Investigations of the Theileria parva carrier-state in cattle at the livestock/wildlife interface of the uPhongolo-Mkuze area in KwaZulu Natal, South Africa

Abstract

Corridor disease (Theileria parva infection in cattle associated with carrier buffaloes) was not reported to cause serious outbreaks prior to 1994. From 2002-2004, outbreaks in cattle have increased in the areas where the disease is endemic in buffalo populations. In this study, the occurrence of Corridor disease outbreaks in the Zululand district municipality was closely monitored from 2004-2009. The observations included the number of cattle involved in the outbreaks, clinical signs, parasitological and post-mortem examinations while blood for serum and in EDTA were collected for serological (IFA test) and molecular (real-time PCR) tests specific for T. parva. Samples were collected from cattle involved in the outbreak, the sick and presumed recovered cattle. Recovered cattle from the farms were brought to the laboratory at the Onderstepoort Veterinary Institute for further investigations. This included tick pick-up and transmission attempts to demonstrate their carrier status as well as assessing their immunity to further experimental challenge using virulent T. parva stabilate. Results were obtained on Corridor disease outbreaks in the study area and ad hoc locations comprising a total of 15 commercial farms and community diptanks in the district from 2004 to 2009. A total of 31 outbreaks were recorded during the study period. The number of outbreaks per year was stable, being 3 or 4 from 2004 to 2007. A 100 percent increase was recorded in the subsequent years, 2008-2009. In one location, Morgenzon farm comprising a commercial and community farmers, had experienced regular outbreaks from 2004-2009. It is also noted that some farms experienced outbreaks for three consecutive years. Three other farms had experienced outbreaks for the first time in either 2008 or 2009. The most severe outbreak occurred in Nyalisa in 2009 where the disease was experienced for the first time in one herd in which 202 cattle were involved and 57 died within 30-40 days after the onset of the disease. Using all the tools mentioned above, the cause of death was confirmed to be due to T. parva infection. The Corridor disease outbreaks that were investigated, have mostly been reported during the months from March-May (88 %) but some (8 %) were encountered during the winter months (June-August). The distribution of outbreaks mainly coincided with the activity period of adult R. appendiculatus. During the investigation period, a total of 846 cattle were tested for Corridor disease and the prevalence was found to be 27 %. The percentage of cattle which were found positive by PCR was 16.5. Seven percent were found positive on both PCR and IFA tests, an indication of the development of a carrier state. However, 10 % of the cattle remained sero-positive with no indication of being parasite-carriers (real-time PCR negative). Five cattle which recovered from an apparent severe T. parva infection in the field and confirmed to be positive by PCR, all became negative before they were used in the transmission experiments. Ticks derived from these cattle were used to infect susceptible bovines but only T. taurotragi was transmitted. The xeno-diagnosis failed to demonstrate the carrier state in these field cattle. The five Corridor disease recovered cattle obtained from different study locations mentioned above, received lethal challenge using T. parva buffalo-derived stabilate. All challenged animals, including the susceptible control, showed schizont parasitosis as detected by the T. parva</i. real-time PCR test starting day 11 to 23. All animals also developed significant antibody titer to T. parva by day 28. Of the field cattle, only one bovine which showed mild reactions manifested by high temperature on day 11 for two consecutive days and schizonts parasitosis in lymph nodes on day 15 for only two days and recovered. The rest of the field cattle did not show any clinical or parasitological reactions during the observation period (103 days). The control bovine had high fever and showed schizonts parasitosis by day 11 for seven consecutive days. The reaction was classified as severe and had to be treated. Unfed R. appendiculatus collected off grass from one of the study sites were applied to feed on a susceptible bovine and only T. taurotragi was transmitted. There were no apparent clinical signs and the animal behavior kept normal during the observation period (60 days). This study suggests that Corridor disease should be considered as an “emerging disease” and more stringent control methods should be implemented. Copyright