Lethal look-alike unmasked, examined - Neospora caninum

Science News, July 29, 1989 by J. Raloff

Lethal look-alike unmasked, examined

A widespread protozoan infection called toxoplasmosis strikes humans and many other warm-blooded animals, including an estimated 40 percent of all cats. Though many of those infected live symptom-free, others suffer spontaneous abortion, severe illness and even death (SN: 2/13/88, p. 102).

Now, a leading toxoplasmosis investigator reports data showing that another, long-unrecognized protozoan -- able to parasitize many of the same hosts--has for decades masqueraded as a particularly virulent form of the more familiar Toxoplasma gondii.

Fourteen months ago, parasitologist Jitender P. Dubey identified and named Neospora caninum, isolated from the stored tissues of 10 dogs that had succumbed to a virulent toxoplasmosis-like disease. Working at the Agricultural Research Service in Beltsville, Md., he eventually grew Neospora in his lab. Last November, he showed it could induce severe toxoplasmosis-like disease in dogs. Says dubey, "We now believe at least 4 percent of dogs are infected."

By infecting laboratory animals with Neospora, he has produced severe toxoplasmosis-like paralysis and death in cats, rats, mice and gerbils over the past year. Since February, Dubey has found the same Neospora in tissues from eight calves and one sheep. He described his ongoing work last week at the American Veterinary Medical Association meeting in Orlando, Fla.

Although Neospora can infect any tissue, Dubey says, "it is most commonly found in the brain and spinal cord," as is Toxoplasma. The two microbes look similar, except that Neospora cysts have a far thicker outer wall. Neospora also induces production of unique antibodies in its hosts -- a finding Dubey and colleague David S. Lindsay employed in designing diagnostic assays. One such test permits researchers to identify Neospora in tissues that have been stored for decades. A newer test involves mixing a special dye with antibodies harvested from rabbits infected with a suspect microbe. If the dye fluoresces, Neospora is confirmed.

Most questions about this protozoan's life cycle, prevalence and susceptibility to treatment remain unanswered. "We also don't know whether it is infectious to people," Dubey notes. "But given its similarity to Toxoplasma" -- which infects an estimated 35 percent of the U.S. population -- "there is at least a potential for it." Toxoplasma can cause central nervous system ailments including paralysis, blindness and retardation.

In future studies, Dubey's group will focus on abortion rates in infected cattle. They also plan to investigate how Neospora spreads. Toxoplasmosis can result from eating the raw or undercooked flesh of infected animals, or even touching the mouth with hands or utensils that have touched such meat. Feces from cats -- the only animals known to shed Toxoplasma's highly infective oocytes, or immature eggs -- is another source of the infection. Dubey, who 20 years ago identified the essential role cats play in Toxoplasma's life cycle, says his experiments with Neospora have now virtually ruled out cats as its primary host.