Rabies in the Eastern Cape Province of South Africa--where are we going wrong?

Rabies is a growing problem in the Eastern Cape Province of South Africa. This study investigated dog ecology, vaccination coverage and rabies neutralising antibody levels in 203 randomly selected dogs within a local municipality in the former Transkei area. Responses to vaccination were also evaluated in 80 of these dogs. The population was remarkably uniform in size, breed and condition. Slightly over 1/5th of the population was between 6 weeks and 1 year of age, while very few dogs reached 10 years or older. According to owner responses, the Animal Health Technicians achieved a total vaccination coverage of 65% of owned dogs over several years, but only 56% within the previous 12 months. Only 32% of dogs had adequate circulating rabies virus neutralisation antibodies (> or = 0.5IU/l). After vaccination, 83% had seroconverted to this level. The magnitude of seroconversion was independent of body condition or age. This study proposes a different approach to vaccination strategies than those currently employed in certain areas of the province.

Infectivity to cattle of metacyclic forms of Trypanosoma (Nannomonas) congolense propagated in vitro. I. Development of localized skin reactions following intradermal inoculation.

Skin reactions similar to those induced by tsetse infected with Trypanosoma congolense were elicited in cattle at sites of intradermal inoculation of in vitro propagated parasites which morphologically resembled metacyclic trypanosomes. The time to detection of the reaction, the time to maximal size and the maximal size attained were dependent on the number of parasites inoculated, although it was possible to induce a skin reaction with as few as 20 trypanosomes. All cattle became infected with the initial detection of the skin reaction preceding parasitaemia by 3 to 7 days.

Cerebral trypanosomiasis in cattle with mixed Trypanosoma congolense and T. brucei brucei infections.

Six Boran steers were infected simultaneously with Trypanosoma congolense and T. brucei brucei while another group of 3 was inoculated with T. b. brucei one year after infection with T. congolense. Three further steers were infected with T. b. brucei alone. Whereas, the six animals which received simultaneous infections developed clinical signs of cerebral trypanosomiasis as evidenced by depression, ataxia and occasional circling, those infected with T. b. brucei alone did not. At necropsy, 4 out of the 6 simultaneously infected animals had a mild to severe disseminated non-suppurative meningoencephalitis. Trypanosoma b. brucei was isolated from the cerebrospinal fluid (CSF) of three out of the four animals with histological lesions. Two of the cattle superinfected with T. b. brucei one year after infection with T. congolense also developed both clinical and histological evidence of cerebral trypanosomiasis. Trypanosoma congolense was isolated from the CSF of one of these 2 animals. Specific antibodies to the variable surface glycoproteins (VSGs) of the infecting T. b. brucei and T. congolense clones were found in the CSF of the 8 animals that developed cerebral trypanosomiasis. In these animals however, there was neither temporal nor quantitative correlation between VSG-specific antibodies in serum and in CSF, implying a de novo synthesis of antibodies to the infecting trypanosomes in the CSF.

Susceptibility and immune responses of zebu and taurine cattle of West Africa to infection with Trypanosoma congolense transmitted by Glossina morsitans centralis.

Following tsetse-transmitted infection with Trypanosoma congolense, major differences in development of localised skin reactions, the ability to control parasitaemia, the degree of anaemia and in antibody response to trypanosomes were found between the reputedly trypanotolerant breeds of cattle (N'Dama, N'Dama/Baoule crosses, Baoule) and the trypanosusceptible West African Zebu. The local skin reactions that developed in the Zebu were large and severe while those that occurred in the other breeds were smaller and less severe or mild. The timing of appearance of parasitaemia and the height of the first peaks were similar in all the animals, but the Zebu were less able to control subsequent waves of parasitaemia. Possibly reflecting these events, it was only in the Zebu that significant anaemia developed. Neutralizing antibody against homologous metacyclic trypanosomes developed between 14 to 18 days after infection in all breeds of cattle; however, marked differences were found when antibody to trypanosomes derived from first peak parasitaemias were tested in the Zebu and Baoule. Neutralizing antibody against these parasites appeared in the Baoule on day 24 but were not detected in Zebu until day 51. Furthermore, the antibody titres were 3 log2 higher in the Baoule. It was concluded that the trypanotolerance exhibited by the West African taurine cattle might be related to a) their ability to control trypanosome numbers in the skin and in the bloodstream, an outcome that was possibly brought about by the earlier and superior immune response and b) failure to develop anaemia which might be associated with their capacity to control parasitaemia.

Parasite kinetics and immune responses in efferent prefemoral lymph draining skin reactions induced by tsetse-transmitted Trypanosoma congolense.

Localised skin reactions (chancres) occurred on the flanks of cattle at the sites of deposition by tsetse flies of metacyclic forms of Trypanosoma congolense. Marked enlargement of the draining prefemoral lymph nodes accompanied the development of the skin reactions. Lymph from these nodes was collected through polyethylene cannulae inserted into the efferent lymphatics, and examined for trypanosomes, cells and antibody content. Within 6-9 days after infected tsetse fly bite, trypanosomes were detected in the efferent lymph; this preceded their appearance in the blood by 3-6 days, indicating that the lymphatic system acted as a major route for the passage of trypanosomes from the skin into the bloodstream. Responses induced in the draining lymph node as a result of trypanosome migration included a 2-3-fold increase in the volume of lymph and up to a 10-fold increase in lymphocyte output, including blast lymphocytes and plasma cells. Neutralising antibodies to metacyclic trypanosomes were detected in lymph and serum by Day 14 after infection, although in 2 out of 4 animals investigated, they were not demonstrated in serum until Day 18. Trypanosomes were also found in small numbers in efferent lymph of the prefemoral lymph node on the flank contralateral to the infected tsetse bites after development of parasitaemia. Increases in lymph flow and cellular output occurred about the same time in the ipsilateral and the contralateral efferent lymphatics, but were significantly less in the latter. Homologous challenge of immunised calves with tsetse-transmitted parasites revealed that trypanosomes were eliminated at the level of the skin or within the draining lymph node, as no parasites were detected in efferent lymph.

Early events following challenge of cattle with tsetse infected with Trypanosoma congolense: development of the local skin reaction.

A local skin reaction (chancre) was elicited in susceptible cattle after the successful feeding of Glossina morsitans morsitans infected with one of two different cloned isolates of Trypanosoma congolense. The chancre first appeared as a small 2 to 3 mm nodule at the site of the challenge as early as day 5 and reached maximum activity by days 10 to 13 when it had developed into a raised, indurated, hot, painful swelling measuring up to 100 mm in diameter. Thereafter it declined in size and activity and by days 20 to 30 was undetectable. Histologically the lesion was characterised by an intense inflammatory reaction and a four- to 10-fold increase in total cellularity. Initially, polymorphonuclear leucocytes were numerous but these were soon replaced by a mononuclear cellular infiltrate consisting mainly of small to medium lymphocytes. Development of the chancre and detection of parasites in the skin preceded by several days parasitaemia and other clinical signs. It was concluded that the skin was acting as a focus not only for establishment of infection but also as a site for localised proliferation of the parasite before dissemination into the bloodstream. The bite of an uninfected tsetse fly produced no detectable reaction and experimental intradermal inoculation of metacyclic T congolense resulted in chancre formation followed by infection. Bloodstream forms given by the same route caused infection but failed to induce a chancre.

Induction of protective immunity in cattle by tsetse-transmitted cloned isolates of Trypanosoma congolense.

Cattle infected with cyclically (tsetse)-transmitted clones of Trypanosoma congolense were treated with the trypanocidal drug diminazene aceturate ('Berenil') at various intervals during development of local skin reactions (chancres), in order to investigate the role played by trypanosomes in the skin in the induction of protective immunity. Cattle, treated on or before Day 12 after infection and prior to detection of trypanosomes in the blood, showed a range of susceptibility to cyclically-transmitted homologous challenge three weeks later. An animal treated on Day 5, when the skin reaction was first detected, was completely susceptible, whereas of the animals treated on Days 10 to 12 at the peak of the skin reaction, some were immune, others exhibited partial immunity, while the rest were susceptible. The susceptible animals, however, showed evidence of sensitization to trypanosomes, as demonstrated by development of Arthus-type reactions following homologous challenge with tsetse-transmitted parasites or intradermal inoculation with lysed or irradiated homologous trypanosomes. Cattle treated 15 or more days after cyclically-transmitted infection, when chancre reaction was subsiding and trypanosomes were present in the blood, were completely immune to challenge. Immunity was achieved by treating cattle on Day 5 and Day 10 after infection with cyclically-transmitted trypanosomes but only if this procedure was repeated on several occasions, indicating that bloodstream forms of trypanosomes were not essential for the induction of protective immunity. Protective immunity appeared to be effective against the metacyclic trypanosomes at the level of the skin, as animals which were immune to homologous challenge with cyclically-transmitted parasites did not develop detectable skin reactions at the site of the tsetse bite. Furthermore, the time of the appearance in the serum of antibody which completely neutralized the metacyclic population (Day 15) coincided with the time at which cattle had developed immunity against homologous challenge. The immunity produced lasted for five months.

Trypanosoma congolense: susceptibility of cattle to cyclical challenge.

Cattle primed by cyclical infection with Glossina morsitans morsitans infected with cloned derivatives of Trypanosoma congolense and treated with the trypanocidal drug Berenil after 3 or 4 weeks were immune to cyclical challenge with homologous clones 3 to 5 weeks later. In these animals, localized skin reactions (chancres) and parasitemia did not develop. The same results were obtained in cattle given a homologous superinfection without prior treatment. On the other hand, cattle subjected to a cyclical challenge with heterologous clones were completely susceptible as demonstrated by the development of chancres. Immunity to homologous challenge was achieved irrespective of the bloodstream variable antigenic types used to infect the tsetse. It was concluded that for a given serodeme the variable antigen composition of the metacyclic population which develops in the tsetse is constant and characteristic. Immunity to cyclical challenge was also obtained with uncloned stocks, providing the same stock was used for challenge. On the other hand, cattle immune to homologous cyclical challenge with cloned material were not always immune to cyclical challenge with parent stock, indicating that certain stocks consist of more than one serodeme. On the basis of these findings, it may be possible to use the chancre as a marker for serodeme analysis.