by: David John Taylor, DVM, Swine Practitioner, specialist in swine veterinary medicine, Glasgow, UK
www.msd-animal-health.com
Infection with leptospira has been demonstrated in all regions of the world where pigs are kept, but information about their prevalence depends upon the nature of the data available and may only apply to some, rather than to all of the serogroups and serovars recorded from pigs. The data have been generated in a number of ways, but the methods used fall into two main groups – those that demonstrate the organisms, their constituents and their products and those which demonstrate immunity to them, principally serum antibody to the organisms.
It is important to specify the husbandry system, as the leptospiral infections in free range pigs differ from those in intensively-housed animals and from those in wild boar or feral pigs. The type of sample used to demonstrate leptospira (blood, aqueous humour, kidneys, or aborted foetuses) may also affect the quality of the data.
PREVALENCE BY REGION
Country reports using serology list the antigens used, the sample size, some information about the source population and give the serogroups and serovars demonstrated. From these data, it is possible to say that leptospirosis has been demonstrated on all continents, although information from Africa is scant. There are reports from Nigeria, Zimbabwe (34%, 1999), Zambia and South Africa (22% slaughter pigs positive, 1995). There are numerous reports from Asia, from Japan, Korea, China (where data frequently refer to pigs from a single province and in a public health context), Vietnam, Laos, Thailand, India (60% positive, 2005) and Iran. In the Americas, there are reports from Argentina where one survey of 20,000 pigs found that 30% were positive in the MAT, mostly to Icterohaemorrhagiae and Castellana. A report from Venezuela found 53% herds to be infected and 26.25% of pigs. Similar reports come from Brazil, Peru, Colombia, Mexico and Trinidad. There is little recent published data from the United States (57% pigs positive, 2000) and Canada, although case material and isolation studies confirm the presence of the agent. Leptospirosis has been reported from most European countries, but extensive surveys are relatively rare. There are recent data from Spain (86% herds have antibody and 34% individual pigs, 2014), Lithuania (42% pigs) Croatia 38% pigs, 2013) Greece (28% pigs, 2003), Serbia (2.7% pigs 2014), Poland (1.2% pigs positive, 2011). Case reports and analysis of past cases confirm the presence of infection in other countries. Records from Australasia report leptospiral infections in New Zealand at levels of up to 90% in slaughter pigs and there are reports of cases and wild boar infections from Australia. Wild boar and feral pigs have been studied worldwide because of their relevance to the domestic pig populations, particularly where there is extensive husbandry as in southwest Spain. The prevalence of infection varies from 3.1% in Sweden, through 10.7%, Poland,13%, the United States, 20.%%, Brazil, 45.8%, Slovenia, 48-53%, Australia to Portugal, 65.4%. The occurrence and prevalence of serogroups depend upon the strains circulating in local wildlife but includes serogroups for which the pig is a maintenance host.
PREVALENCE BY SEROGROUP/SEROVAR
Pomona serogroup contains pig adapted strains of pomona and kennewicki transmissible from pig to pig. These pig adapted strains are found throughout the world, but are less common in Western Europe. Data are often presented as a percentage of total leptospiral infection: thus Pomona antibody was present in 16% of infected pigs in Germany and 33% in Poland (2011). Tarassovi serogroup may be pig maintained in Eastern Europe (Poland 3% of positives, 2014), but has been reported from pigs in Vietnam and was considered common in India. Australis serogroup contains serovar bratislava, an important serovar which can be maintained in pigs. It occurs worldwide and was reported to be present in 66% herds in Mexico (2011), 64% herds in the UK (2006), 52% sows in Lithuania, 8.8% pigs in Quebec, Canada (1999) and 6.2% of herds in Spain (2014). It formed 41% seropositives in Germany (2011) and has been isolated in the United States and in The Netherlands. Its presence has been recorded in Zimbabwe, Sweden, Thailand and Serbia amongst others. Icterohaemorrhagiae is associated with the brown rat, its maintenance host, and infections reflect contact between the species. Prevalences of 19% (Mexico 2011), 16% positive herds, 7% positive pigs (Spain, 2014), 12% (Peru, 2012) are high, but lower prevalences, such as 6% (Poland, 2011) are not uncommon. Grippotyphosa is not reported from some countries (such as the UK), but was recorded in 4% positive herds and 0.3% positive pigs (Spain, 2014). Canicola has been recorded at low prevalence in many countries, but was reported in 3.5% slaughter pigs in Brazil (2013).
CONCLUSIONS
Leptospiral infections are present in pigs all over the world, in intensively managed indoor herds, outdoor herds, backyard pigs and in wild boar and feral pigs. Information about their prevalence comes from serological examination of blood samples or foetal fluids using the classic MAT, or, increasingly, ELISA testing, using antigens which are becoming more serovar specific. PCR is increasingly being used on tissue, urine and foetal material and is also becoming more precise. There are marked differences in prevalence in pig populations between the high levels of 90% slaughter pigs infected in New Zealand through 20% found in Brazil to 1.2% reported from Poland in individual surveys. Where data are presented on a herd basis, it is important to note that a varying proportion of herds in a country can be free from infection. The prevalence of infection with different serogroups also varies from one country to another, depending on the presence of maintenance hosts of the organisms and the contact between these hosts and the pig population. Where the maintenance host is the pig (bratislava, Pomona and, probably, Tarassovi) infections may be transmitted from pig to pig and from herd to herd, with consequent continuing effects on production.