In the USA, disease in cattle is primarily due to the Leptospira serovars hardjo , pomona , and grippotyphosa . However, serovars canicola , bratislava , autumnalis , and icterohaemorrhagiae , among others, also have been isolated. Cattle are the reservoir host for serovar hardjo , type hardjobovis.
Clinical Findings:
Acute leptospirosis can be severe in calves. Serovar pomona results in the most severe disease, however other serovars can cause similar disease. Calves may have fever, anorexia, dyspnea from pulmonary congestion, icterus, hemoglobinuria, and hemolytic anemia. Body temperature may rise suddenly to 105-106°F (40.5-41°C). Hemoglobinuria rarely lasts longer than 48-72 hr. The anemia begins to improve by 4-5 days and returns to normal 7-10 days later. Serovar hardjo being a host-adapted strain, does not typically result in the acute syndrome. Morbidity and mortality are higher in calves than in adult cattle.
In older cattle, signs vary greatly and diagnosis is more difficult. Enzootic infections of naive cattle with serovar hardjo , which usually result in abnormal milk, are more obvious in dairy than in beef cattle. Signs usually are restricted to a sudden drop in milk production; a hemolytic crisis does not occur. The milk is thick, yellow, and blood-tinged, with thick clots and a high somatic cell count; milk production can drop 10-75%, depending on the infecting strain. The udder is typically soft and flabby, which is unique for leptospirosis. Milk production can return to normal in 10-14 days even in the absence of treatment; however, cows with a severe drop in production may not recover to full production during that lactation cycle.
The chronic forms of leptospirosis manifest as abortion and stillbirths, and occur with infections of serovars pomona and hardjo . Abortion generally occurs 6-12 wk after initial infection and is more common during the third trimester. Stillbirths and birth of premature or weak infected calves also occur. An abortion storm in a breeding herd is often the first indication of leptospirosis infection, because the mild initial signs often pass unnoticed. In endemically infected herds, abortions occur mostly in younger animals and are sporadic. Calves reared by previously infected cows are protected by colostral antibodies for up to 6 mo. The calves generally have an antibody titer similar to that of their dams. Infertility may also be a problem in endemically infected herds, possibly as a consequence of localization of infection in the uterus and oviducts.
Lesions:
In the acute form, anemia, icterus, hemoglobinuria, and submucosal hemorrhages are prominent. The kidneys are swollen and dark, with multifocal petechial and ecchymotic hemorrhages, and later develop pale foci of interstitial cell infiltrates. The liver may be swollen, pale, and friable, with minute areas of focal necrosis. Petechiae in other organs are seen in fulminating cases; however, in the more prevalent serovar hardjo infections, the lesions are primarily restricted to the kidneys.
Diagnosis:
Serology with paired serum samples, direct culture in special media, or fluorescent antibody techniques on tissues are used to confirm clinical and postmortem findings. In herd evaluation, sera should be obtained from various age groups. Isolation of the causative agent constitutes the most definitive diagnostic method, but because leptospires are difficult to culture, it is not commonly performed. Elimination of brucellosis, campylobacteriosis, and trichomoniasis as the cause of an abortion outbreak is suggestive of leptospirosis. MAT titers may peak before abortion because the acute infection occurred several weeks previously. Abortion due to serovar hardjo infections may occur with low or negative serologic titers.
Treatment:
Tetracycline and oxytetracycline have been reported to be successful if given early in acute cases. Erythromycin, enrofloxacin, tiamulin, and tylosin are also effective in acute cases. Oxytetracycline, amoxicillin, and enrofloxacin may be useful to treat chronic infections. Blood transfusions may be indicated if anemia approaches a critical level. Treatment has limited effect on the course of disease once uremia has developed.
Management of infected herds merits special consideration. When leptospirosis is diagnosed in pregnant beef cows during the early epizootic phase, further abortions can be prevented by prompt vaccination of the entire herd and simultaneous treatment of all animals with appropriate antibiotics. Antibiotics reduce the number of leptospires in the kidneys and other tissues, at least during treatment, and provide a measure of protection until immunity is induced by vaccination. In dairy herds, generally only the sick animals should be treated with antibiotics because the loss of market milk after treatment must be considered.
Prevention:
Annual vaccinations, confinement rearing, and chemoprophylaxis are used for control. Annual vaccination should be used in closed herds, whereas semiannual vaccination should be considered for open herds. Bacterins may confer protection against abortions and death and reduce renal infections, although some infections do occur. Management methods to reduce transmission include rat control, fencing cattle from potentially contaminated streams and ponds, separating cattle from pigs and wildlife, selecting replacement stock from herds that are seronegative for leptospirosis, and chemoprophylaxis and vaccination of replacement stock. Serology may fail to identify carrier animals, however, as many will have titers
