Mark Walker


 Assistant Professor,

State Extension Water Specialist ,

Department Of Environmental and Resource Sciences

130 Fleischmann Agriculture-MS370,University Of Nevada

Reno, NV. 89557-0013

Off: 702-784-1938

Fax: 702-784-4789



Education, Research Interests, 1997 Publications (and Abstracts) and Interests


 Ph.D., Department of Agricultural and Biological Engineering, Cornell University, 1998

Research Concentration: Methods for Detecting and Assessing Factors Influencing Transport of Oocysts of Cryptosporidium parvum in Soil

Research Concentration: The Effects of Alternative Automatic Calibration Techniques on Parameter Estimation for HEC-1


Research Interests:

  Assessing the risk posed by nonpoint sources of acutely toxic substances, such as

Cryptosporidium. I am exploring ways to develop risk assessment tools that will be

useful for water suppliers.


 1997 Publications (see Abstracts):

 Large Volume Samples For Cryptosporidium And Giardia From Small Streams.
Walker, M., Walter, M., McDonough, K., Journal Of Environmental Engineering, 123(1): 89-92
Assessment Of A Dye Permeability Assay For Determination Of Inactivation Rates Of Cryptosporidium Parvum Oocysts.
Jenkins, M., Anguish, L., Bowman, D., Walker, M., Ghiorse, W., Applied And Environmental Microbiology, 63(10): 3844-3850
Use Of Sentinel Cryptosporidium Parvum Oocysts To Measure Effects Of Passive Manure Storage On Oocyst Inactivation.
Jenkins, M., Bowman, D., Walker, M., Ghiorse, W., 7th International Coccidiosis Conference, September, 1997, London, UK


Large Volume Samples for Cryptosporidium and Giardia from Small Streams

Walker, M., Walter, M., McDonough, K., 1997. Journal of Environmental Engineering, (123):1, 89-92

Sampling the headwaters of tributaries for the parasitic protozoa Giardia and Cryptosporidium presents challenges, especially because the calculated limits of detection are dependent on volumes of water filtered with a sampling apparatus. The design for sampling equipment analyzed here is useful for sampling waters of low to moderate sediment-generated turbidity and has been tested in the Catskill montains of New York State. It is highly portable and operates under gravity head. It can be left unatt ended, provided that conditions at sampled sites are favorable for completing collection of a sample. In all field applications, it met or exceeded federally recommended sample volumes. Tests with suspended sediment indicate that the sampler will filter a t least 140 L (35 gal.) if sediment-generated turbidities are less than 140 NTU and elevation head is between 1-4 m. If turbidity at sites of interest is related primarily to suspended sediments, this testing method may be useful for raw water sampling. < /P>

Assessment of a Dye-Permeability Assay for Determination of Inactivation Rates of Cryptosporidium parvum Oocysts

Jenkins, M.B., L. Anguish, M.J. Walker, D.D. Bowman, W.C. Ghiorse

Applied and Environmental Microbiology, (63)10: 3844-3850, 1997

The ability to detect viable oocysts in feces and environmental samples to discern their rates of deactivation is important for estimating the health hazard of this gastrointestinal parasite in watersheds that include dairy farms. Using complementar y methods of determining oocysts viability that were correlated with animal infectivity, rates of inactivation were calculated for oocysts in naturally infected feces of neonatal calves. Inactivation rates for two pools of oocysts in infected calf feces, stored at 4 deg. C. in the dark were .004 and .0056 per day, as determined by a fluorochrome dye permeability assay. These rates of inactivation were not significantly different from rates of inactivation based on in vitro excystation. Experiments with fo rmaldehye-fixed and unfixed fresh purified oocysts demonstrated that changes in oocysts wall permeability accounted for the inclusion of DAPI and PI and not a metabolic process. In contrast tot he in vitro excystation assay, results from the dye permeabil ity assay indicated changes in the permeability of the oocyst wall over time. Initial results for both pools of infected fees and fresh, purified oocysts indicated a predominance of oocysts impermeable to the fluorogenic stains DAOUI and PI. Infectivity a ssays indicated that these DAPI-/PI- oocysts were infective. Over time, there was a general decrease in proportions of the DAPI-/PI- oocysts and increase in proportions of oocysts that displayed nuclei discretely stained with DAPI but not PI. Increased ra tes in the shift from D-P- oocysts to D+P- oocysts were associated with increases in temperature. Increases in temperature appeared, thus to affect oocysts wall permeability and ultimately oocyst wall inactivation. Neither in vitro excystation nor animal infectivity assays could provide quantitative information about the physiological status (permeability) of the oocyst wall and is relation to viability. The fluorochrome dye permeability assay appeared to be a practical tool for discerning and quantifying the viability of oocysts in environmental samples.

 Use of Sentinel Cryptosporidium parvum Oocysts to Measure Effects of Passive Manure Storage on Oocyst Inactivation.

 M.B. Jenkins(1), D.D. Bowman(2), M.J. Walker(3), and W.C. Ghiorse(1),

 7th International Coccidiosis Conference, September, 1997, London, UK

(1)Section of Microbiology, Division of Biological Sciences; Cornell University, Ithaca, NY (2)Department of Microbiology and Immunology, College of Veterinary Medicine, Cornell University, Ithaca, NY; and (3)Department of Environmental and Resour ce Sciences, University of Nevada, Reno, NV

 Recently purified oocysts of Cryptosporidium parvum were used as sentinels to measure the effect of the passive composting of neonatal calf manure and bedding on oocyst inactivation. The basket of a microfiltration system with a 0.45 m nylon fil ter was filled with dried, sieved, oocyst-free manure. The dried manure was brought to near saturation with sterile distilled water and then inoculated with 2 x 106 freshly purified oocysts. A hole was cut in the cap of the basket, and was used to secure a piece of 60 m nylon mesh filter material. This sentinel system had been tested to demonstrate its ability to equilibrate with exogenous solutes, gases, and water potential, and to retain inoculated oocysts. To facilitate removal of the sentinels from t he manure pile as it increased in height, triplicate sentinels and controls (consisting of 2 x 106 oocysts in 1 ml of sterile PBS in 1.5-ml microfuge tubes) were secured to fishing line and placed at the end of 2.5-m long 6 cm diameter plastic pipe. The e nds of the pipe were cut in half to form a trough into which the sentinels and controls were placed, allowing them to be completely exposed to the manure. Two data loggers for recording ambient manure temperature every 6 hours for the duration of the expe riment were placed next to the sentinels and controls. Oocyst viability was determined using a dye-permeability assay (Jenkins, et al, 1997, Appl. Environ. Microbiol. (63)10:3844-3850). Sentinels and controls were removed from the manure pile on days 14, 25, 47, 61, 103, and 145. Oocysts were extracted from the contents of the sentinels by sucrose flotation. A hemacytometer was used to quantify extracted oocysts. The ambient temper ature of the manure rose from an initial 10C to 29C in the first 5 days of the experiment, fell to 15C after 30 days, was at itslowest, 4C, after 80 days, and then steadily increased until the study's termination. Inactivation of oocysts was significa ntly greater for the sentinels than the controls. On days 14 and 25, viability of sentinel oocysts was significantly less than controls. From day 47 to the termination of the study, 30% of the control oocysts were consistently viable. The percentage of vi able sentinel oocysts continually declined throughout the study with <10% remaining viable after 103 days. These data indicate that storing calf manure in piles can decrease the viability of C. parvum oocysts by >90% in the late fall and early winte r (Nov. through Feb.). Thus, storing C. parvum infected calf manure during the winter months before spreading it on fields could significantly reduce the risk of contaminating surface waters during spring run-off events.


Current Position and Interests

Mark Walker became a member of the faculty of the Environmental and Resource Sciences Department at the University of Nevada - Reno in early September, 1997. Prior to this, he was a hydrologist with the New York State Water Resources Institute at Cornell University.

Mark's research at Cornell involved developing and applying techniques to study transport of oocysts of Cryptosporidium parvum in soils. This parasite infected an estimated 400,000 people in a single outbreak in Milwaukee, Wisconsin in 1993 and led to the deaths of at least 35 people in Las Vegas in 1994. He conducted research to help design animal waste management practices to keep Cryptosporidium from contaminating drinking water supplies of New York City.

While in New York, he carried out several nonpoint source assessment and management programs. These included a statewide sampling survey of groundwater to assess the extent of contamination from normal use of pesticides, application of modeling, water sampling and geographic information system use for data analysis by Soil and Water Conservation Districts and watershed scale management programs sponsored by the U.S. Department of Agriculture.

His research interests include assessing the risk posed by nonpoint sources of acutely toxic substances, such as Cryptosporidium. He is exploring ways to develop risk assessment tools that will be useful for water suppliers. As State Extension Water Sp ecialist, Mark is working with county and regional faculty to identify priorities and opportunities for Extension involvement in community water resources issues.

Mark has several interests that made him eager to relocate to Nevada. First, he loves to hike, bicycle and cross-country ski. He thinks it's great to be in a place where you can do all three in the same day. Second, he has rusty old motorcycle that he's been restoring for several years. He hopes the dry climate will slow the corrosion enough for him get ahead of it. Third, he loves sunshine and hot springs. He can't imagine a place that has a bigger stock of each than Nevada.