Prevalence and Epidemiology of Isospora Suis in Swine

Reports about the presence, prevalence and epidemiology of Isospora suis have come from Italy, Denmark, France, Portugal, Germany, Malaysia, Australia, North and South America, among others. The results of these investigations show a high farm prevalence of the disease in most of the reporting countries (45% to 85%), as well as a high incidence of litters affected by the disease (>30%).

A survey performed during a one-year period in Germany (Meyer et al., 1999) recorded that during the suckling period the infection rate of Isospora suis increased from 18.6 to 37.7%. Diarrhoea was recorded in 78.2% of the Isospora positive litters. These results coincide with surveys performed in Australia, where Isospora was found in 70.9% of piglets with diarrhoea, and with French results (Dellac et al., 1996) that found that litters with diarrhoea problems were 95% positive to oocyst excretion.

During the survey performed in Germany by Meyer (1999), Isospora suis often occurred simultaneously with pathogenic E. coli. This coincides with a study conducted by Dresden et al. (1993) in which the most common organisms involved in neonatal piglet diarrhoea syndrome were identified. Driesen found that

• in almost 36% of cases, I. suis was the only pathogen involved,
• in more than 10% of cases, I. suis was combined with E. coli,
• in 6% of cases, I. suis was combined with rotavirus, and
• in less than 1% of cases, I. suis was found together with both E. coli and rotavirus.

In this work Isospora suis was the most common cause of diarrhoea in piglets. These results coincide with the results obtained by Otten et al. (1995) and by Ilieff (1997), who measured the incidence of different pathogens involved in neonatal piglet diarrhoea and also found that Isospora suis was the most common pathogen involved.

Larson (1995) reported that in Denmark, a high prevalence of Isospora correlated strongly with a young weaning age, which may be explained by the rapid turnover of susceptible piglets in farrowing pens.

The sow plays a minor or nonexistent role in the transmission of Isospora suis. Lindsay et al. (1984) performed a study in the USA in which the oocysts excreted by sows were examined on farms with and without a history of Isospora suis infection in piglets. The study showed a high infection rate with Eimeria species in sows, but less than 1% with Isospora suis.

In other studies, Lindsay and Stuart examined the transmission of Isospora suis on two farms where rectal samples were collected from sows. The samples were taken one week before the farrowing, the day of the farrowing and one week after the farrowing. They also examined the colostrum and placentas microscopically for evidence of parasites. No evidence of parasitic infection was found in the sows, although clinical coccidiosis was found on Farm One (16 of 24 litters) and Farm Two (11of 12 litters). Isospora suis was the only species of Coccidia found in these piglets. The results of these studies indicate that sows are not the primary source of Isospora infection in nursing piglets.

Rather, the most important source of infection is the environment. Infected piglets excrete more than 100,000 oocysts per gram of faeces. In 1992, Henrickson et al. showed that piglets can become infected with a very low dose of Isospora suis oocysts (100). The oocysts are highly resistant to external influences, and the environment in the farrowing houses is favourable for rapid sporulation and transmission of the disease. Therefore, once Isospora is established on a farm, it is most likely transmitted from one contaminated litter to another. Disinfection in farrowing houses is essential to reducing the infection pressure on newborn piglets, but is not enough to control the problem entirely. In piglets, the highest occurrence of the disease is between the second and third week of age.