymph nodes and spleen of infected sheep and goats. The incubation period is very long and infected animals may genetically control the length of the incubation period. The onset of symptoms may be insidious, occur over a long period of time, and may be associated with stress such as change in herd, parturition or sale. Affected animals may isolate themselves or lag behind the herd and startle easily. Hair loss from pruiritis, nibbling, abnormal lip-smacking, and weight loss in spite of good nutrition are common symptoms. All affected animals die. Diagnosis is based on histological examination of characteristic brain lesions and the disease is federally reportable. The Area Veterinary Medical Officer will determine what happens with the herd upon confirmation of the diagnosis, but most herds are quarantined and depopulated.

Further Reading

Pugh, D. G. (ed.) (2002). Sheep and Goat Medicine. Philadelphia: W. B. Saunders Co.

Radostits, O. M., Gay, C. C., Blood, D. C. and Hinchcliff, K. W. (2000). Veterinary Medicine A Textbook of the Diseases of Cattle, Sheep, Pigs, Goats and Horses (9th edn.). London: W. B. Saunders Co.

Smith, M. C. and Sherman, D. M. (1994). Goat Medicine. Philadelphia: Lea & Febiger.