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Dynamics of Trypanosoma evansi outbreaks in the Pantanal, Brazil

A.M.R. Dávila*, R.A.M.S. Silva* & A.M. Jansen**

* EMBRAPA/CPA-Pantanal. E-mail: Fax: 55-67-2311011
** Department of Protozoology, FIOCRUZ. E-mail:


The Pantanal is an area of 140, 000 km2 located in the centre of South America between 16 and 21 S and 55 and 58 W. Its fauna is very rich and diverse. There are 230 species of fishes, 658 species of birds, 80 species of mammals, 50 species of reptiles and 25 species of Tabanidae. Extensive cattle ranches, varying from 10, 000 to 200, 000 hectares, occupy most of this wetland. It is populated by 3, 996, 000 cattle, 4, 966 buffaloes and 139, 760 horses (Cadavid Garcia, 1986). The traditional cattle-raising system is based on calf and yearling production and its marketing involves the animal transport to market-places, river ports or railway stations. The most common means of transportation is on foot, in lots averaging 906 animals and taking on average 11 days to cover 230 km (Cadavid Garcia 1985). Generally 6 cowboys herd the animals and usually each man has one extra horse.

Trypanosomosis due to T. evansi is the principal protozoal disease of horses and wildlife in the Pantanal causing several hundred deaths of horses and other species every year. Recently several outbreaks have caused a high mortality in horses, for example on one farm 50.5% of a herd of 95 horses died (Silva et al., 1995a) and on another 30 horses out of a herd of 40 died, both due to T. evansi infection (unpublished data). In horses, the clinical signs observed were anemia, edema of the legs and lower parts, fever, lethargy, loss of appetite, weakness, lacrimation, abortion and loss of condition (Silva et al., 1995b; 1995c). The diagnosis was made using the microhematocrit centrifuge test (Woo test).

Blood from each sample was taken and the parasites concentrated in the buffy coat of micro hematocrit tubes were examined by preparing thin smears. The trypanosomes were identified based on morphological and biometrical data.

Currently, 18 stocks have been isolated and maintained in nitrogen liquid at -196 °C. Many of them showed slender intermediate forms, slender, stumpy forms were also observed as a minor proportion. The kinetoplast was absent in many of these stocks. According to Ramirez et al. (1997) four isolates from the Pantanal showed significant biometrical variations, and a dog isolate (with a mean of 13.35m m in length) presented smaller measurements than 15-34m m described by Hoare (1972).

A trypanosome survey carried out on domestic and wild animals in the Pantanal by Nunes et al., (1993) demonstrated T. evansi in dogs, coatis (Nasua nasua) and capybaras (Hydrochaeris hydrochaeris). Stevens et al. (1989) verified an infection rate of 27% T. evansi in wild capybaras and 58% in semi-captive animals in the Pantanal. We have found a high prevalence (65%) in coatis during the short rainy season in a subregion of the Pantanal. According to Morales et al. (1976) healthy capybaras can harbour T. evansi and can constitute a reservoir for the infection of horses and dogs in Colombia. Other wild mammals could harbour T. evansi such as the ocelot (Felis pardalis) (Shaw, 1977) and the vampire bat (Desmodus rotundus) (Hoare, 1965) but their role as reservoirs in the Pantanal are still unknown. The vampire bat appears to play an important role in initiating outbreaks of Surra in horses which may then be sustained by flies, acting as effective vectors to spread the infection (Constantine, 1970).

In the Pantanal, studies showed that the vector season occurs in the first half of the wet season, from September/October to December/January. However the tabanids still remain in high numbers until the end of the rainy season. This season represents the period of major risk of trypanosome transmission by these insects due to their abundance and the populational peak of species of high vector potential notably Tabanus importunus.

The increase in cattle trading in the Pantanal over the last years has caused an increase in the movement of horses, cattle and dogs. This could have contributed to the spread the disease. No mixed infections with T. vivax have been recorded in the Pantanal, however this year a mixed infection of T. vivax/T. evansi was recorded in one bovine during a T. vivax outbreak in the Santa Cruz department, Bolivia. The optimum conditions for the animals to acquire or transmit T. evansi are found at transit resting places, mainly near the market-places, when the number of animals from different properties and their close proximity provide an excellent opportunity for disease transmission by the vectors.

We have observed a temporal association between the rainy season, when biting flies, particularly Tabanidae, are abundant, and an increase in the prevalence of T. evansi infections in horses. These outbreaks occured during the prolonged rainy season (September to May) which, in the Pantanal, coincides with the Tabanidae season. During this extensive and prolonged flooding (5 to 6 months) there is a reduction in the area available for the animals. Thus, all animals (domestic and wild) seek refuge in the limited and remaining small areas of dry land forest. As a result disease transmission by vectors may become more intense.

It is for these reasons that we believe there is strong circumstantial evidence to suggest that these outbreaks of "Mal de Caderas" are governed by factors such as the presence of domestic carriers (cattle, horses and dogs), wildlife reservoirs (mainly capybaras and coatis), concentration of animals in dry land areas, abundance of vector population and local husbandry practices such as intense traffic of livestock.

Vampire bats can play an important role in initiating the outbreaks, nevertheless tabanids are fundamental to spread the infection. We believe that the seasonal occurrence of these factors determined the regional epizootiological situation, however more detailed studies are necessary.


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