Leptospirosis is an economically important bacterial infection of livestock that causes abortions, stillbirths, infertility, and loss of milk production. Leptospirosis is caused by pathogenic spirochetes of the genus Leptospira and is an important cause of abortion and infertility in North American cattle.
Leptospirosis is a re-emerging infectious disease that tends to fluctuate in prevalence as a livestock pathogen. Its transmission to livestock and to humans is often associated with wet, unsanitary conditions favouring the collection of surface water infected with organisms shed in the urine of infected animals. Along with dogs, cattle, horses and pigs are commonly infected. A number of wildlife species (skunks, raccoons, rats, muskrats) are incriminated as reservoirs and shed organisms into puddles, ponds and streams that ultimately put livestock at risk, especially those on pasture.
Pathogenic leptospires have been categorized into seven species and approximately 250 serovars. Individual serovars favour specific hosts. For example: serovar Hardjo is commonly found in cattle, serovar Canicola is common in dogs.
Herd prevalence may reach 30 to 40 per cent of animals, most shedding organisms in urine at any one time.
Transmission of the infection involves contact with infected urine, placental fluids, or milk. Leptospira can also be transmitted in semen and during natural breeding or pregnancy. Leptospires invade through intact oral mucosa or water-saturated skin. Environments with standing water sustain bacterial populations and soften the skin of the coronary band to facilitate invasion. Under the right conditions, leptospiral organisms survive for weeks outside of the animal.
Persistently shedding herd mates, wildlife and infected herd additions represent the greatest risk to naive animals. Infected bulls may shed in semen and urine for months. Replacements have been a source of new infections. Infected wildlife has been known to contaminate cattle-holding areas.
Leptospires invade the body through mucous membranes or damaged skin. After an incubation period of three to 20 days, organisms are carried by blood and subsequently invade many tissues, including the liver, spleen, kidneys, reproductive tract, eyes, and central nervous system where they multiply. Antibodies to Leptospira appear soon after infection and with appearance of antibodies infected animals start to clear organisms from most organs except the urinary system.
Leptospira infections in cattle generally result in no or relatively mild clinical signs, but produce a renal carrier state associated with long-term urinary shedding. Persistent infection of the reproductive tract is common in both males and females. Clinical signs of infection can be subtle. Late abortions, stillbirths, or birth of weak calves is an outcome when pregnant females are infected for the first time. Retention of fetal membranes is common. Abortions due to serovar Hardjo infection tend to occur sporadically. Some serovars (Pomona or Grippotyphosa) can be associated with abortion storms.
Perhaps the most economically significant manifestation of Leptospira infections is the subtle impairment of herd reproductive performance. Exposure of non-infected replacement females to organisms shed from chronically infected cows in the herd perpetuates the occurrence of abortions, stillbirths and weak calves.
A good clinical and vaccination history is important. Co-ordination between veterinarian, owner and a diagnostic laboratory with experience in the diagnosis of leptospirosis is necessary to maximize the chances of making an accurate diagnosis. Diagnostic tests include blood tests designed to detect antibodies against the organism, examination of abortion and fetal tissues or tests designed to detect fragments of Leptospira genetic material in tissues or body fluids (PCR or polymerase chain reaction). No single diagnostic procedure can be used in every situation increasing the importance of using combinations of tests. Dependence on blood tests alone is often misleading. Presently, there is a lack of consensus about what antibody levels are indicative of active infection and confusion over interpretation of the cross-reactions that occurs with Leptospira species and vaccines. Up to 50 per cent of cows aborting due to serovar Hardjo will be seronegative at the time of abortion.
The most common approaches to the control of leptospirosis in cattle are based on prevention of exposure, vaccination, and selective treatment.
Important elements of control include: Limiting direct and indirect contact between cattle and carriers of leptospirosis e.g. rodent and wildlife control around buildings, fencing swampy ground or streams. Proper diagnoses of reduced reproductive performance is critical.
Quarantine/treatment of replacements.
Vaccination. Vaccine choice needs to be discussed with a veterinarian.
Antibiotics can be used to treat individual animals and will, in general, eliminate persistent infections. Longacting oxytetracycline (20 mg/kg, IM, two doses 10 days apart) has been shown to be effective in the treatment of serovar Hardjo infections.
Eliminate standing water in high-use areas.