Theileria parva carrier state in naturally infected and artificially immunised cattle.

Rhipicephalus appendiculatus adult ticks, collected in the field from zebu (Bos indicus) and exotic (Bos tarus) cattle with high antibody titres to Theileria parva schizont antigen, transmitted Theileria parva infection typical of East Coast fever to susceptible cattle. Uninfected R. appendiculatus nymphs applied to naturally recovered zebu and exotic cattle kept under tick-free conditions in the laboratory for 16 and 7 months respectively, transmitted fatal theileriosis to susceptible cattle. Cattle immunised by the infection and treatment method were shown to be carriers of Theileria parva by examination of the salivary glands of ticks applied to them and by tick transmission. Three and 7 months after immunisation, Theileria parva infected lymphocytes were established in vitro from peripheral blood lymphocytes. This was the first demonstration, in vitro, of the existence of schizonts in Theileria parva (East Coast fever) infection carrier status. These studies show that cattle from endemic and epidemic areas of East Coast fever (ECF) become carriers thereby maintaining the T. parva population. The relevance of the findings in this study to the control of ECF by dipping, immunisation and treatment is discussed.

The effect of temperature on the rate of transmission of Theileria parva parva infection to cattle by its tick vector, Rhipicephalus appendiculatus.

A steer was infected with Theileria parva parva Kilae stabilate; nymphal Rhipicephalus appendiculatus ticks were applied to its ears so that they completed repletion when the steer had a high piroplasm parasitaemia. The engorged nymphs were subsequently incubated at 28 degrees C for 26-29 days to complete moulting, when the adult ticks were divided into two groups; one was incubated at 18 degrees C for 20 days and the other at 18 degrees C for 14 days and then at 37 degrees C for 6 days. Groups of ticks incubated at 37 and 18 degrees C were triturated and each resultant supernatant fluid inoculated into a steer. Both steers became infected, but the 37 degrees C supernatant group showed a much shorter pre-patent period to schizonts. Groups of ticks incubated at 37 or 18 degrees C were applied to pairs of cattle for 24, 48 and 72 h and then removed. There was a more rapid transmission of theileriosis to cattle by ticks kept at high ambient temperatures compared to those kept at low ambient temperatures. All cattle on which ticks treated at 37 degrees C were applied developed acute and fatal T. parva infection irrespective of the duration of tick application, while only 1 animal receiving ticks treated at 18 degrees C and fed for 72 h developed infection. The pre-patent period for macroschizonts was very short in all the groups receiving ticks incubated at 37 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Development and survival of Theileria parva parva in Rhipicephalus appendiculatus exposed in the Trans-Mara, Kenya.

Nymphal Rhipicephalus appendiculatus (Trans-Mara) were fed on a steer infected with a Theileria parva parva (Kilae 1) stock isolated from an indigenous steer in the Trans-Mara Division, Kenya, which had a high piroplasm parasitaemia. A total of 5000 engorged nymphs which had dropped on one day were enclosed in elongated nylon bolting silk tubes in groups of 200-300 and were transported immediately to the Trans-Mara where they were suspended vertically in the grass cover with one end touching the ground. Over 98% of the nymphs moulted into adult ticks and 50% moult occurred by day 28 after exposure. The ticks showed over 80% survival up to 308 days post-exposure but thereafter showed a marked mortality so that only 22.5% of the ticks were alive after 439 days. Theileria parasites were detected in the salivary glands by day 35 post-exposure and infection rates and levels increased markedly between 180 and 235 days post-exposure. Thereafter, the infection rates and levels generally decreased. Groups of ticks were triturated and the resultant supernatant fluid inoculated into pairs of susceptible cattle, and these proved infective from day 44 to 145 after exposure. Three subsequent attempts to induce infections with supernatant fluid were unsuccessful. From 294 days after exposure, groups of 50 ticks were applied to cattle and caused lethal T. p. parva infections up to 439 days post-exposure. Climatic observations showed a relatively even monthly rainfall as well as mean maximum and minimum monthly temperatures.(ABSTRACT TRUNCATED AT 250 WORDS)

Transmission of Theileria parva by a population of Rhipicephalus appendiculatus under simulated natural conditions.

A 2 hectare paddock on the Muguga Estate, Kiambu District of Kenya (altitude 2100 m) free of Theileria parva-infected ticks was seeded by applying Rhipicephalus appendiculatus nymphs to cattle infected with Theileria parva (Kiambu 4). It was estimated that over 50 000 engorged nymphs fell onto the pasture from 4 cattle with high parasitaemias during the cold season (June). Samples of these ticks were placed in plastic cylinders under the vegetation in the paddock and sexual stages of T. parva were detected in gut lumen smears in these samples up to 21 days after repletion. Zygotes were first observed to transform into kinetes on day 55 and parasites were first detected in salivary glands of adults on day 64. Moulting of the nymphs started on day 60 after repletion and was completed by day 87. Cattle introduced into the paddock showed their first infestation with adult R. appendiculatus on day 64 after repletion of the nymphs and the infestation level gradually increased. On day 76 after repletion, 17% of a sample of adult ticks infesting cattle showed salivary gland infections with T. parva and this increased to over 70% by day 150. From day 360 onwards, a decrease in both the percentage of ticks infected and the number of salivary gland acini infected/tick was noted. In addition, T. parva infections within the salivary glands required a longer period of feeding in the older ticks before they developed into sporozoites. Cycling of Theileria through ticks was prevented by the removal of female ticks before they completed repletion. Lethal challenge levels of T. parva for cattle existed in the paddock up to day 547 after repletion, after which introduced cattle showed no infection or only a sub-lethal infection. Cattle introduced into the paddock on day 808 after repletion showed no evidence of T. parva infection. The tick infestation of introduced cattle decreased markedly from day 368 and infestation had virtually ceased by day 808.

Induction of infective stages of Theileria parva by exposure of host ticks to high temperature.

It was demonstrated that Theileria parva parasites infective to cattle could be induced by exposure of infected adult Rhipicephalus appendiculatus ticks to 37 degrees C. The production of the sporozoites of T parva was not as efficient as by feeding ticks on rabbits.

Theileria parva piroplasmosis in mice, and its attempted transmission with Rhipicephalus appendiculatus ticks.

Theileria parva piroplasm infection of mouse erythrocytes was detected, following intraperitoneal inoculation of mice with irradiated cultures of T. parva-infected bovine lymphoid cells. Infection could not be transmitted to cattle using Rhipicephalus appendiculatus ticks.