The pig is a reservoir host for Leptospira pomona, L. tarassovi, L. bratislava and L. muenchen, the last two being very closely related. It is not a reservoir host for L. icterohaemorrhagiae but it can be infected from rats urine and become ill. It can also become infected by other serotypes from other animals urine, for example L. canicola from dogs and L. hardjo from cattle but the infections are subclinical and do not result in disease. The pig is then an incidental host i.e. does not perpetuate the infection and is only responsible for minimal spread.

L. pomona causes important reproductive problems in female breeding pigs spreading slowly through the herd. It remains in the herd permanently unless steps are taken to eradicate it. It is not in the UK or Ireland and seems to have disappeared from Western Europe but is widespread throughout the rest of the pig rearing world. In America the skunk is an alternative reservoir host.

L. tarassovi causes a similar syndrome (i.e. a collection of signs and lesions) to L. pomona but tends to be milder and to spread more slowly. It is found in Eastern Europe and the Antipodes. It is thought that some wild animals are also reservoir hosts.

The pig is also a reservoir host for certain subtypes of L. bratislava and L. muenchen which are widespread throughout the pigs of the world. They causes a different syndrome to L. pomona and L. tarassovi and affect mainly pregnant gilts and second litter females because they will not previously have encountered it.

Once these organisms are introduced into a herd the pigs become permanent carriers with infection of the kidneys and intermittent excretion of the organism into the urine. L. bratislava/muenchen also permanently inhabit the fallopian tube of sows and the reproductive organs of boars and are spread in semen.

Infection can enter the herd in one of three ways:

1. Introduction of infected gilts and boars.
2. Infection brought into the herd by other animals.
3. Exposure of the herd to indirect sources of contamination, e.g.: contaminated water.

Leptospirosis can be a very difficult disease to diagnose because pigs are often infected but there are no clinical signs to be seen. Thus if you carry out a serological test in your herd and the result is positive, for example to L. bratislava, this does not necessarily mean you have disease - only that the animal has been infected and responded by producing antibodies. If you carry out a serological test with positive results for L. pomona, this may be a cross reaction to other non pig serotypes in the same group.

Conversely, if you have an infertility problem that clinically suggests leptospira as the cause, then such test results would support a diagnosis of disease if there were rising titres in the serum of the affected sows. However this may not be the case because pregnant females seroconvert early in the infection and by the time they abort or show symptoms the serum levels may be falling.

Methods of spread within the herd are as follows:

• Infection is by mouth, through the mucous membranes.
• Most leptospira are inhabitants of the kidney and found in urine.
• Introduction and spread within the herd can occur by various forms of wildlife.
• Introduction and spread within a herd can occur through the introduction of carrier boars and gilts.
• Pig to pig transmission via urine is common.
• Venereal infection is commonplace particularly with L. bratislava/muenchen.
• Contaminated water, floor surfaces , pools and streams.
• Wallows used in outdoor production if there are pools of fresh water.

Leptospira may become localised in the uterus during pregnancy, causing either abortions or increases in stillborn piglets. L. bratislava may persist in the oviduct and uterus of non pregnant sows, and in the genital tracts of boars. This may be an important medium for the maintenance of infection in the herd and be responsible for sows failing to conceive.

Clinical signs
In acute outbreaks inappetence and depression may be observed but chronic low grade disease is more common with abortions, stillbirths and an increase in poor, non-viable pigs. If abortions in a herd are more than 1% then investigations for leptospirosis should be considered. A reduction in farrowing rates and numbers of live pigs born per sow is also an associated factor particularly with L. bratislava infection.

It is important to appreciate that many infections are subclinical but the organisms may persist in the kidneys and reproduction tracts to cause problems later.

Signs associated with acute L. bratislava disease:

• Repeat breeders are common particularly in first and to some extent second pregnancy gilts.
• This often follows embryo loss and there may be copious vaginal discharges.
• Late term abortions.
• An increase in premature piglets.
• An increase in stillbirths.
• Mixed litters of live poor pigs and dead piglets at birth.
• An increase in mummified pigs.
• An increase in repeat breeding animals.
• Often there is a two year cycle of disease.
• Reproductive failure occurs in second litter females, rather than gilts following their introduction to older carrier boars.
• Disease is less common in older animals.
• In long standing carrier herds disease can be difficult to recognise.