Just because a cat has antibodies in its blood indicating previous infection with Toxoplasma gondii is no assurance that it can't become reinfected with the parasite. That's in sharp contrast to the long-standing belief that once infected, a cat could not become reinfected later and begin shedding an egg-like form of T. gondii again in its feces. Now, ARS researchers are testing a commercially developed vaccine for cats that makes them immune to infection by T. gondii and stops the parasite-egg shedding. Cats would be vaccinated annually, especially weaned kittens that are a main source of infection. Although T. gondii infects animals and humans worldwide, cats are the only creatures known to excrete T. gondii oocysts, a form of the parasite that easily withstands nature's harshness. Humans can become infected by swallowing the oocysts. Healthy people other than pregnant women can weather T. gondii infection with few ill effects. But if a pregnant woman becomes infected, there is a 20 to 50 percent probability that her baby will be infected, possibly resulting in blindness, mental retardation or other medical problems for the child. The national cost of raising children infected this way was estimated in 1993 at $5.3 billion.
Parasite Biology and Epidemiology Laboratory, Beltsville, MD
J.P. Dubey, (301) 504-8128

A vaccine for baby pigs may be one step closer in the fight to eradicate a disease that costs U.S. pork producers $60 million a year. Pseudorabies is fatal to young pigs and causes abortion in pregnant sows. Until now, it's been difficult to protect baby pigs by vaccinating them. Current vaccines haven't protected young pigs during a critical time--when immunity from their mothers' milk begins to decline but before immunity from vaccination takes effect. ARS researchers evaluated combinations of proteins delivered by vaccinia virus--the one used to vaccinate humans against small pox. They found that one protein called gB stimulated an immune response in baby pigs with high maternal antibodies. A gB protein-vaccinia virus combination also reduced the amount of shedding of the pseudorabies virus, which cuts down on the spread of the disease. That combination could be the basis for making a new pseudorabies vaccine for young pigs.
Swine Virology Research, Ames, IA
Susan L. Brockmeier, (515) 239-8371

Quick identification of another virus that kills unborn and weakens newborn baby pigs can further help reduce losses to pork producers. Previously known as mystery swine disease, porcine reproductive and respiratory syndrome (PRRS) is one of the most costly diseases to the swine industry. It has been estimated to cause losses that range from $50 to $250 per affected sow per year. ARS researchers used a technique called restriction enzyme analysis to characterize 90 isolates of the PRRS virus from pigs. The test separated those isolates from pigs with PRRS virus strains versus vaccinated pigs. Until now, producers had no means to differentiate the vaccine strain of PRRS virus from field strains. Humans cannot be infected by PRRS virus. (Patent Application 08/609/334)
Virology Swine Research, Ames, IA
Ronald D. Wesley, (515) 239-8305

An outbreak of human leptospirosis in Nicaragua was traced to its source--domestic dogs--using a diagnostic test developed by ARS veterinarians. Prior to ARS diagnostic work, other researchers and health officials suspected rats might be the source of the bacteria that produced pulmonary hemorrhaging leading to illness and death in thousands of Nicaraguans. ARS research indicates that domestic dogs rather than rats were the source of infection. Humans can get leptospirosis from being exposed to animal urine through contaminated soil or water. Over 200 different strains of bacteria cause leptospirosis. A polymerase chain reaction test confirmed the exact strain that caused the outbreak.
Zoonotic Diseases Research, Ames, IA
Richard L. Zuerner/Carole A. Bolin, (515) 239-8325

Nature lends a hand to animals infected with gastrointestinal worms. That finding by ARS scientists could lead to a new way to treat parasite-infected humans and animals. While all the mechanics of this process aren't understood, it's known that a natural product of immune cells, interleukin-4 (IL-4), helps start the worm expulsion process. Production of IL-4 is stimulated by worm infections. In lab tests, researchers injected IL-4 into normal mice. The result: The mice's intestinal muscle contractions increased and additional fluids were secreted. The combination of these actions could dislodge worms from the intestine. Researchers say this discovery could lead to man-made copies of natural IL-4 for use in treating parasite-infected humans and animals.
Immunology and Disease Resistance, Beltsville, MD
Joseph F. Urban, Jr., (301) 504-8765

From the parasite in drinking water that sickened thousands in Milwaukee in 1993, more bad news: Cold doesn't necessarily stop Cryptosporidium parvum. It was once thought this waterborne, infectious parasite could be killed by freezing. But new ARS studies show oocysts--egg-like forms of the parasite--are still potent even after days of freezing temperatures. Oocysts were alive and infectious after being frozen in water at 14 degrees F for a week, at 5 degrees F for a day, and at 4 degrees below zero F for eight hours. Most household freezers run between +5 and -4 degrees F. So, ice made from oocyst-contaminated water must be considered infectious until it's been held at least 24 hours before use. Another unhappy conclusion: Oocysts have the potential to survive mild freezing temperatures in the environment and might be able to overwinter. Cryptosporidium infection in humans can be triggered by as few as 30 oocysts. C. parvum also wreaks havoc in livestock, causing severe diarrhea called "scours" in baby beef and dairy calves. In one study, the parasite was found in virtually all the calves checked during the first 30 days of their life.
Immunology and Disease Resistance Laboratory, Beltsville, MD
Ronald Fayer, (301) 504-8750

On another front, a product from cattle's white blood cells appears to be a promising shield against cattle infection caused by Cryptosporidium parvum. In lab tests, ARS researchers found the product made from cattle's white blood cells blocked C. parvum infection in highly susceptible white mice. The researchers have cloned the gene for the white blood cell product and will test it in cattle. C. parvum causes heavy losses in beef and dairy herds nationwide. In one survey of 28 states, cryptosporidium was found in all 28 states, on 59 percent of the 1,100 farms surveyed, and in 22 percent of the 7,400 calves checked. In another study, it was found in virtually all of the calves checked during the first 30 days of their life.
Immunology and Disease Resistance Laboratory, Beltsville, MD
Ronald Fayer, (301) 504-8750

A genetic immune disorder in Holstein cattle has been effectively eliminated from the breed by removing carrier bulls. Holsteins are the mainstay of the $19 billion U.S. dairy industry. ARS researchers were the first to confirm that bovine leukocyte adhesion deficiency (BLAD) was an inherited disorder. They further developed a diagnostic test to identify carriers of the BLAD gene. If a bull that has the BLAD gene is mated with a cow carrying the same gene, one in four of their offspring will inherit two copies of the gene and usually will die by seven months of age. ARS made the test available to the National Association of Animal Breeders and the Holstein Association to check 20,000 Holsteins, mostly bulls, worldwide. Bulls that were identified as carriers of the BLAD gene were removed from herds. Positive test results in purebred U.S. Holstein cattle have dropped from 23 percent of bulls tested in 1989 to zero.
Metabolic Diseases and Immunology, Ames, IA
Marcus E. Kehrli, (515) 239-8462

An enzyme produced naturally by Trichuris suis--the swine whipworm--could open the door to controlling that internal parasite in pigs. ARS research indicates the enzyme, called a thiol protease, plays a significant role in the worms' ability to digest and absorb nutrients. Blocking the action of the enzyme with a chemical could be an effective way to control the parasites. Swine whipworms cause weight loss, anemia and diarrhea in pigs, and can kill the animal if the infection is heavy. The worms form lesions in the surface of the intestinal lining of infected pigs. This allows bacteria to invade underlying tissues. Whipworms aren't completely controlled by commercially available worming medicines.
Parasite Biology and Epidemiology Laboratory, Beltsville, MD
Dolores E. Hill, (301) 504-8444

ARS parasitologists are using a fingerprint-like pattern of surface cuticular ridges to revolutionize the identification of the most important parasitic pest of cattle and sheep. The new method has made it possible to identify individual specimens of the large stomach worm of cattle and sheep as separate species--ending a century-long controversy. In the southeastern United States, these parasites are called "the bankrupt worm" because of the severe losses of young animals, cost of treatment and loss of contaminated pasture. The pattern of ridges on the outer surface of the worm, called the synlophe, also has been used by ARS parasitologists to develop identification keys to the highly pathogenic species of medium stomach worms in cattle and sheep. Recognizing the worms in cattle and sheep as separate species will improve control measures for these parasites. The method also can be used to recognize exotic species of both groups imported into the United States with exotic hosts.
Biosystematic Parasitology Laboratory, Beltsville, MD
J. Ralph Lichtenfels, (301) 504-8530