Viruses of the Serengeti: patterns of infection and mortality in African lions

Association and host selectivity in multi-host pathogens

The distribution of multi-host pathogens over their host range conditions their population dynamics and structure. Also, host co-infection by different pathogens may have important consequences for the evolution of hosts and pathogens, and host-pathogen co-evolution. Hence it is of interest to know if the distribution of pathogens over their host range is random, or if there are associations between hosts and pathogens, or between pathogens sharing a host. To analyse these issues we propose indices for the observed patterns of host infection by pathogens, and for the observed patterns of co-infection, and tests to analyse if these patterns conform to randomness or reflect associations. Applying these tests to the prevalence of five plant viruses on 21 wild plant species evidenced host-virus associations: most hosts and viruses were selective for viruses and hosts, respectively. Interestingly, the more host-selective viruses were the more prevalent ones, suggesting that host specialisation is a successful strategy for multi-host pathogens. Analyses also showed that viruses tended to associate positively in co-infected hosts. The developed indices and tests provide the tools to analyse how strong and common are these associations among different groups of pathogens, which will help to understand and model the population biology of multi-host pathogens.

Plagues and adaptation: Lessons from the Felidae models for SARS and AIDS

Research studies of infectious disease outbreaks in wild species of the cat family Felidae have revealed unusual details regarding forces that shape population survival and genetic resistance in these species. A highly virulent feline coronavirus epidemic in African cheetahs, a disease model for human SARS, illustrates the critical role of ancestral population genetic variation. Widespread prevalence of species specific feline immunodeficiency virus (FIV), a relative of HIV-AIDS, occurs with little pathogenesis in felid species, except in domestic cats, suggesting immunological adaptation in species where FIV is endemic. Resolving the interaction of host and pathogen genomes can shed new light on the process of disease outbreak in wildlife and in humankind. The role of disease in endangered populations and species is difficult to access as opportunities to monitor outbreaks in natural populations are limited. Conservation management may benefit greatly from advances in molecular genetic tools developed for human biomedical research to assay the biodiversity of both host species and emerging pathogen. As these examples illustrate, strong parallels exist between disease in human and endangered wildlife and argue for an integration of the research fields of comparative genomics, infectious disease, epidemiology, molecular genetics and population biology for an effective proactive conservation approach.

Quantification of Feline immunodeficiency virus (FIVpco) in peripheral blood mononuclear cells, lymph nodes and plasma of naturally infected cougars

Infection of domestic cats with Feline immunodeficiency virus (FIV) results in a fatal immunodeficiency disease, similar to Human immunodeficiency virus 1 (HIV-1) in humans. Elevated plasma viral loads in domestic cats are correlated to decreased survival time and disease progression. However, FIV is also maintained as an apathogenic infection in other members of the family Felidae including cougars, Puma concolour (FIVpco). It is not known whether the lack of disease in cougars is a result of diminished virus replication. A real-time PCR assay was developed to quantify both FIVpco proviral and plasma viral loads in naturally infected cougars. Proviral loads quantified from peripheral blood mononuclear cells (PBMC) ranged from 2.90 x 10(1) to 6.72 x 10(4) copies per 10(6) cells. Plasma viral loads ranged from 2.30 x 10(3) to 2.81 x 10(6) RNA copies ml(-1). These data indicate that FIVpco viral loads are comparable to viral loads observed in endemic and epidemic lentivirus infections. Thus, the lack of disease in cougars is not due to low levels of virus replication. Moreover, significant differences observed among cougar PBMC proviral loads correlated to viral lineage and cougar age (P=0.014), which suggests that separate life strategies exist within FIVpco lineages. This is the first study to demonstrate that an interaction of lentivirus lineage and host age significantly effect proviral loads.

Feline lentivirus evolution in cross-species infection reveals extensive G-to-A mutation and selection on key residues in the viral polymerase

Factors that restrict a virus from establishing productive infection in a new host species are important to understand because cross-species transmission events are often associated with emergent viral diseases. To determine the evolutionary pressures on viruses in new host species, we evaluated the molecular evolution of a feline immunodeficiency virus derived from a wild cougar, Puma concolor, during infection of domestic cats. Analyses were based on the coding portion of genome sequences recovered at intervals over 37 weeks of infection of six cats inoculated by either intravenous or oral-nasal routes. All cats inoculated intravenously, but only one inoculated orally-nasally, became persistently viremic. There were notable accumulations of lethal errors and predominance of G-to-A alterations throughout the genome, which were marked in the viral polymerase gene, pol. Viral structural (env and gag) and accessory (vif and orfA) genes evolved neutrally or were under purifying selection. However, sites under positive selection were identified in reverse transcriptase that involved residues in the nucleotide binding pocket or those contacting the RNA-DNA duplex. The findings of extensive G-to-A alterations in this cross-species infection are consistent with the recently described editing of host cytidine deaminase on lentivirus genomes. Additionally, we demonstrate that the primary site of hypermutation is the viral pol gene and the dominant selective force acting on this feline immunodeficiency virus as it replicates in a new host species is on key residues of the virus polymerase.

Prediction of parasite infection dynamics in primate metapopulations based on attributes of forest fragmentation

Although the effects of forest fragmentation on species and ecological processes have been the focus of considerable research in conservation biology, our capacity to predict how processes will be altered and which taxonomic or functional groups will be most affected by fragmentation is still poor. This problem is exacerbated by inherent temporal and spatial variability in fragment attributes. To improve our understanding of this interplay, we examined how various fragment attributes affect one potentially important ecological process, parasite infection dynamics, and considered how changes in this process affect host metapopulations. From August 1999 to July 2003 we surveyed red colobus (Piliocolobus tephrosceles) metapopulations inhabiting nine fragments (1.2 to 8.7 ha) in western Uganda to determine the prevalence and richness of strongyle and rhabditoid nematodes, a group of potentially pathogenic gastrointestinal parasites. We used noninvasive fecal flotation and sedimentation (n = 536) to detect parasite eggs, cysts, and larvae in colobus fecal samples. To obtain an index of infection risk, we determined environmental contamination with Oesophagostomum sp., a representative strongyle nematode, in canopy (n = 30) and ground vegetation plots (n = 30). Concurrently, physical (i.e., size, location, and topography) and biological (i.e., tree diversity, tree density, stump density, and colobine density) attributes were quantified for each fragment. Interfragment comparisons of nine potential factors demonstrated that an index of degradation and human presence (tree stump density) strongly influenced the prevalence of parasitic nematodes. Infection risk was also higher in the fragment with the highest stump density than in the fragment with the lowest stump density. These results demonstrate that host-parasite dynamics can be altered in complex ways by forest fragmentation and that intensity of extraction (e.g., stump density) best explains these changes.

CR, Poss M. Examining effects of persistent retroviral infection on fitness and pathogen susceptibility in a natural feline host

Many animal populations carry endemic (i.e., permanently present) viruses but few studies have assessed the demographic consequences of these infections under natural conditions. We examined the effects of chronic infection with FIVPco, a feline retrovirus, on the fitness and pathogen susceptibility of its natural host, the cougar (Puma concolor (L., 1771)), in the wild. Based on data obtained through intensive monitoring of 160 cougars from two populations, we estimated survival and different measures of host fecundity of infected and uninfected individuals. In addition, we used serological data collected from 207 cougars to test whether FIVPcopredisposes individuals to a higher probability of infection with other pathogens. We found no evidence for an overall reduction in survival due to FIVPcowhen accounting for other sources of demographic variation (age, sex, and population). There was a consistent but nonsignificant trend towards poorer reproductive performance in FIVPco-infected females. We found no serological evidence for a higher probability of secondary infections associated with FIVPco. Overall, these results support the premise that chronic FIVPcoinfection is asymptomatic in its natural cougar host, probably because of a long evolutionary association between virus and host. However, results of stochastic simulations indicate that only larger reductions in annual survival (>20%) can be excluded with confidence. Also, the possibility of a so far unrecognized cost of FIVPcoinfection on cougar fecundity remains.

Seroprevalence and genomic divergence of circulating strains of feline immunodeficiency virus among Felidae and Hyaenidae species

Feline immunodeficiency virus (FIV) infects numerous wild and domestic feline species and is closely related to human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV). Species-specific strains of FIV have been described for domestic cat (Felis catus), puma (Puma concolor), lion (Panthera leo), leopard (Panthera pardus), and Pallas' cat (Otocolobus manul). Here, we employ a three-antigen Western blot screening (domestic cat, puma, and lion FIV antigens) and PCR analysis to survey worldwide prevalence, distribution, and genomic differentiation of FIV based on 3,055 specimens from 35 Felidae and 3 Hyaenidae species. Although FIV infects a wide variety of host species, it is confirmed to be endemic in free-ranging populations of nine Felidae and one Hyaenidae species. These include the large African carnivores (lion, leopard, cheetah, and spotted hyena), where FIV is widely distributed in multiple populations; most of the South American felids (puma, jaguar, ocelot, margay, Geoffroy's cat, and tigrina), which maintain a lower FIV-positive level throughout their range; and two Asian species, the Pallas' cat, which has a species-specific strain of FIV, and the leopard cat, which has a domestic cat FIV strain in one population. Phylogenetic analysis of FIV proviral sequence demonstrates that most species for which FIV is endemic harbor monophyletic, genetically distinct species-specific FIV strains, suggesting that FIV transfer between cat species has occurred in the past but is quite infrequent today.

Reflecting on ethical and legal issues in wildlife disease

Disease in wildlife raises a number of issues that have not been widely considered in the bioethical literature. However, wildlife disease has major implications for human welfare. The majority of emerging human infectious diseases are zoonotic: this is, they occur in humans by cross-species transmissions from animal hosts. Managing these diseases often involves balancing concerns with human health against animal welfare and conservation concerns. Many infectious diseases of domestic animals are shared with wild animals, although it is often unclear whether the infection spills over from wild animals to domestic animals or vice versa. Culling is the standard means of managing such diseases, bringing economic considerations, animal welfare and conservation into conflict. Infectious diseases are also major threatening processes in conservation biology and their appropriate management by culling, vaccination or treatment raises substantial animal ethics issues. One particular issue of great significance in Australia is an ongoing research program to develop genetically modified pathogens to control vertebrate pests including rabbits, foxes and house mice. Release of any self-replicating GMO vertebrate pathogen gives rise to a whole series of ethical questions. We briefly review current Australian legal responses to these problems. Finally, we present two unresolved problems of general importance that are exemplified by wildlife disease. First, to what extent can or should 'bioethics' be broadened beyond direct concerns with human welfare to animal welfare and environmental welfare? Second, how should the irreducible uncertainty of ecological systems be accounted for in ethical decision making?

Should we expect population thresholds for wildlife disease?

Host population thresholds for the invasion or persistence of infectious disease are core concepts of disease ecology and underlie disease control policies based on culling and vaccination. However, empirical evidence for these thresholds in wildlife populations has been sparse, although recent studies have begun to address this gap. Here, we review the theoretical bases and empirical evidence for disease thresholds in wildlife. We see that, by their nature, these thresholds are rarely abrupt and always difficult to measure, and important facets of wildlife ecology are neglected by current theories. Empirical studies seeking to identify disease thresholds in wildlife encounter recurring obstacles of small sample sizes and confounding factors. Disease control policies based solely on threshold targets are rarely warranted, but management to reduce abundance of susceptible hosts can be effective.

Predictive thresholds for plague in Kazakhstan

In Kazakhstan and elsewhere in central Asia, the bacterium Yersinia pestis circulates in natural populations of gerbils, which are the source of human cases of bubonic plague. Our analysis of field data collected between 1955 and 1996 shows that plague invades, fades out, and reinvades in response to fluctuations in the abundance of its main reservoir host, the great gerbil (Rhombomys opimus). This is a rare empirical example of the two types of abundance thresholds for infectious disease-invasion and persistence- operating in a single wildlife population. We parameterized predictive models that should reduce the costs of plague surveillance in central Asia and thereby encourage its continuance.

Comparative Tests of Parasite Species Richness in Primates

Some hosts harbor diverse parasite communities, whereas others are relatively parasite free. Many factors have been proposed to account for patterns of parasite species richness, but few studies have investigated competing hypotheses among multiple parasite communities in the same host clade. We used a comparative data set of 941 host-parasite combinations, representing 101 anthropoid primate species and 231 parasite taxa, to test the relative importance of four sets of variables that have been proposed as determinants of parasite community diversity in primates: host body mass and life history, social contact and population density, diet, and habitat diversity. We defined parasites broadly to include not only parasitic helminths and arthropods but also viruses, bacteria, fungi, and protozoa, and we controlled for effects of uneven sampling effort on per-host measures of parasite diversity. In nonphylogenetic tests, body mass was correlated with total parasite diversity and the diversity of helminths and viruses. When phylogeny was taken into account, however, body mass became nonsignificant. Host population density, a key determinant of parasite spread in many epidemiological models, was associated consistently with total parasite species richness and the diversity of helminths, protozoa, and viruses tested separately. Geographic range size and day range length explained significant variation in the diversity of viruses.

Epidemiology, genetic diversity, and evolution of endemic feline immunodeficiency virus in a population of wild cougars

Within the large body of research on retroviruses, the distribution and evolution of endemic retroviruses in natural host populations have so far received little attention. In this study, the epidemiology, genetic diversity, and molecular evolution of feline immunodeficiency virus specific to cougars (FIVpco) was examined using blood samples collected over several years from a free-ranging cougar population in the western United States. The virus prevalence was 58% in this population (n = 52) and increased significantly with host age. Based on phylogenetic analysis of fragments of envelope (env) and polymerase (pol) genes, two genetically distinct lineages of FIVpco were found to cooccur in the population but not in the same individuals. Within each of the virus lineages, geographically nearby isolates formed monophyletic clusters of closely related viruses. Sequence diversity for env within a host rarely exceeded 1%, and the evolution of this gene was dominated by purifying selection. For both pol and env, our data indicate mean rates of molecular evolution of 1 to 3% per 10 years. These results support the premise that FIVpco is well adapted to its cougar host and provide a basis for comparing lentivirus evolution in endemic and epidemic infections in natural hosts.

Family and population effects on disease resistance in a reptile

Despite its importance in evolutionary biology, studies of the pattern of disease resistance in natural populations are rare. In this paper, we report patterns of infection of a viral eye disease in juvenile Swedish common lizards (Lacerta vivipara). Females were sampled at random from natural populations immediately prior to parturition with equal exposure of pathogens for all lizards once in captivity. No causative agents could be found that linked risk of disease to maternal/interfollicular transfer of pathogens. The results show that a major factor influencing offspring susceptibility is family identity, suggesting heritable variation in pathogen resistance. Our interpopulation comparison provides additional support for a link between genetics and disease resistance. Lizards in northern Sweden were not only more susceptible to the disease but were also more health compromised once infected, with relatively more reduced growth rate and increased mortality than lizards from the south. This scenario suggests that southern lizards have been under selection for resistance to this pathogen, whereas northern lizards have not, or at least not to the same degree. Thus, this study confirms the importance of genetic (family) effects on pathogen resistance with variation in this trait among natural populations.

Climate warming and disease risks for terrestrial and marine biota

Infectious diseases can cause rapid population declines or species extinctions. Many pathogens of terrestrial and marine taxa are sensitive to temperature, rainfall, and humidity, creating synergisms that could affect biodiversity. Climate warming can increase pathogen development and survival rates, disease transmission, and host susceptibility. Although most host-parasite systems are predicted to experience more frequent or severe disease impacts with warming, a subset of pathogens might decline with warming, releasing hosts from disease. Recently, changes in El Niño-Southern Oscillation events have had a detectable influence on marine and terrestrial pathogens, including coral diseases, oyster pathogens, crop pathogens, Rift Valley fever, and human cholera. To improve our ability to predict epidemics in wild populations, it will be necessary to separate the independent and interactive effects of multiple climate drivers on disease impact.

Pathogenesis of two strains of lion (Panthera leo) morbillivirus in ferrets (Mustela putorius furo)

Canine distemper virus (CDV) was previously considered to have a host range restricted to the canid family. In 1994, the virus was associated with sporadic outbreaks of distemper in captive felids. However, after severe mortality occurred in the Serengeti lions (Panthera leo), attention became focused on the pathogenesis of the virus and a concerted effort was made to identify the virus as CDV or a closely related feline morbillivirus. The present study was designed to explore the susceptibility of ferrets to challenge with two morbilliviruses isolated from lions and the protective effects of a modified-live mink distemper vaccine. Because mortality in ferrets infected with pathogenic CDV approaches 100%, the ferret was selected as a test animal. Two strains of lion morbillivirus were used as a challenge, A92-27/20 (California lion isolate) and A94-11/13 (Serengeti lion isolate). The two strains of lion morbillivirus were antigenically related to CDV (Rockborn strain), and ferrets were susceptible to both of the viruses when inoculated intraperitoneally. The inoculated ferrets were anorectic at 5-6 days postinoculation (PI), exhibited oculonasal discharge at 9-12 days PI, and became moribund at 12-22 days PI. Severe bilateral conjunctivitis was the typical clinical sign. Inclusion bodies characteristic of morbillivirus (eosinophilic, intranuclear, and intracytoplasmic) were distributed in many epithelial cells, including those of the skin, conjunctiva, gallbladder, liver, pancreas, stomach, trachea, lung, urinary bladder, and kidney. Virus was reisolated from selected lung tissues collected at necropsy and identified by CDV-specific immunofluorescence. Ferrets vaccinated with the mink distemper vaccine (Onderstepoort strain) were protected from challenge with the two lion strains, adding further support to the premise that the viruses are closely related to CDV.

Serological and demographic evidence for domestic dogs as a source of canine distemper virus infection for Serengeti wildlife

Following an epidemic of canine distemper virus (CDV) in Serengeti lions in 1994, the role of domestic dogs in the epidemiology of the disease was investigated by serological and demographic analyses. From 1992 to 1994, data were collected from two domestic dog populations bordering the Serengeti National Park. Several lines of evidence indicated that patterns of CDV infection differed significantly between higher-density dog populations of Serengeti District to the west of the park and lower-density populations of Ngorongoro District to the south-east: (a) CDV age-seroprevalence patterns differed significantly between years in Ngorongoro District populations but not in Serengeti District populations; (b) CDV seropositive pups (<12 months of age) were detected in Ngorongoro District only in 1994, whereas a proportion of pups in Serengeti District were seropositive in each year of the study; (c) in Ngorongoro District, the proportion of deaths attributed to disease was significantly higher in 1994 than in 1993, whereas in Serengeti District, there was no significant difference in disease-related mortality between years; (d) in Ngorongoro District, significantly more CDV seronegative dogs than seropositive dogs died in 1994, whereas there was no difference in survival of CDV seropositives and seronegatives between years in Serengeti District. We concluded that, between 1992 and 1994, CDV persisted in higher-density dog populations of Serengeti District, but occurred only sporadically in lower-density Ngorongoro District populations. Data from Ngorongoro District are consistent with exposure of dogs to CDV in 1991 and 1994, but not in 1992 and 1993. These findings suggest that higher-density domestic dog populations to the west of the Serengeti National Park were a more likely source of CDV infection for wildlife during 1994 than lower-density pastoralist dogs to the south and east of the park.