Let’s Talk about Lepto!
By : Dr. Michelle Arnold, UK Veterinary Diagnostic Laboratory
What is Leptospirosis or “Lepto”? Leptospirosis is a complicated bacterial disease commonly associated with abortions, stillbirths and drop in milk production in cattle. However, this bacterium also causes sickness and death in cattle, dogs, sheep and horses worldwide and is an important zoonotic disease affecting an estimated 1 million humans annually. Farmers and those working in meat processing facilities are at highest risk.
What causes leptospirosis? The disease is caused by a unique, highly coiled, Gram negative bacterium known as a “spirochete” belonging to the genus Leptospira. These “leptospires” are highly motile due to their spiral shape and, once inside a host animal, they enter the bloodstream and replicate in many different organs including the liver, kidney, spleen, reproductive tract, eyes and central nervous system. The immune system will produce antibodies that clear the organism from the blood and tissues except from the kidney. Leptospires take up residence primarily in the kidney and are excreted in the urine for months to even years after infection. Less frequently, leptospires persist in the male and female genital tract and mammary gland of females and may be excreted in semen, uterine discharges and milk.
How do cattle become infected with leptospires? Transmission of the organism is most often through direct contact with infected urine, placental fluids, semen or milk. However, transmission may also occur by coming in contact with areas contaminated with infected urine, such as stagnant ponds or swampy areas with standing water. The leptospires survive in the environment for long periods of time (approximately 6 months in the right conditions) in stagnant water as well as in warm and moist soils but die quickly when dry or in cold temperatures. Entry into the animal may be through penetration of intact mucous membranes such as through the mouth and the conjunctiva of the eye, or through damaged or water-softened skin. The organism may also be transferred during breeding and also during pregnancy from dam to fetus.
Which animals carry (“host”) this organism and are responsible for spread of disease? This is where the complicated life cycle of this organism must be explained in order to understand the wide range of disease symptoms that may be observed in cattle. To begin, it is important to distinguish two different types of “hosts”; 1) maintenance or reservoir hosts and 2) incidental or accidental hosts. A “maintenance host” is an animal that can carry the leptospirosis organism but not experience any obvious sickness from it. These are also known as “reservoir hosts” because this animal’s immune system allows the leptospires to happily live and duplicate themselves then be excreted and spread to other animals. Maintenance hosts for leptospires are often wildlife species including skunks, rats, raccoons and opossums but can be domestic animals (dog, cats) or livestock (pigs, cattle), depending on which kind of leptospire (known as a “serovar”) is involved (Table 1). For example, cattle serve as the maintenance host for the Leptospira serovar called “Hardjo type hardjo-bovis”. Transmission from one infected cow to another healthy cow with L. hardjo is efficient and the infection rate can be very high in an unvaccinated herd. When a cow is initially infected with L. hardjo, she may exhibit a few mild signs such as low fever but there will be very little antibody production by the immune system and the leptospires will stay primarily in the kidney and be shed in her urine for a prolonged period of time. In addition, the organism can also localize in male and female reproductive tracts and be shed in semen and uterine discharges.
An “incidental host” or “accidental host” is an animal that gets infected with a Leptospira serovar not normally found in that animal (infected “by accident”) that results in clinical disease that may be severe. Incidental hosts are not reservoirs of infection and transmission of the organism is uncommon within a herd. Infection of an incidental host usually occurs in areas contaminated with urine from maintenance hosts. For example, cattle are incidental hosts for the Leptospira serovar “Pomona” which is carried normally by feral swine (the maintenance host) and transmitted to cattle from water contaminated with swine urine. Once infected, cattle (especially calves) often show significant signs of disease, the immune system rapidly produces antibodies and there is a short period or no prolonged carrier state in the kidney.
What are the symptoms of leptospirosis? Clinical signs or symptoms of disease in cattle depend on which Leptospira serovar is involved and if cattle serve as a maintenance host or incidental host for this specific type. There are over 250 serovars of Leptospira but the two most important serovars affecting cattle in North America are Hardjo and Pomona, with Grippotyphosa, Canicola and Icterohaemorrhagiae much less frequently diagnosed. Most bovine leptospirosis is caused by the L. Hardjo serovar for which cattle serve as the maintenance host.
Most commonly, infection in pregnant cows results in abortion (usually late term), stillbirth, or birth of premature and weak infected calves. Retention of fetal membranes may follow abortion. Infertility may occur if leptospires continue to live within the genital tract of L. hardjo-infected cattle, especially in younger females. Lactating dairy cows may exhibit “milk drop syndrome”, characterized as a drop in milk production for 2-10 days where the milk has the consistency of colostrum, thick clots, yellowish color, and high somatic cell count, but the udder remains soft.
Severe, rapidly progressing disease may occur in calves infected with incidental serovars, especially Pomona. Symptoms of high fever, extreme weakness, red urine, rapid breathing due to anemia and death are all possible. Cows may experience a loss of milk production with very prolonged recovery.
How is leptospirosis diagnosed and treated? Diagnosis of this disease is not necessarily a simple task. Traditionally, a blood sample (red top blood tube) is taken from a cow that recently aborted and submitted to measure antibodies against the most common serovars. Incidental infections (for example, Pomona) will have highly elevated antibody numbers (called “titers”) that are diagnostic. Unfortunately, since cattle are the maintenance host of L. hardjo, the antibody numbers remain low even in the face of infection. Vaccination also confuses the interpretation of results because tests do not differentiate antibodies due to infection or antibodies due to vaccine. Therefore, multiple types of tests may be required to rule this disease in or out. Currently, urine samples can be tested for leptospires through a variety of assays to help identify the organism.
Animals diagnosed with leptospirosis can be treated with injectable long-acting oxytetracycline to remove the organism from the kidney. Research is ongoing if additional treatment is needed to clear infections within the genital tract. Consult your veterinarian for detailed advice regarding diagnosis and treatment options.
What methods are used to control and prevent leptospirosis in cattle? New infections are best prevented through vaccination with products containing the most common serovars affecting cattle. The leptospirosis fraction of the vaccine is often denoted as “L 5” in the vaccine name, representing Hardjo, Pomona, Grippotyphosa, Canicola and Icterohaemorrhagiae. In addition, several vaccine manufacturers have added extra protection against L. Hardjo type-bovis and this is denoted with “HB” in the vaccine name. It is worth mentioning that cattle already infected with leptospirosis must be treated first to remove the organism before vaccination is effective.
Control is accomplished by prevention of exposure, annual vaccination and treatment as needed. Reduction of cattle exposure to infected urine, especially fencing off stagnant ponds and swampy areas, will significantly reduce transmission of the organism. Personal protective equipment should be used when working with cattle suspected to be infected to prevent human disease.