Enemies and turncoats: bovine tuberculosis exposes pathogenic potential of Rift Valley fever virus in a common host, African buffalo (Syncerus caffer).

The ubiquity and importance of parasite co-infections in populations of free-living animals is beginning to be recognized, but few studies have demonstrated differential fitness effects of single infection versus co-infection in free-living populations. We investigated interactions between the emerging bacterial disease bovine tuberculosis (BTB) and the previously existing viral disease Rift Valley fever (RVF) in a competent reservoir host, African buffalo, combining data from a natural outbreak of RVF in captive buffalo at a buffalo breeding facility in 2008 with data collected from a neighbouring free-living herd of African buffalo in Kruger National Park. RVF infection was twice as likely in individual BTB+ buffalo as in BTB- buffalo, which, according to a mathematical model, may increase RVF outbreak size at the population level. In addition, co-infection was associated with a far higher rate of fetal abortion than other infection states. Immune interactions between BTB and RVF may underlie both of these interactions, since animals with BTB had decreased innate immunity and increased pro-inflammatory immune responses. This study is one of the first to demonstrate how the consequences of emerging infections extend beyond direct effects on host health, potentially altering the dynamics and fitness effects of infectious diseases that had previously existed in the ecosystem on free-ranging wildlife populations.

Beyond mice and men: environmental change, immunity and infections in wild ungulates.

In the face of rapid environmental change, anticipating shifts in microparasite and macroparasite dynamics, including emergence events, is an enormous challenge. We argue that immunological studies in natural populations are pivotal to meeting this challenge: many components of environmental change--shifts in biotic assemblages, altered climate patterns and reduced environmental predictability--may affect host immunity. We suggest that wild ungulates can serve as model systems aiding the discovery of immunological mechanisms that link environmental change with parasite transmission dynamics. Our review of eco-immunological studies in wild ungulates reveals progress in understanding how co-infections affect immunity and parasite transmission and how environmental and genetic factors interact to shape immunity. Changes in bioavailability of micronutrients have been linked to immunity and health in wild ungulates. Although physiological stress in response to environmental change has been assessed, downstream effects on immunity have not been studied. Moreover, the taxonomic range of ungulates studied is limited to bovids (bighorn sheep, Soay sheep, chamois, musk oxen, bison, African buffalo) and a few cervids (red deer, black-tailed deer). We discuss areas where future studies in ungulates could lead to significant contributions in understanding the patterns of immunity and infection in natural populations and across species.

Rift valley Fever in Kruger national park: do buffalo play a role in the inter-epidemic circulation of virus?

Rift Valley fever (RVF) is a zoonotic mosquito-borne virus disease of livestock and wild ruminants that has been identified as a risk for international spread. Typically, the disease occurs in geographically limited outbreaks associated with high rainfall events and can cause massive losses of livestock. It is unclear how RVF virus persists during inter-epidemic periods but cryptic cycling of the virus in wildlife populations may play a role. We investigated the role that free-living African buffalo (Syncerus caffer caffer) might play in inter-epidemic circulation of the virus and looked for geographic, age and sex patterns of Rift Valley fever virus (RVFV) infection in African buffalo. Buffalo serum samples were collected (n = 1615) in Kruger National Park (KNP), South Africa, during a period of 1996-2007 and tested for antibodies to RVF. We found that older animals were more likely to be seropositive for anti-RVFV antibody than younger animals, but sex was not correlated with the likelihood of being anti-RVFV antibody positive. We also found geographic variation within KNP; herds in the south were more likely to have acquired anti-RVFV antibody than herds farther north - which could be driven by host or vector ecology. In all years of the study between 1996 and 2007, we found young buffalo (under 2 years of age) that were seropositive for anti-RVFV antibody, with prevalence ranging between 0 and 27% each year, indicating probable circulation. In addition, we also conducted a 4-year longitudinal study on 227 initially RVFV seronegative buffalo to look for evidence of seroconversion outside known RVF outbreaks within our study period (2008-2012). In the longitudinal study, we found five individuals that seroconverted from anti-RVFV antibody negative to anti-RVFV antibody positive, outside of any detected outbreak. Overall, our results provide evidence of long-term undetected circulation of RVFV in the buffalo population.

Inter-epidemic and between-season persistence of rift valley fever: vertical transmission or cryptic cycling?

Rift Valley fever (RVF) is an emerging zoonotic mosquito-borne infectious disease that has been identified as a risk for spread to other continents and can cause mass livestock mortality. In equatorial Africa, outbreaks of RVF are associated with high rainfall, when vector populations are at their highest. It is, however, unclear how RVF virus persists during the inter-epidemic periods and between seasons. Understanding inter-epidemic persistence as well as the role of vectors and hosts is paramount to creating effective management programmes for RVF control. We created a mathematical model for the spread of RVF and used the model to explore different scenarios of persistence including vertical transmission and alternate wildlife hosts, with a case study on buffalo in Kruger National Park, South Africa. Our results suggest that RVF persistence is a delicate balance between numerous species of susceptible hosts, mosquito species, vertical transmission and environmental stochasticity. Further investigations should not focus on a single species, but should instead consider a myriad of susceptible host species when seeking to understand disease dynamics.

Evaluation of hematologic values in free-ranging African buffalo (Syncerus caffer).

As part of a large-scale disease screening program, blood samples were collected from 534 African buffalo (Syncerus caffer) in South Africa's Hluhluwe-iMfolozi Park in October 2005 and May 2006 to establish age- and sex-specific reference intervals for erythrogram and leukogram values. Sixty-seven of the animals were positive for bovine tuberculosis (TB), allowing for comparisons between TB-positive and TB-negative groups. Positive animals had basopenia and slight lymphopenia compared to TB-negative animals. Blood values were compared to those reported for captive African buffalo, American bison (Bos bison), and cattle (Bos taurus). The free-ranging buffalo sampled in this study had higher white blood cell counts than captive buffalo, and this difference was driven by lymphocytes. Free-ranging buffalo also had higher red blood cell counts, mean corpuscular hemoglobin concentration (MCHC), white blood cell counts, neutrophils and lymphocytes, and lower mean corpuscular volume (MCV) than cattle. Demographic and environmental factors strongly affected hematologic values in the study population. Older animals had significantly higher hemoglobin, hematocrit, MCV, and mean corpuscular hemoglobin (MCH), while younger animals had a higher red blood cell count, red cell distribution width (RDW), and white blood cell count, which was due to lymphocytes and basophils. Females had a higher hemoglobin concentration, hematocrit, MCV, MCH, and basophils than males. At the end of the wet season, hemoglobin, red blood cell count, hematocrit, MCHC, RDW, white blood cell count, and neutrophils were all significantly higher, while basophils and MCV were lower, than at the end of the dry season. Our results emphasize the need to use species-specific data when interpreting hematologic values and point to important differences in hematology between captive and free-ranging animals of the same species. Strong variability in hematologic values with animal age and sex, season, and herd affiliation indicates that ''normal'' hematologic values in wild animals vary throughout their lives and subject to fluctuating environmental conditions.