Agent.

Bluetongue virus (BTV) is an arthropod-transmitted orbiviral disease of domestic and wild ruminants. In North America, it is found endemically in the southern and western United States, as well as in parts of the Caribbean, Mexico, and Central America.¹ Serotypes known to occur in these regions include 10, 11, 13, and 17. In Canada, occasionally serologic reactors (mostly bovine) to serotype 11 have been identified in the Okanagan Valley of British Columbia.² At least 24 serotypes have been recognized worldwide.

Transmission.

The virus requires an arthropod midge vector, Culicoides veriipennis, for transmission. The subspecies C. veriipennis sonorensis, found mostly in the Southwest, is thought to be the most important vector. Clinical expression of the disease tends to be seasonal (fall) and linked to the midge’s seasonal cycle. Preferred breeding conditions include predominantly wet conditions and a warm environment. Cattle—considered to be an important vertebrate reservoir—can remain viremic for up to 90 days and show no clinical signs. Infected sheep clear the virus within 21 days but may experience a case fatality rate of 10% to 50%. BTV also has been isolated from bovine semen. Although the vector is found across southern Canada, whether it can propagate infection is questionable.

Clinical picture.

Sheep infected with BTV show signs of high fever; swollen ears, face, and tongue; oral and nasal ulcers; and lameness. Up to 20% of fetuses infected in early gestation are found to have various degrees of hydranencephaly and skeletal deformities and may be aborted or carried to term. Some experimental studies have failed to show disease in fetuses born to infected ewes.³ Plant toxicity and Cache Valley virus (CVV) infection are important to rule out in the diagnosis.

Diagnosis and control.

Some areas of the United States (e.g., California) may have serotype-specific attenuated virus vaccines available. Little to no cross-protection occurs between serotypes. Sheep need to be vaccinated once before the season of risk and again before breeding to avoid congenital central nervous system (CNS) lesions.^4,5 Control measures also can include housing sheep at night and/or pasturing away from wet areas during the season of peak vector activity. In Canada, suspected cases of bluetongue must be reported to the local office of the Canadian Food Inspection Agency. Virus isolation and serologic studies in the aborting dam are done to rule out infection with BTV.

Agent.

Border disease virus (BDV) is a pestivirus closely related to bovine viral diarrhea virus (BVDV) and classical swine fever virus (CSFV). It has been reported sporadically across North America and in many parts of the world. Many different strains of BDV have been isolated from animals with clinical border disease.⁶ Some outbreaks of border disease have been associated with persistent infection with BVDV.⁷

Transmission and pathogenesis.

The virus invades the ewe and causes a transient viremia of 7 days’ duration; it does not cause disease in the ewe but attacks the placenta. During the viremia, BDV is shed in the urine, feces, and saliva. Necrosis occurs at the junction of the villous trophoblast and maternal caruncle, with resultant perivascular necrosis. This necrosis leads to separation of the fetal-maternal-placental bond, with subsequent interference with fetal nutrition and blood gas exchange.⁸ Most fetuses infected in utero before day 60 to 85 of gestation die because of this damage and are either reabsorbed, aborted, macerated, or mummified.⁹ If they survive, various degrees of damage may be seen. Myelination of nerve cells is disturbed, particularly in the cerebellum. Hair follicles also can be affected and produce hair, rather than wool. These fetuses also are born persistently infected with BDV, shed virus continuously, and fail to clear the infection. Fetuses infected after day 85 may abort, or the lamb may be born weak or unaffected, or may be virus negative and with a precolostral titer to BDV. It is possible for several levels of signs to be present in the same litter of lambs.

Clinical picture.

An increase in the number of open ewes, as well as abortion, is seen in the flock, followed by the birth of lambs with or without clinical signs, including weak lambs that are persistently infected.⁶ These lambs are small, with shortened facial and long bones, and have a wool coat that frequently is hairy and darkly pigmented, particularly over the shoulders, neck, and head. They also may show mild to severe body tremors. Accordingly, affected lambs commonly are termed “hairy shakers.” Nervous signs diminish in severity in those lambs that survive. Persistently infected lambs grow more slowly than do their healthy cohorts and appear to be more susceptible to disease.¹⁰ Some evidence indicates that persistently infected lambs may succumb to a disease similar to mucosal disease of cattle.¹¹ At necropsy, findings include gross thickening of the distal ileum, cecum, and colon resulting from focal hyperplastic enteropathy, and cytopathic BDV can be recovered from the gut.⁶ Usually a history of a recent introduction of animals either before, during, or shortly after breeding season can be elicited from the farmer, or the disease may follow contact with cattle, suggesting exposure to BVDV.

Diagnosis.

BDV can be isolated from aborted fetuses and from the buffy coat in centrifuged blood specimens from affected lambs. Antigen-capture enzyme-linked immunosorbent assay (ELISA) can be used on the sera of suspected persistently infected adults and lambs older than 2 months.^6,12 CNS changes, both gross (cerebellar hypoplasia, hydranencephaly) and microscopic (hypomyelination, microgliosis), help make the diagnosis. Immunohistochemical analysis is equally sensitive to virus isolation, except in autolytic fetuses.¹³ In autolytic fetuses, use of formalin-fixed, paraffin-embedded fetal brain for immunohistochemical analysis is more sensitive than virus isolation. It also appears to be more sensitive than antigen-capture ELISA on fetal tissues.¹³ Serum virus neutralization or ELISA titers of antibody from the aborted ewes tend to be high, although this finding in itself is not diagnostic. Demonstration of a rising titer in aborting ewes is difficult because infection generally is historical—preceding abortion by at least 2 months. If an aborting ewe is persistently infected, then antibody levels will be low or absent and virus can be isolated or detected from the serum or buffy coat.

Control and prevention.

After an outbreak of border disease–related abortion, all lambs with congenital pathologic conditions and those demonstrating poor performance should be sent to slaughter when they are as young as possible and, before that, should be kept separate from the breeding flock. Replacement lambs older than 2 months should be screened for virus using virus isolation or antigen capture ELISA.⁶ New additions to the flock should be isolated for at least 2 weeks before introduction to the breeding flock, and consideration should be given to screening them as well. Cattle and sheep should be kept well separated, with no shared feeders or water sources, and an effective vaccination program for BVDV should be maintained in the cattle herd. Available evidence suggests that cross-protection is poor between BVDV and BDV; therefore, vaccinating sheep with a killed or modified live BVDV vaccine confers no advantage.⁶ At present, one border disease vaccine is commercially available.¹⁴ It is a killed adjuvanted vaccine that contains strains of BDV as well as of BVD-1.

The agent.

The Bunyaviridae family, of which CVV is one species, contains several species that are associated with ovine abortion. Organisms in this family of viruses are arthropod borne. Akabane virus is considered to be exotic to North America but is a very important cause of congenital ruminant deformities in Australia, Japan, Korea, and Israel and probably also is present in the Middle East and Asia. In North America, CVV has been implicated in outbreaks of congenital deformities in the U.S. Southwest, particularly Texas, and occasionally in the Northeast (Illinois, Michigan, New York, Maryland, and Pennsylvania) and southern Ontario.¹⁵ Experimentally, Main Drain virus and, to a lesser extent, San Angelo and LaCrosse viruses also cause ovine fetal malformations.¹⁶ LaCrosse virus is an important cause of viral encephalitis in children.¹⁶

Transmission and pathogenesis.

CVV is transmitted mainly by mosquitoes that prefer to feed on large ruminants. Akabane virus mostly is transmitted by Culicoides midges. The vertebrate reservoir for CVV is not known but is speculated to be wild or domestic large ruminants (e.g., deer, cattle). Two days after infection of the sheep by the infected mosquitoes, a viremia of 2 to 5 days’ duration develops. At this time the infected sheep may demonstrate fever, depression, and a reluctance to move. The virus invades the placenta and fetus if implantation has occurred. Little inflammatory response is observed in the placental tissues. The virus then invades the fetal brain, spinal cord, and skeletal muscles. The severity of the lesions is dependent on gestational age at the time of infection. Early infection from day 28 to 32 generally results in early embryonic death and mummification. Infection between days 32 and 37 manifests as CNS and musculoskeletal deformities characteristic of severe arthrogryposis. Infection from day 37 to 48 manifests as milder musculoskeletal lesions with nonsuppurative encephalitis and encephalomyelitis.

Clinical picture.

The dominant clinical picture is one of stillbirth or live birth of congenitally deformed lambs. The predominant deformities are arthrogryposis, brachygnathia, hydranencephaly/porencephaly, microencephaly, agenesis or hypoplasia of the spinal cord, and possibly other CNS deformities. It is possible to see all syndromes in one flock outbreak and more than one syndrome in a litter. Morbidity rates are variable depending on viral load in the mosquito population. Fetuses from cattle and goats also may be affected.

Diagnosis.

It is important to rule out ingestion of locoweed as a cause for arthrogryposis. Osteochondrodysplasia (spider lamb) is inherited in an autosomal recessive fashion and affects mainly Suffolks and Hampshire sheep; neurologic disease and arthrogryposis are not seen. By the time of parturition, CVV has been eliminated from the dam and fetuses, so that virus isolation is unrewarding. In endemic areas, ewes may have background titers to the virus. Diagnostic titers can be obtained only from fetal heart blood or precolostral serum from affected lambs. Absence of a diagnostic titer in the fetus or lamb will not rule out CVV infection but in the dam does rule out this disease.

Control and prevention.

No vaccine is available to prevent CVV in sheep, although an inactivated vaccine for Akabane virus is available in Australia and parts of Asia. Immunity from natural infection is long term and may be for life. It is difficult to predict an epizootic of CVV infection. In geographic areas with a history of CVV infection, the farmer is advised to avoid breeding sheep in mosquito-infested pastures; in temperate parts of North America, the farmer should wait until after a killing frost before turning in the ram.

Agent.

Brucella ovis is a common cause of epididymitis in rams and more rarely causes abortion in ewes. It is widely distributed in the United States and to a lesser extent in Canada, and more commonly in the western regions. Brucella melitensis also can cause ovine abortion and is found in Mexico but not in the United States or Canada.¹⁷

Transmission and pathogenesis.

The organism is spread through contact between mucous membranes (vaginal, preputial, conjunctival), both ram to ewe and ram to ram, but not ewe to ewe. Rams infected with this organism are not always infertile and pass the organism in the ejaculate. Ewes clear the organism within a few weeks of aborting and usually are not sources of infection to the flock the following year. Sources of infection may be the carrier ram and infected material from abortions.

Clinical picture.

Abortion levels of 25% to 35% have been reported. Stillbirths and lamb weakness also may be seen; affected animals may show lesions of suppurative pneumonia.¹⁸ Placentitis is severe, with a thickened, necrotic placenta—a common finding. Examination of the breeding rams will reveal clinical or subclinical epididymitis.

Diagnosis.

Titers in the sera of aborting ewes are variable, and such tests may be misleading because a negative titer does not rule out the disease. The sensitivity of the complement fixation test ranges from 96.3% to 100% and the specificity from 98.4% to 99.9%. The ELISA specificity is reported at 99.4% and the sensitivity at 100%.¹⁹ Culture of or use of a polymerase chain reaction (PCR) assay on the stomach contents of the fetus, as well as culture of the cotyledons, generally is successful.²⁰ Serologic testing and examination of the rams also should be performed. B. melitensis infection should be a consideration in the differential diagnosis, particularly in states bordering Mexico.

Prevention and control.

In endemic areas, such as the western part of North America, a control program using a combination of serologic testing (ELISA or complement fixation test or both), routine scrotal palpation of breeding rams, and vaccination of ram lambs with a killed bacterin will reduce the prevalence of disease.²¹ The vaccine is regarded as having poor efficacy, and the serologic tests and scrotal palpation have only fair sensitivity. Rams should not be shared between flocks, nor should they be housed at any time with rams used in other flocks. In Canada, serologic testing is not readily available for screening of rams. Rams from affected flocks should be slaughtered and replaced with virgin rams from clean sources.

The agent.

Both Campylobacter jejuni and Campylobacter fetus subsp. fetus have been implicated in ovine abortion, and both are also pathogenic to humans. They can reside in the intestinal tract of sheep and many other species, including dogs and birds. C. jejuni is implicated more commonly when abortion is sporadic, although it can affect up to 20% of the pregnant flock. C fetus subsp. fetus is more frequently associated with large outbreaks of abortion that affect a significant percentage of the flock and may occur from year to year. Several serotypes of C fetus subsp. fetus have been recognized. In the United States, most of the isolates associated with abortion are A2 and C.²² Occasionally other species and biovars of Campylobacter have been isolated from aborted ovine fetuses.

Transmission and pathogenesis.

The organisms are harbored in carrier sheep in the intestine and gallbladder.²³ Sources of contamination are feces, aborted fetuses and placentas, and vaginal discharges from aborted ewes. Carrion-eating birds such as crows may transmit the organisms between flocks. Incubation ranges from 8 to 60 days.²⁴

Clinical picture.

Ewes generally start to abort in the second half of gestation. Most abortions occur during the third trimester about 3 days after fetal death.²⁵ Fetuses are expelled well preserved with the placenta. Ewes are not ill, although transient diarrhea has been reported. Ewes infected 2 weeks before lambing may give birth to stillborn lambs, or to weak lambs with a very poor chance of survival. The placenta of aborted fetuses tends to be edematous, with congested swollen cotyledons. The fetus may have subcutaneous edema and an enlarged abdomen with pleuritis, peritonitis, and hepatitis, with occasional target lesions of hepatic necrosis. Abortion levels of up to 70% have been reported, but levels of 10% to 20% are common in enzootically infected flocks. Immunity from one species of Campylobacter species is not cross-protective for another, but natural immunity to the infecting species can occur without abortion and lasts at least 3 years.²⁶

Diagnosis.

Stain impression smears of cotyledons or fetal stomach contents will demonstrate the organism. Darkfield or contrast microscopy will demonstrate the live organism from fetal stomach contents. It also can be cultured from the placenta or stomach contents. Flexispira rappini infection should be a consideration in the differential diagnosis.²⁷ Antibiotic sensitivity testing should be done on the isolate, because resistance to penicillins and tetracyclines can occur.²⁸

Prevention and control.

In the face of an outbreak, all pregnant ewes should receive treatment with an antibiotic that is likely to be efficacious. Long-acting oxytetracycline at the label dose (20 mg/kg) can be used successfully. For large flocks, for which individual injections may be more difficult, tetracyclines may be added to the feed at a level of 250 to 300 mg/head/day until lambing is finished.²⁹ Daily injections of penicillin (400,000 IU) and dihydrostreptomycin (0.25 g) at a dose rate of 6 to 10 ml per ewe also have been recommended.³⁰ Pregnant ewes should be removed from the aborted ewe flock and the contaminated area. Vaccination with a multivalent bacterin can be done in the face of an outbreak, but sufficient immunity to stop abortions takes 2 weeks to develop.²⁹ In flocks in which campylobacteriosis has been diagnosed, vaccination should be routinely done according to label directions. This protocol involves vaccinating all breeding ewes with a bivalent killed bacterin twice, the first before breeding and the second injection 60 to 90 days later. Annual revaccination before breeding should be maintained.

A lack of efficacy of the vaccine may be due to any of several factors. The vaccine may not contain the correct serotype of C. fetus subsp. fetus, or the responsible pathogen may be another species of Campylobacter or F. rappini.²² Culture and typing should be done to rule out this possibility.

Agent.

The genus Chlamydia has recently undergone reclassification.³¹ Chlamydia psittaci immunotype 1, the abortion strain for sheep and goats, is now classified as Chlamydophila abortus type strain ATCC VR 656. Chlamydia pecorum, a fecal chlamydial species that may be nonpathogenic or cause arthritis and conjunctivitis and often cross-reacts on tests with Chlamydia abortus, is now classified as Chlamydophila pecorum type strain ATCC VR 628, Bo/E58. Organisms belonging to this genus are obligate intracellular bacteria that replicate in the phagosome.³²

Transmission and pathogenesis.

Transmission may occur from exposure to aborted materials or vaginal discharge or from environmental contamination and ingestion.³² Experimental oral inoculation of the tonsillar crypts, but not intraruminal inoculation, produces abortion.³² Contact of the nasopharynx with infected placentas in recently lambed ewes can cause abortion in those ewes during the subsequent pregnancy.³³ Chlamydia-naive pregnant sheep infected vaginally will give birth to infected weak lambs, suggesting that the venereal route also may be important in the transmission of this agent.³⁴ When naive nonpregnant sheep were experimentally infected, the organism could be detected in a variety of organs, including the abomasum and jejunum, and lymph nodes, but shedding in the feces was not detected.³⁵ C. pecorum strains, frequently present in large numbers in the feces of healthy ewes, can cause abortion when injected into the muscle of healthy ewes, but not when ingested.³² In nonpregnant ewes, at approximately 1 week after infection, C. abortus becomes undetectable by any means.³⁶ In infected rams, C. abortus may be isolated from the semen and seminal vesicles.³⁶ When the infected ewe becomes pregnant, the organism moves by a hematogenous route to the trophoblast cells of the chorionic epithelium.³⁶ By 95 days of gestation, the infection has spread from the cotyledons to the intercotyledonary areas.³⁹ The fetus also becomes infected, but histopathologic changes are minor. The primary target is the placenta.

Ewes rarely abort at less than 100 days’ gestation, but fetal loss and resorption before 100 days can occur.³⁴ Latent infections occur when ewes are infected either while not pregnant or during late gestation. These ewes will abort during the subsequent pregnancy.³² Ewes that abort once do not abort again for at least 3 years. Although maternal immunity prevents repeated abortion, ewes that have aborted from C. abortus infection subsequently shed the organism in vaginal secretions during estrus and have persistent low-level titers.³⁷ Contamination of mucous membranes or of feed by vaginal secretions, or of the ram’s penis during breeding, may be a mechanism of continuing the infection in the flock.

Clinical picture.

Abortion tends to occur in late term pregnancies, but early fetal death and resorption also occur.³⁷ The placenta is necrotic, with lesions affecting both cotyledons and intercotyledonary spaces. The placenta becomes thickened and necrotic, as well as hemorrhagic at the edges of the lesions.³⁹ Fetuses may be aborted necrotic, well preserved, or rarely mummified. Weak and stillborn lambs also are commonly seen. Abortion levels can be high (up to 30%) in the first year of the disease but decrease in subsequent years to 10% to 15%, with only ewe lambs and new introductions affected in subsequent years. Ewes infected in very late pregnancy or when open may not develop protective antibodies and may be susceptible to infection in the next pregnancy. Chronic infection of the uterus or uterine tubes occurs after abortion and may eventually impair fertility.³⁴

Diagnosis.

The severe placentitis of both the cotyledons and intercotyledonary zones is typical but not diagnostic. Smears made from the chorionic villi and stained with appropriate stains (e.g., Machiavello, Wright-Giemsa, xanthine dye–thiazine dye mixture, Giminez, modified Ziehl-Neelsen, or Brucella differential) will demonstrate clumps of small, red, coccoid intracellular elementary bodies.⁴⁰ The organisms can be confused with C. burnetii. Immunologic staining methods such as those using immunoperoxidase can be used on fixed tissues.⁴¹ Various tissues (fetal lung, spleen, liver, placenta, and vaginal swabs) also can be submitted for culture using either a Chlamydia isolation procedure modified from Stortz or embryonated chick eggs.⁴⁰ Chlamydiae are difficult to isolate, however, because they lose infectivity during transport to the laboratory.³⁹ Fluorescent antibody can be used to detect the organisms in smears and culture, but this technique appears to be less sensitive but perhaps more specific than a chlamydial genus–specific ELISA that detects lipopolysaccharide.⁴²

Infected ewes that have not yet aborted do not have significantly elevated antibody titers. Serologic testing of aborting ewes should always use paired acute and convalescent sera. Low-level titers detected in screening the breeding flock may be due to a lack of test specificity, or to chronic C. abortus infection.

The complement fixation test is not very specific because it detects genus-specific lipopolysaccharide, so that cross-reaction occurs with other species of Chlamydophila, notably C. pecorum.³⁴ This test should not be used for diagnosis in individual aborting sheep or in lambs or rams, or beyond 3 to 6 weeks after abortion.³² Many ELISAs that detect lipopolysaccharide or outer membrane or unstated antigens failed to perform adequately with respect to either specificity (range of 51% to 96%, depending on the test), sensitivity (range of 51% to 81%, depending on the test), or both.⁴³ A new indirect ELISA based on detection of the outer membrane protein of C. abortus has improved sensitivity (84.2%) and specificity (98.5%) over the complement fixation test.⁴⁴ PCR assay is highly sensitive, but again specificity may be poor owing to the contaminating presence of C. pecorum.