Mothballs, which contain either naphthalene or paradichlorobenzene, are toxic to all species. Toxicity may occur whether the mothballs are ingested, if the chemicals are absorbed through the skin, or if the chemical vapors are inhaled. Of note, paradichlorobenzene is considered less toxic than naphthalene (found in older mothball formulations).

Signs and symptoms of toxicity: Clinical signs of toxicity include severe vomiting, diarrhea, refusal to eat, lethargy, loss of balance, tremors or seizures. Hemolytic anemia, liver damage, renal damage, or death can occur.

Toxic consumption:
In dogs, ingestion of 1.5 g/kg (0.6 g/lb) of naphthalene mothballs can cause hemolytic anemia. For reference, mothballs typically weigh 2.7-4g each. There is no established threshold for toxicity for cats. There is also no known toxic threshold for paradichlorobenzene ingestion. All incidents of exposure should be reported immediately.

Dogs: Naphthalene Mothball Toxic Consumption
X-Small Yorkie, Chihuahua	Small Pug, Boston Terrier, Poodle	Medium Beagle, Scottish Terrier	Large Boxer, Cocker Spaniel	X-Large Retriever, German Shepherd	XX-Large Great Dane, St. Bernard
1 – 10 lbs. (0.45 – 4.6 kg)	11 – 25 lbs. (5 – 11.4 kg)	26 – 40 lbs. (11.8 – 18.2 kg)	41 – 70 lbs. (18.6 – 31.8 kg)	71 – 90 lbs. (32.3 – 40.9 kg)	91 – 110 lbs. (41.4 – 50 kg)
> 0.6 g (~ 0.25 mothballs)	> 7.5 g (~ 2.5 mothballs)	> 17 g (~ 6.5 mothballs)	> 27 g (~ 10 mothballs)	> 48 g (~ 17 mothballs)	> 62 g (~ 23 mothballs)

References:
Osweiler, G, et al. (2011). Blackwell’s five-minute veterinary consult clinical companion. Small Animal Toxicology. [Kindle version]. Retrieved from Amazon.com