Leptospirosis and other infections of Rattus rattus and Rattus norvegicus

Abstract

A survey was carried out over 16 months to determine the leptospiral status of rats in urban and rural Waikato. One hundred and sixty rats, 112 urban and 48 rural, were examined for leptospiral infection and also for the presence of other bacterial pathogens, dermatophytes, encephalomyocarditis (EMC) virus, and ecto- and endoparasites. Data relating to sex, species, maturity, environment of the rats and the season in which they were caught, were subjected to statistical analysis to ascertain what effects these had on the prevalence of the various agents in a population of rats.

Sixty three rats (39.4%) had recent leptospiral infection, with 33.1% excreting leptospirae in the urine. Leptospira interrogans serotype copenhageni was isolated from 36 rats and ballum from 15 animals. Of the urban rats, 38.4% were infected compared with 41.7% of the rural rats. Infection with both serotypes was found in both Rattus norvegicus and in Rattus rattus. Of 51 animals from which leptospirae were cultured, 47.1% had titres to the microscopic agglutination test (MAT) of less than 1/20. The maturity of the rats had a highly significant effect (P<.001) on the prevalence of leptospiral infection.

Staphylococcus aureus was the organism most commonly isolated from external abscesses and from the swabs taken from the buccal cavity. Pasteurella pneumotropica was cultured from 20.9% of pharyngeal swabs. No Salmonella species, Streptobacillus moniliformis or Trichinella spiralis were recovered. Trichophyton mentagrophytes and Microsporum gypseum were isolated from the rats but the infections were non-clinical. The commensal Trypanosoma lewisi was observed in blood smears from 12.1% of rats. Ectoparasites were found in 39.2% of animals with Nosopsyllus fasciatus present in 25.3% of the rats. This flea is an important vector for Yersinia pestis (the plague bacillus) but this organism was not isolated from any rats in our survey. The endoparasites were those commonly found in feral rodents.

The survey indicated that the feral rats were carriers of potential pathogens for man and animals, although the leptospiral serotypes copenhageni and ballum, common in the rats, are not the ones causing the majority of cases of leptospirosis in man or domestic animals, in the Waikato area. The EMC virus has been isolated from an outbreak of disease in pigs in New Zealand, and there was serological evidence of its presence in a rat in our survey.