Leptospirosis in Ruminants
Leptospiral serovars of major importance in cattle are Hardjo and Pomona in North America, with serovars Grippotyphosa, Bratislava, Icterohaemorrhagiae, and Canicola occasionally implicated. The most commonly documented cause of leptospirosis among cattle in the USA and throughout much of the world is serovar Hardjo, for which cattle are the maintenance host. Risk factors for Hardjo infection in cattle have been reported to include open herds, access to contaminated water sources, co-grazing with sheep, and use of natural breeding. Serovar hardjo has the ability to colonize and persist in the genital tract of infected cows and bulls.
Many leptospiral infections in cattle are subclinical, particularly in nonpregnant and nonlactating animals. Acute or subacute leptospirosis is most commonly associated with incidental host infections and occurs during the leptospiremic phase of infection. Clinical signs associated with chronic infections are usually associated with reproductive loss through abortion and stillbirth. Persistent colonization by serovar Hardjo of the uterus and oviducts may be associated with infertility characterized by increased services per conception and prolonged calving intervals.
Uncommonly, severe acute disease occurs in young stock infected with incidental serovars, particularly serovar Pomona and less commonly Icterohemorrhagiae. Clinical signs include high fever, hemolytic anemia, hemoglobinuria, jaundice, pulmonary congestion, occasionally meningitis, and death. In lactating cows, incidental infections may be associated with agalactia with small quantities of blood-tinged milk. A less severe form of this “milk drop syndrome” may occur in Hardjo-infected lactating cows in the absence of other clinical evidence of infection. In lactating cows, incidental infections have been reported to cause blood-tinged milk.
The chronic phase of disease is associated with fetal infection in pregnant cows presenting as abortion, stillbirth, or birth of premature and weak infected calves. Infected but healthy calves also may be born. Abortion or stillbirth is commonly the only manifestation of infection but may sometimes be related to an episode of illness up to 6 wk (Pomona) or 12 wk (Hardjo) earlier. Abortions associated with incidental host infection tend to occur late term and in groups or so-called “abortion storms.” In contrast, abortions occurring after infection with serovar Hardjo tend to be more sporadic and can occur mid- to late pregnancy and several months after initial infection.
Diagnosis of incidental host infections in cattle is relatively straightforward. In general, infected animals develop high titers to the infecting serovar; an antibody titer >1:800 at the time of abortion is considered evidence of leptospirosis. Leptospires can be demonstrated in placenta and the fetus in some cases by immunofluorescence, PCR, and immunohistochemistry. Diagnosis of serovar Hardjo infection is more difficult and requires a combination of approaches. Serology alone often fails to identify animals infected with serovar Hardjo, because seronegative shedders are common in infected cattle herds. The recommended diagnostic testing strategy includes the primary use of a test (immunofluorescence or PCR) to detect the organism in the urine from a sample of cattle in the herd followed by serologic testing to provide insight into the likely infecting serovar of Leptospira.
Cattle with acute leptospirosis can be treated with the label dosage of tetracycline, oxytetracycline, penicillin, ceftiofur, tilmicosin, or tulathromycin. Leptospires also are highly susceptible to erythromycin, tiamulin, and tylosin, although these antibiotics cannot be relied on to remove the renal carrier state. Injectable, long-acting oxytetracycline (20 mg/kg) and sustained-release ceftiofur have been shown to effectively eliminate shedding in cattle infected with serovar Hardjo. Vaccination can be combined with antibiotic treatment in the face of an outbreak of leptospirosis, but vaccination alone will not reduce urinary shedding. All appropriate withdrawal times should be observed.
Bovine leptospirosis vaccines available in the USA and Canada are pentavalent and contain leptospiral serovars Pomona, Grippotyphosa, Canicola, Icterohaemorrhagiae, and Hardjo. These vaccines provide good protection against disease caused by each of these serovars, with the possible exception of serovar Hardjo. Experimental and field evidence indicates that some traditional five-way leptospirosis vaccines do not provide good protection from serovar Hardjo infection. New vaccines have been introduced to address this issue. If a primary goal of a vaccination program is protection of cattle against Hardjo, care should be taken in selection of a vaccine product. In general, annual vaccination of all cattle in a closed herd or low incidence area, or twice-yearly vaccination in an open herd or high incidence area, is the most effective approach to control.
Relative to cattle and pigs, sheep and goats have been considered resistant to leptospiral infection, with low seroprevalences and only a small number of serogroups being implicated in clinical disease. Sheep can serve as a maintenance host for serovar Hardjo and therefore spread infection to cattle. Incidental infections may cause sporadic outbreaks of acute disease characterized by hematuria, hemoglobinuria, jaundice, and death (usually in lambs), and occasional abortions.