Leptospirosis in New Zealand pig herds : an epidemiological study and a computer simulation model of endemic leptospiral infection in New Zealand with particular reference to Leptospira interrogans serovar pomona : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Veterinary Science at Massey University
A serological survey of pig sera from six regional areas throughout New Zealand indicated that 60% had titres to Leptospira interrogans serovar pomona and 13% to serovar tarassovi. Pig sera from the North Island districts had higher titres to pomona than those from the South Island districts, however the converse was true for titres to tarassovi. A serological survey of bacon weight pigs at slaughter revealed that 60% had titres to pomona, 53% to bratislava, while titres to tarassovi were undetectable. There was no significant linear association between the magnitude of corresponding pomona and bratislava titres. Pomona was isolated from 53% of pigs kidneys, however attempts to isolate bratislava were unsuccessful. The median prevalence of infection in bacon pigs from farms with endemic pomona infection, at the time of slaughter was 80%. A retrospective study of farming practices revealed that vaccination of breeding pigs had no effect on the infection status of their grower pigs at slaughter. It was also found that farms which reared their grower pigs to pork weight for slaughter were free of leptospiral infection, as were farms with less than fifty breeding sows. Cross sectional serological and cultural prevalence studies of grower pigs on farms with endemic pomona infection revealed that pigs less than ten weeks of age were not leptospiruric and had low or undetectable titres to pomona. Pigs between ten and twenty weeks of age showed an increasing prevalence of both leptospiruria and pomona titres. Further prospective studies indicated that piglets acquire passive immunity from their dams, which has a half life of around sixteen days. The majority of pigs less than fourteen weeks of age appear to be resistant to infection, thereafter the level of their passive immunity wanes and they become infected and leptospiruric. The weekly incidence of leptospiruria for pigs in an infected grower house was usually between 10% and 20%. Following infection, the intensity of leptospiruria was greatest in the first three to four weeks and it lasted for at least six weeks. Infection is believed to occur by both direct and indirect transmission of leptospires between infected and susceptible pigs. It was shown that grower pigs are at the centre of the endemic cycle of infection which is perpetuated by the transmission of infection from older infected pigs to younger susceptible pigs. This cycle of endemic infection can persist independently of the breeding herd. Experimental evidence following the artificial exposure of grower pigs to either serovar pomona or bratislava supported an hypothesis that the occurrence of bratislava titres were associated with early infection of pigs with serovar pomona. The heterologous titres were believed to be a serological cross reaction with homologous IgM antibody to which pigs had been exposed. Experimental evidence demonstrated that leptospires could survive in droplets of less than 50 um, however hamsters exposed to a leptospiral aerosol containing droplets of less than 50 um failed to become infected. Infection via the intranasal route in both hamsters and pigs showed that the infective dose of pomona was between 104 and 106 leptospires, indicating the intranasal route as a natural route for infection. Transmission of infection could therefore occur directly by infective droplets lodging in the nasal cavity. The vaccination of pigs, commencing at ten weeks of age on a farm with endemic pomona infection, revealed that multiple inoculations of a commercially available bacterin can be used to control the level of endemic infection within a grower pig herd. There was evidence to suggest that persistent passive immunity in young pigs could interfere with the efficacy of vaccination. A computer simulation model of endemic pomona infection in a pig herd [Simulated Leptospiral Infection within a Pig herd, SLIP89] was developed using the results of investigations described in this thesis by sequentially breaking down the cycle of endemic pomona infection into a series of logical events. The model repeats a number of predetermined independent and dependant events for each pig within a simulated herd. The outcome of each event is randomly determined from an appropriately selected probability distribution. Each cycle of repetition represents one week in time. The outcome generated by the simulation can be used to observe varying patterns of infection which are due to either the element of chance or the alteration of key variables within the model. The results generated by the SLIP89 must be viewed with the structure and limitations of the model in mind.