Health status, infection and host population dynamics: the use and assessment of haematology

BELDOMENICO PM; GEBERT S; TELFER S; BIRTLES R; BENNETT, M; BEGON M

Hanoi

Conferencia; 3rd International Conference on Rodent Biology and Management; 2006

CSIRO, National Institute of Plant Protection (Vietnam)

Our research focuses on the use of haematology as a proxy of general health condition and immunocompetence in wild rodent populations, and its usefulness in assessing susceptibility and response to infection. The haemogram is the cornerstone of laboratory clinical analyses and yields useful information about organismal well-being and disease by evaluating the number and health of the cellular constituents of blood. However, it has very seldom been used to assess health status in ecological studies of natural populations. Our goal was to describe the relationship between haematological parameters, rodent demographics and population dynamics. Subsequently, these parameters were evaluated in relation to infection with selected pathogens to investigate host susceptibility and response. The haematological parameters evaluated were divided into those that indicate health condition and those that show immunological response to parasitism. Health condition was evaluated as erythrocyte numbers (RBC) in blood, reflecting mainly oxygen uptake efficiency and nutritional status, and lymphocyte counts in blood, reflecting general immunocompetence. The response to parasitism was assessed by total white blood cell (WBC) counts and numbers of neutrophils, monocytes, and eosinophils in blood. A longitudinal study was conducted sampling four field vole populations from Kielder Forest (Northumberland, UK). Samples were taken every 4 weeks from March 2005 to March 2006 (except in December and February). Every field vole was uniquely identified with a subcutaneous transponder, weighed, sexed, and a blood sample was taken from the tail tip. Haemograms were produced with haemocytometers and blood smears. Infection with various pathogens/parasites (namely, Cowpox virus, Bartonella spp., Anaplasma phagocytophilum, Babesia microti, Trypanosoma microti, and gastroenteric coccidians and helminths) was assessed by serology, PCR or examination of microscopic preparations. Blood cells varied with sex and age in a similar manner to what has been described for laboratory rodents. Red blood cells increased with age and their counts were slightly higher in males than in females. Lymphocytes were the predominant WBC type, and their numbers decreased with age. Neutrophils and monocytes, conversely, tended to increase with age. A marked seasonality was observed in most cell types. RBC counts decreased from the onset of the breeding season (April) until late July, with a much more pronounced decline in females. Later in the breeding season (August), the RBC numbers began a quick recovery, and then declined again for both males and females in wintertime. Total WBC counts followed an inverse pattern. They peaked at the beginning of the breeding season, followed by a consistent decline until January. WBC numbers were higher for males while they were not breeding, but the levels were similar for both sexes during the reproductive season. Lymphocytes were highest during the second half of the breeding season, after which lymphocytes abruptly dropped and reached the lowest counts in early spring. Neutrophil numbers were highest at the onset of the breeding season, but then they declined consistently until November. They began increasing again after January. Monocytes followed a similar pattern, but they peaked later (July). The relationships between haematological parameters and current and previous density are presented, as well as relationships with infection dynamics. The onset of breeding appeared to have a major impact on the voles? condition, as evidenced by the decline in RBC and increment of WBC as a result of neutrophil production (indicative of infection). Haematology proved to be a useful tool in assessing condition in natural populations of field voles and it might help construct a more sound understanding of the distribution and dynamics of disease in wildlife populations.